1 U.S. Geological Survey, Water Resources Division, 1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402
2 Environment Canada, Vancouver, British Columbia, Canada
The rate and mechanism of nitrate removal along and between groundwater flow paths were investigated using a series of well nests screened in an unconfined sand and gravel aquifer. Intensive agricultural activity in this area has resulted in nitrate concentrations in groundwater often exceeding drinking water standards. Both the extent and rate of denitrification varied depending on the groundwater flow path. While little or no denitrification occurred in much of the upland portions of the aquifer, a gradual redox gradient is observed as oxic upland groundwater moves deeper in the aquifer. In contrast, a sharp shallow redox gradient is observed adjacent to a third-order stream as oxic groundwater enters reduced sediments. An essentially complete loss of nitrate concurrent with increases in excess-N2 provide evidence that denitrification occurs as groundwater enters this zone. Electron and mass balance calculations suggest that iron sulfide (e.g., pyrite) oxidation is the primary source of electrons for denitrification. Denitrification rates were found to vary considerably between the two areas where denitrification occurs. Denitrification rates in the deep, upland portions of the aquifer were found to range from <0.01 to 0.14 mM of N per year based on mass balance calculations using nitrate and excess-N2 coupled with groundwater travel time estimates. Denitrification rates in groundwater adjacent to the stream are at least an order of magnitude faster, with potential denitrification rates up to 140 mM per year based on in-situ experiments conducted in this zone.
Water Resources Research, Vol. 36, No. 6, Pages 1545-1559, June 2000
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