25C - AGMT - OCWD CENTENNIAL PARK

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Section 5 An injection rate of 3 VIGD was assumed in the Simulation for existing MBI-1 and for each of the four proposed Centennial Park injection wells. This rate represents the maximum likely injection rate of the injection wells based on the preliminary testing of the existing MBI-1 demonstration well, local hydrogeological conditions, and the availability of G,WRS water. The targeted screen intervals for the proposed injection wells would be similar to those at MBI- 1, encompassing two model aquifer layers (Layers 9 and 11), representing the Omicron/Upper Rho and Main aquifers, respectively. The modeled injection rate was apportioned across the two layers according to their respective local transr-nissivity values. The particle tracking code MODPATH was used to estimate the underground retention time. The results of the MODPATH simulations show that particles in the Omicrom/Upper Rho aquifers (Layer 9) moved much faster than those in the Main aquifer (Layer 11) due to the larger calibrated hydraulic conductivity value in Layer 9. Therefore, only results from Layer 9 are presented here to represent the shortest residence time to a drinking water well. As shown in Figure 17, the primary buffer area was generated by connecting the, model-derived particle locations three months after they would be released. The buffer area is a composite area that encompasses all five injection wells. The closest wells to the primary buffer area would be IRWD-11 2 and IRWD-1 7, The particle tracks in Figure 6 show that particles released at the injection wells would initially radiate away from the injection well locations in all directions, but would predominantly travel to the south and southeast toward Irvine Ranch Water District (IRWD) production wells IRWD-12 and IRWD-17, both of which are screened in Layer 9 and Layer 11. Each small arrow depicted in Figure 17 along the particle tracks represents one month of subsurface travel, The earliest particle arrival time to production wells would be well RWD -17 in approximately 16 months, The estimated injection water arrival time to well IRWD-12 would be approximately 34 months. As shown in Figure 18, the travel direction of the particles would be consistent with the known Principal aquifer groundwater flow gradient in the area. The travel times to wells IRWD-1 2 and IRWD-1 7 calculated by the model would be well in excess of the CDPH1 RRT requirement of three months, Secondary Boundary As mentioned above, a secondary boundary is required by the GRRP Regulations, and its extent would be based on the potential primary boundary expansion when new well and/or increased pumping occurs in the zone between, the primary boundary and the secondary boundary, potentially resulting in groundwater flow gradient changes that could expand or otherwise affect the primary boundary. The secondary boundary was generated using the same Mid! Basin Centennial Park 2jMZMProject Final EiR 5-115