Date of Award

2006

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Andrew S. Reeve

Second Committee Member

Martin G. Yates

Third Committee Member

Robert G. Marvinney

Abstract

High mean arsenic concentrations up 26.6 ¹mol/L (1990 ¹g/L) occur in ground water within a watershed at Kelly's Cove, Northport, Maine, USA. The Kelly's Cove watershed is a fractured-bedrock system composed of sul¯dic schist with granitic to dioritic intrusions. Arsenic is enriched in these rocks up to 1050 mg kg¡1 (average: 68 mg kg¡1). The distribution of arsenic in the bedrock appears to be controlled by the presence of arsenopyrite and arsenian pyrite, that occur primarily in post-metamorphic, tourmaline + quartz § carbonate veins and the Kelly's Cove granite. Based on the metamorphic signature of the tourmaline chemistry and the similarity in ±34S values of the sulfides, these veins probably derived from hydrothermal remobilization of surrounding metamorphic rock. Chemical analyses of water from 35 bedrock wells throughout the watershed reveal spatial clustering of wells with high arsenic concentrations. Stiff diagrams and box plots distinguish three distinct water types; calcium bicarbonate-dominated water with low arsenic concentrations (CaHCO3 type), sodium bicarbonate-dominated water with moderately high arsenic concentrations (NaHCO3 type), and calcium bicarbonate-dominated water with very high arsenic concentrations (High-As type). Di®erences in recharge area, ground-water evolution, and possibly bedrock composition contribute to the chemical distinctions within the watershed's ground water. Lack of correlation of aqueous arsenic concentrations with pH indicates that desorption of arsenic is an insignificant control on arsenic concentration. Correlations of aqueous arsenic concentrations with increasing Fe(II)/Fe(III) and decreasing Eh indicates that reductive dissolution of ferric oxyhydroxides plays a role in the occurrence of high arsenic concentrations in the NaHCO3 and High-As type water. Ground water with high arsenic concentrations contains sulfate with enriched sulfur and oxygen isotopes. The range of ±34S[SO4] and ±18O[SO4] values at the Kelly's Cove watershed (+3.4 to +4.9 ?? and -2.01 to +6.72 ?? , respectively) are strikingly similar to that of another Maine watershed (+3.7 to +4.6 ?? and -2.56 to +7.47 ?? , respectively), that has different oxidizing ground-water conditions. The association of high arsenic concentrations and high ±18O[SO4] is not due to oxidizing conditions or reduction of sulfate, but may be related to paleo-aeration of iron oxyhydroxides that are now reducing and releasing arsenic.

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