Red River of the North Basin
National Water-Quality Assessment Program
Tornes, L.H., Puckett, L.J., Porter, S.D., and Stoner, J.D., 1995, Variations of nitrate beneath a riparian
wetland in Minnesota and its relation to stream biota: American Geophysical Union, EOS Transactions,
Abstracts for Fall Meeting -- December 11-15, 1995, p. 186.
Abstract
Nitrate concentrations were measured during August 1994 and 1995 in
water from a sand and gravel alluvial aquifer that discharges to an
overlying riparian wetland and adjacent stream. The study area measures
45 by 120 meters. An apparent relation was determined among stream
biota, stream habitat, and water quality at specific locations within the
study area. Most of the ground-water recharge to this northwestern
Minnesota alluvial aquifer is enriched with nutrients from agricultural
applications and comes from an adjacent surficial outwash aquifer
upgradient from the study area. Nitrate concentrations and hydraulic head
in 1994 were laterally variable and generally decreased toward the stream.
Concentrations ranged from below detection (0.1 milligrams per liter as
N, mg/L) to more than 22 mg/L. Samples collected in 1995 had similar
concentrations, hydraulic-head range, and spatial distribution as those
from the previous year with several site-specific differences. For example,
measurable nitrate was found in areas where previously it had not been
detected, and in other areas nitrate concentrations had decreased.
Measured nitrate concentrations, hydraulic heads, dissolved oxygen,
specific conductance, and oxidation-reduction couples show that the most
likely causes of nitrate variability are (1) local variations in nitrate inputs
to ground water upgradient and (2) variable rates of ground water
movement beneath the wetland where denitrification occurs. Variability of
nitrate probably is not caused by dilution of the shallow ground water
through discharge of deeper ground water having less nitrate.
Evaluation of the Otter Tail River adjacent to the wetland where it is
hydraulically connected to the alluvial aquifer suggests that stream biota
may be useful for locating where ground water that is enriched with
nitrate seeps into streams. In areas where nitrate probably discharges to
the stream with ground water, benthic algae and invertebrate communities
were different than in other parts of the stream. Dense growths of blue-
green algae, indicative of nitrogen enrichment, were found on submerged
portions of wild rice stems in areas adjacent to ground-water discharge,
whereas sparse to moderate growths of other algal taxa were found on
wild rice growing in the middle of the stream channel. Rheocrene
invertebrate taxa (Amphipoda and Isopoda) were found at the stream margin, whereas lotic, surface-water taxa (Ephemeroptera and Trichoptera) were found at the middle of the 70 meter wide channel.
Citation
Index-Published Meeting Abstracts | Study Unit Publications | Red River of
the North Basin, NAWQA | U.S. Geological Survey in
Minnesota | NAWQA Program