Date of Award

Spring 1992

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Program/Concentration

Geology

Committee Director

Joseph H. Rule

Committee Member

Carl Koch

Committee Member

Dennis A. Darby

Committee Member

Gary Speiran

Call Number for Print

Special Collections LD4331.G4M32

Abstract

Thirty-seven Early cretaceous sediment samples of the Potomac Formation were obtained from four cores to determine if fluoride bearing minerals are responsible for elevated fluoride concentrations in Potomac Formation groundwaters of southeastern Virginia. Investigative methods included total sediment fluoride and particle size analysis, semiquantitative mineralogical analysis (achieved by X-Ray Diffraction) and statistical methods (cluster, canonical and regression analysis). The sediments showed wide variation in textures, ranging from clean sands and gravels to silty clays. Sediment mineralogies were dependent on the prevailing size fraction, with quartz and feldspar dominating the sand and silt fractions, muscovite primarily being found in the silt fraction, and the clay minerals kaolinite/chlorite, illite and smectite being predominant in the clay fraction. Minor amounts of siderite, garnet and pyroxene were quantified in the sand fraction. Sediment fluorides occur in highest concentrations in the silt and clay size fractions, most likely in the phyllosilicates muscovite and smectite. The presence of trace amounts (not quantifiable) of humite, chondrodite, lepidolite, and other minerals suggest that these fluvio-continental Early cretaceous sediments were derived from a metamorphic environment similar to the Piedmont.

Static leaching experiments suggest that sediment fluoride is more soluble in sodium bicarbonate waters than in sodium chloride waters. This trend is generally supported by regional hydrochemical data, in which higher average fluoride concentrations occur in sodium bicarbonate waters than in chloride waters. Fluoride concentrations are highest in sodium bicarbonate water with a pH of 7.9 to 8.6, but are lower at pHs outside of this range. Groundwater fluoride concentrations decrease eastward as the water flows from the sodium bicarbonate hydrochemical facies into the sodium chloride hydrochemical facies, suggesting that the sediments of the former facies might act as a fluoride source, and sediments in the latter hydrochemical facies may act as a fluoride sink. However, statistical results do not support this characterization, implying that only very small amounts of fluoride are released/absorbed by the sediments over time.

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DOI

10.25777/6b2p-1167

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