TRACE ELEMENT AND ISOTOPIC TRENDS IN SEDIMENT CORES FROM CLEARWATER LAKE, SOUTHEAST MISSOURI

Clearwater Lake is a 6.5-km2 flood control reservoir that acts as a sink for sediments derived from the Black River basin. The basin contains six of the ten mines of the Viburnum Trend, a major lead-zinc producing region of the world for nearly forty years. Sediment cores representing the period from impoundment (1948) to the date of collection (2002) were taken from three areas of the reservoir that represent subbasins 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. Temporal trends in the concentration of lead and zinc show relatively little change in the subbasin where no mining has taken place and increasing concentrations in the subbasins where active mining occurs. Analysis of pre- and post-mining concentrations in deep lacustrine sediments shows a doubling of the lead concentration and a nearly fifty percent increase in zinc concentration following the development of mines along the Viburnum Trend. All of the lead-zinc concentrations however, remain well below the probable effects concentration derived from consensus-based sediment quality guidelines. Lead isotopic analyses indicate a clear upward trend in the Pb206/Pb207 ratio indicating changing sources within the watershed. Recent sediments have Pb206/Pb207 ratios greater than 1.30 which is characteristic of the anomalously high radiogenic southeast Missouri lead ores.