To: Honorable Mayor and Members of City Council
From: Joy Eldredge, Utilities Director
Prepared By: Mark Holderbein
TITLE:
Title
Lake Hennessey and Milliken Reservoir Watershed Water Quality Monitoring
LABEL
RECOMMENDED ACTION:
Recommendation
Approve and authorize the Utilities Director to execute Amendment 2 to the Memorandum of Understanding with Napa County for Water Quality Monitoring of the Lake Hennessey and Milliken Reservoir Watersheds.
Body
DISCUSSION:
The City and County have a shared interest in gathering additional data to learn how land use changes in the Hennessey and Milliken watersheds might impact the drinking water in the related lake and reservoir. The City owns the Lake Hennessey Reservoir and the Milliken Reservoir which serve as municipal drinking water supplies for the citizens of Napa and over 2,000 customers in unincorporated Napa County. The Hennessey watershed drainage area is composed of approximately 34,000 acres reaching as far north as Angwin. Of this total area, the City owns just 2,822 acres. The Milliken watershed drainage area is composed of 6,200 acres of which the City owns nearly 2,200 acres. Both watersheds are located in the unincorporated area of the County; thus, the County approves zoning and land use laws, and processes land use permits for private development, on these important lands.
On June 20, 2017 the County and City entered into a Memorandum of Understanding (MOU) regarding a study of the watersheds that would be cost shared equally.
On September 26, 2017 the City Council and County Board of Supervisors approved a professional services agreement with Systech to develop a calibrated watershed analysis risk management framework (WARMF) model simulating hydrology and water quality, develop a water quality sampling and analysis plan, and provide a tool which the City and County can use for watershed management on an ongoing basis. The WARMF model combines the physical characteristics of the watershed (topography, land use, soils, vegetation, stream locations etc.) with historical weather data (rainfall, wind etc.), known hydrology (stream flow and depth, lake elevations, diversions etc.) and available water quality data (total dissolved solids, pesticides, nitrogen, phosphorous, sulfates, dissolved oxygen, etc.) collected over time at various sampling points in the watersheds. The model was calibrated by comparing model simulations with known past events to verify accuracy. If the model accurately represents known events, it can be relied upon to predict future events. On March 19, 2019 staff presented the report that documents the inputs to the model, how the model works, and recommended Water Quality Monitoring and Analysis Plan. Council gave staff direction to field verify sampling locations.
On November 19, 2019 the County and City entered into a MOU to gather the samples and perform analyses during the winter rainy seasons 2019/20 through 2021-2022.
Historical rainfall data shows that most reservoir recharge occurs during a handful of large storms during winter months. City and County staff have identified sites, and field-verified logistics to gather water samples from the main tributaries that feed the drinking water reservoirs. The sampling sites are accessible and repeatable to ensure consistency of data. Samples are gathered approximately monthly (generally late October through late May) depending on the rain year when tributaries are flowing due to rainfall runoff. As stated in 2019, it is recognized that it takes 5-10 years of data to establish baseline water quality data and start to recognize trends. There are currently 21 established sites in the Hennessey watershed and 5 in the Milliken watershed. Samples are gathered throughout the rainy season representative of first flush, large runoff events, and average winter flows. The first two drought years 2019-2020 and 2020-2021 resulted in limited sampling opportunities with just 2 and 4 sampling events respectively. 2021-2022 was the first rain year that had consistent runoff sufficient for 8 sampling events to take place from October through May and the following two years yielded similar sampling opportunities. Staff will return to City Council in January 2026 with an update on the sampling results.
To continue to understand runoff water quality in watersheds that contribute to municipal drinking water supplies, staff recommends the City Council approve and authorize staff to execute Amendment 2 to the MOU with the County to continue gathering data in accordance with the water quality sampling and analysis plan. The terms of the MOU ensure both City and County staff are involved in all decisions, review information simultaneously, coordinate all efforts such that both City and County staff participate and are included on all verbal and written communications with any and all contracted parties. City and County staff have proven mutual cooperation throughout this process. All staff efforts and costs will continue to be shared equally (50% City: 50% County) for the scope of work under the next phase that includes three years of tributary water sampling. The scope includes joint stakeholder outreach as needed. Finally, after additional rain years of water quality data, it will be integrated into the existing WARMF model, and City and County staff will be trained to use the model for predictive scenarios.
FINANCIAL IMPACTS:
Similar to the past three years, City and County (Parties) will cost share this effort in equal portions at 50% each. The Scope of Work includes cost estimates for 30 field and laboratory analyses of water quality parameters. (see Attachment 1). Labor efforts for sampling will be contributed 50% by each party. In the event staff from one entity is unavailable, costs for labor hours contributed above 50% will be reimbursed by the Party who is unable to provide labor. Outside contracts for consultants or laboratory services will be reimbursed 50% to the contracting Party. The estimated cost to each entity is $200,000 annually and $50,000 of in-house labor for a period of three years. Budget is available in the Water Utilities Operating Fund’s Water Supply account (53060110 / 60WTSHDMOU).
CEQA:
The Utilities Director has determined that the Recommended Action described in this staff report is exempt from CEQA, pursuant to CEQA Guidelines Section 15060(c) since the action does not result in a direct or reasonably foreseeable indirect change in the environment.
DOCUMENTS ATTACHED:
ATCH 1 - Amendment No. 2 Memorandum of Understanding for Water Quality Monitoring of the Hennessey and Milliken Watersheds
NOTIFICATION:
None.