Geology Deserves Greater Role in Groundwater Assessment

This Guest View was contributed by M. Lee Allison, state geologist and director, Arizona Geological Survey. Mr. Allison joined the agency in June.

Geology is playing an increasingly important role in understanding our water resources. Let me share some examples of what is happening now, and what more needs to be done.

The demands on our water resources and especially groundwater are increasing at an alarming rate. For much of the past 150 years, it was relatively easy to drill a hole in the valley floor and you could usually find water. Today you may have to go much deeper than your grandparents did. As we tap more of the water-bearing sediments in the basins and expand exploitation of our bedrock aquifers, it will be increasingly important to characterize the geologic framework in order to balance maximizing resource recovery with aquifer sustainability.

The petroleum industry has relied on reservoir characterization for the past 30+ years to drain every last drop from declining fields. Prior to that, it was typical to drill wells on a regular spacing and treat the oil reservoir like a giant sponge. Today, geologists and engineers recognize the geologic heterogeneities that control porosity, permeability, and the ultimate recovery of oil from a field.

We are only starting to apply reservoir characterization techniques to groundwater resources. Too often, our aquifer models are idealized and simplified. This raises the question of whether our estimates of water resources may be unrealistically high or low, or perhaps both high and low, at different locations. It’s going to take significantly more effort to realistically map the aquifer geology in order to accurately assess our groundwater resources.

One of the things we can do right now is start using more realistic basin models for Arizona. Open virtually any geology or hydrology publication for the state, including those of my own agency, and you will find a cartoon-like cross section of a “typical” Arizona sediment-filled basin. There are matching high-angle normal faults bounding the basins against the bedrock mountain fronts, descriptions of coarse grained sediments near the mountain fronts, with finer grained sediments in the valley center, perhaps along with playa or axial stream deposits. All of the sediments are symmetrically arrayed across the basins.

The reality for Arizona is much more complex. Many basins are strongly asymmetric, with low-angle normal (“detachment”) faults dominating their shapes. Sediment distribution can be equally asymmetric with coarser grained materials on one side and finer grained on the other. Plus there are deposits of salts reaching thicknesses of thousands of feet in some areas. Piecing together an accurate subsurface picture is essential for understanding water volumes, storage capacity, producibility, and recharge rates.

A second thing we can do is to get geologists, hydrologists, and engineers talking together so our hydrologic models are consistent with the best geologic reservoir characterization we have.

Arizonans rely heavily on bedrock aquifers for water in some parts of the state, although there seems to be little public recognition of this. Good sized cities such as Payson rely entirely on bedrock aquifers. Identifying water resources in bedrock has its own challenges. Many of the geologic units are severely deformed after hundreds of millions of years of tectonic activity. Some of these rock units are twisted, broken, and offset along faults. In many cases, the water is concentrated in fractures and faults cutting through otherwise “tight” formations. Finding and assessing the water resource in these cases demands detailed geologic mapping.

On a different tack, there is a growing demand for detailed geologic maps of surficial deposits, delineating Holocene-aged sediments (10,000 years old and younger) from older Quaternary (up to 1.6 million year old) sediments. Water law in Arizona assumes that water from Holocene sediments is not drawn from underlying aquifers but is subflow from nearby or adjacent rivers and streams. This has significance to anyone who wants to drill for water in a riverine environment, and it means there is need for more detailed mapping to delineate the extent and depth of Holocene sediments.

Lastly, the rapid depletion of groundwater from some of our basins produced substantial subsidence in recent decades, reaching over 18 feet in some locations. A consequence of this is the development of the essentially human-induced geologic hazard of earth fissures, cracks that may reach hundreds of feet down from the surface and extend across the countryside for hundreds of feet to miles. Geologists are still learning how fissures form, propagate, and evolve. In response to publicly prominent fissures that opened overnight following monsoon rains in 2005 in the Phoenix region, the Arizona legislature recently approved a bill for the Arizona Geological Survey to undertake a comprehensive mapping effort to precisely map all the earth fissures in the state. Users will be able to create their own customized maps through an interactive GIS website run by the State Land Department, and also distributed by the Arizona Department of Real Estate. Beyond mapping of the fissures themselves, subsidence mapping by the Department of Water Resources is critical to developing predictive capabilities for new fissures as groundwater depletion continues.

Taken together, these examples show the necessity for geology to be more integral to our assessment of potential groundwater resources and to respond to the consequences of our current practices in using this incredible resource.

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