Horizontal Anisotropy Determined by Pumping in Two Powder River Basin Coal Aquifers, Montana

Abstract

ABSTRACT Hydraulic anisotropic behavior was determined for the Sawyer‐A and Anderson coal aquifers (Tongue River Member of Paleocene Fort Union Formation) by five‐well and four‐well aquifer tests, respectively. The average maximum hydraulic conductivity of 0.65 m/d (2.13 ft/d) for the Sawyer‐A coal aquifer bears N. 85° E. The average minimum hydraulic conductivity, which is perpendicular to the maximum, is 0.26 m/d (0.85 ft/d). Aquifer heterogeneity in the form of poorly developed coal cleat is believed to be responsible for a wide range of principal‐axis orientations determined by a five‐well test performed on the Sawyer‐A coal aquifer. The maximum hydraulic conductivity of the Anderson coal aquifer is 0.20 m/d (0.66 ft/d) and bears N. 32° W. The minimum hydraulic conductivity is 0.07 m/d (0.23 ft/d). When coal cleat orientation and development are unknown, a five‐well aquifer test proves more beneficial than a four‐well test in analyzing the reliability of the results. Well storage and aquifer dewatering affected the drawdowns of the pumped well during both tests. However, such effects were minimal, as indicated by drawdowns in observation wells at a distance as small as 13m (44 ft) from the pumped well. Average hydraulic conductivity determined from drawdowns observed at the pumped wells alone were lower than average values determined from a multiple‐well analysis.