United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89114
Research and Development
EPA-600/S4-84-046 June 1984
Project Summary
Geothermal Environmental
Impact Assessment:
An Approach to Groundwater
Impacts from Development,
Conversion, and Waste Disposal
J. W. Hess, S. W. Wheatcraft, J. E. Edkins, R. L. Jacobson, and D. E. Zimmerman
Groundwater monitoring for the
impacts of geothermal energy
development, conversion and waste
disposal is similar to groundwater
monitoring for other purposes, except
that additional information is needed
concerning the geothermal reservoir. In
the research described here, a six-step
methodology, including failure analysis
and computer-based contaminant
transport and geochemical models to
design groundwater monitoring plans,
was developed. Failure data analyses
indicate that production and injection
wells have the highest probability of
failure and should be the focal points for
groundwater monitoring. Groundwater
monitoring techniques fall into four
categories: 1) monitoring the
injection/production well, 2)
monitoring in the saturated zone, 3)
monitoring in the unsaturated zone, and
4) monitoring on or above the ground
surface. Location of the monitoring
wells and the variables to measure in
them may be determined with the aid of
computer contaminant transport and
geochemical models. POLUTE is a
nondispersing contaminant transport
groundwater computer model that
enables the user to locate a
contaminant plume at any given time.
Prediction of resultant fluid
chemistries from a leaked geothermal
fluid may be essential to unambiguous
evaluation of geothermal fluid impact at
remote groundwater monitoring sites.
This can be accomplished through the
use of geochemical models.
Illustrative examples were run to
demonstrate the first four steps of the
six-step methodology. Results indicate
that the key to groundwater monitoring
is the continuous measurement of
physical, and chemical characteristics
of injection fluids in the production and
injection wells.
This report was submitted in fulfill-
ment of Grant No. R806457 by the
Desert Research Institute under the
sponsorship of the U.S. Environmental
Protection Agency. This report covers
the period from April 17, 1979 to July
16, 1982 and work was completed as
of July 16, 1982.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The research described here used the
guidelines developed by Weiss, Coffey,
and Williams (1979) as a base and refined
a six-step methodology to include failure
analysis and computer-based contami-
nanttransport and geochemical models.
This groundwater monitoring methodol-
ogy enables one to design a monitoring
plan to predict and detect changes in
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groundwater quality due to geothermal
development.
Methodology
The variations in physical settings and
human activities at or near potential
geothermal energy areas preclude one
monitoring plan that is applicable to the
release of geothermal fluid on the surface
as well as below the surface. Thus, the
monitoring method developed empha-
sizes groundwater impacts resulting from
the release of geothermal fluid into
groundwater from a production or injec-
tion well. Inherent in the methodology is
prediction of the possible impacts if a geo-
thermal fluid release should occur. The
methodology contains the following steps:
1. Define baseline conditions, includ-
ing geology, hydrology, water
quality, geothermal system, plant
design, existing surface and
groundwater usage, and other
existing waste disposal systems.
2. Perform failure analysis: compare
the site characteristics to the results
of industry-wide failure analysis to
predict the most likely mode of fluid
release.
3. Forecast aquifer conditions: predict
travel times and chemical concen-
trations in the geothermal reservoir
and overlying aquifers through the
use of solute transport and geochem-
ical models. Predict potential
impacts, define limits of detection
and use to design monitoring plan.
4. Select monitoring sites and tech-
niques.
5. Design monitoring plan and
alternatives, based on the above
steps and legal and institutional
constraints.
6. Implement monitoring plan and
modify as experience dictates.
Failure data analysis indicates that
production and injection wells have the
highest probability of failure and should
be the focal points for groundwater
monitoring. Additional years of operating
experience using currently developed
technologies are needed to estimate
failure rates confidently. Groundwater
monitoring techniques fall into four
2
categories: 1) monitoring the
injection/production well, 2) monitoring
in the saturated zone, 3) monitoring in the
unsaturated zone, and 4) monitoring on
or above the ground surface.
Monitoring the injection/production
wells can best be accomplished by
borehole geophysics and measurements
of annulus fluid properties. Techniques
include acoustic borehole televiewers,
flow meters, cement logs, conductivity
probes, temperature probes, and
pressure gauges.
Monitoring in the saturated and
unsaturated zones generally will involve
the use of monitoring wells in which
chemical and physical measurements
can be made, water samples collected, or
borehole geophysics run. Location of the
monitoring wells and variables to
measure in them may bedetermined with
the aid of computer contaminant
transport and geochemical models.
POLLUTE is a nondispersing
contaminant transport groundwater
computer model that can be applied to an
anisotropic aquifer which is bounded by
impermeable and/or equal potential
boundaries. The location distribution
coupled with the isochron distribution
output from POLLUTE enables the user
to locate a contaminant plume at a given
time. The well flow distribution enables
the user to determine, under a given
scenario, what will be contaminated, how
much contaminant will be intercepted,
and how long it will take for the well to
become contaminated.
Advective contaminant transport
models (which ignore dispersion) are
normally adequate for purposes of
designing geothermal monitoring
networks. POLLUTE, the model
developed for this study, is specifically
designed to be of maximum usefulness
and minimum cost to run and intercept
for monitoring network design. Use of an
advective model such as POLLUTE
eliminates the need for large amounts of
hydrologic data and permits optimization
of the geothermal monitoring network.
Care must be taken to understand that
the model will not predict exact arrival
times of contaminants at discharge
areas. The arrival times predicted by
POLLUTE will be equivalent to the
breakthrough of the 50% contaminant
concentration. This is not seen as a great
disadvantage, since advective-dispersion
models are probably inaccurate even
when dispersivity values have been
measured.
For any given geothermal system, there
are five possible geochemical
mechanisms by which leaked geothermal
fluid chemistries might be altered. Four of
the five mechanisms are related to
solution and precipitation reactions
resulting from 1) aqueous redox potential
changes, 2) temperature changes, 3)
pressure change effect on dissolved
gases, and 4) exposure of fluids to new
mineral assemblages. The fifth
mechanism involves cation exchange
reactions. Prediction of resultant fluid
chemistries may be essential to
unambiguous evaluation of geothermal
fluid impact at remote groundwater
monitoring sites.
Computerized geochemical modeling
methods are recommended for
application to geothermal monitoring
problems when used by experienced
personnel on a site-specific basis. Steps
should include 1) selection and field
supervision of essential baseline data
collection directed to modeling needs, 2)
determination of the degree of model
sophistication required and selection of
appropriate geochemical program types
based on prior field data evaluation for
the Known Geothermal Resource Area
(KGRA) in question, and 3) interpretation
of results, including selection of
diagnostic remote monitoring parame-
ters and identification of information
gaps requiring further evaluation.
Because adequate redox information was
generally lacking in the analyses taken
from the literature, more effort should be
made to quantify this parameter in
geothermal fluids. Redox information
should be evaluated and comparisons
made between a number of direct and
indirect methods. Redox information is
important to studies involving corrosion
of plant facilities as well as to impacts on
nonthermal environments. It is very
important to measure comparable data at
all sites at a given KGRA for both thermal
and nonthermal waters.
Monitoring on or above the surface
includes tests on surface equipment,
surface water measurements, surface
geophysics and remote sensing. Surface
geochemistry techniques such as radon
and mercury detectors may have
monitoring applications.
Three illustrative examples are
presented to demonstrate the six-step
methodology. The methodology was
applied to the Raft River KGRA in Idaho
and to hypothetical examples of leakage
from a lagoon and contaminant plume
tracking and interception. The results of
these examples indicate that the key to
groundwater monitoring is to measure
physical and chemical characteristics
continuously on production and injection
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wells. As an example, a change in
annulus pressure may indicate leakage
through the injection tubing and would be
the first indication of a leak into a ground-
water system. At most sites, the
immediate groundwater systems are
already naturally affected by the
geothermal fluid. This makes early
detection of the geothermal fluid more
difficult in monitoring wells. Monitoring
wells can be used to assess the impact of
a leak, that is, to determine if there is a
detectable change in water quality.
J. W. Hess, S. W. Wheatcraft, J. E. Edkins. R. L Jacobson. D. E, Zimmerman, are
with Water Resources Center, Desert Research Institute, University of Nevada
System, Las Vegas, NV 89109.
Leslie Dunn is the EPA Project Officer (see below).
The complete report, entitled "Geothermal Environmental Impact Assessment:
An Approach to Groundwater Impacts from Development, Conversion, and
Waste Disposal," (Order No. PB 84-198 639; Cost: $23.50, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89114
U.S. GOVERNMENT PRINTING OFFICE; 1984 — 759-015/7731
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