Randy L. Stotler

Strontium isotope geochemistry of groundwater in the central part of the Dakota (Great Plains) aquifer, USA

Abstract The Dakota aquifer of the central and eastern Great Plains of the United States is an important source of water for municipal supplies, irrigation and industrial use. Although the regional flow system can be characterized generally as east to northeasterly from the Rocky Mountains towards the Missouri River, locally the flow systems are hydrologically complex. This study uses Sr isotopic data from groundwater and leached aquifer samples to document the complex subsystems within the Dakota aquifer in Nebraska and Kansas. The interaction of groundwater with the geologic material through which it flows has created spatial patterns in the isotopic measurements that are related to: long-term water–rock interaction, during which varying degrees of isotopic equilibrium between water and rock has been achieved; and the alteration of NaCl fluids by water-rock interaction. Specifically, Sr isotopic data distinguish brines from Kansas and western Nebraska from those in eastern Nebraska: the former are interpreted to reflect interaction with Permian rocks, whereas the latter record interaction with Pennsylvanian rocks. The Sr isotopic composition of groundwater from other parts of Nebraska and Kansas are a function of the dynamic interaction between groundwater and unlithified sediments (e.g., glacial till and loess), followed by interaction with oxidized and unoxidized sediments within the Dakota Formation. This study illustrates the power of combining Sr chemistry with more conventional geochemical data to obtain a more complete understanding of groundwater flow systems within regional aquifer systems where extensive monitoring networks do not exist.

Interpretation of water level changes in the high plains aquifer in Western Kansas.

Water level changes in wells provide a direct measure of the impact of groundwater development at a scale of relevance for management activities. Important information about aquifer dynamics and an aquifers future is thus often embedded in hydrographs from continuously monitored wells. Interpretation of those hydrographs using methods developed for pumping-test analyses can provide insights that are difficult to obtain via other means. These insights are demonstrated at two sites in the High Plains aquifer in western Kansas. One site has thin unconfined and confined intervals separated by a thick aquitard. Pumping-induced responses in the unconfined interval indicate a closed (surrounded by units of relatively low permeability) system that is vulnerable to rapid depletion with continued development. Responses in the confined interval indicate that withdrawals are largely supported by leakage. Given the potential for rapid depletion of the unconfined interval, the probable source of that leakage, it is likely that large-scale irrigation withdrawals will not be sustainable in the confined interval beyond a decade. A second site has a relatively thick unconfined aquifer with responses that again indicate a closed system. However, unlike the first site, previously unrecognized vertical inflow can be discerned in data from the recovery periods. In years of relatively low withdrawals, this inflow can produce year-on-year increases in water levels, an unexpected occurrence in western Kansas. The prevalence of bounded-aquifer responses at both sites has important ramifications for modeling studies; transmissivity values from pumping tests, for example, must be used cautiously in regional models of such systems.

Hydrogeology, Chemical and Microbial Activity Measurement Through Deep Permafrost

Little is known about hydrogeochemical conditions beneath thick permafrost, particularly in fractured crystalline rock, due to difficulty in accessing this environment. The purpose of this investigation was to develop methods to obtain physical, chemical, and microbial information about the subpermafrost environment from a surface-drilled borehole. Using a U-tube, gas and water samples were collected, along with temperature, pressure, and hydraulic conductivity measurements, 420 m below ground surface, within a 535 m long, angled borehole at High Lake, Nunavut, Canada, in an area with 460-m-thick permafrost. Piezometric head was well above the base of the permafrost, near land surface. Initial water samples were contaminated with drill fluid, with later samples <40% drill fluid. The salinity of the non-drill fluid component was <20,000 mg/L, had a Ca/Na ratio above 1, with δ(18) O values ∼5‰ lower than the local surface water. The fluid isotopic composition was affected by the permafrost-formation process. Nonbacteriogenic CH(4) was present and the sample location was within methane hydrate stability field. Sampling lines froze before uncontaminated samples from the subpermafrost environment could be obtained, yet the available time to obtain water samples was extended compared to previous studies. Temperature measurements collected from a distributed temperature sensor indicated that this issue can be overcome easily in the future. The lack of methanogenic CH(4) is consistent with the high sulfate concentrations observed in cores. The combined surface-drilled borehole/U-tube approach can provide a large amount of physical, chemical, and microbial data from the subpermafrost environment with few, controllable, sources of contamination.

Challenges for Coring Deep Permafrost on Earth and Mars

A scientific drilling expedition to the High Lake region of Nunavut, Canada, was recently completed with the goals of collecting samples and delineating gradients in salinity, gas composition, pH, pe, and microbial abundance in a 400 m thick permafrost zone and accessing the underlying pristine subpermafrost brine. With a triple-barrel wireline tool and the use of stringent quality assurance and quality control (QA/QC) protocols, 200 m of frozen, Archean, mafic volcanic rock was collected from the lower boundary that separates the permafrost layer and subpermafrost saline water. Hot water was used to remove cuttings and prevent the drill rods from freezing in place. No cryopegs were detected during penetration through the permafrost. Coring stopped at the 535 m depth, and the drill water was bailed from the hole while saline water replaced it. Within 24 hours, the borehole iced closed at 125 m depth due to vapor condensation from atmospheric moisture and, initially, warm water leaking through the casing, which blocked further access. Preliminary data suggest that the recovered cores contain viable anaerobic microorganisms that are not contaminants even though isotopic analyses of the saline borehole water suggests that it is a residue of the drilling brine used to remove the ice from the upper, older portion of the borehole. Any proposed coring mission to Mars that seeks to access subpermafrost brine will not only require borehole stability but also a means by which to generate substantial heating along the borehole string to prevent closure of the borehole from condensation of water vapor generated by drilling.

Spatial and temporal characteristics of stable isotopes in the Tarim River Basin

By using 233 isotope samples, we investigated the spatial and temporal variations of δ18O and δ2H in precipitation and surface water, and the contribution of different water sources in the rivers within the Tarim River Basin (TRB), which receives snow/glacier meltwater, groundwater, and rainfall. Our study revealed a similar seasonal pattern of precipitation δ18O and δ2H at both the north and south edges of the basin, indicating the dominant effect of westerly air masses in the summer and the combined influence of westerly and polar air masses during the winter, although the southern part showed more complex precipitation processes in the summer. River water in the basin has relatively large temporal variations in both δ18O and δ2H showing a distinct seasonal pattern with lower isotope values in May than in September. Higher d-excess values throughout the year in the Aksu river and the Tizinafu river suggest that water may be intensively recycled in the mountains of the TRB. Based on isotopic hydrograph separation, we found that groundwater is the main water source that discharges the entire basin although individual rivers vary.

Insights gained from geochemical studies in the Waterloo Moraine: Indications and implications for anthropogenic loading

The Waterloo Moraine is a historic and continuing groundwater resource for the growing Region of Waterloo in southern Ontario, Canada, providing hundreds of thousands of litres of fresh water daily to the regional water supply. The complex stratigraphy of the Moraine is characterized by a system of multiple aquifers separated by regionally discontinuous aquitards, complicating resource assessments. The Region’s extensive monitoring and production well network has been sampled repeatedly over the last four decades, providing insights into how long-term pumping and urban development have affected aquifer geochemistry. Various environmental tracers were used to identify groundwater residence time and mixing including 3H, 3H/3He, CFCs, SF6, 14C, and 4He. Studies on the Waterloo Moraine provided important contributions to the application of these environmental tracers by validating unsaturated zone transport theory and documenting the effects of localized atmospheric CFC contamination. These studies have also provided important information about groundwater flow and mixing in the Moraine. Both historical and recent hydrogeological studies identified urban applications of road deicers (salt) and rural agricultural practices (nitrate loadings) as potential problems for future use of groundwater as a domestic water supply for the region, with Cl and NO3-N concentrations in some areas approaching 900 and 17 mg/L, respectively. The effects of these urban and rural contaminant issues are explored, and efforts to mitigate and reduce the impact, including the effects of best (beneficial) management practices, are discussed. The potential impacts of these non-point source contaminants on the public water supply should be considered when planning new residential developments.

A Black Hills-Madison Aquifer Origin for Dakota Aquifer Groundwater in Northeastern Nebraska

Previous studies of the Dakota Aquifer in South Dakota attributed elevated groundwater sulfate concentrations to Madison Aquifer recharge in the Black Hills with subsequent chemical evolution prior to upward migration into the Dakota Aquifer. This study examines the plausibility of a Madison Aquifer origin for groundwater in northeastern Nebraska. Dakota Aquifer water samples were collected for major ion chemistry and isotopic analysis ((18)O, (2)H, (3)H, (14)C, (13)C, (34)S, (18)O-SO(4), (87)Sr, (37)Cl). Results show that groundwater beneath the eastern, unconfined portion of the study area is distinctly different from groundwater sampled beneath the western, confined portion. In the east, groundwater is calcium-bicarbonate type, with delta(18)O values (-9.6 per thousand to -12.4 per thousand) similar to local, modern precipitation (-7.4 per thousand to -10 per thousand), and tritium values reflecting modern recharge. In the west, groundwater is calcium-sulfate type, having depleted delta(18)O values (-16 per thousand to -18 per thousand) relative to local, modern precipitation, and (14)C ages 32,000 to more than 47,000 years before present. Sulfate, delta(18)O, delta(2)H, delta(34)S, and delta(18)O-SO(4) concentrations are similar to those found in Madison Aquifer groundwater in South Dakota. Thus, it is proposed that Madison Aquifer source water is also present within the Dakota Aquifer beneath northeastern Nebraska. A simple Darcy equation estimate of groundwater velocities and travel times using reported physical parameters from the Madison and Dakota Aquifers suggests such a migration is plausible. However, discrepancies between (14)C and Darcy age estimates indicate that (14)C ages may not accurately reflect aquifer residence time, due to mixtures of varying aged water.

Comparison of δ81Br and δ37Cl composition of volatiles, salt precipitates, and associated water in terrestrial evaporative saline lake systems

ABSTRACT This study provides the first characterization of the variability of bromine and chlorine stable isotopic composition in evaporites, associated residual brines, and shoreline gases in terrestrial evaporative saline lakes. The lakes investigated here are groundwater discharge locations, and include both potash-rich alkaline lakes and sodic-dominated neutral pH lakes at a variety of salinities and evaporative stages. The chlorine and bromine isotope systems behave consistently different during evaporative salt precipitation, with 37Cl more enriched in the salt than in the fluid, but 81Br more enriched in the fluid compared with the precipitated salt. The 81Br concentration of shore off-gassing was even smaller than mineral precipitate compositions. The trends observed for bromine isotopes are surprising compared with published laboratory studies, indicating that a process besides inorganic mineral precipitation is affecting δ81Br. Additional processes explored include diffusion, salt deflation, microbial and photolytic conversion to the gas phase, and oxidative bromination of organic matter. Dedicated to Professor Peter Fritz on the occasion of his 80th birthday

Interpreting temporal variations in river response functions: an example from the Arkansas River, Kansas, USA

Groundwater/surface-water interactions can play an important role in management of water quality and quantity, but the temporal and spatial variability of these interactions makes them difficult to incorporate into conceptual models. There are simple methods for identifying the presence of groundwater/surface-water interactions; however, identifying flow mechanisms and pathways can be challenging. More complex methods are available to better identify these mechanisms and pathways but are often too time consuming or costly. In this work, a simple method for interpreting and identifying flow mechanisms and sources using temporal variations of river response functions is presented. This approach is demonstrated using observations from two sites along the Arkansas River in Kansas, USA. A change in flow mechanisms between the rising and falling limbs of river hydrographs was identified, along with a second surface-water source to the aquifer, a finding that was validated with stable isotope analyses.RésuméLes relations nappe–rivière peuvent jouer un rôle important dans la gestion de la quantité et de la qualité de l’eau, mais la variabilité spatiale et temporelle de ces interactions rend difficile leur prise en compte dans les modèles conceptuels. Il y a des méthodes simples pour identifier l’existence de relations nappe-rivière. Cependant, l’identification des mécanismes et voies d’écoulement peuvent constituer un défi. Des méthodes plus complexes sont disponibles pour mieux identifier ces mécanismes et voies de transfert, mais elles sont souvent trop chronophages et coûteuses. Dans ce travail, une méthode simple est présentée pour interpréter et identifier les mécanismes de transfert et leurs origines, en utilisant les variations temporelles des fonctions de réponse d’une rivière. Cette approche est démontrée en utilisant les observations effectuées pour deux sites sur le cours de la rivière Arkansas, au Kansas (Etats-Unis d’Amérique). Un changement des mécanismes d’écoulement entre les montées et descentes des hydrographes de la rivière a été identifié conjointement avec une deuxième origine d’eau de surface alimentant l’aquifère, un résultat qui a été validé par les analyses d’isotopes stables.ResumenLas interacciones agua subterránea–agua superficial pueden desempeñar un papel importante en el manejo de la calidad y cantidad del agua, pero la variabilidad temporal y espacial de estas interacciones hace que sea difícil incorporarlas en los modelos conceptuales. Existen métodos sencillos para identificar la presencia de las interacciones agua subterránea/agua superficial; Sin embargo, la identificación de los mecanismos y trayectorias de flujo puede ser un desafío. Existen métodos más complejos para identificar mejor estos mecanismos y trayectorias, pero a menudo insumen demasiado tiempo o son costosos. En este trabajo, se presenta un método sencillo para interpretar e identificar los mecanismos de flujo y los aportes utilizando las variaciones temporales de las funciones de las respuestas fluviales. Este enfoque se demuestra usando observaciones de dos sitios a lo largo del río Arkansas en Kansas, EEUU. Se identificó un cambio en los mecanismos de flujo entre las ramas ascendentes y descendentes de los hidrogramas del río, junto con un segundo aporte de agua superficial hacia el acuífero, hallazgo que fue validado con análisis de isótopos estables.摘要地下水/地表水相互作用在水的数量和质量管理中可以发挥重要的作用,这些相互作用的时空变化使其很难包含在概念模型中。有确定地下水/地表水相互作用是否存在的简单方法;然而,确定水流机理和水流通道具有挑战性。有更复杂的方法能够更好地确定这些机理和通道,但通常太费时或者代价太高。在本研究中,提出了采用河流响应功能的时间变化解译和确定河流机理和来源的一个简单方法。利用美国堪萨斯州阿肯色河两个地点的观测数据展示了该方法。确定了河流水位图上升和下降翼之间河流机理的变化,以及确定了第二个到含水层的地表水源,这个发现得到了稳定同位素分析的验证。ResumoInterações entre águas subterrâneas e superficiais podem desempenhar uma função importante na gestão da qualidade e da quantidade da água, mas a variabilidade temporal e espacial dessas interações as torna difíceis de incorporar à modelos conceituais. Existem métodos simples para identificar a presença de interações entre águas subterrâneas e superficiais; entretanto a identificação de fluxos e caminhos preferenciais pode ser desafiador. Métodos mais complexos estão disponíveis para uma melhor identificar esses mecanismos e caminhos preferenciais mas são geralmente dispendiosos em tempo ou custo. Nesse trabalho, apresenta-se um método simples para interpretar e identificar mecanismos de fluxo e fontes usando variações temporais de funções de resposta fluviais. Essa abordagem é demonstrada usando observações de dois locais ao longo do Rio Arkansas, Kansas, EUA. Uma mudança nos mecanismos de fluxo entre os membros ascendentes e descendentes dos hidrogramas do rio foi identificada, junto com uma segunda fonte de águas superficiais ao aquífero, uma discoberta que foi validada com análises de isótopos estáveis.

Marinobacter bacteria associated with a massive sulphide ore deposit affect metal mobility in the deep subsurface

A microorganism of the Marinobacter genus capable of Fe-oxidation at near-neutral pH, both in the presence and absence of oxygen, was found at a depth of 1.4 km in proximity to a Cu-Zn Volcanogenic Massive Sulphide (VMS) deposit, within the Triple 7 mine, Flin Flon, Manitoba, Canada. The microorganism was isolated from saline groundwater emanating from boreholes at that depth, which contained a small microbial community consisting of only two organisms. To examine biogeochemical trace metal cycling in this deep subsurface setting, incubation experiments were carried out with the Marinobacter isolate and mineralized (metal-containing ore) material in batch and column flow-through settings. The activity of the Marinobacter isolate resulted in an increase in the mobilization of major elements (Fe, S) and trace metals (Cu, Zn) from the solid ore material. These results indicate that Fe-oxidation may be an important biogeochemical process in the deep subsurface, which affects the mobilization of Fe and trace elements from buried mineralization.