Abstract
As part of the U.S. Geological Survey’s National Water
Quality Assessment (NAWQA) program nitrate transport in
groundwater was modeled in the mid-Snake River region in
south-central Idaho to project future concentrations of nitrate.
Model simulation results indicated that nitrate concentrations
would continue to increase over time, eventually exceeding the
U.S. Environmental Protection Agency maximum contaminant
level for drinking water of 10 milligrams per liter in some
areas. A subregional groundwater model simulated the change
of nitrate concentrations in groundwater over time in response
to three nitrogen input scenarios: (1) nitrogen input fixed at
2008 levels; (2) nitrogen input increased from 2008 to 2028
using the same rate of increase as the average rate of increase
during the previous 10 years (1998 through 2008); after 2028,
nitrogen input is fixed at 2028 levels; and (3) nitrogen input
related to agriculture completely halted, with only nitrogen
input from precipitation remaining.
Scenarios 1 and 2 project that nitrate concentrations
in groundwater continue to increase from 10 to 50 years
beyond the year nitrogen input is fixed, depending on the
location in the model area. Projected nitrate concentrations in
groundwater increase by as much as 2–4 milligrams per liter in
many areas, with nitrate concentrations in some areas reaching
10 milligrams per liter. Scenario 3, although unrealistic,
estimates how long (20–50 years) it would take nitrate in
groundwater to return to background concentrations—the
“flushing time” of the system.
The amount of nitrate concentration increase cannot
be explained solely by differences in nitrogen input; in fact,
some areas with the highest amount of nitrogen input have the
lowest increase in nitrate concentration. The geometry of the
aquifer and the pattern of regional groundwater flow through
the aquifer greatly influence nitrate concentrations. The
aquifer thins toward discharge areas along the Snake River
which forces upward convergence of good-quality regional
groundwater that mixes with the nitrate-laden groundwater in
the uppermost parts of the aquifer, which results in lowered
nitrate concentrations.
A new method of inputting nitrogen to the subregional
groundwater model was used that prorates nitrogen input
by the probability of detecting nitrate concentrations greater
than 2 mg/L. The probability map is based on correlations
with physical factors, and prorates an existing nitrogen input
dataset providing an estimate of nitrogen flux to the water
table that accounts for new factors such as soil properties.
The effectiveness of this updated nitrogen input method was
evaluated using the software UCODE_2005.
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First posted December 20, 2012
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