CENTRAL ARIZONA SALINITY STUDY --- PHASE I Technical Appendix F HYDROLOGIC REPORT ON THE HARQUAHALA BASIN Prepared for: United States Department of Interior Bureau of Reclamation Prepared by: Brown and Caldwell 201 East Washington Street, Suite 500 Phoenix, Arizona 85004 F-1 TABLE OF CONTENTS PAGE TABLE OF CONTENTS ................................................................................................................ 2 LIST OF TABLES .......................................................................................................................... 3 LIST OF FIGURES ........................................................................................................................ 3 1.0 INTRODUCTION .............................................................................................................. 4 2.0 PHYSICAL SETTING ....................................................................................................... 5 3.0 GENERALIZED GEOLOGY ............................................................................................ 6 3.1 BEDROCK GEOLOGY ......................................................................................... 6 3.2 BASIN GEOLOGY ................................................................................................ 6 4.0 HYDROGEOLOGIC CONDITIONS ................................................................................ 7 4.1 GROUNDWATER OCCURRENCE AND MOVEMENT ................................... 7 4.2 GROUNDWATER QUALITY .............................................................................. 8 5.0 SURFACE WATER CONDITIONS ................................................................................ 10 5.1 SURFACE WATER OCCURRENCE ................................................................. 10 5.2 SURFACE WATER QUALITY ........................................................................... 10 6.0 GENERALIZED SALT BALANCE ................................................................................ 11 7.0 REFERENCES ................................................................................................................. 13 F-2 LIST OF TABLES TABLE 1 TDS CONCENTRATIONS IN THE HARQUAHALA BASIN ......................... 4-3 2 GENERALIZED SALT BALANCE FOR THE HARQUAHALA BASIN ....... 6-1 LIST OF FIGURES FIGURE 1 LOCATION MAP 2 KEY FEATURES 3 TDS CONCENTRATIONS IN THE HARQUAHALA BASIN 4 GROUNDWATER PUMPAGE AND CAP DELIVERIES IN THE HARQUAHALA BASIN 5 ANNUAL SALT ACCUMULATION IN THE HARQUAHALA BASIN F-3 1.0 INTRODUCTION The Central Arizona Salinity Study (CASS) is a coalition of water and wastewater agencies evaluating salinity issues in Central Arizona. The mission of CASS is to provide its members with workable alternatives for a quality, cost effective, sustainable, and reliable water supply through partnerships and cooperative efforts in regional salinity planning and management. CASS was formed in November 2001. CASS is a study group and not a legal entity. Salinity from local and imported sources is increasing the salinity of groundwater in localized areas and the salinity of reclaimed water in Central Arizona. The magnitude of the salinity issue is unclear and water providers in Central Arizona decided to work together to assess the problem and, if necessary, develop regional strategies for managing it. Central Arizona water providers must work together to protect, preserve, and develop these shared resources and to respond to issues of: increasing water quality and water supply regulation; increasing reclaimed water utilization; increasing levels of salinity into water reclamation facilities; developing brine disposal strategies; deteriorating groundwater quality in localized areas; and managing costs. If no workable solution is implemented, salinity increases may result in greater water and wastewater treatment costs, decreased agricultural production, and some water sources may become unsuitable for their intended uses. • Increasing salinity levels may reduce the ability of water providers to use groundwater and reclaimed water to meet customer water demands. Some communities may not have enough supply to meet demand. Growth and development in these communities may become limited. • Water reclamation plants may have water quality permit compliance problems. High salinity levels in reclaimed water supplies may make this resource unsuitable for some of its intended uses. Retrofitting water reclamation plants to manage salinity and dispose of brine may significantly increase wastewater treatment costs. • Water customer complaints may increase due to increasing salinity of the drinking water supply. Retrofitting potable water treatment plants to manage salinity and dispose of brine may significantly increase water treatment costs. The following white paper describes hydrologic conditions in the Harquahala Basin. This report is based on literature review of publicly available information, and site-specific fieldwork was not conducted as part of this study. The purpose of the report is to provide a general framework of the physical, geologic, and hydrologic aspects of the basin. The report will also discuss surface water quality and groundwater quality over time, with a primary focus on salinity. F-4 2.0 PHYSICAL SETTING The Harquahala Basin is located in west-central Arizona within the Sonoran Desert section of the Basin and Range physiographic province (Figure 1). The basin is adjacent to the Phoenix Active Management Area (AMA) to the east, Lower Gila Basin to the south, Ranegras Plain Basin to the west, and McMullen Valley Basin to the north. The Harquahala Basin comprises approximately 765 square miles, and precipitation in the basin averages approximately 6 inches per year (Hedley, 1990). The principal natural surface drainage feature is Centennial Wash, which enters the basin between the Harquahala and Little Harquahala Mountains in the northwest, and exits the basin between Saddle Mountain and the Gila Bend Mountains in the southeast (Figure 2). The land surface elevation ranges from 1,000 feet above mean sea level (amsl) where Centennial Wash exits the basin to over 5,000 feet amsl in the surrounding mountains. The basin supports a sparsely populated agricultural community of approximately 900 people, and the population is projected to increase slightly to 1,000 people by the year 2040 (ADWR, 1993). F-5 3.0 GENERALIZED GEOLOGY The Harquahala Basin is thought to have formed during the Tertiary Basin and Range Disturbance. During this disturbance, widespread extensional deformation in southern and western Arizona resulted in northwest trending mountain ranges separated by alluvial filled troughs. 3.1 BEDROCK GEOLOGY The Harquahala Basin is bounded by the Harquahala Mountains on the north, Big Horn Mountains on the northeast, Saddle Mountain on the southeast, Gila Bend Mountains on the south, Eagletail Mountains on the southwest, and Little Harquahala Mountains on the northwest (Figure 2). The bounding mountain ranges are predominantly comprised of Precambrian granitic and metamorphic rocks, Mesozoic and Paleozoic sedimentary rocks, Jurassic to Late Cretaceous granitic rocks and volcanics, and Tertiary volcanics and basalt (Reynolds, 1988. Hedley, 1990). In general, the sedimentary and metamorphic rocks primarily occur in the north/northwest portion of the basin (Harquahala and Little Harquahala Mountains), and the granitic rocks and volcanics dominate the bedrock geology elsewhere in the basin. 3.2 BASIN GEOLOGY The Harquahala Basin is comprised of a relatively deep, northwest trending alluvial valley that is typical of the Basin and Range physiographic province. Based on gravity modeling (Oppenheimer and Sumner, 1980), the depth to bedrock in the majority of the basin is greater than 1,600 feet below land surface (bls). The gravity modeling also suggests the deepest portion of the basin is near the center, with a depth to bedrock greater than 8,000 feet bls. Graf (1980) reported that the alluvial deposits are less than 300 feet thick near the mountains, and greater than 2,000 feet thick near the center of the basin. Hedley (1990) reported that the alluvial deposits are up to 5,000 feet thick in the center of the basin. Although differences in depth to bedrock values are noted in each study, the alluvial deposits are thickest near the center of the basin, and thin towards the bounding mountain ranges. Based on the study by Hedley (1990), the basin fill alluvium in the Harquahala Basin is composed of heterogenous deposits of clay, silt, sand and gravel. The most prominent alluvial unit in the basin is an extensive fine-grained (clay-dominated) deposit. The approximate extent of the fine-grained deposit as modified from Graf (1980) is shown on Figure 2. The fine-grained deposit occurs in the majority of the basin, and is known to be absent only in the extreme southeastern portion of the basin. According to Graf (1980), the thickness of the fine-grained deposits increases towards the northwest, and is more than 1,000 feet thick in the center of the basin. Relatively coarse-grained sand and gravel deposits occur in the southeastern portion of the basin, and also underlie the fine-grained deposits in the majority of the basin. F-6 4.0 HYDROGEOLOGIC CONDITIONS 4.1 GROUNDWATER OCCURRENCE AND MOVEMENT The alluvial basin fill deposits represent the regional aquifer in the Harquahala Basin. Groundwater in the alluvial deposits is generally unconfined, although local semi-confined to confined conditions exist. Based