Abstract

Artificial recharge of the Equus Beds aquifer is part of a strategy implemented by the city of Wichita, Kansas, to preserve future water supply and address declining water levels in the aquifer of as much as 30 feet caused by withdrawals for water supply and irrigation since the 1940s. Water-level declines represent a diminished water supply and also may accelerate migration of saltwater from the Burrton oil field to the northwest and the Arkansas River to the southwest into the freshwater of the Equus Beds aquifer.

Artificial recharge, as a part of the Equus Beds Ground-Water Recharge Project, involves capturing flows larger than base flow from the Little Arkansas River and recharging the water to the Equus Beds aquifer by means of infiltration or injection. The geochemical effects on the Equus Beds aquifer of induced stream-water and artificial recharge at the Halstead and Sedgwick sites were determined through collection and analysis of hydrologic and water-quality data and the application of statistical, mixing, flow and solute-transport, and geochemical model simulations.

Chloride and atrazine concentrations in the Little Arkansas River and arsenic concentrations in ground water at the Halstead recharge site frequently exceeded regulatory criteria. During 30 percent of the time from 1999 through 2004, continuous estimated chloride concentrations in the Little Arkansas River at Highway 50 near Halstead exceeded the Secondary Drinking-Water Regulation of 250 milligrams per liter established by the U.S. Environmental Protection Agency. Chloride concentrations in shallow monitoring wells located adjacent to the stream exceeded the drinking-water criterion five times from 1995 through 2004. Atrazine concentrations in water sampled from the Little Arkansas River had large variability and were at or near the drinking-water Maximum Contaminant Level of 3.0 micrograms per liter as an annual average established by the U.S. Environmental Protection Agency. Atrazine concentrations were much smaller than the drinking-water criterion and were detected at much smaller concentrations in shallow monitoring wells and diversion well water located adjacent to the stream probably because of sorption on aquifer sediment. Before and after artificial recharge, large, naturally occurring arsenic concentrations in the recharge water for the Halstead diversion well and recharge site exceeded the Maximum Contaminant Level of 10 micrograms per liter established by the U.S. Environmental Protection Agency for drinking water. Arsenic and iron concentrations decreased when water was recharged through recharge basins or a trench; however, chemical precipitation and potential biofouling eventually may decrease the artificial recharge efficiency through basins and trenches.

At the Sedgwick site, chloride concentrations infrequently exceeded regulatory criteria. Large concentrations of atrazine were treated to decrease concentrations to less than regulatory criteria. Recharge of treated stream water through recharge basins avoids potentially large concentrations of arsenic and iron that exist at the Halstead diversion site.

Results from a simple mixing model using chloride as a tracer indicated that the water chemistry in shallow monitoring well located adjacent to the Little Arkansas River was 80 percent of stream water, demonstrating effective recharge of the alluvial aquifer by the stream. Results also indicated that about 25 percent of the water chemistry of the diversion well water was from the shallow part of the aquifer. Additionally, diverting water through a diversion well located adjacent to the stream removed about 75 percent of the atrazine, probably through sorption to aquifer sediment, and decreased the need for additional water treatment to remove atrazine.

A flow and solute-transport model was developed using water-level and chloride concentration data to simulate and better evaluate the quantity of stream-water flow to the pumping diversion well. Simulation results indicate that chloride concentrations in the diversion well are dependent on the chloride distribution in the aquifer during the first few years of pumping. About 75 percent of the water in the diversion well originates from stream water after long-term continuous pumping for decades.

A theoretical geochemical model was developed to simulate the effects of artificially recharging fully oxygenated, treated stream water, by injection into the aquifer. Results indicate that chemical precipitation of calcite and iron oxyhydroxide are likely to occur and the potential increase of iron bacteria may combine to cause reduced efficiency of injection wells.

Contents

Abstract
Introduction

Purpose and Scope
Previous Studies

Description of Study Area

Halstead Recharge System
Sedgwick Recharge System

Methods

Data Collection and Analysis

Water-Quality Data
Continuous Data

Statistical and Mixing Models
Flow and Solute-Transport Model
Geochemical Analysis and Simulations

Geochemical Effects of Induced Stream-Water Recharge at the Halstead Diversion Well Site

Major Ion Chemistry and Tracers of Stream Water
Selected Trace Metal Chemistry
Geochemical Simulations
Stream-Water Mixing in Adjacent Aquifer and Solute-Transport Model

Hydrologic Conditions
Simple Mixing

Flow and Solute-Transport Model

Model Definitions and Flow Simulations
Solute Transport

Geochemical Effects of Artificial Recharge at Halstead Recharge Site

Major Ion Chemistry and Tracers of Recharge Water
Selected Trace Metal Chemistry
Geochemical Simulations

Geochemical Effects of Artificial Recharge at Sedgwick Recharge Site

Major Ion Chemistry and Tracers of Recharge Water
Selected Trace Metal Chemistry
Geochemical Simulations

Summary and Conclusions
References Cited

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