Diminished Wastewater Treatment: Evaluation of Septic System Performance Under a Climate Change Scenario.

The effects of climate change are expected to reduce the ability of soil-based onsite wastewater treatment systems (OWTS) to treat domestic wastewater. In the northeastern U.S., the projected increase in atmospheric temperature, elevation of water tables from rising sea levels, and heightened precipitation will reduce the volume of unsaturated soil and oxygen available for treatment. Incomplete removal of contaminants may lead to transport of pathogens, nutrients, and biochemical oxygen demand (BOD₅) to groundwater, increasing the risk to public health and likelihood of eutrophying aquatic ecosystems. Shallow narrow drainfield OWTS, which include pre-treatment steps and provide unsaturated drainfields of greater volume relative to conventional OWTS, are expected to be more resilient to climate change. We used intact soil mesocosms to quantify water quality functions for two advanced shallow narrow drainfield types and a conventional drainfield under a current climate scenario and a moderate climate change scenario of 30 cm rise in water table and 5°C increase in soil temperature. While no fecal coliform bacteria (FCB) was released under the current climate scenario, up to 18 CFU FCB/mL (conventional) and up to 20 CFU FCB/mL (shallow narrow) were released under the climate change scenario. Total P removal rates dropped from 100% to 66% (conventional) and 71% (shallow narrow) under the climate change scenario. Total N removal increased from 14% to 19% under climate change scenario in the conventional, but dropped from 5.6% to less than 0% in the shallow narrow under the climate change scenario, with additional leaching of N in excess of inputs indicating release of previously held N. No significant difference was observed between scenarios for BOD removal. The data indicate that all three drainfield types experience some diminished treatment capacity.