pXRF Geochemical Analyses of Continuous Cores C38S, C59, L63N and L63S: Implications for the Lake Okeechobee, Florida, Watershed Restoration Aquifer Storage and Recovery Project

Aquifer storage and recovery (ASR) wells along northern Lake Okeechobee, Florida, have been proposed as part of the Comprehensive Everglades Restoration Plan. However, researchers have linked elevated concentrations of As, Hg and transition metals in groundwater because of ASR in carbonate rocks. This study presents portable X-ray fluorescence (pXRF) analyses of four continuous cores (C38S, C59, L63N, and L63S) recovered from the Floridan Aquifer System north of Lake Okeechobee, Florida. A total of 5078 pXRF measurements were collected at approximately 30.5 cm along the four cores. Measurements along the cores included the Intermediate Confining Unit, Upper Floridan Aquifer (UFA), Avon Park Permeable Zones (APPZ), and portions of the Lower Floridan Aquifer (LFA). Emphasis was placed on the concentrations of mercury (Hg), arsenic (As), molybdenum (Mo), chromium (Cr), and nickel (Ni), which have been identified as elements of concern during ASR operations. Major-element trends in the cores often reflect lithologic changes in the Floridan Aquifer System. This includes decreases in Ca and Sr and increases in Mg associated with dolomitization in the Avon Park Formation. Trace-metal concentrations vary systematically within hydrostratigraphic units. Mercury commonly exhibits elevated concentrations near the base of the UFA, whereas Mo is often enriched near the top of the UFA. Elevated concentrations of Hg, As, Mo, Ni, and other trace metals frequently occur at hydrostratigraphic boundaries, formation contacts, clay-rich intervals, and especially within organic-rich layers. Organic layers contained metal concentrations several times greater than surrounding carbonate rocks and hosted the highest measured concentrations of Hg, As, and Mo in the cores. The paucity of organic layers and generally lower metal concentrations in the UFA suggest it may represent a favorable zone for ASR storage. In contrast, the APPZ and LFA contain numerous metal-enriched organic layers which may increase the potential for trace metal mobilization during ASR operations. These results suggest pXRF characterization provides an effective tool for geochemical characterization of storage zones and evaluating water-quality risks potentially associated with ASR development.