Major sources of nitrogen input and loss in the upper Snake River basin, Idaho and western Wyoming, 1990

Total nitrogen input and loss from cattle
manure, fertilizer, legume crops, precipitation,
and domestic septic systems in the upper Snake
River Basin, Idaho and western Wyoming, were
estimated by county for water year 1990. The purpose of these estimations was to rank input of
nitrogen by source, determine the amount of total
nitrogen potentially available to both ground and
surface water through leaching and runoff, and
identify areas in the basin where excess nitrogen
is produced.
The results of the input estimations suggest
that domestic septic systems account for less
than 1 percent of the total nitrogen input in the
basin and precipitation accounts for 6 percent.
The remaining 93 percent is produced by cattle
manure (29 percent), fertilizer (45 percent), and
legume crops (19 percent). Input from cattle
manure, fertilizer, and legume crops varies widely
among counties and reflects differences in land-use practices such as different cropping patterns
and numbers of dairies and feedlots.
Residual total nitrogen was estimated by subtracting loss due to cattle manure storage and
application, crop uptake, and decomposition of
previous-year nonleguminous crop residue (chaff)
from all nitrogen input in the basin. Positive mean
values of residual total nitrogen in most counties
suggest that more total nitrogen is input than is
lost. This residual total nitrogen is available for
runoff to surface water or leaching to ground
water. Three out of four counties where mean values of residual total nitrogen were highest (Cassia,
Gooding, and Twin Falls) are located in the western part of the basin, where eutrophication in
the Snake River is evident and ground water
from many wells contains anomalously high
nitrate concentrations. Ground water in the fourth
county (Bingham), which includes the Fort Hall
area north of Pocatello, also contains high nitrate
concentrations.
A mass balance of total nitrogen input and loss
in Gooding, Jerome, Lincoln, and Twin Falls
Counties suggests that more than 6,000,000 kg
(6,600 tons) of total nitrogen is input in this
four-county area than is discharged by the Snake
River. This excess nitrogen probably is utilized by
aquatic vegetation in the Snake River (causing
eutrophication), stored as nitrogen in soil, stored
as nitrate in the ground water and eventually discharged through the springs, utilized by noncrop
vegetation, and lost through denitrification.