Jean M. Bahr

Two-way cluster analysis of geochemical data to constrain spring source waters

Abstract The purpose of the study was to use geochemical characteristics and apparent ages of sampled groundwaters to determine which of the two regionally extensive bedrock aquifers, the lower bedrock aquifer or the upper bedrock aquifer, is a more likely source of water discharging to springs in the Nine Springs watershed. The use of summary statistics and our knowledge of the regional hydrostratigraphy resulted in the identification of three groups of monitoring points that are representative of groundwaters with distinct geochemical characteristics. Two-way cluster analysis of the geochemical data supports these groupings and further identifies subtle geochemical characteristics of the groups. One spring, which is representative of smaller springs and seeps found in the watershed, belongs to a group that is characterized by variable nitrate and chloride concentrations. Water discharging from this spring has a groundwater residence time of approximately 8 years based on the tritium/helium 3 dating method. The water discharging to this small spring is thought to have traveled primarily through the unlithified aquifer, as opposed to either of the major bedrock aquifers. Most of the springs in the watershed belong to a group that is characterized by elevated, but consistent, nitrate, sodium, and chloride concentrations. In addition, cluster analysis revealed that potassium and alkalinity concentrations are somewhat low. Apparent groundwater ages for this group range from 10 to 15 years. The water discharging from the majority of the springs in the watershed is thought to have traveled primarily through the unlithified aquifer and the upper bedrock aquifer before discharging into the former glacial lakebed wetland complex. Due to the relatively short groundwater residence times, spring water quality and flow in the Nine Springs watershed are likely to be vulnerable to the rapid urban expansion occurring within the watershed.

The deep structure of a sea-floor hydrothermal deposit

Hydrothermal circulation at the crests of mid-ocean ridges plays an important role in transferring heat from the interior of the Earth. A consequence of this hydrothermal circulation is the formation of metallic ore bodies known as volcanic-associated massive sulphide deposits. Such deposits, preserved on land, were important sources of copper for ancient civilizations and continue to provide a significant source of base metals (for example, copper and zinc). Here we present results from Ocean Drilling Program Leg 169, which drilled through a massive sulphide deposit on the northern Juan de Fuca spreading centre and penetrated the hydrothermal feeder zone through which the metal-rich fluids reached the sea floor. We found that the style of feeder-zone mineralization changes with depth in response to changes in the pore pressure of the hydrothermal fluids and discovered a stratified zone of high-grade copper-rich replacement mineralization below the massive sulphide deposit. This copper-rich zone represents a type of mineralization not previously observed below sea-floor deposits, and may provide new targets for land-based mineral exploration.

Active Thermal Tracer Tests for Improved Hydrostratigraphic Characterization

Subsurface heterogeneity in hydraulic properties and processes is a fundamental challenge in hydrogeology. We have developed an improved method of borehole dilution testing for hydrostratigraphic characterization, in which distributed temperature sensing (DTS) is used to monitor advective heat movement. DTS offers many advantages over conventional technologies including response times in the order of seconds rather than minutes, the ability to profile temperature synoptically in a well without disturbing the fluid column, sensitivity to a wider range of flow rates than conventional spinner and heat pulse flow meters, and the ease of interpretation. Open-well thermal dilution tests in two multiaquifer wells near Madison, Wisconsin, provided detailed information on the borehole flow regimes, including flow rates and the locations of inflows from both fractures and porous media. The results led to an enhanced understanding of flow in a hydrostratigraphic unit previously conceptualized as homogenous and isotropic.

Nitrate-enhanced bioremediation of BTEX-contaminated groundwater: parameter estimation from natural-gradient tracer experiments.

Two natural-gradient pulse tracer tests were conducted in a petroleum-contaminated aquifer to evaluate the potential for benzene, toluene, ethylbenzene, and xylenes (BTEX) biodegradation under enhanced nitrate-reducing conditions. Addition of nitrate resulted in loss of toluene, ethylbenzene, and m,p-xylenes (TEX) after an initial lag period of approximately 9 days. Losses of benzene were not observed over the 60-day monitoring period. Tracer breakthrough curves (BTCs) were analyzed to derive transport and biodegradation parameters, including advective velocities, retardation factors, dispersion coefficients, biodegradation rate constants, and nitrate utilization ratios. Using the parameters derived from the BTC analysis, numerical simulations of one of the tracer experiments were conducted using BIONAPL/3D [Molson, J., BIONAPL/3D User Guide, A 3D Coupled Flow and Multi-Component Reactive transport model. University of Waterloo, Waterloo, Ontario, Canada]. Simulations using the BTC-derived transport and biodegradation parameters successfully reproduced benzene, TEX, and nitrate concentrations measured during the tracer experiment. Comparisons of observed and simulated nitrate concentrations indicate that the mass ratio of nitrate-N utilized to TEX degraded increased over time during the experiment, reaching values many times that expected based on stoichiometry of TEX oxidation coupled to nitrate reduction. Excess nitrate loss is likely due to oxidation of other organics in addition to TEX.

Analysis of nonequilibrium desorption of volatile organics during field test of aquifer decontamination

Abstract A two-well injection-withdrawal experiment was conducted at a hazardous waste disposal site near Ottawa, Canada to assess the feasibility of aquifer restoration by means of a purge well network. The six-day test involved injecting tracer-labeled clean water into one well while withdrawing contaminated water from a well located 5m away. Samples taken from multilevel sampling points located along flow lines between the two wells were analyzed to determine the concentrations of tracers and volatile organic contaminants. Tracer breakthrough data were fitted to an approximate analytic solution to determine average linear velocities and dispersivites. The concentration histories obtained for three volatile organic contaminants deviate significantly from that predicted using local equilibrium-based transport models. Comparison with results of a one-dimensional, kinetics-based transport model indicates qualitative agreement between observed transport behavior and that expected for solutes affected by a first-order reversible sorption process for which the rate constants are small relative to the groundwater velocity in the induced flow field.

Reactive transport modeling of redox geochemistry: Approaches to chemical disequilibrium and reaction rate estimation at a site in northern Wisconsin

The purpose of this study is to investigate the hydrology and redox geochemistry of shallow groundwater discharging to a stream in northern Wisconsin. In this organic-rich aquifer, we observe both oxygen reducing zones and iron reducing zones whose boundaries are roughly constant over time. To investigate the apparent steady state between solute fluxes and redox reaction rates, we develop a reactive transport model of carbon oxidation. We use a “quasi-kinetic,” “partial-equilibrium” approach to modeling redox reactions, a hybrid approach between traditional equilibrium approaches and fully kinetic approaches that require large computer resources. Our model suggests that observed trends in redox sensitive elements can only be explained by oxidation rates that are both dependent on the predominant electron acceptor and are spatially variable. Our coupled models provide field-based estimates of redox kinetics, which are otherwise difficult to obtain in hydrologically complex systems.

Characterization of fracture connectivity in a siliciclastic bedrock aquifer near a public supply well (Wisconsin, USA)

In order to protect public supply wells from a wide range of contaminants, it is imperative to understand physical flow and transport mechanisms in the aquifer system. Although flow through fractures has typically been associated with either crystalline or carbonate rocks, there is growing evidence that it can be an important component of flow in relatively permeable sandstone formations. The objective of this work is to determine the role that fractures serve in the transport of near-surface contaminants such as wastewater from leaking sewers, to public supply wells in a deep bedrock aquifer. A part of the Cambrian aquifer system in Madison, Wisconsin (USA), was studied using a combination of geophysical, geochemical, and hydraulic testing in a borehole adjacent to a public supply well. Data suggest that bedrock fractures are important transport pathways from the surface to the deep aquifer. These fractured intervals have transmissivity values several orders of magnitude higher than non-fractured intervals. With respect to rapid transport of contaminants, high transmissivity values of individual fractures make them the most likely preferential flow pathways. Results suggest that in a siliciclastic aquifer near a public supply well, fractures may have an important role in the transport of sewer-derived wastewater contaminants.Résumé Afin de protéger des puits publics d’alimentation contre une large gamme de polluants, il est impératif de comprendre le flux physique et les mécanismes de transport dans le système aquifère. Bien qu’un flux à travers des fractures ait été typiquement associé à des roches soit cristallines soit carbonatées, il y a une évidence grandissante qu’il peut être une composante importante de flux dans des formations gréseuses relativement perméables. L’objectif de ce travail est de déterminer le rôle que les fractures jouent dans le transfert de contaminants de sub-surface, tels que eaux usées d’égouts fuyants, à des puits d’alimentation dans un substrat aquifère profond. Une partie du système aquifère de Madispn, Wisconsin (USA) a été étudié en utilisant une combinaison de tests géophysique, géochimique et hydraulique dans un forage adjacent à un puits d’alimentation publique. Les données suggèrent que les fractures du substrat sont des chemins importants de passage depuis la surface à l’aquifère profond. Ces intervalles fracturées ont une transmissivité d’ordre de grandeur plusieurs fois supérieure à celle des intervalles non fracturés. En raison du transport rapide des contaminants, des valeurs de transmissivité élevées des fractures singulières en font les chemins préférentiels les plus vraisemblables. Les résultats suggèrent que dans un aquifère silicaté près d’un puits d’alimentation public, des fractures peuvent avoir un rôle important dans le transport de contaminants d’eau résiduaire provenant d’égouts.Resumen Con el objeto de proteger a los pozos de abastecimiento público de un amplio rango de contaminantes, es imperativo entender los mecanismos físicos de transporte y flujo en el sistema acuífero. Aunque el flujo a través de las fracturas ha sido típicamente asociado con rocas cristalinas o bien carbonáticas, hay una evidencia creciente que ello puede ser una importante componente de flujo en formaciones de areniscas relativamente permeables. El objetivo de este trabajo es determinar el rol que desempeñan las fracturas en el transporte de contaminantes en las proximidades de la superficie, tales como aguas residuales proveniente de las pérdidas de alcantarillas, a pozos de abastecimiento público en un acuífero de basamento profundo. Se estudió una parte del sistema acuífero cámbrico en Madison, Wisconsin (EEUU), usando una combinación de ensayos geofísicos, geoquímicos, e hidráulicos en una perforación adyacente a un pozo de abastecimiento público. Los datos sugieren que las fracturas del basamento son trayectorias importantes de transporte desde la superficie al acuífero profundo. Estos intervalos fracturados tienen valores de transmisividad de varios órdenes de magnitud más altos que los intervalos de rocas no fracturadas. Con respecto al transporte rápido de los contaminantes, altos valores transmisividad de las fracturas individuales hacen a ellas probablemente las mayores trayectorias de flujo preferencial. Los resultados sugieren que en un acuífero silicoclástico cercano a un pozo de abastecimiento público, las fracturas pueden tener un rol importante en el transporte de contaminantes provenientes de aguas residuales de las alcantarillas cloacas.摘要为防止公共供水井受到各种污染物污染,了解含水层系统中物理流动和运移机理是必要的。尽管裂隙中的流动代表性地与结晶岩或碳酸岩相联系,越来越多的证据表明它可以是相对可渗透的砂岩地层中流动的重要组成部分。本文旨在确定近地表污染物运移中裂隙所起的作用,比如从渗漏下水道到深层基岩含水层中的公共供水井的污水。利用地球物理、地球化学以及公共供水井附近钻孔液压测试相结合的手段对美国威斯康星州麦迪逊市部分寒武系含水层系统进行研究。数据表明基岩裂隙是从地表到深部含水层的重要运移路径。有裂隙的区间导水系数比无裂隙区间高若干个数量级。考虑到污染物的快速运移,个别裂隙的高导水系数使它们成为最可能优先运动路径。结果表明在靠近公共供水井的硅质碎屑含水层中,裂隙在下水道起源的污水污染物运移中起重要作用。ResumoCom o objetivo de proteger os furos de captação de abastecimento público de uma grande variedade de contaminantes, é imperativo compreender o fluxo físico e mecanismos de transporte no sistema aquífero. Embora o fluxo através de fraturas tenha sido associado tipicamente a rochas cristalinas ou a rochas carbonatadas, existe uma evidência crescente de que ele possa ser uma componente importante do fluxo em formações granulares relativamente permeáveis. O objetivo deste trabalho é determinar o papel que as fraturas assumem no transporte subsuperficial de contaminantes, tais como águas residuais provenientes da drenagem de esgotos para furos de abastecimento público que captam um aquífero rochoso profundo. Uma parte do sistema aquífero Câmbrico, em Madison, Wisconsin (EUA), foi estudada usando uma combinação de ensaios geofísicos, geoquímicos e hidráulicos num furo adjacente a uma captação de abastecimento público. Os dados sugerem que as fraturas no maciço são importantes vias de transporte a partir da superfície para o aquífero profundo. As zonas fraturadas têm valores de transmissividade várias ordens de magnitude mais elevados que as zonas não-fraturadas. Valores elevados de transmissividade em fraturas individuais torna-as nos caminhos preferenciais de fluxo em relação ao transporte rápido de contaminantes. Os resultados sugerem que, num aquífero siliciclástico perto de uma captação de abastecimento público, as fraturas podem ter um papel importante no transporte de contaminantes com origem no esgoto de águas residuais.

Removal of organic wastewater contaminants in septic systems using advanced treatment technologies

The detection of pharmaceuticals and other organic wastewater contaminants (OWCs) in ground water and surface-water bodies has raised concerns about the possible ecological impacts of these compounds on nontarget organisms. On-site wastewater treatment systems represent a potentially significant route of entry for organic contaminants to the environment. In this study, effluent samples were collected and analyzed from conventional septic systems and from systems using advanced treatment technologies. Six of 13 target compounds were detected in effluent from at least one septic system. Caffeine, paraxanthine, and acetaminophen were the most frequently detected compounds, and estrogenic activity was detected in 14 of 15 systems. The OWC concentrations were significantly lower in effluent after sand filtration (p < 0.01) or aerobic treatment (p < 0.05) as compared with effluent that had not undergone advanced treatment. In general, concentrations in conventional systems were comparable to those measured in previous studies of municipal wastewater treatment plant (WWTP) influent, and concentrations in systems after advanced treatment were comparable to previously measured concentrations in WWTP effluent. These data indicate that septic systems using advanced treatment can reduce OWCs in treated effluent to similar concentrations as municipal WWTPs.

Using reactive solutes to constrain groundwater flow models at a site in northern Wisconsin

We propose a method for constraining a groundwater flow model both by head observations and concentrations of nonconservative solutes such as calcium, using reaction-path modeling. When calibrating flow models in small watershed in northern Wisconsin using head data alone, we encountered problems of nonuniqueness. However, by coupling the flow models with a plagioclase dissolution model, we were able to greatly reduce the number of plausible flow models. First, by using flow modeling and reaction path modeling in parallel, we tested the consistency of residence times predicted by the flow models with solute concentrations predicted by the geochemical models. Mineral dissolution rate parameters were assumed to be spatially uniform; without this condition the geochemistry data would not provide additional constraints to the flow modeling process. For a more comprehensive test of our models, we used reactive-transport modeling to predict the spatial distribution of ions at each site. The models qualitatively reproduced the observed data and our calibrated silicate dissolution rates closely matched those reported in a field study of nearby site. There were also discrepancies between predictions and observations. We attribute these to transient effects and sediment heterogeneities that were not included in the models. While the resulting models are not unique, our approach demonstrates the ability of fairly simple models to explain much of the observed variability in a complex system.

Spatial Electron Acceptor Variability: Implications for Assessing Bioremediation Potential

An extensive network of multilevel samplers was established in a hydrocarbon-contaminated wetland aquifer. Results of groundwater sampling for benzene, toluene, ethylbenzene, and xylenes (BTEX), and electron acceptors show that both pristine and contaminated groundwater have spatially variable chemical signatures, owing primarily to microbially mediated oxidation-reduction reactions. Due to these spatial variations, estimates of the efficiency of intrinsic bioremediation can vary significantly depending on how geochemical data are collected. Use of data collected from monitoring wells with screens longer than the vertical extent of the plume will generally underestimate the potential for intrinsic bioremediation for the most chemically active horizon of the plume. A comparison of pristine and contaminated redox patterns demonstrates that, although BTEX exerts the highest demand for electron acceptors, oxidation of natural organic matter also contributes to electron acceptor utilization. If natural and oth...