Total organic carbon concentrations in clastic cave sediments from Butler Cave, Virginia, USA: implications for contaminant fate and transport

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.

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 (F_oc) 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.

LABORATORY TESTING OF THE POTENTIAL FOR THE INFLUENCE OF SUSPENDED SEDIMENTS ON THE ELECTROCHEMICAL REMEDIATION OF KARST GROUNDWATER

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.

Assessing the relative vulnerability of sensitive karst habitats containing rare, threatened, and endangered species in the Chesapeake and Ohio Canal National Historical Park

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.

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.

A Field-Based Method for Determination of Dissolved Inorganic Carbon in Water Based on CO2 and Carbonate Equilibria

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.

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.