Groundwater Basics

The water beneath us

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.

Groundwater is connected to surface water

Groundwater can be connected to surface water in several ways. Connections between groundwater and surface water can include instances where the water flows from groundwater into a surface water body. This is typically observed at lower elevations within the Napa Valley floor, often along the mainstem Napa River. In other instances, connections occur so that surface water flows into groundwater. In Napa Valley, these conditions are more common where streams flow out of the hillsides and form alluvial fans (areas of course sediment deposits) that are more elevated compared to the valley floor near the mainstem Napa River. The nature of connections between surface water and groundwater can vary by location; the nature of the connection can also vary over time at the same location. Not all streams in Napa Valley flow year round due to physical conditions like topography and geology. Studies are underway to better understand how stream and river systems in the valley interact with one another.

We all depend on groundwater 

Groundwater provides clean, cool water to streams and wetlands during the dry season and in dry years. This is critical for the survival and health of aquatic wildlife and plants. Groundwater is also an important source of water for humans. We use it for our homes, agriculture, and industry.  

There are several advantages to using groundwater. Groundwater is often available right underfoot. It is stored beneath land, leaving the land available for other uses whether developed or undeveloped. The quality of groundwater is often quite good and suitable for most uses. The great storage capacity of groundwater provides greater flexibility and reliability as a source of supply compared to surface water reservoirs when recharge and discharge are holistically managed together.

Taking care of groundwater today for future generations

Groundwater is a limited resource. There can be many negative effects to removing too much groundwater.   Nearby streams and wetlands may receive reduced contributions of flow from groundwater that affects beneficial uses of surface water. Groundwater levels may not fully recover during the rainy season and begin to drop over time. Groundwater reliant plant communities may die or change. The land surface may begin to subside, which can lead to ground failures and permanent loss of groundwater storage.

To make sure that groundwater is available for future generations, we need to study it, monitor it, and manage its use to avoid causing undesirable results.