Formatted Title
Revisiting the Role of Steeply Dipping Extension Fractures in the Newark Basin to Improve CSMs for Remedial Design
Background/Objectives
The leaky multi-unit aquifer model for the dipping Newark basin sedimentary bedrock in Connecticut, New York, New Jersey, and Pennsylvania emphasizes that the bulk of groundwater is transmitted through bedding-parallel fractures that form discrete tabular aquifers. The model also predicts that groundwater flow will typically be in a direction parallel to bedding strike. This model is generally accepted by state and federal regulators for conducting remedial investigations and designs at Newark basin bedrock sites. The New Jersey Department of Environmental Protection has issued guidance documents with this as the default model for Newark basin bedrock sites. While this model is generally a good starting point for characterizing plumes and groundwater flow on a relatively large scale, we have found significant deviations on a smaller scale that affects remedial source area injection design and performance.
Approach/Activities
Steeply dipping extension fractures are pervasive in the Newark basin and in the early 1980’s they played a significant role in Newark basin bedrock models. In the late 1990s leaky multi-unit aquifer system (LMAS) models began being used to characterize groundwater flow, with the steeply dipping extension fractures being the vertical “leaky” part of LMAS models. We will present a remedial case study in the Passaic formation of New Jersey where on the scale of thousands of feet the LMAS model explains flow directions and VOC contaminant migration. However, we found that locally, on a scale of a few hundred feet, steeply dipping extension fractures exert significant control on horizontal flow direction, sometimes 180 degrees different than predicted by using a LMAS model. Our approach included tracking the migration of injected fluorescent tracers, as well as bioremediation amendments and geochemical changes. These methods were complemented with borehole geophysical logging to build the hydrostratigraphic fracture framework and flow model.
Results/Lessons Learned
We found that in the remedial injection area (source zone) that flow is significantly controlled by a set of steeply-dipping en echelon extension fractures and that flow is not along strike. Lesson learned is that the default LMAS model accepted by regulators does not always apply on a remedial injection scale of hundreds of feet. Some of the extensional fractures show small-scale normal faulting with a few feet of vertical displacement, which has also imparted stratigraphic dips of a few degrees on bedding units. Data supporting our model are derived from rock coring, borehole geophysics and sequence stratigraphy work including lithologic and natural gamma marker beds. We will present our conceptual site model (CSM) using structural geology plan maps, plume and tracer visualizations, cross-sections, and an isometric projection, and relate the updated CSM to the optimization of the in situ bioremediation design.