Future inundation of coastal on-site wastewater treatment systems in a region with pronounced sea-level rise.

• SEAWAT is used to model coastal groundwater sensitivity to climate change. • Groundwater inundates up to 27% of OWTS in modelled climate change scenario. • Saltwater-freshwater interface movement depends on climate change scenario. • Recommendations for climate-resilient septic system guidelines are proposed. An often-overlooked impact of rising sea levels is the concurrent rise in coastal groundwater tables, which can result in subsurface inundation of below-ground infrastructure. This is an important consideration for rural coastal communities, many of which rely on on-site wastewater treatment systems (OWTS). Subsurface inundation and flooding of OWTS leads to treatment performance issues, potentially resulting in enhanced contaminant transport to coastal groundwater resources and the ocean via submarine groundwater discharge. We use SEAWAT to develop a variable-density groundwater flow model for the groundwater system underlying a densely populated rural community on the north shore of Nova Scotia, Canada. We then impose various climate change scenarios (sea-level rise, changes in recharge) to evaluate the risk of OWTS inundation from rising groundwater. We find that under current conditions as many as 9 % of OWTS in this small but densely populated watershed are either inundated or completely flooded. This number that could grow to 27 % of OWTS for the climate change scenario with the highest recharge and sea-level rise. As a secondary objective, we track the location of the modeled saltwater-freshwater interface, and 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. We also use the modelling results to develop adaptation recommendations for OWTS installation regulations. 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. [ABSTRACT FROM AUTHOR]