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.
Mr. Logsdon raised several concerns about our recent article on metals geochemistry in the Pinal Creek aquifer. While he questions some details of our analysis, it is possible that he overlooked a large body of data related to in situ mineralogic buffering of acidic ground waters.The major issues presented by Mr. Logsdon include:1. concern about the applicability of aluminum-bearing mineralogic phases used in our analysis; 2. confusion about the mechanisms and kinetics of buffering reactions; 3. disagreement of the applicability of saturation indices (SI) in data interpretation; and 4. possible misunderstanding about the use of iron sulfate phases in our analysis.Our responses to these items are as follows:1. The role of these phases is well supported by the data. 2. The data clearly show that kinetic limitations are insignificant. 3. Saturation indices are important in data interpretation, in light of the large universe of data. 4. We have extensively described the role of iron-sulfate phases in the system.Mr. Logsdon is apparently skeptical that anthropogenic aluminum and iron sulfate minerals in the Pinal Creek aquifer will depress the groundwater pH for long periods of time during remediation. Our analysis of chemical buffering demonstrates that historical precipitated metal and acid releases are still affecting the system, and that these releases drive the remedial duration for groundwater neutralization.In addition, Mr. Logsdon mistakenly states "Quantitative importance of pH buffering by AlOHSO 4 is intrinsic to using total mass metal loading as the basis for cost-allocation as proposed by Dr. Davis and his coworkers." While our analysis of remedial timeframes of the Pinal Creek aquifer elucidates the nature of the technical issues that provide a basis for cost allocation, the loading calculations and fingerprinting work presented in Parts I and III of our analysis Moomaw et al., 2003) stand alone as valid methods of identifying sources and determining mass loading from facilities in the Pinal Creek drainage. Specifically, our work shows that remediation will take a long time, with our estimates ranging from 22 years to over 140 years in the acidic core of the plume. It also demonstrates that acid and metal releases over the entire timeframe of industrial discharges contribute to the formation of acid-and metal-storing minerals that impede remedial progress.Overall, our conclusions regarding the buffering of pH at acidic levels by aluminum and iron hydroxysulfate minerals are well founded in the scientific, peer-reviewed literature, supported by field and laboratory data, and are consistent with our modeling results. In fact, many other researchers have discussed the specific relevant aluminum and iron hydroxysulfate phases, and have reached the consensus that they not control sulfate, aluminum, and iron solubility at low pH but also play an important role in controlling the chemistry of acidic systems.
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