Abstract
The upper Blackfoot River in southeastern Idaho receives
runoff from 12 large phosphate mines. Waste shales that are
removed to access the phosphate ore are highly enriched with
selenium, resulting in elevated selenium in runoff from the
mine waste dumps. In 2001, in cooperation with the Bureau
of Land Management, the U.S. Geological Survey (USGS)
began monitoring streamflow, selenium, and other water-quality
parameters at a single location near the outlet of the upper
Blackfoot River to the Blackfoot Reservoir. Water samples
primarily were collected by a flow triggered, automated pump
sampler, supplemented by manual point and equal-width
integrated manual samples.
The approach to monitoring concentrations and streamflow
over time at a fixed location is ideal for evaluating temporal
trends, but provides no information about the relative source
contributions from the mine waste dumps draining into various
tributaries. In 2001, the Idaho Department of Environmental
Quality (IDEQ) began an annual, mid-May, synoptic survey of
selenium concentrations and streamflow at 21 locations along
the main stem Blackfoot River and its tributaries. Individually,
neither the intensive USGS sampling at the outlet nor the IDEQ
annual synoptic sampling provides a comprehensive view of
selenium runoff in the Blackfoot River watershed. Together, the
efforts are complementary; therefore, in this report, results are
presented from both sampling efforts.
The USGS collected time-series data from 2001 to 2012
at a fixed location, the Blackfoot River near the outlet of the
reservoir, near Henry, Idaho (USGS streamgage 13063000).
Dissolved selenium concentrations from 450 filtered samples
collected at this site ranged from 0.5 to 11.4 micrograms per
liter (μg/L). The State of Idaho chronic aquatic life criterion
concentration of 5 μg/L was exceeded in 31 percent of the
samples, with most exceedances occurring during May of each
year. No exceedances of the selenium criterion were recorded
in months other than April, May, or June. Concentrations of
selenium in unfiltered and filtered samples were similar, and
concentrations from samples collected by depth and width
integrated methods were similar to those collected by grab
(point) samples, indicating that the grab samples adequately
represent selenium concentrations across the entire river cross
section. In speciation analyses made during 2003 and 2004, the
median percentage of total selenium as selenate was 81 percent,
ranging from 17 to 98 percent, and the median percentage of
total selenium as selenite was 19 percent, ranging from 2 to
83 percent of the total selenium. During the period of study,
selenium concentrations had an upward trend during the lowflow
season of August–October. Time trends were not obvious
during other seasons. Selenium daily loads varied by more
than a factor of 900 during the study period and ranged from
0.03 kilograms per day (kg/d) to more than 24 kg/d. Annual
maximum daily loads of selenium varied over nearly a factor of
12, ranging from about 2 to 24 kg/d.
For the annual spring synoptic samples collected by
the IDEQ along the main stem Blackfoot River and major
tributaries, selenium concentrations ranged from less than 2 to
870 μg/L in 176 samples. In most years, the synoptic sampling
showed that the majority of the selenium loads passing the
USGS streamgage at the outlet of the watershed could be
attributed to a single tributary, East Mill Creek, which enters
the Blackfoot River through Spring Creek. Selenium loads
decreased by about half from East Mill Creek before reaching
the Blackfoot River, suggesting that much selenium is at least
temporarily removed from the water column by uptake by
aquatic vegetation or by losses to sediment. Similar decreases in
selenium loads occurred through the main stem Blackfoot River
before reaching the outlet in low flow years, but not in high
flow years.