Malcolm S. Field

A two-region nonequilibrium model for solute transport in solution conduits in karstic aquifers.

Abstract A two-region nonequilibrium model was used to calibrate initial solute-transport parameter estimates generated from tracer-breakthrough curves (TBCs) developed from tracer tests conducted in uni-axial solution conduits in karstic aquifers. Two-region nonequilibrium models account for partitioning of solute into mobile- and immobile-fluid regions to produce a more representative model fit to the strong tails associated with TBCs than do equilibrium models. The nonequilibrium model resulted in an increase in average flow velocities and a decrease in longitudinal dispersion coefficients over comparable estimates using an equilibrium model. Increases in velocity and decreases in dispersion were obtained at the expense of including parameters that describe solute partitioning and mass transfer rate for the mobile- and immobile-fluid regions. In addition, nonidentifiable sorption and mass transfer parameters for the immobile-fluid regions could only be described in terms of upper and lower bounds using readily determined identifiable ratios representing solute partitioning and system constraints based on known physical properties. The identifiable ratios and system constraints serve to minimize model nonuniqueness and renders the nonidentification problem trivial.

Review: Groundwater flow and transport modeling of karst aquifers, with particular reference to the North Coast Limestone aquifer system of Puerto Rico

Karst systems have a high degree of heterogeneity and anisotropy, which makes them behave very differently from other aquifers. Slow seepage through the rock matrix and fast flow through conduits and fractures result in a high variation in spring response to precipitation events. Contaminant storage occurs in the rock matrix and epikarst, but contaminant transport occurs mostly along preferential pathways that are typically inaccessible locations, which makes modeling of karst systems challenging. Computer models for understanding and predicting hydraulics and contaminant transport in aquifers make assumptions about the distribution and hydraulic properties of geologic features that may not always apply to karst aquifers. This paper reviews the basic concepts, mathematical descriptions, and modeling approaches for karst systems. The North Coast Limestone aquifer system of Puerto Rico (USA) is introduced as a case study to illustrate and discuss the application of groundwater models in karst aquifer systems to evaluate aquifer contamination.RésuméLes systèmes karstiques ont un grand degré d’hétérogénéité et d’anisotropie, qui les fait se comporter de façon très différente des autres aquifères. D’une lente infiltration à travers la matrice rocheuse et d’un flux rapide à travers les conduits et fractures résulte une grande variation dans la réponse de la source aux événements pluvieux. Une rétention de polluant a lieu dans la matrice rocheuse et dans l’épikarst, mais le transport de polluants a lieu principalement suivant des cheminements préférentiels qui sont typiquement localisés de façons inaccessibles, ce qui fait de la modélisation des systèmes karstiques un challenge. Les modèles informatiques pour la compréhension et la prévision dynamique du transport du polluant dans les aquifères font que les hypothèses sur la distribution des structures géologiques et leurs propriétés hydrauliques peuvent ne pas toujours s’appliquer aux aquifères karstiques. Cet article passe en revue les concepts basiques, descriptions mathématiques et approches de modélisation des systèmes karstiques. Le système aquifère Calcaire Côtier Nord de Porto-Rico (USA) est introduit comme cas d’étude pour illustrer et discuter de l’application des modèles d’aquifères aux systèmes karstiques pour en évaluer la contamination.ZusammenfassungKarstsysteme sind hochgradig heterogen und anisotrop, weshalb ihr Verhalten sich sehr von anderen Grundwasserleitern unterscheidet. Langsames Durchsickern der Gesteinsmatrix und schnelles Durchfließen von Karströhren und Klüften führen zu einer hohen Variabilität der Reaktion von Quellen auf Niederschlagsereignisse. Die Speicherung von Schadstoffen findet in der Gesteinsmatrix und im Epikarst statt, der Transport von Schadstoffen hingegen überwiegend entlang von präferenziellen Fließwegen. Weil diese in der Regel unzugänglich sind, ist die Modellierung von Karstsystemen eine Herausforderung. Computermodelle zum Verständnis und zur Vorhersage der Hydraulik und des Schadstofftransports in Grundwasserleitern machen Annahmen über die Verteilung der hydraulischen Eigenschaften von geologischen Strukturen, die in Karstgrundwasserleitern nicht unbedingt gültig sind. Dieser Beitrag bespricht grundlegende Konzepte, mathematische Beschreibungen und Ansätze zur Modellierung von Karstgrundwasserleitern. Das North Coast Limestone Grundwassersystem in Puerto Rico (USA) wird als Fallbeispiel vorgestellt, um die Anwendung von Grundwassermodellen zur Beurteilung von Grundwasserverschmutzungen in Karstsystemen zu veranschaulichen und zu diskutieren.ResumenLos sistemas kársticos tienen un alto grado de heterogeneidad y anisotropía, lo que los hace que se comporten muy diferentes de otros acuíferos. El escurrimiento lento a través de la matriz de la roca y el flujo rápido a través de los conductos y fracturas dan como resultado una alta variación en la respuesta de los manantiales a los eventos de precipitación. El almacenamiento de los contaminantes ocurre en la matriz de la roca y en el epikarstico, pero el transporte de contaminantes ocurre mayormente a lo largo de trayectorias preferenciales que son de ubicación típicamente inaccesibles, lo cual convierte al modelado de los sistemas kársticas en un desafío. Los modelos de computadoras para entender y predecir el transporte hidráulico y de contaminantes en acuíferos hacen suposiciones acerca de la distribución y propiedades hidráulicas de los aspectos geológicos que no pueden siempre aplicarse a acuíferos kársticos. Este trabajo revisa los conceptos básicos, las descripciones matemáticas, y los enfoques de modelados para sistemas kársticas. Se introduce el sistema acuífero de North Coast Limestone de Puerto Rico (EEUU) como un caso de estudio para ilustrar y discutir la aplicación de modelos de agua subterránea en sistemas acuíferos kársticas para evaluar la contaminación del acuífero.摘要岩溶系统具有高度的非均质性与各向异性,这使得它与其它含水层的差别很大。流过岩石介质的慢速流与流过导水管道和裂隙的快速流导致泉对降水事件的响应大有不同。污染物储存在岩石介质和表层岩溶中,但污染物的迁移大多沿着优先流径发生,通常情况下这样的优先流径都处于人力无法达到的地方,这给岩溶含水层系统的模拟带来了很大的挑战。为了了解和预测含水层的水力学特性和污染物的迁移,电脑模型会假设地质体的分布和水力学性质,然而这种假设并不总能应用到岩溶含水层中。本文综述了可用于岩溶含水层的基本概念、数学描述方法与模拟方法。波多黎各(美国)的北海岸灰岩含水层系统作为本文一个实例,解释并讨论了应用地下水模型评估岩溶含水层系统中的污染。ResumoOs sistemas cársicos têm um elevado grau de heterogeneidade e anisotropia, o que faz com que se comportem de maneira muito diferente da dos outros aquíferos. O escoamento lento através da matriz da rocha e o rápido fluxo através de condutas e de fraturas resulta numa elevada variação das respostas das emergências a eventos de precipitação. O armazenamento de contaminantes ocorre na matriz da rocha e no epicarso, mas o transporte de contaminantes ocorre principalmente ao longo de caminhos preferenciais, que são tipicamente locais inacessíveis, o que faz da modelação de sistemas cársicos um desafio. Os modelos de computador, para entenderem e preverem a hidráulica e o transporte de contaminantes em aquíferos, assumem suposições sobre a distribuição e as propriedades hidráulicas das caraterísticas geológicas, as quais nem sempre podem ser aplicadas aos aquíferos cársicos. Este trabalho faz a revisão dos conceitos básicos, das descrições matemáticas e das abordagens de modelação de sistemas cársicos. O sistema aquífero Carbonatado do Litoral Norte de Porto Rico (EUA) é apresentado como um caso de estudo, de forma a ilustrar e discutir a aplicação de modelos de águas subterrâneas em sistemas aquíferos cársicos para avaliação da contaminação de aquíferos.

An assessment of the potential adverse properties of fluorescent tracer dyes used for groundwater tracing

The potential ecotoxicity of fluorescent dyes used in tracing, and their possible effects on human health, were evaluated by reviewing available toxicological information for 12 dyes — fluorescein, Lissamine Flavine FF, Rhodamine WT, Rhodamine B, Sulpho Rhodamine G, Sulpho Rhodamine B, eosin, pyranine, Phorwite BBH Pure, Tinopal 5BM GX, Tinopal CBS-X, and Diphenyl Brilliant Flavine 7GFF — and a dye-intermediate, amino G acid. This evaluation used available toxicological information, test data on analogous substances, and mathematical expressions for biological activity. Based on set criteria for human health and acute ecotoxicity, the evaluation indicated that these tracers have low to moderate levels of concern. The use of these tracers for the study of groundwater flow is appropriate if consideration is given to the overall human health and environmental effects. Their use in the environment requires tracer concentrations not exceeding 1–2 mg 1−1 persisting for a period in excess of 24 h in the groundwater at the point of groundwater withdrawal or discharge. A simple calculated potential dose was used in a comparison of the estimated acute toxicity of Rhodamine WT in rats to the known acute oral toxic dose in humans for several known acutely toxic chemicals. This comparison showed that none of the fluorescent dyes evaluated would present an acutely toxic threat at or substantially above the recommended 2 mg 1−1 concentration.

Risk assessment methodology for karst aquifers : (1) Estimating karst conduit-flow parameters

Quantitative ground-water tracing of conduit-dominated karst aquifers allows for reliable and practical interpretation of karst ground-water flow. Insights into the hydraulic geometry of the karst aquifer may be acquired that otherwise could not be obtained by such conventional methods as potentiometric-surface mapping and aquifer testing. Contamination of karst aquifers requires that a comprehensive tracer budget be performed so that karst conduit hydraulic-flow and geometric parameters be obtained. Acquisition of these parameters is necessary for estimating contaminant fate-and-transport. A FORTRAN computer program for estimating total tracer recovery from tracer-breakthrough curves is proposed as a standard method. Estimated hydraulic-flow parameters include mean residence time, mean flow velocity, longitudinal dispersivity, Peclet number, Reynolds number, and Froude number. Estimated geometric parameters include karst conduit sinuous distance, conduit volume, cross-sectional area, diameter, and hydraulic depth. These parameters may be used to (1) develop structural models of the aquifer, (2) improve aquifer resource management, (3) improve ground-water monitoring systems design, (4) improve aquifer remediation, and (5) assess contaminant fate-and-transport. A companion paper demonstrates the use of these hydraulic-flow and geometric parameters in a surface-water model for estimating contaminant fate-and-transport in a karst conduit. Two ground-water tracing studies demonstrate the utility of this program for reliable estimation of necessary karst conduit hydraulic-flow and geometric parameters.

Risk assessment methodology for karst aquifers : (2) Solute-transport modeling

Ground-water flow and solute-transport simulation modeling are major components of most exposure and risk assessments of contaminated aquifers. Model simulations provide information on the spatial and temporal distributions of contaminants in subsurface media but are difficult to apply to karst aquifers in which conduit flow is important. Ground-water flow and solute transport in karst conduits typically display rapid-flow velocities, turbulent-flow regimes, concentrated pollutant-mass discharge, and exhibit open-channel or closed-conduit flow. Conventional ground-water models, dependent on the applicability of Darcy`s law, are inappropriate when applied to karst aquifers because of the (1) nonapplicability of Darcian-flow parameters, (2) typically nonlaminar flow regime, and (3) inability to locate the karst conduits through which most flow and contaminant transport occurs. Surface-water flow and solute-transport models conditioned on a set of parameters determined empirically from quantitative ground-water tracing studies may be effectively used to render fate-and-transport values of contaminants in karst conduits. Hydraulic-flow and geometric parameters developed in a companion paper were used in the surface-water model, TOXI5, to simulate hypothetical slug and continuous-source releases of ethylbenzene in a karst conduit. TOXI5 simulation results showed considerable improvement for predicted ethylbenzene-transport rates and concentrations over qualitative tracing and analytical ground-water model results. Ethylbenzene concentrations predicted by TOXI5 simulations were evaluated in exposure and risk assessment models.

Combined physical and chemical nonequilibrium transport model for solution conduits.

Solute transport in karst aquifers is primarily constrained to relatively complex and inaccessible solution conduits where transport is often rapid, turbulent, and at times constrictive. Breakthrough curves generated from tracer tests in solution conduits are typically positively-skewed with long tails evident. Physical nonequilibrium models to fit breakthrough curves for tracer tests in solution conduits are now routinely employed. Chemical nonequilibrium processes are likely important interactions, however. In addition to partitioning between different flow domains, there may also be equilibrium and nonequilibrium partitioning between the aqueous and solid phases. A combined physical and chemical nonequilibrium (PCNE) model was developed for an instantaneous release similar to that developed by Leij and Bradford (2009) for a pulse release. The PCNE model allows for partitioning open space in solution conduits into mobile and immobile flow regions with first-order mass transfer between the two regions to represent physical nonequilibrium in the conduit. Partitioning between the aqueous and solid phases proceeds either as an equilibrium process or as a first-order process and represents chemical nonequilibrium for both the mobile and immobile regions. Application of the model to three example breakthrough curves demonstrates the applicability of the combined physical and chemical nonequilibrium model to tracer tests conducted in karst aquifers, with exceptionally good model fits to the data. The three models, each from a different state in the United States, exhibit very different velocities, dispersions, and other transport properties with most of the transport occurring via the fraction of mobile water. Fitting the model suggests the potentially important interaction of physical and chemical nonequilibrium processes.

Application of robust statistical methods to background tracer data characterized by outliers and left-censored data.

Accurate analysis of tracer-breakthrough curves is dependent on the removal of measured background concentrations from the measured tracer recovery data. Background concentrations are commonly converted to a single mean background concentration that is subtracted from tracer recovery data. To obtain an improved estimate for the mean background concentration, a statically-robust procedure addressing left-censored data and possible outliers in background concentration data is presented. A maximum likelihood estimate and other robust methods coupled with outlier removal are applied. Application of statically-robust procedures to background concentrations results not only in better estimates for mean background concentration but also results in more accurate quantitative analyses of tracer-breakthrough curves when the mean background concentration is subtracted.

Assessing Aquatic Ecotoxicological Risks Associated with Fluorescent Dyes Used for Water-Tracing Studies

Hydrological tracer testing is the most reliable diagnostic technique available for identifying and quantifying hydrodispersive transport processes. As such, hydrologic tracing is an essential tool that is commonly used to establish flow trajectories, to understand solute-transport processes, and to develop human health and ecological risk assessments. Unfortunately, the use of anthropogenic materials to trace the flow of water may also impart another source of risk to human health and the environment. In general, attempts are usually made to deliberately release tracer agents at concentrations far below their recognized toxic levels. Ecotoxicologically safe levels for injection concentrations of fluorescent tracer agents are generally set at levels far below that which are necessary to maintain measurable downstream concentrations. Appropriate tracer test design is important, because incorrect tracer-mass estimates may result in the release of larger tracer masses than are necessary and that exceed expected environmental concentrations (EECs). To maintain tracer concentrations at or below accepted levels, optimal tracer-test design is essential and may be achieved using the Efficient Hydrologic Tracer-Test Design methodology. By applying an optimal tracer-test design, it is more likely that downstream tracer EECs will be maintained at or below accepted concentrations while maintaining sufficiently high downstream EECs necessary for positive tracer detection.

Modeling solute reactivity in a phreatic solution conduit penetrating a karst aquifer

A two-dimensional model for solute migration, transformation, and deposition in a phreatic solution conduit penetrating a karst aquifer is presented in which the solute is anthropogenic to the natural system. Transformation of a reacting solute in a solution conduit has generally been accepted as likely occurring but actual physical measurements and mathematical analyses of the suspected process have been generally minimally investigated, primarily because of the logistical difficulties and complexities associated with solute transport through solution conduits. The model demonstrates how a reacting solute might decay or be transformed to a product solute some of which then migrates via radial dispersion to the conduit wall where it may become adsorbed. Model effects vary for laminar flow and turbulent flow in the axial direction. Dispersion in the radial direction also exhibits marked differences for both laminar flow and turbulent flow. Reaction zones may enhance subsequent reactions due to some overlap resulting from the longitudinal dispersion caused by flow in the axial direction. Simulations showed that varying the reaction rate coefficient strongly affects solute reactions, but that varying deposition coefficients had only minimal impacts. The model was applied to a well-known tracer test that used the tracer dye, Rhodamine WT, which readily converts to deaminoalkylated Rhodamine WT after release, to illustrate how the model may be used to suggest one possible cause, in addition to other possible causes, for less than 100 tracer-mass recovery. In terms of pollutants in a karst aquifer the model also suggests one possible mechanism for pollutant transformation in a solution conduit.

Hydrogeological and geochemical evidence for the origin of brackish groundwater in the Shabestar plain aquifer, northwest Iran

Shabestar plain aquifer is located in the northeast of the hypersaline Urmia Lake, northwest Iran. There are two types of the aquifer in the plain: an unconfined aquifer that covers the plain and a confined aquifer that is just in the vicinity of the lake. In recent years, some of the agricultural wells have become salinized by saline water due to unrestricted groundwater pumping. Groundwater in the confined aquifer in comparison with the above unconfined aquifer is of good quality. The salty Urmia Lake is considered the most probable source of groundwater salinization. Other potential sources of groundwater salinization could include halite dissolution, and halite is exposed at the southern end of Shabestar plain, and evaporation from the shallow water table. The water samples, based on their total dissolved solid and chloride contents, are classified in the brackish group. The hydrogeological setting and boreholes log interpretation suggest that the saltwater is the result of Urmia Lake water that is entrapped within the fine-grained matrix from when the lake reached its greatest extent. The ratios of Na/Cl, Br/Cl, (Ca + Mg)/SO4, Mg/Cl, (2Ca + Na)/Cl and Rittenhouse diagram preclude halite dissolution as a salinity source and confirm that the lake water with the composition of seawater is the main cause of groundwater salinization. In addition, Li/Cl ratios indicate that the original briny water was somewhat affected by evaporation. However, the effect of evaporation was found to be, at most, a minor influence only.