WATER QUALITY IN THE LITTLE ARKANSAS RIVER AND EQUUS BEDS AQUIFER, SOUTH-CENTRAL KANSAS, 2001‒16

The Equus Beds aquifer storage and recovery project was developed to help the city of Wichita, Kansas meet increasing future water demands. Artificial recharge source water comes from the Little Arkansas River during above-base-flow conditions, is treated using National Primary Drinking Water Regulations as a guideline, and is injected into the aquifer for later use. Water-quality samples were collected from 2 Little Arkansas River surface-water sites and 63 Equus Beds aquifer groundwater sites, including 38 areal assessment index wells (IWs) during 2001–16. Federal drinking-water criteria were used to evaluate aquifer water quality. Less than 1% of chloride and nitrate, 7% of dissolved iron, 48% of dissolved manganese, 12% of dissolved arsenic, and 39% of atrazine detections in surface-water samples exceeded their respective criteria. Constituents of concern in the aquifer that exceeded their respective criteria included chloride, sulfate, nitrate, Escherichia coli, total coliforms, iron, and arsenic. About 5% of shallow (<80 ft) and 7% of deep (>80 ft) IW chloride sample concentrations exceeded the Federal criterion of 250 mg/L; chloride exceedances tended to occur in shallow and deep wells along the Arkansas River and near Burrton, Kansas, an area with past oil and gas activities. Shallow IW mean nitrate was substantially larger than mean deep IW nitrate and about 15% of shallow and <1% of deep IW nitrate sample concentrations exceeded the Federal criterion of 10 mg/L. Iron concentrations exceeded the Federal criterion of 0.3 mg/L in 38% of shallow and 46% of deep IW samples. Arsenic concentrations generally were larger in the deep parts of the aquifer and about 12% of shallow and 34% of deep IW arsenic sample concentrations exceeded the Federal 10 µg/L primary drinking criterion. Shallow IW arsenic concentrations were larger near the Little Arkansas River and the center of the aquifer; large shallow IW arsenic concentrations (10–50 µg/L) in the center of the study area correspond to areas that have had the most water level recovery since the historical low in 1993. Mean oxidation-reduction potential (ORP) in shallow IWs generally decreased with increasing water-level depths and were inversely related to mean arsenic concentrations due to more reducing conditions at larger depths below the land surface.