GEOCHEMICAL ANALYSIS OF A CONTINUOUS CORE: LAKE OKEECHOBEE WATERSHED RESTORATION PROJECT AQUIFER STORAGE AND RECOVERY WELL L-63N, FLORIDA

Aquifer storage and recovery (ASR) wells have been proposed as part of the Comprehensive Everglades Restoration Plan. However, numerous researchers have linked elevated concentrations of chalcophile elements in carbonate rocks to high levels of As, U, Pb, and transition metals in groundwater as a result of ASR. A continuous core of approximately 1500 feet (457.2 m) was drilled north of Lake Okeechobee along canal L-63N as an ARS site. The recovered core begins at a depth of 500 feet (152.4 m) in the confining Hawthorn Group, transects the Upper Floridan Aquifer, the Avon Park permeable zones, and ends at 2000 feet (609.6 m) below the surface. A portable X-ray fluorescence (pXRF) was used to analyze the 1500 foot core approximately every foot. After data quality control, 1338 pXRF analyses of the core were recorded.

The confining unit of the Hawthorn Group has an average Si:Al of 5.98; Rb:Sr of 52; and higher Fe, P, K, and Ti then the underlying carbonate aquifers. The Upper Floridan aquifer (UFA) within the Suwanee and Ocala Limestones has an average Mg:Ca of 0.10; Rb:Sr of 0.006; As of 7 ppm; Mo of 107 ppm; and Hg of 8 ppm. The Avon Park Permeable Zone no. 1 (APPZ1), within the Avon Park Formation, has an average Mg:Ca of 0.72; Rb:Sr of 0.011; As of 215 ppm; Mo of 175 ppm; and Hg of 9 ppm. The Avon Park Permeable Zone no. 2 (APPZ2) has an average Mg:Ca of 0.80; Rb:Sr of 0.013; As of 403 ppm; Mo of 310 ppm; and Hg of 10 ppm. An ash-rich layer was located within the semi-confining unit between APPZ1 and APPZ2 at around 1466 feet. This ash layer has a marked increase in Si, Al, S, Sr, and Mo, and a decrease in Mg and Ca, compared to the dolostones above and below it.

The Si:Al, Rb:Sr, Fe, P, K, and Ti suggest a larger terrestrial input into the confining unit of the Hawthorn Group than the carbonate rocks below. The lower Rb:Sr of the UFA, APPZ1, and APPZ2 suggest they formed in a continental margin setting. The increase of the Mg:Ca from the UFA to the APPZ is indicative of the compositional change from limestone in the UFA to dolostone in the APPZ. The lower As, Mo, and Hg in the UFA suggests this aquifer is a preferable storage unit for ASR than the APPZ1 or APPZ2. The ash layer at 1466 feet is a potential marker bed in the Eocene Avon Park Formation and may be a result of distal volcanic activity or debris fallout from the Chesapeake Bay meteor impact.