Hydrogeology and hydrologic system of Pinal Creek Basin, Gila County, Arizona

Abstract

Acidic water that contains elevated concentrations of metals has contaminated a stream and alluvial aquifer in a mining district near Globe, Arizona. The contaminated aquifer is a narrow layer of unconsolidated alluvium along Miami Wash and Final Creek. The alluvium overlies basin fill, which extends throughout most of the Final Creek basin. The alluvium and basin fill compose the primary aquifer in the basin. Horizontal hydraulic conductivities were estimated at about 200 meters per day in the alluvium, and average linear ground-water flow velocities are about 5 meters per day. Water levels in the aquifer respond rapidly to extended periods of runoff in the basin and variable rates of ground-water pumping. Fluctuations of as much as 2.5 meters in 6 months have been measured. Ground-water levels were near record highs during spring 1985 and declined as much as 13 meters by spring 1989. Measured hydraulic gradients indicate that flow generally is upward from the basin fill to the alluvium. From 1980 to 1984, 155 liters per second of seepage from Webster Lake and 279 liters per second of streamflow infiltration were estimated to be the two largest sources of inflow to the regional aquifer. Significant outflows from the aquifer were 166 liters per second of pumpage and 240 liters per second of ground-water discharge to Final Creek. INTRODUCTION Acidic waters that contain elevated concentrations of metals drain from areas disturbed by mining activities in Final Creek basin, Arizona (fig. 1), and have created a contaminant plume about 15 km long in a stream and aquifer (Eychaner, 1988, p. 291). Mining began in the study area in the late 1800's and continued through the time of this study. From about 1940 to 1986, acidic-waste solutions were stored in Webster Lake; seepage from the lake was a major cause of aquifer contamination. Although the movement of the acidic plume is primarily in ground water, stream-aquifer interactions are important because acidic water can discharge to the land surface when runoff from large storms recharge the regional aquifer. The presence of contaminated ground water has long been recognized in the basin but was not studied until 1979 (Envirologic Systems, Inc., 1983). The U.S. Geological Survey (USGS) began studies in the area in 1984, and in 1987, cleanup of the contaminated ground water began under the supervision of the U.S. Environmental Protection Agency (USEPA). This study was undertaken to increase understanding of the movement of the contamination plume by a comprehensive examination of the hydrogeology and streamaquifer system in the area. Sources of hydrogeologic data included published literature, unpublished data files of the U.S. Geological Survey and mining companies, and field studies. Purpose and Scope The purpose of this report is to describe the hydrogeology and stream-aquifer system in Final Creek basin. The report discusses the lithologic and hydraulic characteristics of major waterbearing units and includes a water budget for the regional aquifer for August 1980 to September 1984. Important components of the water budget