Abstract

Investigations of the transport and fate of contaminants in fractured-rock aquifers require knowledge of aquifer hydraulic and transport characteristics to improve prediction of the rate and direction of movement of contaminated ground water. This report describes an approach to estimating hydraulic and transport properties in fractured-rock aquifers; demonstrates the approach at a sedimentary fractured-rock site in the Newark Basin, N.J.; and provides values for hydraulic and trasnport properties at the site. The approach has three components: (1) characterization of the hydrogeologic framework of ground-water flow within the rock-fracture network, (2) estimation of the distribution of hydraulic properties (hydraulic conductivity and storage coefficient) within that framework, and (3) estimation of transport properties (effective porosity and dispersivity). The approach includes alternative with increasingly complex data-collection and analysis techniques.

The local geologic structure of the site, located in Hopewell Township, Mercer County, is dominated by a gently northwest-plunging syncline. Bedding planes in the main part of the site strike approximately east-west and dip to the north. The two dominant fracture sets in the study area are bedding-plane partings and esast-west-striking structural fractures that dip steeply to the south. Transmissive layers correspond to bedding-plane zones and contain bedding-plane separations and near-vertical structural fractures. The transmissive layers are separated by massive rock zones through which water flows vertically at very low rates, apparently through near-vertical fractures. Transmisssive zones were identified using single-well hydraulic tests and water-level data collected under static and pumping conditions. Transmissive zones occur about every 9 meters, on average.

A 9-day, site-scale aquifer test was designed and conducted to test the basic concept of hydrogeologic framework and to estimate the distribution of hydraulic properties. Application of an analytical solution, in which an equivalent homogenous, anisotropic porous medium was assumed, provided estimates of principal values of hydraulic conductivity of 6.4, 0.30, and 0.0043 m/d (meters per day) and specific storage of 9.2 x 10-5 meters-1; the maximum principal direction of hydraulic conductivity was nearly aligned with strike and nearly horizontal in space. A three-dimensional numerical ground-water-flow model with model layers aligned with the bedding planes provided best-fit, average values of hydraulic conductivity of about 7, 3, and 4 x 10-5 m/d for the strike, dip, and normal-to-bedding plane directions, respectively. The numerical model results indicate that heterogeneities and boundary conditions significantly affect estimates of the hydraulic properties.

Three non-recirculating doublet tracer tests were conducted at spacings of 30.5, 91.4, and 183 meters in approximately 40-meter-long open boreholes using a pulsed bromide injection. Longitudinal dispersivity was found to increase with the scale of the experiment, indicating that a minimum scale (spacing) tracer test is required to provide values of transport properties representative of processes on the order of tens to hundreds of meters, the scale of many contaminant plumes. For the tracer tests conducted at the 183-meter spacing, effective porosities of 3.7 x 10-4 to 7.6 x 10-4 and a longitudinal dispersivity of 12.8 meters were obtained using an analytical technqiue. An effective porosity of 1.2 x 10-3 and a longitudinal dispersivity of 12.8 meters were obtained using a two-dimensional numerical solute-transport model. An effective porosity of 1.4 x 10-3 was estimated from tracer-test data using particle-tracking methods and the three-dimensional numerical flow model used to interpret the site-scale aquifer test.

The hydraulic and tracer tests were successfully evaluated using the approach presented, including mathematical models developed for porous-media applications. This success indicates that flow and transport through fractured sedimentary rocks such as those in the Newark Basin can be simulated as flow and transport through an equivalent porous medium at the scales considered in this study.

Posted August 2009