IMPACTS OF HISTORIC LEAD, ZINC, AND BARITE MINING ON TRACE METAL CONCENTRATIONS IN THE BIG RIVER, MISSOURI, USA

The Big River drains historic Pb, Zn, and barite mining districts in Southeast Missouri including the Old Lead Belt (OLB) and Barite District. Underground mining in the OLB (1864-1972) resulted in large chat piles and tailings impoundments that have been a major source of metal contamination to Big River bed and floodplain sediments. Carbonate terrain buffers pH to circumneutral constraining the solubility and mobility of metals, but environmental impacts from metal contamination are widely documented in the watershed. The present objective is to further understand controls on metal mobility and bioaccessibility in this and other mining-impacted, neutral pH environments.

Water was sampled during base flow at 14 locations on the Big River from above the OLB to near its mouth at the Meramec River. Selected tributaries, tailings seeps, and artesian mine water flows were also sampled. Discharge was measured to assess mass flows of mining related elements (S, Pb, Zn, Cd, Ba). Discharge increased from 1.0 m³ s^-1 above the OLB to 6.3 m³ s^-1 near the mouth. All samples were circumneutral and saturated with respect to calcite. Concentrations of SO₄ increased downstream from 9 mg L^-1 to a peak of 68 mg L^-1 within the OLB. Mass balance attributes this increase to dissolved SO₄ in inflowing tailings seepage and mine water. Sulfate decreased downstream from the OLB and lower SO₄ was found in tributaries draining the Barite District. Zinc and Cd also peaked within the OLB at 130 and 1.2 μg L^-1, respectively. Increases in Zn could potentially be attributed to mine water and tailings seeps, but this was a less plausible source of Cd based on mass balance. Stream water Zn and Cd concentrations tracked elevated metal concentrations in previously sampled flood drape and bed sediments suggesting active metal transfer between aqueous and solid phases. Lead increased within the OLB similarly to SO₄, Zn, and Cd (peak concentration 18 μg L^-1) but did not decrease downstream of the OLB and was still elevated near the mouth. While concentrations of other elements were similar in filtered (0.45 μm) and unfiltered samples, Pb was higher in most unfiltered samples. Big river samples were saturated with respect to barite from the barite district to river mouth. Work is ongoing to understand the residence and solubility of these elements in bed and suspended sediment.