Formatted Title
Putting Conduits to Work: Evidence of Efficient Vapor Extraction in Karst Aquifers
Background/Objectives
The complex architecture of void spaces in karst aquifers creates technical hurdles for remediation, particularly attempts to establish hydraulic control or create sustained in situ reactive zones. The extreme permeability and complex interconnections of karst conduit networks frequently drive remediation at karst sites toward impracticability. Surprisingly, these same challenging characteristics present an opportunity for highly effective remediation of VOCs in bedrock vadose zones. The vapor permeability of karst aquifers, like the hydraulic permeability, can be extremely high; but unlike groundwater, subsurface gas in karst systems can be more effectively controlled and captured. Vapor extraction in karst has been documented previously, notably regarding a highly successful application in Kentucky in the 1980s. In that case, VOC vapors were present in over 50 buildings, including homes, businesses, and two elementary schools. Extracting vapors from the shallow epikarst addressed the issue in all cases. Despite the favorable findings from early studies, the technology remains underutilized at karst sites. This presentation draws on data from two recent case studies that illustrate how the basic principles of soil vapor extraction can be applied to efficiently mine VOC mass stored in karst systems. The studies highlight both the potential usefulness of vapor extraction in karst, and the variables unique to karst systems that influence its effectiveness.
Approach/Activities
Two case studies are presented illustrating vapor extraction applications at karst sites with contrasting geologic settings and contaminant release histories. The first describes an emergency response to a large gasoline release from a tank farm into a shallow, tidally influenced karst system on the island of Bermuda. Vapor extraction wells were installed to control vapor migration, recover VOC mass in the 50-foot-thick vadose zone and mine free-product floating on the water table. The second site is an industrial facility in a karst region of the central United States with a decades-old chlorinated solvent plume. Here, significant VOC mass is trapped in a thick (100 to 150 foot) epikarst underlying the plant, serving as an ongoing source to groundwater. A vapor extraction remedy is being implemented to reduce VOC source mass as a component of a broader remedial program.
Results/Lessons Learned
Pilot and operational data from the sites show that vapor extraction can be highly effective at accessing and removing VOCs in unsaturated karst. Observed commonalities include high airflow at low vacuum and a non-uniform radius of influence. Conventional design calculations and modeling proved unreliable; however, empirical data demonstrated complex interconnections and vapor pathways, some characteristic of open-channel flow. In both studies, evidence showed that a small network of wells could effectively flush the source zone, at times removing multiple pounds of VOC mass per day. In the Bermuda study, rapid implementation of vapor extraction after the release contributed to its success, culminating in a “no-further-action” letter in less than three years. Perched zones and mass trapped in clayey residuum at the central U.S. site are expected to extend that system’s operating lifespan; however, pilot study results look favorable, and the system is expected to provide a meaningful reduction in mass flux from the source zone. Collectively, the results show that vapor extraction is a rare instance of a remedial technology that is effective in karst; and a straightforward tool to apply to these notoriously complex aquifers, where most other technologies struggle.