Hydrogeochemical processes controlling the transport of dissolved organic carbon through a forested hillslope

The subsurface transport of dissolved organic carbon (DOC) through a proposed waste burial site during rain events was investigated in order to assess the role of colloid-mediated contaminant mobility. A sub-watershed (0.45 ha) located on a forested hillslope in eastern Tennessee, U.S.A., was instrumented with an isolated soil pedon for one-dimensional transport studies, and a subsurface weir monitoring system for three-dimensional transport studies. The source of DOC in the soils resulted from dissolution of organic matter in the surface horizon during, and between, rain events, as well as from a highly reactive B horizon which stored significant quantities of DOC in small pores. During large storms, the concentration of DOC was similar on ascending and descending limbs of the subface hydrograph, with a maximum concentration occuring at peak flow. During small storms however, chemical interactions with soil solution SO 4 2- caused DOC concentrations to be greater on the ascending limb of the hydrograph, with maximum DOC concenrations occuring before peak flow. Because subsurface lateral flow through preferential paths predominated in the Bt2 and the Bt3 horizons of the soil during storm events, the total cumullative flux of DOC downslope was generally much greater through the lower soil horizons. A significant component of mobile DOC consisted of hydrophobic organic solutes, even though this material was selectively adsorbed with soil depth relative to hydrophilic organic solutes. The implications of these findings on subsurface contaminant transport are discussed.