Department or Program

Geology

Abstract

As cities across the United States have urbanized, the amount of impervious surface (pavements, rooftops, cement, etc.) has risen steeply. Increases in imperviousness impact natural hydrologic processes in a watershed and can lead to a decrease in water quality (high sediment loads and low dissolved oxygen levels). In Lewiston, Maine, the Hart Brook watershed has 22% impervious surface cover (ISC) and is classified as an urban impaired watershed due to dissolved oxygen levels below 75% saturation and excessive nutrient loads during the peak of summer. Wastewater infrastructure has been surveyed and repaired in some locations, but very limited research has addressed sedimentation patterns. The purpose of this study is to evaluate the timing and extent of urbanization and its impact on sediment dynamics and water quality. Satellite images were analyzed using data from the National Land Cover Dataset to re-estimate total impervious surface cover within the Hart Brook watershed at 32.87%. A separate analysis of historical imagery was performed to estimate change in ISC over time within the watershed and found a 400% increase between 1953 and 1998. Additionally, a 150 cm sediment core was collected from a central floodplain and analyzed for grain size, %C, magnetic susceptibility, and lead concentrations. A geochemical age model using [Pb] and 14C was used to constrain sedimentation rate over time. Three predominant theories are proposed to explain sedimentation rates in the watershed. A modern sedimentation model, based on normalized lead data and a 14C date of 1950+ at 74cm, showed a rapid, minimum sedimentation rate of 2.24 (cm/yr) from 1923 to 1972 and a rate of 0.85 (cm/yr) from 1972 to 2017. A Holocene sedimentation model, based on raw lead data and the nature of the sediments, implied a sedimentation rate of 0.48 cm/yr between 1923 and 1972, and 0.85 cm/yr between 1972 and 2017. Finally, a historical model showed a sedimentation rate of 0.48 cm/yr between 1923 and 1972, and 0.85 cm/yr between 1972 and 2017, but was also extrapolated to the bottom of the 150cm core or the year 1781 using an average of the two rates, 6.4cm/yr.

Level of Access

Open Access

First Advisor

Johnson, Beverly

Date of Graduation

5-2018

Degree Name

Bachelor of Arts

Number of Pages

73

Components of Thesis

1 pdf

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