The Local Groundwater Basin

Unlike most other Bay Area communities, the Tri-Valley benefits from local water-storage capacity in an underground basin that provides us with increased water-supply reliability.

Zone 7 Water Agency is the Groundwater Sustainability Agency (GSA) for the Livermore Valley Groundwater Basin.


Water pipe labeled finished water

Local Groundwater Basin

If you think of our water storage like money in the bank, then we are fortunate to have a local “checking account” water source in the Livermore Valley Groundwater Basin. While our groundwater basin does not hold enough water to sustain the Tri-Valley community long term, it is a valuable source of natural and artificially stored water that we manage carefully to ensure it can provide water when we need it. So how in the world does the water get in there? And how do we get it out?

The formation of an aquifer

The water beneath your feet in our groundwater basin does not flow like a river, but moves slowly, generally from east to west in layers of sand and gravel known as aquifers. You can imagine it like water moving through coffee grounds and a filter. These layers of sand and gravel are separated from each other by silts and clays that act as barriers to water flow. Collectively these layers are known as aquifers. The aquifers have been naturally created by erosion of the surrounding hills over thousands of years. During rain events, streams and landslides move sediment downhill and redeposit it. Faster water deposits gravels, then sands, and slow water deposits silts and clays. Water moves through the small spaces that exist between the sands and gravels like water does when you pour it between ice cubes in your glass.

Infographic of an Aquifer








Infographic of an Aquifer


blue line pointing to wells on aquifer image

Confining Layer

(silts and clays)

Purple line pointing to layers on aquifer image

Near-surface Soil

orange line pointing to surface soil on aquifer image

Sands and Gravels

blue line pointing to gravel on aquifer image

Water entry

white line point to screens on aquifer image

Aquifer Water!

Green line pointing to water on aquifer image

Layers of an Aquifer

  1. We walk upon the surface soil, the top layer, which allows water to soak into the ground.
  2. Layers of rich soil hold the moisture in the ground under the surface.
  3. Gravity pulls water into a layer of sand and gravel deposited thousands of years ago which filter the water naturally as it travels deeper under our feet.
  4. The layers of silts and clays act as a confining layer for the sands and gravels that are beneath them. This confining layer creates pressure in the water beneath it, along with the weight of the world above, and makes it easier to pump water out of lower levels.
  5. “Artesian” is a term that means that water is under pressure and rises to the surface of the earth or even above it.
  6. In the deepest layers of the basin miniscule rocks leave enough space between them for water to gather in an underground basin.

How in the world do we get the water out?

Wells are drilled deep into the Earth to tap into our groundwater basin. Open pipes are placed in the drilled hole to keep the hole open and allow water to be pumped from the ground. The bottom of the well shaft has screens along the sides that allow the water to flow into the well so it can be pumped to the surface. We must be careful in our extraction of groundwater so we do not deplete the resource in the basin, known as overdraft. In addition, it is important we recharge the water we extract from the basin to ensure it continues to be available as a resource. You can imagine the extraction from a well similarly to drinking through a straw in a glass of ice water.

Recharging our local groundwater basin

Part of sustainably managing the groundwater basin is ensuring we refill or recharge the aquifers on the basin. This is done naturally with rainfall and flow from local streams. We also recharge the basin “artificially” with imported water purchased from the State Water Project. Special turnouts allow water to be released from the South Bay Aqueduct to local streams that channel the water into the soil where it can begin the process of filtering through the layers into the aquifers below.


Learn more about how Zone 7 sustainably manages this precious resource.