PRESERVATION OF WATER QUALITY NEAR A SURFACE COAL MINE, NORTHWESTERN COLORADO

The Colowyo Coal Mine has operated as a surface coal mine in northwestern Colorado since the 1960s. Multiple seams of coal have been removed from the upper portions of the Cretaceous Williams Fork Formation. Water quality and quantity parameters have been obtained from 1974 until the present. The Colorado Division of Reclamation, Mining, and Safety requires environmental protection actions including water quality and quantity monitoring.

Bedrock groundwater is present in localized pockets within the Williams Fork Formation and over a wide area within the underlying Trout Creek Sandstone. Unconfined groundwater is present in valley fill (alluvium and colluvium) along streams. Surface water is present in several streams and in isolated stock ponds and springs.

Background surface water samples were obtained from 11 stream and 20 spring locations, and background groundwater samples were obtainded from 14 monitoring wells. Samples were obtained monthly for at least 12 consecutive months at all locations, and were analyzed for up to 10 ions, total dissolved solids (TDS), and up to 12 metals. Surface water was also analyzed for oil and grease and total suspended solids. Field pH, conductivity, and temperature were recorded, as was surface water flow or groundwater depths.

To monitor impacts from mining, a reduced analyte list has been monitored monthly at key locations near mine impacts. No statistically-significant trends in water quality or quantity are evident at any sampling location. Downstream surface water parameters plotted against time had coefficients of determination (r²) less than 0.16; this was similar to the r² of upstream surface water parameters. Downstream valley-fill aquifer water parameters plotted against time displayed a weak increasing trend in water elevation, TDS, conductivity, and some ions; this may be due to localized effects raising the groundwater table into vadose soils with an abundance of salts rather than by mining.

These data indicate that regulations governing surface mining protect surface water and groundwater from potential impacts caused by mining.