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Overburdened Water & Sewer Infrastructure

Our local water and sewer infrastructure is old and in need of significant repairs and upgrades. Many pipes throughout both systems are over 40 years old and major assets have approached or exceeded their projected lifetimes and must be replaced. As the risk of failure increases with age, it is compounded by the impacts of climate change. Maintenance and capital improvement costs are also ever increasing, forcing the agencies to fall further behind in maintaining reliable service without significant increases in capital and maintenance costs, most of which comes from customers. Capacity of both systems is also limited, as indicated below.

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Water Supply for the Future?

MWSD’s current reliable water capacity is 758,880 gpd and they estimated in 2017 that they could support up to 1,000 new water connections (Source: MWSD 2017 Water System Master Plan, Table ES-5). If reliable system capacity is maintained, a 1% growth rate in population will not max out the system until the year 2050. However, year after year of seasonal droughts has reduced the volume of water available in local aquifers. An assessment of the Alta Vista Well (the largest water source for MWSD) in 2019 indicated a significant depletion in available water causing a drawdown in output after multiyear droughts, although the well did appear to recharge following normal rainfall seasons. Climate change has increased the likelihood and severity of droughts going forward, which also reduces the probability of finding adequate new sources of water on the Midcoast. The increasing threat of wildfires has also revealed the need to dramatically increase firewater storage. These risks to future water supply and quality bring into question whether the system can actually support the estimated number of new connections.


Aging and failing infrastructure also jeopardizes water supply and quality. To increase the pace of needed repairs, MWSD established an annual water reliability charge of $558 per connection in July 2020, adding a significant financial burden to many coastside residents. Despite the added revenue from this charge, it covers only half of the needed capital improvement costs, meaning that risks to reliable supply will remain without additional rate increases. Water financial reserves are also below the policy requirements at present, in spite of a $500,000 transfer from Sewer Reserves in 2020.

 Our Recommendations

  • Large commercial and multi-unit residential development applications must provide a more thorough assessment of risks to ongoing water supply and quality, as well as risks of exceeding sewer capacity that threaten environmental safety and service interruptions.

  • Large commercial and multi-unit residential developments must be required to add on-site storage of fire water to reduce the burden on the community’s fire water storage and drinking water supply.

  • SAM must adopt new governance guidelines to improve working relationships among member agencies and better ensure that they meet their legal, regulatory and financial obligations in order to avoid excessive costs, agency consolidation and/or loss of local control of sewer service.

Overburdened Sewer Infrastructure

MWSD’s current sewer capacity allocation from SAM is 510,000 gpd of dry weather season flow according to the JPA. According to SAM Monthly Manager Reports, dry season flows over the last several years have averaged around 250,000 gpd, approximately half of MWSD’s capacity allocation. Assuming the system continues to operate at these flow rates in the dry seasons, MWSD’s capacity allocation will not be exceeded for the foreseeable future. However, wet season flows present a challenge to the sewer system due to inflow and infiltration of storm water runoff. Under such conditions, old pipes and pumps have a higher probability of failure, which can lead to sewage backup and Sanitary Sewage Overflows (SSOs).


SAM has a long history of SSOs, most of which have been directly attributable to failing infrastructure. A total of 101 SSOs occurred in the SAM service area from January 2011 to May 2017. See Midcoast ECO's SAM Status Update and SAM/MWSD Flow Analysis Report – March 7, 2018 for details.


The largest of these spills (344,000 gal) resulted in a $300,000 fine and regulatory enforcement action by the San Francisco Regional Water Quality Control Board (RWQCB) directing SAM to complete replacement of the Intertie Pipeline System (IPS) and add more sewage storage capacity in order to reduce excessive wet weather flow into the SAM treatment plant. IPS sections 1-3 were replaced in 2018 and SAM also initiated a preventative maintenance program. Wet weather sewage storage capacity was also increased from 200,000 gallons to 400,000 gallons next to the Portola Pump Station in 2021. This storage capacity, along with the 434,000 gallon capacity of the Walker tank at the Montara Pump Station, allows SAM greater flexibility in regulating flow into the plant from GCSD and MWSD. However, since HMB has no storage capacity and they have routinely averaged more than 60% of the total sewage flow into SAM, the risk of overflows during significant storm events remains high. In October 2022, the Portola tanks were filled to within 6 inches of overflowing during a significant storm event. Furthermore, from June 2017 to December 2022, there were 28 additional SSOs that spilled a total of over 10,000 gallons of raw sewage with less than half of that volume recovered.


Although other upgrades and repairs continue, the failure rate of old pipes and equipment throughout the system continues to increase, threatening environmental safety and further increasing costs which must be paid almost exclusively by ratepayers. MWSD already raised sewer rates by 47% from 2017-2019, with additional increases in sewer rate charges of 9% per year for 2020-2022. These additional increases will fund $40M in sewer operation and maintenance costs over the next 20 years, barring any catastrophic failures.

Overview of Local Water and Sewer Service Providers


Water service is provided by Montara Water and Sanitary District (MWSD) for Montara and Moss Beach and by Coastside County Water District (CCWD) for El Granada and Half Moon Bay (HMB). Primary sewage collection service is provided by MWSD for Montara and Moss Beach and by Granada Community Service District (GCSD) for El Granada. HMB provides primary sewage collection within its city limits.


The Sewer Authority Mid-Coastside (SAM) provides secondary sewage collection and treatment for all 3 agencies according to a Joint Powers Agreement (JPA). SAM is governed by a board of 8 directors (4 from HMB, 2 from GCSD and 2 from MWSD) and expenses are shared according to sewage volume. From July 2020 to December 2022, relative flow volumes into the SAM plant averaged 61% for HMB, 20% for GCSD and 20% for MWSD.

According to a San Mateo County Sea-level Rise Vulnerability Assessment from 2018, the SAM sewage treatment plant is highly vulnerable to sea level rise and creek backup and is subject to ground water infusion in the event of flooding. Inundation would likely cause a loss of service. For these reasons, the plant’s adaptive capacity was rated as “low”.

The plant has indeed been challenged to the point of catastrophic failure on at least two occasions in the last two years. In December 2021, a storm caused an overflow at the plant, which nearly shorted out the entire electrical building. On December 31, 2022, a major storm event caused Pilarcitos Creek to flood into the plant, resulting in a partial shutdown and near total failure. Flow was stopped from GCSD and MWSD, which caused an overflow of the Walker tank in Montara and sewage flow to the ocean for several hours. The next day, major breaks in the force main in Moss Beach produced additional SSOs. The associated costs for these unplanned emergency repairs have already exceeded $1 million as of the beginning of 2023 and the risks for further impacting events remain.

Age of Coastside’s Only Sewage Treatment Plant Exceeds Projected Lifetime

The SAM plant, built in 1976, has exceeded its 40-year projected lifetime and will need to be replaced within the next few years. A new plant could cost $100-500 million, depending on the chosen site and design criteria. Despite the fact that there are no alternatives for sewage treatment on the Coastside if SAM should fail, there are currently no plans to replace the plant.


In addition, HMB’s 2017 lawsuit against the other SAM partner agencies and HMB’s refusal to pay their share for improvements in infrastructure outside of HMB city limits have disrupted the SAM partnership and delayed critically important projects. HMB’s delay of the Wet Weather Storage Project added almost 100% to the project’s budgeted cost. Summary judgment was granted in favor of GCSD & MWSD in March 2022. HMB has appealed and they continue to protest costs associated with IPS maintenance and repairs. Such budgetary and governance issues at SAM remain significant, further jeopardizing key decisions and timely repairs, as well as adding unnecessary operational and legal expenses to the 3 partner agencies.

Undue Risk of Large Development Projects

Overall, these issues raise significant questions as to whether our local water and sewer infrastructure can accommodate new large development projects that significantly increase new connections, without added risk of water service interruptions or sewage system failure. In addition, large development projects should require additional fire water storage, rather than relying only on water storage of the overall system.

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