CHARACTERIZATION OF ANISOTROPIC AQUIFER BEHAVIOR IN THE PEN ARGYL MEMBER OF THE MARTINSBURG FORMATION, PENNSYLVANIA

Horizontal anisotropy in aquifer properties is common in deformed terrains. When structural fabrics and resulting secondary porosity are controlled by regional tectonics, principle transmissivity tensor (T_α) orientation (θ) may be similar over a wide area. Lithologic variability within a terrain may affect fracture characteristics and thus the magnitude of T_α may vary. Recognition of domains with similar orientation and magnitude of transmissivity tensors is important for effective groundwater management.

The Pen Argyl Member of the Martinsburg Formation is a thick sequence of claystone slate with subordinate intercalated quartzose slate and subgraywacke. Time-drawdown data from four aquifer tests in the Pen Argyl Member exhibited anisotropic behavior at a scale of at least 600 m. The horizontal anisotropic properties were determined using analytical models and the results were statistically characterized allowing for uncertainty estimation and testing for differences. The uncertainties quantify deviation of the real aquifer from the idealized model aquifer and arise from aquifer heterogeneity, deviation from radial flow conditions with likely extended well behavior, and the presence of multiple dipping fracture zones as well as measurement error.

Principle transmissivity tensor orientation for four pumping tests within the Pen Argyl Member of the Martinsburg Formation were statistically indistinguishable (mean θ_α = 41.5^o±8.8^o(3σ)) and aligned parallel to the strike of bedding, axial planar cleavage, and a joint set. Faults were prominent water-bearing zones in the wells. The fault orientations in the wells are unknown but it is likely that they represent bedding plane faults evident in a nearby quarry and identified as important groundwater flow paths in the member. An example of results from a single test show the aquifer is anisotropic (H₀: T_α=T_β rejected at P<0.005) with θ=47.7^o±19.0^o(3σ), T_α/T_β=7.9±5.5(3σ), T_α=218 ±441(3σ) m²/d, and T_β=27±53(3σ) m²/d. The study is a step toward delineation of hydrostructural/hydrogeologic units in the Martinsburg Formation.