Effects of mixing and geochemical reactions on the water quality of an aquifer storage and recovery project at Myrtle Beach, South Carolina

An aquifer storage and recovery (ASR) test was carried out at Myrtle Beach, S.C., in 1992 to identify and quantify geochemical processes that controlled the quality of the stored water. Potable water was injected into the Black Creek aquifer and stored to accommodate peak water-demand periods. Although the recovered water met drinking water quality standards, its chemical makeup was different from that of the injected water.;Chemical analysis of water samples and the code NETPATH were used to simulate the geochemical system by applying an inverse approach. Solutions to the mass balance requirements were not unique and required validation with mineralogic information.;Three distinct zones of reaction were distinguished outward from the well. In the aerobic zone, pyrite and organic-matter oxidation (bacterial mediated) near the well caused an increase in sulfate and ferrous iron and an initial decrease of pH. In the anaerobic zone, calcite dissolution and calcium-sodium exchange decreased calcium but increased sodium, alkalinity, and pH. Iron, from pyrite oxidation, exchanged with sodium and decreased in solution. Fluoride, from anion exchange with apatite hydroxyl ions, increased in solution. Several reactions involving aluminum silicate phases were identified, but their mass transfer contributions were orders of magnitude smaller than from the other reactions. Beyond the anaerobic zone, changes in water quality resulted from mixing with ambient aquifer water.;Alkalinity and pH increases lessened the aggressiveness of the potable water, but sodium and fluoride increases, in the mixing zone, limited the volume of recoverable water. These changes, however, became less significant with each injection-recovery cycle.