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CHARACTERIZING FRACTURED ROCK AQUIFERS USING RADAR, TRACER AND HYDRAULIC DATA

F. D. Day-Lewis
Dept. of Geology, Bucknell University, Lewisburg, PA 17837, USA

J. W. Lane, Jr.
U.S. Geological Survey, 11 Sherman Place, U-5015, Storrs Mansfield, CT 06269 USA

P. A. Hsieh
U.S. Geological Survey, Bldg 15, McKelvey Building, 345 Middlefield Road, MS 496, Menlo Park, CA 94025, USA

S. M. Gorelick
Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA

INTRODUCTION

Aquifer characterization in fractured-rock settings is difficult because measurements of hydraulicproperties are local and sparse, permeability varies by orders of magnitude over short distances, and the three-dimensional configuration of transmissive fractures and fracture zones is complex. Innovative strategies for the improved interpretation and combination of available data are needed to facilitate the development of accurate predictive models of ground-water flow and solute transport. To this end, we present and demonstrate approaches to (a) incorporate hydraulic-connection data into a geostatistical simulation procedure, and (b) analyze cross-borehole radar data collected during saline tracer tests. We use experimental data collected at the FSE well field at the U.S. Geological Survey Fractured-Rock Hydrology Research Site near Mirror Lake, in Grafton County, New Hampshire (fig. 1).

The FSE well field is a 120 by 80-meter (m) area consisting of 13 wells. The bedrock at the site is schist intruded by granite, pegmatite and lamprophyre. The overburden consists of about 20 m of glacial deposits. Based on results of single- and multiple-well hydraulic tests, Hsieh and Shapiro (1996) identified four high transmissivity (high-T) zones in the bedrock underlying the well field. Borehole intervals isolated by hydraulic packers exhibit similar drawdown responses if they are connected by a high-T, whereas intervals not connected by a high- T zone show very different drawdown response. Hsieh and Shapiro (1996) developed a conceptual model of heterogeneity at the site (fig. 2), in which high-T zones consist of a number of connected, highly transmissive fractures that are embedded within a surrounding network of less transmissive fractures. To analyze the hydraulic test data, Hsieh et al. (1999) constructed and calibrated a deterministic ground-water flow model. Day-Lewis et al. (2000) presented a geostatistical approach to generate alternative models consistent with inferred hydraulic connections, and identified several models that approximate the field data.

Day-Lewis et al. (2000) and Hsieh et al. (1999) demonstrated that consideration of tabular high-T features could explain hydraulic-test data from the FSE well field. However, to explain tracer data collected at the site, a more detailed description of heterogeneity is necessary. One approach to identifying preferential flow paths in fractured rock is to combine radar tomography and saline tracer tests (e.g., Olsson et al., 1991). The presence of electrically conductive saline tracer illuminates fractures or high-permeability pathways for tomographic imaging.

In this extended abstract, we review the results of Day-Lewis et al. (2000) and present a combined interpretation of experimental difference-attenuation and tracer-test data collected at the FSE well field. Taken together, these two studies provide insight into several scales of heterogeneity at thesite, and the different data requirements necessary to reproduce (a) hydraulic data and (b) tracer data.

Final copy as submitted to Fractured Rock 2001 for publication as: Day-Lewis, F.D., Lane, J.W., Jr., Hsieh, P.A., and Gorelick, S.M., 2001, Characterization of fractured-rock aquifers using radar, tracer, and hydraulic data: in Fractured Rock 2001 Conference, Proceedings, Toronto, Ontario, March 26-28, 2001, CD-ROM.

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