Table of Contents

Problem
Background
Objectives
Approach
Anticipated Results
Selected References

PROBLEM:

Bedrock formations, like those in New England, have a dominant secondary porosity because of discontinuities such as fractures, joints, and faults. These discontinuities in the rock, collectively referred to as fractures, transmit water more readily than the surrounding solid rock. The fractures represent channels or avenues of high permeability relative to the low permeability matrix. They have the ability to transport fluid and contaminants over relatively large distances very rapidly, which could result in extensive contamination of ground water and surface water.

The usefulness of models to predict fluid movement and chemical transport in heterogeneous fractured bedrock is constrained by our limited understanding of fracture systems and by the difficulty and expense of physically identifying rock types and fractures underground. Information on bedrock lithology, fractures and structure are needed to evaluate fluid movement and chemical transport and to quantify ground-water flow between the unconsolidated glacial deposits and bedrock. By developing methods that incorporate information on the occurrence of fractures, tectonic history, structure, and lithology, interpretive ground-water-flow investigations can be improved.

BACKGROUND:

Research of the flow in fractured rock of the Mirror Lake area was initiated in 1990 by the USGS Toxic Substance Hydrology Program. The study area is in the Pemigewasset River Valley and surrounding uplands near the towns of Woodstock and Thornton, in Grafton County, New Hampshire. The entire study area is approximately 2 square kilometers in size. Part of the study area is within the Hubbard Brook Experimental Forest in the White Mountain Region of New Hampshire. The purpose of this ongoing research is to develop methods to detect and describe fractures and to characterize how water and dissolved chemicals are transmitted through these fractures. This interdisciplinary research project is comprised of 14 research initiatives that make use of geologic, geochemical, geophysical and hydrologic methods. Collaborative studies are being conducted by scientists from the USGS, universities, and research institutes. An overview of the research project is provided by Shapiro and Hsieh (1991). Descriptions of geologic, hydraulic, geophysical, and geochemical methods used in this investigation is provided by Hsieh and others (1993).

This research initiative focuses on characterizing the lithology and fractures in the subsurface and evaluating the geologic controls on ground-water flow. Bedrock in the Mirror Lake Area in Grafton county, New Hampshire is characterized by schists of Silurian to Devonian age, which were generally metamorphosed to sillimanite grade during the Acadian orogeny. The bedrock is predominantly pelitic schists and gneisses that have been complexly folded, intruded by anatectic granites, pegmatites, and basalts (Lyons and others, 1986), and fractured. Aquifers in crystalline bedrock consist of a complex network of interconnected fractures that have developed in response to local and tectonic stresses. The geometry of fracture networks and openness of individual fractures control flow of fluids and the advection, dispersion, and storage of solutes.

OBJECTIVES:

The primary objectives of this research initiative are to:

• provide a site characterization of the subsurface based on borehole data in the Mirror Lake area
  
• develop techniques and interpretive methods for detecting and describing fractures and lithology in the subsurface bedrock for the purpose of characterizing fluid flow and chemical transport in fractured rock
• determine the interrelations between characteristics of fractures, lithology, and hydraulic properties of bedrock and provide an assessment of geologic controls on ground-water movement and the transport of dissolved chemicals
• evaluate the effect of scale on these interrelations and the implications for regional ground-water flow
• evaluate transferability of findings and techniques to other sites
• incorporate geologic information into hydrologic modeling.

Approximately 97 percent of the bedrock is covered with discontinuous layers of glacial deposits. Bedrock exposures are generally limited to the stream beds, ridges, and outcrops exposed in highway excavations. Hence, the characterization of the rock types and fractures relies on subsurface exploratory drilling and other geophysical techniques. Forty bedrock wells, ranging in depth from 60 to 305 meters (m), were drilled from 1979 through 1995 by use of the percussion rotary method. A submersible color video camera was lowered into the wells to visually survey the walls of the wells and improve the interpretation of the subsurface. The video images were used to describe texture, grain-size, color, contacts of rock types, occurrence and description of fractures, foliation, folds, and faults, as well as the condition of the borehole wall. Borehole images provide a direct verification and exact location of contacts between rock types and the location of fractures (Johnson, 1994). Solid bedrock core was obtained at 3 of the 40 wells to obtain representative subsurface samples of rock units and fracture surfaces. The core samples were compared to fracture and lithology interpretations based on standard borehole geophysical and borehole imaging methods.

Submersible cameras were used in all boreholes to characterize the subsurface that was observed in the boreholes for two purposes: (1) to provide a detailed site characterization of the Mirror Lake site and (2) to evaluate field techniques and optical imaging tools. The video images along with drill cuttings (rock fragments that were sampled in the process of drilling the well) were used to construct detailed logs of the boreholes. Click on the image below to see a full size version of an example log.

The most sophisticated down-hole cameras, such as one manufactured by Raax¹, provide an oriented and digitized 360-degree view of the borehole wall. The image can be viewed as an unrolled, flat image that shows the depth along the vertical axis and magnetic direction along the horizontal axis. The orientation (strike and dip) of planar features, such as fractures or lithologic contacts, can be determined from the image. Also the image can be "rolled" into a virtual core. ¹ Use of tradenames is for descriptive purposes and does not constitute an endorsement by the USGS.

The results of core logs and various optical and acoustic imaging tools will be compared for purposes of improving the interpretation of image logs and evaluating the effectiveness of the tools. For a more detailed description of borehole geophysical tools click here.

Statistical analyses will be performed in order to draw conclusions on the distribution of fractures and hydrogeologic factors affecting ground-water flow. The following questions will be addressed:

• What is the vertical distribution of fractures observed in the wells? Is there a correlation between fracturing and rock type? Or is there a relation between fracturing and depth? How does the vertical distribution compare to fracture distributions in surface exposures?
• Is there a correlation between measured hydraulic conductivity and physical factors such as lithology, alteration of the host rock, elevation, depth below land surface, depth below bedrock surface, structural setting, topographic setting, or proximity to identified regional lineaments.
• Is there a correlation between fracture density determined from geophysical and lithologic mapping in boreholes and measured hydraulic conductivity? Is there a preferred orientation to the conductive fractures?

ANTICIPATED RESULTS:

The statistical analyses will yield insight into the fracture domain and allow us to draw conclusions about conceptual fracture systems. These findings can be used to constrain, or improve, models of ground-water flow. It is anticipated that the knowledge gained in performing the statistical analyses can be applied to similar fractured rock environments, by identifying which parameters have the most affect on bedrock aquifers.

A methodology for incorporating geologic factors to flow and transport studies will be developed for application to other fractured-rock research sites. The purpose of this research is to provide guidance for future hydrogeologic characterizations in similar crystalline rock regimes in support of protection and remediation programs for toxic substances in bedrock aquifers.

SELECTED REFERENCES

Hsieh, P.A. and Shapiro, A.M., 1994, Hydraulic characteristics of fractured bedrock underlying the FSE well field at the Mirror Lake site, Grafton County, New Hampshire, in Morganwalp, D.W. and Aronson, D.A., eds., U.S. Geological Survey Toxic Substance Hydrology Program-- Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4015.

Hsieh, P.A., Shapiro, A.M., Barton, C.C., Haeni, F. P., Johnson, C.D., Martin, C.W., Paillet, F.L., Winter, T.C., and Wright, D.L., 1993, Methods of characterizing fluid movement and chemical transport in fractured rock, in Chaney, J.T., and Hepburn, J.C., eds., 1993, Field trip guidebook for Northeastern United States, Geological Society of America, Annual Meeting, Boston, Massa., October 25-28, 1993, p. R1-29.

Johnson, C.D., 1994, Use of a borehole color video camera to identify lithologies, fractures, and borehole conditions in bedrock wells in the Mirror Lake Area, Grafton County, New Hampshire, in Morganwalp, D.W. and Aronson, D.A., eds., U.S. Geological Survey Toxic Substance Hydrology Program-- Proceedings of the Technical Meeting, Colorado Springs, Colo., September 20-24, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4015.

Lyons, J.B., Bothner, W.A., Moench, R.A. and Thompson, J.B. Jr., eds, 1986, Interim geologic map of New Hampshire: Concord, N.H., New Hampshire Department of Resources and Economic Development, Open-File Report 86-1, 1 sheet, scale 1:250,000.

Shapiro, A.M., and Hsieh, P.A., 1991, Research in fractured rock hydrogeology: Characterizing fluid movement and chemical transport in fractured rock at the Mirror Lake drainage basin, New Hampshire, in Mallard,G.E., and Aronson,D.A., eds., Toxic Substances Hydrology Program Proceedings of Technical Meeting, Monterey, Calif. March 11-15, 1991: U.S.Geological Survey Water-Resources Investigations Report 91-4034, p. 155-161.

Winter, T.C., 1984, Geohydrologic setting of Mirror Lake, West Thornton, New Hampshire: U. S. Geological Survey Water-Resources Investigations Report 84-4266, 61 p.

For further information, contact:

Tom J. Mack
U.S. Geological Survey
Water Resources Division
361 Commerce Way
Pembroke, NH 03275

(603)-226-7800

Related USGS links:

Geology -- Water -- Mapping