Irene Farnham

Treatment of nondetects in multivariate analysis of groundwater geochemistry data

Abstract Principal components analysis (PCA) is used to evaluate similarities in the trace element chemistry of groundwaters. Many of the trace elements, however, occur at concentrations below the detection limits (DL), which presents problems for statistical analyses. Since the optimal methods for dealing with the ‘ In this study, a new approach was developed to determine the best substitution methods when dealing with the ‘DL’ values for a given data set. Monte Carlo simulation experiments, using a mixture multivariate model, were performed to test the effects of substitution of the ‘ When ‘

Rare earth element fractionation and concentration variations along a groundwater flow path within a shallow, basin-fill aquifer, southern Nevada, USA

Abstract Rare earth element (REE) concentrations were measured in 5 well water samples and 3 springs located along a groundwater flow path in a shallow, tuffaceous alluvial aquifer from southern Nevada, USA. The REE concentrations in these groundwaters decrease in the direction of groundwater flow. A previous investigation demonstrated that REE solid-liquid phase partitioning coefficients (i.e., Kd’s) for groundwaters from tuffaceous alluvial aquifers in southern Nevada are relatively high (mean Kd = 102.6). Our groundwater REE data, in conjunction with these Kd’s, support strong sorption of aqueous REEs to aquifer surface sites as the primary removal mechanism of REEs from these groundwaters. In addition, relatively high aqueous REE concentrations occur at distinct locations along the groundwater flow path. The elevated REE concentrations are explained by addition of deeper groundwaters, influx of geothermal waters from a hot spring system, differences in solution complexation, and/or mixtures of regional and local recharge sources. Solution complexation modelling of REEs in the groundwaters indicate that carbonate complexes account for more than 99% of each REEs in solution. Moreover, groundwater Yb/Nd ratios (a measure of REE fractionation) are associated with alkalinity (HCO3− + CO32−; r = 0.71). The data and speciation model results indicate that REE fractionation (i.e., the observed heavy REE, HREE, enrichments compared to rock-sources) is controlled by formation of progressively stronger carbonate complexes in solution with increasing atomic number, which inhibits HREE sorption compared to light REEs (LREE); and a greater affinity for the LREEs to sorb to surface sites in the local tuffaceous alluvial aquifers compared to the HREEs.

Deciphering groundwater flow systems in Oasis Valley, Nevada, using trace element chemistry, multivariate statistics, and geographical information system

The origin of groundwater discharging via evapotranspiration and from springs within Oasis Valley, Nevada, is of concern owing to the close proximity of the Nevada Test Site (NTS) and the possible contamination of groundwater as a result of underground nuclear testing. Principal components analysis, cluster analysis, and population partitioning, along with a Geographical Information System, were used to decipher groundwater flow patterns in Oasis Valley, Nevada. These multivariate statistical techniques were applied to the trace element chemistry of groundwater samples collected from 26 springs and wells within Oasis Valley, the NTS, and the Nellis Air Force Range. The results of all statistical analyses showed similar geographical trends in the trace element chemistry of the groundwaters included in this study. Differences are observed between the groundwaters from the NTS and those of Oasis Valley based on the concentrations of the elements Li, Ge, Mo, Rb, Ba, U, and Ru. A concentration gradient is observed from lower concentrations in the NTS to increasing concentrations toward Oasis Valley suggesting groundwater flow in an overall southwestward direction from the NTS. Also, a different trace element signature is observed for the waters collected in the northern and western region of Oasis Valley, suggesting another source of groundwater to this area.

Geochemical and statistical evidence of deep carbonate groundwater within overlying volcanic rock aquifers/aquitards of southern Nevada, USA

Samples collected from springs and wells in southern Nevada were analyzed for major solutes and trace elements as part of a larger study to characterize the geochemical signatures of these groundwaters. In this study, principal component analysis (PCA) was used to reduce the large data sets, including the four major cations (Ca, Mg, Na, K) and 27 trace elements, analyzed in these groundwater samples. Principal components analysis of the major cation data indicates that groundwaters from Cenozoic felsic volcanic rock aquifers/aquitards of southern Nevada exhibit strong chemical associations to each other but weak relationships to groundwaters from the regional carbonate aquifer (which were instead chemically similar to each other). However, PCA of the trace element data demonstrates that some groundwaters from the volcanic aquifers/aquitards are chemically similar to those of the underlying regional carbonate aquifer. The PCA also reveals that these groundwaters from the volcanic aquifers/aquitards have significantly different trace element compositions than perched groundwaters contained within similar felsic volcanic rocks. Moreover, rare earth element (REE) data from groundwaters collected from wells finished in the volcanic aquifers/aquitards of southern Nevada have similar concentrations and similar shale-normalized patterns to the carbonate aquifer groundwaters as well as local carbonate rocks. These same southern Nevada well waters do not exhibit REE concentrations or shale-normalized signatures that resemble the perched volcanic groundwaters or the tuffs of southern Nevada. The REE data and trace element PCA, along with previous carbon isotope analyses, water temperature data, hydraulic head relations, and results of a recent pump test of a well near Yucca Mountain, suggest close contact of the regional carbonate groundwaters and groundwaters from the overlying volcanic rocks of southern Nevada and possible upwelling of the carbonate groundwaters into the overlying volcanic rock units in the vicinity of Yucca Mountain.

Using multivariate statistical analysis of groundwater major cation and trace element concentrations to evaluate groundwater flow in a regional aquifer

Groundwater samples were collected from 11 springs in Ash Meadows National Wildlife Refuge in southern Nevada and seven springs from Death Valley National Park in eastern California. Concentrations of the major cations (Ca, Mg, Na and K) and 45 trace elements were determined in these groundwater samples. The resultant data were subjected to evaluation via the multivariate statistical technique principal components analysis (PCA), to investigate the chemical relationships between the Ash Meadows and Death Valley spring waters, to evaluate whether the results of the PCA support those of previous hydrogeological and isotopic studies and to determine if PCA can be used to help delineate potential groundwater flow patterns based on the chemical compositions of groundwaters. The results of the PCA indicated that groundwaters from the regional Paleozoic carbonate aquifers (all of the Ash Meadows springs and four springs from the Furnace Creek region of Death Valley) exhibited strong statistical associations, whereas other Death Valley groundwaters were chemically different. The results of the PCA support earlier studies, where potentiometric head levels, δ18O and δD, geological relationships and rare earth element data were used to evaluate groundwater flow, which suggest groundwater flows from Ash Meadows to the Furnace Creek springs in Death Valley. The PCA suggests that Furnace Creek groundwaters are moderately concentrated Ash Meadows groundwater, reflecting longer aquifer residence times for the Furnace Creek groundwaters. Moreover, PCA indicates that groundwater may flow from springs in the region surrounding Scottys Castle in Death Valley National Park, to a spring discharging on the valley floor. The study indicates that PCA may provide rapid and relatively cost-effective methods to assess possible groundwater flow regimes in systems that have not been previously investigated. Copyright

Major ion geochemistry of groundwaters from southern Nevada and eastern California, USA

The dissolved ionic constituents of groundwaters are, in part, a record of the minerals and rocks in aquifers through which the water has flowed. The chemical composition and association of these major ions in groundwaters have been used to trace groundwater flow paths and sources. In general, the chemical composition of water in carbonate-rock aquifers is dominated by calcium, magnesium, and bicarbonate, whereas sodium, chloride, and sulfate can be dominant ions in the water that comes from volcanic aquifers or clay minerals. Since the 1990’s, we have dealt with the geochemistry of groundwaters from more than 100 springs and wells in southern Nevada and eastern California, USA for major solutes and trace elements. This paper compiles the hydrochemical data of major ions of these groundwaters. Based on major ion geochemistry, groundwaters from southern Nevada and eastern California can be classified as carbonate aquifer water, volcanic aquifer water, and mixing water (either mixing of carbonate and volcanic aquifer waters or mixing with local recharges). Piper and Stiff diagrams of major ions have graphically shown the general chemical characteristics, classifications, and mixing relationships of groundwaters from southern Nevada and eastern California.