Improper disposal of oil-field brine and other wastes has adversely affected the naturally diverse chemical quality of much of the water in the Walnut River basin, south-central Kansas. The basin is an area of about 2,000 square miles in the shape of a rough triangle with its apex toward the south. The Whitewater River, a principal tributary, and the Walnut River below its junction with the Whitewater River flow southward toward the Arkansas River along courses nearly coincident with the contact of the Chase and overlying Sumner Groups of Permian age. The courses of many minor tributaries are parallel to a well-developed joint system in the Permian rock. Thick interbedded limestone and shale of the Chase Group underlie the more extensive, eastern part of the basin. Natural waters are dominantly of the calcium bicarbonate type. Shale and subordinate strata of limestone, gypsum, and dolomite of the Sumner Group underlie the western part of the basin. Natural waters are dominantly of the calcium sulfate type. Inflow from most east-bank tributaries dilutes streamflow of the Walnut River; west-bank tributaries, including the Whitewater River, contribute most of the sulfate. Terrace deposits and alluvial fill along the stream channels are assigned to the Pleistocene and Holocene Series. Calcium bicarbonate waters are common as a result of the dissolution of nearly ubiquitous fragments of calcareous rock, but the chemical quality of the water in the discontinuous aquifers depends mainly on the quality of local recharge. Concentrations of dissolved solids and of one or more ions in most well waters exceeded recommended maximums for drinking water. Nearly all the ground water is hard to very hard. High concentrations of sulfate characterize waters from gypsiferous aquifers; high concentrations of chloride characterize ground waters affected by drainage from oil fields. Extensive fracture and dissolution of the Permian limestones facilitated pollution of ground water by oil-field brine and migration of the polluted water into adjacent areas. Ground water containing more than 1,000 mg/o=l (milligrams per liter) dissolved solids .and more than 100 mg/o=l chloride is common near oil fields but is exceptional elsewhere. The concentration of nitrate in about 25 percent of the sampled well waters exceeded the recommended maximum for drinking water. High concentrations of nitrate generally were associated with shallow aquifers, local sources of organic pollution, and stagnation. Sodium and chloride are the principle ionic constituents of oil-field brine but are minor constituents of natural surface waters or shallow ground water in the basin. The ratios of the concentrations of sodium to chloride in brine from different oil fields varied within a narrow range from a mean of 0.52. Concentrations of chloride exceeding 50 mg/o=l in streamflow and 100 mg/l in ground water generally signified the presence of oil-field brine if the sodium-chloride ratios were less than 0.60. Higher sodium-chloride ratios characterized relatively rare occurrences of high concentrations of the ions that might have originated in evaporite minerals or in sewage. The concentration of chloride during low flow of the major streams generally increased, and the sodium-chloride ratio decreased, in a downstream direction from about 0.65 near the headwaters to about 0.51, which is characteristic of oil-field brine. The changes were most abrupt where polluted ground-water effluent augmented low streamflow adjacent to old oil fields. With increased direct runoff, the sodium-chloride ratio normally increased, and these ions constituted a smaller percentage of the dissolved-solids load. Annual runoff .decreased progressively from above normal to below normal during water years 1962-64. Higher concentrations .of the ions in streamflow persisted for longer periods during the periods of low runoff