Groundwater Record of Halocarbon Transport by the Danube River

Environ. Sci. Technol. 1997, 31,3293-3299

Groundwatel Becod of HalocarhonTransport by the Danube BiverI . K . e 0 F I L K E , * ' t K . R E V E S Z , iE . B U S E N B E R G , T I . D E A K , +T j , . D E S E O , + A N D M . S T U T E SU.S. Geological Survey, 431 National Center,Reston, Virginia 20192, WTUKI, H-1095 Budapest, Hungary,and Lamont- Doherty Earth Ob seruatory,Palisades, New York 10964

Groundwater dating studies have supported the conceptthat aquifers with low coefficients of dispersion may containcoherent records of past conditions in recharge areas.Groundwater records can provide unique information aboutnatural or anthropogenic changes in the atmosphere andhydrosphere where long-term monitoring data are notavai lable. Here we descr ibe a 40-year record of halocarboncontamination in the Danube River that was retrieved froma shallow aquifer in northwest Hungary. The time scale isbased on 3H and He isotope dating of groundwaters thatwere recharged by the Danube River and movedhorizontally away from the river in a surficial gravel aquiferwith minor dispersion at a maximum rate of at least 500m/yr. Analyses of dated groundwaters along a flow pathindicate that the r iver loads of selected compounds( including CFC-12, CFC-113, and tr ichloroethane) werenegl ig ib le before about 1950, rose rapidly to peak values inthe 1960s and 1970s, andthen decreased byvarying degreesto the present. Peak concentrations are tentatively at-tributed to point s0urces in upstream urban-industrial centers;while recent decreases presumably resulted from decliningmanufacturing rates and(or) improvements in control ofurban-industrial runoff and sewage effluent entering the riverin upstream areas.

lntroductionAquifers with low coefficients of dispersion may containcoherent records of past conditions in recharge areas. Recentadvances in groundwater dating have permitted retrieval ofthose records at time scales ranging from tens of thousandsofyears (glacial-interglacialtime scales) to tens ofyears (recentanthropogenic time scales) (f -O. In the absence of real-time monitoring over the same time scales, the groundwaterrecords can provide valuable information about causes andeffects of global change and regional contamination.

long-term groundwater records of regional and globalclimate change have been retrieved largely from confinedaquifers with long travel times in which piston flow isdominant, isochrons are approximately normal to the direc-tion of flow, and longitudinal dispersion is relatively low.Records of paleotemperatures and changing precipitationpatterns have been derived from aquifers with those char-acteristics (1,2). Records of change over time scales of yearsto decades more commonlv have been retrieved from

* Corresponding author phone: 703-648-6325; fax 703-648-527 4;e-mail: jkbohlke@usgs.gov.

i U.S. Geological Survey.+ VITUKI.s Lamont-Doherry Earth Observatory.

unconfined aquifers in which areally distributed rechargecontaining non-point-source contaminants results in verticalstratification of groundwater ages and contaminant concen-trations. In those situations, both the isochrons and thedirection of fl ow may b e nearly horizontal, but vertical (lateral)dispersion must be low. Records of groundwater contami-nation by atmospheric tritium, chlorofluorocarbons (CFCs),and sulfate and by agricultural nitrate, chloride, and mag-nesium have been derived largely from vertical gradients inaquifers with those characteristics (3-O. The purpose ofthis paperis to describe the retrievalof acontaminationrecordfrom an aquifer that differs from those summarized above innvo important ways: (f) the source of recharge and of thecontamination record is a major river (the Danube); and (2)although the aquifer is largely unconfined, the record ofchange is derived from the areal distribution of contaminantsin the ground water because the source of recharge isessentially linear.

The surficial gravel aquifer underlying the Little HungarianPlain (including the Szigetkriz region of northwest Hungaryand the nvryr Ostrov region of southern Slovakia) is one ofthe largest sources of accessible potable water in centralEurope. Scientific investigations of the aquifer intensifiedrecently because of concerns about potential environmentaleffects of a dam and diversion project completed on theDanube River in 1992, as the river is believed to be the sourceof much of the aquifer water (6-9). The current results wereobtained as pilt of a multidisciplinary investigation of theextent and rate of river water infiltration in the Szigetkdzregion by use of environmental isotopes and tracers (10-13).

Experimental SectionSite of Investigation. The surficial gravel aquifer underlyingthe Little Hungarian Plain was formed in Quatemary time byaccumulation of sediments that were carried down from theAlps by the Danube River and deposited on the LittleHungarian Plain, forming a broad alluvial fan (7,9, 14). Theaquifer varies in thickness up to about 400 m and consistslargely of coarse sand and gravel. The aquifer is underlainby relatively impermeable Quatemary and Tertiary sediments.The potential for groundwater movement away from theDanube Riverinparts of the Little HungarianPlainis indicatedby the confi guration of the water table, which d i p s away fromthe river on both sides (Figure 1). The presence of formerriver water within the aquifer has been confirmed bymeasurements of d18O, d2H, and 3H in groundwaters fromthe Szigetkoz artd Zifiyr Ostrov regions (10-12, 15, 16).

Sample Collection and Analysis. Regional groundwaterisotope surveys were conducted in the Szigetkciz region in1990- 1992 (10, 11). Based on those results, additional watersamples were collected in late April and early May 1993 fromwells located along a path of relatively rapid groundwaterflowawayfrom the river (Table 1;Figures I and 2). Analysesof drsO and dzH were used to confum the distribution of riverwater in the aquifer; 3H and He isotope concenuationsprovided nvo independent estimates of the rate of horizontalflow; and N2, Ar, and Ne concentrations provided estimatesof recharge temperatures. The distributions of selectedhalocarbons were measured along the same transect.

Water-supplywells, screened between 40 and 120 m belowthe water table and outfitted with a variety of permanentlyinstalled high-volume pumps, were s:rmpled from blpasstubes near the well heads. One deep well was screened inthe under$ing Pannonian aquifer. Observation wells, con-structed of PVC pipe and screened less than 20 m below thewater table, were sampled near screen depth with a Grundfo selectric submersible pump. All wells were purged of at least

S0013-936X(97)00336-2 CCC: $14.00 o 1997 American Chemical Society VOL. 31, NO. 11, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY r 3293

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3294 J ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31 , NO. 11 , 1997

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FIGURE 1. Map showing the SzigetkUz region of northwest Hungary.Elevation contours for the water table correspond to averageconditions during the period 1976-1985. The dlr0 contour (heavysolid or dashed line at -10.5 %.) indicates the areal efient ofpenetration of Danube Riverwater into the surlicial aquifer, whilethe 3H peak contour (hearry dotted line, with shading foi sH > 50 TUIindicates approximately the distance traveled by groundwaterc thatf eft the river in the middle 1960s [aquifer data from 19!lt to IfiZltAJ.Symbols indicate the locations of wells (circles indicate municipaland cooperative water-supply wells screened between rCI and l2tlm below the water table; triangles indicate observation wellsscreened less than Zl m helow the water tablel and surface watersites (squareslthatwere sampled between April 25 and May 6,1!R3.0pen symbols are used for groundwater sites that are relatively farfrom the "fast llow path" indicated by the large arrow. Site 14 (froma deep well in the underlying Pannonian aquifer) and site 2l (fromthe Mosoni Dunal were not considered to be part of the flow path.

5- 10 well volumes before samples were taken. Surface watersamples were taken from a boat anchored in mid stream attwo locations in the Danube River and from a bridge acrossthe Mosoni Duna with the Gmndfos pump held approximately1.5 m below the surface.

Samples for stable H and O isotope ratio analysis werecollected in glass bottles with Polyseal caps. Measurementsof d2H and drsO were made in the Reston Stable Isotopela.boratory, U.S. Geological Survey, by H, eqtrilibration andCO2 eqrrilibration, respectively [d is defined with respect tothe VSMOW standard as 1000 x (R/Rvsr,,row - l), where R:2HIL}I or r8O/1601. Samples for Ne and He isotope analysiswere collected in copper tubes and measured by massspectrometry at the lamont-Doherty Earth Observatory (/S).Tritium measurements were made at the Institute of WaterQuality (PIC) in Budapest, Hungary, by liquid scintillationcounting after electrolytic enrichment. Samples for dissolvedNz and Ar analysis were collected in evacuated glass vesselsand equilibrated with low-pressure headspace, which wasthen subsampled for concentration measurement by gaschromatography and for isotopic measurement (drsNlNzl)by isotope ratio mass spectrometry (4, 5).

Samples for CFC analysis were collected in flame-sealedglass ampules with ultrapure N2 headspace and analyzed bypurge-and-trap gas chromatography using an electron capturedetector (12. All sampling equipment and tubing wereconstructed of stainless steel, copper, and aluminum withTeflon ferrule fittings. The concentrations of CFC-12, CFC-11, and CFC-1I3 were quantified using an air standardcalibrated on the NOAA/CMDL scale (tS). CFC concenua-tions were comp ared to those expected in air- satwated watersof appropriate ages at 10 oC (average apparent rechargetemperature estimated from N2, Ar, and Ne concentrations)and 14'C (temperature of the river at the time of samplin$,where the atmospheric partial pressures were assumed to bethose of hemispherically averaged midlatitude air, as rep-resented by volume fraction data from Niwot Ridge, CO (f g-

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FIGURE 2. Vertical section of the surficial gravel aquifer showingthe locations of wells and selected lgg3 isotope data atong agroundwater llow path from the Danube River to the south (seeFigure 11. Groundwaterflow is from rightto left. Well locations areproiected onto the section according to their flow path distancesfrom the river (symbols as in Figure 11. All data are plotted accordingto their projected distances, while the age scale corresponds to aconstant linear velocity of 500 mlyr, consistent with the position ofthe 3H bomb peak. Unceftainties in time and distance for individualwells are estimated to be in the order of 0-5 yr and 0-5 km,respectively. 3H values for precipitation are annual values derivedfrom the 0ttawa correlation l47t tor lg5il-1960, weighted annualaverages for 1961-1987 lZt, and unweighted annual ayerages for1988-1992 (0. Rank and V. Rajner, written communication, lgg4l, alladjusted for decay to 1993. 3H values for the Danube River areunweighted annual ayerages for 1964-1nZ lU D. Rank and V.Rajner, written communication, l!F4l, adjusted for decay to 1993.3H-3He ages for groundwaters are from ref 13. ,1s0 yalues forprecipitation are weighted annual averages from lg61 to 11fi7 l24t,and unweighted annual averages are from 1!fB8 to lggl (D. Rank andW. Papesch, written communication, t9g4). d180 values forthe DanubeRiver are unweighted annual averages for 1gl5-ttf!12 (2S D. Rankand W. Papesch, written communication, 1gg4l.

2/ ; E. Busenberg, unpublished data), adj usted for the elevationof the Danube River near Dunakiliti (f20 m). The relativeabundances of trichloroethane and some other halocarboncompounds in different samples were estimate d qualitativelyfrom digital output from the peak integrator.

100

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1940 1950

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Danube River at Vienna

VOL. 31 , NO. 11 , 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY I 3295

Besults and lliscussionConfiguration of Groundwater Flow. Danube River water,local precipitation, Vienna precipitation, and shallow Sziget-koz groundwater all exhibit similar correlations between drsOand d2H values that are roughly consistent with the correlationexhibited globatly by precipitation (the global meteoric waterltne) (22; T. B. Coplen, written communication, l9g3 ; D. Rankand W. Papesch, written communication, lgg4). However,long-term means of weighted annual drsO values of DanubeRiver water (-f 1.6 * 0.3%o in Vienna in t9Z5-1991 (23; D.Rank and W. Papesch, written communication, l9g4)) aresignificantly lower than those of local precipitation [-9.2 AI.0Too in Vienna in 1961-199I (22)l and of locally rechargedshallow ground waters near the Austria-Hungary borderlabout -9.7 L l.OVoo Q4l and elsewhere in Hungary (iZ)because much of the river water originates as precipitationat higher elevations. Isotope data from the gravel aquifercan be compared with historical records of the isotopiccomposition of the Danube River in Vienna (approximately70-120 krn upstream from the Szigetkdz region), as the rivervalues generally are not signifi cantly different between Viennaand Medve, Hungary (near the downstream end of theSzigetkriz region) 0. Deak et al., unpublished data). Thedistribution of O isotopes in the Szigetkdz region defines aplume of river-derived groundwater (dtaQ < -10.5%o) thatfills the surficial gravel aquifer to as far as 10-20 km from theriver (Figwe l). The distribution of d18O values along the1993 transect corresponding to the fastest flow path (Table1; Figure 2) is consistent with the regional pattem (Figure l)and indicates that river water dominates the surficial aquiferto a flow path distance of at least 22 \{rr from the river.Seasonal variations in the drsO values of the river (typicalannual r€rnge is about l-2Voo) apparently are not wellpreserved, presumably because of dispersion in the aquifer.

Measured concentrations of 3H in the Danube River atVienna (2O responded rapidly to the injection of bomb-produced 3H into the atmosphere, which began in the lg50s,peaked in the 1960s, and then decreased rapidly afterwards.High 3H concentrations (>50 TU, with peak values as highas 80 TU), representing I960s river water, now occur withinthe gravel aquifer in a narrow zone that extends across theSzigetkciz region in an east-west direction (Figure f). 3Hconcentrations in the gravel aquifer are not stratified as theyare commonly in aquifers recharged by areally distributedinfiltration but rather indicate that infiltrating water flowshorizontally away from the river such that isochronoussurfaces representing groundwater recharge ages are es-sentially vertical in the zone of maximum flow. The distribu-tion of peak 3H concentrations indicates that maximumhorizontal groundwater flowvelocities were about 500 m/yrat middepth in the aquifer over the 3O-year period ending inL993 (12). Regional variations in both the extent of the riverwater plume and the appiuent groundwater flow velocitiesare attributed to variations in the hydraulic properties of theaquifer sediments, which are generally finer grained and lessconductive toward the southeast (26) . The longest flow pathsand highest velocities are inferred for ground waters thatinfiltrated from the Danube River north of Dunakiliti andthen flowed within the aquifer along a slightly curved pathto the south (approximately normal to the water tablecontours) , where the 1993 samples were taken (arrow in Figwer).

High groundwater 3H concentrations (>50 TU) corre-sponding to the mid-lg60s bomb peak were found in the1993 transect at flow path distances of about I2-LS km. Agood match can be obtained betr,veen the distribution of 3Hin the groundwater from 0 to 12 km and the historical recordof 3H in the Danube River from 1969 to 1993 by assuming aconstant horizontal linear velocity of 500 m/yr (Figure 2).Groundwater ages calculated from 3H and He isotopemeasurements in the deeper wells generally are in good

agreement with the direction and velocity of flow inferredfrom the distribution of 3H alone (Figure 2). Some of theshallow wells appear to have anomalously low 3H-3He ages,but those are considered to be less reliable. For example,two shallow samples (sites 9 and 1l) that were farther fromttre river (and therefore presumablyolder) than the 3H bombpeak had apparent 3H-3He ages in the lg60s; however, 3H-3He dating of waters older than the 3H bomb peak may beproblematic (13), and the 3H concentrations of samplesbeyond the peak are consistent with precipitation before thepeak Another shallow sample (site 10, screened near thewater table) had an anomalously high nitrate concentrationthat may be attributed to contamination caused by minorlocal infiltration (Table l). Therefore, an overall average flowvelocity of 500 m/yr was assumed in order to match the timeand distance scales in Figure 2. The highest 3H concentrationsmeasured in the aquifer labout 80 TU (12)] were significantlylower than the peak annual values of the river in the mid-1960s (about 200 TU after correction for decay to 1993) andimply that some dispersion has occurred (J.3, 26). Neverthe-less, the systematic correlations among the 3H-3He ages, 3Hconcentrations, and flow path distances indicate that disper-sion, while apparently sufEcient to remove seasonal variationsin d 18O, has not been sfficient to alter pattems in the rechargesignatures of the groundwaters with time scales of severalyears or more. The groundwater isotope data are thereforeinterpreted as a slightly smoothed record of the isotopiccondition of the Danube River north of Dunakiliti, Hungary(including its anthropogenic 3H load) for about a 4O-yearperiod ending in f993.

Distribution of Halocarbons. The distributions of CFCsalong the Szigetkriz groundwater transect (Table l) aresummarized and comparedwith the concentrations erpectedin air-saturated waters in Figure 3; qualitative comparisonsfor some other compounds in selected samples are shown inFigure 4. The concentrations of CCI3F (CFC- I t), CClzFz (CFC-12), and CzClsFs (CFC - I 13) in 1993 were higher than expectedfor eqrrilifrium with hemispherically mixed air and indicatethat the river was receiving excess CFCs from localized ornon-atmospheric sources.

By analogy with the 3H record, and despite some scatterin the data, the generalized trends in the abundances of atleast some of the halocarbons with groundwater age areinterpreted as records of the halocarbon contaminationloadsof the Danube River between the 1940s and the time ofsampling in f993. Concentrations of CFC-I2 apparentlyincreased rapidly in the 1950s, peaked in the tg60s at up to20 times the equilibrium values, and then decreased to about1.3 times the normal equilibrium values in 1993 (Figure 3).The trend followed by CFC-I13 is roughly similar to that ofCFC-12 but appears to be shifted to slightlylater times: CFC-113 did not appear in the river until the 1960s, it did not peakuntil the 1970s and 1980s (also at about 20 times theequilibrium values), and it was more supersaturated (about1.8 times the equilibrium values) than CFC- t2 in 1993. Severalother halocarbons including CzHgCls (TCA), CCI+, and CzChhad relatively low concentrations in the I950s, higher andmore variable concentrations in the lg60s and lg70s, andthen lower concentrations again in the I g80s and 1 g90s (Figure3). tn contast, the concentrations of CFC-If were lower inmost of the groundwaters than in the 1gg3 river water andmostdo not appearto have exceededthe normal equilibriumvalues. The overall trend for CHzClz was qualitatively similarto that of CFC-I1.

The most likely sources of halocarbon contamination inthe river water now residing in the gravel aquifer areconsidered to have been local discharges of highly contami-nated water, such as urban runoff, sewage effluent, orindustrialwastes, into the Danube orits tributaries atlocationsupstream from the Szigetkriz region. High concentrations ofCFCs in rivers elsewhere have been attributed largely to point

II

I

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3296 r ENVIRONMENTAL SCIENCE & TEcHNoLoGY i VoL. 31 , No. 11 , 1997

Year of Recharge1960 1970 1980

r t l

3 0 2 0 1 0 0Flow-Path Distance (km)

FIGURE 3. Distribution of halocarbon concentrations along a transectparallelto groundwaterflow awayfrom the Danube River. Symbols,distances, and ages are as in Figures 1 and 2. Groundwater flowis from right to left. GFC concentrations are compared to thoseexpected in air-saturated waters (ASW) of appropriate ages at 10(upper curve) and 14 oC (lower curvel. The open triangle at 1.8 km(site 17) represents a reduced (denitrified) water in which bothCFC-I1 and GFC-I13 appear to have been completely degraded.

sources such as urban or industrial runoffand sewage effluententering the rivers near the sampling sites (17,27,28). Riverconcentrations more than twic e the atmo spheric equilibriumvalues are common, and order of magnitude supersaturationshave been observed. The investigated groundwater flowpathleaves the Danube River at about lxn 1845, to which the nearestmajor cities upstream are Bratislava at about km tB70 andVienna at about kn f930. The average flow velocity of theDanube River in the investigated reach is in the order of 2-3mi s or about U0-260 km/d (9, 29). At those flow rates,waters contaminated by point sources in major urban-industrial :ueas could reach places of entrance to the gravelaquifer within hours. Gas transfer rates for rivers as large asthe Danube are not well known; given a range of first-orderrate constants observed elsewhere in large streams and rivers[commonly in the order of 0.I-10/d (30-33)], travel timesless than I day could be short enough to preclude full re-equilibration of contaminated river water with relativelyuncontaminated air.

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Another possible source of high halocarbon concentrationsin surface waters is by exchange with contaminated air. Long-term atmospheric anomalies in the order of 10-50% abovehemispheric backgound may be possible at regional (sub-continental) scales (34);while average local anomalies in theorder of 100-200% with short-term spikes more than 1000%above background have been reported (35-32). However, itis considered relatively unlikely that the peak river concen-ffations in the 1960s and 1970s were caused by contaminatedair, in part because it may be expected that enrichmentscaused by short traverses through highly contaminated urbanair would largelybe lost duringlonger traverses through ruralareas with less contaminated air.

voL. 31 , NO. 11 , 1997 / ENVTRONMENTAL SCTENCE & TECHNOLOGY r 3297

Itis possible that the record of halocarbon concentrationshas been altered in part by microbial degradation or sorptionof some of the compounds in the aquifer. Experimentsindicate that the halocarbons are susceptible to microbialdegradation in anaerobic environments, that CFC-ll andCFC- f f 3 are more readily degraded in anoxic sediments thanCFC-12 (38, 39), and that rates of biostimulated anaerobicde gradation of CCla, CFC- 1 1, CFC - I I 3, and TCA are relativelylow in the presence of NOe- but increase when denitrificationis complete (40). Experiences with groundwater flow systemselsewhere indicate that CFC-II, CFC-12, and CFC-ll3 com-monly behave as conseryative tracers as long as the waterscontain measurable Oz or NO3-, that CFC-ll and CFC-lI3are more susceptible to degradation than CFC-12 when thewaters enter sulfate-reducing or methanogenic environments,and that CFC-I 1 degrades more readily than CFC-113 asaquifer conditions become more reducing {4, 41-43; E.Busenberg, L. N. Plummer, and I. K. Brihlke, unpublisheddata). There was no evidence of HzS (by odor) or methane(by GC) in the analped samples from the Szigetk6z region,and chemical data indicate that the bulk of the groundwaterrecharge probably occwred in the main channel of theDanube and not in the relatively stagnant side arms or ox-bow channels, which commonly are underlain by relativelyreduced waters (9). One shallow groundwater sample (site1l) from a wooded lowland near the river that contained Fe,Mn, NHa+, and excess non-atmospheric N2 indicative ofdenitrification was anomalously depleted in CFC- 1 I and CFC-113 (FigUre 3). In contrast, most of the groundwatersunder$ing the Szigetkriz region contained at least traceamounts of Oz and significant amounts of NOr- Gable 1), asdid the surface waters in the main river channel, and thoseconditions are favorable for preservation of at least some ofthe halocarbon compounds. Exceptfor CFC-ll and CHzClz,there was not a systematic decrease in the concentrations ofthe halocarbon compounds with water ages or flow pathdistances. Thus, while degradation or sorption could beresponsible for some of the scatterinthe data, major featuresof the overall trends for CFC-12, CFC-I13, and TCA (possiblyalso CCI+ and CzCla) apparently exist independently of thoseprocesses and may reflect changes in recharging river waterover time. In contrast, it is considered more likely that thedistribution of CFC-ll (and possiblyCH2CI2) mayhave beenaltered within the aquifer and may not reflect changes inrecharge.

The concentrations of CFCs in the gravel aquifer are fartoo low to be considered as water quality problems, but theyappear to provide a unique record of river contaminationhistory. Rapid increases in the halocarbon loads of theDanube River in the 1950s and 1960s are consistent with timesof rapid growthinmanufacture and use while environmentalconsequences of releases were not widely accepted. Theearlier introduction and wide spread use of CFC - f 2 be ginningin the late 1930s) and TCA (beginning in the 1950s) relativeto CFC-I13 (beginning in the 1960s) appear to be recordedin the aquifer data (Figure 3). The timing of the peaks andsubsequent decreases in the levels of contamination of thosecompounds in the river could reflect changes in local ratesof manufacture and use or changes in the treatment anddischarge to the river of urban and industrial wastes in areaslike Vienna and Bratislava. The global production rates ofCFC-I l and CFC-12 peaked in the middle l970s,leveled off,and then decreased abruptly after the late 1980s; whereas,the production rate of CFC-113 peaked in the late 1980s (44,4O. Historical records indicate that the average concentra-tions of NFI4+ and NOz- in the Danube River peaked in theearly 1980s and decreased subsequently GA, possibly inresponse to improved sewage treatment in upstream areassuch as Vienna since the early 1980s [P. Liebe and D. Rank,oral communicationl.

3298 r ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31 . NO. 1 1 . 1997

IABIE 2. Estimated CFC loads for dre llanube Biver at km| 845 near llunakiliti'

compd l9g3 (kg/yr) peak (kg/ryrlt cumulative (kgl

cFc-12 32c F c - 1 1 3 1 5

85 (late 1960s)25 (mid 1970s)

1700500

I Estimates are based on the long-term average annual flow rate ofthe Danube [2075 m/s (OI and approximately the midranges of theshaded regions representing trends in the CFC concentrations overtime in Figure3. Uncertaintiesare unknown but large. b Thetime periodis given in parentheses.

The inferred CFC loads of the Danube River (Table 2) wereinsignificant as compared to the peak glob al production rates,which were in the order of 300-500 x 106 kg/yr for CFC-12(1970s) and 100-300 x 106 kg/yr for CFC-r13 (r980s) (44, 45).The total amounts of CFCs stored within the gravel aquiferappeil to be larger than those commonly found in areallyrecharged stratified aquifers (4, 5, 37, 41-43) but do notrepresent a significant sink or future source of CFCs, as thetotal amount of Danube River water lost to the aquifer onboth sides of the river from Bratislava to Medve was in theorder of 50 m3/s, only about 2-3Vo of the total flow (7, 29).

While infiltration of river water into shallow groundwatersystems is not unco[lmon, the river water plume underlyingthe Szigetktiz region appears to be unusually well developed,and it is believed that this is the first description of such asituation providing a coherent record of several decades ofcontamination in the contributing river. Records of otherstable compounds in the Danube could be investigated byresampling the flow path transect. Conversely, it should bepossible to predict the areal distributions of compoundsconsidered in this study in other portions of the aquifer (e.g.,elsewhere in the Szigetktiz region or on the Slovakian side ofthe river) by combining the regional isotope data with thehistorical contaminant Uends. Further investigations mayidentiff analogous records in similar hydrogeologic settingselsewhere in the world.

AcknowledgmentsSupport for this study was provided in part by the U.S.-Hungaryloint Fund of the U.S. Department of State and theNational Research Program of the Water Resources Divisionof U.S. Geological Survey. Oxygen and hydrogen isotopeanalyses of water samples were provided by T. B. Coplen.Isotope monitoring data for Vienna were provided by D. RankReviews by L. N. Plummer, B. G. Katz, T. A. Abrajano, Jr., R.L. Michel, and K. Peretin helped to improve the manuscript.

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Receiued for reuiew April 14, 1997. Reuised manuscript re-cei.ued August 6, 1997. Accepted August B, 1997.8

ESg70336H

E Abstract published in Ad u ance ACS Ab str ac*, September I 5, 1 997.

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