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 water 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. 

To learn more about groundwater monitoring occuring in Napa County visit the Groundwater Trends and Conditions 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.