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Napa water supply at Lake Hennessey (copy)

The city of Napa and Napa County want to team up on a study of environmental contaminants in the watersheds of the city's two local reservoirs, Lake Hennessey, above, and Milliken. How to do this study is part of ongoing discussions. 

Cautiously, cautiously – that’s Napa County’s approach to creating a watershed computer model that could someday influence rural land use decisions in an effort to keep contaminants out of city of Napa reservoirs.

Given the stakes, supervisors want stakeholders such as the wine industry and environmentalists involved in various decisions.

An initial big decision is where to do water sampling in the Lake Hennessey and Milliken Reservoir watersheds to accurately calibrate the model.

“This is about the credibility of the process ... I don’t want to spend time –years—collecting data and then a key stakeholder questioning the modeling that’s producing this data,” Supervisor Alfredo Pedroza said.

Napa County and the city of Napa are teaming up on the Lake Hennessey and Milliken Reservoir watersheds study. The firm Systech Water Resources is creating a model that looks at the watershed’s land uses, soils, vegetation, streams, weather and other factors.

This is being combined with water quality sampling data from streams feeding the reservoirs that will test for pesticides, nitrogen, dissolved solids and other potential contaminants. Sampling data will be used to calibrate the model so it can look at current watershed conditions and predict future conditions.

Accurately calibrating the model requires taking stream water samples under various winter rainfall conditions. The model won’t immediately start yielding dependable results, county officials said.

“It will probably take five years, assuming we have decent weather, to obtain any useful universe of data,” county Planning, Building and Environmental Services Director David Morrison said.

After reading the latest version of Systech’s water quality monitoring plans, Supervisor Diane Dillon had the same concerns she voiced at the March 19 Board of Supervisors meeting.

An earlier, city study mentions older septic systems as a major water quality concern in the Hennessey watershed, followed by vineyards, spills on Highway 128, wildfires and erosion from roads. Yet in Systech’s new Hennessey and Milliken reports totaling 100 pages, the word “septic” appears once and the word “vineyard” appears 70 times, Dillon said.

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“I want to do the right thing here, but I’m really, really concerned about this model,” Dillon said.

During public comments, local farmer Harvest Duhig said she’s concerned assumptions will be made about where nutrients are coming from.

“I don’t know how you will differentiate if nitrogen is coming from an expansive golf course-like residential lawn versus a percentage of nitrogen that might be added to a different ag system,” she said.

Debra Dommen, who works for Treasury Wine Estates, said where monitoring takes place in the watershed is important. The effort needs to tease out the differences between what’s going on with rural residences, roads and agriculture, she said.

“If you don’t tease them out and they get co-mingled, it’s going to all come down on ag regulations that might not actually solve anything,” Dommen said.

Everyone wants information, she said. But everyone wants to have faith in the information they get.

City of Napa Water General Manager Joy Eldredge addressed supervisors.

“Any assumptions that are in the model are superseded by actual data,” Eldredge said. “So if there are concerns about any of the assumptions being invalid in the model, the best thing to do is populate that with real data.”

Supervisors took no vote, but told staff to work with stakeholders in coming months on the program. County officials will work with the city of Napa on amending the agreement between the two jurisdictions to expand the scope of work related to monitoring and return with a cost estimate at a future meeting.

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Napa County Reporter

Barry Eberling covers Napa County government, transportation, the environment and general assignments. He has worked for the Napa Valley Register since fall 2014 and previously worked 27 years for the Daily Republic of Fairfield.