Wichita's Aquifer Recharge and Water Quality in the Equus Beds, 1995–2012

Average chloride in deep wells, 2006–2012. The red line marks 250 milligrams per liter, the federal secondary drinking-water level. USGS.

Why Wichita recharges its aquifer

Wichita draws its water from the Equus Beds aquifer and Cheney Reservoir.

• After drought years in the late 1980s and early 1990s, aquifer levels in 1993 were as much as 50 feet below their predevelopment (1940) levels.
• Worse, a large chloride (salt) plume near Burrton — left by past oil and gas exploration — and naturally salty water from the Arkansas River were moving toward the city's well field.
• So beginning in 1997, the city began artificial recharge: pumping Little Arkansas River water into the aquifer when the river runs above base-flow needs.

The USGS and Wichita have studied the aquifer together since 1940, and the effects of recharge since 1995. This fact sheet sets out water quality from 1995 through 2012, including Phase I (2007–2012), which put about 1 billion gallons into the aquifer. It is the baseline for judging Phase II, which in 2013 raised recharge capacity by 30 million gallons a day.

More than 8,800 river and groundwater samples were analyzed for more than 400 compounds, covering most of the EPA's primary and secondary drinking-water standards. The main concerns: chloride, arsenic, coliform bacteria and atrazine, and, to a lesser degree, sulfate, nitrate, iron, manganese, oxidation-reduction potential and specific conductance.

Water quality at a glance

"Wells" here are the study's areal assessment index wells.

Chloride

Chloride persists in the aquifer from the Burrton plume, and in the river its level swings widely. The federal secondary limit of 250 mg/L is for taste and odor.

• River: from under 5 to 932 mg/L; over the limit in about 11% of samples.
• Aquifer: over the limit in 5.6% of shallow and 7.4% of deep well samples. The high deep-well values, 2006–2012, were near Burrton and along the Arkansas River.
• Across all index wells, average chloride fell from 2001–2006 to 2007–2012, but the median rose.
• At well IW-05, about half a mile west of recharge well RRW-03, the plume was held back or diluted: chloride stabilized while Phase I recharge ran (2007–2010) and rose again after 2010, when recharge was minimal.

Movement through the aquifer is complicated by discontinuous clay and sand layers: clay slows groundwater moving downward, sand lets it flow.

Chloride in shallow and deep IW-05 and Phase I recharge at RRW-03, near Burrton, 2001–2012. USGS.

Arsenic

Arsenic occurs naturally in clay layers with iron sulfide minerals and can harm health. The federal limit is 10 µg/L.

• River (near Halstead and Sedgwick): median and average about 5 µg/L — half the limit; range under 1 to 15.9 µg/L; over the limit in about 11% of samples.
• Aquifer: over the limit in 12% of shallow and nearly 35% of deep well samples — highest on the west side of the study area and in the east along the Little Arkansas River.
• The source is natural, but changes in groundwater chemistry can change arsenic levels.

Average arsenic in deep wells, 2006–2012. The red line marks the federal limit of 10 micrograms per liter. USGS.

Coliform bacteria

Coliform bacteria indicate that disease-causing organisms may be present. For systems taking at least 40 routine samples a month, the EPA goal is that no more than 5% test positive in a month.

• About 95% of river samples had coliform bacteria, so the goal was rarely met in the river — with counts up to 2,000,000 colonies per 100 mL near Halstead and 9,000,000 near Sedgwick.
• Groundwater was far cleaner. Detections occurred in 25% of shallow and 12% of deep wells — many in the first samples after drilling, so at least some came from the drilling itself.
• In 2009, bacterial and viral indicators were elevated in monitoring wells near the recharge basin RB-1.

Atrazine

Atrazine, a herbicide used on corn and grain sorghum, does not occur naturally. The federal limit is 3.0 µg/L as an annual average.

• River: average 3.6 µg/L, with levels above the limit generally from late spring to early fall.
• Aquifer: averages under 0.1 µg/L in shallow and deep wells.

Did recharge change the aquifer?

Comparing 2001–2006 with 2007–2012 across the index wells:

Averages rose for everything except chloride, arsenic and oxidation-reduction potential — most likely because of the 2011–2012 drought: less rain means less fresh recharge to dilute the groundwater.

Conclusions

• Phase I recharge has not substantially affected groundwater quality. The 1 billion gallons recharged are small beside the aquifer's roughly 990 billion gallons of storage (winter 2012).
• The exception: chloride stabilized for a time in the north-central study area near the Burrton plume.
• The constituents of concern have not risen substantially, and are more affected by climate (natural recharge from rain) and natural processes (oxidation and reduction, metabolism and decay) than by artificial recharge.
• Arsenic remains a concern: it naturally and persistently exceeds the federal limit, especially in the deep aquifer.

Sources

Based on Daniel J. Tappa, Jennifer L. Lanning-Rush and Andrew C. Ziegler, "Water quality of the Little Arkansas River and Equus Beds Aquifer before and concurrent with large-scale artificial recharge, south-central Kansas, 1995–2012," U.S. Geological Survey Fact Sheet 2015–3010 (2015), prepared with the City of Wichita as part of the Equus Beds Groundwater Recharge Project; a work of the United States government in the public domain. The maps, chart and tables are taken from the fact sheet's PDF.