A Detailed Geochemical Investigation for the Mineralogic Associations of Arsenic and Antimony within the Avon Park Formation, Central Florida: Implications for Aquifer Storage and Recovery

The mineralogic associations of arsenic and antimony were investigated for the Avon Park Formation in central Florida to determine its viability for aquifer storage and recovery (ASR). Rock samples were taken from fourteen different cores, and samples were taken according to core length and based on areas suspected to contain high levels of arsenic and antimony, such as molds, dissolution fractures, pyrite, clays, and organic matter. Targeted samples were also taken from high permeability zones. In total 373 samples were described in hand specimen and analyzed for bulk rock chemistry. Inductively coupled plasma optical emission spectrometry and atomic fluorescence spectrometry were used to determine Fe, Al, Si, Mg, Ca, S, P, Mn, Sr, As, and Sb in bulk samples, while electron-probe microanalysis was used to analyze Sb, As, Fe, S, Zn, Ca, Mo in discrete minerals.

The mineralogic and geochemical investigation showed: (1) bulk As and Sb concentrations within the Avon Park are low; (2) As values ranged from <0.1 to 30.8 mg/kg and Sb from <0.1 to 6.76 mg/kg; (3) average values were 2.07 mg/kg As and 0.26 mg/kg Sb; (4) pyrite was present as framboids, hollow framboid rings, small irregularly shaped pieces with topography, and euhedral crystals; it was typically associated with oxidized laminations, fractures, and molds; (5) the framboid rings may have been precipitated by a bacteria making them biogenic; (6) euhedral pyrites contained significantly less As than other morphologies; (7) individual pyrites ranged from <0.1 to 1.43 wt% As while Sb ranged from <0.1 to .44 wt%; (8) the samples from zones of high permeability had lower As and Sb averages than those of the other samples: 0.98 mg/kg As and 0.18 mg/kg Sb versus 1.32 mg/kg As and 0.19 mg/kg Sb for interval and 2.9 mg/kg As and 0.34 mg/kg Sb for targeted samples.