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Draft Supplemental Environmental Impact Report Page IV.G-21 Forward Inc. Landfill 2018 Expansion Project carry the 100-year flow within its banks)in the realigned section of the creek; and provide a stable channel design that meets or exceeds the functions and values of the existing creek. The realigned channel has been designed to carry the 100-year flood flows within its banks. Erosion protection would be provided in areas with high velocities or sharp bends. The U.S. Army Corps of Engineers HEC-RAS hydraulic model was used to determine design water surface elevations and estimate channel velocities and other pertinent flow parameters for stable channel design. The following measures are proposed as part of the project, as described in the Project Description and design study for the proposed creek realignment: • The channel must function as a natural corridor, require little or no maintenance once the vegetation is established, and should provide 100-year flood protection. • The channel slope and depth will be appropriate to the 100-year flood protection. The channel slope and depth are based on the invert elevations of the existing channel at the start and end of the new channel. The slope between these two points along this alignment is designed for 0.00055 ft/ft which translates into a ground surface profile along the alignment a channel depth between 10 and 12 feet. • The appropriate responsible agencies must review and approve the updated April 2018 design for the relocation of the South Branch of South Littlejohns Creek. Implementation of these procedures would reduce the impact to a less-than-significant level. Mitigation Measure G.5:None required. Impact G.6:Adding significant new landfill volume could potentially contribute to the known VOC-contaminated plume and other groundwater contamination. The new expansion would add approximately 8.1 million CY over the additional 17.3 acres of landfill in the stages proposed,which would allow for more leachate generation over time. This additional leachate could potentially contaminate the underlying groundwater. This is because additional waste accumulation could result in some additional risk of potential spills, leaks, and leachate control failures, despite the controls in place. Thus, the project could contribute over time to the known VOC-contaminated plume associated with the unlined portion of the former Austin Road Landfill extending north and northeast from the northern edge of the landfill border, and associated potential to contaminate groundwater. That could result in the need to continue to supply water to affected offsite users for a longer time period. The following measures are proposed as part of the project to minimize this potential impact: • Forward Landfill has agreed to a short-term and long-term mitigation of the offsite impacts of the existing VOC plume, to provide an alternative source of drinking water to those residents in the downgradient area who are using domestic water wells for drinking water and whose domestic wells may be adversely affected by the VOC plume. A long-term solution currently being investigated by Forward to assist those residents on Newcastle Road,who are already being provided with bottled drinking water by Forward, is for