Systematic sampling of springs, tiles, and wells in Kentucky, as part of a recent statewide program to assess agricultural impacts on water quality, showed that NO3 concentrations in these shallow ground water sources varied tremendously. The NO3 concentration could be correlated with flow rate; higher when ground water recharge flushed NO3 from soil in winter and spring, and lower or non detectable in summer and fall when less NO3 leaching occurred. Depending on the season, NO3 concentrations ranged from < 1 to > 10 ppm NO3-N in almost half of the sites. For example, the water in one site, a shallow well over a naturally occurring spring in Bourbon county, varied from 0 to 12 ppm NO3-N during the year (Figure 1 ).

There is an alternative explanation for this variability, an explanation that isn't based on ground water recharge events. An interaction between flow rate and biological activity could explain some of the variability of NO3 concentration in this and similar sites. Since the water percolated through a sediment layer in the Bourbon county well before it could be sampled, it seemed likely that biological denitrification (a microbial process in which bacteria convert NO3-N to N2 gas) during low flow periods might account for the low NO3 concentrations. When water flow was high, NO3 movement through the sediment layer would be too rapid for complete biological removal. We tested this idea by recreating flow-dependent NO3 concentrations in a series of laboratory studies.

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