1. Septic system failures in North Carolina

Millions of Americans, especially those in rural and marginalized communities, rely on septic tanks and other forms of onsite sanitation (OSS) systems to meet their sanitation needs [1, 2]. Roughly half of North Carolina’s (NC) population uses septic systems, compared to about a quarter nationwide [3, 4]. NC has the second highest rural population among all states [5] but ranks 17^th in terms of the share of its population classified as rural—thus, rurality is not the sole reason that such a large portion of NC’s population relies on septic systems [6]. Although septic systems are common in widely dispersed rural areas where piped sewer systems are impractical, they are also found in densely populated communities near cities with centralized services [7, 8]. Extending water and sewer infrastructure to these areas may create jobs and boost local economies, though key challenges associated with extending public services include lack of financing, government support, community engagement, and the presence of existing septic system infrastructure [9, 10]. As population growth, development pressure, and costs of centralized wastewater treatment systems continue to increase, the use and construction of OSS systems in NC are projected to increase as well [3].

In NC, an estimated 10–20% of septic systems experience a failure annually, which occurs when untreated wastewater seeps to the surface of the drain field, directly into groundwater, or backs up into the home [9]. Septic systems may fail for a variety of reasons, including inappropriate design, poor maintenance, and local hydrogeologic and climate factors [11, 12]. Septic systems can only be used in soils that adequately absorb the effluent—those installed in areas with high groundwater tables and in soils with high clay content may be more susceptible to flooding during extreme precipitation events, especially under global climate change [13–15]. Additionally, failure to perform routine maintenance (e.g., desludging every 3–5 years) may lead to clogged drain fields, contaminating the environment and posing risks to human health [16].

Higher septic system densities in communities have been associated with higher rates of diarrheal diseases and acute gastrointestinal illness [17]. When septic systems malfunction, wastewater containing pathogens and excess nutrients can enter groundwater or surface waters, increasing the risk of illness when that water is used for human consumption or recreation [18, 19]. In addition to human health consequences, excess nitrogen and phosphorus in surface water can lead to harmful algal blooms, damaging ecosystem balance and aquatic life [18]. Septic users have also cited decreased property values, difficulty selling homes, high repair costs, and bad odors as concerns associated with failing systems [20].

2. Sanitation challenges in under-resourced areas

Septic systems are common in rural areas due to the impracticality of installing piped sewer systems in areas that are widely dispersed [9]. However, septic systems are also used in peri-urban communities near cities with centralized services, which have been historically excluded from regulated community water and sewer services [21]. Counties with higher Social Vulnerability Index (SVI) scores—a measure of 16 different social factors used to determine overall vulnerability [22]—rely more heavily on septic systems, especially in the eastern part of NC (Fig 1). Factors influencing vulnerability, such as race and socioeconomic status, may have contributed to the zoning patterns excluding these communities from piped sewer services [2, 9].

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Fig 1.

A) Septic system density by county in NC, 1990 [3]. B) Percent rural by country in NC, 2010 [23]. C) Social Vulnerability Index by county in NC, 2020 [22]. North Carolina county shapefiles were obtained from NC OneMap (https://www.nconemap.gov/); figure was constructed using ArcGIS Pro software.

https://doi.org/10.1371/journal.pwat.0000304.g001

Key challenges associated with septic systems in NC include:

1. Climate change. The risks associated with failing septic systems are also compounded by unpredictable precipitation patterns and warming temperatures associated with global climate change. Septic systems in NC’s coastal region are at particular risk of flooding, and high levels of fecal contamination have been measured in the area [24]. Wastewater management systems routinely become overwhelmed by heavy rains, causing raw sewage to leak into the surrounding environment. Across the US, around one-third of newly constructed homes are served by septic systems, indicating a need to monitor their use and design climate-resilient infrastructure [25]. For example, climate-resilient septic systems should be located on lower ground than drinking water sources, constructed with durable materials (e.g., concrete), and emptied every 3–5 years to prevent overflows [14]. In areas with high groundwater tables, mounded drain fields—drain fields built above the adjacent land grade, allowing septic effluent more soil percolation prior to entering groundwater—may offer additional climate resilience [26].
2. Gaps in sanitation monitoring. The NC Department of Health and Human Services (NCDHHS) collects information on septic system failures throughout the state, though there are gaps and discrepancies in county-level reporting. For example, from 2018 to 2019, only 39 of the state’s 100 counties reported septic system malfunctions, totaling around 4,000 failures—a failure rate of only 0.2%—across 24 months (S. Pradhan, personal communication, NCDHHS). This is likely a significant underestimation based on septic system failure rate estimations of about 10–20% from both NC and the whole southeastern US [9, 27]. Though some counties report malfunctions geo-coded to specific addresses, others are only available at the county level. Additionally, nationwide census figures have not included the number of households using septic systems since 1990—resulting in reliance on data which are now over 30 years old [3].
3. Financing of water and sewer services. Funding availability is the primary factor influencing government decisions to extend municipal services to underserved communities in NC [9]. Awareness of failed septic systems may prompt government officials to extend sewer services to these areas; however, underreporting of malfunctions may limit their ability to carry out cost-benefit analysis of sewer service extensions. Residential growth strains utilities that are unable to keep up with the rate of development, and increasingly strict water quality regulations will require them to upgrade treatment systems [9].
4. Lack of septic system user involvement in agenda setting. It is primarily the responsibility of homeowners or neighbors to report failures, which can lead to underreporting and data inaccuracies. The consequences of a failed septic tank—high repair costs or even home condemnation—may lead homeowners to avoid reporting problems with their system [20].

3. Future directions

Septic systems are—and will continue to be—an essential part of sanitation service delivery in NC; however, addressing gaps in septic system monitoring, maintenance, and repair is a significant future need. We propose a focus on the following aspects to reduce the environmental and human health risks of malfunctioning septic systems:

1. Development of policies that target communities not connected to centralized sewer systems. For example, New Hanover County, NC, used door-to-door anonymous surveys to collect data on septic system failures, rather than the typical complaint-based system, which resulted in more robust monitoring and ultimately persuaded government officials to extend sewer services to high need areas [9]. Policies may also include investment in new approaches and technologies to ensure that wastewater infrastructure can withstand or quickly recover from climate-related hazards. Sanitation services must be prioritized at the local and state government levels to reverse the current inadequate state of OSS in NC.
2. Improved monitoring of septic system locations and reporting of malfunctions. In 2018, the U.S. Environmental Protection Agency (EPA) submitted a proposal to the U.S. Census Bureau for the American Community Survey (ACS) to include questions related to septic systems [28]. New data on septic system use and condition may be available by 2025, an important first step in providing necessary information for decision makers to meet community needs and protect human health and the environment. In addition to improved monitoring, there must be coordination among the various actors involved in sanitation monitoring and priority setting (e.g., community members, health officials, elected officials), though their various roles and priorities can make the process difficult [9].
3. Development of tailored educational programs for septic system users. For example, the EPA’s SepticSmart program provides online resources for homeowners, local organizations, and government leaders to understand proper care and maintenance of septic systems to protect health and the environment [29]. The program has been successfully adapted in Washington [30], Ohio [31], and Minnesota [32], resulting in improved water quality, increased septic system maintenance awareness, and a better understanding of homeowner attitudes, beliefs, and behaviors.
4. Involvement of septic system users in meaningful citizen science, agenda setting, and research design. In order to raise awareness about the benefits of proper septic system use, and risks of malfunctions, it will be especially important to include those for whom geography, language barriers or historical exclusion from research and public engagement might discourage their involvement in research [2, 33]. As the communities in question are often socioeconomically disadvantaged, it is important to consider that engagement is not costless for community members and involves both direct costs (e.g., transportation to meetings) and time investments.

Acknowledgments

We would like to thank Sushama Pradhan, Ph.D., for providing access to data on septic system malfunctions in North Carolina.