EPA 910/9-90-020	Alaska
United States Region 10 Idaho
Environmental Protection 1200 Sixth Avenue Oregon
Agency	Seattle WA 98101	Washington
Water Division	Office of Ground Water	August 1990
vvEFft Support Document
For the EPA Designation of the Eastern
Snake River Plain Aquifer as a Sole
Source Aquifer

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SUPPORT DOCUMENT FOR DESIGNATION OF THE
EASTERN SNAKE RIVER PLAIN AQUIFER
AS A SOLE SOURCE AQUIFER
PREPARED BY THE OFFICE OF GROUND WATER
U.S. ENVIRONMENTAL PROTECTION AGENCY, REGION 10
SEATTLE, WASHINGTON
August 1990

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CONTENTS
Page
Introduction	1
Purpose 	1
Sole Source Aquifer Program ... -	1
Petition history	2
General description of the Eastern Snake River Plain	4
Geography	4
Climate 	5
Population 	5
Economy 	6
Hydrogeology of the Eastern Snake River Plain 	7
Stratigraphy 	7
Structure 	9
Ground-water movement	9
Ground-water quahty	 11
Potential for contamination 				 13
Description of boundaries	*.	 14
Drinking water use	 15
Alternative drinking water sources	 15
Conclusions 	*	 16
References cited	 17
ILLUSTRATIONS
(Maps at end of report)
Figures 1-4. Maps showing:
1.	Location of sole source aquifer and streamflow source areas
2.	Topography of sole source aquifer and streamflow source areas
3.	Surficial geology of sole source aquifer and streamflow source areas
4. Ground-water level contours and flow directions
Figure 5. Cross section of vertical ground-water flow	 10
Figure 6. Mean composition of ground water	 12
TABLES
Table 1. Average monthly temperatures (degrees Fahrenheit) . 	5
2. Urban and rural population	6

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SUPPORT DOCUMENT FOR DESIGNATION OF THE
EASTERN SNAKE RIVER PLAIN AQUIFER
AS A SOLE SOURCE AQUIFER
INTRODUCTION
Purpose
This document summarizes readily available information about the eastern Snake
River Plain, and will serve as the technical basis for U.S. Environmental Protection
Agency (EPA) designation of the Eastern Snake River Plain Aquifer as a sole source
aquifer. Those interested in more detailed information may consult the references listed
at the end of the report Additional.references may be found in a bibliography
regarding the geology and hydrology of the Snake River Plain, which contains over a
thousand entries, published by the U.S. Geological Survey (Bassick, 1986).
Sole Source Aquifer Program
The Sole Source Aquifer Program is authorized by the Safe Drinking Water Act of 1974
(Public Law 93-523 42 U.S.C. 300 et. seq.V Section 1424(e) of the Safe Drinking Water
Act states:
"If the Administrator determines, on his own initiative or upon petition that an
area has an aquifer which is the sole or principal drinking water source for the
area and which, if contaminated, would create a significant hazard to public
health, he shall publish notice of that determination in the Federal Register.
After the publication of any such notice, no commitment for federal financial
assistance (through a grant, contract, loan guarantee, or otherwise) may be
entered into for any project which the Administrator determines may contaminate
such aquifer through a recharge zone so as to create a significant hazard to public
health; but a commitment for federal assistance may, if authorized under another
provision of law, be entered into to plan or design the project to assure that it
will not so contaminate the aquifer."
EPA currently has a longstanding policy of not initiating sole or principal source
aquifer designations; the Agency only responds to petitions. Untr! 1987, EPA accepted
sole or principal source aquifer petitions which contained a minimum amount of
information. The Sole Source Aquifer Petitioner Guidance Document, released in
February of 1987, set forth criteria which clarifies the definition of a sole or principal
source aquifer, and describes how to petition EPA. The requirements of the new
guidance only apply to petitions submitted after February of 1987.

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EPA defines a sole or principal source aquifer as otfe which supplies at least 50
percent of the drinking water consumed in the area overlying the aquifer (U.S. EPA,
1987). Current EPA guidelines also stipulate that designated sole or principal source
aquifer areas have no alternative source or combination of sources which could
physically, legally, and economically supply all those who depend upon the aquifer for
drinking water (U.S. EPA, 1987). For convenience, all EPA designated sole or principal
source aquifers are often referred to simply as "sole source aquifers".
Petition History
On March 16, 1977, the Region 10 EPA Office received a petition from Raleigh
W. Stevens of the R Bar S Ranch near Hagerman, Idaho. The petition requested "aid
of any and all types" to study and protect the aquifer which "feeds Gooding County and
all of Hagerman Valley." In response, EPA met with Mr. Stevens and appropriate
federal, state, and local government agencies on June 1, 1977, in Twin Falls, Idaho. At
that meeting, sole source aquifer designation under Section 1424(e) of the Safe Drinking
Water Act was discussed as one of many possible measures designed to protect the
aquifer. A consensus emerged to develop a plan of study for the aquifer underlying the
eastern Snake River Plain, and to seek funds from a combination of federal, state, and
local sources. Mr. Stevens agreed to hold the sole source aquifer petition in abeyance
while funds were sought for an extensive aquifer study with the understanding that the
petition would be reinstated if funds were not obtained (Scott, 1977). EPA has no
record of any federal, state, or local ground-water protection strategies developed in
response to this meeting.
On July 1, 1982, EPA received a letter from John A. McDaniel, President of the
Hagerman Valley Citizens' Alert, Inc., inquiring about the status of a "petition filed
several years ago" to' designate the "Snake Plain Aquifer as a 'Sole Source Aquifer'
under Section 1424 of the Safe Drinking Water Act." EPA responded by pointing out
that although the 1977 meeting participants had agreed upon the need for a
comprehensive aquifer study, no funding had been made available (Burd, August 3,
1982). McDaniel wrote again on September 25, 1982, and formally requested that EPA
reinstate the sole source aquifer petition filed by Mr. Stevens in 1977. The Hagerman
Valley Citizens' Alert, Inc., submitted information requested by EPA to complete this
petition on November 1, 1982.
EPA arranged a meeting in Twin Falls on December 8, 1982, to discuss the
purpose and scope of the sole source aquifer program. The meeting was attended by 37
people who represented the Hagerman Valley Citizens' Alert, a number of federal and
state agencies, several health districts, several irrigation companies, a local conservation
group, and the news media (Marshall, December 20, 1982). Shortly after the meeting in
Twin Falls, the petitioners formally notified EPA of their continued interest in sole
source aquifer designation (McDaniel, December 10, 1982). In light of the petitioners'
continued interest, a Federal Register notice announcing receipt of the petition and
requesting public comment through April 11, 1983, was published on February 9, 1983.
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Informational meetings were arranged for April 13, 1983, in Burley, Idaho and
April 14, 1983, in Idaho Falls, Idaho in order to describe the sole source aquifer
program and discuss the petition to designate the eastern Snake River Plain as a sole
source aquifer area. The 23 people who attended the meeting in Burley, and the 19
who attended the Idaho Falls' meeting, represented local environmental groups,
irrigation companies and other businesses, federal, state, and local government agencies,
and the news media (Marshall, May 4, 1983).
A Federal Register notice, published on March 22, 1984, announced the draft
publication of a support document for designating the "Snake River Plain Aquifer" as a
sole source aquifer. The notice also announced that public hearings would be held only
if sufficient public interest was expressed. Copies of the Federal Register notice and
support document were sent to local, state, and federal officials, public libraries, and
representatives of environmental and agricultural interest groups (Marshall, April 2-3,
1984).	Press releases which summarized the Federal Register notice were issued by
EPA and the Idaho Department of Water Resources (U.S. EPA, March 29, 1984 and
IDWR, April 18, 1984).
Despite widespread notification about the proposed hearings, no requests were
received for a hearing in the Idaho Falls area (Marshall, May 10, 1984). EPA received
ten requests for a public hearing in'Twin Falls from citizens who supported sole source
aquifer designation. The supporters were agreeable to cancellation of the hearings
(Marshall, May 10,1984). Accordingly, both hearings were cancelled.
After cancellation of the hearings, EPA received a number of requests to
reschedule them (Marshall, June 12, 1984). Consequently, hearings were rescheduled
and conducted in Idaho Falls on August 13, 1984, and Twin Falls on August 14, 1984
(Barnes, August 1, 1984 and Sceva, August 17, 1984).
Shortly after completion of the public hearings, the Governor and Attorney
General of Idaho sent EPA a joint letter, dated August 20, 1984, which expressed strong
opposition to sole source aquifer designation- The EPA Region 10 Administrator
responded with a letter to Governor Evans on September 19, 1984, and met personally
with the Governor on November 7, 1984.
On January 16, 1985, the EPA Regional Administrator announced her decision to
postpone making a decision on the sole source aquifer petition (U.S. EPA, January 16,
1985).	The Magic Valley Aquifer Coalition wrote the EPA Regional Administrator on
April 4, 1985, and asked her to reconsider her decision to postpone sole source aquifer
designation (Couch, April 4, 1985).
The petitioners wrote EPA on November 10, 1986, to inquire about the status of
their petition (Bowler, November 10, 1986). The agency responded by confirming that
their petition was complete and would be processed at some point in time (Burd,
December 16, 1986). The petitioners followed up this response with letters to either
EPA or the state governor on the following dates: January 15, 1987; June 11, 1987;
November 2, 1987.
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The EPA Regional Administrator received an unsolicited letter from Governor
Andrus on February 11, 1988. The letter requested that EPA continue to delay a
decision on the eastern Snake River Plain sole source aquifer petition. Later that year,
the Shoshone-Bannock Tribes and the Committee for Idaho's High Desert wrote the
EPA Regional Administrator, and urged EPA to act upon the sole source aquifer
petition (Osborne, April 1, 1988 and OCrowley, April 25, 1988).
On July 12, 1990, the Acting EPA Regional Administrator wrote Governor
Andrus to inform him that EPA was proceeding with sole source aquifer designation for
the eastern Snake River Plain. In response, the Region 10 Office of Ground Water has
prepared this updated support document which makes use of information published in
the last few years, and which more rigorously evaluates alternative drinking water
sources.
GENERAL DESCRIPTION OF THE EASTERN SNAKE RIVER PLAIN
Geography
The eastern Snake River Plain of southeastern Idaho covers almost two-thirds of
the greater Snake River Plain (fig. 1). The arc-shaped Snake River Plain, which
contains most of the population of southern Idaho, extends from near the Wyoming
border westward into eastern Oregon. The geologic and hydrologic rationale for division
of the Snake River Plain into eastern and western portions are described in the
"Description of Boundaries" section of this report
The 45 to 60 mile wide eastern Snake River Plain cuts almost perpendicularly
across the north-south trend of the surrounding mountain ranges and intermontane
valleys. The mountain ranges rise thousands of feet above the area; mountain peak
elevations vary from about 7500 feet to over 12,000 feet. The Snake River drainage
area upstream from King Hill, Idaho, but outside of the Eastern Snake River Plain
Aquifer boundary, is defined as the streamflow source area. The hydrologic significance
of the streamflow source area is described in the "Ground-water Movement" section of
this report
The land surface of the eastern Snake River Plain exhibits little topographic relief
compared to the surrounding mountains, but does contain some locally impressive buttes
and rugged volcanic scabland areas (fig. 2). Overall, the surface of the area slopes
westwardly from an elevation of about 6,000 feet near the eastern margin of the Plain to
about 3,200 feet where the eastern and western parts of the Snake River Plain meet
(Mundorff, 1967).
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Climate
An arid to semi-arid continental climate prevails across the eastern Snake River
Plain. Annual precipitation, which averages 6-12 inches over most of the area, tends to
be evenly distributed throughout the year but often varies significantly from year to year
(Mundorff et al, 1964). Considerably more precipitation (mostly as snow) fails on the
mountainous drainage areas which surround the eastern Snake River Plain on three
sides. The higher elevation areas north and south of the eastern Snake River Plain
receive average annual precipitation of 20-35 inches, whereas higher elevations in the
mountains east of the Snake River Plain average 50-70 inches of precipitation each year
(Mundorff et al, 1964 and Kilburn, 1964).
Average temperatures vary across the eastern Snake River Plain according to
elevation (table 1) (Mundorff et al, 1964). The average growing season ranges in length
from about 150 days at the western part of the eastern Snake River Plain near Bliss to
about 100 days near the eastern margin of the Snake River Plain at Ashton (Stearns et
al, 1938).
Table 1.™Average Monthly Temperatures (Degrees Fahrenheit)
Station Elevation(fO Jan. Mar. May July Sept. Nov.
Twin Falls
3770
27
40
56
71
60
38
Idaho Falls
4830
19
34
53
69
57
34
Ashton
5220
18
29
50
65
54
32
Population
Approximately 273,000 people live in the eastern Snake River Plain (table 2),
(Gaia Northwest, 1988). Population centers are clustered almost exclusively in a band
within 10 miles of the Snake River. Cities with 2,500 or more people, defined as "urban"
areas by the U.S. Census Bureau, contain just over half of the eastern Snake River
Plain's population. Two cities, Idaho Falls (pop. 41,774) and Twin Falls (pop. 28,168)
hold half of the eastern Snake River Plain's urban population and about one quarter of
its total population. About 39 percent of the people in the eastern Snake River Plain
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live in unincorporated areas, many of them on farms and ranches.
People living in the streamflow source area of the eastern Snake River Plain
reside almost exclusively in river valleys. The major population centers are Pocatello in
the Portneuf River Valley (pop. 45,334), and the towns of The Wood River Valley
(Bellevue, Hailey, Ketchum, and Sun Valley) in Blaine County, Idaho.
Table 2.-Urban and rural population
People Percent of
Population
69,942 26
74,391 27
144,333 53
107,238 39
21,077	8
128,315 47
272,648 100
Economy
Irrigated agriculture and associated industries dominate the economy of the
eastern Snake River Plain and many of its tributary valleys. The Idaho National
Engineering Laboratory and an expanding recreation industry account for much of the
remaining economic activity in the area.
Irrigated acreage covers about 3,200 square miles of the 10,800 square mile
eastern Snake River Plain (Lindholm and Goodell, 1986). Crops grown include potatoes,
wheat, dry beans, corn, barley, sugar beets, and hay (Cenarrusa, 1987). Livestock
operations include beef and dairy cattle, sheep, and hogs (Cenarrusa, 1987). Trout
farms are the principal aquacultural activity in the area (Cenarrusa, 1987). Irrigation
began in the later half of the 19th century and grew rapidly after the Carey Act of 1894
and The Federal Reclamation Act of 1902 provided the "means and incentives" for
Idaho Falls and Twin Falls
Other Cities of at Least 2,500
Urban Subtotal
Unincorporated Areas
Towns of Less Than 2,500
Rural Subtotal
Eastern Snake River Plain Total
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building extensive surface irrigation networks (Norvitch et al, 1969). Use of ground
water for irrigation was rare before 1945 but has expanded rapidly since then. Ground
water now accounts for about one-third of the water used for irrigation within the
eastern Snake River Plain (Kjelstrom, 1986).
The Idaho National Engineering Laboratory (INEL) and spinoff industries
account for much of the economic development in the northeastern part of the Snake
River Plain. Nuclear research and production activities within the 890 square mile
reservation employ about 3,700 federal civilian and military employees, and contractor
activity associated with the center brings the total work force directly dependent upon
INEL to over 10,000 people (Cenarrusa, 1987).
Outdoor recreation and associated service industries provide the economic
backbone of some higher elevation valleys in the streamflow source area. Recreation
centers in the Wood River Valley of Idaho and Jackson Hole, Wyoming account for the
strongest recent economic growth in the streamflow source area. For instance, the
population of Blaine County, Idaho increased 34.2 percent from 9841 to 13,200 people
between 1980 and 1986 (Cenarrusa, 1987).
HYDROGEOLOGY OF THE EASTERN SNAKE RIVER PLAIN
Hydrogeological characteristics of the eastern Snake River Plain are used for
defining areal boundaries of the scile source aquifer system, in defining individual
aquifers underlying the plain, and in understanding the ground-water flow system.
Descriptions of each geologic map unit, water yielding characteristics, mapped structures,
and ground-water flow patterns, are taken from the U.S. Geological Survey Regional
Aquifer Study and Analysis of the Snake River Plain publications.
Stratigraphy
The eastern Snake River Plain is composed of volcanic rocks extruded during
Tertiary and Quaternary Periods overlain by Quaternary alluvial deposits (Whitehead,
1986) (fig. 3). Volcanic rock map units include the following: "older silicic volcanics,1'
"older basalt," "basalt," and "younger basalt." Quaternary alluvial strata include: "older
alluvium," "windblown deposits," and "alluvium."
"Older silicic volcanics", underlie the entire Snake River Plain, and crop out at
the land surface in three small isolated areas in the eastern part of the plain. The
Idavada Volcanics are included in this map unit. These massive and dense rhyolitic,
Iatitic, and andesitic rocks occur as thick flows, and blankets of welded tuff that contain
fine- to coarse-grained ash and pumice beds. Jointing ranges from platy to columnar,
with local areas of folding, tilting, and faulting in the streamflow stturce area. The
maximum thickness is greater than 3,000 feet. A number of wells located on the plain
withdraw water from the "older silicic volcanics". Hydraulic conductivity and well yields
are highly variable. Principal production zones include joints, fault zones, and interstices
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of coarse-grained ash, sand, and gravel.
The "older basalt" consists of flood-type basalt flows that are dense, folded and
faulted, and columnar jointed in many places. Flows of vesicular olivine basalt and
some rhyolitic and andesitic rocks are included in this map unit. The age of these
volcanic rocks (Miocene Epoch) is equivalent to the Columbia River Basalt Group and
the Banbury Basalt of the Idaho Group. These basalt flows cover large areas of the
streamflow source area but crop out in only one small area of the western end of the
eastern Snake River Plain. Maximum thickness is greater than 1,000 feet. Where
saturated, these rocks generally yield small to moderate amounts of water to wells.
Hydraulic conductivity is variable, and may be high in places. Specific capacities of
wells finished in the unit range from 3 to 900 gal/min/ft
"Older alluvium" consists of compacted to poorly consolidated, subaerial and
lacustrine deposits of clay, silt, sand, and gravel. The deposits are poorly to well
stratified, and contain beds of ash with intercalated basalt. This map unit occurs
predominantly along the southeastern boundary of the area, and has a maximum
thickness greater than 5,500 feet. The "older alluvium" generally contains water under
confined conditions, has variable hydraulic conductivity, and is an important source of
water in some areas. Well yields range from a few gallons per minute from clay-rich
intervals to several hundred gallons per minute from sand and gravel beds.
"Basalt" occupies small isolated areas throughout the plain, and consists of olivine
basalt that has a well developed soil cover where exposed. Thickness is counted with
the maximum thickness of the "younger basalt" below. Hydraulic conductivity and well
yields are also similar to the "younger basalt".
"Younger basalt" covers the vast majority of the eastern Snake River Plain land
surface. This map unit consists of many flood-type basalt flows of dense to vesicular,
irregular to columnar jointed, olivine basalt. Maximum thickness is greater than 4,000
feet (including "basalt"). Individual flow thickness varies from 10 to 50 feet and averages
20 to 25 feet (Mundorff et al, 1964). Beds of basaltic cinders, rubbly basalt, and
interflow sedimentary rocks occur between the flows. This deposit, along with the
"basalt" unit, comprise the major aquifer units that contribute water to wells in the
eastern Snake River plain. Transmissivity determined from aquifer pump tests averages
6.7 x 10s ft2 /d. Hydraulic conductivity is variable and extremely high in places where
jointing and rubbly contacts between flows occur.
"Windblown deposits" include lake and glacial-flood deposits that mantle much of
the lowland areas, and local occurrences of active sand dunes in the northern section of
the eastern Snake River Plain. This unit is usually found above the water table.
Maximum thickness is unknown, but estimated to be greater than 100 feet
"Alluvium" consists chiefly of flood-plain deposits, but may contain some glacial
and colluvium deposits in the uplands. These sediments occur on the surface of
tributary valleys and flood plains of the main streams, and form alluvial fans at mouths
of some valleys. The deposits are composed of a mixture of unconsolidated to well
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compacted clay, silt, sand, gravel, and boulders. Stratification ranges from unstratified to
well stratified. Maximum thickness is estimated at greater than 250 feet. Hydraulic
conductivity is variable, but moderately high in coarse-grained deposits. Sandy and
gravelly alluvium yields moderate to large quantities of water to wells.
Structure
The eastern Snake River Plain is a structural downwarp filled with a thick
sequence of the Tertiary and Quaternary sediments. Structural characteristics such as
faults and fractures occur throughout the eastern Snake River Plain streamflow source
area, but few are evident on the surface of the Plain (fig. 3). These structures can
influence ground-water flow direction and velocity, and affect well-yield capacities.
Faults may provide avenues for vertical movement of water, or impede and change the
direction of horizontal movement of water (Lewis and Young, 1982).
Outcrops and well logs indicate that basalt flows underlying the plain are highly
fractured. However, few major faults are mapped because they are obscured by
Holocene basalt flows and sedimentary rocks that mantle much of the plain. Seismic
studies suggest the presence of high angle vertical faults along much of the eastern plain
boundary, with displacement being at least 13,000 feet along the northwestern boundary.
Although these faults are not shown on published maps, the Great Rift zone and other
rift zones shown on the maps may be indicative of such deep-seated faulting. If so, most
of the rift zone faults are approximately perpendicular to the boundary faults of the
plain.
Ground-water Movement
The ground-water flow regime beneath the eastern Snake River Plain is an
important factor in determining the potential for ground-water contamination, and for
predicting the movement of contaminants that reach the ground-water system. Ground-
water movement under the plain is determined from water levels measured in the
"regional aquifer system" in 1980 during the U.S. Geological Survey Regional Aquifer
Study and Analysis, (Lindholm and others, 1986). This "system" includes all aquifers
except those considered perched or parts of small, shallow systems, and is representative
of the regional flow in the sole source aquifer.
Ground water moves generally horizontally near the center of the plain, and
vertically in regions of recharge and discharge. Horizontal movement of ground water
in the aquifer is from northeast to southwest, with deviations along gaining reaches of
the Snake River and its tributary basins (fig. 4). Horizontal hydraulic gradients range
from about 3 to 100 feet per mile and average about 12 feet per mile. Hydraulic
gradients are lowest in the central part of the plain, which is undeMain by thick highly
transmissive basalt. Horizontal movement in basalt is primarily through rubbly basalt
flow-tops where hydraulic conductivity values are high. Perched water tables have
formed, particularly near the boundaries of the plain, where fine grained sedimentary
9

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rocks impede the vertical movement of water downward. These perched aquifers
provide a significant source of water to the local inhabitants. Small, isolated flow
regimes may be present in areas of intensive ground-water pumpage or high topographic
relief.
Recharge to the ground-water system occurs from surface-water irrigation (60%),
underflow from tributary drainage basins (25%), direct precipitation (10%), and Snake
River losses (5%) (Kjelstrom, 1986). Surface-water irrigation recharges the aquifer via
infiltration of water from irrigated fields, and losses through irrigation canals. Cross-
sections showing vertical flow paths illustrate the influx of tributary drainage basin
underflow that occurs along the aquifer boundaries (fig. 5).
EXPLANATION
			Water i«Wt
	2400	 Li— oi hypatmmHa*-	B'
50 MILES
VERTICAL SCALE GREATLY EXAGGERATED
0 25 50 KILOMETERS
Figure 5.-Cross section of vertical ground-water flow through the eastern Snake River
Plain aquifer (from Lindholm and others).
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Discharge from the Eastern Snake River Plain Aquifer occurs as spring flow
along the Snake River, direct seepage into the Snake River, and well pumpage.
Although ground-water discharge from evapotranspiration of plants probably occurs
where the water table is shallow, data on total volume released is unavailable. Total
discharge from spring flow and Snake River seepage is estimated to be approximately
7.6 million acre-feet, with most outflow occurring along the Snake River between Milner
and King Hill. Ground-water discharge from well pumpage equals approximately 1.92
million acre-feet (Gaia Northwest, 1988).
Present estimates of average annual surface and ground-water inflow and outflow
on the eastern Snake River Plain are compared to pre-development estimates in a water
budget analysis published by the U.S. Geological Survey (table 3) (Kjelstrom, 1986).
Under present conditions, total inflow equals approximately 10.82 million acre-feet of
water from recharge of the Snake River near Heise, Idaho, water yield from tributary
basins, and direct precipitation on the plain. Outflow from the eastern Snake River
Plain equals approximately 10.77 million acre-feet of water from discharge into the
Snake River at King Hill, evaporation from the Snake River and reservoirs, and
evapotranspiration of irrigation water. The remaining .05 million acre-feet contributes
to ground-water storage. Irrigated "agriculture and other development has resulted in an
approximate 6% decrease in water circulating through the eastern Snake River Plain.
Although ground-water recharge from irrigation and ground-water discharge from
pumping have local impacts on the ground-water system, those volumes of water are
each recycled within the plain, and thus are taken into account in the budget totals.
Ground-Water Quality
Non-thermal ground water beneath the eastern Snake River Plain is generally of
naturally high quality relative to drinking water standards. Available data suggest that
the background water quality of basalt and alluvial aquifers is similar. Concentrations of
ground-water total dissolved solids average 282 milligrams per liter (mg/L) in basalt and
263 mg/L in Quaternary sediments (Yee and Souza, 1987). The relative proportion of
dissolved solids are also quite similar; calcium accounts for about 50 percent of the
cations and bicarbonate about 80 percent of the anions (fig. 6).
Man-induced contamination has been documented in widespread areas at levels
below drinking water standards, and in more localized areas at levels which exceed
drinking water standards. Documented instances of ground-water degradation above
drinking water standards have occurred in both urban and rural areas, from a variety of
land-use and waste water disposal practices.
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100 I—
Basalt of the
Snake River
Group
80
z
o
K
<75
£
s
o
u
UJ
a
?
z
8
cc
a!
60
40
20






:,Jg



Quaternary
sediments
1	2
Mean composition of ground water
Cations
EXPLANATION
Anions
Sodium plus
potassium
Magnesium
Calcium
Chloride plus
fluoride
Sulfate
Bicarbonate
plus carbonate
1	Basalt of the Snake River Group
2	Quaternary sediments
Silica
Figure 6.--Mean composition of ground water (from Yee and Souza, 1987).
Distinct and areally extensive differences in ground-water quality have been
documented between areas of high and low population densities. A ground-water
quality reconnaissance study published nearly 20 years ago shows that chloride levels of
about 10 mg/L were widespread in remote areas of the eastern Snake River Plain
whereas chloride levels of about 100 mg/L were common in ground-water beneath
intensively irrigated areas (Dyer and Young, 1971). Likewise, nitrate levels of about 2
mg/L were common in remote areas but nitrate levels in excess of 15 mg/L were
widespread in ground water beneath intensively irrigated areas (Dyer and Young, 1971).
The documented chloride concentrations have no direct public health significance, but
the elevated nitrate levels are of concern since the primary (health based) drinking
water standard for nitrate is 10 mg/L More recent studies have also documented high
levels of nitrogen (usually as nitrate) in ground water beneath intensively irrigated areas
(Young et al, 1987a; Young et al, 1987b).
Waste disposal practices at INEL have resulted in widespread and well
documented ground-water contamination beneath part of the 890 square mile
reservation. In 1978, a 28 square mile tritium plume was mapped (Barraclough et al,
1982). The plume was estimated to have advanced about five feet per day after
radioactive waste disposal through injection wells began in 1952 (Barraclough et al,
1982). Injection of radioactive waste was halted in 1984, but waste disposal lagoons at
INEL continue to leak contaminants to ground water.
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INEL has received EPA designation as a National Priority List (NPL)
"Superfund" site under the Comprehensive Environmental Remediation, Cleanup, and
Liability Act (CERCLA). This EPA designation is not related to EPA sole source
aquifer designation of the Eastern Snake River Plain Aquifer under the Safe Drinking
Water Act. After EPA sole source aquifer designation, any federal financial assistance
at INEL will be subject to review under Section 1424(e) of the Safe Drinking Water Act.
However, as far as EPA knows, INEL operates entirely upon direct federal fanding; the
Agency is unaware of any federal financially-assisted projects at the facility.
Numerous spills associated with the transportation or storage of petroleum
products and other hazardous materials have been documented on the eastern Snake
River Plain by the Idaho Division of Environmental Quality (IDEQ). IDEQ began a
Contamination Log in 1985 to keep track of spills which could adversely impact ground-
water quality.
Local Health Districts and IDEQ regional field offices often receive complaints
about suspected bacterial contamination of water from private well owners (IDEQ,
1985a). Documented instances of bacterial contamination have occurred in or near the
cities of Paul, Groveland, Collins, and Blackfoot (IDEQ, 1985a).
Potential For Contamination
A combination of natural and man-made factors determine the potential for
ground-water contamination within a given area. Natural factors, attributable to climate
and geologic history, include the thickness and nature of the unsaturated zone, depth to
ground water, and ground-water movement Qualitative measures of some combination
of natural factors is often called "hydrogeologic susceptibility". Man-induced factors
include water withdrawal, waste-water disposal, hazardous material handling and
disposal practices, and other land-use practices. Qualitative evaluation of man-induced
factors combined with hydrogeologic susceptibility is often termed "ground-water
vulnerability" or "potential for contamination".
Hydrogeologic susceptibility varies considerably within the eastern Snake River
Plain. Significant differences in the thickness and nature of the unsaturated zone
accounts for much of the variation. For instance, areas where fractured basalt crops out
at the land surface are more susceptible to ground-water contamination than areas
where thick deposits of clay-rich soil provide a degree of natural protection.
Some practices, such as the use of injection wells or inadvertent use of leaking
underground storage tanks, partly or entirely override whatever degree of natural
protection is afforded by the unsaturated zone. This is of particular concern in the
eastern Snake River Plain because of the widespread use of drain wells (Class V
injection wells) to dispose of excess irrigation water, urban storm Mnoff, and onsite
sewage system effluent (Yee and Souza, 1987; Parliman, 1988).
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Plumes of dissolved contaminants within the basalt tend to spread laterally, in the
direction of ground-water flow, within zones of high hydraulic conductivity. Interflow
zones of fine-grained sediment may almost entirely prevent vertical migration on a local
scale (IDEQ, 1985a).
Open-hole well construction is a common practice in the eastern Snake River
Plain which may have ground-water quality impacts. When much of the borehole is
uncased, water can mingle freely between producing zones. This could be a significant
concern wherever hydraulic head relationships are such that a stratigraphic interval of
poor quality water could contaminate zones of high quality water.
IDEQ has ranked most known and potential sources of ground-water
contamination (statewide) on the basis of human health risk (IDEQ, 1985b). From
greatest risk to least risk, the ranking is as follows: petroleum handling and storage;
feedlots and dairies; landfills and hazardous waste sites; land application of waste-water;
hazardous material handling; pesticide handling and use; land spreading of sludge and
solid or liquid septic tank pumpage; surface runoff; pits, ponds, and lagoons; radioactive
substances; fertilizer application; septic tank systems; mining, including oil and gas
drilling; wells (injection, geothermal, and domestic); and forestry practices. Within the
eastern Snake River Plain, injection and geothermal wells would be of higher concern
than on a statewide basis. In fact, the U.S. Geological Survey has emphasized the
importance of ground-water quality concerns associated with drain wells and geothermal
resource development (Yee and Souza, 1987).
DESCRIPTION OF BOUNDARIES
Areal boundaries of the Eastern Snake River Plain Sole Source Aquifer and
streamflow source area (fig. 1) are coincident with the boundaries of the eastern Snake
River Plain and contributory drainage area as delineated in the U.S. Geological Survey
Snake River Plain Regional Aquifer Study and Analysis. The U.S. Geological Survey
used a combination of geologic contacts and topography to delineate the eastern Snake
River Plain (Whitehead, 1986).
Generally, the aquifer boundary is the contact between Quaternary sedimentary
and volcanic rocks and the surrounding Tertiary and older rocks (fig. 3). Where rocks
equivalent in age to those in the eastern Snake River Plain extend up river valleys, a
topographic contour was chosen to define the boundary. The boundary separating the
eastern and western portions of the Snake River Plain follows a drainage divide from
the northern boundary of the plain to the Snake River at King Hill, follows the Snake
River to Salmon Falls Creek, and follows Salmon Falls Creek to the southern boundary
of the plain. Distinct changes in geology and hydrology that occur along the dividing
line make a hydrogeologic division feasible. The sole source aquifer area extends
vertically from the land surface through all geologic units that have the potential to
supply significant amounts of drinking water to wells.
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The streamllow source area boundary coincides with the topographic divides that
delineate the eastern part of the Snake River Plain drainage basin (fig. 2). The
boundary encompasses the drainage area that contributes surface and ground-water
recharge to the Eastern Snake River Plain Aquifer. Contaminated recharge water
originating in the streamflow source area could adversely affect ground-water quality of
the eastern Snake River Plain aquifer. Therefore, the project review area includes both
the aquifer and streamflow source areas.
DRINKING WATER USE
Ground water, withdrawn from wells and springs, supplies 100 percent of the
drinking water consumed within the eastern Snake River Plain (Gaia Northwest, 1988).
Total domestic water consumption is approximately 46,000 acre-feet per year. Twin
Falls obtains about 50 percent of its summer water supply from springs along the north
side of the Snake River; all other public drinking water purveyors in the eastern Snake
River Plain withdraw ground water from wells. About 39 percent of the eastern Snake
River Plain's population live outside of incorporated areas. Almost all rural residents
rely upon private wells for drinking water, but some receive drinking water from wells
operated by small water systems.
ALTERNATIVE DRINKING WATER SOURCES
Under EPA's 1987 Petitioner Guidance, an aquifer which serves as the sole or
principal source of drinking water for an area may not be designated as such if an
alternative source or combination of alternative sources can physically, legally, and
economically supply all those who depend upon the petitioned aquifer for their drinking
water (EPA, 1987). Aquifers petitioned before 1987 are not subject to this formal
alternative source feasibility criteria, but the feasibility of using alternative drinking
water sources has long been a consideration in sole source aquifer designation decisions.
In the past, EPA has considered that alternative sources of drinking water could
not economically supply the entire population of the eastern Snake River Plain. This
determination rested largely upon the informal assessment that the large numbers of
small towns, farms, and ranches in the area could clearly not be economically served
from other drinking water sources. A formal definition of "economical" did not seem
necessary to make this assessment.
In response to those who questioned EPA's alternative source determination, the
Agency hired a contractor to assess the legal and economic feasibility of supplying
drinking water from an alternative source or combination of sourc'efs to all those living in
the eastern Snake River Plain. This analysis was conducted as if the petitioned aquifer
were to be evaluated using the 1987 EPA Petitioner Guidance, which contains a formal
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definition of economic feasibility, to determine its eligibility for sole or principal source
aquifer designation.
The alternative drinking water source study conducted for EPA concluded that
alternative drinking water supplies are physically and legally available from surface
streams, such as the Snake River, and from aquifers within the streamflow source area
(Gaia Northwest, 1988). However, the study concludes that those living on farms or
ranches, and in towns of less than about 7000, cannot be served drinking water
economically from an alternative source. Therefore, only about 40 percent of the people
living in the eastern Snake River Plain can economically be supplied by an alternative
drinking water source.
CONCLUSIONS
An aquifer must supply 50 percent or more of the drinking water consumed over
the aquifer area in order to receive EPA designation as a sole or principal source
aquifer. Ground water supplies 100 percent of the drinking water consumed within the
eastern Snake River Plain. Further, no alternative source or combination of sources can
economically supply all those who obtain drinking water from the Eastern Snake River
Plain Aquifer. Therefore, the Eastern Snake River Plain Aquifer meets the criteria for
EPA designation as a sole source aquifer under Section 1424(e) of the Safe Drinking
Water Act.
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REFERENCES
Barnes, Eraesta B., Regional Administrator, U.S. Environmental Protection Agency,
Seattle, Washington, August 1, 1984, letter to U.S. Senator Steve Symms.
Barraclough, J.T., Lewis, B.D., and Jensen, R.G., 1982, Hydrologic Conditions at the
Idaho National Engineering Laboratory, Idaho - Emphasis: 1974-1978: U.S. Geological
Survey Water Supply Paper 2191, 52 pages.
Bassick, M.D., 1986, Compilation of references on geology and hydrology of the Snake
River drainage basin above Weiser, Idaho: U.S. Geological Survey Open File Report
86-245, 133 pages.
Bowler, Ned, November 10, 1986, letter from the Hagerman Valley Citizens' Alert to
Robie G. Russell, Regional Administrator, U.S. Environmental Protection Agency,
Seattle, Washington.
Burd, Robert S., Water Division Director, U.S. Environmental Protection Agency,
Seattle, Washington, December 16, 1986, letter to Ned Bowler, President, Hagerman
Valley Citizens' Alert.
Burd, Robert S., Water Division Director, U.S. Environmental Protection Agency,
Seattle, Washington, August 3, 1982, letter to John A. McDaniel, President, Hagerman
Valley Citizens' Alert.
Cenarrusa, P. T., 1987, Idaho Blue Book 1987-88, Compiled and published by the
Secretary of State, Boise, Idaho, 318 p.
Couch, Doris F., April 4, 1985, letter from the Magic Valley Aquifer Coalition, Buhl,
Idaho to Ernesta Barnes, Regional Administrator of the U.S. Environmental Protection
Agency, Seattle, Washington.
Dyer, K.L. and Young, H.W., 1971, A Reconnaissance of the Quality of Water from
Irrigation Wells and Springs in the Snake Plain Aquifer, Southeastern Idaho: U.S.
Geological Survey Open-File Report.
Frenzel, S.A., 1990, Idaho Water Supply and Use ]n National Water Summary 1987—
Water Supply and Use, compiled by Jerry E. Carr, Edith B. Chase, Richard W. Paulson,
and David W. Moody: U.S. Geological Survey Water-Supply Paper 2350.
Gaia Northwest, Inc., 1988, Drinking water consumption and alternative sources for the
eastern Snake River Plain, Idaho, 28 p.
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Hagerman Valley Citizens' Alert, October 27, 1982, material sent to Robert S. Burd,
Water Division Director, U.S. Environmental Protection Agency, Seattle, Washington.
Idaho Department of Health and Welfare, 1985a, Snake Plain Aquifer Technical
Report: Division of the Environment, Water Quality Bureau, 117 p.
Idaho Department of Health and Welfare, 1985b, Groundwater Quality Management
Plan for Idaho: Division of the Environment, Water Quality Bureau, 23 p.
Idaho Department of Water Resources, April 18, 1984, news release #30.
Kilburn, Charbot, 1964, Ground water in the upper part of the Teton Valley, Teton
Counties, Idaho and Wyoming: U.S. Geological Survey Water Supply Paper 1789, 60 p.
Kjelstrom L.C., 1986, Flow characteristics of the Snake River and water budget for the
Snake River Plain, Idaho and eastern Oregon: U.S. Geological Survey Hydrologic
Investigations Atlas HA-68, 2 sheets.
Lewis, R.E., and Young, H.W., 1982, Geothermal resources in the Banbury Hot Springs
area, Twin Falls County, Idaho: U.S: Geological Survey Water Supply Paper 2186, 27 p.
Lindholm, G.F., Garabedian, S.P., Newton, G.D., and Whitehead, R.L., 1988,
Configuration of the water table and depth to water, Spring 1980, water-level
fluctuations, and water movement in the Snake River Plain regional aquifer system,
Idaho and eastern Oregon.
Lindholm, G. F. and Goodell, S. A., 1986, Irrigated acreage and other land uses on the
Snake River Plain, Idaho and Eastern Oregon: U.S. Geological Survey Hydrologic
Investigations Atlas HA 691, 1 sheet.
Marshall, Wendy, December 20, 1982, memo to the Snake Plain Aquifer File, Drinking
Water Programs Branch, U.S. Environmental Protection Agency, Seattle, Washington.
Marshall, Wendy, May 4, 1983, memo to the Snake Plain Aquifer File, Drinking Water
Programs Branch, U.S. Environmental Protection Agency, Seattle, Washington.
Marshall, Wendy, April 2 and 3, 1984, mailing list, Drinking Water Programs Branch,
U.S. Environmental Protection Agency, Seattle, Washington.
Marshall, Wendy, May 10, 1984, letter to Sherl L. Chapman, Executive Director, Idaho
Water Users Association.
Marshall, Wendy, June 12, 1984, letter to Denton Darrington, State! 'Senator, District 26,
Idaho.
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McDaniel, John A., June 29, 1982, letter from the Hagerman Valley Citizens' Alert to
Donald P. Dubois, Regional Administrator, U.S. Environmental Protection Agency,
Seattle, Washington.
McDaniel, John A., September 25, 1982, letter from the Hagerman Valley Citizens'
Alert, to Robert S. Burd, Water Division Director, U.S. Environmental Protection
Agency, Seattle, Washington.
McDaniel, Jack, December 10, 1982, letter from the Hagerman Valley Gtizens Alert to
Wendy Marshall, Drinking Water Programs Branch, U.S. Environmental Protection
Agency, Seattle, Washington.
Mundorff, MJ., 1967, Ground Water in the vicinity of American Falls reservoir, Idaho:
U.S. Geological Survey Water Supply Paper 1846, 55 p.
Mundorff, M J., Crosthwaite, E.G., and Kilburn, Charbot, 1964, Ground Water for
Irrigation in the Snake River Basin in Idaho: U.S. Geological Survey Water Supply
Paper 1654, 224 p.
Norvitch, R. F., Thomas, C.A, and Madison, R. J., 1969, Artificial recharge to the Snake
River Plain aquifer in Idaho; An evaluation of potential and effect: Idaho Department
of Reclamation Water Information Bulletin No. 12, 59 p.
OCrowley, Janet, April 21, 1988, letter from the Committee for Idaho's High Desert,
Boise, Idaho, to Robie Russell, Regional Administrator, U.S. Environmental Protection
Agency, Seattle, Washington.
Osborne, Marvin D., April 1, 1988, letter from the Fort Hall Business Council, Fort
Hall, Idaho, to Robie Russell, Regional Administrator, U.S. Environmental Protection
Agency, Seattle, Washington.
Parliman, DJ., 1988, Idaho Ground-Water Quality in National Water Summary 1986—
Hydrologic Events and Ground-Water Quality, compiled by David W. Moody, Jerry
Carr, Edith B. Chase and Richard W. Paulson: U.S. Geological Survey Water-Supply
Paper 2325.
Sceva, Jack, Regional Geologist, U.S. Environmental Protection Agency, Seattle,
Washington, August 17, 1984, memo to Bill Schmidt, Acting Regional Chief, Field
Operations and Technical Support Branch, U.S. Environmental Protection Agency,
Seattle, Washington.
Scott, Harold, Sanitary Engineer, U.S. Environmental Protection Agency, Seattle,
Washington, June 16, 1977, memo to the "Sole Source File", Drinkihg Water Programs
Branch, U.S. Environmental Protection Agency, Seattle, Washington.
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Stearns, H.T., Crandafl, L., and Steward, W.G., 1938, Geology and ground water
resources of the Snake River Plain in southeastern Idaho: U.S. Geological Survey
Water Supply Paper 774, 268 p.
Stevens, Raleigh W., March 14, 1977, petition to the U.S. Environmental Protection
Agency, Seattle, Washington.
Whitehead, R.L., 1986, Geohydrologic framework of the Snake River Plain, Idaho and
eastern Oregon: Hydrologic Investigations Atlas HA-681, sheet 1 of 3.
U.S. Environmental Protection Agency, Region 10, March 29, 1984, news release
#84-29.
U.S. Environmental Protection Agency, Region 10, January 16, 1985, news release
#85-6.
U.S. Environmental Protection Agency, 1987, Sole Source Aquifer Designation
Petitioner Guidance: Office of Ground-water Protection, 30 p.
Yee, JJ.S. and Souza, W.R., 1987, Quality of Ground Water in Idaho: U.S. Geological
Survey Water Supply Paper 2272, 53 p.
Young, H.W., Parliman, DJ., and Jones, M.L., 1987, Selected Water-Quality Data for
the Burley Irrigation District, South-Central Idaho, March-April, 1987: U.S. Geological
Survey Open-File Report 87-240.
Young, H.W., Parliman, DJ., and O'Dell, I., 1987, Selected Water-Quality Data for the
Minidoka Irrigation District, South-Central Idaho, June 1987: U.S. Geological Survey
Open-File Report 87-465.
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