Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Robinson, Clare

2nd Supervisor

Roy, Jim

Affiliation

Environment and Climate Change Canada

Co-Supervisor

Abstract

Agricultural practices can cause excess inputs of soluble reactive phosphorus (SRP) to freshwater catchments leading to eutrophication. This study evaluates spatio-temporal variability of SRP in an agricultural streambed including the influence of groundwater-stream interactions, and examines the associated geochemical and hydrological controls. High porewater SRP (>1000 µg/L) and high SRP sediment fractions were observed in low groundwater discharge zones, specifically in the shallow (≤ 0.1 m) streambed sediments. Porewater SRP concentrations were linked to iron redox cycling with SRP concentrations concordant with more reducing conditions. Porewater SRP varied temporally but distinct seasonal changes were not observed. The conditions in the shallow streambed sediments switched from less-reducing to more-reducing conditions between sampling times with SRP release to the stream more likely when the shallow streambed was more reducing. The study findings are needed to improve water quality predictions and management practices aimed at reducing P stream loads.

Summary for Lay Audience

Agriculture practices introduce large amounts of excess nutrients (including phosphorus (P)) into the environment. This is an issue as high amounts of P lead to harmful algae blooms in surface waters including Lake Erie. Algae blooms can have negative impacts on the environment, economy, and human health. In recent years, actions have been taken to try to reduce the amount of P that accumulates in the environment, but so far these actions have been largely unsuccessful. “Legacy P stores” – large build-ups of P that occur over many years in the environment such as in soils and streambed sediments – may be one reason why excess P inputs persist. Unfortunately, our understanding of how P moves in the environment is incomplete. Specifically, we need to better understand how P may be trapped or released from streambed sediments, as these sediments act as an important legacy P storage zone. This study aims to assess which conditions – seasonal, chemical, or geological – allow for P to be released from agriculture streambed sediments.

This study collected water and streambed sediment samples from a 40 m stream reach located in Kintore Creek, Thames watershed, Ontario, Canada, to examine what influences SRP mobility in streambed sediments. The main study findings include (i) high SRP concentrations were found in the streambed particularly in areas of low sediment permeability; (ii) high SRP concentrations were located in the shallow (≤ 0.1 m deep) sediments; (iii) SRP retention and release from the streambed sediment was closely linked with redox conditions; (iv) seasonal changes in the flow of water into and out of the streambed may, depending on the direction, either cause SRP to accumulate or to be released from the shallow streambed sediments. The study findings are needed to improve water quality predictions and to guide management practices implemented to address high P storage in streambed sediments.

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