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Clastic cave deposits are representative of sediments throughout the karst aquifer and thus are an abundant and accessible resource through which to study the chemistry of karst aquifer. Clastic cave sediments are attributed to depositional facies based on cave location, sorting, and particle size. These facies settings may influence different chemical parameters of the sediments, like concentrations of total organic carbon (TOC). The TOC concentrations in clastic cave sediments have not been well constrained nor has the role of clastic sediments in contaminant fate and transport through karst systems been well described. In this study, particle size, TOC, and total nitrogen were measured in sediments representing different facies in Butler Cave, Virginia, USA. TOC concentrations ranged from 0.08 - 0.87 weight percent and C:N molar ratio ranged from 3 - 15, indicating a possible terrestrial source of organic carbon in these sediments. The diamicton facies was sandier and but had similar TOC concentrations compared to the channel facies. TOC concentrations measured in Butler Cave were within the same range as those observed in above water, eogenetic clastic cave sediments from two caves in Puerto Rico. Estimated retardation factors calculated based on the TOC concentrations in the Butler Cave sediments indicate the range of TOC in this cave could be responsible for 39 - 987% increase in retardation of selected contaminants. This study highlights the importance of measuring the ranges of TOC in clastic cave sediments across different facies and their role in contaminant fate and transport.

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Interactions of karst aquifer sediments with organic contaminants or microplastics (MPs) have received relatively little attention even though the susceptibility of karst aquifers to contamination and their ability to store and transport sediment is well documented. Studies using polystyrene microspheres as surrogate tracers for bacteria transport in karst systems have generally observed low recovery of microspheres and attributed this to microsphere adsorption onto aquifer sediments. In addition to being used as surrogate tracer for bacteria, microspheres have the potential to be used as surrogate material for organic contaminants and MPs. Using cave sediments as a proxy for karst aquifer sediments, the adherence of two types of microspheres (carboxylated and nonfunctionalized) was measured in three different types of solutions: deionized water, a calcium carbonate solution, and a karst spring water. Both types of microspheres adhered to the sediments; the most influential factor in adherence was solution type not microsphere type. Average adherence ranged from 51%-94% with average adsorption coefficients (KD) ranging from 11.8-442. Average estimated organic-carbon water partition coefficients (KOC) and retardation factors (RF) ranged from 1.64×103 - 6.13×104 and 6.20×101 - 2.29 × 103, respectively. KD, KOC, and RF were an order of magnitude higher in the karst water than in DI or CaCO3 solution. The results illustrate the importance of sediment interactions with potential organic or MP contaminants in karst systems.

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The clastic sediments that accumulate in cave settings can be an important storage reservoir for organic carbon. This study reports on grain size, total organic carbon (TOC) concentrations, and total organic carbon:total organic nitrogen (TOC:TON) ratios measured in sediments from two caves in Puerto Rico. El Tallonal Cave (TAL) is a small cave with a flowing stream; the sediments in TAL were collected from a deposit that is being actively eroded. Clara Cave (CAM) is an upper level of the Río Camuy Cave System; the sediments from CAM were newly deposited by an internal river that rose in response to Hurricane Maria. Sediments collected from both caves were poorly sorted and contained no apparent stratigraphic correlation. CAM sediments contained a larger range in TOC concentrations but were overall lower than TOC in the TAL sediments. In TAL, the TOC concentrations were higher in sediments collected from below the erosional terrace. TOC:TON ratios from sediments at both caves were highly variable, highlighting the heterogeneous deposition and storage of organic matter. Despite the observed variation, TOC concentrations in both cave systems could cause retardation of organic contaminants by up to two orders-of-magnitude, implying that deposited sediments influence the fate of organic contaminants in the groundwater; therefore, cave sediments could facilitate long term storage of organic carbon and associated contaminants.

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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.

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Due to the complicated nature of karst aquifers, many groundwater treatment technologies are difficult to implement successfully. A particular challenge arises because sediments are ubiquitous and mobile in karst systems and may either facilitate contaminant transport or act as long-term substrates for storage via sorption. However, electrochemical remediation is a promising technology to be optimized for karst aquifers due to easy manipulation and control of groundwater chemistry as well as low cost, ability for in situ application, and performance under alternative power sources. This study investigates the effects of suspended karst sediments on the electrochemical remediation of groundwater via electro-Fenton (EF) mechanism. The EF mechanism relies on direct electrolysis (i.e., water electrolysis and ferrous iron release) and indirect, electrochemically-induced processes (i.e., Pd catalyzed H2O2 production). These processes can be optimized for H2O2 generation and support of its activation to hydroxyl radicals - a powerful oxidant capable of degrading and transforming a wide range of contaminants (e.g., chlorinated solvents). In this study, we tested sediments varying in concentrations of Fe, Mn and buffering capacities. When the sediments were introduced into the EF experiments, there were adverse effects on the H2O2 content: at steady state (120 min), Pd catalyzed formation of H2O2 decreased by 60%, 57%, and 75% in the presence of suspended sediment collected from three separate karst locations. Presented results imply that sediments' presence influences EF mechanism in electrochemical systems, but given the flexibility of the technology, it can be optimized in terms of electrode materials, current intensities and current regimes to address these challenges.

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We were asked to provide the Chesapeake and Ohio Canal National Historical Park with information and guidance to help park managers protect and manage karst-related rare, threatened, and endangered species on park land. To do this we developed a vulnerability risk matrix based on a variety of data collected. The purpose of the matrix was to provide the park with an interactive means of evaluating the relative vulnerability of the different sites. The data collected included (1) an inventory of karst resources in the park, (2) collection of water chemistry data, and (3) an RTE assessment. Useful outcomes included a standardized scoring system for the RTE species in the park for each site, an assessment of relative risk (vulnerability of site to negative events) and impact (a measure of the damage to RTE species if a negative event would occur), and a vulnerability matrix that identifies the sites needing management or future assessment. This matrix can be easily modified and used to assess other scenarios or to accommodate the addition of new data. Other parks and sites could reproduce this type of matrix in order to manage their resources.

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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.

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The processing, storage, and flux of inorganic carbon in rivers and streams play an influential role in the lateral transfer of atmospheric and terrestrial carbon to the marine environment. Quantifying and understanding this transfer requires a rapid and accurate means of measuring representative concentrations of dissolved inorganic carbon (DIC) and CO2 in field settings. This paper describes a field method for the determination of DIC based on the direct measurement of dissolved CO2 using a commercial carbonation meter. A 100-mL water sample is combined with 10 mL of a high ionic strength, low-pH, citrate buffer, mixed well, and the dissolved CO2 concentration is measured directly. The DIC is then calculated based on the dissolved CO2 concentration, buffer-controlled ionic strength, pH, and temperature of the mixture. The method was accurate, precise, and comparable to standard laboratory analytical methods when tested using prepared sodium bicarbonate solutions up to 40 mM DIC, North Atlantic seawater, commercial bottled waters, and carbonate spring waters. Coal mine drainage waters were also tested and often contained higher DIC concentrations in the field than in subsequent laboratory measurements; the greatest discrepancy was for the high-CO2 samples, suggesting that degassing occurred after sample collection. For chemically unstable waters and low-pH waters, such as those from high-CO2 mine waters, the proposed field DIC method may enable the collection of DIC data that are more representative of natural settings.

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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.

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Removal of selenate from solution is investigated in batch electrochemical systems using reactive iron anodes and copper plate cathode in a bicarbonate medium. Iron anodes produce ferrous hydroxide, which is a major factor in the removal of selenate from solution. Iron anodes also generate a significant decrease in the oxidation-reduction potential (ORP) of the solution because it prevents generation of oxygen gas at the anode by electrolysis. The removal rates varied from 45.1 to 97.4%, depending on current density and selenate concentration. The transformation of selenate by the process is modeled based on a heterogeneous reaction coupled with electrochemical generation of ferrous and hydroxide. The rates are optimized at lower initial concentrations, higher electrical currents, and the presence of anions. Presence of dissolved oxygen does not cause any significant effects the removal of selenate.

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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, Na(2)SO(4) and NaHCO(3) 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.

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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.