Lead-rich sediments, Coeur d'Alene River Valley, Idaho: area, volume, tonnage, and lead content

In north Idaho, downstream from the Coeur d?Alene (CdA) silver-lead-zinc
mining district, lead-rich sediments, containing at least 1,000 ppm of lead, cover
approximately 61 km2 (or 73 percent) of the 84-km2 floor of the CdA River valley, from
the confluence of its North and South Forks to the top of its delta-front slope, in CdA
Lake. Concentrations of lead (Pb) in surface sediments range from 15 to about 38,500
ppm, and average 3,370 ppm, which is 112 times the mean background concentration (30
ppm) of Pb in uncontaminated sediments of the CdA and St. Joe River valleys.
Most of the highest concentrations of Pb are in sediments within or near the river
channel, or near the base of the stratigraphic section of Pb-rich sediments. Ranges of Pb
concentration in Pb-rich sediments gradually decrease with increasing distance from the
river and its distributaries. Ranges of thickness of Pb-rich sediments generally decrease
abruptly with increasing distance from the river, from about 3 + 3 m in the river channel
to about 1 + 1m on upland riverbanks, levees and sand splays, to about 0.3 + 0.3 m in
back-levee marshes and lateral lakes. Thickness of Pb-rich dredge spoils (removed from
the river and deposited on Cataldo-Mission Flats) is mostly in the range 4 + 4 m, thinning
away from an outfall zone north and west of the river, near the formerly dredged channel
reach near Cataldo Landing. We attribute lateral variation in ranges of thickness and Pb
content of Pb-rich sediments to the dynamic balance between decreasing floodwater flow
velocity with increasing distance from the river and the quantity, size, density, and Pb
content of particles mobilized, transported, and deposited.
We present alternative median- and mean-based estimates of the volume of Pbrich
sediments, their wet and dry tonnage, and their tonnage of contained Pb. We
calculate separate pairs of estimates for 23 Estimation Units, each of which corresponds
to a major depositional environment, divided into down-valley segments. We favor
median-based estimates of the thickness and thickness-interval weighted-average Pb
concentration, because uncommonly thick and Pb-rich sections may excessively
influence mean estimates. Nevertheless, data from partial sections of Pb-rich sediments
are included in most estimates, and these tend to reduce both median- and mean-based
estimates.
Median-based estimates indicate a volume of 32 M m3 of Pb-rich sediments in the
CdA River valley, with a dry tonnage of 47 + 4 M t, containing 250 + 75 kt of Pb
(considering analytical uncertainties only). An equivalent tonnage of dry CdA River
valley sediments of the pre-mining era, with the mean background concentration of 30
ppm of Pb, would contain about 1.4 kt of Pb. Thus, the amount of Pb added to CdA River
valley sediments deposited since the onset of mining is estimated as 249 + 75 kt of Pb, or
about 99.5 percent of the estimated Pb contained. Of an estimated 850 + 10 kt of Pb lost
to streams as a result of mining-related activities, an estimated total of 739 + 319 kt of Pb
has been deposited in sediments of the South Fork drainage basin, the CdA River valley,
and the bottom of CdA Lake (combined). Based on mid-range values from a set of
preferred estimates with uncertainty ranges up to + 50 percent, roughly 24 percent of the
850 + 10 kt of mining-derived Pb lost to streams has been added to sediments of the
South Fork drainage basin, 29 percent to sediments of the CdA River valley floor, and 34
percent to sediments on the bottom of CdA Lake. This amounts to roughly 87 percent of
the Pb lost to streams, not including Pb contained in sediments of the North Fork drainage basin and the Spokane River valley, the tonnages of which have not yet
estimated.