Quantification of nitrate fate in a karst conduit using stable isotopes and numerical modeling.

Nitrate (NO3⁻) fate estimates in turbulent karst pathways are lacking due, in part, to the difficulty of accessing remote subsurface environments. To address this knowledge and methodological gap, we collected NO3⁻, δ15NNO3, and δ18ONO3 data for 65 consecutive days, during a low-flow period, from within a phreatic conduit and its terminal end-point, a spring used for drinking water. To simulate nitrogen (N) fate within the karst conduit, the authors developed a numerical model of NO3⁻ isotope dynamics. During low-flow, data show an increase in NO3⁻ (from 1.78 to 1.87 mg N L-1; p < 10-4) coincident with a decrease in δ15NNO3 (from 7.7 to 6.8‰; p < 10-3) as material flows from within the conduit to the spring. Modeling results indicate that the nitrification of isotopically-lighter ammonium (δ15NNH4) acts as a mechanism for an increase in NO3⁻ that coincides with a decrease in δ15NNO3. Further, numerical modeling assists with quantifying isotopic overprinting of nitrification on denitrification (i.e., coincident NO3⁻ production during removal) by constraining the rates of the two processes. Modeled denitrification fluxes within the karst conduit (67.0 ± 19.0 mg N m-2 d-1) are an order-of-magnitude greater than laminar ground water pathways (1-10 mg N m-2 d-1) and an order-of-magnitude less than surface water systems (100-1000 mg N m-2 d-1). In this way, karst conduits are a unique interface of the processes and gradients that control both surface and ground water end-points. This study shows the efficacy of ambient N stable isotope data to reflect N transformations in subsurface karst and highlights the usefulness of stable isotopes to assist with water quality numerical modeling in karst. Lastly, we provide a rare, if not unique, estimate of N fate in subsurface conduits and provide a counterpoint to the paradigm that karst conduits are conservative source-to-sink conveyors.

Water quality characteristics of discharge from reforested loose-dumped mine spoil in eastern Kentucky.

Surface mining is a common method for extracting coal in the coal fields of eastern Kentucky. Using the Forestry Reclamation Approach (FRA), which emphasizes the use of minimally compacted or loose-dumped spoil as a growth medium for trees, reclamation practitioners are successfully reestablishing forests. Yet, questions remain regarding the effects FRA has on the quality of waters discharged to receiving streams. To examine the effect of FRA on water quality, this study compared waters that were discharged from three types of spoils: predominantly brown, weathered sandstone (BROWN); predominantly gray, unweathered sandstone (GRAY); and an equal mixture of both aforementioned sandstones and shale (MIXED). The water quality parameters pH, EC, Ca, K, Mg, Na, NO-N, NH-N, SO, Cl, TC, suspended sediment concentration (SSC), settleable solids (SS), and turbidity were monitored over a 2-yr period on six 0.4-ha plots (two replications per spoil type). Generally, levels of Cl, SO, Ca, NO-N, NH-N, SS, SSC, and turbidity decreased over time. The pH for all spoils increased from about 7.5 to 8.5. The EC remained relatively level in the BROWN spoil, whereas the GRAY and MIXED spoils had downward trajectories that were approaching 500 µS cm. The value of 500 µS cm has been reported as the apparent threshold at which certain taxa such as Ephemeroptera (e.g., Mayfly) recolonize disturbed headwater streams of eastern Kentucky and adjacent coal-producing Appalachian states.