No swimming allowed

People who have seen too many movies often envision an aquifer as some kind of underground cavern filled with water. It doesn’t work that way. You can’t scuba dive in the Lower Tuscan because it’s made mostly of wet sand and gravel. When you drill a well into it, it’s like you’re inserting a giant straw into a pressurized mud puddle. The lower the pressure, the harder you’d have to drive your pumps to get the water out. Pumping one part of an aquifer can have unexpected results in another part.

There are several aquifers underneath Butte County, which are layered more or less on top of each other. In the foothills, you can actually see where ancient lava flows begin to descend beneath the topsoil, creating the Upper Tuscan Formation, which underlies the relatively shallow alluvial formation. The Lower Tuscan is about 800 to 1,000 feet below the surface. Because its “roof” is composed of ancient volcanic mud, it is a confined aquifer and therefore can be pressurized, as evidenced by year-round artesian wells that flow from the Lower Tuscan.

Geologists have only recently begun to map the complex system of aquifers beneath us. They do so by drilling wells and testing water quality and flow, as well as using “e-log” data from previously drilled natural-gas wells. E-logs can be used to define each aquifer’s electronic “signature,” which is then used to figure out the extent and depth of each aquifer. The very lowest aquifer under the Sacramento Valley is full of ancient sea water, a remnant of the time when the whole area was under the ocean.

Aquifers are naturally recharged by rainfall and by water percolating through porous areas like those under streams and rivers. Butte County is thought to contain most of the “recharge zones” for the Lower Tuscan, a recent discovery that has led to a flurry of speculation and debate over what should be done to protect or augment those zones. But scientists studying the aquifer say they need a lot more information before they can say definitively where the aquifer is refilled. —J.I.