RADIOCARBON IN DISSOLVED ORGANIC-CARBON, A POSSIBLE GROUNDWATER DATING METHOD - CASE-STUDIES FROM WESTERN CANADA

This paper explores the feasibility of using C-14 in dissolved organic carbon (DOC) as an alternative isotopic groundwater dating method. Two hydrogeologically contrasting groundwater systems were tested; the Cretaceous age Milk River aquifer, and low-permeability, organic rich, Wisconsinan age Prairie tills in southern Alberta, Canada. Comparisons of radiocarbon data were made between DOC fractions, dissolved inorganic carbon (DIC), and several DIC geochemical age correction models along well defined flow paths. The data presented demonstrate that C-14 dating of DOC fractions can indeed provide an alternative method for determining isotopic groundwater ages, under suitable conditions. However, detailed information may be required regarding (1) the geologic nature of the aquifer and its flow system, (2) the isotopically conservative behavior of DOC, (3) the initial C-14 activity of DOC in recharge, and (4) the effect of bacterial redox processes on the C-14 activity of DOC. In the Milk River aquifer, DOC C-14 ages were successfully used to estimate groundwater residence times, as well as to further refine input parameter assumptions for the DIC method. In the Prairie tills, DOC C-14 ages were used to establish a maximum age for the pore waters in an environment where the DIC method is especially problematic.