GROUNDWATER INUNDATION OF COASTAL ONSITE WASTEWATER TREATMENT SYSTEMS: INVESTIGATING PRESENT AND FUTURE IMPACTS TO COASTAL WATERS

Abstract

Coastal groundwater is a critical freshwater resource that supports human life, while coastal surface water supports both aquatic life and coastal industries (e.g., tourism, aquaculture). The quality of these critical groundwater and surface water resources is threatened by anthropogenic perturbations such as increased groundwater extraction and pollution, as well as climate change forcing such as sea-level rise. Onsite wastewater treatment systems (OWTS), which are common for wastewater disposal, can become inundated by rising groundwater tables as a result of rising sea levels, resulting in decreased performance of the OWTS. Contaminants that are not attenuated before reaching the elevated groundwater table can migrate through the coastal aquifer and be delivered to coastal waters via submarine groundwater discharge. In this thesis, we use field techniques (piezometers, seepage meters, radon analysis) to characterize the groundwater flow and submarine groundwater discharge at a popular public beach in Nova Scotia, Canada that is lined with cottages using OWTS. Fieldwork is combined with water quality sampling of coastal surface water and submarine groundwater discharge, with a focus on comparing the effectiveness of a novel viral tracer of human fecal contamination (crAssphage) with classic bacterial indicators. The effects of climate change (changing recharge and sea-level rise) on groundwater table elevations and the saltwater-freshwater interface across the same study site are assessed using a coupled groundwater flow and solute (salt) transport model, SEAWAT.

Increased use of OWTS during the summer cottage season at the study site coincided with widespread detections of crAssphage in submarine groundwater discharge (4/4 samples) and coastal surface waters (3/8 samples). Conversely, classical fecal pollution indicators based on bacterial targets were sparsely detected in the samples in the coastal environment (2/12 E. coli samples, 0/12 HF183 samples), likely due to greater attenuation of bacterial contaminants within the subsurface environment. Results from this first application of crAssphage in coastal groundwater contribute to a growing body of research reporting the application of this emerging tracer in various environments impacted by sewage pollution sources.

Results from the SEAWAT modelling indicate that as many as 9% of OWTS in this small but densely populated coastal watershed are either inundated or completely flooded by groundwater. This number could grow to 27% of OWTS for the climate change scenario with the highest recharge and sea-level rise. The location of the modeled saltwater-freshwater interface was also tracked in the model to investigate the potential salinization of groundwater resources used for drinking water supply. The modeled interface moved landward by ≤20 m and proved to be less of a concern than OWTS inundation, except for shoreline dwellings. This research contributes to an increasing number of groundwater modelling studies focusing on the impacts of sea-level rise on coastal subsurface infrastructure and provides important insight for rural coastal communities reliant on OWTS and fresh groundwater for drinking water.

The results of this thesis are used to locate at-risk OWTS across the province, and areas of high OWTS use, high potential sea-level rise, and low elevation are highlighted. These communities in particular should consider the implications of climate change for OWTS vulnerability, but also consider employing novel tracers such as crAssphage to provide early detection of low levels of surface water contamination from OWTS.