Environmental Health - Toxic Substances
U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C
Integrating Multidisciplinary Investigations in the Characterization of Fractured Rock
By Allen M. Shapiro, Paul A. Hsieh, F. Peter Haeni
Because site conditions and project objectives vary from site to site, there is no single approach to the characterization of fractured rock for problems of environmental assessment. There is, however, a unifying theme that is consistent in problems of site characterization; that is, most problems depend to a great extent on advective ground-water movement. Once the spatially heterogeneous hydraulic properties of the bedrock are conceptualized to define advective ground-water flow in the detail consistent with project objectives, other physical, chemical, and biological processes of interest can then be the focal point of characterization. Without first characterizing advective ground-water movement, it is likely that ambiguous and perhaps erroneous interpretations of complex physical, chemical, and biological processes will result. Because of the extreme heterogeneity that is anticipated in fractured rock, no single method of characterization can explicitly and unambiguously map the spatial distribution of hydraulic properties that control advective fluid movement. The integration of geologic, geophysical, hydrologic and geochemical information is a necessity in developing a defensible model of heterogeneity in fractured rock. A multiple-method hierarchial approach to characterization in fractured rock is presented, where in regional geologic and hydrologic information is first synthesized to act as a background to more detailed site-specific reconnaissance using fracture network mapping and surface geophysics. These noninvasive reconnaissance techniques can be used to observe or infer the location and orientation of fractures and fracture zones and fracture properties that are critical in siting bedrock boreholes in which to conduct in situ investigations using single- and multiple-borehole techniques. Single-hole techniques that can be used to characterize heterogeneity in the bedrock include conventional geophysical logging, borehole scanning, borehole flow, borehole radar, hydraulic testing, and chemical sampling. Multiple-borehole methods, such as seismic and radar tomography, borehole flow, hydraulic and tracer testing, are conducted to infer the connectivity and spatial extent of transmissive fractures. Although the characterization methods summarized in this article can be conducted independently, the synthesis of information from multiple characterization methods is the only means of developing an accurate understanding of heterogeneity in bedrock.