The biogeoche
mistry at the interface between sedi
ments in a seasonally ponded wetland (slough) and an alluvial aquifer conta
minated with landfill leachate was investigated to evaluate factors that can effect natural attenuation of landfill leachate conta
minants in areas of groundwater/surface-water interaction. The biogeoche
mistry at the wetland-alluvial aquifer interface differed greatly between dry and wet conditions. During dry conditions (low water table), vertically upward discharge was focused at the center of the slough fro
m the fringe of a landfill-derived a
mmoniu
m plu
me in the underlying aquifer, resulting in transport of relatively low concentrations of a
mmoniu
m to the slough sedi
ments with dilution and dispersion as the pri
mary attenuation
mechanis
m. In contrast, during wet conditions (high water table), leachate-conta
minated groundwater discharged upward near the upgradient slough bank, where a
mmoniu
m concentrations in the aquifer where high. Relatively high concentrations of a
mmoniu
m and other leachate constituents also were transported laterally through the slough porewater to the downgradient bank in wet conditions. Concentrations of the leachate-associated constituents chloride, a
mmoniu
m, non-volatile dissolved organic carbon, alkalinity, and ferrous iron
more than doubled in the slough porewater on the upgradient bank during wet conditions. Chloride, non-volatile dissolved organic carbon (DOC), and bicarbonate acted conservatively during lateral transport in the aquifer and slough porewater, whereas a
mmoniu
m and potassiu
m were strongly attenuated. Nitrogen isotope variations in a
mmoniu
m and the distribution of a
mmoniu
m co
mpared to other cations indicated that sorption was the pri
mary attenuation
mechanis
m for a
mmoniu
m during lateral transport in the aquifer and the slough porewater. A
mmoniu
m attenuation was less efficient, however, in the slough porewater than in the aquifer and possibly occurred by a different sorption
mechanis
m. A stoichio
metrically balanced increase in
magnesiu
m concentration with decreasing a
mmoniu
m and potassiu
m concentrations indicated that cation exchange was the sorption
mechanis
m in the slough porewater. Only a partial
mass balance could be deter
mined for cations exchanged for a
mmoniu
m and potassiu
m in the aquifer, indicating that so
me irreversible sorption
may be occurring.
Although wetlands commonly are expected to decrease fluxes of contaminants in riparian environments, enhanced attenuation of the leachate contaminants in the slough sediment porewater compared to the aquifer was not observed in this study. The lack of enhanced attenuation can be attributed to the fact that the anoxic plume, comprised largely of recalcitrant DOC and reduced inorganic constituents, interacted with anoxic slough sediments and porewaters, rather than encountering a change in redox conditions that could cause transformation reactions. Nevertheless, the attenuation processes in the narrow zone of groundwater/surface-water interaction were effective in reducing ammonium concentrations by a factor of about 3 during lateral transport across the slough and by a factor of 2 to 10 before release to the surface water. Slough porewater geochemistry also indicated that the slough could be a source of sulfate in dry conditions, potentially providing a terminal electron acceptor for natural attenuation of organic compounds in the leachate plume.