RAINFALL SIMULATION STUDIES ON THE MANCOS SHALE, SOUTHWESTERN COLORADO: RUNOFF SALINITY AND SELENIUM

Simulated rainfall onto the Mancos Shale provides an important model of natural salinity and trace element runoff. Rainfall studies were conducted in the Gunnison Gorge National Conservation Area (GGNCA) northeast of Montrose, Colorado. Sites were in the Candy Lane (CL) area near the center of the GGNCA, with slope aspects at each of the four principal cardinal directions, and a west- and east facing slope from the Elephant Skin Wash (ESW) area. Rain application equaled the 100-year storm event rate. Runoff had conductivities that averaged 900 and ranged from 36 to 2500 µS/cm, all with circumneutral pH. Calcium was the most abundant cation, with lesser amounts of sodium. Sulfate was the most abundant anion, averaging 5 mM, with lesser amounts of bicarbonate. Selenium averaged 7 and ranged from below detection to 30 µg/L. Sites in the CL area generally have higher runoff volumes and release amounts of salinity and selenium compared to those in the ESW area. Runoff volumes were 30 to 40 percent of the applied rain at the south- and north-facing sites and only a few percent of applied rainfall at the east- and west-facing sites. Sites in ESW have somewhat greater concentrations of salinity and selenium but lesser volumes of runoff. Intentional fresh disturbance at the south- and north-facing sites decreased the runoff amounts to 20 and 4 percent, respectively, of the applied rainfall, suggesting that this disturbance initially increases the water-holding capacity of the soil. At the south-facing CL site, fresh disturbance initially increased the water-holding capacity of the surface, then stimulated higher salinity and selenium release during later rainfall compared to the undisturbed site. In next day repeat rainfall experiment on the same plots, east-facing sites in ESW, disturbed and undisturbed, retained their water-holding capacity. For these runoff samples, conductivity is a reliable predictor of salinity, sulfate and, with somewhat greater scatter, selenium. The largest releases of selenium in the runoff occur on south-facing slopes in the CL area, with concentrations ranging from 10 to 30 µg/L. Released selenium was 400 µg in a volume of 20 L. The consistent relationship between runoff selenium and the major, highly soluble elements indicates that there is a minor but readily soluble phase of selenium in the soil.