The effects of land-use change on water quality and speleogenesis in Ozark cave systems: A paired cave study of Civil War and Copperhead Caves, northwestern Arkansas.

摘要

Civil War Cave is located in a rapidly developing and highly impacted area of Benton County, Arkansas. This area is covered by shallow mantled karst. In this setting, caves typically are well connected hydraulically to the surface and show a rapid response to surface water changes. Land use may affect the water quality and speleogenesis of a cave. The delicate ecosystem of caves makes them an excellent indicator of changes in ground water conditions. Civil War Cave is home to the threatened Ozark cavefish (Amblyopsis rosae ), making any impact on Civil War Cave more significant. This study characterizes the effects of land use change on water quality and speleogenesis using a paired cave study. In this paired cave study, two caves were evaluated: Civil War Cave, located in a rapidly developing and highly-impacted area in Benton County, Arkansas and Copperhead Cave, which lies within a nearly pristine forested watershed of the Buffalo National River in Newton County, Arkansas. The geological characteristics of Civil War and Copperhead caves are similar, with both caves contained in the Boone Formation. The hydrology of the two caves is similar, with both caves being recharged through the Springfield Plateau aquifer. In both caves, the actual recharge area is larger than the apparent topographic recharge area. The topographic recharge areas are 0.18 square kilometers (km2) for Civil War Cave and 0.35 km 2 for Copperhead Cave. Using normalized base flow approximation, the recharge area was determined to be approximately 7.5 km2 for Civil War Cave, with a possible range of 4.7-10 km2. The normalized base flow indicates a recharge area of 1.75-3.7 km2 for Copperhead Cave. In both cases, the regional flow is modified and/or enhanced by fractures and karst features and groundwater flow-paths cross topographic divides. Sedimentation was observed to be a problem in Civil War Cave beginning in 2005. The sedimentation is likely a result of surface excavation in the immediate recharge area and has subsided since this excavation ended. Copperhead Cave exhibits only minimal sedimentation. Since Civil War Cave is developed in the epikarst, the cave is more vulnerable to sedimentation. Examination of the water quality corroborates the idea that both caves are recharged through the Springfield Plateau Aquifer. Using nitrate as nitrogen (NO3-) to evaluate human impact, Civil War Cave was determined to be impacted, with NO3 - values averaging 6.33 mg/L in the upper level and 6.49 mg/L in the lower level. Copperhead Cave is not impacted, with less than 0.2 mg/L N0 3-. Civil War Cave is developed entirely in the epikarst and as a result contains speleothems formed by a stream flowing on the cave floor. No active speleogenesis was observed in the cave, erosion of speleothems is common, and large amounts of sediment are entering the cave. Copperhead Cave was developed in a deeper karst setting with drip-waters being abundant and, therefore, has speleothems on the ceilings and walls of the cave. Active speleogenesis is observed in Copperhead Cave. At Civil War Cave, development has increased the hydrologic threat, the threat of nutrient stress, and the threat of chemical pollution. In addition, increased sediment related to development presents a threat to the cave ecosystem. The greatest threat in Copperhead Cave is human interaction by visitation. Despite this, the biodiversity in both Civil War and Copperhead Caves are great. Several species that were observed historically were also observed during the present study. Land-use around Civil War Cave has impacted the hydrology, sedimentation, and water quality of the cave. The only impact observed in Copperhead Cave was the result of direct human interference within the cave.