Submitted by Molly Mitchell, PhD, and Julie Herman, PhD, Virginia Institute of Marine Science from CBES Shoreline Newsletter
Flooding is an increasing problem in Virginia, especially in low-elevation areas such as on the Eastern Shore.
Sea level is rising faster in Virginia than anywhere else on the Atlantic Coast, due to a unique combination of land subsidence and ocean dynamics. As sea level rises, the water table beneath the land also rises. This can create wet patches in fields, and can affect in-ground septic systems, creating problems for some property owners. Accelerating sea-level rise in Virginia will significantly increase the flooding threat to low-lying roads, residences, and septic fields. Although most adaptation work in Virginia has focused on urban economic centers, the majority of the state’s coastline is rural and faces different threats, due to reliance on infrastructures such as private wells and septic fields.
In Miami, approximately 1,000 properties with septic systems are already impacted by high water tables; over 67,000 systems are expected to be compromised by 2040.
Large-scale failures result in contamination of nearby creek waters with bacteria, viruses, and nutrients, and can lead
to the closure of shellfish harvesting areas. In Virginia, the number of septic systems impacted by high water tables
is unknown. To help understand the scope of the problem, researchers at the Virginia Institute of Marine Science used
data from the Virginia Department of Health to identify septic-failure “hot spots” that could be caused by high water
A “hot spot” analysis identifies areas where there are clusters of septic failures within a given area. Although septic failures can be caused by internal reasons, such as poor system maintenance or the age of the system, clustered failures are likely due to external factors, such as sea-level rise or changing soil conditions. This analysis can also look at the change in those patterns over time, to see if they are occurring regularly or becoming more frequent.
Regular “hot spots” suggest that the area has underlying conditions that stress septic systems (such as high groundwater tables). “Hot spots” that are increasing in frequency suggest that the system is changing (groundwater is getting higher) or that the septic systems in the area are getting too old and need replacing.
“Hot spots” were relatively rare on a regional scale, but were found in most of the Tidewater localities where sewer is limited or unavailable, and were fairly evenly distributed along the length of the Eastern Shore. But are these septic
failures caused by high groundwater tables? To answer this question, the researchers looked at the land elevations. For
a septic system to function properly, it needs at least 3 feet of dirt between the system and the groundwater table. If the land elevation is less than 3 feet above the water table, there is not enough room for an in-ground septic system to function.
With sea-level rise, water tables have risen, and a septic system installed 30 years ago may no longer work properly.
Across Virginia, 23% of “hot spots” were found in areas with high water tables. Other “hot spots” may be caused by
poor soil conditions or aging septic systems.
At this time, it is impossible to know the water quality impacts from these “hot spots” of septic failures. In areas where “hot spots” only occurred occasionally, it is likely that the impacts were temporary and disappeared when the systems were fixed. Areas with continuous “hot spots” suggest that systems are failing on a regular basis, so even though each issue is repaired, there may be water quality impacts in adjacent creeks. In some areas, studies have been able to find septic issues by looking for optical brighteners from laundry detergent in the creeks. Targeting sampling in creeks near “hot spots” may help identify where septic failures are leading to environmental impacts.
In rural areas, there is a continuing need for waste management. To reduce the potential for human health and environmental impacts, management should focus on removing septic systems from potential interactions with groundwater, using alternative, above-ground, and mounded septic systems, or through centralized wastewater treatment systems (such as sewer systems or community septic systems). Although this study focused on rising groundwater impacts to septic fields, the majority of the properties included also use private wells, so the impacts of sea-level rise in rural areas may go beyond failing septic systems. Contamination can be carried to adjacent wells, polluting the drinking water. In addition, saltwater intrusion into well water can have serious impacts on human health.
The best approach for managing this issue will depend on financial resources available and community interest.
The development of more targeted datasets and enhanced sea-level rise/groundwater models will allow for improved
understanding of future vulnerabilities of the vast rural septic infrastructure at risk from sea-level rise.
1 Miami Dade County. 2018. Septic Systems Vulnerable To Sea Level Rise. https://www.miamidade.gov/green/library/
2 Mitchell M, Isdell RE, Herman J, Tombleson C. 2021. Impact Assessment and Management Challenges of Key Rural
Human Health Infrastructure Under Sea Level Rise. Frontiers in Marine Science, 8, p.278. https://www.frontiersin.org/