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Abstract:
Groundwater generally becomes increasingly reduced (decreasing Eh) with depth from the soil surface, and therefore seepage is a potential source of dissolved Mn, Fe, and NH4 to caves. In Lechuguilla Cave, both abiotic and biotic processes have contributed to the origin of a vast array of secondary speleogenetic features which are enriched in Fe and Mn oxides. Existing chemical and physical properties of Lechuguilla Cave pool water indicates oxidizing conditions, with dissolved Fe and Mn below detection, and N existing primarily as NO3. However, the redox chemistry of the cave seepage has not been well-studied. The objective of this study was to characterize the redox status of Lechuguilla Cave seepage and to test the hypothesis that seepage entering the cave from the overlying vadose zone is a potential source of dissolved Fe, Mn, and inorganic N (as both NH4 and NO3). If present in seepage, Fe, Mn, and NH4 will oxidize in the cave environment, resulting in non-detectable concentrations in cave pools. Seepage was collected from eight locations in the cave and analyzed for field parameters (pH, EC, dissolved O2(g), Eh, temperature) and concentrations of dissolved Fe (Fe2+ + total Fe), Mn, NO3 + NO2-N, and NH4-N. Results indicate that low organic C concentrations prevent the occurrence of complete anaerobic conditions in seepage, but the concentrations of Mn and NH4 indicate that slightly reducing conditions can exist. Iron concentrations were below detection (,0.06 mg L21) in all samples, and N existed primarily as NO3. Field-measured Eh values obtained using a Pt electrode (Ehm) did not correlate with computed Eh values for various redox couples (Ehc), and the poor agreement between Ehc values for the different couples indicates the absence of redox equilibria in the samples. Rather than characterization of redox status according to Eh, seepage is classified as ranging from oxic to suboxic. This redox classification indicates that Lechuguilla Cave seepage can generally be expected to contain low concentrations of organic C and dissolved Mn, dissolved O2(g) ranging from 1 mM to .30 mM, but with Fe below typical analytical detection limits.