Transport and Fate of Nitrate in a Glacial Outwash Aquifer in Relation to Ground Water Age, Land Use Practices, and Redox Processes

Abstract

A combination of ground-water modeling, chemical and dissolved gas analyses, and chlorofluorocarbon age dating of water was used to determine the relation between changes in agricultural practices, and NO 3 -concentrations in ground water of a glacial outwash aquifer in west-central Minnesota. The results revealed a redox zonation throughout the saturated zone with oxygen reduction near the water table, NO 3 -reduction immediately below it, and then a large zone of ferric iron reduction, with a small area of sulfate (SO 4 2-) reduction and methanogenesis (CH 4 ) near the end of the transect. Analytical and NETPATH modeling results supported the hypothesis that organic carbon served as the electron donor for the redox reactions. Denitrification rates were small, 0.005 to 0.047 mmol NO 3 -yr -1 , and were limited by the small amounts of organic carbon, 0.01 to 1.45 percent. In spite of the organic carbon limitation, denitrification was virtually complete because residence time is sufficient to allow even slow processes to reach completion. Ground-water sample ages showed that maximum residence times were on the order of 50-70 years. Reconstructed NO 3 -concentrations, estimated from measured NO 3 -and dissolved nitrogen gas showed that NO 3 -concentrations have been increasing in the aquifer since the 1940s have been above the 714 µmol L -1 maximum contaminant level at most sites since the mid-to late-1960s. This increase in NO 3 -has been accompanied by a corresponding increase in agricultural use of fertilizer, identified as the major source of NO 3 -to the aquifer.

Using chemical, hydrologic, and age dating analysis to delineate redox processes and flow paths in the riparian zone of a glacial outwash aquifer‐stream system

“…Like many rivers in western Minnesota, Otter Tail River chemistry (Table 2) is dominated by Ca 2+ , Mg 2+ , and HCO 3 − [ Tornes et al , 1997]. Glacial deposits in the area have been shown to contain calcite and dolomite, which controls the evolution of water chemistry [ Berndt , 1987; Puckett and Cowdery , 2002; Tucillo et al , 1999]. As a consequence, pH and concentrations of Ca 2+ , Mg 2+ , and HCO 3 − are about what would be expected for waters near equilibrium with calcite and dolomite, and saturation indices calculated in WATEQ were 0.3 and 0.4, respectively.…”

“…Given reported hydraulic conductivities for the aquifer materials of 30.5–73.2 m d −1 , average porosity of 0.25–0.3 [ Cowdery , 1997; Puckett and Cowdery , 2002], and the measured heads and water levels, it is possible to estimate the average velocity of pore waters beneath the wetland. Gradients between piezometers OF1 and A4 ranged from 0.0042–0.026 resulting in estimated average velocities of 0.4–7.6 m d −1 .…”

“…Groundwater samples were collected from the aquifer at site OF1 (Figures 1 and 4) at the terminus of the upland transect described by Puckett and Cowdery [2002]. Site OF1 consisted of a nest of 5 permanent piezometers, installed at depths ranging between 3 and 13.5 m below land surface, and made of 5 cm inside diameter, polyvinyl chloride (PVC) pipe with slotted PVC screens 0.76 m long; the exceptions being the shallowest and deepest piezometers at this location, which were 1.5 m long.…”

“…Methodology followed that previously described by Pfenning and McMahon [1996] with the exception of the NO 3 − and C concentrations, and had a detection limit of 2 nmol L −1 N 2 O with a prescision of ±5%. These sediments were incubated in solutions containing 375, 714, or 1785 μmol L −1 NO 3 − with no additional C to test whether denitrification was limited by NO 3 − alone, or 0, 125, or 375 μmol L −1 C with 375 μmol L −1 NO 3 − added to test whether denitrification was being limited by C. These concentrations are typical for groundwater in the area [ Puckett et al , 1999; Puckett and Cowdery , 2002].…”

“…In this paper, we examine the hydrogeologic and biogeochemical controls on the evolution of water chemistry at a site in west central Minnesota. Previous work in the area examined nitrogen cycling in the 212 km 2 agro‐ecosystem [ Puckett et al , 1999] and hydrogeologic and biogeochemical processes controlling the transport and fate of nitrogen in an upland portion of the aquifer [ Puckett and Cowdery , 2002]. The Puckett and Cowdery [2002] study demonstrated a pattern of decreasing NO 3 − concentrations with both increasing depth and groundwater age along a representative 5 km flow system transect, and their modeling results predicted that groundwater from deep in the aquifer should discharge at the terminus of the flow system in the riparian zone adjacent to the Otter Tail River.…”