Klaus J. Stetzenbach

Rare earth elements as geochemical tracers of regional groundwater mixing

Abstract The rare earth elements (REE) were analyzed in a groundwater system from south-central Nevada (i.e., Ash Meadows National Wildlife Refuge, the Spring Mountains, Pahranagat National Wildlife Refuge, and the Nevada Test Site) in order to investigate their potential use as tracers of regional groundwater flow. Previous investigations using conservative tracers (e.g., deuterium and uranium isotopes) identified recharge in local mountains as the primary source (60–70% of total discharge) for the springs in the regional discharge zone (i.e., Ash Meadows) with the remaining contribution being interbasin flow from the northeast. Initial mixing calculations for these groundwaters using shale-normalized REE patterns agreed well with the previous studies; however, because the REEs are not expected to behave conservatively in natural waters, the effect of both solution complexation, which acts to enhance the stability of the REES in solution, as well as surface complexation, responsible for the particle reactive behavior of the REES, were examined in subsequent mixing calculations. In order to assess the roles of solution and surface complexation, relative partitioning coefficients were estimated for each REE in each groundwater by evaluating the ratio of the ionic strength corrected co 3 β 1 REE, co 3 β 2 REE, and [CO 3 2− ] F to the first hydrolysis binding constants for the REES. The relative partitioning coefficients were then used to calculate REE patterns expected to develop and persist in solution as a consequence of solution and surface complexation. The calculated REE values closely resembled the actual measured REE concentrations, suggesting that the REEs are, in fact, controlled by solution and surface complexation in these groundwaters. The calculated REE concentrations were subsequently used to determine mixing ratios, the results of which coincided with the initial calculations as well as the previous studies. The results of this study suggest that solution complexation of the REEs is sufficient to overcome, to a certain degree, the affinity of the REEs to be adsorbed onto surface sites in the aquifers such that distinctive REE signatures develop and persist in solution in groundwaters from different aquifers. The ability of solution complexation to overcome surface complexation is likely related to the formation of the negatively charged dicarbonato complex [i.e., Ln(CO 3 ) 2 − , where Ln is any REE], which accounts for significant fractions of each REE in these groundwaters.

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 complexation behavior in circumneutral pH groundwaters: Assessing the role of carbonate and phosphate ions

Rare earth element (REE) concentrations were determined in circumneutral pH (7 ≤ pH ≤ 9) groundwaters from south-central Nevada and the Mojave Desert in eastern California, U.S.A. The inorganic speciation of the REEs in these water were evaluated primarily to assess the relative importance of carbonate (LnHCO32+, LnCO3+ and Ln(CO3)2−) and phosphate (LnH2PO42+, LnHPO4+, Ln(HPO4)2− and LnPO40) complexes. Other REE complexes with sulfate, hydroxyl, chloride, and fluoride ions were also assessed, as was the significance of the free metal ion species (i.e., Ln3+). Our speciation calculations indicate that REE-carbonate complexes dominate and typically account for more than 99% of each REE in solution. Moreover, carbonato complexes (LnCO3+) were predicted to be the dominant species for the light REEs and dicarbonato complexes (Ln(CO3)2−) were predicted to increase in importance with increasing atomic number. All other complexes were predicted to be negligible. Furthermore, the combined percentage of all REE-phosphate complexes never accounted for more than 0.1% of the dissolved REEs, and generally much less than 0.1%. Phosphate complexes can not compete with carbonate complexes in these groundwaters because of the low ∑PO43− concentrations (< 0.3–1.58 μmol/kg), the much lower concentrations of the free phosphate ion (i.e., [PO43−]F = 10−9−10−6 μmol/kg), due to ion pair formation with Ca2+ and Mg2+, and the much higher free carbonate ion concentrations ([CO32−]F = 0.32−87 μmol/kg).

Geochemistry of the rare-earth elements in hypersaline and dilute acidic natural terrestrial waters: Complexation behavior and middle rare-earth element enrichments

Rare-earth element (REE) speciation was modelled in acid (2.9 ≤ pH ≤ 3.5), hypersaline groundwaters from Australia and from the Palo Duro Basin in Texas, USA, using a combined specific ion interaction (Pitzer model) and ion pairing model. The free metal ion species (i.e. Ln3+) is the dominant form of dissolved REEs in these systems, accounting for 40–70% of the dissolved metal in groundwater from Lake Tyrrell (Victoria, Australia), 50–90% of the REEs in groundwater from Lake Gilmore (Western Australia), and always > 90% of each REE in the Texas groundwaters. (Lakes Tyrrell and Gilmore are actually dry lakes that act as groundwater discharge zones.) The abundance of the free metal ion species increases in these waters with increasing ionic strength and with decreasing pH. The free metal ion species is followed in abundance by REE-sulfate and REE-chloride complexes that account for: 20–50% and 10–15%, respectively, of the dissolved REEs in the groundwaters from Lake Tyrrell; 15–30% and 5–20%, respectively, in the groundwaters from Lake Gilmore; and < 3% and < 7%, respectively, in the Palo Duro Basin groundwaters. The groundwaters of the Lake Tyrrell system and the Palo Duro Basin are enriched in the middle REEs (MREEs) compared to both the light REEs (LREEs) and the heavy REEs (HREEs). (Only Nd, Sm, and Dy were determined in the Lake Gilmore groundwaters and, consequently, it is unclear whether these acid groundwaters are also enriched in the MREEs.) Previous investigators suggested that Fe-rich organic flocs, REE-phosphate complexation, and solid-liquid exchange reactions between terrestrial waters and MREE-enriched surface FeMn coatings, suspended particulates, or secondary mineral phases within aquifer materials, may promote the development of MREE enrichments in natural waters. We propose that organic colloids and REE-phosphate complexes are insignificant in acidic natural waters in regards to MREE enrichments. Instead, solid-liquid exchange reactions or dissolution of surface coatings, suspended particulates, and/or secondary phases as well as sulfate complexation, more likely controls the development of MREE enrichments in acidic natural terrestrial waters.

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.

Origin of rare earth element signatures in groundwaters of circumneutral pH from southern Nevada and eastern California, USA

Concentrations of the rare earth elements (REE) were measured in circumneutral pH groundwaters from southern Nevada and Death Valley, CA. Groundwaters from the regional lower Paleozoic carbonate-rock aquifer (Cambrian–Devonian) have flat shale-normalized patterns that closely resemble the shale-normalized patterns of the aquifer rock samples (principally Cambrian). Groundwaters associated with younger carbonate rocks (chiefly Permian) in the study region exhibit heavy REE (HREE) enriched, shale-normalized REE patterns with substantial negative Ce anomalies that also mimic these carbonate rocks. In addition, groundwaters from the felsic volcanic rock aquifers have the same flat to light REE (LREE) enriched shale-normalized patterns with large negative Eu anomalies as the felsic volcanic rocks. The similar REE patterns of all the groundwaters and associated aquifer rocks studied suggest that the groundwaters inherited REE signatures from the host rocks through which they flow. Because negative Ce anomalies are not an uncommon feature of carbonate rocks of marine origin, the negative Ce anomalies reported here for these groundwaters may reflect a Permian marine Ce signature. Previously, we demonstrated that carbonate complexes dominate REE speciation in southern Nevada and Death Valley groundwaters. Moreover, solid–liquid partitioning coefficients (Kd) indicate that the affinity of LREEs to sorb to aquifer surface sites is substantially greater than for the HREEs in the southern Nevada carbonate- and felsic volcanic-rock alluvial aquifers. Consequently, the HREEs enrichments reported here for groundwaters associated with younger Paleozoic carbonate rocks compared to these source rocks is consistent with REE carbonate complexation and preferential removal of LREEs to aquifer surface sites.

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.

The solubility control of rare earth elements in natural terrestrial waters and the significance of PO 4 3? and CO 3 2? in limiting dissolved rare earth concentrations: A review of recent information

Rare earth element (REE) concentrations in alkaline lakes, circumneutral pH groundwaters, and an acidic freshwater lake were determined along with the free carbonate, free phosphate, and free sulfate ion concentrations. These parameters were used to evaluate the saturation state of these waters with respect to REE phosphate and carbonate precipitates. Our activity product estimates indicate that the alkaline lake waters and groundwaters are approximately saturated with respect to the REE phosphate precipitates but are significantly undersaturated with respect to REE carbonate and sulfate precipitates. On the other hand, the acidic lake waters are undersaturated with respect to REE sulfate, carbonate, and phosphate precipitates. Although carbonate complexes tend to dominate the speciation of the REEs in neutral and alkaline waters, our results indicate that REE phosphate precipitates are also important in controlling REE behavior. More specifically, elevated carbonate ion concentrations in neutral to alkaline natural waters tend to enhance dissolved REE concentrations through the formation of stable REE-carbonate complexes whereas phosphate ions tend to lead to the removal of the REEs from solution in these waters by the formation of REE-phosphate salts. Removal of REEs by precipitation as phosphate phases in the acid lake (pH=3.6) is inconsequential, however, due to extremely low [PO43−]F concentrations (i.e., ∼ 10−23 mol/kg).

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