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

The Big River drains historic Pb, Zn, and barite mining districts in Southeast Missouri including the Old Lead Belt (OLB). Underground mining (1864-1972) resulted in large chat piles and tailings impoundments that have been a major source of metal contamination to the river. Circumneutral pH resulting from carbonate terrain constrains metal solubility, 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 watershed and other circumneutral environments.

Total chemistry was determined for bed, suspended, and flood drape sediments collected along 200 km of the Big River from above the OLB to near its mouth at the Meramec River. Bulk (<2 mm diameter) bed sediments upstream of the OLB were >90 wt. % SiO₂ but within the OLB were as low as 60% SiO₂ due to dilution with dolomitic tailings. Pb concentrations in these contaminated bulk bed sediments were as high as 1800 mg kg^-1 and decreased to near 100 mg kg^-1 near the river mouth. Cd and Zn concentrations reached 40 and 2000 mg kg^-1 and decreased to <1 and ≤50 mg kg^-1, respectively. Pb, Zn, and Cd were all approximately 2 to 3 times more concentrated in the finer grained fraction (<63 μM diameter) within the OLB and up to 7 times more concentrated near the mouth of the river. This trend was also reflected in fine grained suspended and flood drape sediments indicating the hydraulic transport of fine-grained material as an important transport mechanism for trace metals downstream of the OLB.

Aqueous chemistry was determined at several locations on the river on two occasions during near base flow conditions. All samples were circumneutral Ca-Mg-HCO₃ type with increasing SO₄ through the OLB due to inputs from mine water and tailings seeps. Dissolved Pb (<0.45 mm) increased from below detection upstream to as high as 10 μg L^-1 within the OLB and remained elevated to the river mouth. Dissolved Cd and Zn similarly increased to as high as 1.2 and 130 μg L^-1, respectively, but rapidly decreased downstream of the OLB. Dissolved Cd and Zn were strongly correlated with unfiltered concentrations but dissolved Pb was not, indicating different controls on the partitioning of trace metals between the solid and aqueous phases. Work is ongoing to investigate the residence and solubility of these metals in bed and suspended sediment.