Dorothy J. Vesper

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.

Metal transport to karst springs during storm flow: an example from Fort Campbell, Kentucky/Tennessee, USA

Low levels of heavy metals were investigated in a series of springs discharging from the Mississippian limestone aquifer underlying the Fort Campbell Army Base in western Kentucky/Tennessee. Springs were sampled at short time intervals through periods of storm discharge. Unfiltered samples were digested and analysed by inductively-coupled plasma mass spectrometry. Metals detected at the μg/l level included As, Cd, Cr, Ni and Pb. Metal concentrations exhibited a pronounced maximum coincident with the peak of the storm hydrograph in contrast to carbonate species (Ca, Mg) which dipped to a minimum at the peak of the storm hydrograph. Metal concentrations track with aluminium and iron suggesting that the metal transport is mainly by adsorption onto suspended particulates which are mobilized during storm flow.

Estimating preferential flow in karstic aquifers using statistical mixed models.

Karst aquifers are highly productive groundwater systems often associated with conduit flow. These systems can be highly vulnerable to contamination, resulting in a high potential for contaminant exposure to humans and ecosystems. This work develops statistical models to spatially characterize flow and transport patterns in karstified limestone and determines the effect of aquifer flow rates on these patterns. A laboratory-scale Geo-HydroBed model is used to simulate flow and transport processes in a karstic limestone unit. The model consists of stainless steel tanks containing a karstified limestone block collected from a karst aquifer formation in northern Puerto Rico. Experimental work involves making a series of flow and tracer injections, while monitoring hydraulic and tracer response spatially and temporally. Statistical mixed models (SMMs) are applied to hydraulic data to determine likely pathways of preferential flow in the limestone units. The models indicate a highly heterogeneous system with dominant, flow-dependent preferential flow regions. Results indicate that regions of preferential flow tend to expand at higher groundwater flow rates, suggesting a greater volume of the system being flushed by flowing water at higher rates. Spatial and temporal distribution of tracer concentrations indicates the presence of conduit-like and diffuse flow transport in the system, supporting the notion of both combined transport mechanisms in the limestone unit. The temporal response of tracer concentrations at different locations in the model coincide with, and confirms the preferential flow distribution generated with the SMMs used in the study.

Electrode effects on temporal changes in electrolyte pH and redox potential for water treatment

The performance of electrochemical remediation methods could be optimized by controlling the physicochemical conditions of the electrochemical redox system. The effects of anode type (reactive or inert), current density and electrolyte composition on the temporal changes in pH and redox potential of the electrolyte were evaluated in divided and mixed electrolytes. Two types of electrodes were used: iron as a reactive electrode and mixed metal oxide coated titanium (MMO) as an inert electrode. Electric currents of 15, 30, 45 and 60 mA (37.5 mA L−1, 75 mA L−1, 112.5 mA L−1 and 150 mA L−1) were applied. Solutions of NaCl, Na2SO4 and NaHCO3 were selected to mimic different wastewater or groundwater compositions. Iron anodes resulted in highly reducing electrolyte conditions compared to inert anodes. Electrolyte pH was dependent on electrode type, electrolyte composition and current density. The pH of mixed-electrolyte was stable when MMO electrodes were used. When iron electrodes were used, the pH of electrolyte with relatively low current density (37.5 mA L−1) did not show significant changes but the pH increased sharply for relatively high current density (150 mA L−1). Sulfate solution showed more basic and relatively more reducing electrolyte conditions compared to bicarbonate and chloride solution. The study shows that a highly reducing environment could be achieved using iron anodes in divided or mixed electrolytes and the pH and redox potential could be optimized using appropriate current and polarity reversal.

Contamination of Cave Waters by Heavy Metals

Heavy metals are present throughout caves and karst systems due to both natural and anthropogenic sources. The solubility and transport of iron and manganese, two of the most common heavy metals, are controlled by pH and redox conditions. Although typically soluble in water in reducing conditions, the oxidized metals tend to precipitate as cave deposits or on sediments where they contain many other metals in trace quantities. Many metals are never present in the dissolved state and are introduced, stored, and transported through the system as colloids or larger particles. Particulate metal transport is enhanced during storms when high groundwater velocities permit the metals to be entrained and suspended.

Team Science Applied to Environmental Health Research: Karst Hydrogeology and Preterm Birth in Puerto Rico

Understanding the interaction of environmental contamination and its impact on human health stretches the disciplinary demands required for effective research. Team science is required to understand the origin of contaminants, their pathways to human, their health effects, and for development of effective mitigation. We describe a team science model applied to the study of preterm birth in a region of karst hydrogeology, the Puerto Rico Testsite for Exploring Contamination Threats (PROTECT). This research program uses an innovative, holistic, source-to-outcome transdisciplinary approach that integrates epidemiological, toxicological, analytical, fate-transport, and remediation studies, along with a unified sampling infrastructure, a centralized, indexed data repository and a data management system. PROTECT is contributing new knowledge about the risk that contaminants may pose in pregnancy resulting in preterm birth, how these contaminants reach karst aquifers, and what are the biological mechanisms by which environmental contaminants may promote preterm birth. PROTECT also is developing novel remediation approaches that will target removal of contaminants linked to preterm births from ground water. These integrated efforts offer unique opportunities to address a serious public health problem and its solution would result in a healthier population and a healthier environment.

Bulk Chemistry of Karst Sediment Deposits

Sediments are ubiquitous in karst systems and play a critical role in the fate and transport of contaminants. Sorbed contaminants may be stored on immobile sediments or rapidly dispersed on mobile sediments. Sediments may also influence remediation by either enhancing or interfering with the process. To better understand the potential effects of sediments on remediation, we conducted physical and chemical characterizations of 11 sediment samples from 7 cave and spring deposits from karst regions of Tennessee, Virginia, and West Virginia. The samples were analyzed for particle-size distribution using sieves and laser diffraction particle analysis. The sediment size fraction <2 mm (sand, silt, and clay) was analyzed for slurry pH and specific conductivity (SC) using electrodes and for bulk total carbon, organic carbon, nitrogen and sulfur on an ElementarTM Vario MAX Cube CNS. The same <2 mm fraction was subjected to a pseudo-total extraction using aqua regia with subsequent solution analysis by inductively coupled plasma-optical emission spectrometry (ICP-OES). Most of the samples were dominated by the <2 mm size fraction. Their slurry pHs ranged from 6.8 to 8.4 and their SCs ranged from 45 to 206 μS/cm with the exception of two high SC samples (726 and 8500 μS/cm). The fraction of organic carbon (Foc) in the sediments ranged from <0.1 to 2%. The sample from a saltpeter cave historically used for gunpowder production contained the highest concentrations of N and S (~3 g/kg) but lower total C than some of the spring samples. The pseudo-total extractions were analyzed for Al, Ca, Fe, Mg, and Mn. Of those elements, Mg was the most consistent across the locations (2.0-6.1 g/kg), and Ca was the most variable (1.4-52 g/kg). Given the importance of particle size and elemental concentrations in chemical reactions and remediation, more data of this type are needed to predict contaminant fate and transport and to plan successful remediation projects.

Development and Testing of Hydrogel Beads as Potential Floating Tracers of Contaminant Movement in Karst Aquifers

The transport of light non-aqueous phase liquids (LNAPLs) is not well understood in karst settings. Traditional tracers do not predict the movement of free product; therefore, this study was undertaken to develop a better tracer proxy for LNAPL. The floating hydrogel tracer beads were created using alginate polymers and adding fluorescent pigments and density-modifying additives to alter their physical characteristics. Two sets of multi-tracer field tests (beads plus a conservative solute tracer) were completed in a 60-m section of cave stream. The beads were quantified via counting for the first set of tests and using particle image velocimetry (PIV) for the second. During the 2012 tests (170 L/s discharge), the beads travelled faster than the solute tracer; however, in the 2014 tests (9.1 L/s) the results were less conclusive (the beads arrived before the solute but had a later peak time and a lower mean velocity). Most of the particle studies have reported that particles travel faster than solutes, in accordance with our 2012 studies. Although the beads are particles and thus not an ideal proxy for LNAPL contaminants, they hold promise for future experimental studies and highlight the complexity of LNAPL transport in cave systems.

Fate, Transport, and Exposure of Emerging and Legacy Contaminants in Karst Systems: State of Knowledge and Uncertainty

It is well known that the same characteristics that make karst groundwater systems highly productive make them very vulnerable to contamination. Once in the subsurface, many contaminants move along and spread across flow lines, interact with media and environmental compartments, and react with chemical and biological entities. All of these processes occur within a highly dynamic and heterogeneous framework that affects the mobility, persistence, and potential exposure of humans and wildlife. Fundamental knowledge exists on many of these processes, and several predictive and characterization models have been developed and applied to karst systems. Yet tremendous challenges and uncertainty are faced when trying to predict exposure, implement remedial actions, and manage contaminated systems, particularly in a changing world. This paper discusses the state of knowledge, modeling capabilities, and sources of uncertainty when assessing the fate, transport, and exposure of legacy and emerging contaminants in karst systems. Although applicable to many sites, the discussion is framed around particular examples of extensive contamination in the karst region of northern Puerto Rico, and how these compare to more densely lithified karst systems associated with continental karst. It focuses on contaminants related to industrial, agricultural, and personal care activities. Despite the advancements made on understanding and modeling fate and transport processes, large uncertainty remains on source and system characteristics, scale-dependent model applicability, spatiotemporal data resolution, and the effect of hydrologic conditions and anthropogenic intervention.

Passive detection of Pb in water using rock phosphate agarose beads

In this study, passive detectors for Pb were prepared by immobilizing powdered rock phosphate in agarose beads. Rock phosphate has been used to treat Pb-contaminated waters and soil by fixing the metal as an insoluble pyromorphite mineral. Under lab conditions, Pb was rapidly adsorbed from aqueous solution by the beads over time, consistent with the acidic dissolution of rock phosphate, the precipitation of pyromorphite within the pore space of the agarose gel matrix, and surface exchange reactions. Net accumulation of Pb occurred when beads were exposed to simulated periodic releases of Pb over time. Under field conditions, beads in mesh bags were effective at detecting dissolved Pb being transported as surface runoff from a site highly contaminated with Pb. Rates of Pb accumulation in beads under field conditions appeared to be correlated with the frequency of storm events and total rainfall. The rock phosphate agarose bead approach could be an inexpensive way to carry out source-tracking of Pb pollution, to verify the successful remediation of sites with Pb-contaminated soil, and to routinely monitor public water systems for potential Pb contamination.