chapter explores emerging remediation technologies that have yet to receive extensive field testing and evaluation, and it reviews the state of federal funding for relevant research and development and provides recommendations on research topics relevant to the future management of complex sites where groundwater restoration is unlikely.
The decision to transition a site from active remediation to long-term management requires a thorough understanding of the geologic framework, history of contamination events, the current location and phase distribution of contaminants, temporal processes that affect groundwater flow and chemical migration, and interactions at hydrogeologic and compliance boundaries. The combined understanding of these factors, referred to here as site conceptualization, supports the development of specific management tools such as the conceptual site model (CSM, see Chapter 4) and mathematical models. Typically, the site conceptualization and associated tools are updated as the project progresses from discovery of contamination through closure or transition to long-term management, with the degree of detail dependent on the nature of the contamination and the physical dimensions of the site. The development and enhancement of an accurate and suitably detailed site conceptualization is an important component of addressing future management challenges at these sites including the transition to long-term management.
The current cleanup paradigm distinguishes the source zone from the downgradient plume, in terms of treating each region differently with respect to characterization and remediation, and it acknowledges the dominant role of geologic heterogeneity in controlling contaminant removal from both regions. In NRC (2005), hydrogeologic heterogeneity was conceptually captured by identifying five generic geologic environments ranging from nearly uniformly homogeneous, unconsolidated porous media (Type I) to fractured rock and carbonate aquifers (Types IV and V). More recently, a 14-compartment model has been proposed (Figure 4-1; Sale and Newell, 2011; ITRC, 2011), in which contaminants can reside in groundwater, sorbed, and vapor phases, either within the source zone or the plume, and which are further subdivided into high- and low-permeability regions. In the high-permeability regions, advective transport will control contaminant migration, while in the low-permeability regions, the dominant transport mechanism is molecular diffusion. The advantage of such multi-compartment conceptual models is the ability to focus on the exchange of contaminant mass between specific compartments that can limit the rate and extent of remediation, recognizing that the controlling processes can change over time.