Abstract
In Cape Cod, sea level rise causes groundwater levels to rise, which undermines existing wastewater management strategies and creates water quality issues that threaten public health. This is especially concerning for Cape Cod’s unconfined, sole source aquifer that supplies drinking water for the community (Cape Cod Groundwater Guardians, n.d.). Because the soil profile is made up of unconsolidated sand and gravel, the soil readily absorbs pollutants, which puts Cape Cod’s water resources at greater risk for contamination. Precipitation is the only source of freshwater replenishment for the aquifer, so the preservation of groundwater quality is of the utmost importance on Cape Cod (LeBlanc, 2021). The major source of groundwater pollution is nitrogenous waste from septic systems. More than 85% of residential and commercial properties utilize septic systems, accounting for ~80% of the nitrogen pollution in Cape Cod watersheds (Swartz et al., 2006; Cape Cod Commission, 2015). Septic systems treat waste by filtering pollutants from liquid effluent before it reaches the aquifer (EPA, 2023). As groundwater levels rise, septic system functioning is negatively impacted by waterlogged soils. In this project, I investigated this issue through a vulnerability index that pinpoints residences with septic systems lacking sufficient depth to groundwater and exceeding the regulatory limit of 10 mg/L nitrate to groundwater (Department of Environmental Protection, 2016). I identified western Cape Cod as the region with the greatest number of sites that are highly vulnerable to groundwater nitrate contamination. Furthermore, I explored the impact of septic system pollution on nearshore marine waters, comparing fecal indicator bacteria and nutrient concentrations in eastern and western Cape Cod. Using a Wilcoxon rank sum test, I found that Enterococcus exceedances were greater in eastern Cape Cod while western Cape Cod had significantly greater nitrate concentrations. No statistical difference in phosphorous concentrations was found. These findings will assist water resource managers in identifying and addressing septic systems that lack sufficient depth to the unsaturated zone and are affecting water quality through excessive nitrate contributions. Groundwater and nearshore water quality is an issue that will only become more dire as climate change related sea level rise alters coastlines globally.