Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Robinson, Clare E.

2nd Supervisor

James, Roy W.

Affiliation

Environment and Climate Change Canada

Co-Supervisor

Abstract

Fresh surface waters globally are threatened by eutrophication caused by elevated nutrients, particularly phosphorus (P), loads. P that has accumulated in landscapes over decades, referred to as legacy P, makes it challenging to reduce contemporary P loads and to make long-term water quality predictions. This thesis focuses on understanding geochemical and hydrological controls on P availability from different legacy stores and its potential delivery to surface waters. The specific objectives are to i) investigate factors influencing spatial and temporal variability in soluble reactive P (SRP) concentrations in the shallow aquifer of an alluvial riparian zone, ii) explore the potential for stream pH changes to influence retention and release of SRP from streambed sediments, and iii) evaluate long-term P loads to nearshore lake waters from decommissioned septic systems. These objectives were addressed through a combination of field investigations, numerical modeling, and data analysis.

For the first objective, field investigations conducted over a 19-month period illustrate the complexity of SRP retention and release processes in a shallow riparian aquifer. While SRP concentrations were generally low, high SRP concentrations were intermittently observed that generally could not be linked to factors known to control SRP release. The data highlight the need for caution in interpreting SRP inputs to streams based on discrete groundwater sampling performed at locations not proximal to a stream. This is because SRP released in a riparian aquifer may be rapidly re-sequestered. For the second objective, data analyses indicate that stream pH changes at multiple temporal scales (decadal, seasonal, diurnal, hydrological events) may mediate the release and retention of SRP from streambed sediments. The findings highlight the need to reconsider the role of stream pH changes in future models and analyses of stream P dynamics. For the third objective, field and numerical investigations show that groundwater P plumes from decommissioned septic systems are persistent and may contribute P loads to lakes for long periods (thousands of years) after decommissioning. Overall, the thesis provides important insights into factors controlling legacy P and its potential discharge to surface waters. Recognizing the importance of legacy P is critical for developing effective water quality management programs and establishing realistic expectations regarding the timescale for P load reductions.

Summary for Lay Audience

Phosphorus (P) pollution in streams and lakes is a major environmental problem that can lead to harmful algal blooms and deterioration of aquatic ecosystems. Common sources of P pollution include agricultural fertilizers and manure, and human wastewater. Over many decades, P has accumulated in landscapes including in agricultural soils, streambed sediments, and groundwater aquifers. This accumulated P, which is referred to as legacy P, can make it challenging to reduce current P inputs to surface waters and to predict surface water quality trends. The overall objective of this thesis is to better understand factors that may influence the release of legacy P from areas in landscapes where it can accumulate including riparian zones, streambed sediments, and groundwater systems.

The first objective of the thesis investigated factors that influence the retention and movement of soluble reactive phosphorus (SRP) in a riparian zone which is the area of land between a stream and an agricultural field. Field data showed while SRP concentrations were mostly low, high concentrations were sometimes observed. These high concentrations could not be linked to factors commonly known to control SRP movement. The data suggest that caution is needed in analyzing SRP concentrations in the shallow aquifer of riparian zones and predicting associated SRP inputs to the adjacent stream. The second objective of the thesis demonstrated that pH changes occurring in streams over decades, seasons, daily cycles, and during hydrological events may affect the release and retention of P from streambed sediments to streams. These findings suggest the influences of stream pH changes should be considered in models used to understand P concentrations in streams. For the third objective, combined field monitoring and computer simulations showed that high P from historical non-active septic systems can persist in groundwater aquifers and contribute P inputs to lakes over long time periods (e.g. thousands of years). This study highlights the importance of considering non-active septic systems in calculations of P inputs to surface waters. Overall, this thesis provides important insights into factors that control legacy P. Understanding the importance of legacy P is needed to develop effective water quality management plans and set realistic expectations on the time scales over which P loads to surface waters may decrease.

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