Long-term trends of selected pesticides in the Little Arkansas River, south-central Kansas, 1995–2023

The eastern part of the High Plains Aquifer, herein called the Equus beds aquifer, is currently a primary source of water (2026) for the city of Wichita, Kansas. The Equus Beds Aquifer Storage and Recovery Project was developed in the early 1990s to meet future water demands by using the Little Arkansas River as an artificial aquifer recharge water source during above-base-flow conditions in the river. Little Arkansas River water is diverted at an intake structure, treated following National Primary Drinking Water Regulations as a guideline, and delivered to the aquifer through recharge basins or injection wells for later use. The U.S. Geological Survey, in cooperation with the city of Wichita, completed the study described in this report to quantify and characterize Little Arkansas River water-quality data. Data in this report can be used to evaluate changing conditions in aquifer recharge source water, provide science-based information for decision making, and help meet regulatory monitoring requirements.

Continuous streamflow, specific conductance, turbidity, and discrete atrazine data were collected from two Little Arkansas River study sites and discrete glyphosate and aminomethylphosphonic acid was collected from one of the two Little Arkansas River study sites from 1995 to 2023 to evaluate water-quality conditions and assess pesticide trends. Pesticide trends in the herbicides atrazine and glyphosate and the herbicide-associated compound aminomethylphosphonic acid (a glyphosate degradate) were evaluated using the R package seawaveQ.

Pesticides modeled with the seawaveQ R package had increasing and decreasing trends during the study period, depending on the individual pesticide and if there were strong seasonal patterns; larger estimated concentrations occurring in late spring and early summer. Modeled atrazine concentrations exceeded the Federal maximum contaminant level of 3 micrograms per liter about 8 percent of the time at both study sites. Atrazine had a nonsignificant increasing trend (98 percent) at the Highway 50 site and a nonsignificant decreasing trend (31 percent) at the downstream Sedgwick site during the study period. Glyphosate and aminomethylphosphonic acid trends were not evident at the Sedgwick site during the study period.