Water Quality in Equus Beds Aquifer Minimally Affected by Artificial Recharge

Groundwater quality has not substantially changed since 2007 recharge activities began in the Equus Beds aquifer in Kansas, according to a new study by the U.S. Geological Survey, in cooperation with the City of Wichita, Kansas.

The city of Wichita relies on groundwater from the Equus Beds aquifer and surface water from Cheney Reservoir to meet their water needs. To help mitigate declining water levels, the city of Wichita artificially recharged about 1 billion gallons of treated water from the Little Arkansas River and nearby bank-storage wells into the Equus Beds aquifer during 2007–2012. This was done as part of the Phase I recharge of the Artificial Storage and Recovery (ASR) project.

This report summarizes water quality in the Equus Beds aquifer and the Little Arkansas River before (1995–2006) and during (2007–2012) the first phase of ASR recharge. Findings show no substantial effects on groundwater quality in the aquifer, likely because the total amount of water recharged is relatively small (1 billion gallons) compared to the aquifer storage volume, which was measured at more than 990 billion gallons in winter 2012. This study serves as a baseline to detect any subsequent changes dur­ing continued operation of Phase I and Phase II, which increased the capacity for recharge by 30 million gallons per day when water is available in the river and within treatment parameters. The full report is available online.

“Maintaining and improving water quality in the Equus Beds aquifer for long-term water supply in Wichita and surrounding communities is part of the ASR project,” said Alan King, city of Wichita Director of Public Works.

The eastward movement of a previously discovered area of high chloride concentration within the aquifer, known as the Burrton chloride plume, is likely being slowed because of artificial recharge. The plume advanced to less than one half mile from western edge of the Wichita well field area from 2006‒2012. Past oil and gas activities in the 1920s through 1950s near Burrton, Kansas resulted in a plume of high-chloride groundwater northwest of the Wichita well field, which is an area that encompasses most of the groundwater wells used by the city of Wichita. When chloride levels are high, the water is less usable as a drinking-water source and for crop irrigation without additional treatment. 

Arsenic remains a water-quality constituent of concern because of natural and continued persistence of concentrations exceeding the U.S. Environmental Protection Agency’s (EPA) maximum contaminant levels (MCL), especially in the deeper parts of the aquifer. During 1995–2012, arsenic concentrations exceeded MCLs in about 12 percent of shallow wells and nearly 35 percent of samples from deep wells. Arsenic occurs naturally in clay layers and can be a health hazard to humans. The geochemical effects of artificially recharging oxygenated water may decrease arsenic concentrations in the aquifer.

Atrazine concentrations larger than the MCL generally occur in the Little Arkansas River from late spring to early fall. Atrazine is an herbicide used on corn and grain sorghum, which are crops commonly grown in and near the study area, and it does not occur naturally in the environment. Average atrazine concentrations were almost undetectable in all wells tested in the Equus Beds aquifer during 1995–2012. The water treatment processes decrease concentrations to less than the MCL of 3 micrograms per liter before being recharged into the aquifer through recharge wells or recharge basins.

During 1995–2012, more than 8,800 surface water and groundwater water-quality samples were collected and analyzed for more than 400 compounds, including most of the compounds on the EPA’s primary drinking-water standards maximum contaminant level list and secondary drinking-water regulations secondary maximum con­taminant level list.

More information about USGS Equus Beds aquifer studies can be found on the USGS Kansas Water Science Center website.