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

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.]