U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September
20-24, 1993, Water-Resources Investigations Report 94-4015
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Overview of Research on Use of Hydrologic, Geophysical, and Geochemical
Methods to Characterize Flow and Chemical Transport in Fractured
Rock at the Mirror Lake Site, New Hampshire
by
Allen M. Shapiro (U.S. Geological Survey, Reston, Va.) and Paul
A. Hsieh (U.S. Geological Survey, Menlo Park, Calif.)
CONTENTS
Abstract
Research efforts at the Mirror Lake Toxic-Substances Hydrology Program
research site in Grafton County, New Hampshire have focused on the
development of equipment, field testing procedures, and interdisciplinary
interpretive approaches of characterizing ground-water flow and chemical
transport in fractured rock over distances that range from tens of
meters to kilometers. This paper summarizes the results of current
research in bedrock hydrogeology in local- and regional-scale investigations
at the Mirror Lake site. Local-scale investigations are conducted
over distances of tens of meters and focus on identification of (1)
fractures and fracture properties on exposed surfaces, (2) fractures
in the subsurface using borehole and surface geophysics, and (3) hydraulic
and transport properties of fractures by means of hydrologic testing.
In regional-scale investigations, controlled hydrologic testing cannot
be conducted to identify hydraulic and transport properties of the
bedrock. Regional-scale investigations have focused on (1) methods
of collecting hydrologic and geochemical information in heterogeneous
bedrock environments, and (2) interdisciplinary methods of synthesizing
these data by ground-water-flow and chemical-transport modeling to
infer hydraulic and transport properties of the bedrock.
INTRODUCTION
The U.S. Geological Survey (USGS) Mirror Lake Toxic-Substances Hydrology
Program research site is located in Grafton County, at the southern
extent of the White Mountains in central New Hampshire (fig. 1). The
Mirror Lake site was selected for investigations in fractured rock
as a part of a nationwide program to describe the fate of contaminants
in subsurface environments. Although there are no known contaminants
in ground water at the Mirror Lake site, the equipment, field techniques
and interdisciplinary interpretive methods that are being developed
in this study area are directly transferable to other fractured-rock
sites where ground-water has been contaminated. Research at the Mirror
Lake site is focused on characterizing ground-water flow and chemical
transport in fractured rock over distances of tens of meters to kilometers.
The purpose of this paper is to review the current research being
conducted at the Mirror Lake site. An earlier summary of bedrock research
activities at the Mirror Lake site is in Shapiro and Hsieh (1991).
SITE DESCRIPTION
Mirror Lake is located at the lower end of the Hubbard Brook valley
in the southern part of the White Mountains of central New Hampshire
(fig. 1). Hubbard Brook, which drains the valley, flows into the Pemigewasset
River. The topography of the area generally is steep along mountain
sides and flat along the Pemigewasset River. Although ground-water
flow extends beyond the surface-water drainage basin associated with
Mirror Lake, regional field investigations have concentrated on the
Mirror Lake drainage basin. The Mirror Lake drainage basin occupies
85 ha. Land-surface elevations range from 213 m above sea level at
the lake surface to 481 m at the top of the drainage divide. Three
perennial streams flow into Mirror Lake, and Mirror Lake drains through
an outlet stream into Hubbard Brook.
![shapiro2.final.fig1b](shapiro2.final.fig1b.gif)
Figure 1. Location of the Mirror Lake watershed
in New Hampshire and the location of bedrock wells in the vicinity of Mirror
Lake. (33k)
Glacial drift overlies the bedrock in much of the Mirror Lake area.
The drift thickness ranges from 0 to 55 m and consists mainly of silty
and sandy till, with numerous cobbles and boulders. Kame terraces,
consisting of fine- to coarse-grained sandy ice-contact deposits,
are found at several levels on the mountain sides. In the area between
Mirror Lake and Hubbard Brook, the silty sand and gravel form what
is believed to be a delta-type deposit laid down when Hubbard Brook
adjusted its grade to the Pemigewasset River during deglaciation.
Mirror Lake is believed to be a kettle lake overlying a bedrock saddle,
where bedrock valleys descend to the north and south. The ridge north
of the lake is a moraine overlying the northern bedrock valley, where
drift thickness is as much as 55 m.
The bedrock in the Mirror Lake area is a Silurian sillimanite-grade
schist that has been extensively intruded by late Devonian granite
(C.C. Barton, U.S. Geological Survey, written commun., 1993). The
schist and granite are cut by dikes of pegmatite of unknown age, possibly
a residual differentiate of the granitic intrusions. All three rocks
are cut by lamprophyre dikes, whose age is believed to be middle Jurassic
and early Cretaceous. At a road cut along Interstate Highway 93, directly
east of Mirror Lake, the exposed bedrock consists of a complex suite
of rock types. The granitic intrusions are present as dikes, irregular
pods, and anastomosing fingers that range in width from centimeters
to tens of meters. Pegmatite typically is in the form of dikes, centimeters
to meters wide, that cross-cut both the schist and the granite. The
lamprophyre dikes, having widths of centimeters to meters, are less
common than the other three rock types in the Mirror Lake area.
All components of the hydrologic system in the Mirror Lake drainage
basin are being monitored. Flow of all streams leading to Mirror Lake
are measured with flumes having continuous recorders; precipitation
is measured at two locations in the drainage basin; climatological
information for energy budget and evaporation studies have been collected
since 1983; water levels in bedrock wells, drift piezometers, and
water-table wells are continuously recorded (Winter, 1984, 1985).
Bedrock wells have been drilled in two clusters, referred to as the
FSE and CO well fields (fig. 1). Bedrock wells also have been areally
distributed in the vicinity of Mirror Lake. In early 1993, there were
13 bedrock wells in the FSE well field, 4 bedrock wells in the CO
well field and 12 bedrock wells areally distributed in the Mirror
Lake area. Depths of bedrock wells range from 60 to 230 m. At most
bedrock-well sites, piezometers are installed at one or more locations
in the saturated drift above the bedrock surface. Water-table wells,
whose screens intersect the water table, have been installed throughout
the watershed and at most bedrock-well sites.
Ground-water flow in the Mirror Lake area is typical of mountain-valley
terranes of the New England uplands. Water from precipitation and
snowmelt infiltrates into the subsurface, flows through the glacial
drift and fractured bedrock, and discharges into streams, lakes and
rivers. Estimates of recharge to ground water vary from 0.2 m/yr (meters
per year) to 0.45 m/yr (T. C. Winter, U.S. Geological Survey, oral
commun., 1993). The major features controlling ground-water flow in
the Mirror Lake area are (1) the steep slope of the mountain sides,
(2) the low hydraulic conductivity of the drift and the bedrock, and
(3) the presence of surface-water bodies such as streams, Mirror Lake
and the Pemigewasset River. In most of the Mirror Lake watershed,
the water table is close to land surface, typically less than 10 m
below land surface. The only exception is the moraine ridge north
of Mirror Lake where depth to water table exceeds 20 m. The presence
of a shallow water table suggests that local topography affects the
local pattern of ground-water flow.
Hydraulic conductivities of the glacial drift and bedrock
in the Mirror Lake area have been estimated from results
of hydraulic tests. Based on slug tests in piezometers,
the hydraulic conductivity of the till is estimated to
range between 10-6 m/s (meters per second) and 10-5
m/s (Wilson, 1991). Hydraulic conductivity of the sands
in the kame terraces is approximately 10 times higher than
the till. The results of single-hole, straddle-packer tests
using 5-m long test intervals indicate that the bedrock
hydraulic conductivity is highly heterogeneous, varying
over many orders of magnitude (Hsieh and Shapiro, 1996).
The average hydraulic conductivity over the upper 100 m
of the bedrock is 3x10-7 m/s. Single-hole tests, however,
sample the rock in a small region around the bedrock well
and do not necessarily represent the hydraulic conductivity
controlling regional flow through the bedrock.
CURRENT RESEARCH
Current research at the Mirror Lake site is focused on local- and
regional-scale investigations of ground-water flow and chemical transport
in fractured rock. The local-scale investigations are conducted over
distances of tens of meters and focus on identifying (1) fractures
and fracture properties on exposed surfaces, (2) fractures in the
subsurface using borehole and surface geophysics, and (3) hydraulic
and transport properties of fractures through hydrologic testing (Hsieh
and others, 1993). When applied individually, these methods generally
yield limited information on fracture characteristics. However, when
applied together, these methods become powerful tools that enable
an interdisciplinary approach to the study of ground-water flow and
chemical transport in fractured rock. In regional-scale investigations,
controlled hydrologic testing cannot be conducted to identify hydraulic
and transport properties of the bedrock. Hydraulic and transport properties
must be inferred from hydrologic, geologic and geochemical information.
Regional-scale investigations at the Mirror Lake site have focused
on (1) methods of collecting hydrologic and geochemical information
in heterogeneous bedrock terranes, and (2) the synthesis of this information
by means of ground-water-flow and chemical-transport modeling.
Local-Scale Investigations
The principle problem encountered in characterizing ground-water
flow and chemical transport in fractured rock is the physical identification
and mathematical characterization of the heterogeneous hydraulic properties
of the rock. Fractures are the conduits for fluid movement in the
subsurface; therefore, identification of their location, interconnectivity
and hydraulic properties is paramount in the characterization of ground-water
flow and chemical transport over distances of tens of meters. Hence,
a major research effort at the Mirror Lake site is the investigation
of geometrical properties of fractures. Understanding the geometric
properties of fractures provides a basis for making inferences and
predictions of flow and transport in the subsurface. Barton (1996)
conducted an extensive investigation of fracture properties and rock
types on four vertical highway road cuts and one horizontal glacial
pavement. Rock types were delineated, and fractures with trace lengths
greater than 1 m were mapped for orientation, surface roughness, aperture,
trace length, connectivity and signs of mineralization. From this
investigation, the granite was shown to be more heavily fractured
and to have shorter and more planar fractures than schist, whereas
fractures in schist have greater fracture roughness than those in
the granite. In addition, the connectivity of fractures in the granite
and schist was poor in comparison to studies of fracture connectivity
in other areas of the country. This investigation indicates that fluid
moves through highly tortuous paths in the bedrock.
Fracture mapping is conducted only on exposed surfaces. Research
on characterizing fractures in the subsurface is being conducted by
means of surface- and borehole-geophysical surveys. Geophysical surveys
generally yield an inferred distribution of a specific rock-mass property
from which the presence of fractures must be inferred. Anomalies in
the rock-mass properties attributed to fractures, however, also could
be the result of variations in lithology; therefore, the correlation
of several geophysical techniques as well as geologic information
and hydrologic testing must be used to characterize the geometry of
fractures in the subsurface.
Lieblich and others (1991) and Haeni and others (1993, 1996) discuss
results of surface-geophysical surveys using azimuthal-seismic refraction
and azimuthal-DC resistivity to detect the orientation of steeply
dipping fractures. These methods infer the strike of a dominant, saturated,
steeply dipping set of fractures from the azimuthal variation in the
seismic velocity and the apparent resistivity of the rock, respectively.
Azimuthal seismic-refraction data collected near the CO well field
indicate that the principle strike of fractures is approximately N.
22° E.--an orientation similar to that of the principle orientation
of fractures mapped on the road cut east of Mirror Lake. The principle
strike of fractures on the road cut is approximately N. 30° E.
Results from azimuthal-DC resistivity using a square array were conducted
at the same site and yielded a similar orientation for the principle
strike of a steeply dipping set of fractures. An azimuthal-DC resistivity
survey using a linear array also was conducted at the same site. This
survey indicated a principle fracture strike of N. 352° E. The
linear array requires a large current-electrode spacing that may have
emphasized environmental interferences such as a buried telephone
cable. Also, voltage-electrode spacing should be large relative to
the fracture spacing--a condition that may not have been satisfied
at this site. The interpretation of these results indicates that multiple
geophysical methods must be used to obtain an overall interpretation
of fracture orientation (Lieblich and others, 1991).
The results of surface-exposure mapping indicated the importance
of identifying distributions of subsurface rock types and fractures
in characterizing ground-water flow. Techniques for identifying fractures
and rock types in bedrock wells for hydrologic investigations are
discussed by Johnson (1996) and Paillet (1996). Johnson (1996) describes
the use of a borehole-color television camera to identify rock types,
fractures and other conditions in the borehole, including mineralization
in and near fracture surfaces. The borehole camera reduces the need
to core bedrock wells to identify the distribution of rock type. Paillet
(1996) discusses a systematic approach to the characterization of
fractures in boreholes using a combination of standard geophysical
logging tools that identify the general structure and lithology of
the rock, and advanced borehole geophysical techniques that identify
fractures and their potential for conducting fluid. The acoustic televiewer
log shows the location and orientation of fractures intersecting boreholes.
A sensitive heat-pulse flowmeter measures the fluid velocity in the
borehole. When used under ambient hydraulic conditions in the borehole,
permeable fractures having different hydraulic heads in the borehole
can be identified. When used in conjuction with pumping in the borehole,
the location of the most permeable fractures can be identified.
Results from the heat-pulse flowmeter can also be used to infer fractures
forming hydraulic connections between adjacent boreholes (Paillet,
1996). By pumping one bedrock well and measuring the fluid velocity
in an adjacent bedrock well, the location of fractures responding
to the hydraulic stress can be identified. Paillet and others (1992)
interpreted the transient velocity response to identify hydraulic
properties of fractures and propose conceptual models of fracture
connectivity between boreholes
The acoustic televiewer log and borehole television camera can identify
fractures at the face of the borehole wall including their strike
and dip. The distance these fractures extend into the rock and their
interconnectivity with other fractures cannot be identified by these
borehole methods. A major research effort at the Mirror Lake site
has been the evaluation of geophysical methods that image fractures
away from boreholes. Three methods of characterizing fractures in
the rock mass have been applied at the FSE and CO well fields at the
Mirror Lake site: single-hole, directional electromagnetic (EM) imagining;
EM tomography; and seismic tomography.
Haeni and others (1993) discuss results of using a single-hole, directional
EM tool that measures reflections of electromagnetic wave energy radiated
from the transmitter in the tool. Water-filled fractures in the vicinity
of a borehole, and fractures intersecting the borehole can be inferred
from the reflected signal. The results of using this investigative
tool in the CO well field showed that the same fracture zone could
be detected from different boreholes, and the lateral extent and orientation
of fractures could be inferred.
Seismic and EM tomography also infer the location of fractures in
the rock mass away from borehole walls. Unlike the single-hole directional
tool, where the transmitter and receiver are located in the same borehole,
EM tomography is conducted by radiating electromagnetic energy from
one borehole and monitoring its reception at an adjacent borehole
(Wright and others, 1996a,b). The transmitted energy is delayed by
the propagation distance and by the different dielectric properties
of the water and rock matrix. By collecting data of sufficient density
at a variety of angles (between the source and receiver) over the
length of the boreholes, the spatial distribution of dielectric properties
in the rock volume between the boreholes can be inferred by an inversion
algorithm. Wright and others (1996a) also discuss the results of seismic-tomography
surveys conducted between pairs of bedrock wells in the FSE well field.
Seismic tomography is based on principles similar to those of EM tomography;
however, instead of using electromagnetic energy, an acoustic signal
is generated in one borehole, and the first-arrival of the pressure
wave is monitored in an adjacent borehole. A\x11spatial distribution
of the seismic velocity in the volume of rock between the two wells
can be inferred using an inversion algorithm; areas of low-seismic
velocity imply zones of fracturing, and zones of high-seismic velocity
imply intact rock matrix.
Wright and others (1996b) compare the seismic and EM tomograms with
each other and with fractures identified from acoustic televiewer
logs. Results of the seismic- and EM-tomography surveys identified
the same general zones of fracturing in the rock mass in wells separated
by 10 m in the FSE well field. However, the orientation of the fractures
intersecting the borehole (as identified from the acoustic televiewer
log) was not the same as that of the fracture zones inferred from
the seismic and EM tomograms. Results of hydrologic testing confirm
the orientation of the fracture zones as identified by the tomograms,
implying that the orientation of permeable fractures intersecting
boreholes is not necessarily the orientation of permeable zones in
the rock mass.
Seismic and electromagnetic signals respond to both fractures and
lithology; therefore, dielectric permittivity and seismic velocity
may not correlate with hydraulic properties of the rock. Thus, tomography
must be conducted in conjunction with hydrologic testing to identify
permeable structures in the bedrock. The use of hydrologic testing
to characterize fracture locations and their hydraulic and transport
properties is another major research effort being conducted at the
Mirror Lake site. Hydrologic testing, consisting of hydraulic and
tracer testing, is the most direct method of investigating flow and
transport properties in the subsurface. Hydrologic tests generally
involve artificially inducing a perturbation (a change in fluid pressure
or chemical composition) in the subsurface and measuring the resulting
response. The flow metering during pumping, as discussed by Paillet
and others (1992) and Paillet (1996), are examples of hydraulic testing.
Hsieh (1994), Hsieh and Shapiro (1996) and Shapiro (1996) discuss
hydraulic and tracer tests conducted in the FSE well field to quantify
hydraulic and transport properties of the bedrock.
Hsieh (1996) and Hsieh and Shapiro (1996) discuss hydraulic tests
conducted by installing inflatable packers in all bedrock wells in
the FSE well field to eliminate the artificial vertical permeability
induced by open boreholes. An hydraulically isolated interval in one
bedrock well was pumped continuously, and fluid pressure responses
were monitored in hydraulically isolated intervals in other bedrock
wells. The results of these tests indicated the bedrock does not respond
as an equivalent homogeneous porous medium. Instead, there are several
clusters of highly permeable fractures of limited areal extent. These
highly permeable fracture clusters are only connected through less
transmissive fractures.
Shapiro (1996) discusses radially converging tracer tests conducted
in one of the highly permeable fracture clusters identified in the
FSE well field. New equipment and field techniques of conducting tracer
tests in fractured rock were developed for these tracer tests (Shapiro
and Hsieh, 1996), the results of which showed that there is significant
variability in rock properties that can affect chemical transport,
even within a highly permeable fracture zone.
Other interpretive methods of identifying hydraulic and transport
properties of the rock were developed to take advantage of environmental
tracers. Goode and others (1993) used the natural variability in the
radon content of water sampled during an aquifer test to estimate
the effective porosity of the bedrock and the vertical leakage from
the overburden. The temporal variability in radon concentration of
water measured at the pumped well was attributed to the mixing of
water sources; water in the glacial drift was assumed to have a different
radon concentration than water in the bedrock.
Regional-Scale Investigations
On the scale of kilometers, hydrologic tests in fractured rocks become
ineffective because, over such distances, changes in pressure caused
by fluid injection or pumping are generally too small to measure,
and transport times become too long for tracer tests to be practical.
In order to investigate phenomena affecting flow and transport on
the regional scale, the response of the ground-water system to natural
perturbations and long-term human disturbances must be monitored.
Monitoring hydrologic and geochemical phenomena in bedrock terranes
requires the design of equipment and techniques that account for the
heterogeneous hydraulic properties of bedrock.
Hsieh and others (1996) discuss methods of measuring hydraulic heads
in multiple intervals in a single bedrock well. The monitoring equipment
converts an open borehole to the equivalent of a multilevel piezometer.
The equipment is removable, thereby allowing access to the bedrock
well for hydrologic and geophysical testing when desired. The long-term
installation of this equipment in bedrock wells also prevents the
mixing of waters of differing chemistry because inflatable packers
are used to hydraulically isolate hydraulically discrete intervals
in the borehole.
Shapiro and others (1996) briefly discuss methods of collecting water
samples for geochemical and isotopic analysis from bedrock wells.
Because of the large variability of the hydraulic conductivity of
fractures, water samples for geochemical and isotopic analyses cannot
be collected by pumping an open borehole. A sample from an open borehole
would yield water mixed from all permeable fractures intersecting
the borehole. Instead, water samples are collected after isolating
a single fracture or fracture zone in the borehole using inflatable
packers.
A major research effort at the Mirror Lake site is the interpretation
of geologic, hydrologic, and geochemical information to infer regional
properties of ground-water flow and chemical transport in the bedrock.
Rosenberry and Winter (1993) used 10 years of streamflow, lake-discharge,
and climatological data to estimate the components of bedrock recharge
and discharge in the water budget of Mirror Lake. Water from bedrock
is estimated to account for up to 4 percent of all inflows to the
lake and up to 1 percent of all outflows from the lake. These estimates
are approximately 10 percent of ground-water inflows to and outflows
from the lake; however, they are subject to great uncertainty because
they are small relative to the major components of the water budget
for the lake. These estimates of bedrock fluxes to and from the lake
are being used to calibrate ground-water-flow models in the Mirror
Lake drainage basin and its vicinity.
Harte and Winter (1996) used a numerical, cross-sectional-flow model
of the drift and bedrock to investigate processes affecting bedrock
recharge from the overlying glacial drift. Results of model simulations
were compared with the distribution of the hydraulic head along an
hypothesized path of ground-water flow running through the northwestern
catchment area of the Mirror Lake drainage basin. The locations of
bedrock recharge in the Mirror Lake area are attributed to lateral
trends in the bedrock horizontal hydraulic conductivity, which is
hypothesized to decrease with increasing elevations in the drainage
basin to reproduce measured head gradients between the drift and bedrock.
Shapiro (1993) conducted a numerical simulation of areal (two-dimensional)
regional ground-water flow in a heterogeneous bedrock terrane, where
the heterogeneity was assumed to be the result of a random distribution
of constant transmissivity blocks. The random distribution of transmissivity
resulted in the poor connectivity of highly transmissive zones in
the bedrock. The conceptual model was based on results of hydraulic
testing in the FSE well field that illustrated the poor hydraulic
connection between highly permeable fracture zones (Hsieh, 1996; Hsieh
and Shapiro, 1996). The numerical simulations showed that hydraulic
heads increased with increased elevation of upgradient locations in
a manner similar to that observed in the Mirror Lake drainage basin.
Thus, it is hypothesized that the poor connectivity of highly permeable
zones in the bedrock could also play a role in the regional distribution
of hydraulic heads.
A major research effort at the Mirror Lake site has been the analysis
of ground-water chemistry and environmental isotopes to identify transport
properties of the bedrock and processes affecting chemical transport
in bedrock terranes over distances of kilometers. As water flows in
the subsurface, its chemical composition evolves as the water reacts
with the rock along the flow path. Understanding the chemical changes
in ground water can help develop conceptual models of regional ground-water
flow and chemical transport. In addition, environmental isotopes,
such as tritium, and chlorofluorocarbons (CFC's) in ground water can
be used to estimate the residence times of shallow ground-water.
Water samples were collected from discrete intervals in bedrock wells
and analyzed for a variety of dissolved solids, dissolved gases, and
a variety of stable and radioactive isotopes. Busenberg and Plummer
(1996) measured the concentrations of CFC's in ground-water samples.
By using historical records of CFC concentrations in the atmosphere,
the date of ground-water recharge can be estimated. Drenkard and others
(1996) estimated ground-water ages on the basis of the ratio of the
parent-daughter isotopes tritium and helium. Many ground-water samples
contained anomalously high helium concentrations; however, plausible
corrections could be made for other sources of helium, and ground-water
ages were estimated. The ages estimated from the tritium-helium and
CFC analyses were similar. However, these ages do not correspond well
with the presumed date of the maximum atmospheric tritium concentration.
Both the tritium-helium and CFC ages appear to be approximately 6
years younger than ground-water ages estimated by using the maximum
tritium concentration as the date of the maximum atmospheric tritium
input. This difference is still being investigated and may be the
result of ground-water flow through the unsaturated zone where the
air phase can allow further equilibration with atmospheric conditions,
or it may be a result of the mixing of waters of various ages, either
naturally or through the collection of water samples.
Ground-water ages indicate the residence time of the water since
it was recharged; however, the distance the water has traveled must
also be identified to estimate the regional flow velocity. In heterogeneous
bedrock terranes, the distance the water has traveled is not readily
identifiable. Shapiro and others (1996) proposed a simple conceptual
model to use changes in the concentration of the bicarbonate ion as
a second measure of the ground-water-residence time in the bedrock,
and as a means of estimating the average ground-water velocity in
the bedrock. The model considers transport through a single fracture
and diffusion of bicarbonate ions into the fracture from the rock
matrix. When used in conjunction with the ground-water ages estimated
from CFC or tritium-helium concentrations, an average ground-water
velocity can be estimated to reproduce the measured residence time
and bicarbonate concentration at sampling locations.
The basis of the conceptual model proposed by Shapiro and others
(1996) stems from discussions by Wood and others (1996) regarding
the significance of diffusion in defining the ground-water chemistry.
Wood and others (1996) noted that residence times of less than 50
years are not sufficient for dissolution of minerals in the granite
and schist along fracture faces to produce the concentration of dissolved
solids in the water samples collected in the bedrock. Instead, it
is hypothesized that water in the void space of the unfractured granite
and schist is in chemical equilibrium with the associated minerals
in the granite and schist. In particular, small amounts of calcite
(1 percent by weight) have been found in samples of the granite. Thus,
the equilibrium bicarbonate concentration in the rock matrix is significantly
higher than the concentration in the water recharged through the overburden
and into the bedrock. The concentration gradient between water in
the rock matrix and water in the fractures results in the mass flux
of bicarbonate ions into fractures.
Wood and others (1996) conducted laboratory experiments
to identify the effective diffusion coefficient and porosity
of the intact rock matrix to verify the role of diffusion
from the rock matrix. Porosity of granite samples was approximately
1.5 percent, and the effective diffusion
coefficient of 137Cs in granite (including the effects
of retardation, tortuosity and porosity) was approximately
6 X 10-13 m2/s. In addition, iron-staining in
the rock matrix adjacent to fractures has been noted on
outcrops and cores, and this is a further indicator of
the effects of diffusion in the ground-water chemistry (Barton,
1996; Wood and others, 1996). Preliminary calculations
of precipitation rates for ferric hydroxide using the laboratory-estimated
porosity and diffusion coefficients indicate that ferric
hydroxide bands can form over tens of years as the result
of the diffusion of dissolved oxygen into the rock matrix.
These timeframes are consistent with laboratory diffusion
experiments and the variability of bicarbonate concentration
in the ground water in the bedrock, which is assumed
to be the result of diffusion from the rock matrix.
SUMMARY
Research at the Mirror Lake Toxic-Substances Hydrology Research site
has focused on characterizing the physical properties that can define
ground-water flow and chemical transport in fractured rock over distances
that range from meters to kilometers. The characterization of ground-water
flow and chemical transport over tens of meters has focused on identifying
the geometrical and hydraulic properties of fractures. Mapping of
surface exposures, use of surface and borehole geophysical techniques,
and hydrologic testing have been used to identify properties of fractures
that influence ground-water flow and chemical transport. Over distances
associated with regional ground-water flow, processes affecting ground-water
movement and chemical transport are inferred through the synthesis
of hydrologic and geochemical data by means of ground-water and chemical-transport
modeling.
REFERENCES
- Barton, C.C., 1996,
- Characterizing bed rock fractures in outcrop for ground-water
hydrology studies-An example from Mirror Lake, Grafton County, New
Hampshire, in Morganwalp, D.W., and Aronson, D.A., eds., U.S. Geological
Survey Toxic Substances Hydrology Program--Proceedings of the technical
meeting, Colorado Springs, Colo., September 20-24, 1993: U.S. Geological
Survey Water-Resources Investigations Report 94-4015.
- Busenberg, E., and Plummer, L.N., 1996,
- Concentrations of chlorofluorocarbons and other gases in ground
waters at Mirror Lake, New Hampshire, in Morganwalp, D.W., and Aronson,
D.A., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the technical meeting, Colorado Springs, Colo., September 20-24,
1993: U.S. Geological Survey Water-Resources Investigations Report
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- Drenkard, S., Torgersen, T., Weppernig, R., Farley, K., Schlosser,
P., Michel, R., L., Shapiro, A.M., and Wood, W.W., 1996,
- Helium isotope analysis and tritium-helium age dating in the Mirror
Lake basin, Grafton County, New Hampshire, in Morganwalp, D.W.,
and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances
Hydrology Program--Proceedings of the technical meeting, Colorado
Springs, Colo., September 20-24, 1993: U.S. Geological Survey Water-Resources
Investigations Report 94-4015.
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T.F., 1993,
- Concentration history during pumping from a leaky aquifer with
stratified initial concentration, in Shen, H.W., ed., Hydraulic
Engineering, Proceedings of 1993 National Conference, July 25-30,
1993, San Francisco: New York, American Society of Civil Engineers.
- Haeni, F.P., Lane, J.W., Lieblich, D.A., 1993,
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94-4015.
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