GEOCHEMISTRY OF THREE CONTINUOUS CORES ALONG THE NORTHERN SHORE OF LAKE OKEECHOBEE: INSIGHTS INTO ARSENIC, MERCURY, AND MOLYBDENUM CONCENTRATIONS WITHIN POTENTIAL AQUIFER STORAGE AND RECOVERY ZONES

Aquifer storage and recovery (ASR) wells north of Lake Okeechobee 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. Approximately 3,900 feet (1.19 km) of continuous core from three different locations have been recovered north of Lake Okeechobee. Using handheld X-ray fluorescence, two of the cores (L63N & C38S) were analyzed at approximately every foot to determine the concentrations of metals and other elements. Geochemistry of the third core (L63S) is ongoing. Within these cores several flow zones were identified. These flow zones are located in the Upper Floridan Aquifer (UFAFZ) and, lower in the cores, in the Avon Park Permeable Zone (APPZ). Understanding the concentrations of metals within the matrix rock in these flow zones will guide decision making on where to store water in these ASR wells.

The confining unit in all three wells (Hawthorn Gr.) has high Si and Al, indictive of terrestrial detritus in this clay-rich unit. P can also be high in the confining unit, and in the Suwannee Limestone. Mg is high in the confining unit, suggestive of dolomite rhombs in clay; and, does not become significant again until the dolomitic Avon Park Fm. A number of organic layers occur throughout the cores. These organic layers have some of the highest As, Hg, and transition metal concentrations in the cores. However, only a small number occur in potential ASR flow zones. An ash layer at 1466 feet in core L63N appears to be volcanic in origin, and could be a potential marker bed in the Eocene Avon Park Fm.

Hg in the UFAFZ is generally low, < 20 ppm. However, in one core (L63S) it approached 100 ppm near the base of the UFAFZ. Hg in the APPZ in core C38S is < 40 ppm, but is up to 100 ppm in core L63N. As is also low in the UFAFZ, less than 20 ppm. However, in core L63S it approaching 35 ppm near the top of the UFAFZ. As is higher in the APPZ in both C38S and L63N, and can range over 1000 ppm. Mo ranges between 100 and 1000 ppm in the upper parts of the UFAFZ in two cores. But, it is not as elevated in core C38S. Mo is higher in the APPZ in cores C38S and L63N, and can get higher than 1000 ppm in a few places. Data from the APPZ in core L63S has not been measured as of this time.