Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Clare Robinson

Abstract

Waves are known to influence the flux of pollutants to coastal waters via groundwater discharge. This study combines field measurements with numerical groundwater modelling to evaluate the influence of a period of intensified wave conditions (wave event) on nearshore groundwater flows and geochemistry in a sandy freshwater beach. Comprehensive vertical nested pressure transducer data obtained over a 2.5 day isolated wave event reveal the development of transient groundwater flow recirculations through the nearshore aquifer combined with enhanced water exchange across the sediment-water interface (i.e., beach face). The wave-induced groundwater flows were simulated in FEFLOW using a phase-averaged wave setup approach to represent waves acting on the sediment-water interface. The time-varying measured and simulated hydraulic gradients match well indicating that consideration of wave setup alone, rather than instantaneous (phase-resolved) wave effects, is able to adequately capture wave-induced perturbations in nearshore groundwater flows. Additionally, the impact of the wave-induced groundwater flows on geochemical conditions near the sediment-water interface is illustrated by redox and pH fluctuations over the wave event. The observed fluctuations may considerably impact the fate of reactive pollutants discharging and also recirculating through a nearshore aquifer.

In addition to the importance of the phase-averaged effect of waves, the phase-resolved effects of waves also need to be understood to better predict the fate of chemical and microbial constituents close to the sediment-water interface. Additional high frequency field measurements indicate that the phase-resolved effects of waves account for considerably larger fluxes of water and associated constituents exchanged across the sediment-water interface than the phase-averaged flux. Despite the large magnitude, the flux generated by phase-resolved wave effects is rapidly reversing in direction and thus is not expected to considerably influence dissolved constituents due to its short residence time, estimated to be between < 1 and 70 seconds. However, the exchange of particulates may be important as their transport is strongly governed by attachment and detachment processes. It is proposed that the rapid water exchange processes near the sediment-water interface control the fate and transport of particulate organic matter and fecal bacteria and thus are important for regulating nutrient and microbial coastal water quality.

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