Using GIS to Map Groundwater
The city of Peoria, Ariz., is located within the northern Sonoran desert and has a young, vibrant, educated and skilled populace. Peoria offers a great outdoor recreational quality of life that includes hiking, biking, access to open desert, and boating and fishing at Lake Pleasant in the northern portion of the city. In addition, the Peoria Sports Complex is the home of the San Diego Padres and Seattle Mariners annual spring training.
These are some of the reasons that Peoria, which is within the Phoenix metropolitan area, is the fifth fastest growing community in the U.S. Peoria grew more than 65% between 1990 and 2000, and is expected to grow from a current population of 139,000 to more than 250,000 during the next 20 years. In addition to this rapid population growth, nine years of drought have provided significant challenges for Peoria to keep up with the development of a long-term, sustainable water supply.
The city of Peoria obtains its water supply from two surface water treatment plants, and nearly 30 water supply wells are used to both satisfy current daily peaking needs and serve as a long-term backup water supply. Additional wells are needed to continue to maintain a sufficient redundant groundwater supply. Adding to these challenges is the fact that where future growth is projected to occur within the city, natural hydrogeologic conditions indicate that native groundwater resources are limited.
Developing a guidance tool
The city of Peoria recognized the need to develop a screening-level tool to help guide the location of future water supply wells. The tool would be based on work developed through the U.S. EPA in the 1980s and needed to take into account the varied types of information. The tool would seek to identify both the physical and demographic constraints within the region to adequately drive the model.
Physical constraints would include the location and depths of existing water supply wells, the spatially variable subsurface geology and hydraulic characteristics, the three-dimensional thickness and areal extent of the aquifer, and anticipated location of future growth and development. Other constraints to be developed were exclusionary criteria that eliminated particular land within the city from further consideration and demographic criteria ranked and weighted to provide a way to quantitatively compare the most suitable areas remaining in and around the city to site water supply wells.
Exclusionary criteria used for eliminating areas from further consideration were as follows:
- Where bedrock (e.g. granite) is located at land surface;
- Where depth to bedrock is less than 200 ft below land surface;
- Where aquifer thickness is less than 200 ft; and
- Well impact rules from the Arizona Department of Water Resources.
Mapping groundwater resources
To develop the tool, the city of Peoria selected ArcGIS Desktop (ArcInfo) and the ArcGIS Spatial Analyst extension to apply the exclusionary criteria by bringing together the variety of spatially complex, three-dimensional data needed to analyze its groundwater resources available today and in the future. The utility department used the software because ESRI GIS products are the standard for the city, and the software offers the advanced, sophisticated analyses needed to accomplish this type of work for the utility department.
The publicly available information included locations, depths and annual water-level elevations from more than 650 wells from the Arizona Department of Water Resources (ADWR); subsurface geology from ADWR’s Salt River Valley regional groundwater flow numerical model; surficial geology from the Arizona Geological Survey; and the areal extent of the buried bedrock obtained from a gravity geophysical survey conducted by the city of Peoria. This data was then analyzed using ArcInfo to derive a series of maps identifying areas where depths to bedrock were less than 200 ft (or exposed at land surface) and where the thickness of the aquifer was less than 200 ft. These exclusionary criteria alone eliminated nearly 60% of the 160-sq-mile project area from further analysis.
Developing a suitability model
ArcInfo and the ArcGIS Spatial Analyst extension were then used to apply the demographic criteria to the remaining areas. To identify the optimum locations to site future water supply wells, the demographic criteria were then attributed a weight and rank for each criterion, which was used to compare the remaining areas using a raster-based suitability model. The model was based on aggregating all data to a 40-acre grid size and adding all of the total weights and ranks for each criterion within each grid.
The spatially complex, two-dimensional data that was needed to develop the suitability analysis included known and projected locations of future growth, housing densities and water distribution lines from the city of Peoria; the location of 100-year floodplains from the Maricopa County Flood Control District; locations of groundwater recharge projects from the Central Arizona Project; known areas of degraded water quality from the Arizona Department of Environmental Quality; and landownership from the Arizona State Land Department.
Weighting and ranking also were used in developing the suitability analysis for one of the demographic criteria: proximity to recharge projects. Because groundwater is a nonrenewable resource in the deserts of the southwestern U.S., the authors thought it was important to locate future wells in the proximity of recharge projects to ensure the hydrologic sustainability of a water supply well.
The final results in applying the demographic siting criteria in a suitability model analysis indicate that 1% of the study area is most suitable for siting future wells.
This type of GIS-based, screening-level and decision-making analysis provides the city of Peoria, Ariz., with a flexible tool to help guide the location of future water supply wells in the rapidly growing Phoenix metropolitan area.