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F50/51 Limnophysics - Physikalisches Institut Heidelberg

1. For rainy days umbrella rain wear Note in summer the easiest way to walk on the boat is barefoot don t forget a towel please take something to eat with you for the lunch break Attention you are only allowed to do this practical course if you can swim 5 1 Measurements at Lake Willersinnweiher near Ludwigshafen 5 1 2 Measurements at the lake We will start at 9 00 o clock on Monday morning Meeting point is room 202 on the second floor in the Institute of Environmental Physics Usually your supervisor will test your knowl edge with questions like those in chapter 4 If you have done well you are allowed to do this practical course You will pack the car with the help of the list of materials shown above and your supervisor will drive you to Lake Willersinnweiher the ride takes approximately 30 minutes At Lake Willersinnweiher you will inflate the rubber dinghy and load it Water level of the lake On the lake you will measure the water level with the help of a reference level This reference level is an old weather station We know that the top of the steel girder is located at 88 582 m a s l above sea level This information is necessary for the interpretation of your data with respect to older data Figure 5 2 Weather station at Lake Willersinnweiher Calibration of the probe First step is to wheel down 20 m of the cable and insert the batteries mind the polarity Then you con
2. IlImberger J Von Rohden C and Aeschbach Hertig W Tracing and quantifying groundwater inflow into lakes using a simple method for radon 222 analysis Hydrol Earth Syst Sci 11 1621 1631 2007 Lampert W and Sommer U Limno kologie Georg Thieme Verlag Stuttgart 1999 Millard R C Owens W B and Fofonoff N P On the calculation of the Brunt V is la frequency Deep Sea Research 37 167 181 1990 58 Bibliography Quay P D Broecker W S Hesslein R H and Schindler C W Vertical diffusion rates determined by tritium tracer experiments in the thermocline and hypolimnion of two lakes Limnol Oceanogr 25 2 201 218 1980 Reichel T Optimierung eines Verfahrens zur Radonextraktion aus Wasser 2009 Sandler B Die Wirkung von Sanierungs und Restaurierungsma nahmen auf die N hrstoff str me und die biotische Dynamik eines anthropogenen Gew ssers am Beispiel des Willersinnweihers Ludwigshafen Ph D thesis Universit t Heidelberg 2000 Schmid J Calcitf llung und Phosphor Kopr zipitation im Phosphorhaushalt eines eutro phen Hartwassersees mit anoxischem Hypolimnion Willersinnweiher Ludwigshafen am Rhein Ph D thesis Universit t Heidelberg 2002 Sch nborn W Lehrbuch der Limnologie E Schweizerbart sche Verlagsbuchhandlung N gele u Obermiller Stuttgart 2003 Schwoerbel J and Brendelberger H Einf hrung in die Limnologie Spektrum Akademischer Verlag M
3. C T and Millero F J Precise thermodynamic properties for natural waters covering only the limnological range Limnol Oceanogr 31 657 662 1986 Dehnert J Nestler W Freyer K and Treutler H C Messung der Infiltrations geschwindigkeit von Oberflachenwasser mit Hilfe des nat rlichen Isotopes Radon 222 Grundwasser Zeitschrift der Fachsektion Hydrogeologie 99 1 18 30 1999 Durridge C Manual Rad7 siehe auch http www durridge com Manuals htm Bedford 2001 Ebert C Untersuchung neuer Verfahren zur Radonextraktion aus Wasser 2007 Hostetler S W Hydrological and Thermal Response of Lakes to Climate Description and Modeling pp 63 82 Springer Verlag 1995 Hydras3LT s Quick Start Instructions Hach Company 2004 Hydrolab Hydrolab DS5X DS5 and MS5 Water Quality Multiprobes USER MANUAL Hach Company 2005 Imboden D M and W est A Mixing Mechanisms in Lakes pp 83 137 Springer Verlag 1995 Jassby A and Powell T Vertical patterns of eddy diffusion during stratification in Castle Lake California Limnol Oceanogr 20 4 530 543 1975 Jung G Seen werden Seen vergehen Ott Verlag Thun 1990 Kipfer R Aeschbach Hertig W Peeters F and Stute M Noble gases in lakes and ground waters pp 615 700 Rev Mineral Geochem Mineralogical Society of America Geochemical Society Washington DC 2002 Kluge T Radon als Tracer in aquatischen Systemen 2005 Kluge T
4. Op a por K The thermal expansion coefficient has a negative sign because an increase of density means a contraction 2 3 The thermal expansion coefficient a for pure water changes its sign at the temperature of maximal density T max value is approximately 3 98 C For lower temperatures the thermal expansion coefficient a is negative for higher temperatures a is positive shown in figure 2 2 2 1 Limnophysics 998 5 998 0 997 5 0 2 4 6 8 1 1 10 12 14 16 18 20 T C Figure 2 1 The density 0 of pure water as function of temperature T at a pressure p of 1013 mbar 250 200 150 100 50 a 10 1 K 2 4 6 8 10 12 14 T C I r 16 18 20 Figure 2 2 The thermal expansion coefficient a of pure water as function of temperature T at p 1013 mbar calculated as described in Chen and Millero 1986 2 Basics The density of salt water The density of fresh water differs from the density of pure water because it additionally con tains salts and dissolved gases Usually the density is increased by dissolved salts and increased or decreased by dissolved gases Altogether the density can be described approximately as a function of temperature and dissolved matter p T Ci C2 p T 92 Sac 2 4 Thus the salt water density depends on the density of pure water p T the different concentrations of the substances C and the speci
5. Soil water zone Tre Well Ss 2 Intermediate Ber 2 H Vadose water Pellicu sr K g zone gravitational water A U x IW 1 illary water Z 2 2 d aman Water N ek Groundwater Capillary fringe Groundwater table c 53 zone Gere N E Impervious A Figure 2 16 Groundwater zones from Bear 1979 The soil water zone 1 is a zone near the surface in which the roots of the plants hold the water The water content of the soil water zone is dependent on precipitation Below the soil water zone the intermediate or vadose zone 2 is located There water is kept by capillary forces However the vadose zone is not saturated and we can find air embedded in pore spaces At the lower part of the vadose zone the capillary fringe 3 is located Water of the capillary fringe rises from the saturated groundwater zone 4 due to capillary forces The groundwater level is the depth of the saturated groundwater zone in which the hydrostatic pressure equals the atmospherical pressure Groundwater flow occurs in aquifers which are a system of connected pores so that water can flow slowly through it Good aquifers are e g sediments of gravel sand and sandstone 2 4 Radon as a Tracer In this practical course we will investigate in which depth groundwater stratifies into the lake water This is done with the help of a tracer A tracer is a substance or a characteristic property which marks a water body or water mass We are therefore able to de
6. and their interaction with groundwater 2 Bas cs This chapter is similar to the lecture Physics of Aquatic Systems of Prof Dr W Aeschbach Hertig 2007 2008 Students who took part in the lecture will recognize the theory shown in the next sections 2 1 Limnophysics Limnology deals with inland water and flowing water The word limnology has its origin in the Greek word limne which means lake and the Greek word logos which means knowledge or lore Limnology is mainly a biological subject and examines the structure and function of lakes as ecosystems To get a complete understanding of the processes in lakes the physical and chemical relations have to be known For example looking at a special kind of algae which exists mainly in a depth of 3 m beneath the surface of the lake involves all three sciences The biologist detects the algae but the reason for this special depth could not be explained without the chemist and the physicist One reason for this special depth is the right density of the water where the algae can float easily and at the same time the luminosity is bright enough to allow photosynthesis One part of general limnology is physical limnology which deals with the physical structures and processes in lakes One aim of physical limnology is to examine the mixing behaviour of lakes To get information about the mixing behaviour the stability of a water column needs to be known The stability can be calculated wit
7. determine the consequences for its ecosystem The second aim is to examine the influence of the groundwater inflow Gro parthweiher E P N p SB M EE Zo a Weiher Figure 1 1 Lake Willersinnweiher and his neighbour gravel pit lakes from Wollschl ger 2003 In this practical course two basic measurement techniques are applied one limnological and one hydrological technique To record the limnological parameters a probe is used to measure temperature conductivity and dissolved oxygen in different depths of the lake In the hydrological part water samples from the lake and groundwater are taken and their radon activity concentration is determined in the lab The measurement techniques for the radon measurement were developed at the Institute of Environmental Physics by T Kluge 2005 C Ebert 2007 T Reichel 2009 and many more 1 Motivation This practical course gives an overview on different fields of research and measurement techniques of the Aquatic Systems group at the Institute of Environmental Physics The research group is subdivided into groundwater and paleoclimate and physical limnology The members of the groundwater and paleoclimate group examine the noble gas concentration in combination with the age of ground water and analyse the signals with regard to paleocli mate The members of the group physical limnology are interested in the mixing behaviour of lakes with different sizes and shapes
8. energy is emitted to the detector The detector produces a signal with 50 per cent probability This signal is intensified electronically and transformed into a digital signal The microprocessor stores the energy level of the signal and produces the spectrum 37 3 Measuring instruments and techniques Solid state detector A solid state detector consists of an n doped area of a semiconductor in which electrons are able to move in the conduction band and a p doped area in which positive holes can move in the valence band Between these areas the depletion layer is located It is enlarged by connecting the plus pole of the voltage supply with the n doped area and the minus pole with the p doped area When an ionized particle hits the detector it causes holes and electrons which move due to the electric field These moving electrons and holes cause an electric current proportional to the energy of the particle Because the kinetic energy of its a particle is characteristic for every decaying nucleus one is able to discriminate a decays of different parent nuclides Spectrum of the RAD7 After the preparations of the measurement we can pump radon containing air into the RAD7 After a short time we can see some counts in the energy interval A which is the energy interval of the a decay of Po Usually the counting rate increases in the first five minutes because in this period of time the amount of positive ionized Po nuclei increases unt
9. is possible in the window Operate Sonde The Operate Sonde menu is divided into several tabs In the tab Online Monitoring we first choose the Monitoring Mode 1 time series which means that after each time unit choose the time unit in the Monitoring Interval 2 the probe will perform a measurement and will store it Figure 3 3 shows that after every 5 seconds one measurement is stored In the window Parameters 3 we choose the parameters we want to store For this practical course we need temperature Temp in C conductivity SpCond in uS cm dissolved oxygen DO in and DO in mg l depth Dep100 in m and the circulator If you want to change the order of the parameters use the up and down arrows 4 There are different possibilities to present the data 5 In a graphic New Graph or in a table New Table At the end of one whole measurement the data should be exported to an excel file and a text file Please store the files with the following name year month date and measuring site For example if a data series was taken on September the 15th 2008 at measuring site A you store the file with the name 080915A You can observe the data during measurement figure 3 4 and are able to see changes in the parameters accordingly to the time interval you have chosen If you want to change the shape of the graphic just right click For further informations to the Hydrolab probe look into the manual which can be found on the laptop in
10. means stratification of a lake What is the opposite state Can you explain the seasonal change of the temperature profile Why do we need the Brunt V is la frequency Which concept is the derivation based on What are the basic principles of the budget gradient method What temperature density and oxygen profile do you expect for Lake Willersinnweiher at the moment How does radon enter lake water and groundwater In which lake layers do you expect a higher radon value Why What does secular equilibrium mean Why is it important for this practical course 5 Implementation Figure 5 1 Measurements at Lake Willersinnweiher in 2008 43 5 Implementation 5 1 Measurements at Lake Willersinnweiher near Ludwigshafen 5 44 1 1 Materials rubber dinghy boat panels paddle electric motor and a fully charged car battery pneumatic pump and adapter probe Hydrolab and 8x AA batteries laptop fully charged battery USB cable to connect the probe to the laptop pipes to collect water samples approximately 20 m depth water pump for pipes and a fully charged car battery holder for pipe with a pair of pliers ruler barometer deionized water life jackets key to open the groundwater measuring site GWM tape and pencil 4x 250 ml bottles for groundwater samples buckets for lake water samples 4x 12 1 buckets key for the gate to Lake Willersinnweiher blue barrel to fill the buckets
11. measure your first vertical profile You take a profile by lowering the probe slowly and continuously to the ground of the lake with the help of the cable After you took the first profile pull up the probe wait a short while and let it down again in steps of 2 m 1 m or 0 5 m You have to wait at the different depths until the oxygen sensor has stabilised At the lake you have to measure five profiles two at the deepest point of the lake measuring site A two at the smaller part of the lake measuring site B and one at the measuring site C where we take our water samples The student who handles the laptop has to remember in which depth the thermocline is located because we will take the water samples in relation to the thermocline Figure 5 3 Measuring sites A Band C at Lake Willersinnweiher from Wollschl ger 2003 46 5 1 Measurements at Lake Willersinnweiher near Ludwigshafen Taking lake water samples Take lake water samples from at least three different depths using the pipes and the water pump You have to pump a while until you can be sure that the water you sample is from the corresponding depth When filling the buckets take care that there are no air bubbles inside This is very important because if we have air bubbles in the sample we will loose radon according to Henry s law Taking groundwater samples After taking lake water samples you have to pack everything into the car again and we drive to the groundwate
12. measurement with the method RAD H 0 Setup RAD H20 To determine the radon concentration in the groundwater samples the RAD H20 is used see figure 5 5 This setup is able to measure radon activity concentrations in the range above 100 Bq m The RAD is not able to determine the radon concentration of our water samples directly because it can only handle gases Thus an equilibrium between a certain gas volume we use air and a known water volume has to be established first For this purpose we use a glass frit shown in figure 5 6 According to the manufacturer Durridge about 95 of the radon passes to the gas phase within 5 minutes The final concentration of radon in the gas and the fluid phase can be described by Henry s law After reaching the equilibrium state the air is dried with a cold trap and is then pumped into a closed loop to the RAD7 Knowing the volume of the water and the volume of the air in the closed loop we can calculate the activity concentration in the water sample from the measured activity concentration in air Calculation of the radon activity concentration To reconstruct the radon activity concentration in water Cw o from the measured concen tration in air we have to know the volumes of the water and the air loop Vw and V4 and the water temperature T at the time of the measurement At the beginning all radon is located in the water Cw After reaching the equilibrium state the activity concentration is all
13. nchen 2005 von Rohden C and Ilmberger J Tracer experiment with sulfur hexafluoride to quantify the vertical transport in a meromictic pit lake Aquatic Sciences 63 417 431 2001 von Rohden C Wunderle K and Ilmberger J Parameterisation of the vertical transport in a small thermally stratified lake Aquatic Sciences 69 129 137 2007 Weigel F Radon Chemiker Zeitung 102 287 1978 Wiiest A Piepke G and Halfman J D Combined effects of dissolved solids and temper ature on the density stratification of Lake Malawi Toronto Gordon and Breach 1996 Wiiest A Piepke G and Van Senden D Turbulent kinetic energy balance as a tool for estimating vertical diffusivity in wind forced stratified waters Limnol Oceanogr 45 6 1388 1400 2000 Wilkening M Radon in the environment p 137 Elsevier 1990 Wollschl ger U Kopplung zwischen Oberfl chengew ssern und Grundwasser Modellierung und Analyse von Umwelttracern Ph D thesis Universit t Heidelberg 2003 59
14. nitrogen you can determine the weight of the buckets and the water temperature After you have finished purging the air loop you have to make sure that switch 1 lets the air to the RAD7 that the valves 2 3 7 13 6 9 11 and 12 are open while the others are closed After checking you exchange the normal top with the measuring top of the bucket and regulate the water flow of the pump Now you have to set up the RAD7 to five cycles with 10 minutes each and the pump to On immediately During the measurement the water level in the exchanger should be at the same level as the tape You have to look after the water level very carefully because the exchanger must not fill up with water completely This would enable the water to reach the RAD7 and damage it heavily After reaching the equilibrium state we will measure the radon in the air only in a small air cycle without the exchanger Therefore you should turn off the pump of the RAD7 close the valves 6 and 7 and open valve 4 Afterwards you put the pump on Auto and set up three additional cycles with at least four hours Now you measure the water temperature again and empty the exchanger for the next group After all measurements are finished you have to clean the cold traps from ice 52 5 2 Measurements in the hydrology lab Some data for the analysis Volume Weight Bucket 12 1 0 35 kg Bottle 250 0 5 0 ml 0 17 kg Table 5 1 The sampling containers RAD7 V
15. the lab or ask your supervisor 35 3 Measuring instruments and techniques O Internal Battery Oo External Battery Online monitoring Temp C KH Units 0RP mV Sem mv 5000 1000 SpCond mS em SpCond zien 00 Sat 4000 4 3000 4 2000 4 1000 4 o4 16 15 45 1143 2003 4 15 40 PM 16 18 30 11 3 2003 4 16 40 PM Circulator OFF Figure 3 4 Window Graph during measurement from Hydras3LT 2004 36 3 2 Radon measurement instrument RAD7 3 2 Radon measurement instrument RAD7 3 2 1 How the RAD7 works In the interior of the measurement instrument RAD7 from Durridge we find a hemisphere with a silicon solid state detector A representation of the measurement chamber with the detector is shown in figure 3 5 Air inlet Air outlet with filter nn Pump Ba O Detector e Readout Measurement chamber Figure 3 5 Measurement chamber of the RAD7 from Reichel 2009 Through the filter the sample air is sucked in by the pump and reaches the detector chamber There a high voltage of 2000 to 2500 V between the detector and the hemisphere accelerates the positively ionized particles towards the detector If a radon nucleus decays in the chamber into a positively ionized polonium 218 this particle will be accelerated towards the detector On the surface of the detector the short lived Po decays and the a radiation with a char acteristic
16. the lake flows from south west to north east with a relatively slow velocity of 6 1 1073 m d to 0 45 m d The values are taken from the model of Wollschl ger 2003 During autumn 1975 the lake suffered from a oxygen depletion which required an emergency aeration After this emergency aeration the morphology of the lake was reshaped to improve the lake s ventilation Before Lake Willersinnweiher was reshaped it was divided into two parts by an underwater barrier The height of this barrier was decreased to enable a deep circulation between both parts Furthermore the mud of the sediment was dug out and the lake reached its present maximum depth of 20 m In 1989 Lake Willersinnweiher got its official permission to be used as a swimming lake As a consequence the shore of the lake was reshaped with some small isles In figure 2 18 the recent morphology of Lake Willersinnweiher is shown Figure 2 18 The morphology of Lake Willersinnweiher The barrier is marked by a red line From Wollschl ger 2003 32 2 5 Lake Willersinnweiher The lake bottom is irregularly shaped with a very steep shore The shape of the shore is almost a straight line The red line in the figure marks the 8 m depth barrier which impedes the deep water exchange In table 2 4 some important parameters of Lake Willersinnweiher are summarized from the dissertations of Sandler 2000 Schmid 2002 and Wollschl ger 2003 Lake Willersinnweiher is a hard water lake
17. the oxygen values Please make a short note how you determined it At the end you should have gotten the oxygen values for the different depths and plot an oxygen profile Is it necessary to plot the error bars Perform these evaluations for both measuring sites A and B Where are the differences between the two profiles What is similar How can you explain the shapes of the profiles If you want to print the profiles you have to export the pictures from Origin by choosing Datei and then Grafik exportieren 55 7 Notes to the tasks 7 1 2 Notes to task 1 2 We want to compare profiles from different dates Therefore we have to convert the depth in m to the depth in m a s l For this conversion you need the distance from the water level to the steel girder 88 582 m a s l Please copy your values into the data collection you find on the PC in the file FP Limnophysik Pay attention to copy the values of the table and not the functions You can plot more than one profile in one graph with Origin Therefore you have to open the complete data table and choose Liniendiagramm in the menu Zeichnen Afterwards you choose the x and y axis of one date and click on hinzuftigen You have to continue with the next dates in the same way After plotting all the available data sets choose the ones you really need and print them Did you expect this seasonal change Give reasons 7 1 3 Notes to task 1 3 Calculate the stability frequency as a di
18. will increase by 0 87 mg l Like the coefficient of haline contraction the coefficient of the reference conductivity 6 is defined as _ 1 Op uS se Pk ee IG 2 12 After a short calculation the equation 6g 3 n is derived The value of the coefficient of the reference conductivity is calculated to 0 707 1078 uS cm for lakes containing mainly calcium bicarbonate Now the relation between the salinity S and the reference conductivity K29 equation 2 11 as well as the connection between both contraction coefficients are inserted in the density equation 2 8 With this we have an expression of the density as function of the in situ tem perature and the reference conductivity which can be calculated from the in situ conductivity measured by the probe p T r20 p T 1 Brk2o 2 13 11 2 Basics 2 1 2 Stratification of lakes One of the main aspects of physical limnology is the vertical layering called stratification If the temperature is dominating the density the lake is called thermally stratified In figure 2 3 three layers are drawn in the temperature profile The epilimnion is the upper layer with well mixed water because of the wind In the middle layer which is called the thermocline or metalimnion the temperature decreases The deepest layer is the hypolimnion The density is mainly influenced by temperature Therefore it is possible to subdivide the lake into three layers according to the temper
19. 2 6 on the left side In autumn the solar flux is smaller than in summer Therefore the surface temperature decreases until it reaches approximately 4 C In this situation the wind is able to mix the lake completely shown in figure 2 6 After a complete circulation the temperature of the lake is equal in every depth This situation is called a homothermal lake During winter the surface temperature can decrease to values below 4 C and the water at the surface can freeze The ice or cold water stays above the water with a temperature of 4 C due to the density anomaly of water shown in figure 2 1 In this situation called inverse stratification the eplilimnion cannot mix with the hypolimnion without an input of energy If the temperature at the surface decreases below the freezing point the ice layer insulates the lake water Therefore the ice layer grows only slowly from the surface to the ground This effect is very important because it protects the fish and other animals in the lake In spring the ice melts and a homothermal situation is reached again because of the increas ing solar flux Now the wind causes a second complete circulation of the lake Lakes which mix in autumn and spring are called dimictic lakes There are other types of lakes which mix only once a year mainly in spring Lakes can be classified according to their annual mixing behaviour There are amictic holomictic dimictic polymictic and meromictic lakes Am
20. 5 2 1 Groundwater measurement with the method RAD H20 48 5 2 2 Lake water measurement with the method RAD Aqua Plus 50 6 Working tasks 54 6 1 Limnological parameters 2 2 2 2m nn nn 54 6 2 Groundwater lake interaction 2 2 2m m nn 7 Notes to the tasks 7 1 Analysis of the limnological parameters 22 22 2 nn nme 7 1 1 Notes to task 1 1 7 12 gt Notes totale p TTT 11 32 Notesto task bis tage rer ee rer Dr od 7 2 Analysis of the groundwater lake interaction 22 222 2m 72 1 Notes to tasks 21 2 Sure tea eek rein 7 2 2 Bibliography Notes to task 2 2 1 Motivation We are interested in lakes for many reasons People use lakes for relaxing after work to go hiking around them or to just sit on a bench and enjoy nature Furthermore lakes are used for leisure time activities like swimming or fishing But lakes are not only important for recreation but also a very significant ecosystem for plants and animals Humans sometimes disturb the ecological balance of lakes with their various activities To protect the ecology of lakes it is important to know their limnological parameters For example if the oxygen content is too low the fish population of the lake will possibly die off So we need to know the oxygen content and how it is related to the mixing behaviour of the lake One aim of this practical course is to examine the mixing behaviour of Lake Willersinnweiher near Ludwigshafen and to
21. Ag 2 48 27 2 Basics 120 110 100 90 80 70 60 50 40 30 20 10 A Bq Polonium 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 t min Figure 2 15 Secular equilibrium between the activity A of 2 Rn and 8Po in dependence of the time t After a period of time t which is much longer than the half life Tz the term e gt in equation 2 47 is almost zero With this approximations and the activity of the mother nucleus A t AP e we derive mathematically the secular equilibrium A t A lt 2 49 In the case of a much bigger half life of the mother nucleus than of the daughter nucleus we will always see that the activity of both radioactive materials will be equal after a suitable time t The secular equilibrium is important for the calculation of the activity concentrations of radon with our measurement instrument RAD7 We reach the secular equilibrium between 22 Rn and 2 8Po after 20 minutes The secular equilibrium between Rn and 7 4Po see figure 2 14 is reached after three hours This second secular equilibrium is considered for very exact measurements During the practical course we will mainly consider the equilibrium between Rn and 2 Po 28 2 3 Groundwater 2 3 Groundwater Groundwater is subsurface water which flows above or between a watertight aquitard in the ground Figure 2 16 shows the different zones of groundwater Eh Soil water Ground surface 4 ye e
22. D Menu Key Enter Key Left Arrow Key Right Arrow Key LCD Figure 3 8 The RAD7 from Durridge 2001 choose one with the arrow keys and confirm with Enter If you want to go back just click on Menu Test includes commands to collect data series Data contains commands to look at old stored data With the help of Setup the parameters for the measurement can be set Special will not be used during this practical course In the following sections only the commands which are often needed are listed For further information consult the manual of the RAD7 from Durridge 2001 Setup Before we can start a measurement we have to choose the right adjustment The commands usually needed are Setup Cycle Setup Recycle and Setup Pump We choose the time interval of one measurement with Setup Cycle and how often this time is repeated with Setup Recycle To get a small error we usually make several measurements and take into account the error of all counts If we decide to measure three times with a period of 15 minutes we have to set the Cycle to 00 15 to get zero hours just confirm zero with Enter and the Recycle to 3 The different measuring techniques described in the next chapters need different adjust ments of the pump of the RAD7 With Setup Pump we can choose between ON OFF GRAB or AUTO With the help of ON OFF the pump is permanently on off If we choose GRAB the pump is on for five minutes than a five minutes equilibrium phase
23. F50 51 Limnophysics Version March 2011 Sylvia Lorenz Tillmann Kaudse Prof Dr W Aeschbach Hertig Abstract During the practical course F50 51 the students perform measurements at Lake Willersinn weiher an artificial lake near Ludwigshafen Temperature conductivity and oxygen profiles at different locations within the lake are determined Based on these profiles the mixing behaviour of the lake is to be investigated In the second part of the practical course the students take water samples from the lake and a groundwater sampling site to determine the groundwater inflow into the lake In order to examine the interaction of groundwater with the lake the radon concentrations of the samples are measured in the hydrology lab of the Institute of Environmental Physics in Heidelberg Important notes e You won t be allowed to do this practical course if you cannot swim e This practical course starts on Monday at 9 00 a m e In this instruction sheet all theoretical basics are included so you do not need any further literature Have you seen any mistakes during reading Please write an eMail to Tillmann Kaudse iup uni heidelberg de Contents 1 Motivation 5 2 Basics 7 2 Limn physich mus wane Bu eee Se ernennen 7 2 1 Water density a RR R at pi hoe ee oe Sara nel sn aa ee 7 2 1 2 Stratification of lakes 2 2222 Coon nn nn 12 2 1 3 Stability of a water column 2 2 2 2 2 nn nme 16 2 1 4 Vertical mixing behavio
24. adon activity concentration in the water phase and consider the time difference using the radioactive decay law After calculating the values of groundwater and lake water you are able to answer the following question is radon a good tracer 7 2 2 Notes to task 2 2 In the lab you will find a lab book Please write down your radon calculations and the other information which is necessary to interpret the values In the lab book you can look up the last radon measurements Plot these data with your radon data to get a better radon profile What can you see in the profile How can you interpret your graph Are there any relations between the groundwater level and the radon activity concentrations Reasons Figure 7 1 shows a radon profile of Lake Willersinnweiher in summer 2005 Does this profile match your results What are the differences e 20 Juni v 4 Juli ee een m 12 Juli m 19 Juli 26 Juli 5L bU 10 L io 15 e Typ Fehler 1 1 4 L 4 L 4 L 1 1 n L 0 5 10 15 20 25 30 Bq m Figure 7 1 Radon depth profile of the Lake Willersinnweihers in summer 2005 from Kluge 2005 57 Bibliography Aeschbach Hertig W Physik Aquatischer Systeme I Vorlesungsskript 2007 2008 Bear J Hydraulics of Groundwater McGraw Hill Publishing Company 1979 B hrer H and Amb hl H Die Einleitung von gereinigtem Abwasser in Seen Schweizerische Zeitschrift f r Hydrologie 37 2 347 369 1975 Chen
25. and its isotopes can be found in all three of the natural radioactive decay series which is shown in figure 2 14 The different radon isotopes have different half lives Rn which is sometimes called radon from the 7 U decay series has a half life of 3 82 days The isotope 2 Rn which is called thoron from the Th decay series has a much smaller half life of 55 6 seconds and the isotope 2 Rn which is called actinon from the 35U decay series has a even smaller half life of 3 9 seconds The radioactive noble gas radon is colourless odourless and shows a negligible reactivity with other elements Thus it is suitable for the use as a tracer see section 2 4 for example for the interaction between groundwater and lake water The processes we look at take place in a time range of hours or days and thus we choose radon as a tracer due to its suitable half life 2 2 2 Decay law The radioactive decay is a statistical process which can be described by the radioactive decay law equation 2 38 The number N t of nuclei of a radioactive material at time t which have not yet decayed decreases exponentially with time N t No e 2 38 No initial number of nuclei of the radioactive material A decay constant 25 2 Basics Uran Radium decay 234 U SF 234p 2380 230 Th 234 22 rg Th E Figg ine 71061 21760 71960 e ar Ho Thorium decay Th 232 oa pc Th rg T 208php P S T 212 2087 Pb 231 235 Uran Act
26. are almost horizontal while diapycnal mixing takes place perpendicularly to a density layer During isopycnal mixing we have a energy loss only due to friction Diapycnal mixing needs energy to move water against the gravity Therefore isopycnal mixing coefficients are about ten or hundred times bigger than diapycnal mixing coefficients In a stratified lake mixing in the hypolimnion will mainly be caused by eddy diffusion and molecular diffusion Eddy diffusion results from horizontal currents along different layers for example sediment isopycnals These small eddys cause further small eddys and a heat exchange is reached Molecular diffusion needs a concentration gradient and causes also a heat exchange 19 2 Basics The eddy and molecular diffusion together with further mixing processes which are de scribed in Imboden and W est 1995 are included in the mixing coefficient K which is also called turbulent diffusion coefficient There are many methods to determine the mixing coefficient With our equipment we get the best results with the budget gradient method Budget gradient method The budget gradient method is based on the change of a parameter for example temperature below a certain depth caused by the vertical flux We will derive the mixing coefficient by connecting the amount of heat below a layer and the vertical temperature gradient The budget gradient method assumes a horizontally homogeneous lake with no v
27. ature profile For lakes with a high salt content this approximation is not possible In this case the characterisation of epilimnon metalimnion and hypolimnion has to be done with the help of the density profile temperature C 0 5 10 15 20 epilimnion 107 metalimnion Eass POO TT a xe 20 hypolimnion 257 30 Figure 2 3 Characterisation of the different lake layers from Sch nborn 2003 Surfaces of equal density are called isopycnals In lakes isopycnals are approximately horizontal Within isopycnals no work is needed to mix water Therefore horizontal mixing processes are relatively fast and we consider lakes as horizontally homogeneous Vertical mixing needs energy because work against gravity has to be done Therefore vertical or so called diapycnal mixing is much slower for further information refer to chapter 2 1 4 The stratification of lakes changes with the seasons due to different solar radiation fluxes Lakes get their energy mainly through the surface Therefore the surface temperature of a lake depends extremely on the seasonal variations of the solar energy flux In figure 2 4 the seasonal changes of the energy flux and the surface temperature of lakes for different latitudes are shown In our latitude the solar flux has its maximum during summer The surface temperature reaches its maximum a little later during the late summer months Figure 2 5 shows a typical seasonal change of stratificati
28. bsorption of light in the visible spectrum is small plants in greater depth are able to do photosynthesis heat conductivity small heat is transported mainly through turbulences and not through molecular diffusion Table 2 1 Properties of water from Aeschbach Hertig 2007 2008 Density of pure water The dependency of the density of pure water on temperature is not linear and it is not possible to derive a theoretical equation Chen and Millero 1986 give an empirical polynomial which describes the density p in kg m of pure water 999 8395 6 7914 1072 T 9 0894 1078 T 1 0171 1074 T 1 2846 1076 T 1 1592 1078 T5 5 0125 10 T6 p T 2 2 Values of temperature T have to be put into the equation in C This equation is applicable for a pressure p of 1013 mbar and for temperatures between 1 C and 20 C Usually the density of a substance decreases with increasing temperature because of the characteristic thermal expansion Water has a special behaviour because the breaking up of the lattice structure of ice causes the volume to shrink as the distance between the molecules of water is shorter than the distance between the molecules of ice This process and the thermal expansion overlap and lead to the density function of pure water shown in figure 2 1 Another possibility to describe the density is given by the thermal expansion coefficient a defined as 1
29. d solubility a describes the ratio of the equilibrium concentration of the gas in the fluid to the concentration in the gas phase a 2 50 a Ostwald solubility C activity concentration in the fluid phase C4 activity concentration in the gas phase The solubility of radon is dependent on temperature This dependency was found empiri cally by Weigel 1978 a T 0 105 0 405 e 05027 2 51 In equation 2 51 the temperature T has to be put in in C For example if we calculate the solubility at a temperature of 20 C we get a value of 0 25 That means that at 20 C 30 2 4 Radon as a Tracer 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 T C Figure 2 17 Solubility a of radon dependent on temperature T the concentration in the gas phase is four times higher than in the fluid phase at equilibrium conditions Figure 2 17 shows the dependency of the solubility on temperature In the equilibrium state the gas concentration in the water phase is dependent on its con centration in the gas phase Cy see equation 2 50 The concentration of the gas phase is further dependent on the pressure as described in the ideal gas law n p Co V RT eee concentration in the gas phase in mol liter number of mols of the substance volume of the gas in liter pressure of radon in atm ideal constant for gas R 0 08206 atm liter mol K7 temperature in K HJY lt E A Henry s
30. ease starts with the stagnation of the water body The oxygen content in the hypolimnion is dependent on different constraints Oxygen in the hypolimnion can only get there by mixing with the epilimnion Therefore the mixing type of the lake is very important for the oxygen content A meromictic lake has no oxygen in the monimolimnion as the water of the monimolimnion has no contact to the atmosphere and the oxygen inflow by the groundwater is usually negligible In a holomictic lake the amount of oxygen depends on the volume of the hypolimnion A bigger hypolimnion can store more oxygen The strength of oxygen consumption depends on how much organic substances from the epilimnion sink to the deeper layers where they are decomposed In a lake with only small organic production in the epilimnion it is possible that most of the organic substances are decomposed before they have reached the hypolimnion Therefore only little oxygen is con sumed in the hypolimnion The velocity of the decomposition of organic substances depends on temperature At higher temperatures more organic substances are decomposed than at a lower temperature in the same time Further we have to consider that at higher temper atures the absolute content of dissolved oxygen is smaller than at lower temperatures see 22 2 1 Limnophysics temperature T C oxygen O mg l TO spring oligotroph summer eutroph TO T 0 depth ___ depth dept
31. entration for a sand and gravel aquifer is approximately 9 000 Bq m 2 4 2 How does radon get into lake water There are different origins of radon in lake water Inflows and precipitation as well as the surface water are in contact with the atmosphere thus their radon concentration is determined by the solubility equilibrium see below Since the atmosphere has no radon sources but only the sink due to decay its radon content and thus that of surface water is very low A further origin of radon in lakes is diffusion from the sediments Another source is the decay of dissolved radium in the lake water And finally radon in lake water originates from groundwater if there is an inflow of groundwater into the lake Altogether the radon concentration in lake water is much lower than in groundwater due to firstly the exchange of the surface water with the atmosphere and secondly the quite fast decay 2 4 3 Henry s law We take a look at a system of two phases for example air and water In this system we add a gas in our case radon A part of this gas will dissolve into the water and the other part will stay in the air If we wait a sufficient period of time we will see that the gas reaches a dynamical equilibrium state which means that the flux of radon from air to water equals the flux the other way round The amount of gas dissolved in the water phase depends on the solubility of the specific gas and other factors The dimensionless Ostwal
32. ertical advection Now we can calculate the amount of heat W zo in the layer between the depth zo and the maximum depth zm by integration We integrate the product of the temperature profile T z and the cross section area profile A z of the lake W z0 cpp iM A z T z dz 2 31 The amount of heat in the layer changes over time We can calculate this change of heat by differentiating the equation with respect to time In the following step we take into account that we can exchange integration and differentiation ea Cpp f u y 2 32 ot Ot In figure 2 11 the transport of heat through the area A zo during the time between tem perature profiles t and t2 is shown The amount of heat A transported by diffusion has increased the amount of heat and thus the temperature in the lower layer Area A Temperature T Figure 2 11 Diffusive transport of heat through the area A zo from the lecture Aquatic Systems I of Prof Aeschbach Hertig 2007 2008 20 2 1 Limnophysics Below a certain depth we can assume a flux of heat Fin only caused by diffusion and not by advective transport or radiation This vertical heat flux based on turbulent diffusion is described by Fick s first law OT a 2 33 Fin cpp K Fy thermal flux of energy per area and time Cp specific heat p density K vertical mixing coefficient ar vertical temperature gradient We get the additional amount of heat per time by multiplying the t
33. fers from lake to lake but the value can be approximated if the dominating salts are known Now it is possible to calculate the density as a function of temperature and salinity p T 8 p T 1 Bs 2 8 The probe used in this practical course does not directly measure the salinity S but the electrical conductivity The electrical conductivity of a solution is defined as the reciprocal value of the specific electrical resistance pg The unit Siemens S is equal to the unit O71 l E The measured in situ conductivity is dependent on temperature and salinity One aim of the practical course is to compare values of the salinity in different depths Therefore it is necessary to calculate the in situ conductivity to a reference conductivity k20 at a temperature of 20 C B hrer and Ambiihl 1975 give an empirical polynomial for x20 applicable for lakes with calcium bicarbonate as the dominating salt Kan kr 1 72118 0 0541369 T 1 14842 107 T 1 222651 107 T 2 10 with the electrical conductivity kr uS cm and the measured temperature T C Now we can derive the relation between salinity S and reference conductivity Kan the salinity will increase linearly with the salt concentration if it is a highly diluted solution S N Kau 2 11 The value of n is approximately 0 87 mg l uS cm for lakes dominated by calcium bicarbonate This means if the conductivity increases by 1 uS cm the salinity
34. fferential quotient with the help of equation 2 22 After calculating the stability values you can average over five values to decrease the variabilities You have to consider that there are positive and negative values for the stability frequency if you plot a graph in a logarithmic scale You should decide whether you want the stability N in linear or logarithmic scale and add the density to the graph Afterwards it is easy to interpret the graph 7 2 Analysis of the groundwater lake interaction 7 2 1 Notes to task 2 1 Calculation of the radon activity concentration of lake water The first step is to calculate the Ostwald solubility with the help of the measured temper ature Do not forget to consider the error in the temperature Therefore look at equation 2 51 and use the propagation of uncertainty from Gauss The second step is to get the volume of the water Therefore you have to weigh the buckets with the water and subtract the mass of the empty buckets Afterwards you can use the approximation that 1 kg water is equal to 11 Why can we use this approximation Next step is to look up the volume of the air from table 5 2 Now we determine the activity of the gas cycle Therefore you have to add the counts from energy interval A to get the counts in A multiply the complete counts with the per cent value and calculate the statistical error This value with error has to be corrected for the background You find a background meas
35. fic contraction coefficient 6 defined as p 12 S pac kg The specific contraction coefficient H describes the change of density by the change of substance concentration and is dependent on temperature Table 2 2 shows different values of 6 at a temperature of T 25 C Note that a negative signature indicates that the density of the water will decrease when the concentration of the substance increases 2 5 substance 3 10 kg g Ca HCO3 0 813 Fe HCO3 0 838 NH HCO 0 462 CO 0 273 NH 1 250 air 0 090 particle with p 2 65 g cm 0 632 1 10 g cm 0 091 Table 2 2 Influence of dissolved substances on the density of water values from Imboden and W est 1995 In general mainly salts are dissolved in lake water Therefore only the salts are considered and the dissolved gases and other neutral substances are neglected in the following calcula tions To sum up all salts a new parameter is introduced the salinity S defined as the mass of all salts in a volume per total mass of the volume _ mass of all salts in a volume of water g E 2 6 total mass of the volume of water The salinity influences the density To get a qualitive value of the influence the coefficient of haline contraction gs is defined as Bs a E 9 2 7 10 2 1 Limnophysics The value of 8g varies from 0 73 10 to 1 1073 g kg The coefficient of haline concen tration dif
36. follows and the 40 3 2 Radon measurement instrument RAD7 RAD starts counting This setup for the pump will usually be used if you want to determine the radon concentration of the air in your cellar For the practical course the option AUTO is more relevant If you choose AUTO the pump will start after the humidity in the chamber of the RAD7 reaches 10 If the humidity stays less than 10 the pump will be on every five minutes for one minute until the end of the measurement Test We choose the command Test Status to look at the current measurement In the left upper part of the display you see the run and directly behind it the cycle For example a displayed 0503 tells us that we measure the 5th measurement with the RAD7 and are presently in the third iteration On the right side of the cycle you either see Idle or Live Idle means the detector is in stand by mode Live indicates it is working In the lower left side of the display you see the time left and in the lower right side how many counts have been detected in the current cycle After pressing the right arrow key once you can see the next status window On the display you now see the data of the last finished cycle As before you see the run and cycle in the left part The center shows the value of the radon concentration with its statistical error note the RAD7 considers not only the one sigma error but further errors due to the dead time of the detecto
37. h oligotroph winter with ice eutroph T O T O depth depth w m r m m m m m m r m m m m TI Figure 2 12 Seasonal change of the oxygen content from Sch nborn 2003 23 2 Basics section 2 4 3 Therefore the oxygen content will decrease faster at higher temperatures The oxygen profile in a lake has a characteristic seasonal change which is shown in figure 2 12 During summer stagnation there are two typical oxygen profiles the orthograde and the clinograde distribution of oxygen The shape of the orthograde oxygen profile is similar to the oxygen profile after a full circulation That means the oxygen content is nearly constant in every depth An orthograde oxygen profile can be found in deep holomictic lakes with only small production of organic matter Lakes with low nutrient contents and hence low productivity are called oligothrophic The clinograde oxygen profile has a higher oxygen content in the epilimnion and a very low oxygen content in the hypolimnion The decrease of oxygen with depth does not have to be continuous It is possible that there are maxima and minima due to biological activity A clinograde oxygen profile can often be found in nutrient rich and thus highly productive lakes which are called eutrophic lakes Figure 2 13 shows some examples In part A the orthograde oxygen profile of the olig othrophic K nigsee is shown from 5 7 1980 In part B and C two clinograde oxygen profiles are sh
38. h the help of water density 2 1 1 Water density The density p of a water body is defined as mass per volume and is given in kg m p Fa 2 1 m The density of water like the density of other substances depends on pressure and temper ature Additionally the water density depends on the chemical salt dissolved substances and physical isotopes composition The water molecule has a polar structure and is there fore a good solvent Some special properties of water are summarized in the following table 2 1 The density p of water depends on the temperature T in a special way which is very important for lakes Due to the anomaly of density lakes freeze from the surface to the ground The water density in different lake depths is important for calculating the stability of the water body The stability values provide information about the vertical mixing which is important e g for transporting oxygen to the lake bottom 2 Basics property comparison with other substances importance for the geosphere and biosphere specific heat higher than the specific heat of any other natural fluid protects the environment from extreme variabilities of temperature thermal expansion fresh water has the highest density at a temperature above the freezing point anomaly of density fresh water lakes are freezing from the surface to the ground so fish can survive transparency high because the a
39. hden et al 2007 K a N 2 37 Typical values of the parameters a and b for Lake Willersinnweiher near Ludwigshafen were determined by von Rohden et al 2007 a 2 5 0 6 1071 m s3 b 0 81 0 02 Equation 2 37 shows that the stratification is stronger if the vertical turbulence is weaker More information about this subject can be found in the papers of Quay et al 1980 Jassby and Powell 1975 and W est et al 2000 2 1 5 Oxygen content in lakes There are many processes which change the oxygen content of lakes Oxygen gets into the surface water mainly by gas exchange of the surface water with the atmosphere A further important oxygen source are green plants and cyanobacteria blue green algae which perform photosynthesis Production of oxygen by photosynthesis is dependent on light Water absorbs light therefore oxygen can only be produced in the upper layers The deep hypolimnion is reached by almost no sunlight and thus it is not possible to produce oxygen there In contrast oxygen consuming processes are going on In the epilimnion the algae grow due to photosynthesis When algae die they sink to the hypolimnion where these organic substances are decomposed by bacteria The decomposition is an oxygen consuming process The oxygen in the hypolimnion which is needed for this process is mainly the oxygen of the last circulation in which the deep water exchanged oxygen with the atmosphere The oxygen decr
40. he dependency of stability and mixing coefficient Determine with the help of a figure 6 2 Groundwater lake interaction Tasks 2 1 2 2 2 3 2 4 Calculate the different radon activity concentrations for the different methods Plot your measurements and interpret your results In which depth is groundwater located Does this information give evidence of the depth of infiltration into the lake Compare your radon profile with older ones What is the reason for the differences Measuring techniques why do we have to use different methods for groundwater and lake water What are the advantages disadvantages of each method Important notes Please note your results in the lab book that succeeding groups can refer to your data Make sure that you work with a copy of the original data 54 7 Notes to the tasks In the lab you find a lab book Please write down your results into the lab book so that succeeding groups can refer to the data In the following sections you get hints for the working tasks Please use these hints only if you have no own ideas for the solution 7 1 Analysis of the limnological parameters Please copy the original data into a file with your names folder Eigene Dateien FP Limno physik your names In this file you can work with the data and store your figures and origin projects 7 1 1 Notes to task 1 1 In the following sections you will see how to work with the data of the probe Firs
41. hermal flux Fi with the cross sectional area A zo through which it passes The heat flux in equation 2 33 describes the thermal flux in upward direction z is positive upwards We are interested in the change of heat in the volume below the depth zo so we consider this information of direction with the negative sign The equation for the complete heat flux into the volume below the depth zo 15 OW zo t ot A zo Fin Zo 2 34 OT z t A 20 cppKz 20 5z 20 In a first step we solve the equation for the mixing coefficient K In a second step we insert equation 2 32 into the solved equation and get the following dependency OW z0 t 20 A OT z t K z0 ot alin Fa m 2 35 A z Je OT z t A OT z t 0 CpP Bz z 20 Oz Z0 During this practical course the gradient is calculated from the change of two temperature profiles t and t2 which are taken at two different dates during the year For the calculation of the mixing coefficient we therefore take the following equation AW zo t zo AT z t K z Ai Fen 2a 2 36 A A with At b t AW zo W zo t2 W z0 t1 OT z t Oz OT z t2 Oz 20 2 Oz 20 1 Fe 20 21 2 Basics Relationship between stratification and vertical diffusion Stratification and vertical diffusion correlate The relationship of the Brunt V is la frequency N and the mixing coefficient K can be described after von Ro
42. high biological productivity in the epilimnion This is visible in spring when strong algal blooms occur These algae produce a lot of organic matter which sinks to the hypolimnion and is decomposed inducing high oxygen consumption Therefore the oxygen content in the deeper layers is very low during times of stagnation values of 1 4 mg l 33 3 Measuring instruments and techniques 3 1 CTD probe 3 1 1 Design and functionality of the probe The HYDROLAB probe is a portable sensor capable to measure in situ temperature conduc tivity depth and oxygen concentration Such sensors are usually called CTD probes C for conductivity T for temperature and D for depth nN J CS LA 6 44 5 mm N NA ff NF Sy ed 1 78 inches 1 zu H j F N LT C 42 0 C b s NI ol MS 5 724 mm 28 5 inches 1 Calibration Cap 4 Housing 2 Calibration Cup 5 Battery Compartment 3 Locking Screw 6 Connector Figure 3 1 Structure of the CTD probe from Hydrolab 2005 The probe measures temperature with the help of a 30 kQ variable resistance thermistor which is able to measure temperature with an accuracy of 0 1 C and a resolution of 0 01 C in the range of 5 C to 50 C The conductivity sensor consists of four graphite electrodes in an open cell and is able to measure conductivity in a range of 0 to 100 mS cm with an accuracy of 1 uS cm and a resolut
43. ictic lakes never circulate because they are permanently frozen or thermally stratified Examples are lakes in the Arctic Green land in the Antarctic and in the tropics Holomictic lakes completely mix once dimictic lakes twice and polymicitic lakes many times a year Polymictic lakes are often relatively shallow Meromictic lakes do not mix completely over a long time period In meromictic lakes a deep layer can be found the monimolimnion which does not take part in a circulation event for a long time The layers above which take part in the seasonal circulation can be summarised as mixolimnion This special type of stratification is often caused by a high salinity in the monimolimnion Because of the high salinity the density of the monimolimnion is much higher than the density of the mixolimnion even if the temperature of the mixolimnion is approximately 4 C In the following figures the temperature or conductivity profiles for different lake types are shown Lake Alpnach a part of Lake Lucerne is a holomictic lake From figure 2 7 we can conclude that it completely mixed in the spring of 1992 14 2 1 Limnophysics Z i 18 Lake Alpnach 1992 a Ad l l 24 Iid we bf 30 T l i hen i li T T T T 0 5 10 15 20 25 Temperature C Figure 2 7 Temperature profile of Lake Alpnach from Kipfer et al 2002 A meromictic lake is for example the artificial lake Merseburg Ost 1b Lake Ra ni
44. il it reaches a constant level on the detector After 20 minutes the secular equilibrium state between 7 Po and Rn is reached According to chapter 2 2 3 this means the activity of the daughter nucleus is similar to the activity of the mother nucleus At this time almost all counts can be found in the energy level A which you can see in figure 3 6 IR IBIC ID Counts amp E MeV Figure 3 6 Spectrum of the RAD7 after a short period with new air which contains radon from Durridge 2001 After a period of time we find that the counts per time in A is constant but the overall counting rate increase These new counts occur at the energy level C of the spectrum They originate from the decay of Po see figure 2 14 which reaches its equilibrium state after 3 hours see chapter 2 2 3 In the full equilibrium state the height of both peaks is almost equal as shown in figure 3 7 38 3 2 Radon measurement instrument RAD7 IR JBIC ID Counts ER 6 E MeV E Figure 3 7 Spectrum of the RAD7 after 3 hours from Durridge 2001 counts in A are caused by Po and counts in C are caused by Po After each measurement the RAD7 has to be purged to clear the measuring chamber from the old radon concentration In the spectrum we see that the counts with energy field A decrease fast while the counts in C stay for a longer time The reason for this is that the counts in C originate from lead 210 and bis
45. inium deca 227Th P 221 p C4 231Th Ra c N 223 21po Fr 207ph 21Bi 215p K E 211 pb Figure 2 14 The three natural decay series from Kluge 2005 The half life 7 3 is the time after which the number of nuclei is halved Nal Noe D 1 AT ya In 5 AT 2 In 2 In2 T 1 2 2 39 Radon has a half life of 3 82 days Therefore after two half lives or approximately eight days only 25 of the original amount of radon can be measured So the radon measurements should be done in a short time after taking the water samples The activity A of a radioactive material describes the number of decays per time _dN t A N t 26 2 40 2 2 Radioactivity and radon The unit of the activity is Becquerel Bq Curie Ci or pico Curie pCi 1071 Ci One Becquerel is equal to one decay per second One Curie is equal to the activity of 1 g Ra 1Ci 3 7 10 Bq 2 41 In this practical course we will measure activity concentrations that refer to the activity of a certain water volume 2 2 3 Secular equilibrium We will determine the radon concentration of water by using the measurement instrument RAD7 The RAD7 is not able to measure the radon activity concentration directly but it can measure the decay products of radon Therefore we derive the relationship between radon and polonium in this section If we look at a small decay series of a mother nucleus N and a daughter nucleus Na the m
46. ion of 0 1 uS cm Depth information is collected with a pressure sensor that is able to measure depth in the range of 0 m to 100 m with an accuracy of 5 cm and a resolution of 1 cm The oxygen sensor consists of a measuring chamber a so called Clark Cell and a circulator which helps to get a sufficient sample flow across the membrane of the chamber The sensor measures oxygen by electrochemical reduction of oxygen diffusing through the selective membrane of the chamber The oxygen sensor is able to measure with an accuracy of 0 2 mg l in the range of 0 to 20 mg l and with an accuracy of 0 6 mg l in the range of 20 mg l to 50 mg l Both ranges have a resolution of 0 01 mg l 3 1 2 A short introduction to the software of the probe The CTD probe is controlled by the software Hydras 3 LT Therefore connect the probe to the laptop and start the software Usually the software detects the probe in the main menu If this is not the case plug the USB wire out and in again and click on the button Re Scan for Sondes 34 3 1 CTD probe EG HYDRAS3 LT olx File Connected Sondes MiniSonde 4a 41026 19200 Re Scan for Sondes Operate Sonde Terminal Mode Log Files Pot O com1 Log File1 Download Selected Files T Delete files in sonde after reading Figure 3 2 Starting window of Hydras3LT software from Hydras3LT 2004 As a first step we have to tell the software which parameters we like to store This
47. law states the proportionality of the gas concentrations in a liquid and a gas phase in equilibrium as expressed by equation 2 50 Often it is expressed as a relationship between the concentration of the dissolved gas in the fluid Ce and the partial pressure in the gas phase p We can derive Henry s law in that form from equations 2 50 and 2 52 There are different possibilities to write Henry s law The first and commonly used possibility is with the help of the Henry coefficient Ky RT a p Cy Ku 2 53 The second possibility uses the solubility coefficient Ks a RT Cn Ks p 2 54 31 2 Basics The Henry coefficient and the solubility coefficient are dependent on temperature but not on pressure Further the solubility of radon is dependent on the salinity Usually the solubility decreases with increasing salinity 2 5 Lake Willersinnweiher Lake Willersinnweiher is one of four artificial dredging lakes located between Friesenheim and Oppau near Ludwigshafen in the upper Rhine Valley It was created in the beginning of the 1930s by the BASF The gravel pit reached to the upper groundwater layer which reaches down to a depth of 25 m in the Rhine Valley Therefore the groundwater is connected to the lake and filled the gravel pit up with water Soon the newly formed lake was used for swimming Lake Willersinnweiher has no surface inflows or outflows and is only fed by groundwater and rain Groundwater in the surroundings of
48. mperature dp O0p00 00 08 dz 000z 0S dz The partial derivative of the potential temperature can be deduced from equation 2 28 00 _ oT F 2 30 2 29 Oz Oz Oz The adiabatic temperature gradient for lakes is usually small enough to be neglected There fore we can use the in situ temperature instead of the potential temperature for our lake This approximation is not allowed in deep lakes and the ocean Interpretation of the Brunt V is la frequency The inverse of the Brunt V is la frequency N is the oscillation period of a water package deflected by a small distance from its equilibrium state We consider that there is no exchange with the surrounding water during the oscillation The Brunt V is la frequency squared N is a quantitative measure for the stability of a water column gt 0 stable N 0 labile lt 0 unstable For negative density gradients i e if there is lighter water above heavier the square of the Brunt V is la frequency is positive and the water column is stable For positive density gradients the value of N is negative and the water column is instable The stratification is more stable if the density gradient is greater Therefore in a strongly stratified lake turbulences will calm down quickly The Brunt V is la frequency consists of two components N2 and N2 If the density gradient is dominated by temperature the stability N is almost equal to NZ If the density gradient is d
49. muth 214 which have much longer half lives Usually counts in C are called old radon For this practical course we will only consider the counts in A Notice If the peak in C is very high before we start a measurement we will have to take into account that the error of the RAD7 is higher because of a longer down time of the detector Efficiency of the RAD7 The efficiency of the RAD7 is very dependent on the humidity of the chamber A bigger humidity causes a smaller counting rate Reason for this dependency is that an ionized particle reaches a smaller range at a higher humidity because of the Bethe Bloch Ionization The Bethe Bloch Ionization is dependent on the density of particles If the humidity is higher the density of particles will be bigger and the range decrease Each time we start a measurement we have to proof that the humidity in the chamber is less than 10 During a measurement the sample air will be dried out with a cold trap 3 2 2 Manual of the RAD7 This is a short version of the manual of the RAD7 from Durridge 2001 You will find the complete version in the lab Main menu The RAD7 operates with four keys which are Menu Enter and Have a look at the four main groups Test Data Setup and Special by clicking on Menu To get into the groups 39 3 Measuring instruments and techniques Printer Power Socket RS 232 Serial Port Air Outlet Air Inlet Filter On Off Switch Infra red LE
50. nect the probe to the laptop with the USB cable and start the software Hydras 3 LT You have to calibrate the probe before you can use it We first calibrate the oxygen sensor Therefore we fill deionized water to the lower part of the membrane by paying attention not to get a drop on the membrane itself Now you put on the cap of the protecting calibration cup so that a pressure equilibration is still possible After a short period of time we will reach 100 humidity in the cup Now you can measure the air pressure and convert the unit from mbar to Torr The pressure value in the unit Torr has to be filled in the calibration window DO and you have to click on Kalibrierung to calibrate Usually you will see a 45 5 Implementation popup window with the message Kalibrierung erfolgreich which means the probe is calibrated If this is not the case just try it again After calibrating the oxygen sensor you have to exchange the calibration cup for the mea suring cage The measuring cage will protect the sensors of the probe head in case it reaches the lake s ground With the measuring cage you lower the probe to the water until the pres sure sensor is at water level Now you choose the menu Depth in the calibrating window and click on Kalibrierung You should see a value of 0 m for the depth now Now the probe is used as described in chapter 3 1 2 Recording vertical profiles After calibrating the probe and handling the software you can
51. ocated in both phases C asr and Cy 48 5 2 Measurements in the hydrology lab R ckschlagventil Temperatur Isopropanol Trockeneis Mischung Probengef a H ir EN Edelstahl Rad7 Dewargef Wasserfalle Wasser Glasfritte w rmebad Figure 5 5 Overview of RAD H20 from Reichel 2009 Cwo Vw Cair Vair Cw Vw 5 1 We have to take into account that the RAD7 always has a small underground radon con centration Cy Cwo Vw Cu Vair Cair Vair Cw Vw 5 2 The activity concentration in water in the equilibrium state can be described by equa tion 2 50 As mentioned in chapter 2 4 3 the Ostwald solubility coefficient is dependent on temperature Cy a Cin 5 3 With the help of equations 5 2 and 5 3 we can calculate the activity concentration in water at the time of the measurement This value has to be corrected after the radioactive decay law because a part of the radon in the water sample has already decayed during storage time Measuring groundwater For the method RAD H20 we start with an underground measurement For this purpose you connect everything as described above but take an empty bottle instead of a full one 49 5 Implementation Figure 5 6 Glass frit from Durridge 2001 30 minutes measuring time After the underground measurement change the bottles choose a 20 minutes cycle and switch the pump to On to reach the equilibrium sta
52. olume RAD H20 F50 1317 899 3 cm 2224 2225 2409 1092 3 cm RAD H20 F51 1317 897 3 cm 2224 2225 2409 1094 3 cm RAD Aqua Plus 1317 1608 200 cm 2224 2225 2409 1800 200 cm Table 5 2 Volumes of the experimental set up RAD7 Conversion factor 1317 2224 2225 2409 151 7 Bq m 1 cpm 59 7 Bq m 1 cpm 61 4 Bq m 1 cpm 62 5 Bq m 1 cpm Table 5 3 Conversion factors for each particular RAD7 53 6 Working tasks In sections 6 1 and 6 2 you will find tasks which you have to deal with Please note that all students have to deal with tasks 1 1 1 2 and 1 3 for the limnological interpretation of the data and can do tasks 1 4 and 1 5 additionally Further note that tasks 2 1 and 2 2 for the groundwater lake interaction have to be done by all students and 2 3 and 2 4 can be dealed with additionally 6 1 Limnological parameters Tasks 1 1 1 2 1 3 Draw and interpret the profiles of temperature conductivity oxygen and density in dependency of depth Where are the different layers What are the differences between A and B Compare your profiles with older data and show the seasonal change For example put some older profiles with yours into one plot Calculate the vertical stability and interpret your results Calculate the mixing coefficient with the help of the budget gradient method for different depths and interpret your results What is t
53. ominated by salinity the stability N is almost equal to N Usually NZ dominates in the thermocline while N2 characterizes stability in the hypolimnion In meromictic lakes the temperature component N2 may indicate instability in the mon imolimnion but the salinity is high enough so that NZ is much greater than NZ and finally leads to a positive N which corresponds to a stratification Because of the high stability in the salinity component N 2 the monimolimnion cannot take part in the circulation An example for this situation is shown in figure 2 10 below 60 m a s l For further information about stability take a look at Millard et al 1990 or W est et al 1996 18 2 1 Limnophysics height a s l m 10 107 107 10 10 stability N s Figure 2 10 Stability of a water column N and its components for Lake Merseburg Ost from von Rohden and Ilmberger 2001 2 1 4 Vertical mixing behaviour Overview Lakes mix because of many reasons One reason for mixing are wind driven currents Accel erated water with a certain velocity rubs against deeper water with low velocity Therefore we get a velocity gradient which causes turbulences and mixing Another reason for mixing is an unstable water column which causes convection and turbulences By speaking of horizontal and vertical mixing we mean mixing along isopycnal and diapycnal surfaces Isopycnal mixing means mixing within a density layer density layers in lakes
54. on 12 2 1 Limnophysics 10N SURFACE TEMPERATURE C ESTIMATED CLEAR SKY GLOBAL RADIATION W m 0 J FMAM MONTH Figure 2 4 Seasonal change of solar flux and surface temperature from Hostetler 1995 depth spring summer autumn winter m circulation gt stagnation circulation stagnation 6 4 22 4 0 21 4 Pr 18 4 etalimnion 4 4 10 6 4 limnion 5 4 a 4 4 4 4 20 4 4 4 4 Figure 2 5 Seasonal change of stratification in a dimictic lake from Schwoerbel and Brendel berger 2005 wind wind epilimnion hypolimnion Figure 2 6 Circulation only in the epilimnion left and the starting circulation in the hy polimnion right from Sch nborn 2003 13 2 Basics During summer the temperature of the epilimnion increases due to the increasing solar flux Lake water absorbs the energy of the sun but radiation reaches only some meters down and causes no direct heating of water in the deeper part The thermocline is the layer in which only a small part of the light arrives Thus the temperature in the thermocline decreases strongly In the deeper part the hypolimnion lake water with the highest density is found In deep lakes the temperature of the hypolimnion is approximately 4 C In this case we speak of a thermally stratified lake or the summer stagnation Only the epilimnion is well mixed because of the wind shown in figure
55. other nucleus will decay according to the radioactive law differential version _ dN t dt The amount of daughter nuclei is dependent on the amount of recently decayed mother nuclei and is also dependent on the decay of daughter nuclei itself The change of the amount of daughter nuclei can be described in the following way d Ni t 2 42 dNa t A Ni t dt A2No t dt 2 43 If we now insert the dependency between the number of decays N and the activity A as described in equation 2 40 we get the activity of the daughter nucleus Ao dA dt This differential equation is solved in Wilkening 1990 By taking the starting conditions Sue 2 44 A 0 Ag 2 45 A2 0 0 2 46 into consideration the solution is Aa BE EEE A Ay I 2 2 47 A et 2 47 Figure 2 15 shows the activity of the mother nucleus Radon 222 Ty 2 5500 8 min and the daughter nucleus Polonium 218 T a 3 05 min We infer that after a time t of approximately 20 minutes the equilibrium state is reached and that the activity of the daughter nucleus is nearly constant This equilibrium is called secular equilibrium The equilibrium state can be explained mathematically by the different half lives The half life T of radon is much bigger than the half life T of polonium Therefore the decay constant of radon is much smaller than the decay constant of polonium A lt lt Aa and we can make the following approximation Aa l y
56. own Part B shows the oxygen profile of the deep eutrophic Lake Biel in Switzerland from 11 10 1976 and part C shows the oxygen profile of the eutrophic Pu see in Holstein which is sheltered from the wind profile taken on 4 9 1989 Figure 2 13 Orthograde and clinograde oxygen profiles from Lampert and Sommer 1999 24 2 2 Radioactivity and radon 2 2 Radioactivity and radon 2 2 1 Decay series Radioactivity is a characteristic of unstable nuclei They are able to transform themselves into an nucleus with a lower energetic level by emitting a characteristic radiation In table 2 3 the three types of radiation are summarized type of decay transform process emission a decay 4X gt 4y 44He helium nucleus a radiation B decay axs E Y e 7 electron antineutrino y decay ax gt AX h v y radiation Table 2 3 The three types of radiation An unstable nucleus can decay into a stable nucleus or into another unstable one Therefore decay series can arise Altogether only four different a decay series exist because or y decays do not change the mass number but the a decay decreases the mass number by four units for each decay Today only three of the four decay series can be detected the Thorium series the Uranium Radium series and the Uranium Actinium series The Neptunium series with a half life of the starting isotope of 2 14 million years has already almost completely decayed Radon
57. ponents OT OSs N H 2 2 9 fe Oz Oz en N2 N2 2 26 Let s have a closer look at the derivation So far we neglected that the temperature of the water parcel decreases as it is lifted up because the pressure decreases As a result of the decompression the water parcel will expand thereby spending energy Therefore the temperature will decrease The adiabatic temperature gradient dT dz aq describes this change of temperature OE Be Sp oy 2 27 dz od Cp g gravity constant Tabs in situ temperature in Kelvin Cp specific heat capacity T T S p adiabatic temperature gradient a coefficient of thermal expansion Therefore it is necessary to compare two water parcels at the same reference depth The potential temperature O z zo is the temperature of a water parcel from the depth z after lifting it adiabatically to the depth zo correcting the depth dependency of temperature 17 2 Basics dere i Pelz 2 S 2 plz ae 2 28 For temperatures above 4 C it is possible that the temperature increases a little bit with depth but the water column is still thermally stratified Therefore for a correct calculation of the stability frequency we have to use the potential temperature instead of the in situ temperature During the derivation of the Brunt V is la frequency we split up the density gradient At this point of the calculation we have to put in the potential temperature instead of the in situ te
58. pump rate for the RAD Aqua Plus setup Take care that the exchanger is completely empty Any residual water distorts the mea surement Furthermore check that the one way valve points in the right direction 50 5 2 Measurements in the hydrology lab Temperature senso gas cycle Exchanger cold trap Lf RAD7 Sample bucket Figure 5 7 Structure of RAD Aqua Plus from Kluge et al 2007 Calculation of the radon activity concentration The calculations are based on the same ideas as the calculations of method RAD H20 There fore we get the following equation T 5 4 with Cw activity concentration of the water phase CAir measured activity concentration in air VAir measured volume of air in the closed gas cycle a T Ostwald solubility calculated according to equation 2 51 The last step is again to correct the value Cw with the help of the radioactive decay law 51 5 Implementation Measuring lake water First step connect everything and open or close the valves in the air loop see figure 5 8 Figure 5 8 Rad Aqua Plus in the lab Before we start the measurement we have to purge the whole construction with nitrogen for 10 minutes to assure a low background signal For the purging with nitrogen the valves 8 4 and 10 have to be closed and the others have to be open Switch 1 has to be switched in that way that nitrogen reaches the RAD7 While the air circle is being purged with
59. r Right behind the value you see the symbol for the unit Usually there will be a b standing for Becquerels m If you see a p for picoCuries litre you have to change this in the menu Setup You can get to the next status window with the arrow key The upper left part shows the temperature A very important information you find in the upper right part the humidity If the value of the humidity is above 10 please call your supervisor In the lower left part you see the voltage of the batteries This value should be between 6 00 V to 7 10 V If this is not the case call the supervisor In the right lower part the value of the electric current of the pump is shown The value should be between 0 and 80 mA If this is not the case the filter is possibly blocked and needs to be changed by the supervisor Test Start starts the measurement and the RAD7 will automatically print and store the measurement in the end After a measurement you have to clear the air in the RAD7 from old radon and other decay products This is possible with the command Test Purge If you want to stop the purging choose NO with the arrow key and confirm with Enter Al A Questions After reading chapters 2 and 3 you should be able to answer the following questions 42 Which parameters do you have to consider if you want to determine the density of water How many vertical layers in a lake do you know Do you know the names and their origin What
60. r measuring site There you will collect 2 x 250 ml samples at two different depths It is important to have no air bubbles inside your samples as mentioned before With the help of the probe you determine the level of the groundwater and take a vertical profile We know that the top of the groundwater pipe is located at 92 3 m a s l Figure 5 4 Groundwater measuring site B at Lake Willersinnweiher 47 5 Implementation 5 2 Measurements in the hydrology lab The practical course F50 51 always takes place with four students One group will start with the lake water measurement just after the field trip on Monday afternoon The other group will start the lake water measurement Tuesday afternoon We start with the lake water samples because the radon activity concentration is much lower than in groundwater On Wednesday both groups will do the groundwater measurement Because the duration of the measurements of lake water samples is much longer than that of the groundwater analyses the former are run overnight Due to didactical reasons the groundwater measurement is introduced first and afterwards the lake water analysis is adressed You can do the interpretation of the profiles during the radon measurements Materials RAD7 water samples from the lake and groundwater water pump RAD H20 cold traps some plastic pipes and adapters dry ice isopropanol mixture exchanger thermometer scales 5 2 1 Groundwater
61. t step is to clear the table of a profile from the entries you do not need This means you should delete the lines in the table in which you see that the probe was not in the water yet Further you should look at the end of the table and delete the lines in which the probe lay at the ground The last column contains the circulator status Make sure the status of the circulator is 1 in every depth If this is not the case keep the column and include it in your interpretation otherwise you can delete the column From the corrected table we only need the columns of depth conductivity and temperature You can delete the other columns In this table you can calculate the values of the reference conductivity with the help of equation 2 10 The density of lake water is calculated in two steps Step one is to calculate the density of pure water with equation 2 2 and step two is to calculate the density of lake water with the density of pure water and the reference conductivity with equation 2 13 Open this table with the calculated values with the software Origin to plot the profiles Please note that we are drawing profiles which means that the depth z is always located on the y axis be careful with the direction After these profiles are drawn you can open the second data set with the oxygen data We want to determine the oxygen value in mg l for the different depths Therefore we plot the oxygen value of one depth in dependency of time How do you get
62. te After equi librium is reached choose a measuring time of 60 minutes pump on Auto During the measurement don t forget to note the temperature As mentioned before you have to make sure that during the whole measurement no water gets into the RAD7 After the measurements you have to purge the cold traps and the RAD7 5 2 2 Lake water measurement with the method RAD Aqua Plus The radon activity concentration of lake water range of 5 to 30 Bq m is much lower than the one of groundwater range of 5 000 to 10 000 Bq m The errors of RAD H20 method are too large for the small activity values of lake water Therefore people at the Institute of Environmental Physics have developed new methods to determine the radon activity concentration of lake water with an acceptable error in short time Setup RAD Aqua Plus The method RAD Aqua Plus as the method RAD H20 is based on the equilibrium between a closed water loop and a closed air cycle Figure 5 7 shows the method in principle The lake water samples are taken in 12 1 buckets Before the measurement the normal top is exchanged by a prepared top with an integrated pump The water pump is controlled by a power supply The pumped water is sprayed into the exchanger in which the contact between water and air loop takes place Afterwards the radon containing air is pumped through the cold trap to dry out and then into the RAD7 The equilibrium state is reached after 40 minutes at the given
63. tect a certain water mass The group of tracers can be divided into natural and artificial tracers Natural tracers are for example the isotopic composition of the water itself the amount of heat respectively the temperature of a water mass or as in our case the radon concentration of the water Artificial tracers like SF have to be introduced deliberately into the system Radon is a noble gas with a very low chemical reactivity under natural circumstances Further there is a big difference in the concentrations of radon in lake and groundwater Thus radon is a good tracer for the inflow of groundwater into lakes Temperature is not such a good tracer because the as the temperature difference is not as large and temperature influences density and hence the depth of interstratifications of the inflowing groundwater 29 2 Basics 2 4 1 How does radon get into the groundwater Radium is the mother nucleus of radon and is abundant in every type of sediment or rock in different concentrations Radium decays to radon inside the ground or sediment and leaves the sediment grains due to the recoil from the decay conservation of linear momentum see Dehnert et al 1999 Another possibility is that radon diffuses through the sediment into groundwater where it accumulates In a homogeneous sediment a characteristic radon activity concentration can be calculated Sandler 2000 shows that the value of the characteristic radon activity conc
64. tz in Germany Figure 2 8 shows that the temperature below 57 m a s l above sea level stays constant over the whole year The temperature in the mixolimnion decreases to approximately 4 C meaning that the temperature of the monimolimnion is higher than the temperature of the mixolimnion This situation can only be explained by extraordinary high salinity in the monimolimnion which stops the mixing The high salinity causing the long time stability of the monimolimnion is shown in the right part of the figure Gem Mar 25 1999 Apr 28 height a s m C Aug 26 upper chemocline SS Li ss a ee T A i 0 5 10 15 205 6 7 8 10 20 30 40 temperature C el conductivity x mS cm Figure 2 8 Temperature and conductivity profiles of Lake Merseburg Ost 1b from von Ro hden and Ilmberger 2001 15 2 Basics 2 1 3 Stability of a water column In the section above the vertical stratification of lakes was discussed qualitatively Now the Brunt V is la frequency will be introduced to get a quantitative measure for the stability of a water column A column of water is called stable if a water parcel which is deflected from its equilibrium state experiences a restoring force Looking at a water parcel and its surrounding water the density of the water parcel pp and the surrounding water py zo are equal at the depth zo equilibrium state If the water parcel is lifted upwards a small distance z
65. ur 2 2 2 2 2 mE n nennen 19 2 1 5 Oxygen content in lakes 2 2 CC nn 22 2 2 Radioactivity andradon 2 2 2 none 25 2 251 Decay SERIES ic 4 44 ni are a nen ae er 25 2 2 2 Decay law 22 0 4 2 0 a aa ee 25 2 2 3 Secular equilibrium e se c soes ac raati icu d nen 27 2 9 Groundwater a 4 ee h ae E RE E ee es 20 2 4 Radon 589 a Tracers e220 N ur a a denen RR Das 29 2 4 1 How does radon get into the groundwater 2 22222220 30 2 4 2 How does radon get into lake water 2 2 2 one nennen 30 2 4 3 Henry slaw 2 u ols Sons a ana ea then ri 30 2 5 Lake Willersinnweiher 2 2 22 E CC nn nn 32 2 5 1 Mixing behaviour in the last years 2 2 2 nme nenn 33 2 5 2 Oxygen content in the lake 2 2 En nn nn 33 3 Measuring instruments and techniques 34 3 1 GRD Probe 3 4 2 2a Wa det a ea N Gd ie 34 3 1 1 Design and functionality of the probe 2 2 nenn 34 3 1 2 A short introduction to the software of the probe 34 3 2 Radon measurement instrument RADT 2 2 2 n nn 37 3 2 1 How the RAD7 works 2 2 2 2 CL nn 37 3 2 2 Manual of the RAD s oxe soron eee Gm ee an BoP aoe ae ehe 39 4 Questions 42 5 Implementation 43 5 1 Measurements at Lake Willersinnweiher near Ludwigshafen 44 pill Materials tisa 22 20 Ra er ra Po hee ee 2a 44 5 1 2 Measurements at the lake 2 2 22 on on nn 45 5 2 Measurements in the hydrology lab 2 2 2 2 non nn 48
66. urement for each RAD7 we usually use How do you determine the background you need for your individual measurement time Write down how you get your background and the according error Afterwards you have the radon activity concentration of the gas cycle in the unit counts per minute cpm which you have to transfer into Bq m see table 5 3 Next step is to calculate the activity concentration in the water phase at the time of the measurement with equation 5 4 Use the Gaussian error propagation to get your error Which errors have to be taken into account Which are negligible 56 7 2 Analysis of the groundwater lake interaction The last step is to take into account that time has passed since we took the samples Therefore you have to correct the activity concentration with the help of the decay law Calculation of the radon activity concentration of groundwater The first step in this part is similar to the first step in the last section we have to cal culate the Ostwald solubility a Aa The next step is much easier because in the range of groundwater the RAD7 calculates the right value of the radon activity concentration in the gas phase You just have to consider the background measurement and calculate the error Usually the duration of your background measurement and the real measurement is not equal Do you have to consider this difference in time The last two steps are equal to the lake water calculation determine the r
67. with lots of calcium Thus the equations derived in chapter 2 1 1 are valid for Lake Willersinnweiher geographical parameters typical amounts of ions in the lake water volume 1 3 106 m sulfate 2 4 mmol l surface area 16 17ha bicarbonate 2 3 mmol l mean depth 7 7m chloride 2 2 mmol l maximum depth 20 m sodium 1 9 mmol l maximum length 850 m calcium 2 5 mmol l maximum width 325 m nitrate 30 umol l Table 2 4 Parameters of Lake Willersinnweiher summarized from Sandler 2000 Schmid 2002 and Wollschl ger 2003 2 5 1 Mixing behaviour in the last years Lake Willersinnweiher has been examined by the Institute of Environmental Physics for several years Therefore we know that the lake is a holomictic or dimictic lake The dimictic behaviour only occurs if the ice sheet stays over a longer period or the temperatures are very low over a longer period For some time the smaller part of the lake showed a specific behaviour characterized by an increasing concentration of salts towards the ground Furthermore we have detected that the circulation period gets shorter This could be caused by a faster increase of the air temperatures in spring which stops the circulation 2 5 2 Oxygen content in the lake There is a lot of agriculture around Lake Willersinnweiher that causes a high inflow of fertil izers into the groundwater As a consequence Lake Willersinnweiher is an eutrophic lake with a
68. zo shown in figure 2 9 the following forces act on the water parcel z coordinate positive upwards gravity Fo gppV 2 14 buoyancy F4 gpuV 2 15 force Frum Fg Fa Fium g pp pu V 2 16 Fsum acceleration a 2 17 Ger 2 17 Z P Py 2 Zo Pp Py Zo Figure 2 9 Lifted and equilibrium state of a water package Therefore we get the restoring force per unit of mass acting on the water parcel a pu z 2 18 PP The density of the surrounding water py z at the lifted depth z is almost equal to the den sity of the surrounding water py zo at the depth zo Therefore a Taylor series approximation is possible put ml 2 G 2 19 16 2 1 Limnophysics This approximation is put into equation 2 18 Further we consider that the density of the water parcel pp is equal to the density of the surrounding water py zo in the equilibrium state We derive the restoring force per unit of mass pp pee Zl 20 2 20 This is the well known differential equation of a harmonic oscillation oscillator N z zo 2 21 with the oscillation frequency N m 2 2 2 22 pp dz As discussed in chapter 2 1 1 the density of water is a function of temperature and salinity Hence the density gradient can be split up using equations 2 3 and 2 7 dp do dT p s Se l 2 2 dz OT z OS Oz 2 OT Os 2 24 apa Bex 2 24 Now it is visible that the stability frequency has also two com

F50/51 Limnophysics - Physikalisches Institut Heidelberg

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