Date of Award

Fall 9-11-2015

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Department

Civil Engineering

Advisor

Aria Amirbahman

Second Committee Member

Stephen Norton

Third Committee Member

Holly Ewing

Abstract

Lake Auburn, Maine, U.S.A. is a historically oligotrophic lake serving as the primary drinking water source to a population of 40,000. In 2011 and 2012 the local water district observed decreasing water quality, evidenced by increasing epilimnetic total phosphorus (P) concentrations, increasing lake turbidity and the appearance of cyanobacteria. These conditions are especially concerning due to Lake Auburn’s exemption from filtration by the EPA. The decline in water quality has been linked to widespread hypolimnetic summer anoxia, causing internal P loading from the lake sediment. This study evaluated the chemical P speciation in hypolimnetic sediment from Lake Auburn following the Psenner method for sequential chemical extractions to measure P associated with reducible Fe hydroxide (bicarbonate-dithionite extraction), Al hydroxide and organic matter (NaOH extraction), and acid-soluble species (HCl extraction). Results show that Lake Auburn sediments are high in reducible Fe hydroxide (107 to 1267 µmol g-1) and relatively low in Al hydroxide (77 to 242 µmol g-1). Analyses of longer sediment cores (~60 cm) identified concentrated Fe and P in the surficial sediment (0-2 cm). Previous research has shown sediments with molar Al hydroxide:reducible Fe ratios < 3 and Al hydroxide:reducible Fe-bound P < 25 to act as a source of P under anoxic conditions. Lake Auburn sediment exhibits molar Al:reducible-Fe ratios between 0.2 and 1.7 and molar Al:reducible Fe-bound P ratios between 2.0 and 14.5, indicating a risk for internal P loading. An investigation of water chemistry sampled in transect along Lake Auburn tributaries showed that Al removal and dilution in upstream lakes and wetlands paired with seasonal Fe export from upstream wetlands is causing low Al and high Fe concentrations in tributary streams and likely impacting sediment chemistry, resulting in vulnerability to eutrophication.

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