Groundwater Subareas: Napa Valley Floor – Yountville and Napa

The Yountville and Napa Subareas comprise the southern portion of the Napa Valley Floor. The geology of these subareas is distinguished by the alluvial sediments, such as clay, silt, and sand, deposited across the valley and ranging in thickness from a few tens of feet to hundreds of feet. Coarse-grained deposits in the alluvium, such as sands and gravels, form the primary aquifer units in the alluvium. The alluvial sediments lie atop various rock deposits, primarily volcanic formations with lesser occurrences of sedimentary formations, which contain lower aquifer units.

 

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SGMA Representative Groundwater Hydrographs, Northern Napa Valley Subbasin

SGMA Representative Groundwater Hydrographs, Southern Napa Valley Subbasin

 

Over the long-term, groundwater levels have remained broadly stable in the Yountville and Napa Subareas. In the Yountville Subarea, the depths to groundwater in the spring are generally less than ten to 20 feet. In the Napa Subarea, depths to groundwater in the spring are generally less than ten feet to about 30 feet below ground surface. The deepest depths to water are found in portions of the subarea east of the Napa River.

Seasonal groundwater level fluctuations vary by proximity to the center of the valley in the Yountville and Napa Subareas. Along the western and eastern edges of the valley, levels are subject to larger seasonal fluctuations. In the Yountville Subarea, groundwater levels in the center of the valley fluctuate from 10 to 25 feet seasonally, and near the edge of the valley groundwater levels fluctuate by about 25 to 35 feet.

In the northeast portion of the Napa Subarea, east of the Napa River, groundwater levels in two wells monitored by the County have declined by about 20 feet to 30 feet, with groundwater levels in those same wells having stabilized since about 2009. Due to concerns relating to continued groundwater development in the area, and due to the complex hydrogeologic setting which includes mapped faults and the Napa River in relatively close proximity to the area of interest, the County authorized a study to better understand groundwater conditions and potential factors relating to historical groundwater levels in the northeast Napa Area. The study, completed in September 2017, evaluated the effects from pumping in adjacent portions of the Napa Valley Subbasin, mutual well interference in an area of interest near Petra Drive, and effects on streamflow due to groundwater conditions.

The study findings provide a more detailed understanding of groundwater conditions in the area. Among the findings are that groundwater system in the Study Area is in balance, with inflows and outflows nearly equal, over the 28-year period studied. Subsurface outflows of groundwater from the Study Area are primarily in the direction of the MST Subarea, likely due to previously documented lowered groundwater levels in the MST. Simulations involving different levels of groundwater pumping in the Study Area showed very small changes in the water table level and surface water stage when comparing baseline pumping rates to a scenario with no groundwater pumping in the Study Area. Climatic effects were found to have a much greater effect on groundwater discharge to the Napa River for three groundwater pumping scenarios: the baseline pumping simulation, a reduced pumping scenario, and a scenario where pumping rates were doubled relative to the baseline scenario. Groundwater quality is generally good in the Yountville and Napa Subareas, with limited samples exceeding standards for constituents including various metals and minerals. No spatial patterns are evident to explain the existence of elevated values.

Groundwater quality is generally good in the Yountville and Napa Subareas, with limited samples exceeding standards for constituents including various metals and minerals. No spatial patterns are evident to explain the existence of elevated values.

Groundwater Map - Napa Valley Floor – Yountville and Napa

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.

To navigate to other Groundwater subarea pages, use the links below.

Angwin and Pope Valley | Carneros, Jameson/American Canyon, and Napa River Marshes | Eastern Mountains and Western Mountains | Napa Valley Floor – Milliken-Sarco-Tulucay | Napa Valley Floor – Yountville and Napa | Napa Valley Floor – Calistoga and St. Helena