Aquifers are an unseen but critical resource in California’s water supply system.
These natural basins that sit below the surface are found underneath 40 percent of California’s land area.
Aquifers come in two types. Some are formed in the space between porous materials such as sand, gravel, silt or clay and are known as alluvial aquifers or unconfined aquifers. However, in many places in California, there are aquifers beneath a rock layer that does not allow water to permeate in measurable amounts. These are known as confined aquifers.
Confined aquifers under pressure are known as artesian aquifers. This pressure can push water to the surface, which when drilled are called artesian wells. Aquifers can also be several feet thick or several thousand feet thick and can be found across California, from the brackish coast to the Central Valley to the desert interior.
This diverse geography brings with it a range of challenges. Adding to those challenges, California uses more groundwater—the main water source for aquifers—than can be replaced naturally or artificially.
Aquifers play an important role as a source of fresh water for urban areas and agricultural irrigation. Unlike surface water, which is mostly found in the northern and eastern parts of the state, aquifers are widely distributed throughout California. Additionally, they are also often found in places where fresh water is most needed, for instance, in the Central Valley and Los Angeles.
Similar to a below ground sponge, aquifers are the natural accumulation of runoff and precipitation. Below the surface, this runoff then percolates into crevices between rocks, silt, and other material.
Generally, water purveyors prefer to tap aquifers closest to the surface because it is more practical and cheaper to pump. Alluvial aquifers, confined by looser material, are also favorable sources of groundwater rather than the tougher confined aquifers. However, some confined aquifers, if drilled, can contain enough pressure to drive water to the surface without a pump. Such aquifers are called artesian aquifers, and at the surface they connect to artesian wells.
Overall, aquifers—as a sub-surface natural reservoir—also help meet California’s increasing demand for .groundwater. At the moment, for example, roughly 30 percent of the state’s water needs are met by groundwater. Water officials also view depleted aquifers as good places to store water for use in the future, a process known as groundwater banking.
Even so, as California continues to grow and consume more water, and pay increased attention to environmental stewardship, new challenges confront aquifer use.
Surging demand for water, and such related practices as overdrafting, have burdened aquifers and connected ecosystems in recent decades.
In consequence, depleted aquifers are vulnerable to both natural and man made contamination.
With nature, elements such as radon and arsenic can leach into aquifers that have been drawn down. Meanwhile, lowered levels of fresh water in the top layers of aquifers can also expose heavier salt water sitting at the bottom, degrading water quality. At the same time, along the coast, particularly in the highly agricultural Pajaro and Salinas valleys, sea water can also intrude into depleted aquifers.
Similarly, in some cases, salty groundwater can come from ancient seawater that was isolated in the subsurface sediments until pumping began. The northern San Joaquin Valley near Stockton has problems with pockets of sea water that pumps have drawn into the fresh water aquifers. In the northern Sacramento Valley, some pumpers are at risk of tapping into ancient deposits of sea water, which can degrade the quality of the groundwater.
Man made pollutants, once thought to be filtered out by the soil, are now known to harm water quality as well. These contaminants include industrial solvents and pesticides.
In some cases, the challenges to aquifersustainability are being addressed by re-charging aquifers with surface water (rather than waiting for them to re-fill naturally) through a process called conjunctive use.
In other situations, proper management of aquifers is still in development, particularly as courts have established that groundwater may be pumped and used on land far from the original source.