LOW-LYING COASTAL COMMUNITIES AS FUTURE WETLANDS

Increases in rainfall and sea level rise can lead to rising groundwater tables. In low-lying coastal areas, this can cause flooding and impair septic systems. Thus, as in wetlands, knowledge of the height of the water table relative to the land surface is critical. 15 shallow monitoring wells were installed in Beaufort, South Carolina, where septic systems play a vital role in wastewater treatment. Loggers in these wells recorded water levels at 15-minute intervals from May 2022 to August 2023. Pastas groundwater was used to simulate water levels in each well to compute the risk of impairment of septic systems over 34 years. Tidal and rainfall data were downloaded from NOAA’s Ft Pulaski and Beaufort MCAS stations, respectively. Models were calibrated to 13 months of field data. Heavy rainfall events recorded at Beaufort MCAS station in July-August 2023 were not included because they did not affect the whole of the study area.

We present results from 12 wells that were not inundated during the monitoring period. During significant rainfall events between August and September 2022, water levels rose by nearly 1 m and receded to baseline afterward. We defined baseline as the common water levels when we did not have much rainfall. Baseline water levels were consistent with 1D analytic solutions which show a positive non-linear relationship with the distance from the shoreline. Models calibrated to the field observations and then run using 34 years of historical rainfall and tidal data confirm an interannual variability of about 1 m over 34 years, consistent with sparse well data available from nearby SC DNR well records.

Models showed significant annual variability impacting septic systems at least twice a year in most wells, with rising frequency linked to predicted rainfall intensity under climate change scenarios. Water levels in 83% of wells did not rise further after 1.5°C rainfall scenario due to topographic limitation. Contrarily, 17% exhibited flux limitation causing about 10% rise in groundwater levels for 2°C and 4°C scenarios. Risk of impairment did not correlate with the distance from the shoreline but correlated with accommodation space, defined as the difference between water levels and land surface. This simple relationship makes it possible to generate a risk map by extrapolating point data from wells in a GIS framework.