DENITRIFICATION CAN CAUSE VARIABLE NO3- CONCENTRATIONS IN SHALLOW GROUNDWATER

The mechanisms regulating NO3- concentration and dissimilation in a sh allow, spring-fed well were investigated by creating variable flow con ditions in a series of sediment-filled laboratory microcosms. Nitrate concentrations in the laboratory microcosms reproduced field observati ons; effluent concentrations were as high as 100% of the influent conc entration when flow rates were high, less than 10% of the influent con centration when flow rates were low, and of intermediate concentration when flow rates were intermediate. The NH4+ concentration (up to 6.9 mg NH4+-N L(-1)) did not vary despite changes in flow, which indicated that assimilatory and dissimilatory NO3- reduction to NH4+ were incon sequential. Changes in NO3- concentration were consistent with dissimi latory reduction of NO3- to N-2 gas (denitrification) based on the fol lowing evidence: intermediates of denitrification accumulated with sho rt NO3- retention times (flow rates > 2 mL min(-1)) and disappeared wi th long NO3- retention times (flow rates < 0.4 mL min(-1)); between 60 and 68% of the influent NO; was unaccounted for at the end of incubat ion, presumably because it was denitrified; decreasing sediment conten t in the microcosms (from >1000 g to <400 g) to shorten retention time decreased NO3- reduction; sediment pH increased from pH 7.5 to 8.5, w hich would occur during denitrification, during periods of low flow; a cetylene caused N2O to accumulate, indicating that the terminal step i n denitrification was blocked. Forty-seven percent of the springs, wel ls, and tiles from a statewide water assessment in Kentucky demonstrat ed variable NO3- concentration. Denitrification appears to affect wate r quality assessment of NO3- concentration during the year and attenua te NO3- impacts on shallow groundwater.