Controls on the transport and carbon isotopic composition of dissolved organic carbon in a shallow groundwater system, Central Ontario, Canada

Abstract

The sources, fluxes and sinks of dissolved organic carbon (DOC) in a groundwater recharge environment and along a well defined flowpath were investigated in a shallow unconfined aquifer system in Central Ontario, Canada. The initial recharge flux of DOC in the upper vadose zone is estimated to be 49 kg ha⁻¹ yr⁻¹, however, biochemical recycling and retention processes deeper in the vadose zone result in a net DOC flux of about 9.8 kg ha⁻¹ yr⁻¹ to the groundwater. Within the aquifer sequential redox processes observed along the flowpath (OZ consumption, SO₄²⁻ reduction, fermentation) further consume or produce DOC. Characterization of the DOC suggest a predominance of high molecular weight (HMW) aquatic fulvic acids and intermediate molecular weight compounds. Radiocarbon analyses of HMW and low molecular weight DOC fractions in the groundwater indicate: (1) DOC fractions recharging the groundwater system are derived from organic carbon sources in the upper soil zone; (2) ¹⁴C reductions in DOC observed along the groundwater flowpath are a combined result of redox processes (DOC oxidation, production) and the gradual input of bomb ¹⁴C; and (3) DOC oxidation in the groundwater does not significantly impact the inorganic geochemistry or carbon isotopic composition of the dissolved inorganic carbon pool.