Coombsville Vineyards: MST Groundwater, Recycled Water, Boundary and Zoning

By Matthew Martinez, Founder, CEO & Managing Broker, Diamond Real Estate Group. California DRE #01869679.

Coombsville is the horseshoe of hills and benchland on the east side of the city of Napa, closed in by Mt. George and the Vaca Range and open to San Pablo Bay weather from the south. The water question here is unlike anywhere else in the valley: much of the area appears to fall within the County’s Milliken-Sarco-Tulucay groundwater area, where long-term well declines have shaped how the County reviews new water use.

Coombsville at a glance

Where the appellation runs

The rule at 27 CFR 9.223 starts at the 1,877-foot peak of Mt. George and runs south and southeast along ridges, contours and dirt roads past Leonia Lakes, Sugarloaf and Kreuse Canyon. It then turns northwest to Imola Avenue, follows Imola west to the Napa River at the Maxwell Bridge, runs up the river for 3.2 miles, follows Milliken Creek north to Monticello Road, and climbs back to Mt. George. Two USGS maps are named: Mt. George and Napa.

The 2011 final rule describes a horseshoe-shaped landform that is part of the Vaca Range, with a "cup-and-saucer" form attributed to an ancient caldera. It places Coombsville inside both the Napa Valley AVA and the multi-county North Coast AVA. Its corners touch the Oak Knoll District to the northwest and Los Carneros to the southwest without overlapping either, and its eastern line stops short of Wild Horse Valley.

The name has a history buyers sometimes hear about. TTB first proposed an 11,200-acre "Tulocay" area in roughly the same place in 2006, then withdrew it in 2008 after commenters questioned the name evidence and noted that the word was tied to a particular winery. The Coombsville boundary that followed is 125 acres smaller. Because the western line follows Milliken Creek and the Napa River, blocks near the city edge need a close look; our post on Coombsville vineyard blocks and the AVA boundary walks through examples, and our Coombsville neighborhood guide covers the residential side.

Soils, bay influence and what they mean for owners

The 2011 rule cites soils of volcanic origin, with Hambright-Rock outcrop on about 28.5 percent of the area and Coombs soils on about 24.1 percent. It describes marine influence from San Pablo Bay and morning fog that clears on a different schedule from neighboring areas, about 2,550 growing degree-days (Winkler Region II) and average rainfall of about 19 inches a year. Rock outcrop on the slopes and deeper soils on the benches can sit on the same parcel, so a soil survey and ripping history matter as much here as the appellation name. For neighboring cool-climate ground, see our Carneros vineyards guide.

Groundwater: the MST area

This is the section to read twice. Napa County’s baseline groundwater report identifies the Milliken-Sarco-Tulucay (MST) area as the County’s groundwater-deficient area because of long-term declines in groundwater levels. It notes that MST lies outside any DWR-designated basin, and it places a cluster of pumping wells around Third Avenue between Coombsville Road and North Avenue, inside the appellation. An adjacent piece of the state subbasin, the Northeast Napa Management Area, is managed separately.

The County’s groundwater FAQ says that since 2004 the County has not approved a discretionary project in the MST that could not meet a "no net increase" standard for groundwater use. In plain terms, if your plan needs a County discretionary permit, such as a winery use permit or a new vineyard that needs CEQA review, expect to show it will not increase groundwater pumping. Ask the seller for well logs, any County water availability analysis and records of past permit conditions. The County also reports that its groundwater ordinance and water availability analysis are being updated in its water year 2025 report, so check the current standard before you write an offer.

Recycled water as an alternative

The County and the Napa Sanitation District created the MST Recycled Water Project to deliver recycled water for landscape and vineyard irrigation in place of groundwater. A Community Facilities District to finance the pipeline was formed in December 2012, and owners on or near the route can ask to annex; those who join accept a maximum special tax for a set term, plus water charges and their own on-site connection. Participation is voluntary. If a parcel is already in the district, the special tax should appear on the property tax bill; if it is not, ask whether a connection is available. The County page may not reflect current terms, so confirm with the agencies. See recycled water for vineyards and vineyard water rights.

Zoning, hillside rules and winery permits

On unincorporated land, the County’s Agricultural Preservation and Land Use Element sets a 40-acre minimum for Agricultural Resource parcels and 160 acres for Agriculture, Watershed and Open Space. Policies from Measure J (1990) and Measure P (2008) cannot be changed without voter approval until after December 31, 2058. Parcels close to town can carry non-agricultural designations, so confirm the designation and zoning of the exact parcel; see Napa Ag Preserve and AW zoning and building a home on Napa ag land.

New planting on the hillsides toward Mt. George needs an erosion control plan on slopes over 5 percent, per the County’s erosion control plan page. The Conservation Regulations compliance guide requires a Use Permit Exception for disturbance on 30 to 50 percent slopes or inside stream setbacks, and in the AW district 70 percent of the 2016 forest and oak canopy must stay. Milliken Reservoir is among the County’s listed sensitive domestic water supply drainages; check whether any part of a parcel drains to it.

For wineries, the 2014–15 Napa County Civil Grand Jury report summarizes the Winery Definition Ordinance’s 10-acre minimum for new wineries and 75 percent Napa County grape rule; confirm the current code, and in Coombsville pair any winery plan with the MST water standard. See Napa winery use permits and the Winery Definition Ordinance. The County’s Williamson Act program applies 40-acre (non-prime) and 10-acre (prime) minimums.

Fire

The eastern hills are wildland. Look up the parcel on the State Fire Marshal’s hazard maps, whose State Responsibility Area maps took effect April 1, 2024, and on Napa County’s 2025 local maps for land nearer the city. Read wildfire smoke taint and vineyard and winery insurance.

A due diligence list for a Coombsville vineyard

Start with our vineyard due diligence guide, then add:

• Plot the parcel against 27 CFR 9.223, especially near Milliken Creek, the Napa River and Imola Avenue.
• Confirm whether the parcel is in the MST area and what County water standard applies to your plans.
• Get well logs, water level history and any prior water availability analysis or permit conditions.
• Check whether the parcel is in the MST recycled water district, or whether it can annex.
• Confirm General Plan designation, zoning and whether the parcel is near or inside City of Napa limits.
• Map rock outcrop, soils and slopes before planning new blocks.
• Pull any erosion control plans and check for stream setbacks.
• For a winery, review the use permit and how it handled groundwater.
• Check fire hazard zone, access roads and insurance.
• Review grape contracts, vine age, rootstock, clone and virus status.

Questions people ask

Sources & notes

1. eCFR: 27 CFR 9.223, Coombsville
2. Federal Register: Establishment of the Coombsville Viticultural Area (2011)
3. Napa County: Baseline Data Report, Chapter 17, Groundwater Hydrology
4. Napa County: Groundwater FAQs and WY2025 Annual Report Executive Summary
5. Napa County: Milliken-Sarco-Tulocay Recycled Water Project
6. Napa County: General Plan, Agricultural Preservation and Land Use Element
7. Napa County: Compliance with Conservation Regulations and Erosion Control Plan Review
8. Napa County: Williamson Act and Agricultural Preserve Contracts
9. Napa County Civil Grand Jury: 2014–15 report on wineries (WDO summary; confirm against current code)
10. Office of the State Fire Marshal: Fire Hazard Severity Zones; Napa County: 2025 FHSZ Maps

The MST area and the Coombsville AVA are separately defined and do not share a boundary; confirm a parcel’s status in each. Acreage and vineyard counts are 2011 petition figures. General information, not legal or tax advice. Last reviewed November 2026 by Matthew Martinez.

Related: Napa Valley vineyards · Coombsville real estate guide · Stags Leap District vineyards · St. Helena vineyards · Vineyard due diligence · Vineyard water rights · All vineyard and winery guides