Rising groundwater levels in Dare County, North Carolina: implications for onsite wastewater management for coastal communities

Sea level rise (SLR) and coastal storms can cause increased flooding and land loss, saltwater intrusion (surface and subsurface), wetland loss/change, flooding, and impacts to local infrastructure. These changes can impact the resilience of coastal communities. Globally, sea level is projected to rise between 0.32 and 1.01 m by 2100 due to the thermal expansion of ocean waters and the conversion of land ice to water, based on low-very high greenhouse gas emissions scenarios (Intergovernmental Panel on Climate Change Shared Socioeconomic Pathways: SSP1-2.6–SSP5-8.5, Fox-Kemper et al. 2021). Spatial variations in local SLR occur due to variations in ocean currents and local factors such as subsidence and sediment delivery (Fox-Kemper et al. 2021). At the Duck, NC National Oceanic and Atmospheric Administration (NOAA) tidal gage in Dare Co, relative local SLR from 1978 to 2022 is estimated at approximately 4.88 mm/year (±0.56 mm/year) (NOAA 2024), faster than the global mean SLR rate of approximately 1.9 mm/year from 1971 to 2006 (Fox-Kemper et al. 2021). Along the Atlantic Coast of the United States, the southern coast of Virginia and the northern coast of North Carolina (NC) are experiencing some of the highest rates of relative SLR along the Atlantic Seaboard (Piecuch et al. 2018). This region is vulnerable due to higher rates of subsidence than other regions along the Atlantic Coast, with rates ranging from 1 to 3 mm/year (Karegar et al. 2016). The higher subsidence rates have been attributed to post-glacial settling and groundwater abstraction (Johnston et al. 2021). Recent relative SLR rates appear to be accelerating (NC CRC 2010; Ezer & Atkinson 2015; Piecuch et al. 2018; NOAA 2024), however there is uncertainty in future projections associated with potential changes in greenhouse gas emissions, ice sheet processes, ocean currents, subsidence, and other variables (Fox-Kemper et al. 2021).

SLR has led to increased duration and frequency of ‘nuisance’ or sunny-day flooding (temporary inundation of low-lying areas due to high tides) within many of NC's and Virginia's coastal cities and conditions are likely to worsen with additional SLR (Sweet et al. 2014). For coastal areas, it is expected that groundwater levels will rise in response to increasing sea level causing an increase in the occurrence of groundwater inundation (Manda et al. 2015). Low-lying coastal areas, such as within NC, are naturally flood-prone due to shallow water tables, flat topography, and the common occurrence of tropical cyclones. In particular, the risk of flooding appears to be rising as the number of catastrophic tropical storm flooding events has increased along the NC coast since the late 1990s (Paerl et al. 2019; Kunkel et al. 2020). More broadly, recent work by Knutson et al. (2020) suggests that SLR combined with response of tropical cyclones to anthropogenic warming will increase coastal flood risks. It is estimated by the year 2100 that approximately 163 thousand–300 thousand citizens in NC would be directly affected by inundation associated with SLR, based on a NOAA digital elevation model used to simulate expected 0.3 m increment changes in the mean higher high water (MHHW) mark, up to 1.8 m (Hauer et al. 2016). These changes will have a large impact on coastal communities and their infrastructure.

Functioning wastewater infrastructure is critical to public and environmental health and the resilience of coastal communities. Recent work has begun to address the vulnerability of municipal wastewater infrastructure in coastal NC to SLR (Allen et al. 2018; Hummel et al. 2018). These studies observed trends of increasing magnitude, frequency, and impact of flood events on wastewater infrastructure with SLR. Although these studies indicate that climate change will impact above ground wastewater infrastructure, less work has focused on buried wastewater infrastructure, such as onsite wastewater treatment systems (OWTS). Conventional OWTS, also referred to as septic systems, require unsaturated soils for effective wastewater treatment.

Like many coastal regions, eastern NC is heavily reliant on OWTS. For example, it is estimated that for the watersheds draining to coastal NC, over 50% of the population relies on onsite wastewater treatment (Pradhan 2004). Although OWTS are utilized at most residences, limited work has focused on the effects of climate change on onsite wastewater infrastructure. Inadequate onsite wastewater treatment associated with chronic or acute coastal flooding can lead to surface water and shellfish contamination by bacteria and viruses and increased nutrient loading that can promote harmful algal blooms and fish kills (Cahoon et al. 2016; Lapointe et al. 2017). Flooding of decentralized systems can cause acute problems, such as wastewater discharging to the surface, backing up in homes, and inundating OWTS electrical systems. In addition, the chronic problem of rising sea level and groundwater levels can lead to lack of wastewater treatment, poor drainage, surfacing wastewater, saltwater intrusion, reduced setback distance to surface waters, and reduced vertical separation distance (VSD).

One aspect of OWTS that is most likely to be affected by rising sea level and associated groundwater table rise is the VSD, the depth of unsaturated soil that exists between the OWTS drainfield trenches and the water table (or a restrictive soil layer). The VSD is an important factor influencing reduction of environmental pollutants found in septic tank effluent. An adequate VSD is needed to ensure the degradation of organics, removal of bacteria and viruses, and nitrification. Field and column studies have shown that when the VSD is less than 60 cm, there is a reduction in bacteria treatment (Karathanasis et al. 2006a; Stall et al. 2014; Humphrey et al. 2015), nitrification (Karathanasis et al. 2006b; O'Driscoll et al. 2014; Cooper et al. 2016; Humphrey et al. 2017; Al-Atrash et al. 2023), and treatment and retention of pharmaceuticals and personal care products (Del Rosario et al. 2014). To protect the groundwater quality from onsite wastewater impacts, a minimum VSD between the bottom of the drainfield and the groundwater table is required to ensure an adequate thickness of unsaturated soil for treatment. Current regulatory approaches in the U.S. provide a VSD requirement for installation of a system, typically based on the seasonal high-water table. The VSD regulations vary for states and municipalities across the U.S., most states require >60 cm VSD; however, North and South Carolina have lower VSD requirements ranging from 15 cm (South Carolina) to 30 (loam, clay, and silt soils) and 45 cm (sandy soils) (NC) (NC DHHS 2017; Henneman 2020; SC DHEC 2023). In coastal communities, where rising sea level results in rising groundwater levels, the VSD may decline over time, resulting in reduced capacity for onsite wastewater treatment. Based on shallow water tables and the rate of SLR in the region, it was hypothesized that changes in groundwater depth related to SLR and coastal storms would have negative influences on VSD over time. The current study evaluated the influence of SLR and extreme precipitation events on changing water table conditions in coastal Dare County, NC, and the implications for onsite wastewater treatment.

Dare County is in northeastern NC along the Atlantic Ocean and the Albemarle-Pamlico Sound. The county extends from the mainland and includes Roanoke Island, bordered by the Sound, and the barrier island system, also known as the Outer Banks. Tourism plays a major role in the County's economy, as the Outer Banks is a popular resort and vacation area. Dare County had a permanent population of approximately 38,000 in 2022 (US Census Bureau 2023). However, tourism results in a large seasonal population increase which can be almost an order of magnitude larger (225–300 K from June to August) during the summer tourist season (Dare County 2023). The region receives approximately 127 cm of rainfall per year (1984–2022), with an annual average air temperature of 15.8 °C (NOAA-NCEI 2023). Although rainfall is somewhat evenly distributed throughout the year, annual and seasonal variability in precipitation amount can occur due to tropical storms. Evapotranspiration in the region is approximately 74% (∼100 cm/year) of annual precipitation and is at a maximum in the summer months (Sun et al. 2002).

Dare County is located within the tidewater region (Outer Coastal Plain) of the NC Coastal Plain. The region is characterized by low relief and extensive wetlands. Land surface elevation is typically below 4 m, except for sand dunes along the Outer Banks, the highest elevation being at Jockey's Ridge (28 m above sea level). Surface waters are affected by wind and lunar tides across the region, and the tidal fluctuations are greater along the Atlantic Ocean coast relative to the estuarine coastline. The land surface is a broad, eastward dipping plain that contains a series of Pleistocene marine terraces. The geology consists of sediments and sedimentary rocks that generally dip and thicken to the southeast (recent to Cretaceous age) (Lautier 2009). At the surface, the surficial aquifer can range in thickness from 0 to ∼40 m. The depth to the water table in the surficial aquifer across the County is typically less than 4 m and is deepest underlying the dunes along the Outer Banks. The surficial aquifer typically consists of sandy sediments, shell and clay beds, and peat deposits. Underneath the surficial aquifer, a confining unit is commonly observed (Yorktown confining unit). This unit is composed of silt and clay beds that can range from 2 to 30 m. Beneath the confining unit, the Yorktown aquifer is composed of beds of fine medium grained sands including some shelly material, interspersed with layers of clay (Lautier 2009). Since around the 1980s, the County has experienced substantial population growth and increased tourism (particularly for the Outer Banks region) that has resulted in increasing demands on groundwater as a source of public water supply (USGS 2023), particularly in the summer months. The Yorktown aquifer system is the dominant source of potable water for the County. The Dare County Water Department provides water to most of the County, typically utilizing groundwater from the Yorktown aquifer and in cases where the water has elevated salinity, Reverse Osmosis is used to treat the brackish groundwater. Overall, the network of public water supply wells can provide up to 15 million gallons per day (MGD) (Dare County Water Department 2023).

At individual sites, the mean annual depth to groundwater data were assessed for normality using a Ryan-Joiner test. Data from all sites exhibited a normal distribution; with the exception of the BI site, which had a normal distribution after log-transformation. In addition, annual precipitation and sea level data exhibited normal distributions. Linear regression analysis was used to evaluate trends in mean annual groundwater depth, precipitation, and sea level over time. This approach has been used in other recent groundwater trend studies (Smith & Medeiros 2019; Cox et al. 2019). In addition, Sen's slope estimator was used to estimate the magnitude of groundwater depth and sea level trends (Sen 1968). Sen's slope estimates the magnitude of trends in long-term temporal data and is considered to be less affected by outliers in the data (Nam et al. 2022; Jiqin et al. 2023).

For long-term analysis of depth to groundwater (1984–2022), there were data gaps at individual sites, and a change in frequency in sampling (from approximately bi-monthly in the 1980s and 1990s to daily in the early 2000s (Tables 1–3)). Each site had at least 4 data points per year from 2000 to 2022. Several sites had less than 3 data points/year from 1986 to 1999 (Table 3). Other studies (e.g. Smith & Medeiros 2019) have used paired well regressions to fill data gaps. Initial data analysis revealed that a paired well approach was slightly less effective (mean R² = 45%) than a multiple regression approach (mean R² = 56%). Multiple regression analysis revealed that mean annual sea level and annual precipitation totals could be used to estimate mean annual groundwater depth at sites that had years with limited data (Table 2). To approximate mean annual groundwater depths at sites for years with <3 measurements, the complete record of mean annual groundwater depth from 2000 to 2022 was used to develop multiple regression equations to approximate mean annual groundwater depth at individual sites for the specific year (Table 2). The total precipitation and mean annual sea level for the years indicated in Table 3 were used to approximate mean annual groundwater depth for individual sites for years that had <3 measurements based on the equations in Table 2. This approach resulted in 12.8% of mean annual groundwater depth data points as regression estimates. Monthly data analysis at long-term sites focused on synthesis of daily groundwater depth records. Daily groundwater depth data from 2013 to 2022 was used to evaluate seasonal variability of groundwater depth in Dare Co. Daily data was available for all sites during this period, apart from SP (2017–2022).

Groundwater depth across Dare County was estimated for current conditions, and future NOAA SLR intermediate projections in 2060 and 2100 using ArcGIS Pro. A digital elevation model (DEM) was constructed using 5 m QL2 LiDAR data acquired from NCEM (2023), which was the finest elevation resolution available for this county at the time of the study. DEM tiles were integrated into ArcGIS Pro 3.1.3 to create a land surface elevation. A point cloud was generated using a fishnet with a 30 m × 30 m cell size. Since most of the study area is <4 m above sea level, a coarser cell size resolution (e.g. 30 m × 30 m) was selected in lieu of a finer resolution (e.g. 5 m × 5 m) to increase computation practicality (e.g. processing time and data storage). Study areas with more variability in elevation data may require a finer cell size to avoid accuracy issues (ESRI 2021). Points that populated over surface water features (i.e. land surface elevation above NAVD88 = 0 m) were removed from the point cloud. After removing these points, the total number of observations in the point cloud was 1,006,810. The number of observations that were <4 m above sea level was 979,785 (97.3% of the total). In the original 5 m DEM, there were 784,000,000 observations and 770,948,773 of these were <4 m above sea level (98.3% of the total). After creating the point cloud, elevation data were extracted from the DEM. Current groundwater depth was estimated using a linear regression based on land surface elevation above NAVD88 (Figure 7(a)). After creating the raster layer for the current groundwater depths, predicted groundwater depth was compared with the mean measured groundwater depths for monitoring wells listed in Table 1. There was a moderate relationship between the predicted groundwater depth and mean measured groundwater depth (R² = 0.64). The mean difference between the two variables was −8% ± 63%. Groundwater depth in 2060 and 2100 was estimated by subtracting current groundwater estimates by the projected mean groundwater rise for each respective year. Mean groundwater rise projections were estimated to be 0.63 m and 1.6 m for 2060 and 2100, respectively. These estimates were based on intermediate SLR projections by NOAA (2024), static annual rainfall accumulations, and the relationship between sea level and groundwater depth (Equation (1)). Ordinary kriging was used to create a groundwater depth map for current, 2060, and 2100 sea level projections for Dare County.

Wanchese Community Center on Roanoke Island had the most extreme groundwater level rise (approximately 0.66 m from 1984 to 2022) (Figure 3(a)). These data suggest that septic systems in the region have experienced reduced VSD over this time period. Average drainfield depths at four OWTS sites in Nags Head were 0.7 m. Based on the NC VSD requirements of 0.3 (silty/clay soils) − 0.45 m (sandy soils), this would require a minimum of approximately 1 m groundwater depth at sites with conventional septic systems. Overall, only three sites (SR, SP, and MA) had mean annual groundwater depths >1 m where conventional septic systems could be viable. Based on the trend, it was estimated that groundwater levels have risen approximately 285 mm (0.93 ft.) since 1984. The groundwater depth data indicate that VSD for septic systems has been declining over time in Dare Co, associated with rising sea level.

Recent studies indicate that coastal storms and SLR can reduce the effectiveness of OWTS in Atlantic coastal communities from Rhode Island (Cooper et al. 2016; Cox et al. 2019, 2020a, 2020b), New York (Fisher et al. 2016), NC (O'Driscoll et al. 2014; Humphrey et al. 2017), and Florida (Miami-Dade Co. 2018). Cox et al. (2019) focused on changing groundwater levels in coastal Rhode Island and found that on average groundwater levels were rising at 14 mm/year since 1964 and these changes were affecting coastal OWTS. At Cape Cod National Seashore, MA, mean groundwater level rise of 8.2 mm/year was estimated from 2000 to 2017 (Smith & Medeiros 2019). The mean estimate of 7.5–7.6 mm/year groundwater rise in Dare Co. from the current study and range of 0.5–17 mm/year falls within the range found in these recent studies. These and other studies (Masterson et al. 2016; Sukop et al. 2018; Dong et al. 2019) reveal that groundwater level rise in the surficial aquifer is commonly correlated with SLR in coastal communities along the Atlantic Seaboard. Although correlation between rising sea level and rising groundwater levels (declining groundwater depth) does not necessarily indicate causation, the patterns across the Dare County region and in the other studies mentioned suggest a regional mechanism consistent with a SLR influence on rising groundwater levels in the surficial aquifer.

Overall, SLR and annual precipitation could explain 65.5% of variability in mean annual groundwater depth for Dare County from 1984 to 2022. Additional factors that increase groundwater recharge may also contribute to rising groundwater levels over time. These include decreases in evapotranspiration associated with vegetation removal, increases in onsite wastewater recharge associated with increased water use, seasonal changes in the balance between precipitation and evapotranspiration, infiltration from stormwater management infrastructure, leakage from the water supply network, irrigation, and other factors.

Increased groundwater recharge due to subsurface wastewater treatment and disposal has been observed in other studies (e.g. Cox et al. 2019) and can contribute to groundwater level rise. In a similar barrier island setting at Bogue Banks, NC, O'Driscoll et al. (2019) found that OWTS contributed an additional 18% of annual groundwater recharge to the island, which can contribute to local groundwater level rise. Coastal tourism can have a large influence on water use and wastewater discharge to the surficial aquifer via OWTS, with artificial recharge potentially contributing to rising water tables. In addition, deforestation may also have an influence on rising groundwater levels in Dare County. Deforestation can cause reductions in evapotranspiration that can lead to increased groundwater recharge (Ranjan et al. 2006). In humid regions, urbanization has been shown to reduce evapotranspiration (Mazrooei et al. 2021). The influence of land use change on evapotranspiration and groundwater recharge may play a role for portions of Dare County where land cover has been converted from forest to urban and suburban lawns and buildings. Crawford et al. (2013) estimated that newly developed parcels resulted in a loss of 437 m² of vegetated cover per parcel in the northern Outer Banks, NC. Further work is needed to better characterize the water budget over time.

Overall, the groundwater response following periods of heavy rainfall showed that extreme events can result in elevated groundwater levels for weeks to months, reducing VSD and OWTS performance. In cases of major storm events, it can take over a month for groundwater levels to recede to pre-event groundwater depths. During periods following major precipitation events, the VSD will decline and may take days to months to return to suitable conditions for soil-based wastewater treatment. Additional precipitation during the recession periods can prolong the period when VSD is unsuitable for onsite wastewater treatment. These storm and post-storm periods can result in elevated environmental health risk since untreated or partially treated OWTS effluent can be a source of nutrients, fecal bacteria, and viruses to water resources. Additionally, recent work by Housego et al. (2021) indicates that for storm surges during large storms, ocean levels can increase by several months over several days and the changes in hydraulic gradients can result in periods of shallower groundwater depth for several days, particularly in areas closest (within 160 m) to the ocean shoreline. The implications are that large rainfall events >1–2 years recurrence interval (8.6–10.5 cm/day) can be sufficient to cause groundwater level rise greater than the 30–45 cm VSD and inundate systems for periods ranging from weeks to months, depending on storm magnitude and site drainage characteristics. Acceleration of coastal erosion due to storm events (Gopinath & Seralathan 2005) may also increase vulnerability of coastal OWTS by reducing the setback distance (horizontal distance to surface water) which can potentially increase contaminant transport from OWTS to surface waters or lead to acute failure (Rakhimbekova et al. 2023). If surface flooding occurs, there are additional risks of dislodging or damaging OWTS components. SLR interacts with storms over time to increase the inland extent that may experience flooding (Cox et al. 2020a). In the future, this will lead to an increase in coastal OWTS that experience storm-related impairment.

As previously mentioned, it was approximated that 1 m of groundwater depth at a minimum is needed to meet the NC VSD requirements (assuming drainfield depth is <0.7 m). For the 2021–2022 period, daily records of groundwater depth for the Nags Head sites located in septic system drainfields were paired with the NC DEQ long-term monitoring wells to evaluate the duration of time when conventional OWTS would be inundated or have reduced VSD. Mean groundwater depth typically increased with land surface elevation (Figure 7(a)). Daily groundwater depth was <1 m for at least 5 days for 9/13 (69%) sites. For the nine sites with daily groundwater depth <1 m, the percent of time with <1 m groundwater depth ranged from 100 to 0.7% (Figure 7(b)). Two sites (SC and BI) had groundwater within 1 m of the surface for the entire period. Four sites (MB, CSI, MA, and BS) had daily groundwater depth that was >1 m for the 2-year period, indicating that adjacent septic systems would likely have suitable VSD for most of the year. The sites that did not have any days with groundwater <1 m depth were located at land surface elevations of 2.74 m or greater. These data indicate that conventional OWTS in areas in Dare County where land surface elevation is <2.74 m above sea level are less likely to meet the VSD requirements.

Four project sites included wells screened in the septic drainfields (BS, SNH, DP, and MB) providing insight into the direct impacts of groundwater on OWTS. Comparing groundwater depths to drainfield trench depths at each site, only DP and SNH sites included days when groundwater was within 0.45 m of the drainfield. DP had 19 days (2.6%) when groundwater was too shallow for the recommended VSD but never had groundwater shallow enough to inundate the drainfield. The residential site (SNH) had the shallowest groundwater with groundwater found in both the tank and drainfield and was found to be a compromised site due to shallow groundwater depths. At the SNH site, the drainfield trench depth was 0.79 m below the land surface, therefore based on the NC VSD requirement (45 cm) for sandy soils, groundwater depth should be >1.24 m. Specifically, groundwater was recorded at this site from 13 November 2020 to 27 April 2023 and groundwater was found to be inundating the drainfield (<0.79 m depth) 60.5% of the time and the 45 cm VSD requirement was never met (GW depth was never >1.24 m).

NOAA SLR projections (NOAA 2024) at Duck, NC were used with groundwater trends to estimate future groundwater depths to 2100 (Figure 8(b)). Equation (1) was applied to estimate groundwater depths associated with SLR projections for the low, intermediate, and high scenarios (SLR increases ranging between 0.5 m and 2.1 m by 2100). Here an assumption is made of static annual rainfall totals and not considering potential changes in annual rainfall total. For the high scenario, average groundwater depths within the region are projected to be at the land surface before 2060. For the intermediate SLR scenarios, mean groundwater levels are projected to rise by approximately 0.3 m by 2040 and by 1.65 m by 2100. Thus, there is an increased risk of drainfield inundation by 2040, particularly for systems with conventional drainfields buried deeper than 0.5 m, with drainfield groundwater inundation occurring on average for Dare County by 2060. Conventional septic systems in low-lying areas in Dare Co. are being stressed by rising groundwater levels and systems that had adequate VSD in the 20th Century, may no longer have adequate VSD. At a minimum, remedial actions are needed for systems that are currently experiencing prolonged periods of groundwater inundation.

The future projections included in Figures 8(b) and 9 were focused on the groundwater depth response to projected SLR scenarios and based on the assumption of similar annual rainfall and evapotranspiration rates as current. If the balance between rainfall and evapotranspiration shifts significantly in the future, the changes in groundwater recharge may also affect groundwater depth. Similarly, changes in artificial recharge associated with changes in water use and increased wastewater discharge may affect groundwater depth. Here, we illustrate the intermediate scenario with a global SLR of 1 m in 2100 relative to the 2000 baseline. NOAA local relative SLR projections for this scenario indicates a 1.2 m relative SLR for Duck, NC by 2100 (NOAA 2024). This scenario is closely aligned with a global mean surface air temperature increase of 4 °C by end-century (2081–2100) above the pre-industrial period (1850–1900). The intermediate scenario aligns with a likely upper bound of greenhouse gas emissions (Huard et al. 2022) and more closely follows the observation-based extrapolation of current SLR trends found within the Southeast US to 2050 (Sweet et al. 2022). For reference, the closest greenhouse gas emission scenario for the NOAA intermediate SLR scenario is SSP2-4.5 (1 m increase global mean sea level at end-century relative to 2000) (Sweet et al. 2022). Note that there are a range of possibilities for future SLR and this uncertainty increases for longer time horizons because climate pledges and policies are more uncertain.

Modeled estimates of groundwater depth were based on the relationship between land surface elevation and groundwater depth. Local conditions can lead to variability in groundwater depth such as soil type, depth of confining unit, slope, local variations in wastewater discharge, site drainage, and other considerations. Due to these constraints, the future projections are provided as a basis to evaluate relative risk of groundwater inundation across the study area. In the future, additional monitoring wells can help further improve groundwater table mapping.

As discussed in the methods section, earlier data collection efforts between 1984 and 1999 had less consistent data collection at several wells, resulting in some wells having less than 3 measurements per year. For the long-term groundwater depth trends, data gaps were most prevalent in pre-1999 data, multiple regression estimates used for this period may slightly affect trends at individual wells. The multiple regression approach to fill data gaps may result in less accurate mean annual groundwater depth estimates for pre-1999 data. However, we evaluated the effect this approach had on the long-term trend estimates by comparing mean annual groundwater depth trends from 1984 to 2022 with multiple regression estimates (−7.6 mm/year), from 1984 to 2022 without estimates (−7.9 mm/year), and 2000–2022 (−8 mm/year) and they showed similar trends.

The SLR and accompanying groundwater level rise trends measured in Dare County, NC provides evidence that VSD for OWTS is declining in the region, resulting in increased likelihood of failing or compromised OWTS. The estimates suggest that groundwater levels are rising at approximately 7.6 mm/year, on average this would suggest that systems that had adequate separation distance in the 1980s could be experiencing drainfield inundation. There is increased risk for compromised or failing systems in low-lying areas <2.7 m land surface elevation, such as was shown for the SNH site (Figure 8(a)). With local relative SLR rates projected to increase in the future, the areas viable for conventional septic systems will decrease. Intermediate SLR scenarios and groundwater level rise trends suggest that in this setting, OWTS at sites currently meeting the 30–45 cm VSD requirements could experience inundation within several decades. For example, based on the projected average groundwater level rise of 0.3 m from 2020 to 2040 (Figure 8(b)), systems that currently meet the 0.3 m VSD could experience groundwater inundation within two decades and approximately three decades for 0.45 m VSD requirements. In addition, intense precipitation events can also increase the duration of time when VSD is inadequate for proper subsurface wastewater treatment.

More broadly, there is growing evidence that coastal communities along the Atlantic Coast of the US are experiencing rising groundwater levels in the surficial aquifer associated with SLR, resulting in shallower groundwater depths. Shallower groundwater depths can reduce the effectiveness of OWTS by reducing the VSD which results in a decline in soil-based wastewater treatment effectiveness over time, due to limited aeration and shorter soil residence times. Static VSD regulations do not accommodate for the fact that the VSD may decline over time in areas where the water table is rising. Many states in the US have VSD requirements of 91 cm or greater, however there are numerous states with substantial coastal populations where communities rely on OWTS, such as NC and SC, where the VSD requirements are less stringent (30–45 cm NC, 15 cm SC). With these shallower VSD requirements, there is less margin for error. For NC and other coastal states, barriers to revised VSD regulations may exist. In some cases, the regulatory framework may restrict local regulations, such as in NC where OWTS regulations are statewide (Vorhees et al. 2022). Potential barriers to revised VSD regulations may include: economic constraints, perceptions that stricter VSD regulations would constrain housing and economic development, limited data on groundwater depth and OWTS performance in coastal communities, limited data on climate change impacts to OWTS, community preferences, state and local regulatory priorities and constraints, individual property rights, lack of collaboration between state and local agencies, and other variables (Tryhorn 2010; Kirchhoff & Watson 2019; Vorhees et al. 2022).

In coastal communities that rely on OWTS, there is an increased risk that rising sea level will create situations where even properly sited systems may have VSDs that decline over time and create a greater risk to water quality impairment and public health. Health regulators in the coastal Carolinas have indicated that water table depth is a primary concern for OWTS in the region (Vorhees et al. 2022). These and other considerations have led to a growing number of coastal communities implementing onsite wastewater vulnerability assessments and planning efforts to address this issue, including Miami-Dade County, FL (Miami-Dade County 2018), Nags Head, NC (Miller 2022), and St. Augustine, FL (Kyzar 2021).

Moving forward, policy changes that increase VSD requirements could be developed to provide improved resilience for OWTS in areas experiencing rising groundwater. Additionally, digital databases and tracked inspections and maintenance can improve oversight of compromised or failing systems in vulnerable areas. Engineering solutions may include mounded systems, advanced treatment systems, wastewater reuse, water conservation, groundwater drainage and/or pumping, and shifting to municipal treatment systems. Community planning efforts can be enhanced with increased groundwater level monitoring to improve understanding of changing groundwater dynamics, along with adaptive management, education and outreach. Environmental education efforts that include technical and social innovations can help communities to understand options for transitioning to more sustainable management approaches and the potential role of environmental citizenship (Jorgenson et al. 2019; Falih et al. 2022). Funding for onsite wastewater infrastructure adaptation efforts will be an important aspect, since most OWTS are privately owned. Improved understanding of communities at risk can help to plan for resiliency approaches.

In conclusion, the main findings of this study are as follows:

• (1) SLR along the NC coast has led to rising groundwater levels.

• (2) Rising groundwater levels can compromise onsite wastewater treatment by reducing the thickness of aerated soil for soil-based wastewater treatment.

• (3) Large precipitation events can temporarily reduce the effectiveness of septic systems for weeks to months.

• (4) Regulatory approaches should consider the effects of rising groundwater levels on coastal onsite wastewater infrastructure.

• (5) There is a growing need to develop strategies to increase resilience of coastal onsite wastewater infrastructure to mitigate current and future climate change impacts.

This work was supported by funding from the NOAA Climate Program Office NOAA-OAR-CPO-2019-2005530. In addition, the Town of Nags Head provided in-kind support and access to monitoring sites, data, and guidance. We are especially thankful to Holly White, Kylie Shepard, Kate Jones, Andy Garman, David Ryan, Conner Twiddy, and others that assisted with the Nags Head Decentralized Wastewater Management Plan, and Holly Miller and others at Tetra Tech. We appreciate the long-term NC DEQ groundwater depth data and access to wells provided with assistance from Nat Wilson and David May. The State Climate Office of NC provided meteorological data and NOAA provided sea level data for the study. In addition, Lauren Voorhees (NC Sea Grant), Eric Edwards (NC State University), Iain Burnett (NC State University), and Katie Hill (University of Georgia) provided valuable inputs to the project team and insights on climate change influence on septic systems throughout the project. We appreciate the efforts of ECU students (Patrick Bean and others) and staff (John Hoben) who contributed to monitoring efforts. In addition, we are thankful to one Nags Head homeowner that provided access to an OWTS for monitoring and evaluation.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.