, Volume 55, Issue 3, pp 247-268

Relationship between DOC concentration and vadose zone thickness and depth below water table in groundwater of Cape Cod, U.S.A.

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Abstract

Changes in concentration ofdissolved organic carbon (DOC) reflectbiogeochemical processes that determinechemical composition of groundwater and othernatural waters. We found that the deeper thevadose zone, the lower the concentration of DOCin groundwater near the water table, indicatingthat considerable attenuation ofsurface-derived DOC occurred in the vadosezone. Under vadose zones <1.25 m, DOCconcentrations at the surface of the watertable ranged to >20 mg C l−1, while forvadose zones >5.0 m, DOC never exceeded2.0 mg C l−1. DOC concentrations alsodecreased exponentially with increasing depthbelow the water table, most notably in theupper two meters, implying continuedattenuation in the upper layer of the saturatedzone. Ninety-nine percent of the DOC wasattenuated by the time the water reached adepth of 19 m below the water table. A stronginverse relationship between DOC and nitrateconcentrations suggests that nitrate isdepleted where DOC supplies are high, providingevidence that some portion of the DOC losses ingroundwater are due to microbialtransformations, including denitrification. DOC concentrations in shallow groundwater showconsiderable spatial variability, butconcentration of DOC at any one site issurprisingly stable over time. The largestsource of variation in DOC concentration ingroundwater therefore is spatial rather thantemporal, suggesting that local heterogeneitiesplay an important role in DOC delivery toshallow groundwater. Our results highlightboth the importance of shallow vadose areas inDOC delivery to groundwater and the need todistinguish where samples are collected inrelation to flow paths before conclusions aremade about mean groundwater DOC concentrations. The substantial losses of DOC in the vadosezone and in shallow depths within the aquifersuggest quite active biogeochemical processesin these boundary environments.