FATE AND TRANSPORT OF HEAVY METAL CONTAMINANTS IN THE BIG RIVER, OLD LEAD BELT, SOUTHEASTERN MISSOURI

Over a century of mining in the “Old Lead Belt” has resulted in the accumulation of 250 million tons of waste tailings that serve as a potential source for metal contaminants. Water and bottom sediment samples were collected from six sites in order to evaluate contaminant transport processes; three locations on the Big River, a tributary creek, and two small streams draining local tailings piles. Water samples include an unfiltered, and <5, <0.45, and <0.02 micron filtered fractions. Sediment samples were sieve-separated into coarse to medium sand, medium to fine sand, and silt + clay fractions. Waters were analyzed by ICP-MS; sediments by XRD, SEM-EDS, and ICP-ES (following acid digestion). Metal migration occurs in a dynamic and complex fluvial-chemical system. Sand to silt sized galena, pyrite-marcasite, and iron oxides-oxyhydroxide grains occur in the sediments, generally decreasing in size and increasing in degree of roundness downstream. Solution pH values progressively increased from ~7.8 in tailings pile streams to a high of 8.2 on the furthest downstream sample on the Big River, ~20 miles to the northwest. Total metal contents plus the proportion of metals in a dissolved state (<0.02 micron) generally decreased once waters mix into the Big River. Most metals in the main river channel were associated with >5.0 micron sized particles. These particles are easily suspended during flooding and thus contribute to the overall metal transport process. Zinc contents were highest in streams near tailings piles (11,830 ppb) with more than 90% passing through a 0.02 micron filter. Zinc in the Big River was reduced to 150-225 ppb, with the <0.02 micron fraction representing only 2 to 8% of the total. Concentrations of Zn were greatest in the silt- to clay-sized sediments, ranging up to 1600 and 26,400 ppm for Big River and tailings pile sediments, respectively. Distributions of Cd closely paralleled those of Zn, although Cd concentrations were considerably lower. Lead concentrations in streams from the tailings piles were 95-210 ppb, with up to 14% occurring in the <0.02 micron filtrate, while Big River waters ranged from 195-285 ppb with <0.3% passing through the filter. Lead concentrations in bottom sediments also were also highest in the finer sediment fractions with concentrations ranging up to 2800 and 24,800 ppm for Big River and tailings pile sediments, respectively.