SHALLOW GROUNDWATER DENITRIFICATION

In domestic and irrigation wells in a 324-square kilometer area, nitra te concentrations greater than the maximum contaminant level (MCL) for potable water almost always were associated with relatively oxidizing groundwater (measured Eh > +0.28 +/- 0.02 volts). Removal of nitrate, most probably via bacterial reduction (denitrification), occurred bel ow +0.28 v. The observed Eh for this reduction is in reasonable agreem ent with thermodynamics. In these wells the variability in areal nitra te concentrations is strongly influenced by location with respect to t he sharp redox front although there is a significant negative associat ion between nitrate-N concentrations and well depth. This purification process removes an estimated 46 metric tons N/year from the groundwat er at this reducing front. On a global scale similar redox fronts coul d remove thousands of tons of nitrate from groundwater annually. In sp ecially installed nested wells in the vicinity of the redox front, gro undwater nitrous oxide (N2O) concentrations were highest at or near th e water table and decreased with depth indicating that most of the N2O loading was derived from nitrification and denitrification processes in the vadose zone and transported via recharge to the water table. Th e vertical stratification of both nitrate and N2O indicates that denit rification was removing both compounds. Nitrous oxide concentrations i n shallow groundwater were as much as three orders of magnitude higher than those expected in water at equilibrium with the atmosphere.