APPLICATION OF BENCH-SCALE LEACHING STUDIES TO PREDICT HYDROGEOCHEMICAL PROCESSES DURING AQUIFER RECHARGE

Mobilization of As and other metals during artificial recharge into Upper Floridan Aquifer System carbonates is a cause for concern. During initial recharge, oxygen-rich surface waters displace native, reduced ground-water within permeable storage zones. Water-quality in the aquifer is controlled by changes in physio-chemical conditions that affect water-rock interactions and mineral stability. As a result, naturally-occurring As may be released into stored water with concentrations exceeding 150 ug/L. A principal mechanism for the release of As and associated metals (e.g., Ni, Co, V) is oxidation of arsenian pyrite, which occurs locally in trace amounts in the aquifer matrix. Additional forms of As identified by sequential extraction studies include soluble As and As bound to carbonates, oxides and organics.

Bench-scale leaching studies have been employed to assess metals mobilization during simulated recharge and recovery using aquifer core chips (Eocene Avon Park Formation) and both deionized/distilled and source (recharge) waters. Each leaching apparatus, which contains 1000ml of water and 300g of core chips, is designed to minimize evaporation, allow measurement of physical parameters (pH, DO, etc.), and vary DO concentrations through aeration or N2 (gas) saturation. Cycle test simulations involve a water change every month. Leachate samples are collected for trace metal analyses. In addition, whole-rock analyses of major, trace and rare-earth elements, as well as microprobe studies provide characterization aquifer matrix compositions.

Results of three simulated cycle tests confirm that 1) the experimental water-rock ratio yields leachate concentrations comparable to levels observed in field studies; 2) DO saturation causes an increase in the release of As; and 3) successive cycles result in a step-wise decrease in mobilized metal concentrations. The leaching studies also demonstrate that low-DO (< 0.8 mg/L) conditions exhibit increasing As concentrations with time. This ongoing research also includes microprobe element mapping of pre- and post-leached thin sections. Results of these bench-scale leaching and extraction studies will hopefully prove to be a cost-efficient technique for predicting hydrogeochemical conditions and processes at the field scale.