Carbon-14 measurements and characterization of dissolved organic carbon in ground water

Abstract

Carbon-14 was measured in the dissolved organic carbon (DOC) in ground water and compared with ¹⁴C analyses of dissolved inorganic carbon (DIC). Two field sites were used for this study; the Stripa mine in central Sweden, and the Milk River Aquifer in southern Alberta, Canada. The Stripa mine consists of a Precambrian granite dominated by fracture flow, while the Milk River Aquifer is a Cretaceous sandstone aquifer characterized by porous flow. At both field sites, ¹⁴C analyses of the DOC provide additional information on the ground-water age. At the Stripa site the DIC from the recharge area probably precipitates at around the 300 m level of the mine, never reaching the deeper ground waters. In this case, ¹⁴C analyses of the DOC provides a better estimate of the ground-water age. The dilution of the DIC by carbonates and microbial processes in the Milk River Aquifer is so great that geochemical corrections of ¹⁴C data are difficult. This is another example where ¹⁴C analyses of the DOC provide more information on ground-water age. Carbon-14 was measured on both the hydrophobic and hydrophilic organic fractions of the DOC. At the Stripa site, the hydrophobic organic compounds in the V2 borehole ranged from 7,500 to 15,500 years before present, suggesting a young component of ground water. Other hydraulic and isotopic evidence supports relatively recent ground water mixing with older brines in this borehole. The δ¹³C values of the DIC in the V2 borehole are light and similar to the stable carbon isotope values for the DOC, supporting a biogenic origin of the DIC. The organic compounds in the hydrophobic and hydrophilic fractions were also characterized. The DOC may originate from kerogen in the aquifer matrix, from soil organic matter in the recharge zone, or from a combination of these two sources. Carbon-14 analyses, along with characterization of the organics, were used to determine this origin. Carbon-14 analyses of the hydrophobic fraction in the Milk River Aquifer suggest a soil origin, while ¹⁴C analyses of the hydrophilic fraction suggest an origin within the Cretaceous sediments (kerogen) or from the shale in contact with the aquifer.