Baseline and Historic Depositional Rates and Lead Concentrations, Floodplain Sediments, Lower Coeur d'Alene River, Idaho

Lead-rich sediments, containing at least 1000 ppm of lead (Pb), and derived
mainly from discarded mill tailings in the Coeur d'Alene mining region, cover
about 60 km2 of the 80-km2 floor of the main stem of the Coeur d'Alene River
valley, in north Idaho. Although mill tailings have not been discarded directly
into tributary streams since 1968, frequent floods continue to re-mobilize
sediment from large secondary sources, previously deposited on the bed, banks,
alluvial terraces, and natural levees of the river. Thus, lead-rich sediments
(also enriched in iron, manganese, zinc, copper, arsenic, cadmium, antimony and
mercury) continue to be deposited on the floodplain. This is hazardous to the
health of resident and visiting human and wildlife populations, attracted by
the river and its lateral lakes and wetlands.

This report documents and compares depositional rates and lead concentrations
of lead-rich sediments deposited on the bed, banks, natural levees, and flood
basins of the main stem of the Coeur d'Alene River during several
time-stratigraphic intervals. These intervals are defined by their
stratigraphic positions relative to the base of the section of lead-rich
sediments, the 1980 Mt. St. Helens volcanic-ash layer, and the sedimentary
surface at the time of sampling. Four important intervals represent sediment
deposition during the following time spans (younger to older): 1. Baseline,
from 1980 to about 1993 (after tailings disposal to streams ended, but before
any major removals of lead-rich sediments); 2. Early post-tailings-release,
from about 1968 to 1980; 3. Historic floodplain-contamination, from about 1903
to 1968; and 4. Background, before the 1893 flood (the first major flood after
large-scale mining and milling began upstream in 1886).

Medians of baseline depositional rates and lead concentrations in levee
sediments vary laterally, from 6.4 cm/10y and 3300 ppm Pb on riverbanks and
levee fore-slopes to 2.8 cm/10y and 3800 ppm Pb on levee back-slope uplands. In
lateral flood basins, baseline medians increase with water depth, from 2.2
cm/10y and 1900 ppm Pb in lateral marshes, to 2.9 cm/10y and 2100 ppm Pb in
littoral margins of lateral lakes, and 4.0 cm/10y and 4400 ppm Pb on limnetic
bottoms of lateral lakes.

The median of lead concentrations in baseline sediments is 82 percent of the
median for early post-tailings-release sediments, with a 69-percent probability
that the two data sets represent statistically different populations. By
contrast, the median of lead concentrations in baseline sediments is 57 percent
of the corresponding median for historic-interval sediments, and these two data
sets definitely represent statistically different populations. The
area-weighted average of medians of lead concentrations in baseline sediments
of all depositional settings is 2900 ppm Pb, which is 1.6 times the 1800 ppm Pb
that can be lethal to waterfowl. It also is 2.9 times the 1000-ppm-Pb threshold
for removal of contaminated soil from residential yards in the Coeur d'Alene
mining region, and 111 times the 26-ppm median of background lead
concentrations in pre-industrial floodplain sediments.

During episodes of high discharge, lead-rich sediments will continue to be
mobilized from large secondary sources on the bed, banks, and natural levees of
the river, and will continue to be deposited on the floodplain during frequent
floods. Floodplain deposition of lead-rich sediments will continue for
centuries unless major secondary sources are removed or stabilized. It is
therefore important to design, sequence, implement, and maintain remediation in
ways that will limit recontamination.

[Summary provided by the USGS.]