Evaluating the impact of failing septic systems on water quality and acute gastrointestinal illnesses in North Carolina

Despite increasing awareness of public health risks associated with septic system failures, significant knowledge gaps remain regarding the drivers of these failures and their impacts on water quality and public health. This dissertation aims to identify the environmental and socioeconomic determinants of septic system failure and to estimate their association with groundwater contamination and acute gastrointestinal illnesses (AGI) in North Carolina.The first study systematically reviewed 82 studies to characterize groundwater contamination from septic systems globally. Common contaminants included E. coli, nitrate, Enterococcus spp., total coliforms, ammonium, phosphate, chlorides, and pharmaceuticals. Contamination risk was influenced by soil and hydrogeological conditions, climate, septic system density, and operation and maintenance practices. The second study developed a composite septic system vulnerability index using principal component analysis to identify at-risk areas in Dare County, North Carolina. The vulnerability index was driven by exposure to flooding, shallow water tables, and soil limitations. The index was strongly associated with septic system repairs, demonstrating its use for targeting interventions and informing climate-resilient management strategies.The third study used propensity score methods to evaluate the relationship between septic system failures and groundwater quality. Areas with septic failures had significantly higher nitrate concentrations (β = 0.17 mg/L, p < 0.05) and increased odds of total coliform contamination (OR = 1.79; 95% CI: 1.17–2.77; p = 0.007), highlighting failing systems as important nonpoint sources of groundwater pollution. The fourth study assessed the combined effects of hurricane-related flooding and septic failures on AGI emergency department visits using a quasi-Poisson model. Septic system failure was associated with a slight decrease in the AGI ED visit rate (RR = 0.97, 95% CI: 0.97–0.98, p < 0.05). However, after adjusting for septic failures, the AGI ED visit rate among private well users increased significantly (RR = 1.06, 95% CI: 1.01–1.11, p = 0.03). The interaction effect between flooding and septic failure burden was amplified, resulting in a fourfold increase in AGI risks (RR = 4.03, 95% CI: 3.13–5.18, p < 0.001), underscoring the compounded public health risks associated with environmental and infrastructure vulnerabilities.