Groundwater Subareas: Eastern Mountains and Western Mountains

These two subareas adjoin the Napa Valley Floor and comprise the upland portions of the Napa River Watershed. A wide variety of volcanic rock types are present in the Eastern Mountains Subarea, including andesitic, basaltic, and dacite lava flows as well as various tuffs and breccias. North and east of Lake Hennessey the Eastern Subarea also includes an exposure of the older, metamorphic Franciscan Complex. The Western Mountains Subarea includes lesser outcrops of volcanic rocks, largely adjacent to the Calistoga Subarea and to the west of Yountville, with additional exposures of the sedimentary Great Valley Sequence and metamorphic Franciscan Complex.

Recent efforts to expand the Napa County monitoring network have resulted in five wells volunteered for monitoring between the two subareas. Groundwater level monitoring data for these wells are include three years of semi-annual measurements. The depths to groundwater in these wells ranged from 36 feet to 240 feet from ground surface elevations ranging from 390 feet to 1,660 feet.

From the limited amount of groundwater quality data in subareas east and west of the Napa Valley Floor, the groundwater is generally of good quality. Elevated levels of iron and manganese occur, along with lower than average pH values indicating more acidity than the Napa Valley Floor.

Groundwater Map - Eastern Mountains and Western Mountains Subareas

Using the map below, click on specific areas to find more information and links to subarea groundwater monitoring results, levels and trends. Enter your address, city or zip above the legend to zoom to an area of interest.

To navigate to other Groundwater subarea pages, use the links below.

Angwin and Pope Valley | Carneros, Jameson/American Canyon, and Napa River Marshes | Eastern Mountains and Western Mountains | Napa Valley Floor – Milliken-Sarco-Tulucay | Napa Valley Floor – Yountville and Napa | Napa Valley Floor – Calistoga and St. Helena