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.
APPROACH:
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?
Lithologic, fracture, and hydraulic
data, and the results of statistical analyses described above, will be used
to conceptualize fluid movement in bedrock. These data and statistical parameters
will be applied to models (deterministic and stochastic) with the intent
of answering the general question: Can we use the results of the statistical
ananlyses to improve flow and transport models on small or regional scales?
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
5/30/00