Hydrogeology at three test-well sites in Garrett County, Maryland
2017, Drummond, D.D., Bolton, D.W., and Andreasen, D.C
Open-File Report 15-02-03
Seven test wells were drilled at three sites in Garrett County, in the Appalachian Plateau Physiographic Province of Maryland. The purpose of this study was to (1) provide baseline data on the hydrogeologic characteristics (hydraulic properties, water levels, and water quality) of aquifers at depths typically utilized for water supply, and to (2) investigate the hydraulic connection between the shallow (less than approximately 200 feet) and deep (approximately 500 to 1,000 feet) aquifers and surface water.
Testing indicates that the aquifer system is highly heterogeneous and anisotropic, with distinct differences between sites in fracture orientation (high-angle versus low-angle), fracture density (both transmissive and non-transmissive fractures), hydraulic head gradient, transmissivity, specific yield, water quality, and degree of hydraulic connection to the nearby stream. There are also differences between wells within each site including deep versus shallow aquifer response, hydraulic relation between the wells and stream, head gradient, and water quality. The differences in hydrogeologic characteristics seen in this study illustrates the complexity of the groundwater flow system in the Appalachian Plateau Physiographic Province of Maryland, making prediction of the fate and transport of contaminants in the subsurface very difficult.
At the Buffalo Run test site in northwest Garrett County, open intervals in shallow and deep wells are 40 to 120 ft, and 125 to 230 ft, respectively. Both wells were flowing artesian wells, with hydraulic heads of approximately 87 ft and 5 ft above land surface for the deep and shallow wells, respectively. Flow rates for the deep and shallow wells were 110 and 0.8 gallons per minute, respectively. Most fractures were subhorizontal and associated with bedding. Three transmissive fractures in the deep borehole contributed almost all of the ambient flow. In the shallow borehole, four fractures contributed almost all of the flow. Transmissivities in GA Aa 15 were 710 ft2 /d during the drawdown phase, and 945 ft2/d during the recovery phase. The head difference between the two wells suggests that a hydraulic connection between the two zones is unlikely, and the stream gage in Buffalo Run showed no response from the flowing aquifer test of the deep well, indicating little or no hydraulic connection between them. The shallow aquifer responds to rainfall events, indicating a direct hydraulic connection with the surface and suggesting a connection with Buffalo Run. Water quality was similar between the two wells and Buffalo Run, although Buffalo Run had a higher percentage chloride and sulfate, which may reflect input of road salt and other processes.
At the Savage River test site in northeast Garrett County, open intervals in shallow and deep wells are 40 to 120 ft and 500 to 986 ft, respectively. The deep well was essentially a dry hole, indicating a very low permeability; this precluded conducting an aquifer test or collecting a water sample. The water level in the shallow well was 75 ft below land surface. Transmissivity of the shallow well was calculated as 6 and 4 ft2/d in the drawdown and recovery phases of the pump test, respectively. The specific capacity of the shallow well was 0.06 gpm/ft. Log analysis indicated that the borehole penetrated transmissive bedding and higher-angle fractures. There were virtually no transmissive fractures below 500 ft. A lack of water-level response during aquifer testing indicated no direct hydraulic connection between the deep and shallow aquifers. The relation between shallow groundwater and streamflow in the Savage River could not be evaluated due to lack of proximity (the gage was about 3 miles from the drill site) and the relatively low discharge rate of the test (5 gpm). However, the shallow well and the Savage River stream gage showed similar hydrograph patterns. GA Bf 29 had a mixed-cation, oxygen-rich water type, and had a low dissolved-solids content; Savage River had sodium and chloride as the dominant ions (suggesting an anthropogenic source) and had higher overall dissolved solids.
At the Nydegger test site in southwest Garrett County, open intervals in three wells range from 20 to 32 ft, 40 to 200 ft, and 500 to 985 ft, respectively. Water levels in all three wells were below the level of the adjacent Nydegger Run, indicating that it is a losing stream, and that there is some other control on groundwater flow in the area. The specific capacity in the deep and middle wells was 0.03 and 11.3 gpm/ft, respectively; transmissivity was 2 and 2,300 ft2/d, respectively. Most fractures were subhorizontal and associated with bedding. The 535-ft fracture zone accounted for more than 90 percent of the total transmissivity in the deep well. Water-level response during aquifer testing indicates a strong hydraulic connection between the deep, middle, and shallow aquifers. The deep, middle, and shallow aquifers respond to rainfall events, indicating a direct hydraulic connection with the surface. The deep well had a sodium-bicarbonate water type, was lower in dissolved solids and had a higher methane concentration compared to the shallow and middle wells, whose water were calcium-chloride or calcium-mixed anion types. Water from both the deep and middle wells differed from the Savage River water samples which were predominantly calcium-sulfate water.