United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S7-81-004 Mar. 1981
Project Summary
Hydrologic Impacts From
Potential Coal Strip Mining —
Northern Cheyenne
Reservation
William W. Woessner, Thomas J. Osborne, Edward L. Heffern, Charles
Andrews, Jason Whiteman, Wesley Spotted Elk, and Daniel Morales-Brink
This research project was initiated
with the overall objective of assessing
the possible impacts of coal strip
mining to the groundwater and surface
water resources of the Northern Chey-
enne Indian Reservation.
Meteorological, groundwater and
surface water monitoring networks
were established to evaluate the oc-
currence, quantity, movement and
quality of the region's water resources.
Available geologic data were gathered,
and field mapping and drilling pro-
grams were undertaken to delineate
the near surface stratigraphy of the
coal bearing Fort Union Formation.
Three priority study sites containing
strippable coal and representing varied
hydrogeologic settings were selected
for more detailed study.
Results of this project show the
presence on the Reservation of a
complete section of the Tongue River
Member of the Fort Union Formation
which contains 11 major coal seams.
A stacked groundwater flow system
exists in which extensive clinker
deposits, thick coal seams, channel
sandstones and unconsolidated al-
luvium in the major valleys store and
transmit the major portion of ground-
water. Water quality is variable and is
principally the result of the aquifer
mineralogy and the presence or ab-
sence of sulfur reducing bacteria.
Hydrologic and salt balance calcula-
tions indicate that in the Tongue River
along the Reservation, mean discharge
increased by eight percent and dis-
solved solids load increased by six per-
cent. In that part of Rosebud Creek
located within the Reservation, mean
discharge increased by 150 percent
and the dissolved solids load increased
by 39 percent. Postulated coal strip
mining at three priority study sites in
the Tongue River Valley, high central
divide region and Rosebud Creek
Valley would have impacts on the local
surface water and groundwater re-
sources. Water quality degradation
from the leaching of spoils would
affect the quality of the major streams.
A large percentage of the teachable
constituents in mine spoils would be
removed by the first pore volume of
spoil recharge and groundwater which
passes through the spoil. Mining all
strippable reserves of the Knoblock
coal seam system along both sides of
the Tongue River Valley from Birney
Day Village to a point eight kHometers
north of the town of Ashland would
have a major impact on the regional
groundwater quality and a significant
impact on the quality of the Tongue
River. Water quality impacts from coal
strip mining would last for hundreds of
years.
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This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal Research Laboratory, Cincinnati.
OH to announce key findings of the
research project which is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
This is a summary of the results of an
investigation designed to address the
questions of the Northern Cheyenne
Indian Tribe concerning the potential
hydrological impacts of coal strip mining
on their Reservation in southeastern
Montana.
At the initiation of this research effort
in 1975, all available data describing the
natural geologic, meteorologic, ground-
water and surface water systems oper-
ating in the Reservation area were col-
lected and reviewed. It was concluded
that existing basic data were inadequate
for developing a meaningful mining
impact analyses. In an attempt to obtain
necessary baseline information, mete-
orologic and hydrologic data collection
networks were designed and imple-
mented. Geologic mapping and field
programs were initiated. Many of these
programs still continue for it is realized
that initial programs have only provided
sufficient information to establish sys-
tem trends and further data needs. A
long period of record is necessary to
characterize more completely the natu-
ral systems of the Reservation which
are believed typical of the western coal
region.
Once adequate data were collected,
interpretation and analyses of the in-
formation base were started. The types
and extent of geologic formations found
in the area were delineated. The coal
stratigraphy and geomorphology of
three areas with the potential for eco-
nomical coal mining were described.
Analyses of precipitation and evapora-
tion records yielded a correlation with
topography and a base for groundwater
recharge evaluation and surface water
system definition. Rock units which
form the complex groundwater system
operating within the Reservation were
identified and hydrogeologic properties
of coal, clinker, sandstone and alluvial
material were described. Precipitation
and surface water discharge records
were correlated to define the surface
water systems. Seepage run data were
used to identify the contribution of
groundwater to the surface water sys-
tems. Most of the project period was
used to identify the existing natural
system, since it is firmly believed that
without careful delineation of the natu-
ral undisturbed or pre-mining systems,
prediction of the coal strip mining
impacts to the water resources would
not be meaningful.
Intensive analyses of the potential
impacts coal strip mining could have on
the water resources of the Reservation
was initiated in the last year of the
project. After evaluation of geologic and
hydrologic data, general mine sites
were selected in three areas. Pre-
mining input/output models of the hy-
drologic system in the area were devel-
oped at each site. The emphasis of the
mining impact section is on an evalua-
tion of the post-mining water quality
and quantity changes. Aquifer dewater-
ing during mining and mine effluent
quality impacts were recognized, but
the absence of detailed mining plans
and hydrogeologic data precluded de-
tailed analyses. Post-mining impact
analyses included laboratory saturation
extract testing of drill cuttings, and
leaching tests of representative samples
for each mine. Inputs to spoil hydrology
modeling were obtained from available
literature. Impact analysis techniques
were evaluated and the use of these
methods for regional, large-scale coal
development impact prediction was
discussed. A brief discussion of man-
agement possibilities of the Tribe's
water resources in light of potential
impacts from mining on the Reservation
and from regional mining was prepared.
Conclusions and
Recommendations
Conclusions
Geology
Approximately 90 percent of the
Reservation surface lies in the coal
bearing Tongue River Member of the
Paleocene Fort Union Formation. The
Member, where complete, is about 500
m (1640 ft) thick.
Fine-grained elastics and coals domi-
nate the Tongue River Member. Nearly
all sediment is medium to fine grained
sandstone, siltstone, or shale. Shale is
often found near the tops and bases of
coal seams. Channel sands are common.
Thin limy zones are often present.
Eleven major coal bed systems have
been identified on the Reservation.
From highest to lowest stratigraphic
level, they are the Roland, Smith, Ander
son. Canyon, Cook, Wall, Pawnee
Sawyer-Knobloch-Lee-Nance, Rosebud
McKay, Flowers-Goodale, and Robinson
Alluvium up to 37 m (120 ft) thick ir
places fills the valleys of Rosebud Creek
Tongue River, and their tributaries.
Clinker beds, formed by the burning o
major coal seams, cover about one-thirt
of the Reservation surface and reach
thicknesses of up to 70 m (230 ft). Th(
most extensive clinker beds are thost
related to the Anderson and KnoblocF
coal seams.
Individual coal seams are continuou;
over much of the Reservation, but ma)
split, pinch out, or merge with othe
seams. Interburden units of shale, silt
stone and sandstone are, for the mos
part, fairly discontinuous.
The Eocene Wasatch Formation cap;
highland areas in the south-central par
of the Reservation.
Strata are nearly flat lying with i
general dip of less than one degree t<
the southeast.
Minor northwest-southeast anc
northeast-southwest trending fault!
and joints occur. Measured vertica
displacement so far has been limited tc
50 m (160 ft) or less.
Present-day Reservation topography
has been formed by rivers dissecting ar
ancient plateau surface.
Groundwater
Alluvium, clinker, sandstone and coa
are the major near-surface aquife
lithologies on the Reservation.
Four hydrogeologic units, the Uncon
solidated, Anderson, Knobloch, am
Lower Tongue River, have been identi
fied. These units store and transmi
groundwater in the near surface hydro
geologic system.
Clinker deposits are major compo
nents of the identified flow systems. Th<
fractured nature of these deposits resul
in a range of hydraulic conductivitiei
and recharge potentials.
The hydraulic properties of clinker an
dependent on the thickness of the coa
burned and on the quantity of coa
clinker produced.
In the water years 1975 and 1976
recharge was greatest to the uncon
solidated deposits in the major strean
valleys and clinker deposits. Rechargi
to the confined system was negligible
Hydraulic conductivities estimated fo
coal aquifers fall within the range o
those reported in the literature.
Groundwater quality relationship!
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are influenced by sulfur reducing bacte-
ria in deeper flow systems and by ion
exchange.
The principal constituents of the
groundwater are sulfate, calcium, and
bicarbonate ions. The presence of cal-
cium and magnesium usually is inter-
preted to indicate recharge is entering
the system.
The Tongue River receives ground-
water discharge from bedrock and
alluvial aquifers along the reach of the
Reservation.
Rosebud Creek receives groundwater
discharge along the reach within the
Reservation, although rates are small.
Surface Water
The Tongue River derives its principal
hydrologic and water quality character-
istics in the upper reaches above the
Tongue River Dam. Snowmelt from the
Big Horn Mountains, irrigation in
Wyoming, and streamflow regulation by
the Tongue River Reservoir are dominat-
ing influences on the annual hydrograph
and salinity trends of the Tongue River
in Montana.
Groundwater discharge is a small
component of the total discharge of the
Tongue River below the Tongue River
Dam, comprising about 16 percent of
the streamflow at the northern boundary
during baseflow periods. The mean
groundwater seepage rate to the Tongue
River along the Reservation was found
to be 0.01 cubic meters per second per
river kilometer (0.55 ft3/sec/r mi)
during the fall and winter base flow
periods measured
The mean discharge between Tongue
River gaging stations at either Reserva-
tion boundary increased by 1.13 mVsec
(39.9 ftVsec), or about eight percent
during the study period. The river was
found to gain an average of 0.798
mVsec (28.2 ftVsec) from surface
water discharges. Consumptive uses
from irrigation caused an average an-
nual rate of depletion of 0.095 mVsec
(3.36 ftVsec). Evaporation losses were
calculated to occur at a mean annual
rate of 0.071 mVsec (2.51 ftVsec).
Rosebud Creek is a prairie stream that
derives its principal characteristics from
seasonal climatic patterns, topographic
controls and geologic deposits bordering
the creek or its tributaries. Rosebud
Creek was found to gain flow throughout
its upper half, and lose flow throughout
its lower half, on an annual basis.
. Groundwater discharge to Rosebud
Kreek comprises about 23 percent of the
total discharge at the northern Reser-
vation boundary during baseflow peri-
ods. The mean groundwater seepage
rate to Rosebud Creek on the Reserva-
tion was found to be 0.001 m3./sec.r/km
(0.08 ftVsec/r/mi) during the fall-
winter baseflow periods measured.
The mean discharge between Rose-
bud Creek gaging stations at either
Reservation boundary increased by
0.653 mVsec (23.1 ftVsec) or about
150 percent during the study period.
The creek gained an average of 0.137
mVsec (4.84 ftVsec) from groundwater
discharge and 0.414 mVsec (14.6
ftVsec) from surface tributary discharge.
Evaporation losses were calculated to
occur at a mean annual rate of 0.033
mVsec( 1.17 ftVsec).
The Tongue River at the state line has
already picked up 81 percent of the
average dissolved solids concentration
measured at Miles City, Montana, pri-
marily due to the effects of irrigation
return flows in Wyoming. The average
dissolved solids concentration increased
by 38 mg/1 or eight percent between
the two NCRP sampling stations at the
Reservation boundaries.
Most surface waters on the Reserva-
tion are a calcium-magnesium-bicar-
bonate type normally associated with
streams receiving most of their water
from lower-salinity sources such as
snowmelt, storm runoff or clinker-allu-
vial aquifers. Cook Creek and Muddy
Creek have a calcium-magnesium sul-
fate-bicarbonate water type higher in
salinity due to pronounced effects of
groundwater and spring flow from coal
and sandstone aquifers in the upper
part of the drainages above the clinker
zones.
Total dissolved solids at NCRP sam-
pling stations on the Tongue River
cycled between lows of 200 mg/1 to
highs of 700 mg/1 during water years
1976 and 1977.
Rosebud Creek at the northern Reser-
vation boundary has already picked up
85 percent of the average dissolved
solids concentration measured at the
mouth of Rosebud Creek. The average
dissolved solids concentration increased
by 203 mg/1 or 37 percent between the
two NCRP sampling stations at the
Reservation boundaries.
Dissolved solids concentrations in
Rosebud Creek were usually higher but
less variable at either sampling station
than in the Tongue River. The sampled
range at RBC #2 was from 400 to 650
mg/1 and was 550 to 900 mg/t at RBC
#1. The principal reason for the lack of
salinity variation is due to the dominance
of discharge from clinker and alluvial
aquifers directly to the creek and its
tributaries, and lack of irrigation.
A hydrologic balance equation was
applied to NCRP Tongue River and Rose-
bud Creek data. The calculated down-
stream discharge at TR #1 was 0.7
percent less than the measured value.
The calculated discharge at RBC #1 was
9.2 percent less than the measured
discharge.
A salt balance equation was applied
to NCRP water quality data once the
hydrologic system was balanced. The
calculated dissolved solids concentration
at TR #1 was nine percent higher than
the measured value and the calculated
value at RBC #1 was two percent lower
than the measured value.
Impacts
Pre-mining exploration and mine plan
drilling may impact the natural ground-
water systems.
During mining, reduction of water
levels in wells, destruction of wells, and
the reduction or disappearance of spring
flow are the* major impacts. Water
quality impacts from a nitrate rich mine
effluent can occur.
Post-mining spoil water is higher in
total dissolved solids than the natural
pre-mining groundwater.
Quantification of recharge rates of
the spoil, and the quantity and quality of
resulting leachate entering the ground-
water flow system, is necessary to pre-
dict post-mining impacts to the water
resources.
Single 4.9 to 9.8 million metric tons
per year (5 to 10 million short tons per
year) mines have a local impact on the
quantity and quality of the adjacent
surface water and groundwater system.
Salinity impacts to the Tongue River and
Rosebud Creek are expected to be
minimal.
Extensive mining of the Tongue River
Valley will impact the groundwater and
surface water systems operating in the
region. Groundwater with higher TDS,
and chemical constitutent concentra-
tions, will be unusable for irrigation and
mine site vegetation use. Downgradient
from the mine site, water quality will
gradually improve; however, high sodium
adsorption ratio (SAR) and TDS values
may limit the use of the water. Total
dissolved solids in the Tongue River
would increase from 16 mg/1 during
mean flows to over 500 mg/1 during
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very low flow periods depending on the
rate of recharge to the spoils.
The majority of leachable material is
removed from the overburden samples
by the time one pore volume of water
passes through the samples.
Both groundwater and spoil water
will leach constituents from mine spoils.
Local groundwater used in tests leached
chemical constituents from the over-
burden sample.
The time required to pass one pore
volume of water through spoil material
at the three postulated mine sites
ranged from a low of 100 years at the
Indian Coulee mine site to a high of
1,500 years at the Buffalo Jump mine
site.
The length of time mining impacts
could affect the regional water resources
is measured in hundreds of years.
Recommendations for Future
Research
Geology
The structure of coal clinker beds
needs more intensive study in order to
determine which zones within the
clinker are the main transmitters of
water.
An extensive drilling and geophysical
logging program on the unexplored
portions of the Reservation is needed in
order to define stratigraphic correlations
between coal seams and to define
possible additional reserves of strippable
coal.
Major massive sandstone bodies on
the Reservation need more thorough
surface mapping and subsurface con-
firmation with drill logs, in order to
determine their configuration.
Hydrology
Additional studies of the flow systems
operating on the Reservation, especially
the identification of recharge areas of
the deeper flow systems operating in
the Tongue River Member are needed. A
series of deep wells should be drilled in
the suspected, highland recharge area
and its flanks, to measure head differ-
ences, recharge rates, and water quality
changes with depth.
Detailed pump testing of representa-
tive wells in the coal, clinker sandstone
and alluvial aquifers is needed to refine
hydraulic conductivity calculations.
Special attention should be given to
documenting the hydraulic conductivi-
ties of clinker, as this is an important
hydrogeploqic unit, adjacent and down-
gradient from most proposed mining
operations in the region.
An array of two or more wells finished
in alluvium, clinker, and bedrock mate-
rial, ideally located adjacent to existing
stream gaging stations, should be in-
strumented with continuous water level
recorders for one or more years. Com-
parisons of stream stage hydrographs
with well hydrographs would indicate
trends in groundwater recharge, dis-
charge, and storage in relation to fluctu-
ations in river stage and discharge.
Such data would greatly assist interpre-
tation of seepage run results and
groundwater discharge rates to the
Tongue River and Rosebud Creek.
Water occurrence, water storage, and
use by plants in the unsaturatedzone, is
a significant component of the hydrologic
balance. Field studies using lysimeters
or other soil moisture indicators should
be conducted in conjunction with con-
current precipitation and well measur-
ing programs.
Impacts
Monitoring of groundwater levels
should continue to refine estimates of
recharge to the various aquifers. Accu-
rate data on the rates of recharge for
clinker are critical for determining the
impact post-mine waters would have on
the adjacent water resources.
Additional seepage runs should be
conducted on Rosebud Creek and on
Tongue River to refine groundwater
discharge rates. These rates are very
useful for checking the hydraulic con-
ductivities of alluvium and for predicting
the influence mining will have on
surface water resources.
The impact of pre-mining drilling
operations on the natural groundwater
system should be studied.
Spoil hydrology research is at an early
stage. Accurate predictions of the im-
pacts of a mining operation to the
quality and quantity of the associated
hydrologic system cannot be performed
without representative values of spoil
water chemistry, spoil hydraulic proper-
ties and recharge rates. Therefore,
mining impact research in the immediate
future should focus on spoil hydrology.
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William W. Woessner, Thomas J. Osborne, Edward L. Heffern, Charles
Andrews, Jason Whiteman, Wesley Spotted Elk, and Daniel Morales-Brink
are with the Northern Cheyenne Research Project, Lame Deer, MT 59043.
John Martin is the EPA Project Officer (see below).
The complete report, entitled "Hydrologic Impacts From Potential Coal Strip
Mining—Northern Cheyenne Reservation," (Order No. PB 81-155 061; Cost:
$23.00, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
> US QOVEmMENT PRINTING OFFICE 1961-757-064/0295
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