Recirculation Systems

Abstract

In this chapter we discuss how recirculation systems can be engineered to achieve mixing in order to facilitate in situ remediation. As noted in Chapters 1 and 5, recirculation systems have a number of advantages. First, as active systems, they can be designed to maximize the probability of contaminant control and capture, even under changing hydrological conditions. Second, since the systems use wells, the recirculation zone(s) can be established at depths that can’t be impacted by other systems (e.g., permeable reactive barriers). Third, as will be discussed subsequently, mixing of the amendment and the contaminant occurs in an engineered reactor, either in-well or aboveground, and is therefore relatively complete. Fourth, for systems that rely upon biodegradation, it has been shown that recirculation systems can be used to establish in situ biological treatment zones that are effective in biodegrading the target contaminant, even when initially biodegradation activity is sparse (Hoelen et al., 2006). Finally, as net loss of water is minimized when recirculation is used, the systems are very useful in regions where water needs to be conserved. They can be designed to confine a source of contamination and treat it there, or to act similar to a barrier wall by removing contaminants in a passing plume to prevent downgradient contamination. Of course, as active systems that use wells, there are a number of attendant disadvantages to recirculation; the most obvious being the operation, maintenance, and monitoring costs associated with a “pump-and-treat” system (the fact that the treatment happens to occur in situ notwithstanding). Additionally, as systems which rely on pumping, the water which is captured and amended will come preferentially from the most permeable zones of the aquifer. Contaminated water that is resident in low permeability regions may not be treated.