DNAPL Site Characterization Issues at Chlorinated Solvent Sites

Abstract

Past releases of chlorinated solvents generally occurred as dense nonaqueous phase liquids (DNAPLs). Because DNAPLs are heavier than water and are sparingly soluble, they pose difficult characterization, remediation and long-term management challenges. At such sites, a distinction is made between the source zone, which includes portions of the subsurface where DNAPL is or was present as a separate phase, and the downgradient plume of dissolved contamination resulting from groundwater flow through the source zone. Site management frequently involves attempting to contain or deplete contamination in both of these areas. During the past two decades, several promising in situ technologies (e.g., chemical oxidation, thermal extraction and cosolvent/surfactant flushing) have been applied at many sites to remove or destroy contaminants in DNAPL source zones. Yet the U.S. Environmental Protection Agency (USEPA) Expert Panel on DNAPL Remediation (USEPA, 2003) concluded that: " As far as the Panel is aware, there is no documented peer-reviewed case study of DNAPL source zone depletion beneath the water table where U.S. drinking water standards, or MCLs [maximum contaminant levels], have been achieved and sustained throughout the affected subsurface volume, regardless of the in-situ technology applied. " A key factor limiting remediation success is the difficulty of adequately delineating and characterizing the DNAPL source zone. As an introduction to this chapter, a number of related DNAPL characterization issues/observations are listed. These are discussed throughout this chapter and include: In most cases, DNAPL source zone(s) and architecture must be delineated (and distinguished from downgradient dissolved plumes) in order to effectively manage site contamination. Characterization efforts need to focus on traditional goals (such as defining the nature and extent of contamination) and on acquiring data to evaluate remediation options. Remediation goals and strategy should be formulated based on the conceptual site model (CSM) early in the characterization process to ensure collection of appropriate remediation data. The CSM encompasses the site stratigraphy (layering), DNAPL release areas, depth to groundwater, groundwater flow directions and rates, approxi-mate location of DNAPL, DNAPL architecture (i.e., pooled where free-phase DNAPL occupies most, but not all, of void spaces in more-permeable zones, or at residual saturation where disconnected globules are trapped by capillary forces) and other important site features. An outside-in characterization approach guided by the CSM should be used to mini-mize undesirable spreading of DNAPL. H.F. Stroo and C.H. Ward (eds.), In Situ Remediation of Chlorinated Solvent Plumes, doi: 10.1007/978-1-4419-1401-9_8, # Springer Science+Business Media, LLC 2010 217 Characterization tools (e.g., direct-push technology) have advanced, but many challenges remain, especially for fractured rock and deep DNAPL source zones. Characterization uncertainties and requirements increase with age of release, sub-surface complexity, and at sites with large and multiple DNAPL source zones. For large DNAPL source zones, it only may be possible to define them generally. Direct observation of DNAPL is often difficult (e.g., DNAPL at residual saturation will not flow into a well) and indirect methods are used to infer DNAPL presence based on elevated contaminant concentrations in groundwater, soil gas and soil. There is a recent focus on characterizing the downgradient mass flux coming from the DNAPL source; however, tools to characterize flux are limited and not well tested. DNAPL left in the subsurface after remediation is due to both incomplete characteri-zation and remediation. Given the complexity of DNAPL site characterization, it is advantageous to use a dynamic and flexible investigation workplan based on a CSM that is refined iteratively as new data become available. This is illustrated in Figure 8.1, which shows that characterization continues into remediation. An overarching goal of data collection is to sufficiently understand site stratigraphy, groundwater flow conditions, contaminant distribution and contaminant behavior in order to guide site management decisions. At the outset, DNAPL site investigation depends on the purposes of the characterization effort.