Changes in Discharge Water Quality in a Historic Hard Rock Mining Watershed

Abstract

The chemical composition of nine abandoned mine water discharges at the Tar Creek Superfund Site were examined in 2001-2002 and compared to data collected in the mid-1980s. The site is a portion of the abandoned Tri-State Lead and Zinc Mining District of Oklahoma, Kansas and Missouri. In the initial water quality study conducted in 1983-1986, mine waters were characterized as having low to circumneutral pH and elevated metal concentrations. Fe, Zn and alkalinity concentrations ranged from 140-1836 mg/L, 47-200 mg/L, and 0-560 mg/L as CaCO3 eq., respectively, and pH values ranged from 3.0-6.1. During the summers of 2001 and 2002, sampling of nine previously studied discharges was conducted. Data were compared with 1980s data to help characterize suspected changes in mine water chemistry. In situ parameters included pH, temperature, conductivity, dissolved oxygen, alkalinity, and turbidity. Samples were collected and analyzed for Fe, Zn, Cd, Pb, Ni, Mn, Cu, Ca, Mg, SO4, NO3, NO2, PO4, Cl, F, NH4, K, Na, and Li concentrations. In 2001-2002, mine waters were found to have circumneutral pH and exhibit substantial alkalinity with elevated Fe, Zn, Pb and Cd concentrations. Water quality ranges for data collected 2001-2002 were 51197 mg/L for Fe, 4-11 mg/L for Zn and 124-414 mg/L CaCO3 eq. for alkalinity. The range in pH values was 5.8-6.2. In general, most water quality data showed very little change when compared with data from the 1980s study. However, metal contaminant concentrations changed substantially with average decreases in Fe, Zn, and Mn of 74%, 91%, and 74%, respectively. Changes in mine water chemistry are presumed to have occurred due to filling of mine voids with water, which altered the amount of oxygen available at exposed sulfide surfaces, thus decreasing rates of contaminant production. Prior surface water diversions characteristically deemed as having little impact on mine drainage discharge or receiving streams quality, may have substantially modified mine pool hydrology, thus resulting in a general improvement in water quality. The current mine drainage chemical composition lends itself to implementation of passive treatment systems for effective metal removal. Additional