The properties and controls on the diffusive transport of dissolved organic carbon (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 and nonfractured aquitard, reaching minimum values of between 12 and 16 mg/L below 15 m. Flow field-field flow fractionation analyses showed that the DOC was uniform with respect to hydrodynamic size (2.5 nm), molecular weight (approximately 900 Da), and aqueous diffusion coefficient (1.9 ¿ 10-10 m2/s). Specific UV aborbance (UV/TOC) varied with depth but was significantly lower than surface water DOC. Results of batch experiments showed that DOC exhibited negligible sorption (i.e., kd = 1.1 ¿ 10-3 mL/g) to the aquitard matrix. Double-reservoir diffusion tests showed that DOC diffuses through this clay-rich media. Results of best fit numerical modeling of the diffusion cell data yielded an effective diffusion coefficient of 9 ¿ 10-11 m2/s for the DOC and suggested that the effective porosity for the DOC may be approximated by the total porosity. Pore aperture measurements on core samples and the results of the diffusion cell experiments revealed that straining of DOC by the aquitard matrix does not occur. Overall, our results suggested that DOC can diffuse through clay-rich aquitards in a similar manner to conservative inorganic solutes.