CLIMATE CHANGE AND ITS INFLUENCE ON GROUNDWATER LEVELS IN COASTAL NORTH CAROLINA: IMPLICATIONS FOR ONSITE WASTEWATER TREATMENT

Sea level rise and coastal storms can impact the resilience of coastal communities due to their effects on increased flooding and land loss, saltwater intrusion, wetland loss/change, and impacts to local infrastructure. This talk will focus on the influences of rising groundwater levels on decentralized wastewater infrastructure. In North Carolina, there are approximately 1 million onsite wastewater treatment systems (OWTSs or septic systems) draining to the coast. These systems rely on unsaturated soils to treat waste and reduce the water quality impacts to surrounding waters. Rising groundwater tables can reduce the vadose zone thickness and a system’s capability to effectively treat waste. Recent research indicates groundwater levels are rising in the surficial aquifer along the coast. These changes have been attributed to factors such as: sea level rise; coastal storms; wastewater disposal; and land-use change. Groundwater level data (1983-2021) from 8 NC Department of Environmental Quality surficial aquifer wells in Dare Co. were analyzed to evaluate the temporal variations in groundwater levels. Five additional wells were instrumented to evaluate groundwater interactions with OWTSs in 2019-2020. Groundwater level data were compared with NOAA sea level rise data collected at Duck, North Carolina since the 1970s. The results revealed that groundwater level rise is occurring and positively correlated with sea level rise. This change in surficial aquifer groundwater levels has resulted in a decline in the unsaturated soils available for onsite wastewater treatment. Current regulations in North Carolina require a 45cm vertical separation distance between the drainfield and seasonal high water table for sandy soils. Based on the relationship between land surface elevation and groundwater levels, conventional OWTS at land surface elevations of < ~2.6m above sea level were likely to experience inadequate vertical separation and potential groundwater inundation that would reduce the effectiveness of the OWTS. Current regulatory approaches assume relatively static groundwater depth conditions. Future regulatory approaches in coastal communities should consider rising groundwater tables and their effects on septic systems to ensure adequate wastewater treatment and protection of public health.