TEMPORAL VARIABILITY OF ORGANIC C AND NITRATE IN A SHALLOW AQUIFER

The loading of organic substrates into shallow aquifers may follow sea sonal cycles, which will impact the transport and fate of agrichemical s. The objective of this research was to measure temporal changes in t he groundwater dissolved organic C (DOC) and nitrate concentrations. G roundwater monitoring wells were installed and sediment samples from t he aquifer were collected in 1991. Sediment samples were used to evalu ate denitrification potentials, while water samples were collected at periodic intervals in 1992 and 1993 from the surface of the aquifer. W ater samples were analyzed for nitrate-N and DOC-C. Denitrification wa s observed in sediment amended with nitrate and incubated under anaero bic conditions at 10 degrees C. Addition of algae lazed biomass increa sed denitrification, establishing that denitrification was substrate l imited. In the aquifer, DOC concentrations followed seasonal patterns. DOC concentrations were highest following spring recharge and then de creased. Peak timing indicates that freezing and thawing were responsi ble for seasonal DOC patterns. These findings show that seasonally dri ven physical processes, such as freezing and thawing, influence organi c substrate transport from surface to subsurface environments, and tha t this process should be taken into account when assessing agrichemica l detoxification rates in shallow aquifers.