Groundwater Subareas: Angwin and Pope Valley

The geology of the Angwin & Pope Valley subarea is 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 2020, 6 groundwater level monitoring sites were monitored by Napa County, Howell Mountain Mutual Water Company, and Pacific Union College in the Angwin & Pope Valley subarea, with the oldest records dating back to 2014. 5 sites are located in Angwin and 1 site is in Pope Valley. Groundwater elevations range from 95 to 233 feet below ground surface in Angwin, and 3 to 35 feet below ground surface in Pope Valley, with the deepest water level recorded in fall 2020.

Groundwater quality is good in the Angwin Subarea except for occasional samples with iron and manganese levels above drinking water standards. Out of 6 wells with sufficient periods of record, water quality trends generally show decreases in EC and TDS concentrations and increasing chloride and nitrate concentrations, though 2 remain below drinking water standards. 

Groundwater quality is limited to 2 sites in Pope Valley, and appears to be generally very good, with the exception of iron, manganese, and a slightly acidic pH (6.45 pH units).