CHARACTERIZATION OF LIMESTONE LEACH BED EFFICACY OVER SEASONS AND STORMS

The legacy of coal mines persists across Appalachia and other coal regions, with continuing environmental issues -- including acid mine drainage (AMD) – that require remediation to address. Passive remediation systems are a common tool to remediate AMD because of the relatively low cost and maintenance requirements. These systems generally rely on multiple components, but limestone leach beds (LLBs) are among the most frequently utilized. While the construction and use of LLBs continue to increase, relatively little is understood about how LLB efficacy varies over storms and seasons.

Therefore, we measured the inlet and outlet chemistry of an LLB from March 2022 through March 2023 to determine how remediation efficacy varied over seasons and storm cycles. Measurements were made downstream of Ingrand Mine, a representative below-drainage Appalachian Coal mine in north central WV, which operated from 1981 to 1997 and generate acidic drainage (pH 3 to 4) with high annual average metal concentrations (Fe (II) =7.6 mg/L, Al (III) =22.0mg/L, Mn (II) =0.7mg/L). The LLB was installed as part of a passive remediation system in 2015.

Measurements included in situ field parameters (pH, discharge, specific conductance, temperature) and laboratory analyses of water samples for dissolved metal concentrations (Al, Mn, Mg, Ca), specific conductance, lab alkalinity, and hot acidity. Seasonal samples (n=32) were collected at approximately weekly intervals; storm samples (n=20) were collected during two discrete storm events (August and November) The LLB removed, on average, 97% of acid, 85% of Mn, 97% of Fe, and 95% of Al loads. Removal of all metals was less effective during periods of high flow (June-September), with up to 30% of Mn, 8% of Fe, and 13% of Al loads passing through the LLB; and post-storm, with up to 100% of Mn, 16% of Fe, and 22% of Al loads passing through the LLB. By characterizing how flow, temperature, and AMD concentrations vary over seasons and the associated variability in LLB treatment efficacy and downstream solute loads, these results will help managers better design and construct cost-effective systems that meet remediation goals.