Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor(s)

Clare Robinson

Abstract

Groundwater discharge may be an important pathway for delivering pollutants into nearshore waters of large inland lakes, including the Great Lakes and Lake Simcoe in Southern Ontario, Canada. This pathway however is poorly understood and quantified. While field methods for evaluating groundwater discharge to surface waters in tributary and marine settings have been widely applied and are well tested, there are limited field methods available for evaluating groundwater discharge to large inland lakes, particularly at the regional scale (i.e. 5-100 km). The objective of this thesis was to evaluate suitable field methods for quantifying groundwater discharge into large inland lakes at different spatial scales (regional- and local-scale), and to evaluate the spatial distribution and magnitude of groundwater discharge along shorelines in Nottawasaga Bay, Lake Huron and Lake Simcoe. A combination of the field methods was first evaluated for quantifying groundwater discharge along a 17 km stretch of shoreline in Nottawasaga Bay near the Township of Tiny. Regional-scale radon (222Rn) and electrical resistivity tomography (ERT) surveys were initially conducted along the entire shoreline to identify potential hotspots areas with higher direct groundwater discharge. From a management perspective, identification of direct groundwater discharge hotspots is needed so that water quality management and mitigation efforts aimed at reducing groundwater pollution inputs can target these areas. Following identification of a potential direct groundwater discharge hotspot area, a higher-resolution 222Rn survey in this area. Data from this survey indicated that groundwater discharge is highest close to the shoreline with discharge decreasing offshore. A steady-state mass balance model which considers the various sources and sinks of 222Rn from the coastal water column was applied to estimate groundwater discharge rates based on in-lake 222Rn concentrations. Six beach sites along the 17 km stretch of shoreline in Nottawasaga Bay were characterized more closely with shore-normal transects of groundwater wells installed to determine the groundwater flux towards the lake. Detailed vertical temperature and hydraulic gradient profiles were also collected at some sites to characterize local-scale groundwater discharge patterns. While this work showed the successful application of 222Rn for evaluating nearshore groundwater discharge to large inland lakes, the use of local-scale methods including vertical temperature and hydraulic gradient methods was challenging due to cobble-rock substrate which prevented manual installation of equipment in the nearshore lake bed. Regional-scale 222Rn surveys were subsequently performed in Lake Simcoe to identify groundwater discharge hotspots along 80 km of shoreline. Two potential direct groundwater discharge hotspot areas were identified, as well as two areas where indirect groundwater discharge (i.e. groundwater discharge to creeks which then flows into the lake) may affect the nearshore lake water quality. High-resolution surveys conducted in the groundwater discharge hotspot areas, with data indicating that groundwater discharge in these areas is higher in the nearshore and decreases offshore. Groundwater level measurements along shore-normal transects at six locations along the surveyed shoreline were used validate the 222Rn survey results. Considerable temporal variability was observed for the in-lake 222Rn concentrations with preliminary analysis indicating this variability is due to varying wind speed as well as precipitation. Better understanding of factors contributing to the temporal variability in 222Rn concentrations is needed for more accurate interpretation of the regional-scale 222Rn survey data. The combination of measurement techniques explored in this thesis provides characterization of nearshore groundwater discharge to large inland lakes at multiple scales. This characterization as well as evaluation of appropriate field methods is needed to develop more effective and targeted management plans to mitigate the contribution of groundwater discharge to nearshore waters.


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