Quantifying saline groundwater flow into a freshwater lake using the Ra isotope quartet:

A case study from the Sea of Galilee (Lake Kinneret), Israel

Hadas Raanan1 and Avner VengoshDivision of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, North Carolina27708

Adina PaytanInstitute of Marine Sciences, University of California-Santa Cruz, Santa Cruz, California 95064

Ami NishriIsrael Oceanographic and Limnological Research, The Yigal Allon Kinneret Limnological Laboratory, P.O. Box 345,Tiberias, 14102, Israel

Zbigniew KabalaDepartment of Civil and Environmental Engineering, Pratt School of Engineering, Duke University, Durham, NorthCarolina 27708

Abstract

We investigated the Ra isotope quartet in order to quantify the discharge of saline groundwater into afreshwater lake. The activities of 226Ra, 228Ra, 223Ra, and 224Ra were measured in the Sea of Galilee, Israel,surrounding saline springs, and the Jordan River, coupled with U, Th, and 226Ra measurements in sediment coresand laboratory Ra adsorption experiments under different salinity conditions. The 226Ra activity (0.007–0.008 BqL21) and 226Ra : Cl ratio in the lake were significantly lower than those of the inflowing saline springs, indicatingthat 75–86% of the incoming 226Ra is removed from the lake water. This ‘‘missing’’ Ra is likely adsorbed ontosuspended particles. Given the observed differential depletion of Ra isotopes, we present a model consisting ofadsorption-desorption, recoil, and decay of the short-lived 224Ra and 223Ra isotopes. We predict a removal timeof ,2 weeks for Ra, inferring the apparent residence time of suspended matter in the lake. A mass-balancecalculation of the different 226Ra inventories reveals a saline groundwater flux of 44–61 3 109 L yr21 for ‘‘Fuliya-type’’ water. The residence time of dissolved 226Ra in the lake is estimated as 3–4 yr. The 228Ra : 226Ra ratio of thelake water and a mass-balance calculation show that 228Ra arrives from regeneration though bottom sediments, inaddition to groundwater flux. Mass-balance calculations of the expected saline fluxes before the diversion ofsaline inflows to the lake (early 1960s) provide 226Ra activity estimates in the lake (0.018–0.020 Bq L21) that aresimilar to the value (0.018 6 0.0001 Bq L21) measured in the lake in 1962.

Over the last three decades, radium isotopes have beenused to trace and quantify groundwater discharges intooceans (Knauss et al. 1978; Burnett et al. 2006; Paytan et al.2006) and estuarine environments (Elsinger and Moore1983; Charette et al. 2003; Burnett et al. 2008). All of thesestudies focused on the interface between fresh groundwaterand saline surface or subsurface water. Ra, a decay productwithin the U-Th decay chains, is an alkaline earth elementwith four naturally occurring isotopes, the half-lives of

which range from 3.6 d to 1600 yr (224Ra 5 3.66 d; 223Ra5 11.43 d; 228Ra 5 5.76 yr; 226Ra 5 1600 yr). Given thegeochemical similarity to barium, its predecessor in thealkaline earth family of elements, Ra tends to co-precipitatewith barium minerals, most notably with barium sulfate(Molinari and Snodgrass 1990; Paytan et al. 1996). Sincethe affinity of Ra for adsorption decreases with increasingsalinity (Krishnaswami et al. 1991; Webster et al. 1995;Sturchio et al. 2001), it has been perceived as a conservativetracer (after accounting for radioactive decay) in salineenvironments such as estuaries and coastal zones. Infreshwater lakes, however, Ra isotopes have not beenapplied except for a few studies. Kraemer (2005) used theRa isotope quartet to quantify fresh groundwater fluxes intothe freshwater of Cayuga Lake, New York. In that study, the224Ra : 228Ra and 223Ra : 226Ra ratios were used to trace themixing proportions of groundwater in the lake. Bycomparing ratios of short- to long-lived Ra isotopes,Kraemer (2005) was able to delineate the Ra sources thatentered the lake and the flow rate of water masses in the lake.Moser et al. (1998) measured 226Ra activities in the watercolumn, shallow sediments, and pore water of Zurich Lake,

1 Corresponding author (hr9@duke.edu).

AcknowledgmentsWe thank the two anonymous reviewers for their thorough and

thoughtful reviews, which have significantly improved the qualityof this manuscript. We thank Nir Koren and the team at the YigalAllon Kinneret Limnological Laboratory for their field support,Shay Kronenblom from Ben Gurion University for his help insampling, Irena Pankratov from the Israeli Water CommissionLaboratory. and Gary Dwyer, David Vinson, and DaniellaHirschfeld from the LEARN laboratory at Duke University fortheir help in the laboratory. H.R. was supported by the Fulbrightfellowship.

Limnol. Oceanogr., 54(1), 2009, 119–131

E 2009, by the American Society of Limnology and Oceanography, Inc.

119

Switzerland. They showed that Ra activity in the watercolumn changes seasonally with dissolved oxygen; lowerconcentrations were observed during the winter when thedissolved oxygen levels were high. This was related to theinteraction of Ra with redox-sensitive minerals in thesediment.

In this paper, we explore the potential of using naturallyoccurring Ra isotopes as a proxy for understanding waterbudgets and Ra behavior in freshwater lakes. In contrast toestuarine and coastal zones, where fresh groundwaterdischarges into saline environments, here we focus on anopposite situation, where saline groundwater flows into afreshwater lake. This study aims to explore the validity of Raisotope geochemistry as a tool for quantifying groundwaterdischarge into freshwater lakes. We show that Ra in the lakewater, which is primarily derived from saline groundwaterdischarge, is scavenged from the lake water due to therelatively low salinity of the lake (236 mg Cl L21). We used226Ra variations to quantify the residence time of Ra in thewater and to estimate the saline fluxes entering the lake.

Methods

Study area—The Sea of Galilee (Lake Kinneret, Lake ofTiberias) is a freshwater lake (236 mg Cl L21) located in thenorthern part of the Jordan Rift Valley (Fig. 1). The averagevolume of the lake is 4.3 3 1012 liters over an area of 166 km2,of which 90 km2 are pelagic (Nishri et al. 1999). The salinityof the lake is controlled by a balance between: (1) freshwaterrecharging the lake, mainly through the Jordan River andother surface tributaries; (2) saline springs, emerging mostlyaround the perimeter of the lake; (3) outflows, by pumpingand through its natural outlet; and (4) evaporation.

Since the construction of the National Water Carrier(NWC) in 1964, the lake water has been extensively used tosupply Israel’s water demand. Water pumping from thelake through the NWC has been on order of 420 3 109 Lyr21, approximately one fourth of Israel’s annual nationalconsumption. During the 1930s, the Degania Dam wasconstructed on the southern outlet of the lake. The dam iskept closed, preventing flow to the Southern Jordan Riverduring most years, except in unusually rainy years, when itis opened to prevent flooding of adjacent communities andfields. During the 1950s, when water-quality monitoring ofthe lake began, a constant increase in the Cl2 concentrationof the lake was observed, peaking at ,400 mg Cl L21

during 1965. In an effort to reduce the salinity of the lake, asalinity diversion channel (SDC) was constructed to divertthe flow of some of the saline springs that discharge intothe lake. The SDC diverts three major groups of springs(Fig. 1): the Tiberias Hot Springs (THS, 109 L yr21,,18,400 mg Cl L21); the Tabgha Springs (15 3 109 Lyr21, ,2200 mg Cl L21); and the Fuliya Springs (0.75 3109 L yr21, 1200–17,400 mg Cl L21). Since the construc-tion and operation of the SDC and removal of the majorsaline inflows to the lake, the salinity of the lake hassignificantly decreased. Since the 1970s, Cl2 content hasvaried between 220 and 270 mg L21, which reflectsvariations in the annual precipitation and freshwater fluxesto the lake (Nishri et al. 1999; Gvirtzman 2002).

During the past decade, several attempts have been madeto quantify the nature and extent of the unmonitoredsubmarine saline discharge still entering the lake (Dror etal. 1999; Kolodny et al. 1999; Nishri et al. 1999). Thesestudies have used the ratio of Br : Cl in the lake and thesaline springs to distinguish between the different salinewaters that flow to the lake, and they have suggested that‘‘Fuliya-type’’ water containing ,1200 mg Cl L21 is thepredominant salinity source that is still entering the lake, inspite of the SDC diversion. They estimated the flux of thisunmonitored saline source to be 61–68 3 109 L yr21.

Moise et al. (2000) surveyed the 224Ra, 228Ra, and 226Raactivities in the saline springs surrounding the lake. Theyshowed that the ratio of 228Ra : 226Ra decreases withincreasing Cl2 content, and that high correlation existsbetween Cl2 concentration and 226Ra activity within eachgroup of saline springs. Nishri and Stiller (1997) calculatedthe residence time of radium in the lake and showedsignificant 222Rn contribution to the upper water layer dueto recoil from suspended matter in the thermocline zone;

Fig. 1. A map of the Sea of Galilee, the major groups ofsprings surrounding the lake (Tiberias Hot Springs, TabghaSprings, and Fuliya Springs) and the Jordan River. A and F marksampling locations in the lake.

120 Raanan et al.

this characteristic is more strongly expressed during thesummer months when the lake is stratified.

Water sampling and analysis—Large-volume (50–300 li-ters) water samples from the main water body of the Sea ofGalilee and saline springs entering the lake were collectedduring the course of three sampling campaigns betweenMarch 2005 and June 2006. A 250-liter sample of theUpper Jordan River was collected during February 2008.Water temperature, conductivity, dissolved oxygen, pH,and redox potential were measured in the field. The large-volume samples were run through a plastic columncontaining Mn oxide-covered acrylic fibers (Moore 1976),which efficiently adsorb the radium isotopes. The fibersfrom samples SG1–18 were sent to Stanford University forimmediate measurement of their short-lived 223Ra and224Ra isotopes by delayed coincidence alpha counting(Moore and Arnold 1996). Samples SG21–29 were mea-sured by the same method at the Laboratory of Environ-mental RadioNuclides (LEARN) at Duke University. Thefibers were then incubated in a sealed glass cylinder for 3weeks and measured for 226Ra using a Radon-in-Airmonitor (RAD7, Durridge Inc.), following the method ofKim et al. (2001). Samples SG1–18 were counted at Ben-Gurion University (BGU) in Israel, whereas samplesSG21–29 were measured at Duke University. All the fiberswere then smashed and sealed in 90-mL tin cans, and their228Ra was measured using a Canberra DSA2000 BEGegamma detector at LEARN at Duke University.

Small-volume subsamples were collected for generalchemistry and trace metals analysis. Trace metals ofsamples SG21–29 were measured on a VG PlasmaQuad-3inductively coupled-plasma mass spectrometer (ICP-MS) atDuke University. Trace metals and cations of samplesSG1–18 were measured on a Thermo Jarrell-Ash 61E ICP-ES in the Water Commission Laboratory at the VolcaniInstitute at Bet-Dagan, Israel. Cations of samples SG21–29were measured on an ARL-Fisons Spectraspan 7 DirectCurrent Plasma (DCP) spectrometer at Duke University.Anions of samples SG21–29 were measured on a DionexIon Chromatograph (IC) DX-500 at the Wetland Centre ofThe Nicholas School of the Environment and EarthSciences at Duke University. Anions of samples SG1–18were measured on a Dionex IC model 600, with a CD-20detector, a GP-40 pump, and an AS-14 column at Ben-Gurion University. Cl and HCO3 concentrations ofsamples SG1–18 were measured on a RadiometerTIM800 Titration Manager combined with an ABU 901Autoburette. Cl titration with 0.015 mol L21 AgNO3 wasmonitored by a Radiometer M25Ag silver electrode.Alkalinity titration with 0.02 mol L21 HCl was monitoredby a Radiometer Phc2401-8 pH electrode. Alkalinity ofsamples SG21–29 was measured by titration with 0.02mol L21 HCl using a YSI pH100 meter. The reaction error(i.e., the balance between anions and cations) was checkedfor all samples and found to be below 4%.

Sediment sampling and analysis—During the June 2006sampling campaign, three gravimetric sediment cores werecollected at Sta. A (Fig. 1). The cores (26–28 cm in length)

were then sliced into 3-cm intervals and sealed in plasticcups pending analysis. Each sediment sample was placed ina piston, and a lever was used to force the pore water out.The pore-water samples were collected in plastic vials andstored for chemical analysis. The partially dried sedimentswere emptied into preweighed 90-mL tin cans and placed inan oven at 50uC for 24 h. The dried sediments were thencrushed to a diameter ,2 mm using a mortar and pestle,returned to the tin can, weighed, covered, and sealed withelectric tape to prevent gas escape. The sealed cans wereincubated for at least 3 weeks (to allow 226Ra to reach secularequilibrium with its 214Bi granddaughter), and each samplewas counted in a Canberra DSA2000 BEGe gamma detectorat LEARN in Duke University for 2–3 d to measure nuclidesfrom the U-Th series. The 226Ra activities were obtainedthrough the 609-keV energy line of its radioactive grand-daughter, 214Bi, assuming secular equilibrium. The activitiesof all these nuclides were calibrated using CCRMP U-Th orestandard DL-1a and Canberra Multi-Gamma Ray standardMGS-5C, measured under physical conditions identical tothe samples (e.g., can size, material type). Extracted porewaters were analyzed for anion concentrations using aDionex IC DX-500 with CD-20 detector, a GP-40 pump,and an AS-14 column at Duke University.

Adsorption experiment—Dredge samples of sedimentscollected from the bottom of the lake at Stas. A and F(Fig. 1) were used as the solid material for Ra adsorptionexperiments. Pore waters from sediments of both sampleswere extracted using a centrifuge at 3800 rpm for 20 min.After separating the water from the solid, five samples of2 g each were weighed from each site and inserted into acentrifuge tube. An artificial solution with chemicalcomposition and ionic ratios similar to those of the Seaof Galilee was prepared with Cl content of 1000 mg L21.Subsamples of the initial solution were diluted to contain750, 500, and 250 (i.e., identical to the Sea of Galilee) mg ClL21. Thirty milliliters of each solution were added to one ofthe 2-g samples from each site, and 30 mL of deionizedwater were added to a fifth sample from each site. Theinitial 226Ra activity in the dissolved phase of eachsubsample (,5.3 Bq) was obtained by adding 0.02 g of astandard (NIST 4966) containing 263 Bq 226Ra g21.Additionally, two sediment-free samples were preparedcontaining the same amount of Ra as the other 10subsamples, one using 30 mL deionized water and theother with 30 mL of the 1000 mg Cl L21 solution, todetermine if Ra was lost through adsorption to the surfaceof the plastic centrifuge tube. The 12 tubes were thenshaken for a week in a temperature-controlled room (23–25uC). The liquid and solid phases were then separatedusing a centrifuge for 20 min at 3800 rpm. The pH of thewater was measured, after which the samples were dilutedusing deionized water to a volume of ,150 mL. Thediluted solutions were gravity fed to columns containingMnO2 fibers, and the process was repeated seven times foreach sample to assure that all the Ra from the solution wasadsorbed to the fibers. The fibers were then manuallysqueezed and individually incubated in glass tubes for atleast 3 weeks, after which their 226Ra activity was counted

Quantifying saline groundwater flow 121

on a Radon-in-Air monitor at LEARN at Duke University,following the method described previously.

Results

Water samples—The chloride contents and Ra isotopicactivities in the saline groundwater, freshwater of theJordan River, and fresh lake water of the Sea of Galilee aresummarized in Table 1. The Ra isotopic results of thesaline springs reported here are consistent with themeasurements reported by Moise et al. (2000) for the samesaline springs. The activities of the four Ra isotopes arelinearly correlated with chloride concentrations, particu-larly in the Fuliya Springs (Fig. 2). All samples from theSea of Galilee were collected during the summer monthswhen the lake is stratified (i.e., thermocline depth is,10.5 m, see Table 1). The average 226Ra, 228Ra, 223Ra,and 224Ra activities measured along the lakes’ watercolumn, which are used in our discussions, are 0.0073,0.00058, 0.0000068, and 0.00012 Bq L21, respectively(Table 2). These activities are several orders of magnitudelower than those measured in the saline springs entering thelake (Table 1). The 228Ra : 226Ra ratio in the lake waterranged between 0.069 and 0.108, (average of 0.079) and issimilar to the ratio at the Tiberias Hot Springs (THS;0.061–0.105; average 0.087) and Fuliya Group Springs(0.041–0.092). The ratio in the lake, however, is lower thanthe ratio range in Tabgha Springs (0.179–0.280; an averageof 0.24) (Fig. 3). The 224Ra : 223Ra ratio in lake water (12 to28) overlaps with the ratio in Fuliya B spring (13–19), but itis significantly higher than in Fuliya A (6.5), Tabgha (7.8),and THS (7.3) Springs (Table 1). The 224Ra : 228Ra ratiovaries from ,1.5 in the Fuliya and Tabgha Springs, to ,1at THS spring and 0.25 in the lake water. The Ra : Cl ratiosin the lake water (4.93 3 1027, 2.89 3 1028, 2.27 3 1026,and 3.11 3 1025 for 224Ra, 223Ra, 228Ra, and 226Ra,respectively; Table 2) are significantly lower than those inthe Fuliya, Tabgha, and THS Springs.

The 226Ra, 228Ra, 223Ra, and 224Ra activities of watercollected from the Jordan River are 0.00153, 0.000665,0.0000235, and 0.000591 Bq L21, respectively.

Sediment samples—The 226Ra activities of the sedimentsfrom the two cores systematically increase with depth (Fig. 4).In contrast, the U activity (238U; measured by ICP-MS) in thesediments decreases with depth and shows no apparentcorrelation to the changes in the activity of 226Ra (Fig. 4).

Adsorption experiment—The fraction of Ra that wasadsorbed during the adsorption experiment, Raads, wascalculated by subtracting the dissolved activity measured atthe end of the experiment from the initial Ra activity,corrected to the weight of sediments used for theexperiment to yield activities in units of Bq g21. Thedissolved activities measured at the end of the experiment,Radis, were divided by the volume of the solution to yieldactivities in units of Bq mL21. The experiments show thatthe ratio of Raads : Radis decreases with chloride concentra-tion (Fig. 5), clearly showing that the affinity of Ra toadsorption decreases with increasing salinity.

Discussion

Geochemical indicators for radium sources and sinks in theSea of Galilee—The Ra isotopic composition of the Sea ofGalilee is controlled by (1) the Ra isotopic ratios of thesaline inflows, which are considered the major sources fordissolved constituents in the lake (Kolodny et al. 1999;Nishri et al. 1999); (2) dissolved and particulate Ra enteringthrough the Jordan River; (3) possible recoil fromparticulate matter in the water column (Kraemer 2005);(4) removal through adsorption and/or co-precipitationinto barite minerals; (5) diffusion from bottom sediments(Key et al. 1985; Krest et al. 1999; Kraemer 2005); and (6)radioactive decay. Given the residence time of water in theSea of Galilee (,6 yr), the radioactive decay applies only tothe relatively short-lived nuclides of 228Ra, 224Ra, and223Ra, but not to 226Ra.

The Ra activities and their ratios in the different salinesprings are presented in Fig. 2 and in Table 1. The datashow that 226Ra in the lake water is depleted relative to226Ra activities in the saline springs when extrapolated tothe Cl content of the lake water (Fig. 2). We thereforesuggest that in addition to dilution, which would equallyaffect Ra and Cl, Ra is being substantially removed from theliquid phase. Given that (1) the Sea of Galilee isundersaturated with respect to the mineral barite (SaturationIndex [SI] 5 20.2, calculated by PHREEQC software); (2)there is no evidence for the occurrence of barite in the uppersediment layer (Halitz and Nishri unpubl.); and (3) theresidence time of water in the lake is ,6 yr, we suggest that226Ra is removed from the lake water through adsorptiononto suspended particulates and sedimentation. Our ad-sorption experiment (Fig. 5) confirms that under the salinityconditions of the Sea of Galilee (Cl ,250 mg L21), a largefraction of Ra is removed by adsorption.

Nishri et al. (1999) and Kolodny et al. (1999) suggestedthat ‘‘Fuliya-type’’ water with an estimated salinity of,1200 mg Cl L21 is the predominant unmonitored salinesubmarine input that has continued to flow into the lakeafter the SDC diversion. In spite of that, in the followingdiscussion, we consider all known major saline endmembers (Fuliya, Tabgha, and THS), as well as the JordanRiver, as potential salinity (and Ra) contributors to thelake. For each of the saline end members, the expected226Rainitial value in the specific source (if dilution was theonly process that controls Ra activity in the lake) wasdetermined by multiplying the ratio of each radium isotopeto Cl2 by the salinity of the lake (e.g., 0.0292 Bq L21 forFuliya; Fig. 2; Table 2). The difference between themeasured 226Ra in the lake (0.00735 Bq L21) and theexpected Rainitial suggests that the lake water is depleted by75% to 86% in 226Ra compared to that expected fromsimple dilution of Fuliya and THS water, respectively(Table 2; Fig. 2). Likewise, we show even larger depletionsin 224Ra and 223Ra activities in the lake water relative to theexpected Ra for diluted saline groundwater (95–99% forFuliya and Tabgha water, respectively; Fig. 2C,D). Thedepletion in 228Ra activity in the lake (Fig. 2B) is between57% (Fuliya-type water) and 94% (Tabgha Springs). Thesedepletions indicate a significant sink for Ra in the lake,

122 Raanan et al.

Table1.

Ra

diu

ma

nd

chlo

rid

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the

sali

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spri

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s,th

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(20

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Mo

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00

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22

8R

a(B

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22

3R

a(B

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22

4R

a(B

qL

21)

Cl

(mg

L2

1)

22

4R

a:

22

3R

a

22

8R

a:

22

6R

a

22

4R

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22

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Quantifying saline groundwater flow 123

which changes the isotope ratios from those seen in thesaline springs, and makes the activity of Ra in the lakesignificantly lower than expected from dilution of the salinesprings. While several processes might contribute to thescavenging of Ra through the sediments, in the followingdiscussion, we assume the major scavenging mechanism isvia adsorption.

A possible additional Ra source to the lake (in additionto saline springs) is recoil from suspended particles in thewater column (Kraemer 2005). Given the short residencetime of particulate matter in the lake water column (a fewweeks; Stiller and Imboden 1984), we suggest that recoilcould contribute only short-lived Ra nuclides with similar

half-lives (224Ra, 223Ra), while 228Ra and 226Ra are derivedpredominantly from the saline springs that flow to the lake.Assuming that (1) the upper 3 cm of the sediment coresrepresent the particulate matter in the lake, with U and Thactivities of 0.046 and 0.013 Bq g21, respectively (Fig. 4);(2) the small size of particulate matter results in 100% recoilefficiency to the lake water; (3) nuclides in the U and Thdecay series are in secular equilibrium within the particulatematter (i.e., 224Ra 5 228Ra 5 232Th and 238U 5 226Ra); and(4) the concentration of particulate matter in the watercolumn of the lake is 2.1 mg L21 (Stiller and Imboden1984), we can calculate the expected 222Rn (9.45 3 1025 BqL21), 224Ra (2.73 3 1025 Bq L21), and 223Ra (7.0 3

Fig. 2. Activities of the four Ra isotopes versus chloride concentrations (logarithmic scale) in the saline springs and Sea of Galilee.Radium activities in the lake are significantly lower than the Ra activities in the saline springs extrapolated for the lake salinity.

124 Raanan et al.

Table2.

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(mg

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a:

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Quantifying saline groundwater flow 125

1027 Bq L21) activities and 224Ra : 223Ra ratio (39) in thelake water derived entirely from recoil processes from thenuclides in suspended matter. Given that the adsorptionrate of Ra is extremely fast (i.e., minutes; Krishnaswami etal. 1982), one would expect that Ra adsorption ontoparticulate matter would change the Ra activities in thesuspended matter and thus their recoil products. Forexample, Nishri and Stiller (1997) reported significantly

higher 222Rn levels in the water column of the Sea ofGalilee (an average of 0.0343 6 0.0208 Bq L21), whichcould reflect higher activities of 226Ra in suspended matterderived from adsorption from the lake water. Likewise, themeasured 224Ra (an average of 1.2 3 1024 Bq L21) and223Ra (6.8 3 1026 Bq L21) activities in the lake water arehigher (4.3- and 9.7-fold, respectively) than the expectedactivities derived from recoil. Moreover, the rapid Raadsorption relative to the decay rate of even the short-livedRa isotopes would result in high ratios of the short-lived tolong-lived isotopes (e.g., 224Ra : 228Ra . 1, 223Ra : 226Ra .0.046) upon recoil combined with adsorption (Krishna-swami et al. 1982), when, in fact, the measured 224Ra : 228Raand 223Ra : 226Ra ratios in the lake water are much lower,0.25 and 0.0009, respectively. In a later section, we willquantify the different Ra sources and sinks that affect theRa isotopic ratios in the Sea of Galilee.

The ratio of 228Ra : 226Ra in the lake water (an averageratio 0.079) is slightly higher than the ratio of Fuliya-typewater containing 1200 mg Cl L21 (0.065). Given theresidence time of water in the lake (,6 yr) and the expecteddecay of 228Ra, one would expect the ratio of 228Ra : 226Rain the lake to be lower than the ratio measured in the salinegroundwater source. The relatively higher 228Ra : 226Raratio of the lake water is therefore an indication of anadditional source with a higher 228Ra : 226Ra ratio. Asidefrom Fuliya-type water, other potential water sources tothe lake are the Tabgha Springs (228Ra : 226Ra 5 0.237),THS (0.087), and the Jordan River (0.434; Table 1).Contribution from any of these sources might increasethe 228Ra : 226Ra ratio in the lake above the ratioanticipated from discharge of Fuliya-type water alone.

Fig. 3. 228Ra : 226Ra ratios versus chloride concentrations inthe saline springs and Sea of Galilee. The 228Ra : 226Ra ratio in thelake is higher than that of ‘‘Fuliya-type’’ water, suggesting thelatter cannot be the only source of Ra entering the lake.

Fig. 4. 226Ra, 238U, and 232Th activity variations in sedi-ments along core 2 collected from Sta. A. The data show noapparent correlation between the activities of 226Ra and 238U.

Fig. 5. Variations of the ratio Raads : Radis with chlorideconcentrations as measured in adsorption experiments usingsamples collected at Stas. A and F (Fig. 1). The data suggestthat Ra adsorption decreases with salinity.

126 Raanan et al.

Alternatively, the elevated 228Ra : 226Ra ratio might be theresult of regeneration of 228Ra from the bottom sediments(Key et al. 1985; Krest et al. 1999; Kraemer 2005), wherethe 228Ra : 226Ra ratio (0.61 at the upper 3 cm) issignificantly higher than the ratio measured in both thelake and in Fuliya-type water.

Modeling adsorption and decay of radium isotopes—Thelong-lived 226Ra isotope abundance in the lake is controlledpredominantly by adsorption processes. In contrast, therelative depletion of the other Ra isotopes is controlled byrecoil, radioactive decay, and adsorption. In order toquantify these effects, we consider four possible scenariosto explain the isotopic variations of the four Ra isotopes. Inthe different models described here, we use the average226Ra, 228Ra, 223Ra, and 224Ra activities in the lake waterand assume that Fuliya springs are the predominant Rasource entering the lake (Kolodny et al. 1999; Nishri et al.1999). First, we consider that all Ra isotopes are removedsolely by adsorption (model A). This approach results in75–86% depletion of 226Ra, 57–94% depletion of 228Ra(measuring error up to 25%), and 95–99% for both 223Raand 224Ra (measuring error is less than 10%) relative to theexpected Fuliya input. In our second approach, we considerthat all short-lived 224Ra and 223Ra isotopes are removedonly through radioactive decay (model B). We use the basicradioactive decay equations

D ~ D0e{lt ð1Þ

and

t ~ {1

lln

D

D0ð2Þ

where t is the decay time, l is the decay constant for thespecific Ra isotope, D0 is the initial (calculated) activity attime t0, and D is the measured activity after time t. We defineD0 values as the initial activities in the lake had dilution beenthe only process affecting the activity of radium in the lake(normalized to Cl content in the lake; see Fig. 2B–D;‘‘expected activity’’ in Table 2) and D as the measuredactivities. The calculated decay times using this model are15.6–24.1 and 50.8–82.7 d for 224Ra and 223Ra, respectively.The third approach (model C) combines adsorption andradioactive decay. We consider a model where all isotopesare adsorbed to the same extent, while the remaining Radifferences are the result of removal through radioactivedecay. In this model, D0 becomes the percentage of eachradium isotope that remains in solution after subtracting theadsorbed portion, whereas D remains the activity measuredin the lake water. Assuming that 226Ra is removed only byadsorption, model C uses 75–86% as the fraction of each Raisotope that is removed by adsorption, converting Eq. 2 tothe following form,

t~{1

lln

D

D0~{

1

lln

Rameasured

Raexpected{Raadsorbed

~{1

lln

Rameas

0:25Raexpor{

1

lln

Rameas

0:14Raexp

ð3Þ

This model results in decay times of 8.3–13.8 and 28.0–50.2 d for 224Ra and 223Ra, respectively.

The fourth model considers adsorption of Ra isotopesthat is proportional to the recoil production : activity ratioof the different Ra nuclides, which depends on their decayconstants. This model (model D) follows the approach ofKrishnaswami et al. (1982), which showed that underexchange conditions (e.g., adsorption-desorption reactions)between dissolved Ra and Ra adsorbed onto surfacesediments, the retardation factor (Rf) is equal to the ratioof production (by recoil) to Ra activity. In their model,Krishnaswami et al. (1982) showed that shorter-livedisotopes experience less retardation than longer-lived ones.This is derived from the higher supply rate of the shorter-lived Ra isotopes through recoil. Kraemer (2005) suggestedthat recoil of the short-lived Ra nuclides can occur fromsuspended matter in the water column of the lake. Theretardation of Ra isotopes (Rf) can be described as

Rf ~k1 z k2 z l

k2 z lð4Þ

where l is the decay constant (min21), and k1 and k2 arefirst-order adsorption and desorption rate constants(min21), respectively. This model assumes exchange (ad-sorption-desorption) reactions between suspended matterin the lake water and sufficient time for recoil of the short-lived Ra nuclides to occur from the suspended matter backto the solution. Assuming desorption rate is significantlylower than adsorption in freshwater, Eq. 4 can beapproximated as:

Rf &k1 z l

k2 z lð5Þ

By extrapolating the results of Krishnaswami et al. (1982)to the salinity conditions of the Sea of Galilee, we findvalues of 1022 and 1026 min21 for the k1 and k2

parameters, respectively. Since the l values for the short-lived Ra isotopes are significantly smaller than k1 (for224Ra, 223Ra, and 228Ra, the l values are 1.3 3 1024, 4.2 31025, and 2.3 3 1027 min21, respectively), Eq. 5 can berewritten as

Rf &k1

k2 z lð6Þ

We have used these assumptions for setting the Rf ratiosbetween the different Ra isotopes. Using the 223Ra and224Ra differences to develop the model, the ratio of theretardation factors Rf,223 to Rf,224 (defined as R) is

<~Rf ,223

Rf ,224~

k1

k2 z l223

k1

k2 z l224

~k2 z l224

k2 z l223ð7Þ

Next, we define X as the ratio between the adsorbed Ra andthe expected Ra activity from the saline source that entersthe lake (corrected for dilution):

224Raads

224Raexp~ X ð8Þ

Quantifying saline groundwater flow 127

Hence, the decay of Ra isotopes from the initial saline Ra,corrected for Ra adsorption is

224Rameas ~ 224Raexp { 224Raads

� �:e{l224t

~ 1 { Xð Þ224Raexp:e{l224t

ð9Þ

223Raads ~ <:X :223Raexp ð10Þ

Combining Eqs. 8–10 for all Ra isotopes and solving for tyields

t~1

liln (1{<X )z ln

iRaexp

iRameas

� �� �ð11Þ

where i signifies the specific Ra isotope (224Ra, 223Ra, or228Ra). Application of Eq. 11 to the 224Ra-223Ra and224Ra-228Ra isotope pairs yields decay time of 10.7–13.8 d(12.3 d on average) when using Fuliya-type water as thesaline end member, and 13.4–15.8 d (14.6 d on average)when using THS-type water as the saline end member. Inspite of the fact this model is not based on actualmeasurement of adsorbed Ra : expected Ra, the internalconsistency of the saline end members in model D indicatesthat this model, which includes Ra adsorption-desorptionexchange and preferential recoil ingrowths of Ra isotopesbased on their decay constant, can provide consistent decaytimes for all short-lived Ra isotopes.

Following Simpson et al. (1982), this calculated range ofdecay times is defined as the ‘‘permanent burial time’’ (t) ofRa isotopes in the Sea of Galilee. This ‘‘permanent burialtime’’ in fact refers to an average ‘‘residence time’’ ofsuspended matter in the lake water that consists of themajor sink for Ra nuclides. This estimation of 12.3–14.6days is similar to, though slightly lower than, the residencetime of suspended matter in the lake calculated by Stillerand Imboden (1984) of 53 d.

226Ra mass balance in the Sea of Galilee—Assuming thatthe principal Ra source that enters the lake is saline seepagethrough springs, we use the conservation of mass equationto provide information on the flux of saline water enteringthe lake,

d 226Ra� �

SG

dt~0~226RaIN{226RaOUT ð12Þ

where d[226Ra]SG is the net change in radium activity in thelake over time. Assuming the lake is well mixed and is insteady state, the incoming Ra flux is expected to be equal tothe outgoing flux. The 226Ra sinks in the lake are described inEq. 13 and consist of 226Ra exiting the lake through the NWCand through sedimentation of particles within the lake:

226RaOUT~226RaNWCz226RaSED ð13Þ

where [226Ra]SED is the bulk Ra activity measured insediments and represents the sum of ‘‘supported’’ (i.e., Rain secular equilibrium with U) and ‘‘adsorbed’’ (i.e.,scavenged from the lake water) Ra found in suspendedmatter in the lake. Assuming that sedimentation occurs in thepelagic area where Ra is being permanently removed from the

lake water,

226RaSEDBq=yr½ �~ 226Ra

� �SED

Bq=g½ �| rateð Þ g=m2:yr½ �

|SA m2� � ð14Þ

Stiller and Imboden (1984) measured 0.052 Bq g21 of 226Ra insuspended matter from the lake. We did not collect a sampleof suspended matter from the lake in this study, but wemeasured an average of 0.037 Bq g21 of 226Ra in the mostshallow samples from cores 2 and 3 (the upper 3 cm of thecores; Fig. 4). We therefore use the range of 0.033 to 0.050 Bqg21 of 226Ra to represent what we define as [226Ra]SED. Thesedimentation rate (rate; 1.4 6 0.4 kg m22 yr21) and surfacearea (SA; 90 km2) were calculated by Serruya (1978) and usedby Nishri et al. (1999) to describe sedimentation in the pelagicarea.

Also, 226RaNWC is the amount of Ra lost per yearthrough pumping (FNWC) the lake water to supply theNational Water Carrier:

226RaNWCBq=yr½ �~ 226Ra

� �SG

Bq=L½ �| FNWCL=yr½ � ð15Þ

The 226Ra activity of the water pumped to the NWC is thesame as that of the lake water. On average, a volume of 4203 109 liters is pumped through the NWC each year.

The total incoming 226Ra flux (226RaIN) is composed ofdissolved Ra delivered through the Jordan River (226RaJR,

DISS), particulate Ra entering the lake through the JordanRiver (226RaJR,SED), and the unmonitored saline groundwaterflux (226RaSAL), assumed to be predominantly of Fuliya type:

226RaIN~226RaJR,DISSz226RaJR,SEDz226RaSAL ð16Þ

226RaJR,DISSBq=yr½ �~ 226Ra

� �JR,DISS

Bq=L½ �|FJR,WL=yr½ � ð17Þ

We estimate that the annual water flux from the JordanRiver, FJR,W, is 400 3 109 L yr21 with a 226Ra activity of0.0015 Bq L21 (Table 1). Based on measurements ofsuspended matter arriving from the Jordan River (0.033 Bqg21) conducted by Stiller and Imboden (1984), we estimatethe Jordan River 226Ra sedimentary flux, 226RaJR,SED, as

226RaJR,SEDBq=yr½ �~ 226Ra

� �JR,SED

Bq=g½ �|FJR,SEDg=yr½ � ð18Þ

where FJR, SED is the annual sediment flux from the JordanRiver and is calculated to be 40 3 106 kg yr21. This flux is themain source of allochthonous sediments arriving to the lake(Stiller and Imboden 1984). Using Eqs. 12–18, the amount of226Ra entering through unmonitored saline groundwaterdischarge can be calculated as

226RaSALBq=yr½ �~226RaOUT{226RaJR,DISS{

226RaJR,SED ð19Þ

Assuming that the saline groundwater source to the lake isFuliya-type water containing 1200 mg Cl L21 and 0.12 Bq226Ra L21, the annual Fuliya-type flux entering the lake iscalculated to be between 44 3 109 L yr21 and 61 3 109 Lyr21, corresponding to [226Ra]SED values of 0.033 and0.050 Bq g21, respectively. This calculated Fuliya-type salineflux would insert 52.7 3 106 to 73.4 3 106 kg Cl yr21 to thelake. This is similar, yet slightly lower, than the ,75 3 106 kg

128 Raanan et al.

yr21 estimated by Nishri et al. (1999) using a chloride mass-balance alone.

Dror et al. (1999) used the ratio of Br : Cl in the lake andits sources to show that Fuliya-type water alone cannotexplain the Br : Cl ratio observed in the lake and postulatedan additional contribution from THS-type water with a highBr : Cl ratio. Contribution of THS-type water with a higher228Ra : 226Ra ratio could also explain why the lake water hasa higher 228Ra : 226Ra ratio than that of Fuliya-type water.We show, for example, that for a scenario in which the salinegroundwater flow is composed of 98% Fuliya-type waterand 2% of THS water mixture (Table 3), the saline flux ofFuliya-type water would contribute only 26–37 3 109 L yr21

and 40.7 3 106–56.8 3 106 kg Cl yr21 using [226Ra]SED

values of 0.033 and 0.050 Bq g21, respectively.Since the 226Ra activity measured in the bottom

sediments in the lake represents both ‘‘supported Ra,’’arriving as the sedimentary flux of the Jordan River, and‘‘adsorbed Ra,’’ which is derived from scavenging of salinegroundwater discharge, we calculate the relative proportionof dissolved Ra being lost through scavenging by:

226RaSCAV %½ �~226RaSED

Bq=yr½ �{226RaJR,SEDBq=yr½ �

226RaSALBq=yr½ �

|100

ð20Þ

Our calculations suggest that 54–67% of the dissolved226Ra entering the lake is lost through adsorption. Thisestimate is similar to our independent observation that usesthe difference between the 226Ra : Cl ratio in the lake andsaline springs to show that 75–86% of the 226Ra enteringthe lake through the unmonitored saline groundwaterdischarge is being scavenged to the sediments.

Our flux calculations can now be used to evaluate theresidence time of 226Ra in the Sea of Galilee by dividing theexisting 226Ra reservoir in the lake (standing stock) by theyearly entering or exiting flux (Eq. 21):

TRa yr½ �~ existing

entering=year

~226Ra� �

SGBq=L½ �|Vol L½ �

226RaJR,DISSz226RaJR,SEDz226RaSALð Þ Bq=yr½ �

ð21Þ

where [226Ra]SG is the average 226Ra activity in the lake(0.00735 Bq L21), Vol is its average volume of the lake (4.33 1012 liters), and the incoming fluxes are calculated fromEqs. 18–20. Using Eq. 21, the calculated residence time ofRa in the lake is 4.3 and 3.4 yr for [226Ra]SED values of0.033 and 0.050 Bq g21, respectively. The similarity of thiscalculation to previous estimations made by Nishri andStiller (1997) (residence time in the order of 3.8 years)strengthens the validity of our estimation.

Reconstructing the pre-SDC 226Ra activity in the Seaof Galilee—Assuming the pre-SDC lake (before 1964) was insteady state, we can now use mass-balance equations tocalculate the expected 226Ra activity in the lake during theearly 1960s, several years before the diversion of the salineinflows to the lake took place. Ra sources to the pre-1964lake, as described in Eq. 22, were through the Jordan River(226RaJR,DISS), the now-diverted saline springs of Fuliya,THS, and Tabgha (226RaFUL,pre64, 226RaTHS,pre64, and 226Ra-

TAB,pre64, respectively), and the unmonitored saline flux stillentering the lake and calculated using Eq. 19 (226RaSAL,now):

226RaIN,pre64 ~226RaSAL,nowz226RaFUL,pre64z226RaTHS,pre64

z226RaTAB,pre64z226RaJR,DISS ð22Þ

We assume the saline flux that entered the pre-1964 lake toequal the average saline flux diverted away from it today; thatis, 0.75 3 109, 1 3 109, and 15 3 109 L yr21 for the Fuliya,THS, and Tabgha Springs, respectively. We assume that all ofthe diverted Fuliya Springs are of ‘‘Fuliya type,’’ bearing0.122 Bq 226Ra L21. THS and Tabgha activities are taken asthe averaged values measured in these springs (4.211 and0.444 Bq 226Ra L21, respectively; Tables 1, 2). The flux ofdissolved Ra entering through the Jordan River is assumed tobe the same as today (Eq. 17). To account for the flux of Racurrently entering the lake through unmonitored flow fromthe saline springs, we use the water fluxes calculated after Eq.19 (Table 3). As discussed earlier, we consider a scenariowhere all the unmonitored saline flux is of Fuliya-type water,as well as other scenarios where up to 2% of the water isderived from THS-type water.

Ra outputs in the pre-SDC lake are through outflow tothe southern part of the Jordan River and local use

Table 3. Flux values calculated for different contribution proportions of Fuliya and THS groundwater discharge to the Seaof Galilee.

AssumptionFuliya-type flux*

(3109 L yr21)THS-type flux{

(3109 L yr21)Cl2 flux

(3106 kg yr21)

228RaREG

(3107 Bq yr21)

100% Fuliya type{ 43.90 N/A 52.68 5.1599% Fuliya type, 1% THS type{ 32.92 0.32 45.21 6.0798% Fuliya type, 2% THS type{ 26.34 0.51 40.74 6.62100% Fuliya type1 61.16 N/A 73.39 2.4899% Fuliya type, 1% THS type1 45.87 0.44 62.99 3.1298% Fuliya type, 2% THS type1 36.69 0.71 56.76 3.50

* Fuliya-type water is taken as having 1180 mg Cl L21, 0.122 Bq 226Ra L21.{ THS type water is taken as having 4.21 Bq 226Ra L21.{ [226Ra] in lake suspended matter is 0.033 Bq g21.1 [226Ra] in lake suspended matter is 0.050 Bq g21.

Quantifying saline groundwater flow 129

(226RaSJR,DISS) and scavenging to the sediments:

226RaOUT,pre64~226RaSJR,DISSz

226RaSCAV,pre64 ð23Þ

Using Eq. 20, we calculate the current scavenging fraction(54–67%) for the lake. However, the salinity of the Sea ofGalilee in the early 1960s was higher than today (more than350 mg Cl L21; Gvirtzman 2002), and, thus, following ouradsorption experiment (Fig. 5), we expect lower adsorptionfraction under the higher salinity of the Sea of Galileeduring the early 1960s. Assuming Cl2 content of 350 mgL21 during the early 1960s, the relative change in the226Raads : 226Radis ratio detected by our adsorption exper-iment (Fig. 5) indicates that the scavenging fraction duringthe early 1960s would have been 94% of the currentscavenging rate (when Cl concentration is 250 mg L21).Following this calculation, the early 1960s scavengingfraction is estimated at 51–63% (226RaSCAV,pre64 [%]). Theamount of 226Ra lost through scavenging to the sedimentsin the pre-SDC lake would therefore be

226RaSCAV,pre64Bq=yr½ �~226RaSCAV,pre64 %½ �| 226RaSAL,now

�z226RaFUL,pre64z

226RaTHS,pre64z226RaTAB,pre64

�Bq=yr½ � ð24Þ

The remaining exiting Ra flux, 226RaSJR,DISS, is assumed todischarge the same amount of water as is currently pumpedthrough the NWC (FNWC[L yr21]; Eq. 15), bearing theaverage activity of the pre-SDC lake ([226Ra]SG,pre64 [BqL21]). Assuming steady state and combining Eqs. 22–24, weuse Eq. 25 to calculate the 226Ra activity in the pre-SDC lake:

226Ra� �

SG,pre64Bq=L½ �~

226RaJR,DISSBq=yr½ �

FNWCL=yr½ �

z 1{226RaSCAV,pre64 %½ �� �

| 226RaSAL,nowz226RaFUL,pre64

�z226RaTHS,pre64z

226RaTAB,pre64

�Bq=yr½ �7FNWC

L=yr½ �

ð25Þ

Our calculations suggest that the pre-SDC 226Ra activity in

the lake was between 0.018 to 0.020 Bq L21. This value is inagreement with the actual 226Ra activity (0.018 6 0.0012 BqL21) measured in the Sea of Galilee during 1962 (Gilboa1963). This agreement reinforces the validity of our estimationfor the unmonitored saline flux to the Sea of Galilee.

228Ra mass balance in the Sea of Galilee—A similarapproach to the 226Ra mass balance provides the base forconducting mass-balance calculations for 228Ra in the Seaof Galilee. Sinks for 228Ra in the lake (Eq. 26) consist of228Ra exiting the lake through the NWC, scavenging to thesediments, and radioactive decay:

228RaOUT~228RaNWCz228RaSCAVz228RaDEC ð26Þ

The value of 228RaNWC is defined by Eq. 15 as the annualamount of 228Ra exiting the lake due to pumping through theNational Water Carrier. The activity of 228Ra in suspendedmatter derived from the Jordan River was not reported byStiller and Imboden (1984), and thus we cannot use this directmeasurement as we did for the 226Ra mass balance. We

evaluate the amount of 228Ra lost through sedimentation byassuming that the reactivity of all Ra isotopes with suspendedmatter is identical. We therefore assume 228RaSCAV (%) isequal to the 226RaSCAV (%) determined in Eq. 20:

228RaSCAVBq=yr½ �~228RaSCAV %½ �|228RaSAL

Bq=yr½ �

~226RaSCAV %½ �|228RaSALBq=yr½ �

ð27Þ

Since the half-life of 228Ra (5.7 yr) is in the same scale as theresidence time of water (,6 yr) in the Sea of Galilee, we mustaccount for radioactive decay as an additional sink for 228Ra.As described in Eq. 28, 228RaDEC accounts for the annualdecay of all the sources and sinks of 228Ra:

228RaDECBq=yr½ �~ 228RaNWCz228RaSCAV

�z228RaJR,DISSz

228RaSAL

�| 1{e{l228t� �

ð28Þ

The incoming flux of 228Ra (228RaIN) consists of dissolved228Ra entering through the Jordan River (228RaJR,DISS) anddissolved 228Ra entering through the saline springs (228Ra-

SAL). Any excess 228Ra found in the system is considered tobe regenerated through the sediments (228RaREG), mainly bydiffusion, as suggested earlier in our discussion, so

228RaIN~228RaJR,DISSz228RaSALz228RaREG ð29Þ

To account for the 228Ra arriving through the saline springs(228RaSAL), we considered the saline flux calculated usingEq. 19 to be all of ‘‘Fuliya type,’’ as well as scenarios whereup to 2% of the Ra arrived from THS-type water (Table 3):

228RaSALBq=yr½ �~F226Ra,SAL

L=yr½ �| 228Ra� �

SALBq=L½ �~

226RaSALBq=yr½ �

½226Ra�SALBq=L½ �|

228Ra� �

SALBq=L½ �

ð30Þ

Combining Eqs. 26–30m we can find the amount ofregenerated 228Ra as

228RaREGBq=yr½ �~228RaOUT{228RaJR,DISS{

228RaSAL ð31Þ

Our calculations predict that of the 228Ra inventory in theSea of Galilee, a range of 2.48 to 6.62 3 107 Bq yr21

(Table 3) originates from regeneration and/or diffusionthrough bottom sediments. Consequently, we estimate that4% to 10% of the dissolved 228Ra in the lake is derived viaregeneration from the underlying sediments, 34% to 43% isfrom the Jordan River, and 48% to 61% comes through theunmonitored saline groundwater discharge.

This study investigates the Ra isotope geochemistry ofthe fresh lake water of the Sea of Galilee and the salinesprings that constitute the major salt source to the lake.Integration of the data shows that a large fraction of the Raentering the lake via the saline inflow is removed throughscavenging to suspended matter and sedimentation. Wepropose that the removal is by adsorption onto suspendedmatter in the lake water, yet validation by directmeasurements of Ra in suspended matter in the lake is

130 Raanan et al.

necessary. The relationships between Ra isotopes withdifferent half-lives indicate that the reactions between lakewater and suspended matter involve adsorption-desorptionexchange, recoil, and decay of the short-lived 223Ra and224Ra isotopes. Our data suggest an average residence timeof suspended matter on the scale of 2 weeks. Mass balancewas used to determine the residence time of the residualdissolved Ra (3 to 4 yr) in the lake system.

Our data suggest that the unmonitored saline ground-water flux varies between 44 3 109 L yr21 and 61 3 109 Lyr21, assuming the entering groundwater is of ‘‘Fuliya type.’’We also estimate that 2.48–6.62 3 107 Bq 228Ra yr21 entersthe lake through regeneration from bottom sediments, whichmakes up 4% to 10% of the overall annual 228Ra flux enteringthe lake. We combined a mass-balance approach with theresults of our adsorption experiment to calculate the expected226Ra activity in the lake before the diversion of saline springsto the lake. Our calculated 226Ra activity, 0.018–0.020 BqL21, is in agreement with the 0.018 6 0.0012 Bq 226Ra L21

that was actually measured in the lake in 1962.In spite of the nonconservative behavior of Ra in

freshwater systems, this study demonstrates that variationsof the Ra isotope quartet can provide essential information forquantifying the flow of saline groundwater into a freshwaterlake and the residence time of dissolved Ra in the lake water.

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Edited by: Stephen P. Opsahl

Received: 20 November 2007Accepted: 22 July 2008

Amended: 12 September 2008

Quantifying saline groundwater flow 131