ASSESSING AQUIFER DENITRIFICATION POTENTIAL

Denitrification may be a significant sink for nitrate in many aquifers. Therefore, understanding the fate and transport of nitrate in hydrologic systems requires an understanding of the denitrification potential in aquifers. Our working hypothesis in assessing aquifer denitrification potential is that the supply of electron donors that can support denitrification in aquifer sediments is the limiting factor. Having a labile supply of electron donors generally insures an oxygen-limited environment and a viable population of denitrifying bacteria – other denitrification requirements. Our research, and that of previous investigators, indicates that the three major suites of electron donors that contribute to denitrification are: 1) organic carbon, 2) sulfide as pyrite, and 3) ferrous iron minerals. Our regional program for assessing the aquifer denitrification potential has two major components: 1) measuring in situ denitrification rates using in situ mesocosms (ISMs) and 2) correlating the results with measurements of electron donor concentrations in the sediments. The integration of both components requires geochemical modeling; for this we use PHREEQC (Parkhurst and Appelo, 1999). A companion paper presents more modeling details. Our network of denitrification ISMs includes data from nine sites in eight glacial-outwash aquifers, with several more sites proposed. Our results so far show that denitrification rates and kinetic orders vary from site to site, as do the proportional contributions of electron donors. Furthermore, the aquifer with the largest supply of electron donors, and therefore the highest denitrification potential, has sediments containing shale fragments apparently from a nearby black shale member of the Pierre Shale (Upper Cretaceous), which is known for relatively high concentrations of organic carbon, pyrite, and ferrous iron. Identifying similar aquifers may serve as a “first-cut” towards determining those with high denitrification potential. In aquifers composed of less reactive sediments, relatively higher electron donor concentrations seem to be available in the aquifer sediments near streams.