Preliminary Evidence That Rising Groundwater Tables Threaten Coastal Septic Systems

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Many communities along the southern Rhode Island coast rely on onsite wastewater treatment systems (OWTS), known as septic systems, to treat and disperse wastewater. System design requires sufficient vertical separation distance between the bottom of the drainfield and the seasonal high groundwater table to ensure an adequate volume of unsaturated soil to treat wastewater before it reaches the groundwater. Based on depth to groundwater table data submitted to the state regulatory agency as part of OWTS permit applications, groundwater tables along the southern Rhode Island coast have been rising at a rate of 14 mm/year since 1964. Communities where potable water is imported have greater rates of rising groundwater tables, up to 17 mm/year. From a mass balance perspective, precipitation, human wastewater inputs, and sea level rise represent the major factors elevating coastal groundwater tables, whereas evapotranspiration, discharge of groundwater to coastal waterbodies, and drinking water extraction are the major components lowering groundwater tables. As water inputs continue to change in the coming decades, rising groundwater tables may reduce OWTS functionality, threatening coastal drinking water aquifers and coastal ecosystems with nutrient and pathogen pollution.

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Journal of Sustainable Water in the Built Environment