Hydrogeochemistry of a Clayey Till: 2. Sources of CO2

The spatial distributions of inorganic and organic carbon in gas, aqueous, and solid phases were studied in an 18‐m‐thick surficial deposit of fractured clayey calcareous till. Large PCO2 increases were observed with increasing depth at the bottoms of vadose zones from which solid organic carbon had been depleted. Hydrochemical reaction simulations indentified a reaction path consistent with PCO2, O2, major element chemistry, 13C, and 14C isotope observations; this path is “open” to a biogenic CO2 reservoir with PCO2 higher than, and 14C composition considerably different from, that of the soil zone. Such CO2 reservoirs, occurring in deep‐vadose/shallow saturated zones and thus isolated by overlying sediments from temperature fluctuations which limit soil‐zone respiration, may be common in the Western Glaciated Plains as evidenced by prevalence of hard, high‐PCO2 waters in the region's intertill aquifers. Carbon 14 data suggest that in the till, both dissolved organic C and dissolved inorganic C have two sources: soil organic matter and kerogen in the till matrix. Regardless of source(s), initial 14C activities of both aqueous C pools at the water table are “diluted” substantially relative to the actual residence time of the water; unappreciated, this could cause large relative errors in the calculation of radiocarbon‐based groundwater age dates.