Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Dr. Clare Robinson

Abstract

This thesis presents field measurements and numerical modeling that provide insight into the nearshore geochemical conditions and groundwater flows controlling the mobility of arsenic (As) in a freshwater beach aquifer and its potential discharge to Lake Erie. Field measurements were performed via shore-normal monitoring transects installed at beaches (Little Beach and Main Beach) located adjacent to a brownfield industrial harbour site that has elevated sediment and groundwater As concentrations. Detailed pore water chemistry analyses revealed elevated As (up to 0.056 mg/L) 1 - 2 m below the shoreline at all transect locations. The distributions of species in the aqueous and sedimentary phases suggest that As mobility is strongly linked with iron (Fe) redox cycling. Sediment analysis by sequential extraction revealed a layer of amorphous and crystalline Fe (hydr)oxides present at the sediment-water interface (SWI) near the shoreline. This Fe hydr(oxide) layer may be accumulating As and preventing its release to nearshore waters. Numerical modeling combined with vertical hydraulic gradient measurements indicated that wave-induced recirculation across the aquifer-lake interface was significant and this likely establishes the redox gradient that led to Fe (hydr)oxides precipitation at the SWI. Numerical and field results showed that the water infiltration/exfiltration across the groundwater-lake interface were sensitive to varying wave intensity and seasonal lake water level fluctuations. The source of As in the nearshore beach aquifers remains unknown. While Little Beach is adjacent to the East Headland of the industrial site where elevated As has been recorded, Main Beach is disconnected hydraulically from the East Headland. If the elevated dissolved As observed is from a natural geogenic source, this findings of this thesis may have widespread implications for As cycling in the nearshore areas of the Great Lakes. Finally, while this study focused on As, the nearshore geochemistry and subsurface flows investigated are be pertinent to understanding the discharge of other chemicals (e.g., nitrate, ammonium, phosphorous) to nearshore inland coastal waters via the groundwater pathway.

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