Dissolved organic carbon and methane in a regional confined aquifer, southern Ontario, Canada: Carbon isotope evidence for associated subsurface sources

Abstract

Information regarding the origin, composition and transport of natural dissolved organic carbon (DOC) in groundwater is necessary to understand the transport of metals and organic pollutants, as well as for the use of 14C in DOC as an isotopic groundwater dating method. Previous research in several groundwater systems has suggested soil organic C is the predominant source of high molecular weight DOC to the subsurface. Through the use of stable isotopes, 14C and geochemical analyses, this study shows that significant concentrations of DOC and CH 4 in a regional confined aquifer can be generated in situ from subsurface sedimentary organic sources. The DOC and CH 4 produced is a combined result of degradation of buried peats and bacterial action, resulting in high DOC concentrations and strongly methanogenic conditions in the aquifer. The DOC and CH 4 comprise, on average, nearly 50% of the total dissolved C pool in the central part of the aquifer. Methanogenic conditions complicate isotopic groundwater dating by the conventional dissolved inorganic carbon (DIC) method. Estimates of isotopic groundwater residence time using DOC 14C data are proposed by the application of 14C isotope and mass balance corrections.