About 85% of Cape Cod residents—and 25% of Americans nationwide—rely on septic systems to process their wastewater. So how well do septic systems fare at removing chemicals such as pharmaceuticals, consumer products, and flame retardants? Silent Spring Institute researchers recently compiled the most comprehensive dataset on contaminants of emerging concern (CECs) discharged from septic systems. Using these data, the Institute modeled the amount of CECs that ends up in groundwater that feeds Cape Cod drinking water wells and coastal waters. The results raise concerns about potential health effects from exposure to trace chemicals.
CECs are frequently detected in U.S. rivers, streams, and drinking water supplies, but no regulations and few voluntary guidelines have been developed. Previous studies by Silent Spring Institute on Cape Cod were among the first to measure CECs in public drinking water wells, private wells, groundwater, and freshwater ponds. To investigate the contribution of septic systems, Silent Spring researchers compiled concentration data for 34 emerging contaminants from 16 published studies, and intensely investigated nine of the chemicals.
The team found that septic systems do well at removing some chemicals. For example, more than 99% of acetaminophen (commonly sold as Tylenol) and caffeine is removed. Other CECs were handled less efficiently, however. On the whole, less than half of an anticonvulsant drug, carbamazepine; the antibiotic sulfamethoxazole; and a flame retardant, TCEP, were removed by septic systems. Silent Spring Institute has previously detected all three chemicals in Cape Cod drinking water. Systems that are failing due to age or lack of maintenance may remove less chemicals overall.
The study adds important information as Cape Cod officials debate new wastewater management options to address nutrient pollution, which causes damaging algal blooms in local waters. The study found, for example, that upgrading septic systems to sewers may not directly address CECs. Treated water from septic system and sewer processing plants contained similar levels of CECs, and the chlorination process used to disinfect sewage also causes additional harmful chemicals to form.
The findings suggest that officials can best protect drinking water and human health at the same time as reducing nutrient runoff by restricting septic systems from discharging into areas that supply local drinking water wells. Officials should also make plans to divert wastewater from sewered areas away from zones that supply drinking water.
The Commonwealth of Massachusetts provided support for this study.