Groundwater Subareas: Angwin and Pope Valley
The geology of the Angwin Subarea represents a continuation of the diverse volcanic rock types that are widespread in the adjacent Eastern Mountains Subarea. Andesite and basalt flows and pumiceous ash-flow tuffs predominate, with some limited areas of alluvial sediments also present. The Pope Valley Subarea includes both Great Valley Sequence sedimentary rocks and, in the Pope Valley Floor, thin alluvial sediments. The alluvium in the Pope Valley Subarea appears to be relatively thin, generally less than 100 feet thick and more commonly less than 50 feet thick.
In 2017, groundwater level monitoring in the Angwin Subarea was performed at five wells by Napa County, Howell Mountain Mutual Water Company, and Pacific Union College at recently volunteered wells. Groundwater level monitoring data for the Angwin Subarea wells are available from 2014 to 2017. Depths to groundwater in these wells ranged from 95 feet to 233 feet from relatively high ground surface elevations ranging from 1,608 feet to 1,747 feet. The only current groundwater level monitoring site in Pope Valley is a single well in the Pope Valley Basin with data available from 2014 to 2017. Depths to water have ranged from 3 to 16 feet below ground surface over that time.
Groundwater is generally of good quality in the Angwin Subarea except for occasional samples with iron and manganese levels above drinking water standards. Out of six wells with sufficient periods of record, water quality trends generally show decreases in EC and TDS concentrations and increasing chloride and nitrate concentrations, though the latter two remain below drinking water standards.
Groundwater quality data are limited to two sites in the Pope Valley Subarea. From the limited amount of data, however, it appears that groundwater quality is generally very good, with the exception of iron, manganese, and a slightly acidic pH (6.45 pH units).
Groundwater Map - Angwin and Pope Valley Subarea
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.
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