Cienega Creek Basin Groundwater Evaluation Frank Postillion, CGWP, Chief Hydrologist David Scalero, Principal Hydrologist Jacob Prietto, Hydrologist Jennifer Becker, Principal Hydrologist Pima County Regional Flood Control District July, 2015 Draft Executive Summary Overview: Pima County Regional Flood Control District (District) has evaluated the historical and current water balance and effects on regional groundwater conditions, and potential impacts on the hydro- and meso-riparian vegetation along the watercourses of the Cienega Creek Watershed (CCW) . The CCW study area is approximately 475 square miles in eastern Pima County, western Cochise County and northeastern Santa Cruz County, Arizona, with most of the basin in Pima County (Figure 1-1, Area Map). The amount of the CCW in the Arizona Department of Water Resources (ADWR) Tucson Active Management Area is 80 square miles, mostly within the southern portion of Davidson Canyon (Figure 1-1) Pima County (County) property within the CCW includes the Cienega Creek Natural Preserve (CCNP) in the lower part of the watershed, Bar V Ranch, Empirita Ranch, and Sands Ranch and Clyne Ranch. Cienega Creek stream flow supports rare and endangered fish and frogs as well as dense areas of riparian vegetation including cottonwoods, mesquites, and willow that provide shelter and foraging habitat for a wide variety of birds, bats, and other wildlife species. These unique natural resources are under threat from man-made land use changes and natural changes, including global climate change. The County and the US Government have invested millions to protect the CCNP in the lower watershed and the Las Cienegas National Conservation Area in the upper watershed and other important conservation areas in the CCW. The County has begun to compile risk assessments of potential threats, including drought, depletion of groundwater, land use and mining, and develop appropriate management strategies to reduce or eliminate impacts from these threats to the sustainability of the Cienega Creek Watershed. Geologic Features : Sedimentary units underlying the area, consist of the Quaternary surficial deposits, Tertiary older basin fill alluvium, Pantano formation and hydrologic bedrock. Differences between the upper and lower basins have to do with the thickness of the formations. The younger alluvium is thin in both basins, not exceeding about 200 feet in the upper basin and may not reach 100 feet in the lower basin. Although productive with significant sands and gravels, the recent alluvium does not provide an appreciable amount of water for large well use. However, it provides an abundant amount of water for riparian trees and vegetation along Cienega Creek, its tributaries and Davidson Canyon. The basin fill can be as thick as 1,200 feet south of I-10 near Empirita Ranch, but productivity of the aquifer diminishes with depth. In the upper basin, the basin fill consists and lenses of sands, gravels and silts and is as deep as 2000 feet near Sonoita, and more productive in the Empire Ranch area along the Cienega Creek corridor and floodplain. In some areas the basin fill of the upper basin could be semi–artesian potentially due to the sequences of silts and clays. The Pantano formation, consisting of mostly consolidated and cemented sandstone, unproductive claystone and mudstone, present in the lower basin, and in some areas such as downstream of Pantano Dam, forms a semi-impermeable layer above the recent alluvium. In areas of the upper basin the Pantano formation may underlie the basin fill in the deeper portions of the aquifer near Sonoita. Bedrock consists of consolidated sedimentary rocks with a mix of plutonic, igneous and other sedimentary rocks. The underlying bedrock complex and the surrounding mountains yield little water to the system but serve as a primary recharge medium for the mountain fronts in areas of large and smaller fractures and can provide movement of water toward the central portions of the basins. Where the bedrock nears the surface along Cienega Creek and Davidson Creek, groundwater is forced to the surface due to the very low permeability of the bedrock. The “Narrows”, Pantano Dam and “Three Bridges” have shallow bedrock underlying Cienega Creek bed and adjacent confining bedrock along the sides of the Creek. Cienega Creek Groundwater Evaluation Page ii Groundwater Conditions: Of the over 2000 wells in the upper and lower Cienega Basin, most are private domestic wells. Well pumping is focused in three main areas: Sonoita, Vail and the Jay-6 area near I-10 and the Cochise/Pima County line. The other area is spread among private wells within the upper and lower basins. Pumping has increased from about 2,290 acre- foot (AF) in 1984 to almost 4,700 AF by 2014. The dominant pumping categories in 2014 include private domestic, at 1,700 AF, 1,850 AF for municipal wells, 885 AF for agriculture and 265 AF for industrial/mining. Groundwater movement generally mimics surface topography, following the path of Cienega Creek in the lowlands and moving north-northwesterly toward the Tucson Basin. The most recent water-level data compiled in 2014-2015 show varying depth to groundwater, ranging from below 50 feet near the channel corridor of Cienega Creek north of I-10 and south of the “Narrows” and in some areas of Davidson Canyon; near 100 feet north of I-10 and on the Empirita Ranch; near 200 feet in the Sonoita area; 200-300 in the Jay Six area near I-10 and the Cochise/Pima County Line; and 300-500 feet in the Vail area near Colossal Cave Road. Some of the areas recognized as designated Shallow Groundwater Areas are now deeper than 50 feet, especially south of I-10 on Empirita Ranch. Historic water levels show a declining trend since the early 1980s, especially in the lower Cienega Basin. Deeper wells show steeper declines over the last 20-30 years. Shallower wells near perennial reaches show less decline, but there is still a declining trend. Overall, the lower Basin shows a water level decline of about 0.9-0.95 feet/year (ft/yr) over the last 20-30 years. The decline in the last 10-years in the lower Basin has been about 0.5 ft/yr. The probable stressors are drought and increased pumping in the Vail, J-6 areas and more private domestic wells in the area. The upper Basin appears more resilient to water-level decline and experienced less increased pumping than the lower Basin. The declines in the last 20-30 years range from 0.2-0.3 ft/yr. However, water-level declines in the last 10 years have increased to 0.6 ft/yr, possibly due to increased pumping and drought. Despite significant flood flow and volumes recorded at Pantano Dam in the last 10 years, lack of significant winter flows have minimized recharge of the upper and lower basins and recoveries of shallow groundwater levels in the area. The declining groundwater levels have also resulted in reduced perennial flow and flow length since 1994. Several local and watershed-wide groundwater modeling studies have been performed over the last 35 years. University of Arizona models developed in the 1980s-1990s projected significant water-level declines could result from high volume pumping in the upper and lower Basins from proposed mining or residential development and suggested levels that could threaten the riparian habitat along the Cienega Creek. In the past 10 years conceptual and three-dimensional flow models have been developed covering the mid-upper and portions of the lower Basin, evaluating the impacts of the proposed Rosemont mining pit, on the water surface elevation of these areas. The major models looked at the specific declines resulting from the Rosemont pit redirecting groundwater and changing gradients for the final Rosemont Environmental Impact Statement. The models showed significant declines would result from pit excavation, even with the importation of water pumped from the Upper Santa Cruz Basin to process. The geographic extent of mine impacts on water levels would increase over time, projecting from 50 to 100 to 1000 years in the future. The models did not account for the current pumping impacts or a range of climatic conditions. These models did not account for the current declining water level conditions in the Cienega Creek Groundwater Evaluation Page iii Basin. As such, these models were not adequate in projecting the overall impacts of current and future pumping, and a range of climatic conditions coupled with mine pit construction. Water Quality and Geochemistry: Groundwater and surface water quality in the Cienega Basin is influenced partially by geology, including gypsiferous bedding in lower Cienega, lower total dissolved solids (TDS) in the Davidson Canyon area and upper Cienega Creek, and different water types. The lower Cienega basin is dominated by calcium-sulfate–bicarbonate waters, except for groundwater at Pantano Dam which is sodium sulfate and dominated by older recharge. Davidson Canyon and the upper watershed groundwater and surface water are dominated by lower TDS, calcium-bicarbonate types and sodium bicarbonate types. Isotopic compositions of water samples can provide a useful tool for investigating hydrological processes in surface water and groundwater systems. Isotopic results, mostly limited to Davidson Canyon and lower Cienega Creek show groundwater wells, springs, and surface water samples having oxygen and hydrogen indicate a mix of summer and winter precipitation contributions, with a dominance of winter recharge and some evidence of evaporation prior to recharge or flow. Davidson Canyon Wash surface water flows generally reflect a winter groundwater source, compared to sites closer to the confluence of Cienega Creek. Some wells in the proposed Rosemont project area, a spring about 2-miles west of the proposed mine site, and a site over 10 miles downstream in Davison Canyon Wash may have less interaction with modern precipitation compared to most other sampled sites.