CONTROLS ON THE DISTRIBUTION AND TRANSPORT OF DISSOLVED ORGANIC CARBON IN A THICK, CLAY-RICH AQUITARD

Dissolved organic carbon (DOC) plays a role in the fate and transport of trace elements in the subsurface. The importance of DOC in the long-term transport of metals in aquitards is, however, poorly understood. DOC characteristics, its fate, and the controls on its transport were studied in an 80-m thick, well-characterized clay-rich till aquitard in Saskatchewan, Canada. Porewater samples were collected on nine occasions from 14 piezometers installed in the upper 50 m of the till in 1995. The DOC concentrations exhibited a well-defined decrease with depth through the till from 150 mg/L at 2-m depth to 7 mg/L at 30-m depth. The depth-concentration profile was invariant with time. Fulvic acids dominated the DOC from all depths. Flow Field-Flow Fractionation (FFF) and tangential filtration (TFF) were used to show that the DOC depth profile was uniform with respect its molecular weight range. The factors controlling the transport of the DOC in the aquitard include (1) the constrictivity of the DOC in the pore spaces, (2) DOC retardation on the till matrix, (3) DOC aqueous diffusion coefficient, (4) effective diffusion coefficient of the DOC within the aquitard, and (5) effective porosity of the DOC within the aquitard. These parameters were quantified using mercury porosimetry, batch Kd testing, centrifuge modeling, FFF, TFF, and dual reservoir diffusion cell testing. The calculated parameters were then used to determine the sources and timing for the development of the DOC profile through the clay aquitard.