Karen H. Johannesson

Factor analytical approaches for evaluating groundwater trace element chemistry data

Abstract The multivariate statistical techniques principal component analysis (PCA), Q-mode factor analysis (QFA), and correspondence analysis (CA) were applied to a dataset containing trace element concentrations in groundwater samples collected from a number of wells located downgradient from the potential nuclear waste repository at Yucca Mountain, Nevada. PCA results reflect the similarities in the concentrations of trace elements in the water samples resulting from different geochemical processes. QFA results reflect similarities in the trace element compositions, whereas CA reflects similarities in the trace elements that are dominant in the waters relative to all other groundwater samples included in the dataset. These differences are mainly due to the ways in which data are preprocessed by each of the three methods. The highly concentrated, and thus possibly more mature (i.e. older), groundwaters are separated from the more dilute waters using principal component 1 (PC 1). PC 2, as well as dimension 1 of the CA results, describe differences in the trace element chemistry of the groundwaters resulting from the different aquifer materials through which they have flowed. Groundwaters thought to be representative of those flowing through an aquifer composed dominantly of volcanic rocks are characterized by elevated concentrations of Li, Be, Ge, Rb, Cs, and Ba, whereas those associated with an aquifer dominated by carbonate rocks exhibit greater concentrations of Ti, Ni, Sr, Rh, and Bi. PC 3, and to a lesser extent dimension 2 of the CA results, show a strong monotonic relationship with the percentage of As(III) in the groundwater suggesting that these multivariate statistical results reflect, in a qualitative sense, the oxidizing/reducing conditions within the groundwater. Groundwaters that are relatively more reducing exhibit greater concentrations of Mn, Cs, Co, Ba, Rb, and Be, and those that are more oxidizing are characterized by greater concentrations of V, Cr, Ga, As, W, and U.

Speciation of rare earth elements in natural terrestrial waters: assessing the role of dissolved organic matter from the modeling approach

Humic Ion-Binding Model V, which focuses on metal complexation with humic and fulvic acids, was modified to assess the role of dissolved natural organic matter in the speciation of rare earth elements (REEs) in natural terrestrial waters. Intrinsic equilibrium constants for cation-proton exchange with humic substances (i.e., pKMHA for type A sites, consisting mainly of carboxylic acids), required by the model for each REE, were initially estimated using linear free-energy relationships between the first hydrolysis constants and stability constants for REE metal complexation with lactic and acetic acid. pKMHA values were further refined by comparison of calculated Model V “fits” to published data sets describing complexation of Eu, Tb, and Dy with humic substances. A subroutine that allows for the simultaneous evaluation of REE complexation with inorganic ligands (e.g., Cl−, F−, OH−, SO42−, CO32−, PO43−), incorporating recently determined stability constants for REE complexes with these ligands, was also linked to Model V. Humic Ion-Binding Model V’s ability to predict REE speciation with natural organic matter in natural waters was evaluated by comparing model results to “speciation” data determined previously with ultrafiltration techniques (i.e., organic acid–rich waters of the Nsimi-Zoetele catchment, Cameroon; dilute, circumneutral-pH waters of the Tamagawa River, Japan, and the Kalix River, northern Sweden). The model predictions compare well with the ultrafiltration studies, especially for the heavy REEs in circumneutral-pH river waters. Subsequent application of the model to world average river water predicts that organic matter complexes are the dominant form of dissolved REEs in bulk river waters draining the continents. Holding major solute, minor solute, and REE concentrations of world average river water constant while varying pH, the model suggests that organic matter complexes would dominate La, Eu, and Lu speciation within the pH ranges of 5.4 to 7.9, 4.8 to 7.3, and 4.9 to 6.9, respectively. For acidic waters, the model predicts that the free metal ion (Ln3+) and sulfate complexes (LnSO4+) dominate, whereas in alkaline waters, carbonate complexes (LnCO3+ + Ln[CO3]2−) are predicted to out-compete humic substances for dissolved REEs. Application of the modified Model V to a “model” groundwater suggests that natural organic matter complexes of REEs are insignificant. However, groundwaters with higher dissolved organic carbon concentrations than the “model” groundwater (i.e., >0.7 mg/L) would exhibit greater fractions of each REE complexed with organic matter. Sensitively analysis indicates that increasing ionic strength can weaken humate-REE interactions, and increasing the concentration of competitive cations such as Fe(III) and Al can lead to a decrease in the amount of REEs bound to dissolved organic matter.

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.

Strontium isotopes and rare earth elements as tracers of groundwater-lake water interactions, Lake Naivasha, Kenya

Abstract Strontium isotope compositions and rare earth element (REE) concentrations are presented for groundwater and surface water samples collected from the Lake Naivasha watershed in the East African Rift, Kenya. The chief objective of the study is to test the suitability of REEs, in conjunction with Sr isotopes, as tools for investigating groundwater–lake water interactions. In general, the REE concentrations and 87 Sr/ 86 Sr ratios support the authors’ earlier investigations where Cl mass balance, δ 18 O, δD, and He isotopes were employed to study groundwater–lake water interactions in the Naivasha watershed. The REE data suggest that a significant amount of the groundwater south of Lake Naivasha (i.e., 50–85%) consists of lake water recharge to the aquifer system. Specifically, mixing calculations conducted using REE data of Lake Naivasha water and groundwaters indicate that between 70 and 85% of groundwater directly south of the lake is likely lake water. These values are somewhat higher than the authors’ previous estimates determined with conservative stable H isotopes (δD, 50–70%). For both cases, however, the data demonstrate that water originating in Lake Naivasha contributes significantly to the underlying groundwater flow system, hence supporting earlier evidence that the lakes freshness reflects rapid loss of water and dissolved solutes to the local groundwater system. Overall, lake and groundwater Sr isotope compositions support seepage of lake water into the underlying aquifer along the lakes south shore. The 87 Sr/ 86 Sr data also provide additional insight into the geochemical evolution of waters of the Lake Naivasha watershed indicating that the initial source of Sr to these waters is likely chemical weathering reactions involving basaltic rocks within the recharge zones of the watershed along the Rift Valley flanks. Furthermore, with increasing residence time of groundwaters within the aquifer system and flow down and along the rift valley floor, Sr isotope compositions of groundwaters become more radiogenic, reflecting rock–water interactions with chemically differentiated and radiogenic peralkaline rhyolite volcanic rocks. The importance of the longer aquifer residence times and radiogenic source rocks is especially apparent for geothermal waters of the Olkaria Geothermal Field that have 87 Sr/ 86 Sr ratios (i.e., 0.70747) similar to local comendites.

Perennial ponds are not an important source of water or dissolved organic matter to groundwaters with high arsenic concentrations in West Bengal, India

excess of 4600 m gk g −1 . Stable isotope ratios of waters from constructed, perennial ponds indicate the ponds are chiefly recharged during the summer monsoon, and subsequently undergo extensive evaporation during the dry (winter) season. In contrast, groundwaters with high As concentrations plot along the local meteoric water line (LMWL) near where the annual, volume‐weighted mean precipitation values for d 2 Ha ndd 18 O would plot. The stable isotope data demonstrate that groundwaters are directly recharged by local precipitation without significant evaporation, and thus are not recharged by, nor mixed with, the pond waters. Furthermore, reactive transport modeling indicates that dissolved organic matter (DOM) derived from pond waters does not fuel microbial respiration and As mobilization at depth in the underlying aquifer because travel times for pond‐derived DOM exceed groundwater ages by thousands of years. Instead, organic matter within the aquifer sediments must drive dissimilatory iron reduction and As release to groundwaters. Citation: Datta, S., A. W. Neal, T. J. Mohajerin, T. Ocheltree, B. E. Rosenheim, C. D. White, and K. H. Johannesson (2011), Perennial ponds are not an important source of water or dissolved organic matter to groundwaters with high arsenic concentrations in West Bengal, India, Geophys. Res. Lett., 38, L20404, doi:10.1029/2011GL049301.

Geochemistry of Rare Earth Elements in Groundwaters from a Rhyolite Aquifer, Central México

Rare earth element (REE) concentrations were measured in groundwaters collected from wells finished in a fractured, rhyolitic (Cuatralba Ignimbrite) aquifer from the La Muralla region of the central Mexican State of Guanajuato. The study site is located within the Faja Volcanica Transmexicano (i.e., Trans-Mexican Volcanic Belt), an extensive region of active volcanism within central Mexico. La Muralla groundwaters are relatively warm (32.2 ± 2.7 °C), dilute Na-Ca-HCO3 waters (5.1 mmol/kg ≤ I ≤ 9.5 mmol/kg) of circumneutral pH (7.27 ≤ pH ≤ 8.01). Concentrations of REEs in La Muralla groundwaters are exceedingly low, as demonstrated by Nd values, which range from ∼ 10 pmol/kg to 34 pmol/kg. La Muralla groundwaters exhibit enrichments in the heavy REEs (HREE) over the light REEs (LREE) compared to Average Shale, as well as volcanic rocks from the Trans-Mexican Volcanic Belt, including rhyolitic volcanic rocks similar to those of the Cuatralba Ignimbrite aquifer. Shale-normalized Yb/Nd ratios of La Muralla groundwaters range from 1.85 to 6.55, with a mean (± standard deviation) of 4.2 ± 1.2. Rare earth element concentrations for La Muralla groundwaters are normalized to the average REE values of 27 different calc-alkaline rhyolites (from the literature) from the Trans-Mexican Volcanic Belt. The average Trans-Mexican Volcanic Belt rhyolite-normalized Yb/Nd ratios for La Muralla groundwaters range from 1.57 to 5.55, with a mean (± standard deviation) of 3.52 ± 1. Speciation calculations predict that REEs occur principally as carbonate complexes in La Muralla groundwaters, with LREEs predominantly in the form of positively charged, carbonato complexes (LnCO3 +), and to a lesser extent, free metal ions (Ln3+), and HREEs chiefly in solution as negatively charged, dicarbonato complexes (Ln(CO3)2 −). The speciation model predictions suggest that the HREE enrichment of La Muralla groundwaters originate from solution and surface complexation reactions within the system. Specifically, the preferential complexation of HREEs as negatively charged, dicarbonato complexes acts to stabilize HREE is solution owing to both the strength of these complexes and their low affinity for aquifer surface sites. Because La Muralla groundwaters are of circumneutral pH, surface complexation sites within the Cuatralba Ignimbrite are expected to predominantly be negatively charged. Therefore, because LREEs occur primarily as positively charged, carbonato complexes in La Muralla groundwaters, they are preferentially removed from solution owing to complexation to aquifer surface sites.

Rare Earth Element Concentrations, Speciation, and Fractionation along Groundwater Flow Paths: The Carrizo Sand (Texas) and Upper Floridan Aquifers

Groundwater samples were collected in two different types of aquifers (i.e., Carrizo sand aquifer, Texas and Upper Floridan carbonate aquifer, west-central Florida) to study the concentration, speciation, and fractionation of rare earth elements (REE) along the groundwater flow path in each system. Major solutes and dissolved organic carbon (DOC) were also measured in these groundwaters. In the Carrizo aquifer, groundwaters in the recharge zone are chiefly Ca-Na-HCO3-Cl type waters and shift into Na-HCO3 waters with flow down-gradient. DOC is generally low (0.65 mg/L) along the flow path. In the Upper Floridan Aquifer, groundwaters are Ca (Mg)-HCO3 waters at the recharge zone, shift into Ca-Mg-SO4-HCO3 waters at the mid-reaches of the flow path, and finally become Ca-Mg-SO4 waters at the discharge zone. DOC is higher (0.64 – 2.29 mg/L) than in the Carrizo and initially increases along the flow path and then decreases down-gradient. Rare earth element concentrations generally decrease along the groundwater flow path in the Carrizo sand aquifer, whereas in the Upper Floridan aquifer, the Nd concentrations increase first and then slightly decrease, Gd concentrations are erratic, and Er tends to decrease along the groundwater flow path. Shale-normalized REE patterns are enriched in the HREEs for groundwaters from the recharge regions of both aquifers and tend to flatten with flow down-gradient. Speciation calculations predict that carbonato (LnCO3 +) and/or dicarbonato complexes (Ln(CO3)2 −) are the main dissolved species for all REEs in groundwaters from the Carrizo sand aquifer, whereas organic complexes of the REEs (LnHM) are dominant in all but two furthest down-gradient groundwaters from the Upper Floridan aquifer. The variations of REE concentrations, speciation, and fractionation patterns along the flow paths reflect water-rock reactions (mainly adsorption and solution complexation) experienced by REEs in groundwaters from both aquifers. The solution complexation of REEs affects the REE concentrations and fractionation patterns in groundwaters along the flow path via the following two ways: (1) the complexation capacity of REEs with carbonate ions and organic ligands varies with increasing atomic number of REEs; and (2) different complexation forms (i.e., negative or positive) at different geochemical conditions of groundwaters (i.e., pH, alkalinity, and DOC concentrations).

RARE EARTH ELEMENT CONTENTS OF HIGH pCO2 GROUNDWATERS OF PRIMORYE, RUSSIA: MINERAL STABILITY AND COMPLEXATION CONTROLS

The rare earth element (REE) geochemistry of cold, high pCO2 groundwaters was studied in springs and boreholes in the Primorye region of the Russian Far East. The gas phase in these waters is dominated by mantle-derived CO2 (up to 2.6 atm.), being introduced to shallow groundwaters along major fault systems. The aggressive nature of these moderately acidic groundwaters has led to unusual trace element characteristics with high concentrations of relatively immobile elements such as Al, Be, heavy REEs and Zr. They are also marked by extremely high concentrations of Fe and Mn, up to 80 mg 1−1 and 4 mg 1−1, respectively. The REE patterns generally show enrichment in the middle to heavy REEs and low concentrations of the light REEs (La-Nd). Most groundwaters show relatively flat shale-normalised middle to heavy REE profiles, with the exception of Eu, which may form positive or negative anomalies depending on local mineralogy. A characteristic of many of the groundwaters is the presence of positive Sm-Eu anomalies. A range of potential ligands is present in the groundwaters and model calculations show that the dominant species are Ln3+, carbonate complexes (i.e., predominantly LnCO3 +), and LnF2+. Concentrations of Cl− and SO4 2− are very low in most waters and nitrate and phosphate are below detection limit. The role of organic completing is not known due to lack of data, but such complexes may be important since limited TOC data show that organic contents may be high. The middle to heavy REE enrichment found in the groundwaters is consistent with dissolution of Fe-Mn oxyhydroxides and release of adsorbed REE. Positive Eu anomalies in some groundwaters correlate with high Ca and Sr, pointing to control by plagioclase dissolution. High Y/Ho ratios and positive Y spikes on REY plots in high-F− waters suggest an important control by F− completing, which is confirmed by speciation calculations. Extreme enrichments in the heavy REEs are found in two groups of mineral waters with Yb/La ratios up to 9.8. The extreme enrichments in heavy/light REEs present in these areas are too high to be simply controlled by speciation fractionation and it is suggested that a weathering phase with heavy REE enrichment is responsible. The source is suggested to be zircon, which typically displays such heavy REE enrichments. Although zircon is generally stable during low-temperature weathering, it is know to break down in acidic carbonated solutions. This is supported by high Zr concentrations (as well as high U, Be) in these groundwaters and a correlation between Zr and heavy REE enrichment.

Potential contaminants at a dredged spoil placement site, Charles City County, Virginia, as revealed by sequential extraction

Backfills of dredged sediments onto a former sand and gravel mine site in Charles City County, VA may have the potential to contaminate local groundwater. To evaluate the mobility of trace elements and to identify the potential contaminants from the dredged sediments, a sequential extraction scheme was used to partition trace elements associated with the sediments from the local aquifer and the dredged sediments into five fractions: exchangeable, acidic, reducible, oxidizable, and residual phases. Sequential extractions indicate that, for most of the trace elements examined, the residual phases account for the largest proportion of the total concentrations, and their total extractable fractions are mainly from reducible and oxidizable phases. Only Cd, Pb, and Zn have an appreciable extractable proportion from the acidic phase in the filled dredged sediments. Our groundwater monitoring data suggest that the dredged sediments are mainly subject to a decrease in pH and a series of oxidation reactions, when exposed to the atmosphere. Because the trace elements released by carbonate dissolution and the oxidation (e.g., organic matter degradation, iron sulfide and, ammonia oxidation) are subsequently immobilized by sorption to iron, manganese, and aluminum oxides, no potential contaminants to local groundwater are expected by addition of the dredged sediments to this site.

The Lake St. Martin bolide has a big impact on groundwater fluoride concentrations

The majority of residents of Manitoba (Canada) outside of the capital, Winnipeg, rely on groundwater for their drinking water. Between lakes Winnipeg and Winnipegosis, most aquifers occur in Paleozoic carbonate lithologies. Proximal to the town of Gypsumville, however, lithologies associated with the Lake St. Martin impact structure and younger basin-filling red bed and evaporite (gypsum/anhydrite) sedimentary rocks complicate the hydrology and hydrochemistry. Here, domestic wells have elevated salinities (up to 8000 mg/L total dissolved solids), elevated sulfate (up to 4000 mg/L), and elevated fluoride concentrations that are in excess of health limits (F − up to 15.2 mg/L, with 20% over 1.5 mg/L). Groundwaters with elevated fluoride occur exclusively within the impact structure. The impact melt rocks and younger red beds consistently have the highest fluoride abundances, up to 2160 ppm. Groundwater pH values are alkaline, ranging up to 10.7, with highest groundwater pH from wells in the impact melt rocks. The spatial associations of impact melt rocks and red beds with elevated fluoride, strong positive correlations between fluoride and pH, sodium, chloride, sulfate, boron and lithium, greater Fe 2 O 3 and Al 2 O 3 concentrations of the host rocks, and cation exchange capacity (CEC) all indicate that fluoride concentrations in groundwaters are enhanced as a result of anion exchange wherein OH − and \(CO_{3}^{2{-}}\) displace F − adsorbed onto Fe- and Al-oxyhydroxide surfaces. Thus, the elevated fluoride contents of groundwaters at Gypsumville are a consequence of the composition of the impact melt rocks and enhanced permeability and grain-size reduction produced by bolide impact.