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U.S. Geological Survey (USGS) research in characterizing fractured rock hydrology in the Mirror Lake watershed

 

IN 1990, the Toxic Substances Hydrology Program of the USGS initiated research activities in the bedrock of the Mirror Lake watershed, which falls nearly entirely within the Hubbard Brook Experimental Forest. 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 including 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.

Other than the multibillion dollar investigations associated with high-level radioactive-waste isolation (which are not particularly relevant to issues of near-surface anthropogeneic contamination), there has been a lack of detailed field investigations in fractured rock. 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. Furthermore, hydraulic conductivity of fractures can vary over many orders of magnitude in contrast to the range associated with unconsolidated geologic media. Also, 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.

Because of the extreme heterogeneity in fractured rock, there is no single method that can explicitly and unambiguously map 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.

Also, at the Mirror Lake site, an infrastructure has been developed 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. In addition, bedrock wells and overburden piezometers have been installed over a 4 square kilometer area for the purpose of investigating regional ground-water flow and chemical migration. Characterization over distances of kilometers is also important in many problems of environmental assessment, because fractures usually have low porosity, and thus have the potential to transport fluid and constituents over large distances in a short period of time. Because of the detailed multidisciplinary investigations 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 in conceptualizing heterogeneity in fractured rock.

More detail on the USGS Toxic Substances Hydrology Program at Mirror Lake can be found at http://toxics.usgs.gov/sites/mirror_page.html

The USGS has also been involved in hydrology and biogeochemistry studies on Mirror Lake in collaboration with Hubbard Brook Ecosystem Study scientists.

Map of Mirror Lake wells