Statistical Clustering of Major Solutes: Use as a Tracer for Evaluating Interbasin Groundwater Flow into Indian Wells Valley, California

Many previous studies have demonstrated use of specific solutes or isotopes as tracers of groundwater flow. We present a technique that uses standard hydrochemical information to create statistically based hydrochemical facies, which are then used as tracers of groundwater flow. This approach reduces potential subjective bias during interpretation of single tracer data in complex systems with multiple sources or conflicting multiple tracer data. Standard hydrochemical data from 1,368 water samples that spanned more than 80 years were analyzed using cluster analysis to decipher groundwater flow paths in Indian Wells Valley (IWV), California. The statistically derived hydrochemical facies form distinct spatial patterns in which all major-ion concentrations increase progressively from Sierra Nevada (recharge) to China Lake playa (discharge), consistent with the topographically driven flow of groundwater (the typical case for basin and range flow systems). However, once individual samples could be placed in the context of the normal hydrochemical evolution, anomalies are readily identifiable. The distribution of water chemistry in the southeastern part of the IWV does not conform to the regional trend. Groundwater from that part of the IWV is statistically more similar to waters from the Kern Plateau area (in the high Sierra Nevada outside the local watershed) than to waters from the local watershed. The groundwater is interpreted to originate from the fracture-directed interbasin flow from the Kern Plateau area that is directly recharging the alluvial aquifer in the subsurface. Inclusion of this flow could substantially alter current water budgets and water resources management approaches. INTRODUCTION Water resources play an increasingly critical role in our world as population continues to grow and the related development places even more stress on the hydrologic system. The demand for greater volumes of fresh water is further complicated as the disposal of hazardous waste affects the limited supply of potable water. This situation makes an improved understanding of groundwater circulation patterns especially important. In this study, evidence of interbasin flow through fractured metamorphic and igneous rocks that have been previously considered to be impermeable has significant implications regarding both water resources and the effects of fractures on the permeability of fractured igneous and metamorphic rocks. During the past 20 years, many studies have used tracers to delineate oceanic circulation patterns (Smethie et al., 1986; Measures and Edmond, 1988; Krysell et al., 1994; and Moore et al., 1998), water movement in watersheds (Rose et al., 1996; Clow et al., 1997; Negrel et al., 1997; Leibundgut, 1998; and Gamlin et al., 2001), interactions between ground and surface water (Neumann and Dreiss, 1995; Li and Spalding, 1996; Katz et al., 1997; and Rodgers et al., 2004), anthropogenic contamination’s movement, source, and attenuation (Hurst et al., 1991; Thierrin et al., 1992; Guzman and Jarvis, 1996; Komor, 1997; Gaebler and Bahr, 1999; Antich et al., 2000; Paridaens and van Marcke, 2001; Divine et al., 2003; and Hogan and Blum, 2003). A number of these applications have included the use of radioactive tracers for aquifer flow paths and environmental problems (Kuzmenko et al., 1992; Corbett et al., 1997; and Johnson et al., 2000). In most studies a single natural or artificial component, pair of components, or isotopic ratios are used as tracers. For instance, the isotopes of B, C, Cl, H, and O (Desaulniers et al., 1981; Fritz et al., 1990; James et al., 2000; and Kloppmann et al., 2001), rare earth elements (Johannesson et al., 1997), minor and trace elements (Farnham et al., Environmental & Engineering Geoscience, Vol. XII, No. 1, February 2006, pp. 53–65 53 2000), and krypton-85, chlorofluorocarbons, and sulfur hexafluoride (Cook et al., 1996; Cook and Solomon, 1997; and Bauer et al., 2001) have been used to delineate flow paths in aquifers. The spatial variability observed in the composition of these tracers provides insight into aquifer heterogeneity and connectivity and can be used to estimate mean residence times of groundwater. Typically, detection and direct measurement of water transfer across topographic divides (interbasin flow) are usually extremely difficult; however, tracer studies are a powerful tool for detection of this flow component. However, most of the environmental and anthropogenic tracers generally require elaborate sample collection, handling, and analysis techniques due to extremely low abundances in natural aqueous systems, together with good understanding of the hydrochemical background and the chemical behavior of the tracers (Sabatini and Austin, 1989; Krysell et al., 1994; Ball and Trudgill, 1997; Sutton et al., 2001; and Plummer et al., 2004). Often, performing such a tracer study on the regional scale is prohibitively expensive, considering the large number of samples that would be required to effectively determine the flow paths. Although there have been prior applications of multiple components as tracers (Spall et al., 1992; Katz et al., 1997; Negrel et al., 1997; and Swarzenski et al., 2001), the approach of this study differs from the previous studies in that it uses multivariate statistical technique (cluster analysis) to define hydrochemical facies and applies those facies as tracers. In this context, the nonsystematic variations within the normal background pattern were examined to evaluate the hypothesis that there is substantial interbasin flow into the alluvial aquifer. The insight that is gained from the current study can be used to develop a realistic conceptual groundwater flow model for the basin. The technique is particularly useful because it does not require unusual analytical procedures and makes use of the standard hydrochemical data readily available for many locations. The objectives of this paper are (1) to describe the major hydrochemical facies in the study area, (2) to use these hydrochemical facies as tracers to delineate groundwater flow paths in the aquifer system, and (3) to evaluate the existence of an interbasin flow component postulated by authors in recent studies.