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between 0.0 and -0.3 my (extremely reducing conditions). Selenium arriving with irrigation drainage waters at the escarpment springs and ground water systems in the northern and southern portion of the study area most likely contain selenate or selenite. Once this water is discharged at the terrace escarpment the selenate and selenite may be biogeochemically reduced to elemental selenium or selenide in the wetlands. These selenium species are biologically unavailable and since they are insoluble they do not occur in the water column. selenite and other selenium species can also be rendered biologically unavailable by formation of metal selenides, or Se-sulfides, or by selenite adsorption to sediments such as Fe oxyhydroxides. Once in the soil, some selenide can be biogeochemically reduced to hydrogen selenide, which like hydrogen sulfide, is a gas that can volatilize and leave the soil and water column to the atmosphere. Thus, some selenium can be naturally removed from wetlands. Our water samples report total selenium, which as described above occur as several different species in the field. The samples which contained high selenium concentrations occur at springs and a ground water station which most likely have oxic conditions. We. suspect that the selenium was in the form of selenate. selenate received by wetlands may be either taken up by plants and invertebrates or biogeochemically reduced to elemental selenium or selenide which, being insoluble, are removed from the water column. Selenium (Se) distribution follows a similar pattern to that of conductivity and sodium, being highest in the north (Figure 16). Water collected from two stations, 7 and 15, contained high Se concentrations. High concentrations were also found in a small pond just south of the study area. No wetland station had detectable concentrations of Se. Se most likely is moving in agricultural drainage waters which leached salts from the Mancos Shale and flow in a generally east to west direction in the northern and southern portions of the Escalante Ranch area. The salts are concentrating in the briny waters of small ponds and pools. Aerial photographs of the study area illustrate that the terrace area directly east of the main wetlands does not have a center pivot sprinkler. However, center pivots are present on the terrace to the north and south of the wetland. Thus, salt and selenium distributions may relate to the presence of center pivots and their irrigation patterns. 26