Date of Award

2023

Degree Type

Thesis

Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)

Specialization

Environmental and Earth Sciences

Department

Natural Resources Science

First Advisor

Alissa Cox

Abstract

Clean water is integral to human and ecosystem health, and effective wastewater treatment is a critical part of protecting clean water. In regions of the United States where homes are not connected to sewers and centralized wastewater treatment plants, wastewater is treated on site using septic systems. This thesis focuses on two types of pollution from septic systems: nitrogen and per and ploy fluoroalkyl substances (PFAS). The study area for this project, Charlestown, Rhode Island, is in a densely developed coastal zone that is vulnerable to nitrogen pollution because of proximity to coastal ecosystems, like brackish coastal lagoons (salt ponds), and because all homes are reliant on groundwater for drinking water. Nitrogen is a pollutant of concern for both drinking water and marine ecosystems. PFAS compounds are a class of organic chemicals that have been linked to a variety of human health concerns, are found in wastewater from centralized wastewater treatment plants, and are beginning to be regulated in drinking water.

The first manuscript presents the nitrogen reducing performance of an experimental nitrogen removing septic system and an existing, proprietary nitrogen removing technology. The experimental system incorporates layers of sand and sand mixed with sawdust in the drainfield (i.e. soil treatment area) and is called a Layered Soil Treatment Area (LSTA). We collected monthly influent and effluent samples from four LSTA systems installed in Charlestown, RI from July 2022 to April 2023. We also sampled 14 Advantex AX-20 nitrogen-removing septic systems installed within the same watershed. Using standard lab analyses we measured concentrations of nitrogen species (nitrate, ammonium, total nitrogen), and other wastewater parameters (BOD5, TSS, DO). Mean nitrogen concentration in the LSTA effluent between July 2022 and April 2023 was 5.40 mg/L, compared to 22.6 mg/L in proprietary nitrogen removing technologies sampled monthly during summer 2022 and once in November 2022 and April 2023. Although the different sampling periods prevent a direct comparison about whether LSTA outperform the AX-20 in this study, the mean concentration of total N (5.40 mg/L) we observed in the LSTA is lower than long term total N concentrations in AX-20s in other studies. We coupled the concentration data with flow estimates from all septic systems in our study watershed and found that, if installed where appropriate, the LSTA systems could reduce the mass nitrogen load to our study watershed by 78%, at a fraction of the cost of connecting the watershed to a centralized wastewater treatment plant.

Manuscript two investigates the occurrence of PFAS compounds in a variety of septic systems in the same watershed PFAS have not been studied extensively in onsite wastewater treatment, and many regions, like our study area, that use septic systems also rely on groundwater for clean drinking water, so understanding occurrence of PFAS in septic systems is critical to protecting human health. We collected samples from 19 septic systems in a coastal watershed in Charlestown, RI, representing a variety of individual types of septic systems commonly used in the region. Adapting EPA method 8372 for analysis of 24 common PFAS compounds, we developed a method for quantifying PFAS in septic system wastewater by filtration and tandem liquid chromatography-mass spectrometry. We detected the presence of 22 of the 24 PFAS compounds studied in our project. Total PFAS concentrations ranged from 100 to 450 ng/L with a median concentration of 312 ng/L. There was a wide variation in concentrations and in which compounds were present across individual systems. The most common potential sources of the PFAS compounds present in our wastewater were stain and water-resistant fabric coatings and food packaging. When considering the concentrations of compounds regulated in drinking water it is clear that septic systems may be a source of harmful compounds to groundwater, a potential threat to people who draw well water from groundwater in regions where septic systems treat wastewater. These findings indicate the need for additional research of PFAS in septic systems and the mechanics governing concentrations in different types of septic systems. The concentrations we have found in this initial study should advise regulators and policy makers to test PFAS in drinking water supplies in regions where onsite wastewater treatment is common.

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Available for download on Friday, September 05, 2025

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