SALT AND SELENIUM IN MANCOS SHALE TERRANE ON THE GUNNISON GORGE NATIONAL CONSERVATION AREA, WESTERN COLORADO, USA

The release of salt and selenium (Se) under natural weathering conditions is being studied in Mancos Shale terrane on the Gunnison Gorge National Conservation Area (NCA), western Colorado. Gypsum (CaSO₄•2H₂O) and thenardite (Na₂SO₄) are the major salts identified. Gypsum is abundant in fractured shale, whereas thenardite occurs as efflorescent salts on the ground surface and on soil surfaces of newly excavated trenches. Sulfur isotopic data show that the sulfate in these salts is produced by oxidation of pyrite in the Mancos Shale. Gypsum formed within the shale as weathering solutions moved along fractures and bedding planes. Dissolution of the gypsum during soil formation produces Ca that adsorbs onto clays, releasing Na to form the highly soluble thenardite.

Chemistry of saturation paste extracts (SPE) was used to test the mobility of salts and Se in Mancos-derived soils on hillslopes and valley floors in the NCA. Only small amounts of gypsum dissolve in the SPE, whereas thenardite and its hydrated forms were readily soluble. Conductivity of the extracts (ECe) was used as a proxy for the amount of salt readily extracted from soil. The ECe of soil developed on, or transported to, the valley floor systematically increases over an order-of-magnitude (1.5 to 15 mS/cm) from the surface to a depth of 3.7 m. Surface soils (0 to about 25 cm depth) on slopes are relatively low in salt (3 to 6 mS/cm) and extractable Se (10 to 70 ppb dry-weight basis), whereas deeper soil accumulates salt (up to 35 mS/cm) and extractable Se (200-1000 ppb). As the soil salt content increases, the percentage of exchangeable Na and of total Se extracted increases (2 to >60 % and 2 to 40%, respectively). Chloride (probably sourced from the atmosphere) and nitrate (sourced from organics and possibly feldspars in the shale) have high concentrations in soil on slopes armored against rapid erosion with pediment gravel; conversely, these anions do not accumulate in unarmored soils that are more rapidly eroded.

Calculations indicate that 4 to 5 kg extractable salt/cu m and 0.2 to 0.4 g extractable Se/cu m are stored in Mancos soil on the NCA. If salt-laden horizons are disturbed and brought to the surface where they are out of equilibrium with the weathering environment, large amounts of these salt and Se loads will be available for rapid mobilization.