The Arkansas River in southeast Colorado and southwest Kansas is one of the most saline rivers in the U.S. Most of the dissolved salts in the river water entering Kansas remain in the river corridor and contaminate the ground waters in the alluvial and High Plains aquifers. The Upper Arkansas River Corridor Study was formulated and conducted as a Kansas Water Plan project to evaluate the fate and transport of the dissolved solids that are contaminating the ground-water resources in the river corridor. The evaluation addressed needs described in the Kansas Water Plan for determining the relationships among river flow and quality, ground-water levels and quality, and water use in the corridor area in the Upper Arkansas River Basin.
The primary sources of the dissolved constituents in Arkansas River water are from soils and bedrock in Colorado. The dissolved salt concentration in the river water greatly increases across eastern Colorado as evapotranspiration from ditch diversion and storage systems consumes water, while the dissolved salts remain in the residual water. Dissolved solids contents in low flows of the Arkansas River can exceed 4,000 mg/L at the Colorado-Kansas border. The major dissolved constituents in the river water, in the order of decreasing mass concentrations that usually occur, are sulfate, sodium, bicarbonate, calcium, magnesium, chloride, and silica. Sulfate concentration has ranged from 700 to 2,600 mg/L and averaged between 1,900 and 2,000 mg/L during the last couple of decades. The range in chloride content has been about 40-200 mg/L during that period. The water is usually extremely hard. Fluoride and boron can exceed 1.0 and 0.75 mg/L, respectively the maximum limits recommended for irrigation water, in saline low flows. Selenium usually exceeds 5 µg/L, the chronic toxicity level for aquatic life. Nitrate concentrations are always relatively low (usually 1-3 mg/L as nitrate-nitrogen).