Hydrogeology of the Brunswick (Passaic) Formation and Implications for Ground Water Monitoring Practice

Abstract

Fractured shales of the Brunswick Formation provide a major aquifer in the most industrialized region of New Jersey. Numerous cases of ground water contamination have been documented in this formation. However, effectiveness of monitoring and remediation efforts is often hampered by the use of inappropriate concepts regarding ground water flow controls in this complex aquifer system. One such concept presumes that near‐vertical fractures parallel to the strike of beds provide principal passages for the flow and produce an anisotropic response to pumping stress. Field evidence presented in this paper confirms that the Brunswick Formation hosts a gently dipping, multiunit, leaky aquifer system that consists of thin water‐bearing units and thick intervening aquitards. The water‐bearing units are associated with major bedding partings and/or intensely fractured seams. Layered heterogeneity of such a dipping multiunit aquifer system produces an anisotropic flow pattern with preferential flow along the strike of beds. Within the weathered zone, the permeability of the water‐bearing units can be greatly reduced. The commonly used hydrogeologic model of the Brunswick as a one‐aquifer system, sometimes with vaguely defined “shallow” and “deep” zones, often leads to the development of inadvertent cross‐flows within monitoring wells. If undetected, cross‐flows may promote contaminant spread into deeper units and impair the quality of hydrogeologic data. Hydrogeologic characterization of the Brunswick shales at any given site should be aimed primarily at identification of the major water‐bearing and aquitard units. Recommended techniques for this characterization include fluid logging and other in‐well tests.

Fractured‐Aquifer Hydrogeology from Geophysical Logs; The Passaic Formation, New Jersey

“…Results of a preliminary packer test conducted across Wells 6 and 1 indicate that hydraulic communication across bedding planes is much lower than along bedding planes, implying that these high-angle fractures are not continuous vertically. This interpretation is consistent with the field observations of Houghton (1990) and of Michalski (1990). Consequently, the permeability of the formation appears to be anisotropic, as noted by Vecchioli et al (1969), because T values are much lower across bedding planes than within bedding planes.…”

“…These results indicate that certain intervals have very low permeabilities (both horizontal and vertical) and serve as efficient confining units within the aquifer. They also support the outcrop descriptions regarding the vertically truncated high-angle fractures [Houghton (1990); Michalski (1990)]. From this general hydrogeologic framework defined by fracture type, orientation, and distribution, we infer that the subset of high-angle fractures that is categorized as being transmissive introduces some degree of horizontal-transmissivity anisotropy into the aquifer by creating a preferential flow path along strike.…”

“…Thus, although some high-angle fractures provide significant pathways for fluid transport, they are apparently not extensive in the vertical direction and may only be transmissive in specific rock layers primarily along strike. Houghton (1990) and Michalski (1990) report that near-vertical fractures observed at outcrops traverse coarser grained or otherwise fissile beds and terminate in finer grained mudstone beds.…”

“…The basic formulation assumes the aquifer to be homogeneous, confined, isotropic, and of infinite extent, conditions which are sufficiently met in this environment at a borehole scale of investigation. Individual fractures can be regarded as thin, homogeneous layers that behave as confined units because they tend to be filled near the surface with clay residue from weathering (Michalski, 1990). Slight departures from ideal isotropic conditions at the borehole scale may be expected, particularly when considering steeply dipping fractures of variable extent.…”

Effects of crustal stresses on fluid transport in fractured rock: case studies from northeastern and southwestern USA

“…However, packer test results at the New Jersey site indicate that hydraulic communication across bedding planes is much lower than along bedding planes, implying that most steeply dipping fractures are truncated by bedding and are not Q vR dp dx = ς 3 12 continuous vertically. This interpretation is consistent with the field observations reported by Houghton (1990) and by Michalski (1990). In addition, gradational sequences of mudstones, siltstones, and sandstones are relatively thin (<1 m) in the logged wells at the New Jersey site.…”