Water for the towns of Prescott, Prescott Valley, Dewey-Humboldt, Chino Valley, and approximately 13,000 domestic wells is obtained from a groundwater system legally referred to as the Prescott Active Management Area (PrAMA) that covers approximately 485 square miles and is divided into two sub-basins, the Little Chino and the Upper Agua Fria. Hydrologically the PAMA is comprised of a bottom lying aquifer consisting of volcanically derived lava flows separated from an overlying aquifer consisting mainly of silt, sand, and gravel by a poorly permeable deposit of intermixed clay, silt, and volcanic fragments. Water moves vertically between the two aquifers resulting in their comprising a single ground water system.
The amount of water stored in the PrAMA to a depth of 1,000 feet is estimated by the Arizona Department of Water Resources (ADWR) to range from approximately 3 to 5.2 million acre-feet and this estimate, as well as others, is often used to state that the aquifer contains enough water to last hundreds of years at the present rate of usage.
Such a statement is overly simplistic and is not scientifically viable. Simply stated, it is not scientifically possible to estimate the amount of water available from an aquifer or ground water system based solely on the amount of water in storage. There is no scientific basis for such a conclusion.
As stated by ADWR, “Although the volume in storage is seemingly large as compared to annual groundwater pumping, it should be realized that not all of that volume could be practically produced by groundwater withdrawals from wells. Hydrologic and technical issues that ultimately limit the actual volume of groundwater that can be produced include, but are not limited to: Aquifer productivity and heterogeneity, costs to drill new wells, increasing pumping costs and decreasing well yields with increasing depth-to-water, physical availability requirements under the Assured Water Supply Program, legal restrictions on well locations, water quality, land subsidence.”
It is one thing to be able to obtain the entire amount of water in a lake by pumping or draining it. The same can be said for the amount of water in a glass. There is nothing that resists the removal of this water except gravity. Water in a silt, sand, and gravel aquifer is contained within the pore space between the individual grains comprising the aquifer, whereas water in a volcanic aquifer is contained within cracks and fractures in the lava flows and within the space and/or sediments between individual lava flows. As such, water in an aquifer is not a body of water that stands alone. Movement of water toward a well is resisted by the aquifer itself as it moves between these interconnected spaces. Given this alone, the maximum amount of water that can be obtained from a well or wells over time has very little to do with the amount of water stored in the aquifer. Instead it depends on the sustained rate that water can be withdrawn from wells. This, in turn, depends significantly on other factors not related to the amount of water stored in the aquifer, including the resistance of the aquifer to the movement of water.
For instance, the amount of water stored in a deposit of silt and clay contains more water than that stored in a deposit of sand and gravel of the same thickness and extent. It should be readily apparent that the former will offer more resistance to the movement of water than the latter with the result that wells located in the silt and clay deposit will yield much less over time and will go dry much more readily than wells situated in the sand and gravel deposit despite the fact that, other things being equal, more water is stored in the silt and clay deposit. This example illustrates one additional important fact. Given the resistance of the aquifer to the movement of water alone, the water level in a well is always lower than that in the aquifer next to the well thereby making it impossible to remove all of the water from the aquifer by using wells.
Given the size of the PrAMA, removal of the maximum amount of water from it would require a very large number of wells, considerably more than presently exist. Pumpage from multiple wells located near each other or located some distance from the area of natural discharge from the aquifer reduces the individual yield from each well and can actually result in some of the wells going dry. As a result, the maximum amount of water that can be obtained from an aquifer over time is highly dependent on the distribution and rate of pumpage of wells in the aquifer. In order to maximize the amount of water obtainable from a ground water system, the distribution and rate of pumpage from wells would have to be maximized. Even under these circumstances, however, the amount of water available from the PrAMA would be significantly less than the amount stored.
Maximizing the distribution and rate of pumpage in the PrAMA would not be an easy thing. First, the science of hydrology does not readily lend itself to determining this distribution. It would be a trial and error procedure for well placement based initially on projections from a groundwater model. Second, and probably of greater significance, Arizona does not regulate the distribution of and rate of pumpage from wells in order to obtain the maximum amount of water from an aquifer and, as a result, wells presently existing in the PrAMA have not been located with this in mind. Existing wells actually reduce the maximum amount of water obtainable from the aquifer. Maximizing the distribution and rate of pumpage would, in all likelihood, require relocation and/or deepening of some existing wells, both domestic and high yield. These requirements would in turn cause social, legal, political, and economic issues difficult if not impossible to overcome within the State’s existing legal framework.
Finally, the amount of water stored in large ground water systems such as the PrAMA cannot be accurately determined given existing knowledge. Determination of ground water storage requires knowledge of aquifer thickness and of the storage properties of the aquifer and how this varies spatially. The thickness of the rocks underlying the PrAMA is poorly known as is their ability to store water. ADWR readily admits to a lack of knowledge concerning these all important parameters. Given both unknowns, the stated figure for the amount of water stored is simply an “educated guess” at best for which the potential degree of error is unknown and potentially quite large.
In summary, beyond the factors that limit the amount of water that can be produced from storage stated by ADWR, it should be realized that the amount of water actually stored in the PrAMA is not well known. Even if it were, it’s not scientifically possible to determine the amount of water that can be obtained, but this amount would be significantly less than the amount stored. In order to maximize the amount of water obtainable from the PrAMA, the distribution and rate of pumpage of wells would have to be maximized which is not a straightforward process. Wells presently situated in the PrAMA are not based this criteria and would actually reduce the maximum amount of water obtainable from the PrAMA. The actual undertaking of an effort to maximize the amount of water obtainable from the PrAMA could result in a need to relocate and/or deepen many existing wells, both domestic and high yield. This, in turn, would create overwhelming social, legal, political, and economic issues.
- Bill Meyer