Septic Systems and Sea Level Rise

The Potomac Watershed Roundtable met virtually last Friday. The first meeting of the Roundtable in over a year. Though I miss the ability to chat with other attendees, it was still a good meeting. Dr. Molly Mitchell Research Assistant Professor with the Center for Coastal Research Management at the Virginia Institute of Marine Science spoke to the group about the research that she, Robert Isdell, Julie Herman and Christine Wilcox are engaged in: Prediction of Future Septic System Failure Site in coastal Virginia.

From VDH 2019 presentation-M. Mitchell

Sea level is rising very quickly in Virginia. It has already risen eight inches since 1970’s and is anticipated to rise an additional foot and a half by 2050. Higher temperatures are causing seas to rise as glacial ice melts and the volume of water in the oceans expands. In addition, Virginia is sinking- the land has been subsiding for 10,000 years. The combined impact is the most significant effective sea level rise on the eastern seaboard. 

Climate change and subsidence threatens to alter the depth of the water table in the coastal regions. Conventional and AOSS septic systems require the soil to finish the treatment of the septic effluent. This requires that the soil be dry and unsaturated and there be a minimum of three feet between the bottom of the septic drainfield and the water table. When there is not, the system cannot remove bacterial contamination. 

Septic systems are estimated by the Chesapeake Bay model to contribute 6% of the nitrogen contamination to the Bay, but there is data to indicate that when soils become flooded they flush excess nitrogen. The increased nitrogen has been observed, but the source and mechanism is not understood. In the Chesapeake Bay region, many localities are already allocating significant resources to measures to reduce nitrogen and phosphorus contamination as part of the Chesapeake Bay TMDL. Rising sea levels will increase the expense.  

Using the limited data available from the Virginia Department of Health Dr. Mitchell and the other scientists attempted to identify correlations that might be predictive of future system failures. Using the permits for repair of septic systems and the changing depth to groundwater. They found that structural and geologic factors- the age and type of septic system and the rising water table did correlate with system failure. Additional sources of septic system data possibly maintained on a local level will be necessary to advance the work to a predictive model. However, it is clear that as sea level continues to rise, Coastal communities will need to decide how to treat sewage waste from these areas currently serviced by individual septic systems as coastal flooding renders these systems ineffective. Future development has to consider the impact of rising sea levels, the demands of the Chesapeake Bay TMDL and costs to the community.