Theme: 03. Green and Sustainable Remediation

Topic Area: 3c. Climate Resilience and Site Remediation

Formatted Title:

The Surprising Effects of Sea Level Rise on Coastal Biodegradation

Background/Objectives:

Remedial technology evaluations rarely consider the long-term effect of shifting hydrologic and biogeochemical conditions due to climate change. Understanding the impact of these shifting conditions is paramount for coastal sites as sea levels are anticipated to rise in the United States from 3 to 5 feet by 2100.

Under present-day conditions, a saltwater wedge was found to play a large role in the fate and transport of the dissolved-phase contaminant plume associated with a petroleum dense non-aqueous phase liquid (petroDNAPL) body at a former industrial facility in the Caribbean. A saltwater wedge occurs where fresh upland groundwater overlies subterranean marine saltwater. The saltwater wedge controls the downgradient migration of the dissolved-phase contaminant plume by providing (1) a density gradient that prevents the hydraulic advancement of the plume and (2) an excess of sulfate that supports the biodegradation of dissolved phase constituents. Present-day assimilative capacity calculations are one to three orders of magnitude greater than the amount needed to support the biodegradation of the dissolved-phase plume.

To understand future conditions, the effect of sea level rise on saltwater wedge location and dissolved-phase contaminant fate and transport was modeled.

Approach/Activities:

Groundwater flow and transport conditions were modeled using SEAWAT (MODFLOW 2000 coupled to MT3DMS) to simulate variable density groundwater flow. Predictive models were run to account for future conditions associated with climate change. In addition to sea level rise, the effect of extreme rainfall events on groundwater levels were also considered. Dissolved-phase contaminant fate and transport were evaluated. Federal Emergency Management Agency (FEMA)’s preferred method of climate informed science approach (CISA) was used to incorporate the best available hydrologic data that integrate current and future changes in flooding based on climate science.

Results/Lessons Learned:

As sea levels rise, the overall distance available for contaminant attenuation between the source and potential ocean receptors decreases. However, at the same time, the saltwater wedge advances further inland with the most prominent effects observed in shallow aquifer zones. The encroachment of the saltwater wedge toward the petroDNAPL body sufficiently controlled downgradient advancement of the dissolved-phase plume even under the most extreme projections for sea level rise through 2100. Incorporating the influence of sea level rise on groundwater hydraulics and biogeochemical conditions was found to be integral in understanding plume behavior over time. These findings are essential to consider during the remedy selection process to identify a resilient, durable, and effective remedy.

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Jon Ouverson