How Much Groundwater Is Down There?
by Greg Hess, R.G., Clear Creek Associates
Access to Central Arizona Project (CAP) water, declines in per capita consumption, and reuse of treated wastewater have reduced the Tucson area’s reliance on groundwater, putting our region in a better water resources position than it has experienced in decades. But even so, physical access to groundwater remains a requirement for much of the residential growth that will occur in southern Arizona in the coming years. So how do we know if there will be enough groundwater to support this growth?
The demonstration of sufficient water resources is the subject of Arizona’s Assured and Adequate Water Supply (AAWS) regulations. Under the AAWS program, studies are conducted by hydrogeologists before land is subdivided and lots are sold, to evaluate whether enough groundwater will be available to supply the subdivision’s needs for 100 years.
The hydrogeologic studies address the following questions:
How much water will the subdivision use? The Arizona Department of Water Resources (ADWR) provides a simple spreadsheet tool that calculates all the reasonably foreseeable water uses in a proposed subdivision, including indoor water consumption, residential landscape watering, turf and common area irrigation, and other demands on the water supply – even the water that will be used during construction. In addition, existing demands and future demands from other planned subdivisions must be addressed.
What do existing data show? The initial phase of the study involves a review of existing data. This includes an evaluation of whether groundwater supplies are already being depleted. If they are, this must be taken into account.
What is the nature of the aquifer? If the aquifer can’t be characterized using data from previous studies, a field investigation will be needed. This will likely include drilling one or more test wells. When a well is drilled, the depth to water and the minimum thickness of the aquifer are defined. The flow characteristics of the aquifer are also evaluated. This provides a preliminary assessment of how much groundwater is likely to be available. After the well is drilled, an aquifer test is conducted by pumping the well continuously at a constant rate for a specified duration. While pumping, the rate at which the water level drops in the pumped well and in nearby observation wells is monitored. The rate of recovery after the well is turned off is also monitored. The test results generate numerical values for two critical aquifer properties: storativity (its ability to store and release water) and transmissivity (the ability of water to flow through it).
What happens to the aquifer after 100 years? The final step is to enter various data, including the transmissivity and storativity values from pumping tests, into computer programs that simulate an aquifer’s response to long-term groundwater withdrawals. There are two types of simulations: analytical models and numerical models. An analytical model assumes that the aquifer has a single transmissivity value and a single storativity value, and that the aquifer has the same thickness everywhere. This type of model is appropriate for small projects where the geology is uniform. A numerical model is more complex. It consists of a 3-dimensional grid that looks something like an irregularly shaped Rubik’s Cube. Values of transmissivity and storativity are assigned to each individual block throughout the model. A numerical model is necessary for larger projects and more complex environments. Both types of models simulate the effect of pumping groundwater, with the goal of predicting how far the water table will drop after 100 years.
- Will there be enough water? This question is answered by comparing the model simulation results to specific criteria. In the Tucson Active Management Area, a subdivision will not receive a Certificate of Assured Water Supply if the model predicts that, within 100 years, the depth to groundwater will drop to more than 1,000 feet below land surface or to the bottom of the aquifer (whichever is shallower). A subdivision in an AMA must have an Assured Water Supply demonstration for a plat to be approved. If a 100-year supply is not demonstrated, the size of the subdivision must be reduced or certain water uses (turf irrigation for example) must be eliminated. Outside an AMA the maximum depth is 1,200 feet. If the water table will drop below 1,200 feet, a subdivision can still be approved, but the buyers of lots must be informed that a 100-year Adequate Water Supply was not demonstrated.
Arizona’s AAWS program has helped direct growth toward areas where water supplies are more reliable, and it has provided investors with confidence in our state’s long-term economic viability. Without the AAWS program, the answer to the question “How much groundwater is down there?” would in many cases be “We don’t know.” This is not an answer we want to give to anyone considering relocating to southern Arizona.