Ellen K. Herman

Clastic sediment transport and storage in fluviokarst aquifers: an essential component of karst hydrogeology

Carbonate aquifers with well-developed conduit systems carry a flux of clastic sediment as an intrinsic aspect of the functioning of the aquifer. Sources of clastic sediments include sediments carried by sinking streams, soil washdown from the epikarst, plug injection by sinkhole piping failures, residual insoluble material from the dissolution of the limestone, and sediment backflooded from surface streams. The conduit system acts as a mixing chamber where the injected materials are sorted and rearranged. Information on the sediments and their transport processes can be obtained by investigating the source areas, by inspection of cave sediments, and by monitoring clastic sediment discharged from springs as a function of flow conditions. The engine that drives the sediment transport system is storm recharge in the ground water basin drained by the conduit system. Fine-grained clastics move during ordinary storms and can be captured easily at springs, but movement of coarser materials requires high-intensity, therefore infrequent, storms so that most of the sediment flux is episodic with long periods of storage interspersed with short periods of movement. Fluid mechanics provides the basis for calculations of both bedload and suspended load components. However, these calculations become complex because of the need to take into account discharge-dependent shifts from pipe flow to open channel flow and the effect of irregularities in conduit morphology and blockages due to breakdown and other barriers.

Advances in Monitoring to Understand Flow Paths in Karst: Comparison of Historic and Recent Data from the Valley and Ridge of Pennsylvania

We revisited seven karst springs studied in 1971 to examine how advances in monitoring techniques influence our understanding of spring flow paths and recharge. These springs in the Valley and Ridge of Pennsylvania offer a variety of spring discharge volumes and physical appearance (visible conduits at the discharge site versus fracture networks). We focused on temperature for a comparison between historic and recent trends because it is the least likely to differ in value due to updated measurement techniques. High-resolution temperature data showed variations in storm responses (not observed with historic data) which distinguish among springs. We also used automatic samplers to track geochemical changes during storm events. Variations in CO2 concentrations and Mg/Ca ratios provided indicators of fast and slow recharge and changes in flow paths over time. Some springs showed similar response from storm to storm and some varied. Rare earth elements (REEs) were analyzed in baseflow samples to evaluate their potential for distinguishing spring source rock along flow pathways. The REE grouped springs differently than their temperature and storm response geochemistry. This study showed that classification of karst springs varied depending on the parameter monitored. This variation further points out the complexity of karst flow paths and recharge. Multiple methods and long-term monitoring are needed to interpret karst spring discharge and provide sampling schemes for source water protection.

Using Stable Isotopes to Distinguish Sinkhole and Diffuse Storm Infiltration in Two Adjacent Springs

While both springs can be traced to sinks, their isotopic signatures reflect how storm water infiltrates and travels within each spring’s recharge area. Tippery is fed by a perennial sinking stream and more developed conduit network, while Near Tippery has a more diffuse recharge area with mixing of different surface inputs. As stable isotopes are unaffected by redox or dissolution processes, they can provide a conservative tracer with which to characterize how other parameters, such as temperature, alkalinity, and turbidity, are reflected in different spring recharge behaviors.

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.

Contaminated Groundwater in Karst: Why Is It an Issue? An Introduction to the KWI San Juan Conference

The Karst Waters Institute sponsored a conference on karst groundwater contamination and its impacts on public health. The objective was to facilitate communication between hydrogeologists and the biomedical community, especially those dealing with public health issues. This volume contains the papers presented at the conference.