NATURALLY OCCURRING ARSENIC IN THE UPPER FLORIDAN AQUIFER, SOUTHWEST FLORIDA: IMPLICATIONS FOR AQUIFER STORAGE RECOVERY

摘要

Aquifer storage and recovery (ASR) facilities have become the storage solution of choice for many communities worldwide. The Southwest Florida Water Management District (SWFWMD) has recently determined that ASR is a critical component for its 20-year strategy to enhance water supply in southwest Florida where, as of January 2002, 26 facilities are in operation and 19 are permitted for construction. The Comprehensive Everglades Restoration Project (CERP) calls for over 330 ASR wells to store over 1.7 billion gallons of water per day. The future of ASR in Florida, however, is unclear because of the discovery of elevated arsenic (As) levels during recovery cycle testing of several ASR facilities throughout southwest Florida. Concentrations are usually highest during the first recharge-recovery cycles. Concentrations of up to 100 ppb were reached in one of four wells recently undergoing testing in the City of Tampa. Here we report our preliminary findings for arsenic concentrations in the Suwannee Limestone, the ASR storage zone for most ASR systems in southwest Florida. This is the first study in a series that address the arsenic issue during ASR During this study core from 22 wells collected by the Regional Observation Monitor-well Program (ROMP), a division of the SWFWMD, was sampled to gain a better understanding of the arsenic distribution and mineralogy within the Suwannee Limestone. Possible sources for the arsenic include: 1) sulfide minerals, 2) iron oxyhydroxide coatings, 3) organic material, and/or 4) phosphate nodules. Optical microscope descriptions of more than 300 Suwannee Limestone core samples show only little iron oxyhydroxide, while sulfide minerals in the form of framboidal pyrite were abundant. Total arsenic concentrations were determined by hydride generation-atomic fluorescence spectrometry (HG-AFS), which showed much higher values for arsenic in the Suwannee Limestone than previously reported. In addition, the distribution of arsenic throughout the Suwannee Limestone was very heterogeneous. Samples with concentrations in excess of 20 ppm arsenic were chosen for analysis by SEM with the intent to further constrain the mineralogical arsenic association in each sample. We found that framboidal pyrite was again more abundant than expected, being present in more than 50% of the SEM samples. Research is continuing at the University of South Florida to further constrain the mineralogy of the arsenic in question. The next step will be to examine thin sections with apparatus more sensitive than SEM, such as TEM, Microprobe, or SIMS.