Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/1995
Publication Date: N/A
Interpretive Summary: Leaching of nitrate from agricultural fields into groundwater continues to be a problem in many areas of the country. This research investigated the relationship between groundwater nitrate concentration and dissolved organic carbon (DOC) in a shallow aquifer in southeast South Dakota. Spring rains and snowmelt contribute to a rise in groundwater levels and transport a pulse of dissolved organic carbon into the aquifer. DOC levels declined in the summer and fall. Nitrate concentrations were erratic, but did not show a seasonal pattern, remaining between 15 and 30 ppm over the 2-year study. The aquifer microorganisms were capable of converting nitrate to nitrogen gas in a process called denitrification. Experiments with aquifer sediments showed that the denitrification process was limited by the concentration of carbon substrate available to the microorganisms. In the aquifer, declines in the levels of DOC were followed by temporary declines in nitrate, presumably due to denitrification by the aquifer microorganisms using DOC as an energy source. Knowledge of DOC sources and levels may be required in some systems to accurately predict nitrate contamination in groundwater. This knowledge will impact scientific and regulatory programs concerned with maintaining water quality and supply.
Technical Abstract: Temporal changes in the dissolved organic C (DOC) concentration in shallow aquifers may influence aquifer agrichemical detoxification mechanisms. The objective of this research was to measure temporal changes in the groundwater DOC and nitrate concentrations. Groundwater monitoring wells were installed and sediment samples from the aquifer were collected in 1991. Sediment samples were used to evaluate denitrification potentials, while water samples were collected at periodic intervals in 1992 and 1993 from the surface of the aquifer. Water samples were analyzed for nitrate-N and DOC-C. Denitrification was observed in sediment amended with nitrate and incubated under anaerobic conditions at 10 deg C. Addition of algae biomass increased denitrification, establishing that denitrification was substrate limited. In the aquifer, DOC peaks were observed following spring recharges and then decreased. Corresponding peaks in nitrate-N were not observed. Decreases in DOC concentrations may have been due to dilution or microbial activity. The sources of the DOC could only have been from the surface, and therefore, substantial amounts of DOC may be transported from surface to subsurface environments during spring recharge.