NITRATE IN GROUNDWATER AND WATER SOURCES USED BY RIPARIAN TREES IN ANAGRICULTURAL WATERSHED - A CHEMICAL AND ISOTOPIC INVESTIGATION IN SOUTHERN MINNESOTA

This study evaluates processes that affect nitrate concentrations in g roundwater beneath riparian zones in an agricultural watershed. Nitrat e pathways in the upper 2 m of groundwater were investigated beneath w ooded and grass-shrub riparian zones next to cultivated fields, Becaus e trees can be important components of the overall nitrate pathway in wooded riparian zones, water sources used by riparian trees and possib le effects of trees on nitrate concentrations in groundwater were also investigated. Average nitrate concentrations in shallow groundwater b eneath the cultivated fields were 5.5 mg/L upgradient of the wooded ri parian zone and 3.5 mg/L upgradient of the grass-shrub zone. Shallow g roundwater beneath the fields passed through the riparian zones and di scharged into streams that had average nitrate concentrations of 8.5 m g/L (as N). Lateral variations of delta D values in groundwater showed that mixing among different water sources occurred beneath the ripari an zones. In the wooded riparian zone, nitrate concentrations in shall ow groundwater were diluted by upwelling, nitrate-poor, deep groundwat er. Upwelling deep groundwater contained ammonium with a delta(15)N of 5 parts per thousand that upon nitrification and mixing with nitrate in shallow groundwater caused nitrate delta(15)N values in shallow gro undwater to decrease by as much as 19.5 parts per thousand, Stream wat er penetrated laterally beneath the wooded riparian zone as far as 19 m from the stream's edge and beneath the grass-shrub zone as far as 27 m from the stream's edge. Nitrate concentrations in shallow groundwat er immediately upgradient of where it mixed with stream water averaged 0.4 mg/L in the wooded riparian zone and 0.8 mg/L near the grass-shru b riparian zone. Nitrate concentrations increased toward the streams b ecause of mixing with nitrate-rich stream water. Because nitrate conce ntrations were larger in stream water than shallow groundwater, concen trated nitrate in the streams cannot have come from shallow groundwate r at these sites. Water sources of riparian trees were identified by c omparing delta D values of sap water, soil water, groundwater, and str eam water. Soil water was the main water source for trees in the outer 4 to 6 m of one part of the wooded riparian zone and outer 10 m of an other part. Groundwater was a significant water source fur trees close r to the streams where the water table was less than about 2.1 to 2.7 m below the surface. No evidence was found in the nitrate concentratio n profiles that trees close to the streams that took up groundwater th rough their roots also took up nitrate from groundwater. The lack of s uch evidence is attributed to the nitrate concentration profiles being insufficiently sensitive indicators of nitrate removal by trees.