ERDC/EL TR-06-8, Geochemical Models of Water-Quality Changes During Aquifer Storage Recovery (ASR) Cycle Tests, Phase I: Geochemical Models Using Existing Data

Geochemical models were developed using existing water-quality data sets from three permitted, potable-water Aquifer Storage Recovery (ASR) systems in south Florida. All three systems store and recover water in different permeable zones of the upper Floridan Aquifer System (FAS). At the Olga ASR system, water is stored in the Suwannee Limestone; at the North Reservoir ASR system, water is stored in the Arcadia Formation of the lower Hawthorn Group. Both sites are located in Lee County, along the southwest Gulf Coast of Florida. At the Eastern Hillsboro ASR system, water is stored in the basal Hawthorn unit; this system is located in Palm Beach County near the southeastern Atlantic Coast of Florida. The objectives of this study are to use geochemical modeling methods to simulate 1) mixing between native water of the upper FAS and recharge water during cycle testing; 2) geochemical reactions that occur during the storage phase of cycle tests in different lithologies; and 3) controls on arsenic transport and fate during ASR cycle testing. Existing cycle test data sets were developed for permitting purposes, not research; therefore, concentrations of some major dissolved constituents are estimated. Quantitative uncertainty that resulted from the use of incomplete water-quality datasets is defined for these geochemical models. Mixing of recharge and native groundwater end members during cycle testing is simulated using chloride as a conservative tracer. Mixing models show that low-chloride groundwater mixes to different extents during recharge in the Arcadia Formation and Suwannee Limestone. At the North Reservoir ASR system (Arcadia Formation), recharge water is transported as plug flow, as shown by sigmoid-shaped breakthrough curves in monitor wells, and chloride trends that resemble conservative mixing lines. In contrast, at Olga ASR system, recharge water is affected by hydraulic factors because breakthrough curves at the monitor well are not sigmoidal, and chloride trends deviate from conservative mixing curves. Data were insufficient to simulate mixing at the Eastern Hillsboro ASR system. Inverse geochemical models quantified phase mole-transfer between water and rock, which controls water quality during the storage phase of a cycle test. The greatest phase mole-transfer values resulted from reactions of iron and sulfur at the Olga and North Reservoir ASR systems. Specifcially, these reactions included pyrite oxidation with subsequent iron oxyhydroxide precipitation, and sulfate reduction with hydrogen sulfide production. These reactions should proceed in a sequence, not simultaneously, and suggest that the redox evolution of the storage zone exerts a significant influence on stored water quality. Arsenic mobility is a major challenge to ASR feasibility, so inverse geochemical models were developed to simulate redox conditions that facilitate arsenic mobility during ASR cycle testing. Trends in arsenic concentrations measured at ASR and monitor wells, along with additional water-quality data, arsenic speciation analyses, and bulk chemistry and major mineralogy in core samples from the Arcadia Formation and Suwannee Limestone constrain these models. The stability of iron oxyhydroxide phases changes as the storage zones evolve from oxic (during recharge) to sulfatereducing (during storage and recovery). Because iron oxyhydroxide is an effective sorption surface for arsenic, the stability of this mineral is an important control. The onset of sulfate-reducing conditions causes reductive dissolution of iron oxyhydroxide, with subsequent release of sorbed arsenic. The instability of iron oxyhydroxide during recovery is supported by inverse geochemical models at Olga and North Reservoir ASR systems. However, phase mole-transfer values are small (micromoles/kilogram water), and it is unclear if this mass of iron is sufficient for effective arsenic sequestration. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR.