THE ORIGIN AND LONG-TERM TRANSPORT OF DOC IN A CLAY-RICH AQUITARD

^{The origin and transport of dissolved organic carbon (DOC) was investigated in a thick clay-rich till aquitard. DOC, 14C and d13C measurements of fulvic acids (FA) decreased with depth from 168 mg C/L, 87 pmC, and -23.7 ‰ in the oxidized and fractured zone (1.2 m depth) reaching minimum values of between 12-16 mg C/L, 13-15 pmC, and -26 ‰ between 15 and 43 m depth in the aquitard. Numerical simulations of the downward migration of 14C-FA from the soil zone closely approximated the observed values and showed that simple diffusive mixing and radioactive decay fully described the transport of 14C-FA in the till, and that the FA was stable over long periods of time. The data suggested that the DOC in the aquitard was derived from two end members: soil organic material formed since the Holocene (about 10ka BP) and dissolved organic matter originally deposited with the till (calculated to be about 15ka BP). Because transport of the DOC is controlled by diffusive mixing, the application of decay-age calculations to determine the groundwater ages in a diffusion-dominated transport system was shown to be erroneous. The strong linear relation between C-FA and C-DIC data suggested a similar mode of transport and origin, but showed a consistently younger and more representative age for C-FA. Overall, the results of this study showed that DOC and FA transport in the clay-rich aquitard was conservative. As a result, DOC should be considered in studies of DOC facilitated transport of metals and radionuclides in aquitards.}