Aerobic Treatment Units for Widespread Onsite Wastewater Treatment in Coastal Louisiana: Pollution, Policy, and Market-Based Solutions.

Onsite wastewater treatment systems are known to be sources of aquatic pollution; however, scarce data usually prevent system-level assessments. Aerobic treatment units (ATUs) are widely utilized in Louisiana, where groundwater and soil conditions limit septic tanks. By combining an ATU permitting record with housing unit build data, we estimate there were 412,552 permitted ATUs in Louisiana by the end of 2023. We conservatively estimate the annual surface water loading from ATUs in the 24 coastal parishes is 7.5 million pounds of nitrogen and 2.2 million pounds of phosphorus, which are equivalent to 48% and 84%, respectively, of all the nitrogen and phosphorus discharged by major wastewater treatment plants in Louisiana. Despite the use of ATUs for 73.7% of new homes built from 1990-2016, our analysis of policy documents indicates that ATU management in the two coastal parishes with the highest number of ATUs is best described as "basic" with limited compliance monitoring, enforcement, and public awareness. Simultaneously, we estimate that the deployment of Environmental Impact Bonds premised on nutrient recovery and optimized energy consumption could be sufficient to fund routine ATU inspection and maintenance programs. Our findings strongly suggest that the onsite wastewater treatment status quo jeopardizes water quality at scale, demanding the pursuit of engineering and policy solutions.