In 1998, residents of Placitas, N.M., a few miles from the Rio Grande, were dealing with drought and the potential for hundreds of new homes tapping into the underground water supply.

Some of the Placitas domestic wells were fine. Others nearby were going dry.

No one really understood what was happening with the aquifer.

With money from the state Legislature, a group of residents and Sandoval County officials called in the cavalry in the form of hydrogeologist Peggy Johnson and other geologists from the New Mexico Institute of Mining and Technology.

Johnson, manager of the aquifer mapping program at the New Mexico Bureau of Geology and Mineral Resources, began a 2 1/2-year process of collecting data on well locations, water quality and water-table depths. The scientists overlaid existing geologic maps with the new aquifer information.

In vivid color and topographic detail, the new maps illustrated how the jumbled rock layers and fractures underground impacted water levels in the domestic wells. "Some of loveliest places in Placitas overlay rock that did not have good water-bearing capacity," Johnson said.

Sandoval County planners and Placitas residents could look at the maps and see why some wells were in the wrong spot for good water.

"All we do is provide the tools. How they choose to use the information is up to them," Johnson said.

Water flowing underground toward the Rio Grande from the mountains travels through a labyrinth of rock layers. Sand, gravel, shale, granite and other rock were layered one on top of the other over millions of years, then fractured and lifted into a patchwork quilt by multiple heaves of the Earth's crust.

Mapping those geologic layers is key to understanding the quality and flow of underground water, a critical component to managing the state's waters.

For Johnson, mapping aquifers with the bureau is her "dream job." She grew up in Boise, Idaho, fascinated with math and science. She took a geology class and was "astonished." On a field trip to natural springs, she learned how the chemical properties of water can be used to trace its journey through rock layers. She was hooked: "I said that's what I want to do."

Johnson earned a bachelor's degree in geology and a master's in hydrology. She worked as a hydrogeologist with Daniel B. Stevens and Associates in Albuquerque before landing the job at New Mexico Tech in 1996 as one of 200 applicants. "It's one of the only places in the state where I could do exactly what I dreamed of and do it as a public service," Johnson said.

Johnson has managed aquifer-mapping projects for Placitas, Santa Fe and Española. Currently, she's part of a team mapping springs along 80 miles of the Rio Grande Gorge from the Colorado border to Embudo. The team has mapped more than 100 springs so far, a project it loves because most of the members, including Johnson, are boaters.

Johnson also applies her knowledge to proposing solutions in water management. In 2007, Johnson was one of three editors for the book Water Resources of the Middle Rio Grande, created for state legislators and other officials as part of the annual Decision-Makers Field Conference hosted by New Mexico Tech.

Johnson's job mapping aquifers is a collaborative effort with other scientists in the bureau.

The map-making team has focused on areas with the greatest potential for water conflicts, such as the Rio Grande and the Pecos River. They map one 60-square-mile-section — or a smaller section — at a time. Each section is called a quadrangle.

The foundation of aquifer mapping begins in six metal buildings on the outskirts of the New Mexico Tech campus. Housed there are thousands of donated cylindrical rock core samples and cuttings, the only collection of its kind in the state. Some date back to the 1920s, when companies began tapping into the state's oil and gas resources. Some of the cores are taken from thousands of feet below the surface, allowing geologists to begin building a model of the Earth's structure far beneath their feet.

Instruments lowered into deep drill holes measure resistance to electrical currents, porosity and other geophysical aspects of the rock layers.

Thin shavings of the core, stained and studied under a microscope, reveal the pores in the rock, where water, oil or gas gather. Some rock types, like sand, are more porous than others, letting water seep down. Johnson described the geology of Santa Fe's main aquifer as a big bathtub made of bedrock and filled with porous sand and gravel that stores water.

Another map shows a cross-section of the rock layers and the land surface topography. Water tends to flow downhill from mountains to basin, even underground. But how far it gets depends on the layers of rock and fault lines in between.

Aeromagnetic and seismic surveys from the U.S. Geologic Survey can pinpoint the path of fault lines that tend to either divert water or act like a drain. The information is compiled into a geographic map showing topography, the age and depth of rock layers, and the faults.

To gather basic water data, Johnson and others scrounge well logs from drillers, municipalities, community water systems and the Office of the State Engineer. Those logs provide valuable information on rock types and aquifer levels at specific depths.

GPS data pinpoint the location of these wells in relation to the geology.

Water samples are gathered and tested at the bureau's lab for a variety of chemical characteristics — levels of bromide, sulfates, sodium, potassium, tritium and more. "Chemistry tells us a lot about where water is coming from and how it flows through the subsurface," Johnson said.

Ultimately, all the data are combined by computer graphic artist Brigitte Felix Kludt to generate a map showing Johnson's predictions of where water will flow, where it is more easily recharged and where wells are likely to go dry.

In Placitas, Johnson found the groundwater flow in the isolated aquifers beneath the village began as rain or snow, falling in the Sandia Mountains. "Water soaks into the shallow soil and moves ... along cracks and fractures in the hard, crystalline limestone and granites of the rugged mountain terrain," Johnson said. "Because the terrain is steep and these fractures are open conduits, water moves ... at a fast pace until it reaches its first barrier — the Placitas fault zone."

The fault places the fractured limestone and granite carrying water against layers of impermeable mudstone and shale of the deep red Chinle Formation, all rotated and tilted at steep angles, Johnson said. At the Placitas fault, groundwater from the mountains has to find a way through the Chinle Formation along another cross-cutting fault, or it is forced to the surface as a spring.

The water's journey from snowmelt on the mountain to discharge at the village of Placitas or Tunnel Spring may take a few weeks or a few years.

As critical as geologic mapping is to understanding water resources, less than 27 percent of the state has been mapped since the program began 18 years ago, and even fewer of the aquifers have been mapped.

Mapping takes a long time and a lot of money. It can take one to two years to map a quadrangle just for the geology. An Albuquerque aquifer map took seven years to complete at a cost of more than $20 million in federal and state funds. The Española Basin map took more than three years.

"If we had twice the personnel and money in that time, we would have twice as much done," said Paul W. Bauer, associate director and principal senior geologist at the New Mexico Bureau of Geology and Mineral Resources. Bauer is developing a Rio Grande River guide that will cover geology, cultural history, rafting and more.

Rep. Mimi Stewart, D-Albuquerque, has introduced a bill (HB 17) seeking $450,000 for the statewide aquifer-mapping program.

If approved, it will allow Johnson and the team to continue expanding their understanding of the state's complex geography and its impact on water resources.

Contact Staci Matlock at 470-9843 or smatlock@sfnewmexican.com.