Date of Award

Winter 12-21-2018

Level of Access

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

Ivan Fernandez

Abstract

This study explores the sources and mechanisms of phosphorus (P) mobilization during base flow within the Amsden Brook watershed, Fort Fairfield, Maine, USA. Amsden Brook is an agriculturally dominated watershed drained by a spring-fed and perennial first- to second-order stream. We characterized the P concentrations within the watershed to investigate connections between soils, stream sediment, surface water, and groundwater. Waters were monitored monthly during the 2017 snow-free period for temperature, pH, dissolved oxygen, conductivity, soluble reactive P (SRP), total P, strong acid anions, strong base cations, dissolved organic carbon (DOC), Al, Fe, and Mn. Phosphorus speciation within soils and sediment samples was determined by sequential chemical extractions. Soil samples were also analyzed by the Maine Soil Testing Service after being subject to a Modified Morgan nutrient extraction, the conventional method for agricultural soil testing in Maine.

The emerging groundwater was under-saturated by up to 40% with respect to O2, with pH = 7.24, T = 7.0 C, and SRP = 3.0 μg L-1. Groundwater PCO2 was 35x ambient PCO2 (410 ppm). Degassing of CO2 from the emerging groundwater resulted in a significant increase in pH downstream, and an increase in the SRP concentration from 3.0 to a maximum of 40.6 μg L-1.

The composition of stream sediment and surface soils from various landscape positions were compared and showed that agricultural soils and sediment had a similar composition, while surface soils from forested slope buffers were different.

Laboratory experiments using homogenized stream sediment identified a reduction in the P adsorption capacity, and an increase in the desorption of native P with increasing solution pH from 7.25 (emerging groundwater) to 8.50 (air-equilibrated surface water). These data allowed us to identify the pH-dependent desorption from P- laden sediment, sourced from eroded agricultural soils, as the most significant source of dissolved P in Amsden Brook under base flow conditions.

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