Characterization and isotopic composition of organic and inorganic carbon in the Milk River Aquifer

Carbon isotope analyses were performed by accelerator mass spectrometry on fractions of dissolved organic carbon (DOC) in groundwater from the Milk River aquifer, Alberta, Canada. The DOC was separated into high molecular weight (HMW) and low molecular weight (LMW) fractions for analysis. Characterization information and ¹⁴C activities suggest that the HMW fractions (primarily humic substances) originate in the interstitial waters of the soil zone in the area of recharge and are transported through the porous media of the aquifer. Transformations of HMW fractions down the flow path, including an increase in carbon, decrease in oxygen, and a pronounced increase in the aliphatic nature of this fraction, are consistent with decreasing ¹⁴C activity. The LMW fractions show a dominant kerogen origin. This fraction of the DOC has low ¹⁴C activities (generally <1 percent modern carbon), and gas chromatograph/mass spectrometry analyses identified a homologous series of substituted alkanes, further evidence that these compounds may have originated from organic‐rich shales in the aquifer and/or confining beds. Geochemical and stable isotope measurements of the dissolved inorganic carbon (DIC) indicate that microbial activity is occurring in the Milk River aquifer. The alteration of these DIC δ¹³C ratios by microbial activity render corrections for groundwater age in this aquifer difficult, if not impossible. Although ¹⁴C analyses of DOC fractions will not replace other groundwater dating techniques, these analyses do provide additional information on groundwater age and perhaps a more complete interpretation of groundwater age than DIC alone.