Abundance and mineralogical association of arsenic in the Suwannee Limestone (Florida): Implications for arsenic release during water–rock interaction

Abstract

Arsenic (As) is being released from the Suwannee Limestone, Upper Floridan aquifer, during aquifer storage and recovery (ASR) cycle testing with concentrations of up to 130 μg/L in the recovered water. To determine the abundance and mineralogical association of As in the Suwannee Limestone matrix we have conducted a detailed mineralogical and geochemical study of 306 core samples. In addition to random sampling of core material, we also collected “targeted” samples of core material that contained organic material, hydrous ferric oxide, pyrite, phosphate minerals and clays. Bulk As concentrations were determined by acid digestion followed by hydride generation-atomic fluorescence spectrometry (HG-AFS). Specific mineral phases were analyzed using scanning electron microscopy (SEM) and electron-probe microanalyses (EPMA). The average As concentration for all 306 samples is 3.5 ppm, but only 1.7 ppm after exclusion of the “targeted” samples. The average for the “targeted” samples is 9.5 ppm. The detailed lithologic, mineralogical, and geochemical study of As in the Upper Floridan aquifer, Suwannee Limestone shows that: (1) Arsenic is present in the Suwannee Limestone in low concentrations, but is concentrated in minor mineral phases, particularly pseudo-framboidal pyrite. (2) Pyrite is generally As-rich and can contain concentrations between 100 and 11,200 ppm As (average 2300 ppm, n = 25). (3) Compared to pyrite, other trace minerals contain much less As. (4) Pyrite is ubiquitous throughout the Suwannee Limestone, but is most abundant in high porosity zones. (5) Previously suggested hydrous ferric oxide, clay minerals and apatite are not an important source of As. This study also provided some insight into the question whether the interaction (reaction) of water with an aquifer matrix that contains only a few part per million As is capable of producing high-As concentrations in groundwater. Our findings indicate that it can be relatively easy to generate a high-As groundwater despite a low bulk As concentration in the aquifer matrix. A change in physico-chemical conditions that selectively affects the stability of As-bearing minerals is sufficient to increase As concentrations by several orders of magnitude.