COASTAL COMMUNITY WOES: SEPTIC SYSTEM DRAINFIELDS ARE THREATENED BY ELEVATED GROUNDWATER TABLES

Many coastal communities along the U.S. eastern seaboard rely on individual onsite wastewater treatment (i.e. septic) systems to treat and disperse household wastewater. Regulations for systems in coastal areas specify the minimum distance between drainfield components and the seasonal high water table, to ensure there is sufficient unsaturated soil for adequate wastewater treatment. Historical groundwater table data, as well as modeling, indicates that coastal groundwater tables along the southern New England coast are rising, raising questions about whether current in-place drainfields installed decades ago are becoming compromised. Using long-term shallow groundwater monitoring wells and ground-penetrating radar (GPR) surveys of 10 coastal drainfields in southern Rhode Island USA, we investigated whether systems have adequate distance to groundwater below the drainfield. We hypothesized that older systems would be compromised to greater extents than more recently installed systems. Our results indicate that only 20% of systems are not impaired by elevated groundwater tables, while 40% of systems are experiencing inadequate distances between drainfield components and the groundwater table at least 50% of the time. Additionally, 30% of systems in this study do not meet minimum distance requirements between the drainfield and groundwater table at any time of the year. Neither age of system nor distance to tidal water body appears to predict which systems are likely to be compromised. Inaccurate methods of seasonal high groundwater table determination may explain these findings, suggesting that changes in the permitting process for coastal systems may help protect coastal drinking and surface water resources.