Hell and High Water: Diminished Septic System Performance in Coastal Regions Due to Climate Change

Climate change may affect the ability of soil-based onsite wastewater treatment systems (OWTS) to treat wastewater in coastal regions of the Northeastern United States. Higher temperatures and water tables can affect treatment by reducing the volume of unsaturated soil and oxygen available for treatment, which may result in greater transport of pathogens, nutrients, and biochemical oxygen demand (BOD5) to groundwater, jeopardizing public and aquatic ecosystem health. The soil treatment area (STA) of an OWTS removes contaminants as wastewater percolates through the soil. Conventional STAs receive wastewater from the septic tank, with infiltration occurring deeper in the soil profile. In contrast, shallow narrow STAs receive pre-treated wastewater that infiltrates higher in the soil profile, which may make them more resilient to climate change. We used intact soil mesocosms to quantify the water quality functions of a conventional and two types of shallow narrow STAs under present climate (PC; 20°C) and climate change (CC; 25°C, 30 cm elevation in water table). Significantly greater removal of BOD5 was observed under CC for all STA types. Phosphorus removal decreased significantly from 75% (PC) to 66% (CC) in the conventional STA, and from 100% to 71–72% in shallow narrow STAs. No fecal coliform bacteria (FCB) were released under PC, whereas up to 17 and 20 CFU 100 mL-1 were released in conventional and shallow narrow STAs, respectively, under CC. Total N removal increased from 14% (PC) to 19% (CC) in the conventional STA, but decreased in shallow narrow STAs, from 6–7% to less than 3.0%. Differences in removal of FCB and total N were not significant. Leaching of N in excess of inputs was also observed in shallow narrow STAs under CC. Our results indicate that climate change can affect contaminant removal from wastewater, with effects dependent on the contaminant and STA type.气候变化可能会影响美国东北部沿海地区基于土壤的原位污水处理系统（OWTS）的污水净化效能。较高的气温与地下水位会通过缩减非饱和土壤体积与处理所需氧气供应量干扰污水处理过程，进而导致更多病原体、营养物质及生化需氧量（BOD5）向地下水迁移，危及公众健康与水生生态系统安全。OWTS的土壤处理区（STA）可在污水渗滤过土壤的过程中去除污染物。传统型土壤处理区接收来自化粪池的污水，其污水渗滤位置位于土壤剖面较深处。与之相对，浅窄型土壤处理区接收经预处理的污水，其污水渗滤位置处于土壤剖面更浅层，这或许使其对气候变化具备更强的适应性。本研究采用原状土壤中宇宙模拟系统（intact soil mesocosms），量化了当前气候（PC；20℃）与气候变化情景（CC；25℃，地下水位抬升30cm）下，1种传统型与2种浅窄型土壤处理区的水质净化功能。所有类型的土壤处理区在气候变化情景下的BOD5去除率均显著提升。传统型土壤处理区的磷去除率从当前气候下的75%显著下降至气候变化情景下的66%，而浅窄型土壤处理区的磷去除率则从100%降至71%~72%。当前气候条件下未检测到粪大肠菌群（FCB）释放，而在气候变化情景下，传统型与浅窄型土壤处理区的粪大肠菌群释放量分别可达每100毫升17和20个菌落形成单位。传统型土壤处理区的总氮去除率从当前气候下的14%提升至19%，但浅窄型土壤处理区的总氮去除率则从6%~7%降至不足3.0%。粪大肠菌群与总氮的去除率差异并不显著。在气候变化情景下的浅窄型土壤处理区中，还观测到了氮素淋溶量超出输入量的现象。本研究结果表明，气候变化可对污水中污染物的去除效果产生影响，且该影响因污染物类型与土壤处理区类型而异。