Lagoon #1 - Algae Bloom Update
The information below is an update provided by the Public Works Director sent to concerned residents on July 1, 2022
Dear Lagoon Residents,
Over the past few weeks, the Town has received multiple complaints regarding the heightened algae levels at Lagoon #1 (adjacent to Lakeside, Council Crest, and Monona). Coincidentally, the San Francisco Chronicle also published an article about the rising algae blooms in the bay area and statewide (link provided below).
In response to these resident concerns, the Town met with Solitude Lake Management (a contractor specialized in algae mitigation for waterways) and WRA Environmental Consultants to assess the existing conditions and to identify some possible options to improve the situation and their associated constraints.
A summary of these two consultations is summarized in the memorandum prepared by WRA Environmental Consultants (link provided below). The memo states that “Algal blooms occur when there is an abundance of nutrients in a slow‐moving body of water that receives plenty of sunlight (EPA 2022)… [Algal blooms] have become an increasingly prevalent issue in the San Francisco Bay Area in recent years due to the sustained drought conditions. Rainfall that would typically help flush out excess nutrients from the aquatic systems has been critically less than normal with much of the region in severe drought.” The SF Chronicle article and the WRA memo both discuss the potential for toxic and harmful blue-green algae to form in waterways, however WRA’s analysis of Lagoon #1 have “ruled out the potential of cyanobacteria, “blue-green algae”, based on the lagoon’s brackish composition and lack of characteristic odor and color.
Although the harmful algae being ruled out is certainly good news, the Town still recognizes that many of the lagoon residents still feel inconvenienced by the abundance of algae and have requested that Town take immediate action. However, the original grant deed from 1954 identified the Town of Corte Madera as having exclusive rights to the “Lagoon 1” parcel of land strictly for flood control and drainage purposes. Whereas, fronting property owners have rights to boat, bath, fish and other recreational rights. Therefore, these stated land rights are very important in defining the Town’s role and limitations thereof, especially when considering the use of Town funds to improve facilities for recreational use in areas that are not publicly available to the greater community.
In summary, the memorandum evaluated four particular options that could limit or reduce the algae levels; increased flushing, mechanical harvesting, hydro-raking and dredging.
Out of these four options, hydro-raking and dredging were identified as requiring multiple environmental regulatory permits, detailed designs and substantial construction efforts that would likely cost $100k - $500k and take 18-36 months to perform once funding is identified and a project was initiated. Given the determination that these algae blooms are a nuisance and that there is no current evidence that these algae blooms are compromising the flood control system, the Town is unable to contribute funding or resources towards these options at this time. However, the Town did initiate a multi-year Storm Drain Master Plan effort in early 2022 that will evaluate the storm water capacity of Lagoon #1 amongst others in Town to determine if dredging is needed from a flood control protection standpoint and depending on the outcome of that engineering analysis, dredging could be considered in the future.
Increased flushing was also identified as an option that may “limit future algal growth but it would not likely reduce the current extent of algae biomass within the lagoon. No additional regulatory permits would be required.” The current weekly flushing schedule, available on our Town webpage here, starts the process on a Tuesday and restores the water elevations by Friday. The downside of this option is that it would take additional staffing resources and operating costs to double-flush the lagoons and likely would require Town staff members to come in over the weekends in addition to their regular shifts and also would make the lagoons mostly unavailable for recreation for those additional days.
The last and most effective short-term option, if desired by the residents, would be to hire a company like Solitude to mechanically harvest the lagoon algae. This method as proposed does not require regulatory permits and could be accomplished in 2-3 days for approximately $16,750 to address this event. Although this option would not remove the lagoon of the algae in its entirety it would remove a substantial amount, in particular, the majority of the algae that is floating near the top of the water surface. However, given this algae remediation process has little or no benefits to the flood control system, the Town does not have a basis to contribute public funds beyond facilitating the work, if funded by a private third party (i.e. a group of lagoon residents). There is also a possibility that more algae will continue to grow after the initial removal through the summer and fall months.
For members of the public that would like to discuss this further and to ask questions, Town staff will be holding an outdoor meeting at our Lagoon #1 pump station site (adjacent to #65 Lakeside Drive) next Wednesday, July 6th at 4pm.
Until then we hope you have a safe and fun holiday weekend!
R.J. Suokko, PE
Director of Public Works
Town of Corte Madera
Download a PDF of the SF Chronicle Article