In an aquifer as convoluted as Shasta’s, it’s not easy to determine the effects of pumping groundwater or diverting springs for bottled water, says Gordon Grant, a hydrologist with the U.S. Forest Service’s Pacific Northwest Research Station. With most of the 80 inches of annual precipitation seeping underground, some estimates suggest as much as 40 million acre-feet may be stored in the complex volcanic fissures — more than California’s top 100 reservoirs combined.