Strontium isotope ratios, strontium ion
concentrations, major ion concentrations, and stream
discharge were measured within the Middle Oconee River
basin upstream of Arcade, Georgia during the period 2003-
2004. Strontium isotope ratios (87Sr/86Sr) in stream base flow
were between 0.7126 and 0.7172, considerably higher than
rainfall (0.7117) and shallow ground water (0.7120). This
indicates that Sr and by inference other weathering products
in base flow are derived principally from weathering, rather
than ion exchange in the shallow soil zone. Unlike all other
parameters, strontium isotope ratios were temporally
invariant within a given watershed, independent of seasonal
variations with respect to base flow discharge. Strontium
isotope ratios were significantly different in all four
watersheds and therefore provide the best geochemical
“tracer” for base flow. 87Sr/86Sr ratios increased as a
function of basin area, independent of Sr ion concentrations,
probably as the result of the increased contribution from
rubidium-bearing minerals such as K-feldspar, muscovite,
and biotite.Sponsored by:
Georgia Environmental Protection Division
U.S. Geological Survey, Georgia Water Science Center
U.S. Department of Agriculture, Natural Resources Conservation Service
Georgia Institute of Technology, Georgia Water Resources Institute
The University of Georgia, Water Resources Facult

Fullagar, Paul D.

Rose, Seth

Scholarly Materials And Research @ Georgia Tech

STRONTIUM ISOTOPE RATIOS IN STREAM BASE FLOW WITHIN THEGEORGIA PIEDMONT PROVINCESeth Rose  and Paul D. Fullagar1 2AUTHORS:  Associate Professor Department of Geology, PO Box 4105, Georgia State University, Atlanta, GA 30302-4105 and Professor,1 2 Department of Geological Sciences, University of North Carolina-Chapel Hill, CB 3315 Mitchell Hall, Chapel Hill, NC, 27599 REFERENCE:  Proceedings of the 2005 Georgia Water Resource Conference, held April 25-27, 2005, at The University of Georgia,Kathryn J. Hatcher, editor,  Institute of Ecology, The University of Georgia, Athens, Georgia.  Abstract. Strontium isotope ratios, strontium ionconcentrations, major ion concentrations, and streamdischarge were measured within the Middle Oconee Riverbasin upstream of Arcade, Georgia during the period 2003-2004. Strontium isotope ratios ( Sr/ Sr) in stream base flow87 86were between 0.7126 and 0.7172, considerably higher thanrainfall (0.7117) and shallow ground water (0.7120). Thisindicates that Sr and by inference other weathering productsin base flow are derived principally from weathering, ratherthan ion exchange in the shallow soil zone. Unlike all otherparameters, strontium isotope ratios were temporallyinvariant within a given watershed, independent of seasonalvariations with respect to base flow discharge. Strontiumisotope ratios were significantly different in all fourwatersheds and therefore provide the best geochemical“tracer” for  base flow. Sr/ Sr ratios increased as a87 86function of basin area, independent of Sr ion concentrations,probably as the result of the increased contribution fromrubidium-bearing minerals such as K-feldspar, muscovite,and biotite.INTRODUCTION      The strontium isotope ratio ( Sr/ Sr) has been used in87 86watershed studies to trace water pathways and defineweathering reactions that control the chemistry of groundwater and base flow (Bullen and Kendall, 1998). The firstorder control on the Sr/ Sr ratio in water is the degree to87 86which rubidium (Rb-87 is the radiogenic parent of Sr-87) ispresent as a substitute for potassium within mineralscomprising a watershed (Douglas et al., 2002). In thatPiedmont Province watersheds are comprised dominantly ofaluminosilicate minerals (i.e. K-feldspar, biotite, andmuscovite), Sr/ Sr should be relatively high. One87 86complication is that Sr derived from cation exchangereactions at shallow depths typically produces much lowerSr/ Sr ratios (Miller et al., 1993).87 86This study represents the most systematicinvestigation to date of strontium isotope systematics on alarge basin scale (13-860 km ) within the Georgia Piedmont2Province. The principal objectives were to determine theextent of strontium isotope and strontium ion concentrationvariation  within base flow, rainfall, and shallow groundwater and how this variation relates to seasonal factors,stream discharge, major ion geochemistry, basin mineralogy,and basin area.  STUDY AREA AND METHODS    The 860 km  Middle Oconee River basin in Jackson2County upstream of Arcade, Georgia (Figure 1)  was chosenas a study area in that it is relatively undeveloped (-95%forest and pasture) and stream discharge is monitored on acontinuous basis by the USGS.  Shallow ground water wasmonitored within the regolith near the Middle Oconee Riverat a depth of  17 meters and rainfall samples were acquiredat a location -75 km west of the site. Stream base flow(USGS, 2004) and shallow ground water were monitored ona monthly basis during the period between March, 2003 andMarch, 2004. Rainfall totals were obtained near the studyarea from the U. S. Department of Agriculture stationlocated at Watkinsville, Georgia, approximately 20 kmsoutheast of the study area. Specific conductance wasmonitored in the field and pH and alkalinity concentrationswere determined usually within 24 hours after samplecollection. Tritium ( H) and stable oxygen isotope (* O)3 18ratios were measured on selected samples for comparativepurposes.Sr/ Sr ratios and Sr ion concentrations were determined        87 86at the University of North Carolina-Chapel Hill using a VG(Micromass) Sector 54  thermal ionization massspectrometer (TIMS). These techniques produce Sr isotoperatios that have uncertainty of no more than 2 in the 5thdecimal place (2standard deviations); for conciseness, wereport the ratios in the 4th decimal place.RESULTS AND DISCUSSION    The study area received 1467 mm precipitation during thestudy period which is 21% greater than normal and most ofthe excess occurred between June and July, 2003. Totaldischarge was likewise 37% greater than normal during theFigure 1. Middle Oconee River Basin study area showingsampling sites.study period. Rainfall and discharge returned to normalduring the later part of the study and base flow dischargerates declined by a factor of two to three during the periodbetween August and November, 2003 (Figure 2). Thisdecline in base flow discharge is a defining characteristic ofstream flow in the Piedmont region.     Strontium ion concentrations were relatively low in studyarea base flow, ranging from 16.3-26.9 :g/L (Table 1)compared to the global river average of 78 :g/L (Palmer andEdmond, 1992).  Neither strontium ion concentrations norstrontium isotopic ratios varied as a function of base flowdischarge during the sampling period. Strontium ionconcentrations in base flow were significantly higher than inrainfall or shallow ground water (Table 1) which indicatesthat dominant source of Sr (and by inference other ions) inbase flow is the weathering products of minerals, rather thanthe release of exchanged ions in shallow soil horizons.    Sr/ Sr ratios varied between 0.7113-0.7176 and87 86averaged 0.7145 within base flow in the four streams (Table1). This average is higher than the global river average(0.7119; Palmer and Edmond, 1992) and can be attributed tothe dominance of Rb-bearing aluminosilicate mineralswithin Piedmont Province watersheds.  The Sr/ Sr ratios87 86in stream base flow were considerably higher than in theweighted rainfall (0.7117) and in the shallow ground water(0.7120; Table 1). The mean Sr/ Sr of all streams were87 86statistically different at very high levels of confidence("=0.00001; Rose and Fullagar, 2004) indicating that eachstream is characterized by its own strontium isotopicaveragesignature. The highest ratios ( Sr/ Sr  = 0.7172) were87 86observed in the terminal Middle Oconee River basin and thelowest ratios were observed in the two small Indian Creekwatersheds (Table 1).    Strontium ion concentrations and strontium isotopic ratiosFigure 2. Stream dsicharge for the Middle Oconee Riverbasin near Arcade GA for the study period. Samplingdates are shown in triangles. (Data from the U.S.Geological Survey, 2004.did not vary as a function of time within a given watershed(Figure 3). The strontium isotope ratios were the mosttemporally invariant of all parameters in stream base flowincluding specific conductance (coefficient of variation = 5-10%), environmental tritium (coefficient of variation = 4-25%) and * O (coefficient of variation = 5-7%). These18results suggest that those processes responsible forgeochemical and isotopic variation within base flow for agiven basin (such as variable source area contributions) donot affect strontium isotope ratios. Sr/ Sr ratios become87 86integrated and homogenous within a given watershed overa wide range of base flow rates that occur during the courseof a year. Hence, these ratios provide the best geochemicalor isotopic “signature” for base flow within a givenPiedmont basin.     The intra-basin Sr/ Sr variation within stream base flow87 86occurred without much change with respect to strontium ionconcentrations (Figure 4). Average Sr concentrations variedonly between 22-24 ppb in the four basins. Average Sr/ Sr87 86were lowest within the two small Indian Creek watersheds(0.7127 - 0.7133) and were highest within the terminalMiddle Oconee River basin (0.7172).   The lithology of the study area is dominated byamphibolite, mica shcist, and biotite gneiss;however, Sr/ Sr ratios in the underlying bedrock are not yet87 86known. A hypothetical explanation for this trend (increasingratios with increasing watershed area)is that flow paths inthe larger Middle Oconee River basin are likely longer thanwithin the other sub-basins. The longer flow paths, in turn,may integrate a greater contribution from Rb-bearingminerals such K-feldspar, biotite, and muscovite which willresult in increased Sr/ Sr ratios in base flow.87 86Figure 3. Temporal variation for the four stream basinswithin the Middle Oconee basin study area. Note: errorbars are approximately the same size as the symbols.CONCLUSIONS    Three notable features related to the evolution ofstrontium isotope ratios  were inferred with respect to baseflow in the Middle Oconee River basin. First, strontiumisotope ratios were temporally invariant at a given samplinglocation and were not related to the rate of base-flowdischarge or any other seasonal or hydrological trend.Second, each of the four sub-basins analyzed within this860km  study area was characterized by its own unique2Sr/ Sr “signature”. Third, strontium isotope ratios87 86apparently increased with basin area, independent of  Sr ionconcentrations. Such variation might be the result of subtlemineralogical controls (i.e. the integration of more Sr-87from the weathering of K-feldspar, biotite, and muscovite)in the larger basins.     The time-invariant and unique ratios associated with agiven basin should enable future studies to utilize Sr-isotoperatios as “natural tracers” for stream base flow. One possibleapplication might involve the use of Sr isotopes in massbalance studies to analyze flow contributions fromindividual streams within a large integrated watershed.REFERENCES CITED Bullen, T.D. and Kendall, C., 1998. Tracing of                weathering reactions and water flowpaths: a multi-isotope approach. In: (Eds.) Kendall, C. andMcDonnell, J.J., Isotope Tracers in CatchmentHydrology, Ch18, Elsevier, Amsterdam,611-616.Douglas, T.A., Chamberlain, C.P., and Blum, J.D.,        2002. Land use and geologic controls on the major  elemental and isotopic (* N and Sr/ Sr)       15 87 86geochemistry of the Connecticut River watershed,   USA. Chemical  Geology 189, 19-34.       Figure 4. A possible evolutionary path for Sr isotopesin Piedmont watersheds. Note that a principal featureof this suggested path is that Sr isotope ratios increasewith basin area and this is not accompanied by relatedincreases with respect to Sr ion concentrations.Miller, M.L, Blum, J.D., and Friedland, A.J., 1993.             Determination of soil exchangeable-cation loss and      weathering rates using Sr isotopes. Nature  362,           438-444.Palmer, M.R. and Edmond, J.M. 1992. Controls over      the strontium isotopic composition of river water.    Geochemica et Cosmochimica 56, 2099-2111.Rose, S. 2004. A combined hydrological, geochemical    and isotopic approach to understanding the effects   of basin scale on base flow systematics in the           Georgia Piedmont Province. Technical CompletionReport for the Georgia Water Resources Institute FY2003, 90p.Rose, S. and Fullagar, P.D. (2004, in review). Strontium isotope systematics of base flow inPiedmont Province watersheds, Georgia (USA).submitted to Applied Geochemistry.U.S. Geological Survey, 2004. USGS 022174745          Middle Oconee River near Arcade, GA.                  http:waterdata.usgs.gov/          ACKNOWLEDGMENTSWe gratefully acknowledge funding for this study fromthe Georgia Water Research Institute (FY 2003) and GeorgiaState University. We would like to express our greatappreciation to Jerry Waddell and Eugene Carlson of theJackson County Water and Sewage Authority for facilitatingour ground water monitoring in the study area.Table 1. Summary of Key Isotopic and Geochemical Results (after Rose, 2004)                        Middle                    Pond                        Indian               Indian                 Ground                   Rainfall                        Oconee                   Fork                         Creek                Creek                  Water                         Basin                      Creek                    (2  order)          (1  order)nd st  Basin Area (km2)          860                            54                                3.9                      12.7                  -----                           ----- No. of Isotopic                  11                             8                                  8                          9                        6                              5 Analyses  Average [Sr] ppb                23.6                          21.8                             24.2                    24.2                      6.1                          5  Range               0.7170-                      0.7146-                      0.7126-                0.7133-                0.7113-              0.7109-Sr/ Sr             0.7176                       0.7148                       0.7128                  0.7134                 0.7125               0.7122  87 86  Average            0.7172                       0.7147                       0.7127                  0.7133                 0.7120               0.7117Sr/ Sr  87 86AverageSpecific Conductance          81.9                            74.2                          78.4                       92.6                    21.0                   -----[:S/cm]AverageAlkalinity                27.7                            26.5                         30.6                        35.9                       4.0                   ----- [mg/L]AveragepH                            6.70                           6.82                          6.74                         6.56                      5.17                 -----

Strontium isotope ratios in stream base flow within the Georgia Piedmont province

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Strontium isotope ratios in stream base flow within the Georgia Piedmont province

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