Elements Influencing Cost Allocation in the Pinal Creek Aquifer, Arizona, USA. Part I: Geochemical Fingerprinting and Source Delineation

Abstract

This is the first in a three-article series in this volume of Environmental Forensics describing the framework that was developed to quantitatively allocate mass loading of metals to the Pinal Creek alluvial aquifer. This article describes geochemical fingerprinting, followed by spatial and temporal analysis of Pinal Creek monitoring well data (more than 600,000 records from hundreds of locations), which identified three distinct source areas and plumes on the basis of facility-specific differences in process geochemistry, ore mineralogy, and solution handling. The primary Webster Lake/Gulch source originated in 1926 (700 mg/L chlorine, 200 mg/L copper, 3000-9000 mg/L iron). Secondary contributions from the Bluebird/Oxhide/Live Oak Gulch/Davis Canyon leach complex in Upper Bloody Tanks Wash began circa 1962 (30-200 mg/L chlorine, 1500-3500 mg/L copper, <500 mg/L iron), and recent (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) contributions came from Miami Unit hydromining releases to Bloody Tanks Wash (30-70 mg/L chlorine, ∼1000 mg/L copper, ∼2000 mg/L iron). In Kiser Basin, where the plumes commingle, temporal evolution of the plume chemistry is reflected in a shift from a predominantly iron:copper-rich solution (10:1) to an increasingly copper-dominated signature (1:1) with the arrival of the secondary plume. Based on historical records and geochemical fingerprints, the original acid front migrated through the alluvium at ∼0.5 m/day; however, later acidic releases traveled at ∼2.3 m/day due to exhaustion of the available buffering capacity. The divisible nature of the plume supports quantitative allocation of contributions to the aquifer.

Elements Influencing Cost Allocation in the Pinal Creek Aquifer, Arizona, USA: An Introduction and Synopsis of Findings

“…Forensic techniques described by Helgen et al (2003) identified three distinct plumes based on different ratios of Cl, Cu, and Fe that are directly attributable to the loss of mining solutions from large-scale leaching of Cu ore with ferric sulfate and/or sulfuric acid. The utility of these constituents results from their transport at close to groundwater velocities (∼0.5 m/day) in the acidic core of the plume.…”