Investigating and Remediating Contaminated Karst Aquifers

Abstract

Subsurface investigations of contaminated karst aquifers are generally regarded as extremely difficult. The difficulty not only is partly a result of the significant heterogeneity and anisotropy created by the existence of open and plugged ramiform conduit systems, but is also a result of the existence of an overlying epikarst. Even more intractable is the effective remediation of contaminated karst aquifers for basically the same reasons. The difficulties associated with investigating and remediating karst aquifers are further exacerbated when situated in areas with complex folding and faulting of strata. Couple the specifics of various contaminant types of varying degrees of reactivities, densities, and miscibilities (e.g., VOCs, LNAPLs, DNAPLs) with the complexities typical of karst terranes and the limitations of comprehensive karst investigations and effective remediation techniques quickly become evident. Typical remediation techniques, such as pump-and-treat operations, in situ thermal treatments, in situ chemical oxidation, bioremediation, and monitored natural attenuation all exhibit significantly reduced performances relative to other types of aquifers. Partially in recognition of the challenges associated with specific contaminant types and groundwater investigations and remediation techniques when applied to contaminated karst terranes the U.S. EPA developed the concept of a TI (Technical Impracticability) waiver in which remediation below MCLs (maximum contaminant levels) may not be required. Very few TI waivers have ever been issued, however, and obtaining a TI waiver is quite formidable. Remediation down to MCLs is a desirable goal, but the vagaries of karst terranes fully justify the concept of a TI waiver at some complex sites.