Graduation Year

2004

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Pichler, Thomas.

Keywords

phosphate, mining, pyrite, aquifer, trace metal, Florida

Abstract

In order to understand the mineralogical association and distribution of arsenic (As) in the Hawthorn Group in central Florida, I examined in detail the chemical and mineralogical composition of 361 samples that were collected from 16 cores. Geochemical analyses were performed by hydride generation - atomic fluorescence spectrometry (HG-AFS) and inductively coupled plasma optical emission spectrometry (ICP-OES). The identification of discrete minerals was aided by scanning electron microscopy (SEM) and chemical compositions were obtained by electron-probe microanalyses (EMPA). Arsenic concentrations for all Hawthorn samples vary from 0.1 to 69.0 ppm with a mean of 5.6 ppm. Average As concentrations for the individual units of the Hawthorn Group vary significantly from 9.0 ppm in the Peace River Formation to 3.0 ppm in the Tampa Member of the Arcadia Formation.

This detailed mineralogical and geochemical study demonstrates that: (1) Arsenic in the Hawthorn Group varies from the formation to formation and is primarily concentrated in trace minerals, such as pyrite; (2) Concentrations of arsenic in pyrite crystals can vary drastically from a minimum of 0 ppm to a maximum of 8260 ppm; (3) Pyrite occurs in framboidal and euhedral forms and is unevenly distributed throughout the Hawthorn Group; (4) Hydrous ferric oxides can contain up to 540 ppm of arsenic; (5) Phosphate and organic material, and clays contain lower arsenic concentrations that pyrite and hydrous ferric oxides; and (6) Arsenic, sulfur, and iron have lognormal distribution throughout the Hawthorn Group. This study is important because phosphorous, arsenic and sulfur are chemically closely related, and thus they co-occur posing a potential problem for the phosphate industry.

Information about the concentration, distribution and mineralogical association of naturally occurring arsenic is essential to forecast its behavior during anthropogenically induced physico-chemical changes in the aquifer. Recently, aquifer storage and recovery (ASR) facilities in west-central Florida reported arsenic concentrations in excess of 100 micro g/L (100 ppb) in recovered water. The ASR storage zone in this area is in the Suwannee Limestone, which directly underlies the Hawthorn sediments. It is crucial to the future of ASR in this area to understand the source, distribution, and cycling of arsenic in the overlying Hawthorn Group and the Florida Platform.

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