INCREASED COASTAL POLLUTION EXPECTED UNDER FUTURE SEA LEVEL STANDS: CHEMICAL EVIDENCE FOR TIDAL GROUNDWATER INUNDATION OF COASTAL WASTEWATER INFRASTRUCTURE (Invited Presentation)

Climate change-driven sea level rise (SLR) is increasingly stressing and decreasing the efficacy of coastal wastewater infrastructure due to its vulnerability to flooding by tidal groundwater inundation. This can result in flooded sewer mains and onsite sewage disposal systems (OSDS), lowering the efficiency of treatment and increasing the potential for wastewater leaching into groundwater and the coastal ocean. To investigate the potential impact of SLR on coastal wastewater infrastructure, we conducted a field-based study along the coast and in storm drains in Honolulu, Hawaiʻi using perigean spring tides as a proxy for future sea level stands. Geochemical tracers such as radon and salinity were used to monitor the timing and provide evidence of groundwater inundation with respect to tides. In addition, pharmaceuticals and dissolved nutrients were sampled at low and high tide to trace potential wastewater pollution associated with tidal inundation. This study focused on two possible pathways that provide evidence of wastewater leakage from tidally induced groundwater inundation: 1) leakage into storm drains during extreme tidal ranges and 2) leakage into groundwater and subsequent discharge into the coastal ocean. Coincident with increases in salinity, radon concentrations in storm drains increased by 4 to 200 times at high tide compared to low tide, indicating tidally driven groundwater inundation. In addition, multiple wastewater tracers were also detected in different stages of the tide at various locations. For coastal sites, both groundwater and wastewater tracers were detected; suggesting that neighboring OSDS units may be compromised, which will only worsen at higher sea level stands. Nearly 60% of all studied locations (n= 4) showed an increase in concentration in one or more pharmaceutical compounds, indicating that wastewater leakage into storm drains or directly to the coastal ocean is already a prevalent problem. This study demonstrates the potential impact of SLR on water quality and presents comparisons between tides, locations, and mechanisms of coastal flooding.