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Lake Hennessey spillway

The spillway of Lake Hennessey, the city of Napa's main reservoir, overflows during a storm on Feb. 26. Under an agreement between the city and Napa County, a new study would analyze the patterns of water flow over the lands surrounding Lake Hennessey and Milliken Reservoir to better understand the potential impact of development on water quality.

The picture of water quality for Napa residents will be drawn from tests at as many as 32 sites in the streams and woodlands surrounding two reservoirs that supply the city.

An agreement between Napa’s city and county governments lays out the locations for rainy-season water samples that will drive a study of the health of watersheds for Napa’s local water sources – and, officials hope, monitor the potential impacts from farming and development.

In recent years, Napa city leaders have advocated for detailed water monitoring in order to safeguard a watershed area that lies largely outside its direct control. Some 34,000 acres in rural Napa County, as far north as Angwin, drain into Lake Hennessey, but the city owns only 2,822 acres. Milliken Reservoir’s watershed spans 6,200 acres, of which the city owns 2,200 acres, just over a third of the total.

The plan, approved last week by the City Council and county Board of Supervisors, lists 25 potential testing sites around Lake Hennessey east of Rutherford, the larger of the reservoirs, and seven others near Milliken Reservoir northeast of the city. About 20 could be installed during the upcoming rainy season, Phillip Miller, the county’s deputy director of water resources, told supervisors.

Reports shared with supervisors and the council provide more details on the scope of future water monitoring, which is intended to show what impact land-use changes such as new vineyards or home construction may have on the drinking-water supplies for some 80,000 people. Napa County and city first agreed to pursue joint testing in 2017.

Test sites to analyze water flow toward Lake Hennessey may include locations on tributaries and near Conn Creek, Chiles Creek and Sage Creek, according to city Water Manager Joy Eldredge. In the Milliken Creek watershed, samples would be taken at two existing test sites above and below Walt Ranch, as well as new sites for Milliken Creek and other tributaries.

Testing of water samples from a variety of soils, terrains and land uses will eventually will show the differences in runoff between vineyards, rural homes and untouched woodland and grasses, as well as from the more developed community of Angwin, Eldredge told council members. Samples would be gathered during or immediately after storms during the Napa Valley’s rainy season, generally from November to May.

A combination of field and laboratory tests will measure water samples for their oxygen and pH levels as well as amounts of pesticides, bacteria, dissolved solids, nitrogen, phosphorus and other substances. The results will help to define current watershed conditions and predict future water quality.

City and county are to split the cost of the water monitoring, including collection, testing and record-keeping, pegged at $200,000 annually for the next three years. About five to 10 years of data will be needed to produce a full picture of the watershed’s health, according to Miller of Napa County water resources

“We thought we’d take a smaller bite and have a three-year look ahead,” he told supervisors. “The idea was to get our feet wet, get into it early and see how it goes.”

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You can reach Howard Yune at 707-256-2214 or hyune@napanews.com

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Public Safety Reporter

Howard Yune covers the city of Napa and the town of Yountville. He has been a reporter and photographer for the Register since 2011, and previously wrote for the Marysville Appeal-Democrat, Anaheim Bulletin and Coos Bay (Oregon) World.