Groundwater Basics


Groundwater is water located below ground where the voids or pore spaces between sediment grains or rocks are fully saturated. Groundwater originates from a variety of sources, including precipitation, irrigation, infiltration from surface waters, and injection wells to name a few. Groundwater can flow from one part of the subsurface to another; it can also leave the subsurface as outflow to streams or other surface waters, spring flow, evaporation, and discharges from wells. In a sense, all groundwater starts as some form of surface water. The two types of water, surface water and groundwater, are inherently connected.

Groundwater is part of the hydrologic cycle

Depending on location, slope, geology, soil type and vegetation, the ground can receive and retain groundwater similar to the way a sponge soaks up water. The percolation process that moves water from the surface to the subsurface is often a relatively slow process that occurs over several years to over a millennium in some cases. 

Napa County Groundwater Monitoring Plan

Groundwater and surface water are highly important natural resources in Napa County. The Napa County Groundwater Monitoring Plan was put in place in order to address groundwater-related challenges within the County. The Napa County Groundwater Monitoring Plan is a living document that will be updated based upon the data collected. Its purpose is to formalize and augment current groundwater monitoring efforts (levels and quality) to better understand the groundwater resources of Napa County. The plan will aid in making the County eligible for public funds administered by the California Department of Water Resources (DWR), and will regularly evaluate trends to identify changes in levels and/or quality and factors related to those changes that warrant further examination to ensure sustainable water resources.


Over the past several years, Napa County has developed a more focused understanding of the geology that controls the occurrence and availability of groundwater. With this updated geologic information and with assistance from community members serving on the Groundwater Resources Advisory Committee (GRAC 2009-2011), the County expanded the number and distribution of wells that it monitors to improve the understanding of groundwater resources countywide. Between 2011 and 2015 the number of County monitored wells increased from 47 to nearly 100. In 2019, Napa County monitored 97 wells. Also, in 2018, DWR monitored four wells that had been suspended in response to multiple, large wildfires that burned in many areas around Napa Valley in October 2017. Part of the monitoring network expansion in recent years occurred through the construction of 10 dedicated monitoring wells at five sites in Napa Valley, designed specifically to provide data on the interactions between groundwater and surface water. In total, 109 wells were monitored in 2019.

Voluntary Groundwater Monitoring

The Voluntary Groundwater Level Monitoring Program measures groundwater elevation twice per year (Spring and Fall). These measurements improve the understanding of groundwater for the landowner and the County. A comprehensive network of privately owned volunteer wells, along with publicly owned wells, provides a greater understanding of Napa County aquifers. The program will be strengthened by expanding the voluntary well network to areas where data is lacking or nonexistent.  

2017 Groundwater Level Monitoring Sites

2019 Groundwater Level Monitoring Sites
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To learn more about groundwater monitoring occuring in Napa County visit the Groundwater Monitoring page. On this page you can read a brief summary of current monitoring results and trends. You can also use the interactive map below to get more details about groundwater in various parts of Napa County.