Directional hydraulic behavior of a fractured-shale aquifer in New Jersey

Abstract

The principal source of ground water throughout a large part of central and northeastern New Jersey is the aquifer in the Brunswick Shale -- the youngest unity of the Newark Group of Triassic Age. Large-diameter public-supply and industrial wells tapping the Brunswick Shale commonly yield several hundred gallons per minute each. Virtually all ground water in this aquifer occurs in interconnecting fractures; the formation has practically no effective primary porosity. Numerous pumping tests have shown that the aquifer exhibits directional, rather than isotropic, hydraulic behavior. Water levels in wells alined along the strike of the formation show greater magnitude of interference than those in wells alined in transverse directions. Drawdown data evaluated by standard time-drawdown methods indicate computed coefficient of transmissibility in all cases is least in the direction of strike. Because of the distribution of observation wells available for the tests, distance-drawdown methods of evaluation could be used in only one instance -- for just one direction; the computed coefficient compared favorably with that calculated from the time-drawdown method. Computed values of transmissibility may be unreliable owing to the departure of the aquifer from the ideal model. It is even possible that the direction of minimum computed transmissiblity is actually indicative of the alinement of fractures with the greatest permeability. However, the relation of the directional behavior to the structure of the formation has practical significance when locating the new wells near existing wells. Well interference can be greatly minimized, generally, by alining wells perpendicular to the strike.