Hydrothermal contamination of public supply wells in Napa and Sonoma Valleys, California

Article history: Received 11 July 2012 Accepted 15 January 2013 Available online 7 February 2013 Editorial handling by Peter Birkle Groundwater chemistry and isotope data from 44 public supply wells in the Napa and Sonoma Valleys, California were determined to investigate mixing of relatively shallow groundwater with deeper hydrothermalfluids.Multivariate analyses includingCluster Analyses,Multidimensional Scaling (MDS), Principal Components Analyses (PCA), Analysis of Similarities (ANOSIM), and Similarity Percentage Analyses (SIMPER) were used to elucidate constituent distribution patterns, determine which constituents are significantly associated with these hydrothermal systems, and investigate hydrothermal contamination of local groundwater used for drinking water. Multivariate statistical analyses were essential to this study because traditional methods, such as mixing tests involving single species (e.g. Cl or SiO2) were incapable of quantifying component proportions due to mixing of multiple water types. Based on these analyses, water samples collected from thewells were broadly classified as fresh groundwater, salinewaters, hydrothermal fluids, ormixed hydrothermal fluids/meteoric waterwells. TheMultivariateMixing andMass-balance (M3)modelwas applied in order to determine the proportion of hydrothermal fluids, salinewater, and fresh groundwater in each sample.Major ions, isotopes, and physical parameters of thewaterswere used to characterize the hydrothermal fluids as Na–Cl type, with significant enrichment in the trace elements As, B, F and Li. Five of the wells from this study were classified as hydrothermal, 28 as fresh groundwater, two as saline water, and nine as mixed hydrothermal fluids/meteoric water wells. The M3 mixing-model results indicated that the nine mixed wells contained between 14% and 30% hydrothermal fluids. Further, the chemical analyses show that several of these mixed-water wells have concentrations of As, F and B that exceed drinking-water standards or notification levels due to contamination by hydrothermal fluids. 2013 Elsevier Ltd. All rights reserved.