VOL. 14, NO. 6 WATER RESOURCES RESEARCH DECEMBER 1978 The CarbonIsotope Geochemistry of a SmallGroundwater System inNortheastern Ontari ø P. FRITZ, E. J. REARDON, AND J. BARKER Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada R. M. BROWN Atomic Energy of Canada Ltd., ChalkRiver, Ontario, Canada J. A. CHERRY,R. W. D. KILLEY, AND D. MCNAUGHTON Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada The carbon isotopic composition (•8Cand •4C)of the inorganic carbon dissolved in the waters of a small, largely unconfined aquifer in unconsolidated sediments on theCanadian Shield hasbeen investi- gated. Three principal carbon sources arerecognized: soil COa, rock carbonate, andbiogenic CO:.The average •8C value of thesoil COa isclose to -21.0 4- 1.5%, andpresent-day '4Cactivities of thesoil COa vary between 130 and 162% modern •4C. Veryminor amounts (< 1.0%) of carbonate minerals (6'aC = -0.6%o) are present within the aquifer and react with this soil CO:to produce a dissolved inorganic carbon (DIC) with'aC activities which areas much as50% below the initial activities of thesoil COa. Thethird carbon source, a biogenic CO:, could bedetected onlyindirectly, andits presence is primarily deduced from the occurrence of methane in the deeper parts of these aquifers. The largeisotope fractionation which occurs during bacterial coproduction of CO: withthis methane results, however, in a •aC-rich CO•. and thus a DIC with high •'aC values. Values of •'•C as high as +11%ohave been measured in groundwaters onthe Canadian Shield. Since theorigin of thedestroyed organic matter isnot yetknown, no assessment of the importance of thisCO: for the •aC contents of the DIC is possible. INTRODUCTION The development andutilization of northern areas in Can- ada necessitate an understanding of activehydrologic systems within these regions. This paper presents the results of a car- bon isotope study in a small Precambrian watershed on the Canadian Shield. Attempts are made to explain especially the •4C contents observedand thus to contribute to the develop- ment of carbon 14 analyses as a water-age-dating tool for groundwaters in crystalline rocks. The isotope geochemistry of carbon in groundwater systems is controlled primarily by two parameters: (1) the abundance andisotopic composition of soil COo. andits interaction with infiltrating water in the unsaturated zone and (2) processes whichalter the carbon content of the groundwater within the aquifer. Geochemically, the two can be treated separately, sincethe influenceof the soil CO: dominatesthe chemical and isotopic evolution of open systems where exchange with a gas phase occurs, whereas dissolution and precipitation of carbon- ates, oxidation of organic matter,and otherprocesses become controlling in closed systems, aswefindthem in confined and some unconfined aquifers. Closed system processes usually have thegreatest influence in altering thex4C concentrations in groundwaters. They have to be assessed if attempts aremade to obtainabsolute groundwater ages from the measured activities of the dissolved inorganic carbon. Models proposed for the 'correction' of apparent x•Cages have been based on considerations of either the chemical or the isotopic evolution of groundwaters withina given aquifer. They all attempt to quantify the amounts of limestone that dissolved during groundwater flowunder closed system condi- tions, and no attempts haveyet beenmade to includenon- carbonate sources. The useof xaC data is of major importance, Copyright (D 1978 bythe American Geophysical Union. Paper number 8W0634. 0043-1397/78/068W-0634501.00 since limestone carbonate and soil CO: have very different •aC/x2C ratios. Ingerson and Pearson [1964] discussed two pos- sible solutions, one based on a comparison of initial total inorganic carbon versus final totalcarbon and theother based on an isotope mass balance. Mook [1976] expanded this ap- proach by including both dissolution and exchange terms, arguing that previous methods didnot consider exchange with soil CO: after rock dissolution. Smith et al. [1975] studied groundwater flows in theLondon Basin and showed that continuousdissolutionand precipi- tationof carbonate withinan aquifer canbevery important in controlling carbon isotope evolution. Wigley [1976] presented a more quantitative discussion of the data given by Smith etal. [1975, 1976] with special emphasis on the incongruent dis- solution of dolomites. All of these correction procedures utilize aqueous carbonate species or carbon 13contents to assess thegeochemical and isotopic evolution ofgroundwaters. However, the geochemical history of a groundwater is alsoreflected in other ions. For example, Ca 2+, Mg :+, SO• •+, Na+,and Cl- contents can give a rather detailed pictureof the dissolution and precipitation history of carbonate minerals andgypsum and information about ion exchange processes. Garnier [1976], Fontes and Gar- nier [1978],and Reardon and Fritz [1978] present correction models which are based on utilization of these ions as in- dicators for such processes. A Fortran IV computer program attached to thewateranalyses treatment program (WATEQF) by Plumruer et al. [1976] has been presented by Reardon and Fritz [1978]. This program considers corrections based on initial versus final carbonate, simplecarbonate, dissolution carbonate plus gypsum dissolution with andwithout ion ex- change processes, and dissolution-precipitation processes as they mayoccur during the incongruent dissolution of dolo- mites. Chemical and isotope analyses are compared to model calculations for both chemistry and carbon 13. 1059