SOILS AND SELENIUM IN THE LOWER UNCOMPAHGRE AND LOWER GUNNISON VALLEY AREAS

The NRCS Ridgway Soil Survey project staff, in cooperation with the Shavano Conservation District, and in conjunction with the Gunnison Basin Selenium Task Force and the US Bureau of Reclamation (BOR), conducted soil sampling for selenium analysis on selenium-rich Mancos Shale soils within the project boundaries of the Ridgway Soil Survey. Two hundred eleven soil samples from 44 sites were collected, during the soil survey process, for selenium analysis. For study purposes, these sites were broadly classified into three categories; twenty-one sites were irrigated, 19 were non-irrigated, and 3 were waterlogged. Soil samples were prepared by BOR and analyzed by the US Geological Survey. Statistical analysis of the resultant data was conducted by the BOR.

A strong correlation exists between soluble selenium and irrigation. Non-irrigated soils average 34 times more soluble selenium, within the immediate soil profile (upper 5 to 9 feet), than irrigated soils. This correlation supports the concept that the soil-water system within the irrigated areas of the watershed is a flushing system, differing greatly from the soil-water system in areas of California.

Some of the Mancos-derived soils that have been irrigated for the past 50 or more years have undergone rapid weathering and pedogenesis. Most of the soils once identified in the 1967 Delta-Montrose Soil Survey (field work late 1940's) as residual (less than 40 inches from soil surface to paralithic contact) have exhibited increases in depth ranging from of 20 to greater than 60 inches. These soils have also exhibited clay increases on average of 5 to 7 percent total clay within the soil profile control section. This increase in clay, as a result of sediment-laden irrigation waters and weathering of the Mancos Shale soil materials, has caused the soil classification of many of these irrigated soils to change from fine-silty to fine (less than 35% clay to greater than 35% clay). This ongoing weathering of the soil overburden and paralithic Mancos parent material as a byproduct of irrigation, suggests a continuum of labile salts and heavy metals becoming available for mobilization and transport.