Sources of salinity near a coal mine spoil pile, north-central Colorado

A small (1 km2) salt-affected stream drainage on the High Plains north of Denver, Colorado was sampled to determine the near-surface dispersion of soluble salts and metals from low-sulfur coal mining waste (spoil). Surface waters collected along the 0.8-km stream reach, and aqueous leachates of spoil and naturally saline local soil, were analyzed for chemical constituents and sulfur isotopes. In this semiarid setting with abundant carbonate-bearing surficial sediments, the limited, mildly acidic drainage from the spoil pile is quickly neutralized, restricting the mobility of many elements. However, some spoil-derived constituents were clearly traceable within the upper 0.4 km of the stream reach. Spoil leachates and surface water near the spoil pile have distinctive compositions of major anions and cations, and elevated levels of dissolved nitrate compared with downstream waters. Spoil-derived sulfate was traceable because it has generally positive values of delta34S that contrasted with generally negative values of delta34S in soil leachates and evaporite salts from the surrounding area. Spatial-chemical sampling of surface water showed an abrupt increase in dissolved U, Se, B, Li, and Mn in the lower 0.4 km of the stream reach where shallow ground water from surrounding irrigated fields contributed to surface flow. The downstream evolution of surface water chemistry and sulfur isotopic composition is consistent with mixing between spoil-affected upstream water and irrigation-return water. The methods described should be applicable at other sites in similar settings where the environmental effect of low-sulfur coal mining waste must be assessed and where access to samples of shallow ground water is limited.