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.

To better understand how vineyard and housing development could affect its Upvalley water sources, the city of Napa may join forces with the county on a study of runoff and inflow into Lake Hennessey and Milliken Reservoir.

The study, which comes before the City Council on Tuesday afternoon, would expand monitoring and sampling to help officials better understand how water flows into Napa’s two municipal reservoirs – and help the city better predict how runoff from new homes or vineyards in the reservoir watersheds, only a small portion of which the city owns, could affect the quality of the drinking water ultimately piped to Napa residents and businesses to the south.

Council members are scheduled to choose from two funding options for the study: $560,000 for a full slate of water testing sites or $265,000 for a smaller number of locations. The estimates do not include equipment or staffing costs.

While the city owns Lake Hennessey and Milliken Reservoir, it controls only 2,822 of the 34,000 acres surrounding Hennessey – an area stretching as far north as Angwin – and 2,220 of the 6,200 acres bordering Milliken Reservoir. Since both watersheds are in unincorporated areas, Napa County oversees land-use and zoning laws, and, thus, has the final say over development.

The city and county governments in June 2017 agreed to split the cost of a study to gather more precise information about the reservoir watersheds through an analysis that would take into account existing topography, land uses, stream locations and vegetation, as well as historical rain and wind data. The study also will incorporate data on the levels of pesticides, solids, phosphorous, dissolved oxygen and other substances found in past water samples at various locations.

An analysis model developed for the study divides the Lake Hennessey watershed into 214 segments to more precisely track the various paths into the city’s main local water supply, which serves about 84,000 customers in and around Napa, according to the city’s water manager Joy Eldredge. Using that model, the inquiry will better compare the water quality of runoff from undeveloped lands, farmland and residential projects, she wrote in a memorandum to the council.

The move to a closer study of the lands bordering the Napa reservoirs comes amid debates over other vineyards proposals in their vicinity, including Le Colline, a 34-acre vineyard envisioned near the Linda Falls nature preserve outside Angwin. The project would remove 24.5 acres of forest and 9 acres of grasslands.

While the county’s draft environmental report listed no unresolvable impacts from the vineyard’s creation, the city has sought water-quality studies for the rainy winter months to track runoff and pollutant levels in nearby Conn Creek, which flows to Lake Hennessey. In February, Le Colline’s developer offered to pay for water samples to be collected starting with the project’s groundbreaking, and continuing until two years after planting is finished.

Earlier, the city expressed concerns about another vineyard project, Walt Ranch, citing the risk of contaminants from the 209-acre site washing into Milliken Reservoir, Napa’s secondary city water source. After developers promised in 2016 to monitor nutrient runoff at several locations, the city agreed not to oppose the plan. (Walt Ranch’s approval by the county is under appeal by the Center for Biological Diversity and the Sierra Club, who oppose the plan for its potential for groundwater demands, tree losses and destruction and habitat destruction.)

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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.