Groundwater Trends and Conditions
Napa County and other public agencies have been studying groundwater in the area since the mid-1900s. Although groundwater occurs in all areas of the County, the Department of Water Resources (DWR) has delineated several groundwater basins and subbasins where the underlying geology consists of alluvial sediments, which are unconsolidated and tend to hold and yield groundwater more readily. The Napa Valley Subbasin, which coincides approximately with the Napa Valley Floor, is the largest groundwater basin/subbasin in the County. Between the Napa Valley Subbasin and the San Pablo Bay is the Napa-Sonoma Lowlands Subbasin. Three other groundwater basins also occur in the County: Pope Valley Basin, Berryessa Valley Basin, and a small area within the Suisun-Fairfield Valley Basin. The County has also been divided into different groundwater subareas based on geopolitical and hydrogeologic characteristics, to facilitate discussion of conditions in different areas of the County. The general locations and descriptions of the different subareas are included below.
The County’s groundwater monitoring program includes a network of sites for tracking groundwater and related conditions in the County, including wells, streamflow gages, StreamWatch sites (Napa County RCD flow conditions observation sites), land subsidence stations, and more. Monitoring of wells is an important part of the County’s efforts to track trends in groundwater levels, storage, and quality and to support implementation of the Groundwater Sustainability Plan for the Napa Valley Subbasin. Groundwater monitoring in the County began in the early 1900s, conducted by federal, state, and local entities. The County has added many wells to the monitoring network to improve understanding of groundwater conditions and characteristics and the monitoring network continues to evolve to address monitoring needs. Most wells in the County’s groundwater monitoring program are located in the Napa Valley Subbasin and in the Milliken-Sarco-Tulucay area and include a combination of well types with different characteristics (e.g., depth and screen interval) distributed across the County.
Although groundwater levels in the County drop during the dry season and during dry years, they typically recover during wetter periods as a result of recharge from precipitation. Find information about the latest trends and conditions at individual monitoring sites by visiting the interactive Napa County Groundwater Webmap.
The following resources provide additional information on the County’s monitoring program and recent groundwater trends and conditions in the County.
- Annual reports on groundwater conditions
- A video about groundwater monitoring in Napa County and how the results are used
- An interactive storymap about groundwater and surface water connectivity in Napa County
Calistoga, St. Helena, Yountville, and Napa Subareas
The Napa and Yountville subareas encompass the southern portion of the Napa Valley Subbasin and the Calistoga and St Helena subareas align roughly with the northern portion of the Napa Valley Subbasin.
The geology of the Calistoga, St. Helena, Yountville, and Napa subareas is distinguished by the more extensive presence of alluvium, consisting of sediments such as clay, silt, sand, and gravel deposited across the valley and ranging in thickness from a few tens of feet to hundreds of feet. The alluvium is one of the principal aquifers in the Napa Valley Subbasin. Coarse-grained sediments, such as sands and gravels, are the primary water-bearing materials of the alluvium. The alluvium in the Napa Valley Subbasin directly overlies more consolidated rocks, primarily volcanic formations, which include several porous units capable of storing and yielding economically significant groundwater.
In the northern part of the Napa Valley Subbasin, in the Calistoga and St. Helena subareas, available data on historical groundwater levels indicate relatively shallow spring groundwater levels (depths <20 feet below ground surface) with typical seasonal fluctuations in levels (spring to fall) of 10 to 20 feet.
In the Yountville subarea, spring groundwater levels are generally less than 10 to 20 feet below ground, similar to the Calistoga and St. Helena subareas to the north, although seasonal water level fluctuations in the Yountville subarea range more widely from 10 feet to more than 30 feet depending on proximity to the edge of the valley floor and aquifer characteristics. Seasonal fluctuations tend to be less in the center of the valley and in shallower unconsolidated deposits and greater in areas closer to the edges of the valley floor and in deeper more consolidated aquifer units. In the Napa subarea, spring groundwater levels are typically between about 20 to 30 feet below ground surface with seasonal water level fluctuations ranging in depth from 10 to 40 feet. The deepest groundwater levels in the subarea occur in a part of the subarea identified as the Northeast Napa Management Area where historical groundwater level declines have been noted. The Northeast Napa Management Area is located east of the Napa River and west of the Milliken-Sarco-Tulucay subarea, within the Valley Floor in an area of thinner alluvium bounded by geologic faults to the west and east that affect lateral groundwater flow and recharge to the area. Lower groundwater levels in areas adjacent to the Northeast Napa Management Area are also believed to be a contributing factor in historical water level declines exhibited within the Northeast Napa Management Area.
Groundwater quality within the Napa Valley Subbasin is generally good, but it can vary by location. In the northern areas of the valley in the Calistoga subarea and in some areas along the edges of the valley, available water quality data indicate naturally elevated concentrations of boron, arsenic, chloride, total dissolved solids (TDS), sulfate, and fluoride in some wells, mostly related to influence from geothermal waters. In the Yountville and Napa subareas groundwater quality is generally good with inconsistent and occasional occurrences of naturally elevated metals and minerals.
Milliken-Sarco-Tulucay (MST) Subarea
The Milliken-Sarco-Tulucay (MST) subarea overlaps part of the Napa Valley Subbasin and includes the watersheds of the Milliken, Sarco, and Tulucay Creeks on the eastern side of the Napa Valley.
The primary source of groundwater produced in the MST is the tuffaceous member of the Sonoma Volcanics, which occurs in the northern and southern parts of the MST on the eastern side of the Soda Creek Fault. Below the tuff, and underlying the entire MST, are the andesitic and basaltic lava flows of the Sonoma Volcanics, units which generally yield only small amounts of groundwater. Thin deposits of alluvium exist along stream channels in the area and are the basis for the extension of the Napa Valley Subbasin in the subarea. The majority of the MST is outside of the Napa Valley Subbasin and is not part of a groundwater basin designated by DWR.
Depths to groundwater in the MST subarea are highly variable and can range from 30 feet or less in some areas to 200 feet or greater in other areas. The MST subarea has been designated as a water deficient area by Napa County, largely because of historical groundwater level declines observed in the area as early as the 1960s and 1970s and persisting more during recent very dry periods. Proposed development projects involving the use of groundwater are more stringently regulated in the MST than in other parts of the County. Use of alternative water supplies in the MST, including recycled water along with continued land use permitting constraints and water conservation efforts, are expected to aid in maintaining sustainable groundwater level conditions in the MST subarea in the future.
The limited data available on groundwater quality in the MST subarea suggests it is generally good although also highly variable. Naturally occurring constituents including metals like boron, arsenic, and barium and inorganic constituents such as sodium, chloride, and sulfate have been measured at elevated concentrations in some wells in the MST as a result of the geologic environment.
Carneros, Napa River Marshes, and Jameson/American Canyon Subareas
The Carneros, Jameson/American Canyon, and Napa River Marshes subareas are in the southern part of the County and span areas including parts of the Napa Valley and the Napa-Sonoma Lowlands Subbasins.
The geology of the Carneros subarea is marked by the extensive presence of the Huichica Formation with limited areas of volcanic rocks and alluvial deposits. Alluvial deposits in the Carneros subarea tend to be thin and found in the vicinity of Carneros and Huichica Creeks. Although there is limited information on the geology in the Napa River Marshes and Jameson/American Canyon subareas, available data suggest fine-grained alluvial deposits exist at the surface at lower elevations, whereas some higher elevation areas of the Jameson/American Canyon subarea include surface outcroppings of sedimentary rocks.
Available groundwater level data for the Carneros subarea indicate depths to groundwater ranging from 5 feet to 100 feet although the range of groundwater elevations is considerably smaller between 30 feet and -5 feet above mean sea level. Seasonal fluctuations in groundwater levels tend to be relatively small (about 5 feet). Available groundwater level data in the Carneros subarea are limited to the southern part of the subarea and only since 2011. Groundwater level data are even more limited in the Jameson/American Canyon subarea, but available data suggest groundwater levels can be very shallow (<15 feet) in both spring and fall.
Areas within the Carneros, Napa River Marshes, and Jamison/American Canyon subareas where long-term groundwater quality records are available have shown elevated TDS concentrations. The existence of elevated TDS in groundwater in these wells is believed to be a result of salinity derived from marine sediments in which groundwater occurs or from influences on groundwater from higher salinity migrating through well structures without appropriate annular seals.
Eastern and Western Mountains Subareas
The Eastern and Western Mountains subareas cover the upland areas to the west and east of the Napa Valley Subbasin. A wide variety of volcanic rock types exist 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 fewer exposures of volcanic rocks, most of which occur adjacent to the Calistoga subarea and to the west of Yountville, with additional exposures of the sedimentary Great Valley Sequence and metamorphic Franciscan Complex. The tuffaceous member of the Sonoma Volcanics, where present, can yield water more readily than rocks of the Franciscan Complex and Great Valley Sequence. However, the natural water-bearing and yielding properties of most of the other geologic units is limited in the subareas, and groundwater storage and transmission are highly influenced by the degree (and to a lesser extent, orientation) of fracturing.
Groundwater level data in these subareas indicate high variability in depths to groundwater including a range of 4 feet to 250 feet in wells monitored. The limited groundwater quality data in Eastern and Western Mountains subareas suggest the groundwater quality is generally good although some elevated levels of iron and manganese and lower pH (higher acidity) water exist in the areas.
Pope Valley, Berryessa, and Angwin Subareas
The Pope Valley and Berryessa subareas include the Pope Valley Basin and Berryessa Valley Basin in the eastern part of the County. The geology of the Pope Valley subarea is dominated by sedimentary rocks, including sandstones and shales of the Great Valley Sequence, with areas of surficial alluvial sediments that define the Pope Valley Basin, a groundwater basin designated by DWR. 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. Where thicker and more laterally extensive, the alluvium in Pope Valley has characteristics more suitable for yielding larger quantities of groundwater. The Berryessa subarea is also comprised mostly of rocks of the Great Valley Sequence, with only few areas of thin alluvium along the eastern shores of Lake Berryessa, which define the extent of what DWR has defined as the Berryessa Valley Basin. As noted for other subareas, rocks of the Great Valley Sequence can yield limited supplies of water, which tend to occur in association with fracture networks in the rocks. The Angwin subarea consists mainly of andesite and basalt flows and pumiceous ash-flow tuff units of the Sonoma Volcanics, with minor areas of alluvium.
Limited groundwater level data exist in the Pope Valley subarea indicating groundwater levels ranging from 3 to 35 feet below ground surface and seasonal fluctuations of between 10 and 15 feet. No long-term data are available on groundwater levels in the Berryessa subarea, although review of static water level data provided on DWR Well Completion Reports at the time of well construction suggest water levels are variable depending on location and well depth. Some wells closer to Lake Berryessa exhibit groundwater levels of 10 to 30 feet below ground surface, but levels may be considerably deeper in more upland areas away from the lake. In Angwin, groundwater levels tend to be deeper with levels in wells with historical monitoring having depths to groundwater ranging from 95 to 233 feet below ground surface with relatively small seasonal fluctuations in levels (~10 feet).
Groundwater quality data are also very limited in these areas; however, based on data from two wells in Pope Valley, the water quality appears to be generally good with potential for elevated iron and manganese concentrations. In Angwin, groundwater quality also appears to be good with some instances of elevated iron and manganese concentrations; recent data have shown some decreases in TDS (salinity) concentrations, while some increases in nitrate and chloride concentrations have also been noted.
Other Groundwater Subareas
Other groundwater subareas in the County include Knoxville, Livermore Ranch, Central Interior Valleys, and Southern Interior Valleys. These areas include limited development and are dominated by the presence of sedimentary rocks of the Great Valley Sequence with some units of the Sonoma Volcanics. Although the geology of these areas limits yields from groundwater wells, some localized areas of alluvium exist, including part of the Fairfield-Suisun Valley Basin within the Southern Interior Valley subarea, where more groundwater tends to occur. Little information is available on groundwater levels and quality in these areas; however, it is likely conditions are similar to other adjacent subareas with similar geologic settings.