Chemical changes in the injected water and native ground water during artificial recharge through a well have an important bearing on the success or failure of recharge-well operation. Water stability and changes in chemical quality of the water that may be caused by changed environment should be examined critically when contemplating recharge. In this study the principal chemical changes observed that may cause clogging of the recharge well and aquifer were a change in calcium carbonate saturation of the injected and native water, whereby the calcium carbonate precipitated; precipitation of iron when reducing and oxidizing waters are mixed; ion exchange and clay dispersion; and changes in water stability caused by water treatment. The native ground water at the test site was stable with respect to its environment. However, the injected water tended to pick up carbon dioxide as it moved through the aquifer. Native ground water would readjust to the changed environment upon reentry into the zone of carbon dioxide depletion causing deposition of calcium carbonate. The Eh-pH relationship of the native ground water and recharge water could cause iron to be precipitated as ferric hydroxide at the interface of the ground water and injected water in the aquifer. Incomplete removal of the injected water would tend to cause an accumulation of precipitated iron in the aquifer, eventually plugging a large volume of material. The native ground water averaged about 25 percent sodium and in the injection supply averaged about 45 percent. Surface water injected into the aquifer would would displace the native ground water and increase the abundance of monovalent cations surrounding aquifer materials, thus creating favorable conditions for ion exchange. However, most of the aquifer materials are chemically inert, and clay dispersal caused by ion exchange probably was not a serious clogging factor. The chemical treatment of surface water by coagulation with alum and chlorination with chlorine gas caused most of the changes in chemical quality. Although the treated water may have been stable with respect to its aluminum content in the settling canal, mixing with the native ground water in the aquifer could alter the pH and alkalinity of the two solutions. This would cause postinjection flocculation of residual aluminum and subsequent clogging of the aquifer. Heavy chlorination of the injected water also may cause clogging owing to precipitation of iron and other metals. Dissolved iron and organic matter are easily oxidized by chlorine. Iron precipitated in this manner would tend to lower aquifer perEl E2 ARTIFICIAL RECHARGE, GRAND PRAIRIE REGION meability, and oxidation of organic matter would release gases, leading to air entrainment. Well redevelopment is an important phase in water injection and may provide the means for economical and practical operation of a recharge well. INTRODUCTION PURPOSE OF THIS REPORT In 1953 the Grand Prairie region of Arkansas was selected for an investigation of fundamental principles of recharging ground-water reservoirs through wells. The investigation consisted of collecting detailed hydrogeologic information in the vicinity of the recharge site, drilling two recharge wells, constructing water-treatment and conveyance facilities, and making a series of tests by injecting water into the two wells. This report, one of a series covering separate elements of the project, was prepared as the data and interpretative analyses became available. Its purpose is to present information about the geochemical changes observed in the native ground water, injected water, and aquifer during recharge tests; potential recharge problems caused by chemical phenomena; and the results of redevelopment tests made on the recharge wells. LOCATION OF THE AREA The Grand Prairie region in east-central Arkansas is in the Mississippi Alluvial Plain section of the Coastal Plain. The region is an irregular but continuous tract of prairie lying between the White River and Bayou Meto. It extends northwestward, from near the confluence of the White and Arkansas Rivers to a short distance beyond Lonoke in Lonoke County. Nearly all of Arkansas County and parts of Lonoke, Prairie, and Monroe Counties are included in the Grand Prairie region. (See fig. 1.) The Grand Prairie includes approximately 1,000 square miles, of which roughly 700 square miles or 450,000 acres is developed riceland. The project area includes about 210 square miles of land in Arkansas County (fig. 2). The Rice Branch Experiment Station of the University of Arkansas is near the center of the area and is the site of the artificial-recharge tests. PROJECT LAYOUT A brief description of the physical layout of equipment and control points is needed to understand the geochemical aspects of the study. Samples of water for chemical analysis were collected from one or more observation wells at different distances from the recharge wells. Samples of water were collected also from different points along the water-treatment system. The layout of the project is shown in figures 3 and 4. GEOCHEMICAL ASPECTS OF ARTIFICIAL RECHARGE E3