Ground-water hydrology and water-quality data for wells, springs, and surface-water sites in the Bradley-Brumalow creeks area near Arnold Air Force Base, Tennessee, September to December 1999

Abstract

Arnold Air Force Base (AAFB) occupies about 40,000 acres in Coffee and Franklin Counties, Tennessee. The primary mission of AAFB is to support the development of aerospace systems. This mission is accomplished through test facilities at Arnold Engineering Development Center AEDC), which occupies about 4,000 acres in the center of AAFB. Several synthetic volatile organic compounds (VOC’s), primarily chlorinated solvents, have been identified in ground-water samples at AEDC. Private groundwater supplies in the Bradley-Brumalow Creeks area are hydraulically downgradient from AEDC and could be affected by transport of VOC’s in the ground water at AEDC. From September to December 1999, a comprehensive investigation of the groundwater resources in the Bradley-Brumalow Creeks area was conducted to determine if VOC’s from AEDC have affected local private water supplies and to advance understanding of the ground-water-flow system in this area. The investigation focused on locating and sampling all private water wells and springs located within the Bradley-Brumalow Creeks area that are used as a source of drinking water, though not all of the wells and springs sampled are currently used as a source of drinking water. Ground-water-flow directions were investigated by conducting base-flow stream measurements, measuring water levels in wells, and constructing a potentiometric-surface map of the Manchester aquifer in the study area. Data were collected from a total of 150 private and 88 monitoring wells during the course of the study. Depths to ground water were determined for 103 of the private wells and 86 of the monitoring wells. The wells ranged in depth from 14 to 167 feet deep. Water-level altitudes ranged from 946 to 1,081 feet above sea level. Depths to water ranged from 3 to 93 feet below land surface. Water-quality samples were collected from all 150 private wells that draw water from the Manchester aquifer. Additionally, a reconnaissance of 8 springs and 33 surface-water sites was conducted in the Bradley-Brumalow Creeks area. Discharge measurements were made at 5 of the 8 springs and all 33 of the surface-water sites as part of the regional base-flow component of the study. Water-quality samples were collected at 8 of the springs and 9 of the surface-water sites. Water-level-altitude data collected from wells and base-flow data collected from streams and springs were used to construct a regional potentiometric-surface map of the Manchester aquifer in the study area. Several notable features are illustrated on the map, including a groundwater divide that roughly follows the regional surface-water divide, a “saddle” along the groundwater divide lying northeast of AEDC, and two prominent groundwater“troughs” (valleys) extending east and southeast from the divide toward Bradley Creek. Water-quality samples collected from the 150 private wells, 8 springs, and 9 surface-water sites in the Bradley-Brumalow Creeks area were analyzed for major ions and VOC’s. The sampled water is predominantly of the calcium bicarbonate type. Specific conductance for sampled water ranged from 10 to 788 microsiemens per centimeter (µS/cm), with a median of 104 µS/cm. The range and median value for pH in sampled water were 4.5 to 8.0, and 6.3, respectively. Concentrations of most of the VOC’s analyzed for were less than detection limits. None of the sample results exceed drinking water maximum contaminant levels for public water systems. However, some compounds were detected in concentrations exceeding analytical reporting levels. Two wells produced samples containing toluene in concentrations of 1.4 and 1.3 µg/L. Chloroform also was detected in the sample from another well at a concentration of 2.4 µg/L. Other contaminants of concern were detected in estimated concentrations less than their reporting limits, referred to as estimated values. Samples from three wells showed the presence of tetrachloroethylene (PCE). Estimated concentrations of PCE in the samples ranged from 0.13 to 0.74 µg/L. Trichloroethylene (TCE) was detected in a sample from one of the three wells (0.35 µg/L). 1,1,1-Trichloroethane (1,1,1-TCA) was detected in another well (0.13 µg/L). Dichlo-rodifluoromethane was detected in samples from three other wells. Estimated concentrations of dichlorodifluoromethane ranged from 0.23 to 1.1 µg/L. Samples from another well, a spring, and a surface-water station also showed the presence of trace amounts of toluene. Estimated concentrations of toluene ranged from 0.11 to 0.47 µg/L. Benzene was detected in a sample from one well at an estimated concentration of 0.18 µg/L. Xylenes and ethylbenzene were detected in the samples from another well at estimated concentrations of 0.38 and 0.1 µg/L, respectively. For the VOC’s detected, the fre-quency of detections and median concentrations are compared to data from ambient rural ground water. Most of these volatile organic compounds, particularly the chlorinated solvents PCE, TCE, and 1,1,1-TCA, occur at concentrations above these ambient levels in the ground water at several solid waste management unit sites at AAFB. Collectively, data obtained during the study can aid in the understanding of regional ground-water-flow pathways and their relation to activities at AAFB.