In an average year, approximately 30% of California's water needs are met by groundwater. In times of drought, groundwater consumption can rise to as much as 60%! The dependence on groundwater during dry years is based on the existence of hundreds of millions of acre-feet of water in California's 450 groundwater basins compared to approximately 45 million acre-feet in California's 1,200 surface water reservoirs.

So just what is this abundant resource known as groundwater? Groundwater is found under the earth's surface. Most groundwater is a result of rain and melted snow that has soaked into the ground, making it a self-replenishing source. Some groundwater has existed for millions of years. While there are no actual underground rivers, groundwater seeps into the ground by making its way into cracks and spaces in sand, soil, and rocks, forming an aquifer. From the aquifer, the water is brought back to the surface naturally through springs, sometimes discharging into lakes and streams, or via man-made wells.

Aquifers are replenished through a process known as recharge. Recharge can be a naturally occurring process as part of the hydrologic cycle as rainfall infiltrates the land surface and percolates into the underlying aquifers. Natural recharge occurs at different rates in different areas due to variations in properties such as soil types, plant cover, land slope, and rainfall intensity. In addition to rainfall recharge, bodies of surface water may also recharge groundwater aquifers by seeping from the sides of water bodies and percolating into the aquifer. A third source is artificial recharge where aquifers are directly replenished through the pumping or injecting of water into the wells or by spreading water over a land surface, allowing it to seep into the aquifer at a natural rate. Artificial recharge is also a useful tool to dilute contaminated groundwater supplies. Natural recharge is a much longer process than artificial recharge.  An aquifer that has been over pumped could take decades or centuries to recharge and some of these aquifers never recharge fully.

Contamination is one of the major hazards of groundwater. Groundwater is vulnerable to pollution from both natural contaminants and human negligence. The Environmental Protection Agency provides information on activities leading to ground water contamination. Since groundwater is below the surface of the land, it is extremely difficult and expensive, and many times impossible, to remove the contaminants from a contaminated groundwater source. Unfortunately, many groundwater reservoirs remain contaminated for hundreds of years, if not forever, once contamination is detected.

Some of the many resources concerning groundwater include:

Contacts for General Groundwater Information in California