© 2001 Kluwer Academic Publishers. Printed in the Netherlands. Relationship between DOC concentration and vadose zone thickness and depth below water table in groundwater of Cape Cod, U.S.A.

Abstract. Changes in concentration of dissolved organic carbon (DOC) reflect biogeochem-ical 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 i