CLIMATE CHANGE AND RISING WATER TABLES IN LOW-LYING COASTAL SOUTH CAROLINA

Rising sea levels and increasing rainfall intensity can contribute to a rising water table, which can cause flooding and threaten subsurface infrastructure like septic systems in low-lying coastal areas. To assess this threat, fifteen shallow monitoring wells were installed in Beaufort County, South Carolina, where septic systems are an important component of wastewater treatment. The loggers in the wells recorded water levels every fifteen minutes beginning in May 2022. An open-source Python package, pastas groundwater, was used to model the hydraulic head in each well to estimate rates of impairment of septic systems over 34 years. Rainfall and tidal data were downloaded from the Beaufort MCAS station of the South Carolina state climatology division and NOAA’s Fort Pulaski station, respectively.

The results of nine wells that were not flooded during the observation period are presented in this study. For all but one of the wells, the water table rose by about 1 m during a period of increased rainfall between August and September, after which it returned to the “baseline,” which we defined for each well as low water levels that persisted for at least 25 days. In the remaining well, the water table rose by about 0.5 m. Baseline hydraulic heads were consistent with 1D analytic solutions that show that baseline water levels increase with distance from the shoreline. The amplitude of tidal fluctuations in the wells also decreased with distance from the shoreline, consistent with analytical solutions.

The machine-learning pastas model, when calibrated to the field data, confirmed annual average variabilities of about 1 m over 34 years. This range is high enough to impact the septic systems in at least about 80% of the wells. Simple conceptual models suggest the risk of impairment should be greatest near the shoreline, but we found no trend in the risk with distance from the shoreline. Instead, we found a clear trend with the accommodation space, which we defined as the difference between the surface elevation and the water level baseline. This simple relationship allows point data from wells to be extrapolated in a GIS framework to create spatial maps of risk.

The results of this study can help civil engineers and coastal managers plan for likely challenges that may arise due to rising groundwater tables in coastal areas, especially in the design of septic systems.