The Old Lead Belt is a historic lead (Pb) and zinc (Zn) mining sub-district within the Southeast Missouri Lead Mining District which was a global producer of Pb worldwide from 1869 to 1972. Past and ongoing releases of chat, tailings, and other mining wastes to the Big River have resulted in the contamination of channel sediment and floodplain deposits with toxic levels of Pb along 170 river kilometers of the Big River from Leadwood to its confluence with the Meramec River. Previous studies by the USGS and USFWS identified elevated Pb concentrations in the active channel sediments of Big River. However, what is not well understood are the spatial and temporal patterns of the volume or mass storage of mining sediment in channel and floodplain deposits of the Big River and its major tributaries.
The magnitude and impact of mining operations on the sediment load and geochemistry of the Big River has been significant. Active channel bed and bar deposits are contaminated above the aquatic sediment PEC with >128 ppm Pb from Leadwood (R-km 171) to the confluence with the Meramec River (R-km 0). In channel sediments, the highest Pb concentrations (>1,000 ppm Pb) occur from Desloge (R-km 158.1) to St. Francois State Park (R-km 140.3). Similarly, overbank floodplain deposits are contaminated above the residential soil threshold limit of 400 ppm Pb along the entire length of the river below Leadwood to a depth of 1 to 4 meters or more. In floodplain deposits, the highest concentrations (>2,000 ppm) tend to occur between the Bonehole (R-km 165.3) and Browns Ford (R-km 79.5).
Both fine-grained and coarse sediments are contaminated with Pb and other metals in the Big River. XRF Pb analyses for <2 mm fraction of channel sediment typically approach 2,500 ppm in St. Francois County, while larger chat (4-8 mm) fractions can contain over 5,000 ppm Pb. Mill slimes (<63 um) were released directly to the river during mining operations and contained concentrations of Pb typically >10,000 ppm.
The occurrence of mining chat (2-16 mm) deposits is largely limited to channel segments in St. Francois County between Leadwood and Bonne Terre. Dolomite tailings fragments were only detected in the channel from below the Desloge pile (R-km 158.1) to Highway E (R-km 132.9). Calcium analyses (tailings tracer) suggest that the downstream extent of transport for mining chat is probably about 10 km upstream of the Jefferson County line. However, finer tailings sediment fractions (<2 mm) are present further downstream to Browns Ford (R-km 79). Tile probe depths in bar and bed locations of the channel are used to estimate the storage of contaminated sediment. Average unit storage rates are 2,570 +/- 14% (1s) m3/100 m from R-km 171 to 90 and 1,580 +/- 12% from R-km 90 to 15. The storage budget for contaminated sediment and Pb focuses attention on the role of floodplains as sources and sinks of contaminants in mined watersheds. There is about 3,700,000 m3 of contaminated sediment stored in the channel and 86,800,000 m3 stored in floodplains. Following, there is 3,800 Mg Pb
Pavlowsky, Robert T.; Owen, Marc R.; and Martin, Derek J., "Distribution, Geochemistry, and Storage of Mining Sediment in Channel and Floodplain Deposits of the Big River System in St. Francois, Washington, and Jefferson Counties, Missouri" (2010). OEWRI Technical Reports. 1.