Artificially refilling aquifers - even with treated wastewater - may be the best answer for the West's strained water supplies, according to water supply experts.
Still, hidden and complex subterranean geology makes aquifer storage an inexact science, said water officials at the American Groundwater Water Trust forum here two weeks ago.
Rapidly growing cities throughout the West are looking to groundwater supplies to offset shortfalls in streams, the primary water source for most municipalities.
Water levels in major aquifers are falling at alarming levels, water officials say.
Pools in the massive Denver Basin are dropping as much as an inch a day in some areas, for example, requiring ever-deeper wells and prompting concerns over whether there are adequate long-term supplies for municipalities.
In places as varied as Orange County, Calif., and the San Luis Valley, officials have found success injecting excess water back into the ground during wet years to replenish aquifers and even bolster above-ground streams.
"Managed recharge has been going on for years," said Andrew Stone, executive director of the nonprofit groundwater trust.
The Centennial Water District has injected water into existing wells for more than a decade, and Aurora's under-construction Prairie Waters project includes a groundwater recharge effort intended to filter wastewater for reuse.
Storing water underground minimizes evaporation losses typical for surface reservoirs, and the process can serve as filtration because most aquifers aren't open caverns so much as porous "sponges" of cobble and sands.
Environmental concerns with aquifer recharge center on water- quality issues such as contamination, but they can be limited if the water is cleaned first.
"Overall, the environmental trade-offs in many cases are going to be worth doing underground storage," especially compared with dams, said Melinda Kassen of Trout Unlimited.