Survey report reveals groundwater details, dispels myth

JEROME -- Federal scientists have proven once and for all that the persistent myth of a giant underground lake in the Verde Valley is just that -- a myth.

An above-ground lake did cover the Verde Valley at one time, but it dried up millions of years ago.

While the scientists weren't surprised by the lack of an underground lake in the Verde Valley, they were surprised to find a deep sub-basin in the Las Vegas Ranch area of the Williamson Valley north of Prescott that might hold a substantial amount of groundwater.

Unlike the larger Big Chino sub-basin that has evidence of the Big Chino Fault above the ground along the base of Big Black Mesa, evidence of the underground faulting was not visible above ground in Williamson Valley. Scientists previously thought the groundwater system in Williamson Valley was smaller and irregular. City of Prescott test wells helped confirm that the basin exists.

Geophysicist Vicki Langenheim presented these and other results of a long-awaited U.S. Geological Survey report to the Yavapai County Water Advisory Committee Wednesday in Jerome.

The 43-page report with detailed maps provides the most comprehensive look to date of the geology of the upper and middle Verde River watershed. The four-year study precedes and aids the study that the agency now plans to release by Sept. 30.

While the geology study found out that the basin under Williamson Valley is larger than previously thought, other basins in the watershed are about the same size as previously thought, Langenheim said. The Big Chino sub-basin is the largest, at about 4,000 feet deep, 12 miles long and two miles wide. The Williamson Valley sub-basin is about half that size.

The basins may hold water, depending on what kind of sediment fills them. The hydrological study will look at the water side of the equation.

The local governmental members of the Yavapai County Water Advisory Committee helped the USGS and state pay for the $2.5 million studies of the upper and middle Verde watershed geology and hydrology.

Langenheim conducted the new geologic study with two other USGS scientists -- Ed DeWitt, who has been studying the Verde River Basin geology for decades, and Laurie Wirt, who created a stir several years ago when she concluded that 70 percent to 80 percent of the flow in the upper Verde River originates from the controversial Big Chino sub-basin.

Prescott and Prescott Valley plan to pump Big Chino groundwater south to their Little Chino sub-basin, which is only about one-fourth the size of the Big Chino.

The USGS studies are especially important in this watershed because regions such as the Prescott tri-city area are highly dependent on groundwater. The geologic study better defines the boundaries of the ground-water systems, while the hydrologic study will try to estimate how much water might be in those systems.

"Geology can only take you so far," said Langenheim, but such studies are necessary because scientists can't ever dig enough wells to get the information they need more directly. It would require too many costly wells.

Using an analogy to a home, the geology provides the floor and walls to the Verde watershed's groundwater system, while the hydrology will add the rooms and stairs, Langenheim said.

The geologic study estimates the depth of ancient granite and schist rocks that provide an impermeable "basement" to groundwater sub-basins in the watershed. It includes maps showing the basement as well as the younger rocks above it that actually can hold water: older sedimentary rocks such as limestone, topped by volcanics that are topped by younger sediments.

The study also uses the latest technology to locate faults that could provide passage for underground water supplies. Aerial flights to measure underground magnetics and gravity meters on the ground helped map basement rocks and faults, while analysis of more than 1,300 well-drilling logs helped confirm the geological conclusions.

With the help of previous studies, the new study does the best job so far of quantifying the size of sub-basins that could hold groundwater.

Now it's up to the USGS hydrologists to subsequently discern whether the sediments consist of gravel that can hold a lot of water or clay that leaves much less room.

Some of the other results of the geologic report include:

¥ Numerous underground volcanic plugs in the Little Chino sub-basin in the Prescott tri-city area likely impede the flow of ground-water to the upper Verde River. Some of the plugs are exposed above the ground, such as the landmark Thumb Butte next to Prescott. Numerous plugs also are between the Big and Little Chino sub-basins.

¥ A relatively young, narrow and deep fault parallels the west side of the Verde River in the Verde Valley. USGS scientists named it the Bridgeport Fault because it goes through that unincorporated community. The sub-basin along that fault is about 3,000 to 4,000 feet deep, nine miles long and two to three miles wide, Langenheim estimated.

¥ Geology indicates there could be a small, deep groundwater basin northwest of Page Springs and another one southwest of Camp Verde in the Verde Valley.

¥ The fault that crosses Page Springs is about 10 miles longer than scientists previously thought. The springs supply a state fish hatchery. The fault may explain why the springs exist, Langenheim said. Faults are important because they can act as barriers or conduits to groundwater flow.

Copies of the new geology report are available online at http://pubs.usgs.gov/of/2005/1154.