National Water-Quality Assessment (NAWQA) Program
By Larry J. Puckett, Timothy K. Cowdery, Peter B. McMahon, Lan H. Tornes, and Jeffrey D. Stoner[Water Resources Research, vol. 38, no. 8, August 2002]
A combination of chemical and dissolved gas analyses, chlorofluorocarbon age dating and hydrologic measurements were used to determine the degree to which biogeochemical processes in a riparian wetland were responsible for removing NO3- from ground waters discharging to the Otter Tail River in west-central Minnesota. An analysis of river chemistry and flow data revealed that NO3- concentrations in the river increased in the lower half of the 8.3 km study reach as the result of ground-water discharge to the river. Ground-water head measurements along a study transect through the riparian wetland revealed a zone of ground-water discharge extending out under the river. On the basis of combined chemical, dissolved gas, age date, and hydrologic results, it was determined that water chemistry under the riparian wetland was controlled largely by up-gradient ground waters that followed flow paths up to 16 m deep and discharged under the wetland, creating a pattern of progressively older, more chemically reduced, low NO3- water the further one progressed from the edge of the wetland towards the river. These findings pose challenges for researchers investigating biogeochemical processes in riparian buffer zones because the progressively older ground waters entered the aquifer in earlier years when less NO3- fertilizer was being used. NO3- concentrations originally present in the ground water had also decreased in the up-gradient aquifer as a result of denitrification and progressively stronger reducing conditions there. The resulting pattern of decreasing NO3- concentrations across the riparian zone may be incorrectly interpreted as evidence of denitrification losses there instead of in the up-gradient aquifer. Consequently, it is important to understand the hydrogeologic setting and age structure of the ground waters being sampled in order to avoid misinterpreting biogeochemical processes in riparian zones.
Table of Contents