Dissolved cadmium, zinc, and lead loads from ground-water seepage into the South Fork Coeur d'Alene River system, northern Idaho, 1999

The valley of the South Fork Coeur d’Alene
River and some of its tributaries have been heavily
impacted by the dispersion of metal-enriched
materials from the Coeur d’Alene mining district
since 1884. The valley floor, including the unconsolidated
valley-fill/flood-plain aquifers, is a major
holding area for mine tailings. The U.S. Geological
Survey, in cooperation with the U.S. Environmental
Protection Agency, characterized groundwater
and surface-water relations for parts of the
South Fork Coeur d’Alene River Basin and quantified
the loading of dissolved metals into the South
Fork Coeur d’Alene River system from groundwater
seepage. This information can be used to
determine the effects of dissolved metal from
ground-water seepage on the river system and to
evaluate the necessity and feasibility of remediation
along gaining reaches. This study defines a
field approach that can be repeated during and
after the implementation of remediation solutions
to measure the effectiveness of these efforts in
reducing loading to streams.
The study area includes three reaches along
the South Fork Coeur d’Alene River valley in the
Coeur d’Alene mining district in central Shoshone
County, northern Idaho: a 3.3-mile reach of Canyon
Creek at Woodland Park, a 4.8-mile reach
of the South Fork Coeur d’Alene River near
Osburn, and a 6.5-mile reach of the South Fork
Coeur d’Alene River near Kellogg and Smelterville.
Seepage studies were conducted during July
27–29; September 17–19; and October 15–17,
1999. Each seepage study was conducted over a
3-day period, during which each station was measured
on a daily basis for streamflow, and waterquality
samples were collected. The consecutiveday
approach allowed for an evaluation of variability
in streamflow gains and losses and metal loading
that resulted from changing hydrologic
conditions.
During the July, September, and October
seepage studies, ground-water seepage was the
predominant source for gains in dissolved cadmium
and zinc loads in the three study reaches,
whereas tributary inflow loads were a minor
source. The overall average net gain in dissolved
zinc load from ground-water seepage into the
South Fork Coeur d’Alene River near Kellogg and
Smelterville was about 730 pounds per day, compared
with the net gains in Canyon Creek at Woodland
Park and the South Fork Coeur d’Alene River
near Osburn, which were roughly similar at 150
and 218 pounds per day, respectively. The net gain
in dissolved cadmium load from ground-water
seepage into the three river reaches was about two
orders of magnitude less than the gain in dissolved
zinc.
On the South Fork Coeur d’Alene River study
reaches near Osburn and near Kellogg and Smelterville,
no pattern associated with an increase or
decrease in dissolved lead load along gaining or
losing subreaches was recognizable. Canyon Creek
at Woodland Park was the only study reach where
ground-water seepage contributed appreciably to
the dissolved lead load; the average net gain was
1.5 pounds per day.
The average dissolved lead loads leaving
South Fork Coeur d’Alene River study reaches (corrected for tributary inflow along the study
reaches) near Osburn and near Kellogg and
Smelterville were 1.4 and 0.8 pounds per day less,
respectively, than the loads entering the study
reaches. The decrease in dissolved lead could be
the result of lead adsorbing onto organic and inorganic
sediment surfaces and (or) coprecipitating
with iron and manganese oxides. These forms of
lead likely will be resuspended into the water column
at high flows.