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THE IMPACT OF CLIMATE CHANGE ON SEPTIC SYSTEMS

Molly Mitchell

Robert Isdell

Julie Herman

Christine Tombleson

Tami Rudnicky

Collaborating with: Division of Data Management and Process Improvement at VDH

2023 Rappahannock River Symposium

Oct 25 2023

Fredericksburg VA

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DARK GREEN = known sewer lines

Remainder assumed to be septic

Actual numbers of septic systems in the coastal zone are unknown (although there is current work to elucidate)

Actual locations of septic systems within a plat is typically unknown

Functional state of most septic systems is not known

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FAILING SEPTIC SYSTEMS…

Contribute as much as 6% of the total nitrogen load from the Chesapeake watershed (Bay Watershed Model 2009 Scenario, Chesapeake Bay TMDL)

In small waterbodies, the local impact can be much higher.

For example, in Buttermilk Bay, MA, 74% of the nitrogen was attributed to septic systems (Horsley Witten Hegeman. Inc. 1991)

Strength of the relationship depends on tidal flushing or the prevalence of tidal marshes (Giordano et al. 2011)

Can also cause bacterial contamination of groundwater (e.g., Stewart & Reneau, 1981, Arnade 1999)

http://www.coastalgems.org/

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THE PROBLEM WITH COASTAL SEPTIC SYSTEMS

Traditional, in-ground septic systems require unsaturated sediment to function

Minimum 3ft

Tank

Drainfield

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THE PROBLEM WITH COASTAL SEPTIC SYSTEMS

Sea level rise is increasing water table levels leading to septic system failure

Minimum 3ft

Failing septic systems result in an increased loading of bacteria, viruses, nitrogen and possibly phosphorus to adjacent waterways

Tank

Drainfield

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WHAT DO WE NEED TO KNOW?

Where do we have high septic failures that could be targeted for funding repairs?

Where are these related to rising groundwater tables that might get worse over time?

What are some solutions to this issue? Are the solutions robust to climate change?

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IMPORTANT POINTS: REASONS FOR FAILURES�ONLY DATASET AVAILABLE—PERMITS FOR REPAIRS

Human

Poor maintenance
Parking on septic field

Structural

Age of system (20-40 year lifespan)
Type of system

Geologic

High water table (seasonal or permanent)
Low soil permeability

No known proxies—but human predictability is low

Some proxies, but hard to actually know system age

Potential proxies for evaluation & responsive to changing conditions

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UNDERLYING DATA�VDH REPAIR PERMIT DATABASE��9 YEARS OF DATA��Statistically analyze the temporal & geospatial distribution of permit repairs to find underlying patterns that could help inform decision-making. �

Repair

permits

(^.)

Waterfront parcels

Other parcels

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Mitchell, M., Isdell, R.E., Herman, J. and 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

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Intensifying

Sporadic

Adaptva.org

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CONSIDERING MANAGEMENT ACTIONS

Consecutive/Persistent hotspots 38% (current issues)

Target for sewer systems?
Active and continuous monitoring of adjacent waterbodies

Intensifying hotspots 3% (emerging issues)

High risk of failures under increased sea level

Sporadic hotspots 47%

These are likely due to years of high water table
May get worse with precipitation changes or sea level rise

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TRENDS IN WATER QUALITY AT 4 STATIONS WITH POSITIVE TRENDS

annual geometric mean

90^th percentile

closure thresholds for:

annual geometric mean (14 CFUs)

90^th percentile (31.5 CFUs)

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WATER QUALITY IMPACTS

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Adaptva.org

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SOLUTIONS OTHER THAN SEWER?

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QUESTIONS? �molly@vims.edu

All of this data is collected for regulatory/permitting processes, not this type of analysis

There are a lot of unknowns in assessing septic vulnerability

Building resilience in this area will be expensive and difficult

Preventing future issues is an important key to future resilience