The two models presented below have been used for physical-chemical treatment of pollutants. The continuous trench allows open access to water in multidirectional flow (Fig. 15.7a). In this system, the user is not interested

in collecting the product of treatment for further processing. The alternative — funnel-and-gate system — offers the user the option of collecting flow from a localized position for further processing (Fig. 15.7b).

The flow in the continuous trench system, which is perpendicular to groundwater flow direction, needs to be slightly larger than the cross sec­tional area of the contaminated groundwater in order to capture the con­taminants in both vertical and horizontal directions (Gavaskar et al., 2000). The funnel-and-gate system is composed of impermeable walls and at least one reactive zone. The funnel structure could be sheet piles or slurry walls where the function of the funnel is to intercept the contaminated ground­water and lead it to the treatment zone. Phillips (2009) has elaborated on the designs of different reactive barriers, including but not limited to: the thickness of the PRB to provide sufficient residence time for the contami­nants within the treatment zone to be completely treated.

Other complex designs have been tried including the multi-sequenced permeable reactive barriers (MS-PRBs) for multiple contaminants. MS-PRBs use multiple reactive materials in more than one reactive zone as shown in Fig. 15.7c (Dries et al., 2004).

For the purpose of treating nuclear and radioactive waste around waste storage sites, the cheapest option could be the inoculated barrier system. All reactive barriers in the context of heavy metal treatment, unless heavily engineered and expensively constructed (which defeats the purpose), are mainly — if not only — suitable as temporary containment barriers to be operated until the pollutant source is completely removed or a more per­manent solution for continuous remediation is found.