SCOPE OF WORK

GROUNDWATER MODELING AND REPORTING SERVICES

FOR THE WEST COAST BASIN BARRIER PROJECT

West Basin Municipal Water District Page 2 of 4 Scope of Work

This work is in support of the ongoing West Coast Basin Barrier Project (WCBBP). The WCBBP uses a hydraulic barrier to prevent seawater intrusion into groundwater aquifers. The WCBBP is a joint project involving the Los Angeles County Department of Public Works (LACDPW) and the Watershed Replenishment District (WRD). The LACDPW owns and operates the barrier, WRD is the Watermaster for the basin, and West Basin produces advanced-treated recycled water that is injected into the series of barrier wells along the coast.

The project is regulated by the current Barrier permit (R4-2006-0069) issued by the Los Angeles Regional Water Quality Control Board (LARWQCB), the Title 22 Recycled Water Regulations for Groundwater Recharge, and the Recycled Water Policy. Annual groundwater modeling is a requirement of the permit and required topic for the WCBBP Five-Year Engineering Report submitted to regulators.

West Basin is seeking qualified firms to perform groundwater modeling services. Firms must have the ability to study and analyze groundwater interactions at local and/or regional scales, using West Basin’s MODFLOW/SEAWAT groundwater model. An understanding of the West Coast Basin, or other barrier systems is highly desirable. Obtaining any applicable licenses for this modeling work is the responsibility of the Consultant.

The purpose of this work is to update the MODFLOW/SEAWAT groundwater model annually for five consecutive calendar years and run the model each year to produce output files and maps of groundwater elevations and recycled water percentages in each aquifer receiving recycled water. The Consultant must request injection data and pumping data each year from Los Angeles County Department of Public Works and the Watershed Replenishment District in order to update the model. A technical memorandum will be produced each year by mid-March that describes the results and the work performed for the previous year.

The work must meet the requirements of the regulators, which are described below. Work performed under this scope will be under the direction of a qualified engineer registered in the State of California, or a certified hydrogeologist in California, and experienced in the field of advanced wastewater treatment for groundwater recharge. The input and output files must be provided to West Basin at the end of each year’s model update. At the conclusion of the five years, the contractor will assist West Basin with the preparation of the Five-Year Engineering Report and will provide the stamp of a Professional Geologist (PG) upon review completion.

Report Report Covers Due to West Basin 2026 Report 1/1/2026 – 12/31/2026 3/15/2027 2027 Report 1/1/2027 – 12/31/2027 3/15/2028 2028 Report 1/1/2028 – 12/31/2028 3/15/2029 2029 Report 1/1/2029 – 12/31/2029 3/15/2030 2030 Report 1/1/2030 – 12/31/2030 3/15/2031

5-Year Engineering Report 1/1/2026 – 12/31/2030 September 2031

The scope includes the following tasks:

1. Kickoff meeting (web-based): The winning consulting firm will briefly meet with West Basin and other cooperating agencies to discuss the scope of work, project team, key points of contact, project schedule, project management, receive data or describe data transfer protocol, and answer any additional questions.

2. Confirm the current calibration of the current MODFLOW/MT3D/SEAWAT model using existing historic data, and also incorporating new hydrogeologic, water quality, water level, water pumping, and water injection data. Input files of the previous model calibration will be provided by request.

West Basin Municipal Water District Page 3 of 4 Scope of Work

The annual model updates, along with all supporting information about calculations and statistical tests and indices, shall address the following:

• Mass balance to determine the portion of recycled water in the groundwater, and also to demonstrate that blending is occurring in the aquifer. Any recycled water already present in the groundwater due to ongoing Barrier injection should be accounted for as part of the total amount of recycled water in calculating the percent of recycled water in an aquifer.

• Determine groundwater contours and flow directions in the basin’s primary water-bearing units:

200’ Sand, Gage/Gardena, 400’ Gravel (Lynwood), Silverado, and Lower San Pedro (Sunnyside) aquifers.

• Determine injected recycled water flow paths from groundwater elevation contours, and compare with the flow and transport model’s flow paths.

• Predict density dependent seawater/freshwater advancement and recycled water migration in saturated porous media.

• Revise groundwater flow and solute transport model and calibrate results as appropriate to match as closely as possible the actual flow patterns and water quality (e.g., chloride levels) observed within the aquifer.

• Temporal and spatial trends in the data used for the model, with particular reference to comparisons between stations with respect to distances from the monitoring wells, water quality data collected during previous years, recycled water injection, domestic supply well extractions, and attributes of the basin and the Barrier. Include any appropriate statistical tests used.

• Evaluate the travel time (retention time) to the nearest drinking water production well and project the percentage of recycled water and flow to domestic supply wells (active or standby) over the next 20 years.

3. Prepare a technical memorandum of the results of Task 2 for submission to the LARWQCB that meets the reporting requirements for WEST BASIN’s Barrier permit. The report must address, at a minimum, all points under item 2 above.

4. Within 30 days of submitting each technical memorandum, provide a well-documented, fully working version of the model to West Basin along with all necessary input, output, graphics, and calibration files necessary for West Basin and any cooperators to readily open, run, and view the output files of the model.

5. Prepare a technical memorandum summarizing the previous five annual reports. Relevant permit documentation can be found on the following page. The report must address the following, at a minimum:

• Total quantity of water injected into each major aquifer, and the proportions of recycled water and diluent water that comprise the total quantity;

• Estimates of the rate and path of flow of the injected water within each major aquifer;

• Projections of the arrival time of the recycled water at the closest extraction well, and the percent of recycled water at the wellhead.

• Any assumptions and/or calculations used for determining the rates of flow and for projecting arrival times and dilution levels.

• A discussion of the underground retention time of recycled water, a numerical model, tracer or other methods used to determine the recycled water contribution to each aquifer.

• A revised flow and transport model to match actual flow patterns observed within the aquifer if the flow paths have significantly changed.

West Basin Municipal Water District Page 4 of 4 Scope of Work

• Revised estimates, if applicable, on hydrogeologic conditions including retention time and the amount of the recycled water in the aquifers and at the production well field at the end of the five-year reporting period. These estimates must be based upon actual data collected during that year on recharge rates (including recycled water, native water, and portable water), hydrostatic head values, groundwater production rates, basin storage changes, and any other data needed to revise the estimates of the retention time and the amount of the recycled water in the aquifers and at the production well field.

• Significant differences, and the reasons for such differences, between the original estimates presented in the 1993 Engineering Report and the revised estimates.

Both the annual report, and the 5-year summary with engineering report must be prepared in compliance with West Basin’s Barrier permit (R4-2006-0069) issued by the Los Angeles Regional Water Quality Control Board (LARWQCB), the Title 22 Recycled Water Regulations for Groundwater Recharge, and the Recycled Water Policy. All of these can be found online or provided upon request by West Basin.