Assessment of nutrients, suspended sediment, and pesticides in surface water of the upper Snake River basin, Idaho and western Wyoming, water years 1991-95

A water-quality investigation of the upper
Snake River Basin began in 1991 as part of
the U.S. Geological Survey's National Water-
Quality Assessment Program. As part of the
investigation, intensive monitoring was conducted
during water years 1993 through 1995 to assess
surface-water quality in the basin. Sampling and
analysis focused on nutrients, suspended sediments,
and pesticides because of nationwide
interest in these constituents.

Concentrations of nutrients and suspended sediment
in water samples from 19 sites in the upper
Snake River Basin, including nine on the main
stem, were assessed. In general, concentrations of
nutrients and suspended sediment were smaller in
water from the 11 sites upstream from American
Falls Reservoir than in water from the 8 sites downstream
from the reservoir where effects from land-use
activities are most pronounced. Median concentrations
of dissolved nitrite plus nitrate as nitrogen
at the 19 sites ranged from less than 0.05 to
1.60 milligrams per liter; total phosphorus as phosphorus,
less than 0.01 to 0.11 milligrams per liter;
and suspended sediment, 4 to 72 milligrams per
liter. Concentrations of nutrients and suspended
sediment in the main stem of the Snake River, in
general, increased downstream. The largest concentrations
in the main stem were in the middle
reach of the Snake River between Milner Dam and
the outlet of the upper Snake River Basin at King
Hill.

Significant differences (p<0.05) in nutrient and
suspended sediment concentrations were noted
among groups of sites categorized by the quantity
of agricultural land in their upstream drainage
basins. Water samples collected from sites in drainage
basins where agricultural land constituted less
than 10 percent of the land use contained significantly
smaller concentrations of nutrients and suspended
sediment than samples from sites in drainage
basins where agricultural land constituted more
than 10 percent of the land use. Significant differences
in nutrient and suspended sediment concentrations
were inconsistent among groups representing
10 to 19 percent, 20 to 29 percent, and
greater than 29 percent agricultural land use. Seasonal
concentrations of dissolved nitrite plus
nitrate, total phosphorus, and suspended sediment
were significantly different among most of the agricultural
land-use groups. Concentrations of dissolved
nitrite plus nitrate were largest during the
nonirrigation season, October through March.
Concentrations of total phosphorus and suspended
sediment, in general, were largest during high
streamflow, April through June.

Nutrient and suspended sediment inputs to the
middle Snake reach were from a variety of sources.
During water year 1995, springs were the primary
source of water and total nitrogen to the river and
accounted for 66 and 60 percent of the total input,
respectively. Isotope and water-table information
indicated that the springs derived most of their
nitrogen from agricultural activities along the margins
of the Snake River. Aquacultural effluent was
a major source of ammonia (82 percent), organic
nitrogen (30 percent), and total phosphorus (35 percent).
Tributary streams were a major source of
organic nitrogen (28 percent) and suspended sediment
(58 percent). In proportion to its discharge
(less than 1 percent), the Twin Falls sewage-treatment
plant was a major source of total phosphorus
(13 percent). A comparison of discharge
and loading in water year 1995 with estimates
of instream transport showed a good correlation
(relative difference of less than 15 percent) for
discharge, total organic nitrogen, dissolved nitrite
plus nitrate, total nitrogen, and total phosphorus.
Estimates of dissolved ammonia and suspended
sediment loads correlated poorly with instream
transport; relative differences were about 79 and
61 percent, respectively.

The pesticides EPTC, atrazine, desethylatrazine,
metolachlor, and alachlor were the most commonly
detected in the upper Snake River Basin and
accounted for about 75 percent of all pesticide
detections. All pesticides detected were at concentrations
less than 1 microgram per liter and below
water-quality criteria established by the U.S. Environmental
Protection Agency. In samples collected
from two small agriculturally dominated tributary
basins, the largest number and concentrations of
pesticides were detected in May and June following
early growing season applications. At one of
the sites, the pesticide atrazine and its metabolite
desethylatrazine were detected throughout the year.

On the basis of 37 samples collected basinwide
in May and June 1994, total annual subbasin applications
and instantaneous instream fluxes of EPTC
and atrazine showed logarithmic relations with
coefficients of determination (R2 values) of 0.55
and 0.62, respectively. At the time of sampling, the
median daily flux of EPTC was about 0.0001 percent
of the annual quantity applied, whereas the
median daily flux of atrazine was between 0.001
and 0.01 percent.