Napa & Yountville
Geology
The Yountville and Napa Subareas include the southern portion of the Napa Valley Subbasin. 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.
Groundwater Levels
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 10 to 20 feet under non-drought conditions, similar in nature to the Calistoga and St. Helena Subareas to the north. Seasonal fluctuations vary by proximity to the center of the valley. Along the western and eastern edges of the subarea, levels are more subject to larger seasonal fluctuations. Groundwater elevations in the center of the valley fluctuate seasonally approximately 10 to 25 feet, and near the edge of the valley fluctuate approximately 25 to 35 feet. In the Napa Subarea, depths to groundwater ranges from about 20 to 30 feet below ground surface in the spring most years. Seasonal groundwater elevations in this subarea generally fluctuate from 10 to 40 feet. The deepest depths to water are found in portions of the subarea east of the Napa River.
In the northeast portion of the Napa Subarea, east of the Napa River, groundwater levels in 2 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 3 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
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.
Map
Click the link or image below to open an interactive map for this subarea.