A HYDRAULIC TOMOGRAPHY EXPERIMENT IN CONTAMINATED FRACTURED SEDIMENTARY ROCKS, NEWARK BASIN, NEW JERSEY, USA

In contaminated fractured rocks, fine-scale characterization of the hydraulic conductivity (K) distribution is a critical step towards fully understanding contaminant transport behavior and designing remediation strategies. It is important to characterize K variability within the less-fractured rock matrix as well as across the network of higher-permeability fractures. To this end, a hydraulic tomography experiment was performed in July 2015 in contaminated fractured mudstones in the Newark Basin near Trenton, NJ, with the goal of estimating the K distribution of the rocks at fine scale. The spatial arrangement of seven existing wells (in a circle of 9 m radius with one central well), the use of packers to divide the wells into multiple monitoring intervals, and the deployment of fiber optic pressure transducers enabled collection of a hydraulic tomography dataset comprising high-resolution drawdown observations at an unprecedented level of spatial detail for fractured rocks. The experiment involved 45-minute cross-hole aquifer tests, conducted by pumping from a given packer-isolated well interval and continuously monitoring drawdowns in all other well intervals. The collective set of drawdown data from all tests and intervals displays a wide range of behavior suggestive of highly heterogeneous K within the tested volume, such as: drawdown curves for different well intervals crossing one another on drawdown-time plots; variable drawdown curve shapes; variable order and magnitude of time-lag and/or drawdown for intervals of a given well in response to pumping from similar fractures or stratigraphic units in different wells; and groups of well intervals that show similar responses for different pumping tests. Preliminary assessment of these data, together with a rich set of geophysical logs, suggests an initial conceptual model that includes densely distributed fractures of moderate K at the shallowest depths of the tested volume, connected high-K bedding-plane-parting fractures at intermediate depths, and sparse low-K fractures in the deeper rocks. Future work will involve tomographic inversion of the data to estimate the K distribution at a scale of ~1 m³ in the upper two-thirds of the investigated volume, where observation density is greatest.