SEDIMENT BUDGET APPROACH TO ASSESS TAILINGS AND LEAD STORAGE IN CHANNEL AND FLOODPLAIN DEPOSITS IN BIG RIVER, OLD LEAD BELT, SOUTHEAST MISSOURI

The Old Lead Belt in southeast Missouri was a global leader in lead ore production for more than a century (1869-1972) and discharged large quantities mine wastes contaminated with lead (Pb) into Big River (2,500 km²). While mining sites have been remediated, remobilization of contaminated sediment from alluvial storages continues to be a major source of present-day contamination. This study assessed the magnitude and spatial distribution of mining-contaminated sediment stored in channel and floodplain deposits of Big River which drains the Saint Francois Mountains and Salem Plateau in the Ozark Highlands. Government records were used to estimate total tailings discharge for the district to the watershed at 264 million Mg with a peak combined mill capacity of 24,000 Mg ore/day in 1933. About 75% of total tailings were shipped off-site for use as fertilizer and construction materials. However, 22% of the tailings remain in six stabilized piles covering 11 km² and 3% is stored in alluvial deposits between Leadwood and Eureka, Missouri (171 km). The most contaminated floodplain deposits occur 40-50 km downstream from mine sources, typically ranged in depth from 1.5-3.5 m, and averaged >2000 mg kg^-1 Pb. Channel sediments were most contaminated within 20-30 km of the mines where average site concentrations were >1400 mg kg^-1 Pb and >1000 mg kg^-1 Zn. A total of 157 million Mg of contaminated sediment is stored along the Big River, with 92% within floodplain deposits. These contaminated sediments store a total of 188,500 Mg Pb. Interestingly, coarse tailings (2-16 mm) account for 13-20% of the total mass storage of contaminated channel sediment, contain concentrations of >4000 mg kg^-1 Pb, and account for over half of the mining-related Pb mass stored in channel deposits within the Big River.