Surface-water-quality conditions and trends were assessed in the lower Kansas River Basin, which drains about 15,300 square miles of mainly agricultural land in southeast Nebraska and northeast Kansas. On the basis of established water-quality criteria, most streams in the basin were suitable for uses such as public-water supply, irrigation, and maintenance of aquatic life. However, most concerns identified from a previous analysis of available data through 1986 are substantiated by analysis of data for May 1987 through April 1990. Less-than-normal precipitation and runoff during 1987-90 affected surface-water quality and are important factors in the interpretation of results.

Dissolved-solids concentrations in the main stem Kansas River during May 1987 through April 1990 commonly exceeded 500 milligrams per liter, which may be of concern for public-water supplies and for the irrigation of sensitive crops. Large concentrations of chloride in the Kansas River are derived from ground water discharging in the Smoky Hill River Basin west of the study unit. Trends of increasing concentrations of some dissolved major ions were statistically significant in the northwestern part of the study unit, which could reflect substantial increases in irrigated acreage.

The largest concentrations of suspended sediment in streams during May 1987 through April 1990 were associated with high-density cropland in areas of little local relief and medium-density irrigated cropland in more dissected areas. The smallest concentrations were measured downstream from large reservoirs and in streams draining areas having little or no row-crop cultivation. Mean annual suspended-sediment transport rates in the main stem Kansas River increased substantially in the downstream direction. No conclusions could be reached concerning the relations of suspended-sediment transport, yields, or trends to natural and human factors.

The largest sources of nitrogen and phosphorus in the study unit were fertilizer and livestock. Nitrate-nitrogen concentrations in stream-water samples did not exceed 10 milligams per liter; relatively large concentrations in the northwestern part of the study unit were associated with fertilizer application. Concentrations of total phosphorus generally were largest in the northwestern part of the study unit, which probably relates to the prevalence of cultivated land, fertilizer application, and livestock wastes.

Deficiencies in dissolved-oxygen concentrations in streams occurred locally, as a result of discharges from wastewater-treatment plants, algal respiration, and inadequate reaeration associated with small streamflow. Large densities of a fecal-indicator bacterium, Escherichia coli, were associated with discharges from municipal wastewater-treatment plants and, especially in the northwestern part of the study unit, with transport of feca

The largest concentrations of the herbicide atrazine generally were measured where the largest quantities of atrazine were applied to the land. Large atrazine concentrations, 10 to 20 micrograms per liter, were measured most frequently in unregulated principal streams during May and June. Downstream of reservoirs, the seasonal variability of atrazine concentrations was decreased compared to that of inflowing streams.