SALT AND SELENIUM IN THE UPPER COLORADO RIVER: PROCESSES OF SUPPLY AND TRANSPORT FROM THE CRETACEOUS MANCOS SHALE

Salt and selenium in the Colorado River are of concern due to salinity limits mandated by the agreement between the U.S. and Mexico, the need to irrigate salinity sensitive crops, protection of wildlife habitat, and suitability of river water as a municipal supply. One-third of the salt and nearly all the Se in the Upper Colorado River (UCR) at Cisco, Utah, are directly attributed to weathering of the Cretaceous Mancos Shale. In 2005, a total of 1.4 Mt salt and 60 t Se were transferred from Mancos landscapes to the UCR above Cisco. This study investigated how and when solutes move from a Mancos landscape in the Uncompahgre watershed into the UCR. The Uncompaghre River is a tributary to the UCR in southwest Colorado, and over 60% of its annual salt load and 100% of its Se load are derived from natural weathering and irrigation of the Mancos. Rock and derived soil from a trenched weathering profile were chemically and mineralogically analyzed, and the Uncompahgre River water composition was measured during high-water, irrigation, and base flow regimes. Multiple chemical changes occur during weathering of the Mancos. At the onset, iron sulfide minerals oxidize to form gypsum with a unique isotopic composition (δ³⁴S = -20‰) that enabled the tracking of solutes. Progressive weathering leaches this gypsum (14% in shale to 4% in soil), Ca exchanges for Na on the clays, and Na₂SO₄ phases form in the soil. Selenium in unweathered shale resides in pyrite, rare clausthalite (PbSe), and possibly organic matter. Oxidation converts primary, reduced Se to selenate, which is stored as a component of sulfate salts. The amount of Se extracted by water decreases with increasing weathering (40% in the shale compared to 13% in soil), indicating that the metal is labile during early weathering and is continually lost from the weathering profile. During weathering of shale to soil, several elements are depleted: Ca 50%; Na 55%; S 10%; C_CO3 40%; and Se 65%. The amount of Mancos-derived salt and Se that entered the Uncompahgre River in 2005 and was transported downstream to the UCR is 375 t/d and 10 kg/d during high water flow (spring), 1180 t/d and 580 kg/d during irrigation season (summer), and 760 t/d and 11 kg/d during base flow (winter). Similar high rates of transfer were observed in all Mancos Shale watersheds and are influenced by changes in land use and seasonal weathering.