Formatted Title
A Step in the Right Direction: Untangling Groundwater Dynamics in the Face of Climate Change
Background/Objectives
In the Gulf Coast region, a mature chlorinated VOC groundwater plume has migrated downgradient from a historical release. The site features an unconfined, moderately transmissive shallow sand groundwater system intersecting a small stream. To address the plume, a sustainable treatment strategy involved installing a permeable reactive barrier (PRB) using mulch derived from trees downed by Hurricane Ike in 2008. Despite installation downgradient of the source along the “known” groundwater flow direction, a portion of VOC mass was observed to migrate to locations not captured by the PRB. With uncertainty surrounding the cause, carbon substrate injections were applied to address the unexpected arrival of mass but these only partially succeeded. Increased tropical storm and hurricane intensity over the last several years in the region prompted the formation of a hypothesis to explore whether climate change was affecting groundwater flow and plume migration patterns.
Approach/Activities
To test this hypothesis, a detailed hydrologic study was designed and implemented, incorporating historical climate and water-level data to help identify past conditions and data gaps; as well as high-resolution water-level data collected from new groundwater wells and surface water points to address those data gaps and provide further insight. Utilizing the results of time-series analysis applied to the long-term data sets, the high-resolution study involved strategically installing groundwater wells on both sides of the small stream, focusing on the newly inferred plume migration direction. High-resolution water-level data was then collected over 9 months, spanning multiple seasons and capturing the response to precipitation events, and analyzed alongside the historical data via transient horizontal hydraulic gradient analysis. Results of this analysis were used to better understand conditions during tropical storms and hurricanes relative to ‘normal’ conditions to support or refute the plume migration hypothesis.
Results/Lessons Learned
The evaluation yielded multiple lines of evidence supporting the understanding of the dynamic groundwater flow and plume migration patterns at the site. Just as important, the results provided an improved understanding of the groundwater/surface water interaction at the site—a losing stream with groundwater flow converging along the stream centerline. Consequently, plume mass migration is more likely to divert along the centerline stream direction (i.e., area of convergence), suggesting the stream centerline acts as a hydrogeologic boundary. The evaluation established a direct connection between increased groundwater elevation and flow direction during periods of elevated rainfall, primarily resulting from tropical storms and hurricanes. Recognizing these patterns, to be successful, future plume mitigation strategies and hydrogeologic analyses must account for these changing conditions related to ongoing and future climate change.