Occurrence and transport of cadmium, lead, and zinc in the Spokane River basin, Idaho and Washington, water years 1999-2001

A water-quality investigation of the Clark
Fork-Pend Oreille and Spokane River Basins began
in 1997 as part of the U.S. Geological Survey
National Water-Quality Assessment Program. As
part of the investigation, selected streams in the
Spokane River Basin were sampled for trace metals
during water years 1999–2001. These data,
combined with data collected as part of a U.S.
Environmental Protection Agency Remedial Investigation/
Feasibility Study, were used to assess the
occurrence, loads, and transport of cadmium, lead,
and zinc at 21 streamflow-gaging stations in the
Spokane River Basin.
Concentrations of dissolved and total cadmium,
lead, and zinc varied widely both at and
among stations. At most stations, dissolved cadmium
and zinc composed most of the total concentrations;
dissolved lead generally composed less
than 10 percent of the total lead concentration.
From the South Fork Coeur d’Alene River near
Mullan downstream to the South Fork Coeur
d’Alene River near Pinehurst, concentrations of
trace metals increased 2 to 4 orders of magnitude.
The mean flow-weighted concentrations of total
cadmium, lead, and zinc near Pinehurst for water
years 1999–2001 were 5.7, 80, and 810 micrograms
per liter (
µg/L), respectively. On the Coeur
d’Alene River near Harrison, downstream from the
confluence of the metal-enriched South Fork and
the relatively dilute North Fork Coeur d’Alene
River, the mean flow-weighted concentrations of
total cadmium, lead, and zinc were 1.6, 88, and
240
µg/L, respectively. Trace-metal concentrations
were smaller in the Spokane River than in the
Coeur d’Alene River because of dilution and
retention in Coeur d’Alene Lake. The mean flowweighted
concentrations of total cadmium, lead,
and zinc in the Spokane River near Post Falls were
0.32, 3.1, and 71
µg/L, respectively.
Regression models relating the mass transport,
or load, of trace metals to changes in stream
discharge and time were successful in simulating
the variability in trace-metal concentrations and
loads. The median coefficient of determination for
the load models for the 21 stations was largest for
total lead (92 percent) and smallest for dissolved
and total cadmium (82 percent). Whereas most of
the cadmium and zinc loads in the Spokane River
Basin were derived from the South Fork Coeur
d’Alene River, most of the lead load was derived
from the Coeur d’Alene River downstream from
the confluence of the North and South Forks. Major
tributary sources of trace metals to the South Fork
Coeur d’Alene River were Canyon Creek, Ninemile
Creek, and Government Gulch. These three
tributaries contributed about 3,000 pounds of cadmium,
23,000 pounds of lead, and 310,000 pounds
of zinc annually to the South Fork Coeur d’Alene
River. Erosion and transport of sediment-bound
lead in the Coeur d’Alene River was the primary
source of total lead, accounting for almost
400,000 pounds annually during water years
1999–2000. Ground-water discharge in the area of
the Bunker Hill Superfund site was a major source
of zinc in the South Fork Coeur d’Alene River,
contributing more than 250,000 pounds per year.
During water years 1999–2000, the average
annual loads of cadmium, lead, and zinc transported
from the Coeur d’Alene, St. Joe, and St.
Maries Rivers to Coeur d’Alene Lake were 8,900, 500,000, and 1.4 million pounds, respectively.
The Coeur d’Alene River accounted for more than
99 percent of the total load of each of these three
metals entering the lake. About 4,600 pounds of
cadmium, 44,000 pounds of lead, and 980,000
pounds of zinc were transported from Coeur
d’Alene Lake into the Spokane River. Between the
Spokane River near Post Falls, Idaho, and the Spokane
River at Long Lake, Washington, there was
an annual net loss of about 2,600, 20,000, and
250,000 pounds of cadmium, lead, and zinc,
respectively. About 2,000 pounds of cadmium,
24,000 pounds of lead, and 730,000 pounds of
zinc were transported annually downstream from
Long Lake toward the Columbia River.