MWD approves preliminary design to replace Lake Mathews wastewater system

Joe Naiman
[email protected]

The Metropolitan Water District of California has authorized the preliminary design to replace the wastewater system at Lake Mathews.

A May 9 MWD board vote authorized the preliminary design process, appropriated $350,000 for that phase, and found the preliminary design phase to be categorically exempt from California Environmental Quality Act review. The preliminary design activity will include data collection, resource evaluation activities, and potential inspections.

In the late 1930s MWD built the Colorado River Aqueduct which runs from Parker to Lake Mathews, and MWD’s Colorado River supply is distributed from Lake Mathews. The reservoir’s original construction included installation of an on-site wastewater system which serves bathrooms and kitchen facilities at the Lake Mathews maintenance buildings, administrative offices, and repair shops. The wastewater system was expanded in the 1960s.

The wastewater system consists of three components: community septic tanks and leach fields, collector lines to convey wastewater from Lake Mathews facilities to the septic tanks and sewer laterals to convey wastewater from individual buildings to the collector lines. Solid waste in the septic tanks settles to the bottom of multiple chambers and then undergoes biological treatment before being removed by pump trucks at regular intervals, and the liquid effluent is dispersed through perforated pipes into subsurface soils in the leach fields. The system includes five community septic tanks and approximately 6,000 feet of collector lines and sewer laterals.

Although the wastewater system has been given regular maintenance, its age has led to incidents of slow-draining sinks and toilets, broken and clogged pipes, septic tank backups, and clogged leach fields. In the early 21st century municipal sewer service was extended to the area, and MWD staff has recommended that the on-site septic tank treatment be discontinued and that the wastewater system be connected to the Western Municipal Water District sewer system. A nearby WMWD sewer main includes a connection point which was specifically installed for potential Lake Mathews future use, and that connection can accept wastewater by gravity flow from all of the Lake Mathews facilities.

The preliminary design activities will develop design criteria based on projected future wastewater flows and current codes, initiate the permitting for the sewer connection, conduct field investigations and potholing to locate utilities, design a preliminary layout of the collection system, prepare environmental documentation, and develop a cost estimate. MWD will perform all activities; the $350,000 budget includes $240,000 for the technical activities along with $53,000 for project management and permitting and $57,000 for contingency allowance.

The preliminary design phase is expected to be completed in November 2017.

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