Abstract

Declining water levels and concerns about the migration of a known saltwater plume upgradient from public supply wells prompted the City of Wichita to investigate the feasibility of using artificial recharge to replenish the water supply in the Equus Beds aquifer. After preliminary testing, the City of Wichita began Phase I of the Equus Beds Aquifer Storage and Recovery Project in 2006. In 2009, the City of Wichita installed an experimental passive gravity recharge well and trench system to increase artificial recharge at Recharge Basin 1, one of the six Phase І recharge sites.

The U.S. Geological Survey collected water samples from 13 sites and maintained 8 continuous monitors to test the recharge capacity of the experimental passive recharge system, the effect of the recharge on geochemistry of the aquifer, and the fate of bacteria and viruses present in the recharge water. About 576,000 gallons of treated surface water from the Little Arkansas River were recharged through the passive recharge well and trench system into the Equus Beds aquifer during April 2009. In May 2009, U.S. Geological Survey tests detected that bacterial and viral indicators (total coliform, fecal coliform, Escherichia coli, coliphage virus, and Clostridium perfringens) were entering the Recharge Basin 1 wells through the recharge system and recharge was discontinued. The City of Wichita disconnected the trench collection system from the passive gravity recharge well in July 2009, and in July and August 2009 withdrew 1,825,000 gallons of water from the aquifer at Recharge Basin 1 to remove the recharged water and avoid contamination of the aquifer.

The original recharge rate in Recharge Basin 1 was about 10.8 gallons per day per square foot. After installation of the passive recharge system, recharge water entered the aquifer through the passive well at a rate of about 19.2 gallons per day per square foot, a per unit area increase of about 78 percent.

During artificial recharge, continuous monitors recorded rising water-level altitudes in the passive gravity recharge well and nearby monitoring wells as water flowed at about 10 feet per day from the passive recharge well toward nearby downgradient monitoring wells. The increase in water level in this area would have the effect of temporarily slowing the eastward migration of saltwater from the nearby Burrton plume.

Bacterial and viral indicators were detected in water samples from Recharge Basin 1 sites before and immediately after the installation of the passive gravity recharge well and trench system, during artificial recharge, and after artificial recharge. After water withdrawal in August 2009 and through the end of data collection in March 2010, detections of bacterial and viral indicators in groundwater decreased to densities similar to those before installation of the passive recharge system.

Concentrations of chloride in samples collected from the trench, passive gravity recharge well, and nearby monitoring wells increased from an average of 34 milligrams per liter before artificial recharge to an average of 64 milligrams per liter during artificial recharge, reflecting the addition of recharge water with measured chloride concentrations of 62 to 94 milligrams per liter. When water was being pumped out of the aquifer through the passive gravity recharge well, chloride concentrations increased to 94 milligrams per liter in the removed water and increased to 150 milligrams per liter in the deep monitoring well nearest the passive gravity recharge well, indicating that, as water was being pumped from the passive well, water with a large chloride concentration from elsewhere in the aquifer was flowing toward the passive well. Chloride concentrations did not exceed the U.S. Environmental Protection Agency Secondary Drinking Water Regulation of 250 milligrams per liter in any Recharge Basin 1 samples collected as part of the study.

Iron concentrations exceeded the U.S. Environmental Protection Agency Secondary Drinking Water Regulation of 300 micrograms per liter in three wells before, during, and after recharge, and were not substantially affected by the addition of oxygenated treated surface water into the system. Arsenic concentrations exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 micrograms per liter in some deep wells, but did not exceed the Maximum Contaminant Level in shallow wells. The addition of oxygenated treated surface water to the aquifer did not substantially change arsenic concentrations at Recharge Basin 1 monitoring sites.

Atrazine concentrations in samples collected from Recharge Basin 1 well sites during 2009–2010 did not exceed the Maximum Contaminant Level of 3 micrograms per liter and were largest in groundwater samples collected 4 to 6 days after artificial recharge was discontinued. Atrazine concentrations in samples collected after the passive gravity recharge well experiment were similar to those collected before the experiment began.

Saturation indices for calcite (CaCO₃), iron hydroxide (Fe₃(OH)₈), and manganite (MnOOH) increased during artificial recharge while saturation indices for scorodite (FeAsO₄:2H₂O) decreased, indicating that mixing oxygenated artificial recharge water with aquifer water caused conditions favorable for oversaturation and precipitation of some minerals. Saturation indices indicated the potential for calcite and iron hydroxide precipitation in some samples during the study period.

First posted June 16, 2011