Radiocarbon and stable isotopes in water and dissolved constituents, Milk River aquifer, Alberta, Canada

Abstract

Origin and age of groundwater in the Milk River aquifer (Alberta, Canada) was the focal point of the present study. To answer related questions, wells in this aquifer were sampled and 2H and 18O in water, 13C and 2H in methane, 34S and 18O in SO 4, 13C and 14C in dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC), and tritium contents determined. Stable isotope analyses on ground-water permitted the recognition of at least three different zones within the aquifer: starting at the recharge zone modern-type groundwaters are encountered for the first few kilometers. These are followed by groundwater with slightly higher 18O and 2H contents, i.e. waters which do not show a glacial signature and were probably recharged under warmer climatic conditions. Groundwater in the third zone is characterized by higher salinities and a pronounced oxygen isotope shift. They show the gradual admixture of far more saline formation waters from the Alberta Basin. Sulphate-S and sulphate-O isotope values indicate a till source for the SO 4. However, SO 4 in wells close to the area where SO 4 virtually disappears show isotopic values which suggest that SO 4-reduction is occurring, although other studies could not find bacterial populations of SO 4 reducers. Methane is present in most wells and, based on C and H isotope data, has a biogenic origin. Methane production occurs close to some wells and/or it diffuses from the confining shales into the aquifer. Radiocarbon is measurable only in the first 20 km from the recharge zone, however, transformation of measured concentrations to “water ages” is very difficult because of the complexity of the geochemical system. Comparison with 14C data on DOC was, therefore, very interesting and this new technique may complement the DIC radiocarbon dating. Two wells near the recharge zone yielded DIC, DOC-High Molecular Weight (HMW) and DOC-Low Molecular Weight (LMW) results. The DIC dates were much older than DOC dates but the differenve in years between the wells for all three types of C were within 1000 a of each other. The DOC dates led to the conclusion that the initial DIC radiocarbon content was as low as 30% modern when the water entered the aquifer. The age of the water 20 km from the recharge zone was assessed at ∼20,000 a, resulting in a velocity of ∼1 m/a. This result is much higher than estimates by other techniques.