NITRATE IN GROUNDWATER AND WATER SOURCES USED BY RIPARIAN TREES IN ANAGRICULTURAL WATERSHED - A CHEMICAL AND ISOTOPIC INVESTIGATION IN SOUTHERN MINNESOTA
Sc. Komor et Ja. Magner, NITRATE IN GROUNDWATER AND WATER SOURCES USED BY RIPARIAN TREES IN ANAGRICULTURAL WATERSHED - A CHEMICAL AND ISOTOPIC INVESTIGATION IN SOUTHERN MINNESOTA, Water resources research, 32(4), 1996, pp. 1039-1050
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.