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
Research in Characterizing Fractured Rock Aquifers
Other than the multibillion-dollar investigations associated with high-level radioactive-waste isolation (which are not particularly relevant to issues of near-surface anthropogenic contamination), there has been a lack of detailed field investigations in fractured rock aquifers. Addressing issues of contamination in fractured rock also has been hindered by the fact that methods of site characterization applied in unconsolidated near-surface deposits are not necessarily applicable to highly heterogeneous bedrock environments. In fractured rock, geologic structure controls the occurrence of fractures, which are the predominant mechanism for fluid movement. No formation is uniformly fractured, and thus, assumptions of formation homogeneity and even anisotropy that are commonly applied in unconsolidated porous media may not be appropriate for the description of fluid movement in fractured rock. Hydraulic conductivity of fractures can vary over many orders of magnitude in contrast to the range associated with unconsolidated geologic media. Furthermore, because of complex geologic structures and fracture connectivity, hydraulic properties of fractured rock do not vary smoothly in space. It is not uncommon to observe abrupt spatial changes in the hydraulic properties in fractured rock with both depth and areal extent.
In 1990, the Toxic Substances Hydrology Program initiated research activities in the bedrock of the Mirror Lake watershed and its vicinity in central New Hampshire (figure 1). The Mirror Lake watershed falls nearly entirely within the Hubbard Brook Experimental Forest, which is a site for long-term ecosystem studies administered by the Forest Service (U.S. Department of Agriculture). The site is characterized by fractured metamorphic and igneous rocks overlain by a thin veneer of glacial drift. Although the site is uncontaminated, research has focused on developing and testing field techniques and interpretive methods of characterizing the properties of bedrock that affect ground-water flow and chemical migration. The intention is to transfer these techniques to other bedrock terrain and sites of contamination. The basic premise of the investigations at the Mirror Lake site has been that a knowledge of fluid movement and nonreactive transport must be a precursor to the characterization of the more vexing issues associated with the fate of toxic substances in heterogeneous subsurface environments.
Because of the extreme heterogeneity in fractured rock, there is no single method that can map explicitly and unambiguously the spatial distribution of hydraulic properties that control fluid movement and chemical migration. Investigations at the Mirror Lake site have focused on integrating interpretations from geologic and fracture mapping, surface and borehole geophysics, hydrologic testing and geochemical and isotopic methods. Although the individual characterization methods developed and tested at the Mirror Lake site can stand alone, the synthesis of information from multiple characterization methods is the only means of developing a defensible conceptual understanding of heterogeneity in bedrock terrain.
In addition, an infrastructure has been developed at the Mirror Lake site to investigate ground-water flow and chemical migration over distances from meters to kilometers. Two well clusters referred to as the FSE and CO well fields have been constructed for detailed investigations in bedrock over distances up to 100 meters (figure 1). In addition, bedrock wells and overburden piezometers have been installed over a 4 square kilometer area for investigating regional ground-water flow and chemical migration. Characterization over distances of kilometers is also important in many problems of environmental conducted to identify geologic features, fractures and hydraulic and transport properties of the bedrock over distances from meters to kilometers, the data and infrastructure of the Mirror Lake site make it a field-research laboratory that can be used to test new site-characterization tools, field techniques, and interpretive approaches to conceptualizing heterogeneity in fractured rock.
For additional information contact:
Allen M. Shapiro,