An investigation of lead and other metal contaminants in the sediments of Clearwater Lake, Missouri

Clearwater Lake, located in southeast Missouri, is a flood control reservoir that acts as a sink for sediments derived from the Black River Basin. This basin contains six of the ten mines within the Viburnum Trend, a major lead-zinc producing region of the world for over 45 years. Sediment cores were collected from three areas of the reservoir that represent drainage from sub basins with no mining activity, a single mine site, and multiple mines. The sediment cores were dated using 210Pb and 137Cs and chemically analyzed for major and minor elements. The vertical profiles in these cores represent the period from impoundment (1948) to the date of collection (2002). Temporal trends in the vertical concentration of lead and zinc show relatively little change in the sub basin (Webb Creek) where no mining has taken place. By contrast, increasing concentrations were noted in the sub basins (Logan Creek, Black River) where active mining occurs. Analysis of pre- and post-mining concentrations in deep lacustrine sediments from the Clearwater Dam sample site shows a doubling of the lead concentration and a nearly 50 percent increase in zinc concentration between 1965 and 2002. Age dating of the Clearwater Dam core shows there is an increase in the lead concentration from 48.6 mug/g to 77.9 mug/g and zinc concentration from 117 mug/g and 136 mug/g during a time of approximately 2.5 years from the beginning of mining in the Black River Basin. Increased concentrations in younger sediments are also noted for other mining related metals (As, Cd, Co, Cu, and Ni). Isotopic analyses indicate a clear upward trend in the 206Pb/207Pb ratio indicating a change in source of sediment lead within the basin. Recent sediments have Pb206/Pb 207 ratios greater than 1.30, which is characteristic of southeast Missouri lead ores. Isotopic mixing calculations using the Bonneterre host rock and ore lead as end points show the relative percentage of ore lead contribution to the sediment increases from 23 percent in 1953 to 56 percent in 1997.