Estimation of dissolved organic carbon contribution from hillslope soils to a headwater stream

The downward flux of dissolved organic carbon (DOC) in the vadose zone was examined using artificial rain experiments on a soil lysimeter at the White Clay Creek watershed in southeastern Pennsylvania. Refractory DOC (RDOC) and labile biodegradable DOC (BDOC) were transported from the organic rich soil layer and eluted from the base of a soil lysimeter during the experimental water applications. A physically based, distributed model that considers water flow and the transport of heat, RDOC and labile BDOC was developed and used to explore the hypothesis that the delivery of DOC to the water table occurs primarily in the riparian zone. Macropores were considered by adopting a dual permeability modeling approach. The calibrated model successfully replicated the temporal dynamics of a bromide tracer, and of RDOC and labile BDOC eluted from the soil lysimeter during the experiments. An estimated annual DOC delivery to groundwater in the watershed was obtained by scaling up the vertical one-dimensional model to the watershed. The results suggest that the subsurface contribution of DOC accounts for about 72% of the total annual export from the watershed, and the riparian zone contributes more than 90% of total DOC in groundwater.