Carneros, Jameson/American Canyon, Napa Wetlands

Geology

The geology of the Carneros Subarea is marked by the extensive presence of the generally low permeability Huichica Formation with minor occurrences of volcanic rocks and alluvial deposits. The alluvial deposits tend to be thin and found in the vicinity of Carneros and Huichica Creeks. Based on limited available data the Jameson/American Canyon and Napa River Marshes Subareas have fine-grained alluvial deposits present at the surface at lower elevations. Higher elevation areas of the Jameson/American Canyon Subarea also include outcropped sedimentary rocks.  

Groundwater Levels

In 2020, 12 groundwater level monitoring sites were located in the Carneros Subarea. The longest period of record among them extended back to October 2011. All monitored wells are located in the southern half of the subarea at land surface elevations between 100 feet and 15 feet. 

Patterns of groundwater level fluctuations in these wells have shown annual variations of approximately 5 feet from spring to fall. Groundwater elevations range from about 30 feet to -5 feet. Depths to groundwater below ground surface have varied more widely from 5 feet to 100 feet. Groundwater levels have been stable to increasing in 11 of 12 of the currently monitored wells. In 2019, groundwater levels were above levels measured in 2014 and 2015 in the one well that showed groundwater level declines since monitoring began in 2011. 

In the Jameson/American Canyon Subarea the only current groundwater level data are from 1 well recently volunteered for monitoring. Spring and fall measurements recorded in that well between 2014 and 2020 found shallow depths to groundwater ranging from 3 feet in the spring to 14 feet in the fall.  

Groundwater Quality

Subareas south of the Napa Valley Floor may be susceptible to seawater intrusion originating from San Pablo Bay. In the Carneros Subarea, where long-term records are available, 2 wells have shown increasing Total Dissolved Solids (TDS) trends, though all 4 wells with long-term records were initially at or above the secondary Maximum Contaminant Levels (MCL). Nitrate concentrations have been stable to decreasing in all 5 wells with long-term records in the Carneros Subarea and Napa-Sonoma Lowlands Subbasin. 

Construction information for wells with groundwater quality data in the 3 subareas south of the Napa Valley Floor are limited, making it difficult to determine the source and distribution of the elevated TDS and salinity values. For example, it is not clear whether high salinity groundwater observed in some wells in the Carneros Subarea is a result of saltwater intrusion or interaction of groundwater with the geologic units present in and around the subarea.  

Map

Click the link or image below to open an interactive map for this subarea.

Carneros, Jameson/American Canyon, Napa River Marshes 

Cross Sections & Figures

G-G’ Carneros River Marshes Subarea

H-H’ Carneros Subarea

Animation of conceptualized sub-surface geology

Geologic Layers Figure