This Order directs Respondents to perform a remedial design for the Groundwater Operable Unit (OU) described in the Record of Decision, dated September 30, 2002, and to implement the design by performing an interim remedial action. Operable Unit or ���OU��� pertains to the groundwater contamination releasing to the Mississippi River adjacent to Sauget Area 2 disposal Site R and the resulting impact area. The selected remedy includes the installation of a 3,500 foot long, U -shaped, fully penetrating, jet grout barrier wall to be installed between the downgradient boundary of Sauget Area 2 Site R and the Mississippi River (See Figure 9-1) to abate the release of impacted groundwater. Three partially penetrating groundwater recovery wells will be installed inside the U -shaped barrier wall to control groundwater moving to the wall. Groundwater quality, groundwater level, sediment and surface water monitoring will be used to ensure acceptable performance of the interim groundwater remedy. Extracted groundwater will be treated and ultimately discharged to the Mississippi River in compliance with all applicable or relevant and appropriate requirements (ARARs)