Seasonal and weather-related controls on solute concentrations and acid drainage from a pyritic coal-refuse deposit in southwestern Indiana, U.S.A.

Abstract

A small watershed on a barren deposit of pyritic coal refuse was instrumented and intensively monitored for 15 months to determine characteristics of acid runoff, and seasonal and weather-related trends in subsurface aqueous chemistry. Pore waters are most contaminated in the shallow ( <1 m ) subsurface, where total dissolved solids (TDS, mostly sulfate and iron) sometimes exceed 160,000 mg L −1. Chemical fluctuations, which are related to changes in moisture content, are extreme, and concentrations of TDS range across more than an order of magnitude during the course of a water year. In contrast, lower concentrations of TDS ( <65,000 mg L −1 ) are consistently observed below the water table. Samples of overland flow contain concentrations of TDS as high at 8260 mg L −1. Surface runoff is acidified by dissolution of hydrated iron sulfate minerals. These salts, which are ubiquitous on the ground surface and in shallow capillary pathways, are evidence of a strong hydrologic connection between the unsaturated zone and the surface. Preliminary calculations indicate that acidic outflow by surface runoff constitutes a major portion of the total acid drainage from the coarse-refuse deposit.