To: Honorable Mayor and Members of City Council
From: Joy Eldredge, Utilities Director
Prepared By: Sara Gallegos, Management Analyst II
TITLE:
Title
Lake Hennessey and Milliken Reservoir Watershed Sampling and Monitoring
LABEL
RECOMMENDED ACTION:
Recommendation
Receive a presentation providing an update on the Utilities Department Watershed Sampling and Monitoring Program at Lake Hennessey and Milliken Reservoir.
Body
DISCUSSION:
Since 2011, the City and County have shared an interest in gathering 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 80,000 residents 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.
In 2017, the City and County began discussions for monitoring the watersheds and collaborated on a calibrated Watershed Analysis Risk Management Framework (WARMF) model. This model simulates hydrology and water quality, developing a water quality sampling and analysis plan, 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. If the model accurately represents known events, it can be relied upon to predict future events.
In 2019, the City and County presented to Council a report that documents the inputs to the model, how the model works, and entered into a MOU for Water Quality Monitoring to gather the samples and perform analyses during the winter rainy seasons from FY2019-2020 through FY2021-2022.
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 can take 5 to 10 years of data to establish baseline water quality data and start to recognize trends.
There are currently 21 established sampling sites in the Hennessey watershed and 6 in the Milliken watershed. Each sampling event occurs on the last Wednesday of each month between October and late May, when 10 or more sites have flowing water. City and County staff work together to collect samples, up to 10 liters of water, or 141 individual analytes from each location. Each sampling event averages 48 hours of collective work, to gather 4 sample collection groups, or 17 samples per event.
Water quality samples are collected from runoff from various types of land use, including urban, undeveloped bare land, rural residential, and agricultural. Sites are located throughout the watershed at strategic locations so they are representative of upstream tributaries. Water quality analyses test for nitrogen, dissolved solids, phosphorous, sulfates, pesticides, coliforms, and many other parameters. To date, a total of 41 sampling events, or 681 samples have been collected. Samples are gathered throughout the rainy season representative of first flush, large runoff events, and average winter flows. The first two drought years FY2019-2020 and FY2020-2021 resulted in limited sampling opportunities with just 2 and 4 sampling events respectively. FY2021-2022 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.
To continue to understand runoff water quality in watersheds that contribute to municipal drinking water supplies, City and County extended the Water Quality Monitoring MOU in 2025, to continue gathering data in accordance with the water quality sampling and analysis plan. The next phase includes three years of tributary water sampling, including joint stakeholder outreach as needed. 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.
It is critical that staff continue to monitor incoming water quality for continued optimized watershed and reservoir management, invasive species suitability, and potential watershed and water treatment responses. The continued sampling and model will help inform future decisions about our watershed management, the effects of land use, and will ensure continued protection of our waterways and water quality.
FINANCIAL IMPACTS:
None.
CEQA:
City staff recommends the City Council determine the Recommended Action is exempt from CEQA, pursuant to CEQA Guidelines Section 15060(c).
DOCUMENTS ATTACHED:
ATCH 1 - Staff Presentation
NOTIFICATION:
None.