Nitrate Contamination of Drinking Water and Groundwater
A Spatial Analysis of Septic System Impacts on Water Quality in Barnstable County, Massachusetts
Background
While many coastal communities face threats from climate change related sea-level rise, Cape Cod faces unique challenges.
In Barnstable County, MA sea level rise is causing groundwater to rise, which undermines existing wastewater management strategies and creates water quality issues that threaten public health.
The drinking water and groundwater contamination issues are exacerbated by the hydrogeological characteristics of this area.
The soil profile is made up of mostly coarse, porous sand through which water percolates, resulting in an unconfined, sole source aquifer that supplies drinking water for the community (Cape Cod Groundwater Guardians).
On one hand, this is beneficial because the sand allows large amounts of fresh water to be retained; however, the porous sand also readily absorbs pollutants which puts Cape Cod’s groundwater and surrounding coastal resources at greater risk for contamination.
The major source of groundwater pollution is nitrogen from human waste that originates from septic systems, which are the predominant method of wastewater treatment on Cape Cod.
More than 85% of residential and commercial properties utilize septic systems that discharge wastewater into the aquifer, which accounts for approximately 80% of the nitrogen pollution in Cape Cod watersheds (Swartz et al., 2006; Cape Cod Commission, 2015).
Standard septic systems are on-site sewage treatment units buried in the ground that consist of a septic tank which collects waste and a drain field through which liquid effluent filters to remove harmful pollutants before reaching the aquifer (EPA, 2023).
As climate change affects sea level and storm intensity, septic system functioning is negatively impacted by storm flooding and groundwater rise because septic tanks rely on at least 60 centimeters of soil in the unsaturated zone to filter out pollutants (Lusk, 2023).
Research has shown that waterlogged soils hinder treatment of wastes, allowing nitrate, fecal indicator bacteria, and protozoa to infiltrate groundwater supplies that provide drinking water to residents (Lusk et al., 2011).
Due to the health risks that nitrate contamination poses, the EPA regulates the maximum allowable level of nitrate in drinking water at or under be 10 mg/L (EPA, 2024).
However, adverse health effects of drinking chronically nitrate-contaminated water have been noted in levels as low as 2 mg/L, resulting in blue baby syndrome, a variety of cancers, and birth defects (Hamlin et al., 2022).
In addition to human health threats, nitrogen pollution of groundwater can reach the ocean and wreak havoc on the marine ecosystem.
Excess nitrogen in the nearshore offsets the balance of nutrients and results in low oxygen, increased turbidity, and toxic algal blooms that kill marine wildlife and lower biodiversity (Malone & Newton, 2020).
Nitrate contamination of groundwater and marine waters not only threatens the health and safety of Cape Cod’s residents but also the viability of the area’s economy which is heavily reliant on tourism, real estate, and aquaculture.
Objectives
This project seeks to model spatiotemporal trends in nitrate contamination of drinking water and identify areas vulnerable to groundwater nitrate contamination from septic tanks in Barnstable County, MA.
This accomplished by:
- Analyzing the quality of drinking water from public water supplies through an Emerging Hotspot Analyses of maximum and mean nitrate levels in drinking water from 2000-2020.
- Visualizing the results of Emerging Hot Spot Analysis in a three-dimensional Space Time Cube for maximum and mean nitrate levels over the twenty year time period.
- Creating a vulnerability index of areas susceptible to groundwater nitrate pollution from septic systems.
- Raster surface of groundwater depth to identify areas where the unsaturated zone is insufficient to effectively filter pollutants (< 0.6 meters).
- Raster surface of estimated septic system removal efficiency to identify areas where Title V septic systems are present and releasing excessive nitrate to groundwater supplies (> 10 mg/L). The 21-25% net nitrogen removal efficiency of Title V septic systems published by Costa et al. (2002) was used to extrapolate the nitrogen load to the groundwater post-treatment.
I hypothesize that the nitrate contamination of drinking water has worsened and will continue to worsen as a result of climate change related sea level rise. In the three-dimensional Space Time Cube, it is expected that a greater number of hot spots will be located closer to the present day when visualized by the results of the Emerging Hot Spot Analysis.
Because the vast majority of Barnstable County utilizes septic systems to treat wastewater, I hypothesize that areas nearest the coast will be most vulnerable to nitrate groundwater contamination because these areas have a lower elevation and therefore a lower depth to groundwater.
Together the spatiotemporal model of nitrate concentrations in drinking water and the vulnerability index will shed light on areas of Barnstable County, MA that are particularly at risk for nitrate contamination of both drinking water and groundwater. These areas should receive special attention as government officials and water resources engineers begin to tackle the complex issue of wastewater management in the face of climate change.
Data & Methods
Data Type | Name | Source | Format |
|---|---|---|---|
Population | Town Boundary | Shapefile (Polygon) | |
Water Quality | Massachusetts Drinking Water Quality Data | CSV | |
Topographical | Elevation Contours | Shapefile (Line) | |
Hydrological | Cape Cod Water Tables 2 ft Contours | Shapefile (Line) | |
Wastewater & Land Use Management | Waste Water Flow (Watershed MVP) | Shapefile (Point) |
Table 1. Overview of data used in this project's analyses.
Figure 1. Methods used to conduct Emerging Hot Spot Analyses and create a three-dimensional Space Time Cube to visualize mean and maximum drinking water nitrate concentration from 2000 to 2020.
Figure 2. Methods used to create the vulnerability index identifying high-risk areas for groundwater nitrate contamination. Two rasters were generated in this analysis: (1) a depth to groundwater raster and (2) a septic removal efficiency raster.
Results
Emerging Hot Spot Analysis of Nitrate Concentration in Drinking Water (2000-2020)
Emerging Hot Spot Analysis of Mean Nitrate Concentration (mg/L) in Drinking Water
Three sporadic hot spots were identified in Barnstable, Mashpee, and Sandwich.
Two sporadic cold spots were identified in Brewster and Chatham.
One persistent cold spot was present in Orleans.
Emerging Hot Spot Analysis of Maximum Nitrate Concentration (mg/L) in Drinking Water
Five consecutive hot spots were identified in Barnstable, Mashpee, Sandwich, Bourne, and Falmouth.
Two consecutive cold spots were found in Orleans and Brewster.
Two sporadic cold spots were identified in Harwich and Chatham.
3D Space Time Cube Visualization of Nitrate Concentration in Drinking Water (2000-2020)
Hot Spots of Mean Nitrate Concentration (mg/L) in Drinking Water over twenty years
Barnstable, Mashpee, and Sandwich exhibited the most hotspots with 99% confidence intervals.
Though Yarmouth did not register as a hot spot in the Emerging Hotspot Analysis, there are a notable number of hot spots present when visualized in three-dimensions.
The time slices for 2009 and 2019 were common hotspots across towns.
Within the county, the town of Barnstable had the most hotspots with 99% confidence intervals, with eighteen of the twenty time slices containing hotspots.
Hot Spots of Maximum Nitrate Concentration (mg/L) in Drinking Water over twenty years
The five towns that contained consecutive hot spots--Barnstable, Mashpee, Sandwich, Bourne, and Falmouth--contained hot spots with 99% confidence intervals from 2019-2020.
Similar to the results of the mean nitrate concentration Space Time Cube, Yarmouth contains a maximum nitrate hot spot for 2019-2020 though this was not evident in the Emerging Hot Spot Analysis.
Vulnerability Index of Areas Susceptible to Groundwater Nitrate Pollution from Septic Systems
Groundwater Depth Vulnerability Raster
Septic Tank Removal Efficiency Raster
Groundwater Nitrate Contamination Vulnerability Raster
Groundwater Nitrate Contamination Vulnerability Raster with Town Boundaries
The vulnerability index of groundwater nitrate contamination identified twelve highly vulnerable locations where the depth to groundwater is insufficient and septic system removal efficiency exceeds regulatory limits.
As Figure 3 shows, most of these sites were in Barnstable (41.67%) and Harwich (33.33%), while the remaining three sites were in Sandwich (16.67%) and Falmouth (8.33%).
Figure 3. Bar graph summarizing the locations of twelve areas which are classified as highly vulnerable to groundwater nitrate contamination.
Discussion & Conclusion
The results of the Emerging Hot Spot Analyses and Space Time Mining support my hypothesis that water quality is decreasing throughout time, likely due to climate change associated sea level rise which has increased to over 10 mm/year on the east coast (Dangendorf et al., 2023). Overall, hot spots in both mean and maximum nitrate concentration in drinking water from public water supplies have increased over time, with 2019 and 2020 containing the highest nitrate concentrations in drinking water across the county.
The results of the vulnerability index confirm my hypothesis that highly vulnerable areas are located near the coast due to their low-lying elevation and proximity to coastal sea level rise. The towns of Barnstable, Harwich, and Sandwich are particularly notable because they contain the greatest number of at-risk areas and many of these locations are near water bodies, which puts nearshore coastal waters at greater risk for nitrate pollution and contamination from fecal indicator bacteria.
For the twenty year time period between 2000 and 2020, the town of Barnstable contained the most hot spots for mean nitrate concentration, which means that residents have been chronically exposed to nitrate and are at increased risk for nitrate-related health conditions. This community also contains the greatest number of areas that are highly vulnerable to groundwater nitrate pollution.
Nitrate concentrations in drinking water peak in the town of Barnstable, MA
The spatial analyses performed in this project have identified the towns in Barnstable County that are disproportionately impacted by septic system effluent and contain hotspots of nitrate contamination in public drinking water. These results shed light on the impacts that septic systems have on water quality and may inform the mobilization of septic system replacement efforts that are urgently needed to address the waste management issues in Cape Cod.
Future Research
Algae bloom in Harwich's West Reservoir on August 21st, 2023
The results of the vulnerability index demonstrated that areas vulnerable to groundwater nitrate contamination are often located near waterbodies or on the coast due to their lower elevation.
This indicates that nitrate pollution and fecal indicator bacteria from septic systems in these areas likely pose a threat to marine water.
My continued research on this topic will build upon these findings and delve deeper into the spatial relationship between the identified vulnerable areas and wastewater pollution of the nearshore environment.
References
Cape Cod Commission. (2015). Cape Cod Area Wide Water Quality Management Plan Update. https://www.capecodcommission.org/resource-library/file/?url=/dept/commission/team/208/208%20Final/Cape_Cod_Area_Wide_Water_Quality_Management_Plan_Update_June_15_2015.pdf
Cape Cod Commission. (2023). Cape Cod Water Tables 2ft Contours [Data set]. https://gis-cccommission.opendata.arcgis.com/datasets/6942d258fcb24cd39d414971e6044431_8/explore?location=41.786628%2C-70.285155%2C8.89
Cape Cod Commission. (2023). Waste Water Flow (Watershed MVP) [Data set]. https://gis-cccommission.opendata.arcgis.com/datasets/waste-water-flow-watershed-mvp/explore
Cape Cod Commission. (2024). Town Boundary [Data set]. https://gis-cccommission.opendata.arcgis.com/datasets/3a1e10b9efb748ca8c87e7c84b5ed0e2_4/explore?location=0.000125%2C-70.303250%2C0.00
Cape Cod Groundwater Guardians. (n.d.). The Cape Cod Aquifer. https://www.capecodgroundwater.org/learn-more/cape-cod-aquifer/
Costa, J.E, Heufelder, G., Foss, S., Millham, N.P. & Howes, B. (2002). Nitrogen removal efficiencies of three alternative septic technologies and a conventional septic system. Environment Cape Cod 5(1), 15—24.
Dangendorf, S., Hendricks, N., Sun, Q. et al. (2023). Acceleration of U.S. Southeast and Gulf coast sea-level rise amplified by internal climate variability. Nature Communications 14, 1935 (2023). https://doi.org/10.1038/s41467-023-37649-9.
Department of Environmental Protection. (2016, February 22). Aggregation of Flows and Nitrogen Loading Guidance 310 CMR 15.216. Guidelines for Title 5 Aggregation of Flows and Nitrogen Loading. https://www.mass.gov/doc/nitrogen-loading-and-aggregation-of-flows-310-cmr-15216-guidelines-0/download
Environmental Protection Agency. (2023, August 7). Types of Septic Systems. Septic Systems. https://www.epa.gov/septic/types-septic-systems#septictank
Environmental Protection Agency. (2024, January 2). National primary drinking water regulations. Ground Water and Drinking Water. https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
Hamlin, Q.F., Martin, S.L., Kendall, A.D., & Hyndman, D.W. (2022). Examining Relationships Between Groundwater Nitrate Concentrations in Drinking Water and Landscape Characteristics to Understand Health Risks. GeoHealth, 6(5). https://doi.org/10.1029/2021GH000524
Lusk, M.G. (2023). What to Do When Septic Systems Are Impacted by Flooding from Storms or Groundwater Rise. EDIS, 2023(6). https://doi.org/10.32473/edis-AE591-2023
Lusk, M.G., Toor, G. S., & Obreza, T. (2011). Onsite Sewage Treatment and Disposal Systems: Bacteria and Protozoa. EDIS, 2011 (8). https://edis.ifas.ufl.edu/publication/SS552
Malone, T.C. & Newton, A. (2020). The Globalization of Cultural Eutrophication in the Coastal Ocean: Causes and Consequences. Frontiers in Marine Science, 7(670). https://doi.org/10.3389/fmars.2020.00670
Massachusetts Environmental Public Health Tracker Network. (2020) [Data set]. Bureau of Climate and Environmental Health. https://matracking.ehs.state.ma.us/Environmental-Data/Water-Quality/nitrate.html
MassGIS. (2003). Elevation Contours (1:250,000) [Data set]. Bureau of Geographic Information. https://www.mass.gov/info-details/massgis-data-elevation-contours-1250000#downloads-
Swartz, C.H., Reddy, S., Benotti, M.J., Yin, H., Barber, L.B., Brownawell, B., & Rudel, R. (2006). Steroid Estrogens, Nonylphenol Ethoxylate Metabolites, and Other Wastewater Contaminants in Groundwater Affected by a Residential Septic System on Cape Cod, MA. Environmental Science and Technology 40(16), 4894-4902. https://doi.org/10.1021/es052595+
