Hydrological and geochemical analyses of the karstic Bobcat Cave watershed: A case study at Redstone Arsenal, northern Alabama
Type of DegreeMaster's Thesis
Geology and Geography
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Hydrologic, geochemical, and stable isotope methods were applied to study karstic carbonate watersheds of Redstone Arsenical (RSA) in north-central Alabama near Huntsville. Delineation of flow paths in RSA watersheds near the Bobcat Cave is crucial for the protection of the Alabama Cave Shrimp, which are listed as a federal endangered species. Encroaching urbanization and continuing industrial development in the area surrounding Bobcat Cave pose a threat to groundwater quality in the study area. Sources of groundwater recharge to Bobcat Cave as well as groundwater flow directions surrounding the cave must be determined to successfully create a scientifically defensible buffer zone around the cave. Previous dye tracer studies suggest that Indian Creek, the largest body of surface water in the study area, may serve as a groundwater divide that blocks westward migration of contaminants from sources east of the creek. Determining the hydrologic connectivity between the east and west side of the creek is an important aspect of delineation of the watershed. Results from geochemical analysis and in-situ water quality parameter measurements of groundwater in the Bobcat Cave indicate that recharge to the cave is largely from surface runoff entering the cave via conduits during storm events. Groundwater in Bobcat Cave quickly responded to freshwater inputs during storm events. Groundwater temperature and electrical conductivity show step-wise drops, and water level and pressure rose quickly following large rainfall events. These results were consistent with previous water-level studies conducted in the Bobcat Cave. Water level in carbonate bedrock groundwater monitoring wells were measured periodically over a one-year time period. Groundwater levels generally fluctuated less than two meters over the course of the study despite drought conditions that Huntsville experienced during this time period. These results suggest that drought and precipitation have less pronounced effects on groundwater levels of relatively deep bedrock as compared to water level inside the Bobcat Cave. Water-level data indicate a general southward flow in the study area. However, groundwater flow direction locally in the vicinity of Bobcat Cave was determined to be to northward in February and March of 2017, consistent with previous dye studies that determined a localized northerly groundwater flow direction in the area around the Bobcat Cave. Results of trace elements, major ions, stable oxygen, hydrogen, and carbon isotopes, and dissolved organic carbon geochemical analyses provide strong evidence of poor hydrologic connection of groundwater between the eastern and western side of Indian Creek. Elevated concentrations of trace elements such as arsenic, strontium, rubidium, nickel, selenium, boron, and vanadium were only found in groundwater to the east of Indian Creek. Major ion geochemistry suggests the occurrence of two distinct hydrochemical facies in our study area. Groundwater to the east of Indian Creek is dominated by carbonate/bicarbonate, sodium, potassium, and sulfate ions. Groundwater in Bobcat and Matthews caves and monitoring wells to the west of Indian Creek is dominated by calcium-bicarbonate ions. The δ18O and δD isotopic signatures of groundwater samples suggest that precipitation is a major source of recharge to the groundwater in our study area. The variations in oxygen and hydrogen isotope composition seem to reflect “the amount effect” in which δ18O values are typically depleted in precipitation falling during periods of high rainfall. Bobcat and Matthews Caves and the shallow had more depleted δ13C values compared to deeper groundwater monitoring wells, suggesting that the shallow well and caves are influenced more by biogenic carbon dioxide in shallow soil. Very depleted δ13C values (-22 ‰) from a groundwater monitoring well to the east of Indian Creek suggest that this well is contaminated by organic carbon or hydrocarbons dominated by lighter 12C. GC-MS analysis of volatile organic compounds in this well were inconclusive. Groundwater in wells near the airfield to the east of the Indian Creek contain higher dissolved organic carbon concentrations than those in Bobcat Cave and wells to the west of the Indian Creek. The extent of groundwater contamination near the airfield requires further investigation. In summary, these results suggest that Indian Creek may serve as a groundwater divide that blocks westward groundwater migration and contaminant transport from recharge areas east of the creek, protecting Bobcat Cave and the Alabama Cave Shrimp from sources of contaminants to the east of Indian Creek.