These 2 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 5 wells volunteered for monitoring between the two subareas. Groundwater level monitoring data for these wells include measurements recorded semi-annually for the past 7 years. The depths to groundwater in these wells ranged from 4 feet to about 250 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 below average pH values indicating more acidity than the Napa Valley Floor.
Click the link or image below to open an interactive map for this subarea.