Mechanism and rate of denitrification in an agricultural watershed: Electron and mass balance along groundwater flow paths

The rate and mechanism of nitrate removal along and between groundwater flo w paths were investigated using a series of well nests screened in an uncon fined sand and gravel aquifer. Intensive agricultural activity in this area has resulted in nitrate concentrations in groundwater often exceeding drin king water standards. Both the extent and rate of denitrification varied de pending on the groundwater flow path. While little or no denitrification oc curred in much of the upland portions of the aquifer, a gradual redox gradi ent is observed as aerobic upland groundwater moves deeper in the aquifer. In contrast, a sharp shallow redox gradient is observed adjacent to a third -order stream as aerobic groundwater enters reduced sediments. An essential ly complete loss of nitrate concurrent with increases in excess N-2 provide evidence that denitrification occurs as groundwater enters this zone. Elec tron and mass balance calculations suggest that iron sulfide (e.g., pyrite) oxidation is the primary source of electrons for denitrification. Denitrif ication rate estimates were based on mass balance calculations using nitrat e and excess N-2 coupled with groundwater travel times. Travel times were d etermined using a groundwater flow model and were constrained by chlorofluo rocarbon-based age dates. Denitrification rates were found to vary consider ably between the two areas where denitrification occurs. Denitrification ra tes in the deep, upland portions of the aquifer were found to range from <0 .01 to 0.14 mM of N per year; rates at the redoxcline along the shallow flo w path range from 1.0 to 2.7 mM of N per year. Potential denitrification ra tes in groundwater adjacent to the stream may be much faster, with rates up to 140 mM per year based on an in situ experiment conducted in this zone.