Relationship between DOC concentration and vadose zone thickness and depth below water table in groundwater of Cape Cod, USA

Changes in concentration of dissolved organic carbon (DOC) reflect biogeochemical processes that determine chemical composition of groundwater and other natural waters. We found that the deeper the vadose zone, the lower the concentration of DOC in groundwater near the water table, indicating that considerable attenuation of surface-derived DOC occurred in the vadose zone. Under vadose zones <1.25 m, DOC concentrations at the surface of the water table ranged to > 20 mg C l(-1), while for vadose zones >5.0 m, DOC never exceeded 2.0 mg C l(-1). DOC concentrations also decreased exponentially with increasing depth below the water table, most notably in the upper two meters, implying continued attenuation in the upper layer of the saturated zone. Ninety-nine percent of the DOC was attenuated by the time the water reached a depth of 19 m below the water table. A strong inverse relationship between DOC and nitrate concentrations suggests that nitrate is depleted where DOC supplies are high, providing evidence that some portion of the DOC losses in groundwater are due to microbial transformations, including denitrification. DOC concentrations in shallow groundwater show considerable spatial variability, but concentration of DOC at any one site is surprisingly stable over time. The largest source of variation in DOC concentration in groundwater therefore is spatial rather than temporal, suggesting that local heterogeneities play an important role in DOC delivery to shallow groundwater. Our results highlight both the importance of shallow vadose areas in DOC delivery to groundwater and the need to distinguish where samples are collected in relation to flow paths before conclusions are made about mean groundwater DOC concentrations. The substantial losses of DOC in the vadose zone and in shallow depths within the aquifer suggest quite active biogeochemical processes in these boundary environments.