INFLUENCE OF FRACTURE ANISOTROPY ON GROUND-WATER APPARENT AGES: A CASE STUDY FROM THE VALLEY AND RIDGE PROVINCE, PENNSYLVANIA

Authors: BURTON, WILLIAM - USGS; PLUMMER, L - USGS; BUSENBERG, EURYBIADES - USGS; LINDSEY, BRUCE - USGS; Gburek, William

Submitted to: Ground Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2001
Publication Date: 5/20/2002
Citation: Burton, W.C., Plummer, L.N., Busenberg, E., Lindsey, B.D., Gburek, W.J. 2002. Influence of fracture anisotropy on ground-water apparent ages: a case study from the valley and ridge province, pennsylvania. Ground Water. 40(3):242-257.

Interpretive Summary: As part of the efforts to restore water quality of the Chesapeake Bay, regulators and action agencies must determine the time lag between nitrogen remediation at the land surface by agricultural managers and the reduction in nitrate levels in nearby streams. To address this problem, scientists from ARS at University Park, PA, are cooperating with U.S. Geological Survey personnel to evaluate ground water travel times in a small watershe in east-central Pennsylvania. Two naturally occurring ground-water tracers were sampled in wells located near the watershed outlet where observed ground-water age should represent the longest travel times within the watershed. The pattern of bedrock fracturing controlling ground water movement was also determined and found to be quite regular and generally following the bedrock bedding planes that dip to the north. The isotope sampling showed that ground water traveling along paths parallel to the bedding planes is younger in age than that traveling along paths opposite to the dip. Also, the ages observed (5 to 20 years) were significantly longer than those previously calculated by a ground-water flow model (seasonal to <5 years) that assumed uniformly fractured layers parallel to the land surface topography. Thus, ground-water travel times in the watershed appear to be influenced more by the patterns of fracture geometry than was previously thought. These findings will help ground-water modelers in other areas underlain by similarly folded and fractured rock, such as elsewhere in the Valley and Ridge province. They have also helped quantify travel times in the watershed setting so planners can better evaluate the effects of land management on nitrate concentrations in streams.

Technical Abstract: As part of a regional study of ground-water travel times in the Chesapeake Bay Watershed, chlorinated fluorocarbon (CFC) and 3H/3He ground-water apparent ages were obtained from two arrays of nested piezometers located on the north limb of an anticline in fractured sedimentary rocks in the Valley and Ridge geologic province in Pennsylvania. The fracture geometry of the gently east-plunging fold is very regular and consists predominantl of south-dipping to subhorizontal to north-dipping bedding-plane parting, and east-striking, steeply dipping axial planar spaced cleavage. In the area of the piezometer arrays, which trend north-south on the north limb of the fold, north-dipping bedding-plane parting is a more dominant fracture set than is steeply south-dipping axial-plane cleavage. The dating of ground water from the piezometer arrays reveals that ground water traveling along paths parallel to the dip direction of bedding-plane parting has younger 3H/3He and CFC apparent ages, or a greater component of young water, than does ground water traveling along paths opposite to the dip direction. The travel times inferred by these ages are significantly longer than those previously calculated by a ground-water flow model that assumed isotropically fractured layers parallel to topography. Ground water travel times in the watershed therefore appear to be more influenced by anisotropic fracture geometry than previously realized. This could have significant implications for ground-water models in other areas underlain by similarly folded and fractured rock, such as elsewhere in the Valley and Ridge province.