TRANSPORT PROPERTIES OF DISSOLVED ORGANIC CARBON IN A CLAY-RICH AQUITARD

Naturally occurring dissolved organic carbon (DOC; <0.45 µm filtrate) is a common constituent of the total aqueous carbon pool in groundwaters. It can control microbial redox processes in groundwater systems, buffer the pH of waters, participate in mineral dissolution/precipitation reactions, and complex with metals and radionuclides. The properties and controls on the diffusive transport of DOC in a thick clay-rich till aquitard were investigated. DOC was measured in 14 piezometers ranging in depth from 1.2 to 43 m below ground (BG). The DOC data showed a decrease in concentration with depth from 168 mg/L in the surficial, fractured and oxidized zone (1.2 m BG) through the thick underlying unoxidized aquitard, reaching minimum values of between 12-16 mg/L below 15 m. Flow field-field flow fractionation analyses showed that the all of the DOC was uniform with respect to hydrodynamic size (2.5 nm), molecular weight (approximately 900 Da), and aqueous diffusion coefficient (1.9x10-10 m2/s). Results of batch experiments showed that DOC exhibited negligible sorption (i.e., kd=1.1x10-3 mL/g) to the aquitard matrix. Double-reservoir diffusion tests showed that DOC can diffuse through this clay-rich media. Results of best-fit numerical modeling of the diffusion cell data yielded an effective diffusion coefficient of 9x10-11 m2/s for the DOC. Pore-aperture measurements on core samples and the results of the diffusion cell experiments suggested that straining of DOC by the aquitard matrix does not occur. Overall our results show that DOC can diffuse through clay-rich aquitards.