EPA 910^88-193
United Slates
Environmental Protection
Agency
Region 10
1200 Sixth Avenue
Seattle WA 98101
April 1988
Office of Ground Water
&EPA Resource Document
For Consideration of the
Lewiston Basin Aquifer as a
Sole Source Aquifer
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RESOURCE DOCUMENT FOR CONSIDERATION OF
THE LEWISTON BASIN AQUIFER
AS A SOLE SOURCE AQUIFER
Office of Ground Water
U.S. Environmental Protection Agency
Region 10
Seattle, Washington 98101
April 1988
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TABLE OF CONTENTS
Page
INTRODUCTION 1
Sole Source Aquifer Program 1
Sole Source Aquifer Petition 1
Purpose of Report 2
GENERAL DESCRIPTION OF THE LEWISTON BASIN 2
Geographical Overview 2
Climate : 4
Population 4
Geology 5
Pre-Tertiary Rocks 5
Columbia River Basalt 5
Basalt Deformation 8
Unconsolidated Sediments 8
Ground-Water Movement 8
Recharge 10
Discharge 11
Lewi ston Basi n Boundari es 13
Streamflow Source and Project Review Areas 13
Water Qual i ty 13
Potential for Contamination 14
Water Supply Systems 14
Alternative Sources 15
CONCLUSIONS 18
REFERENCES 19
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ILLUSTRATIONS
Page
Figure l...Areal Extent of the Columbia River Basalt 3
Figure 2...Generalized Basalt Stratigraphy of the Lewiston Basin 6
Figure 3...Idealized Basalt Flow Sequence and Basalt Intraflow
Structures 9
Figure 4...Areas of Ground Water and Surface Water Interconnection
for the Upper 800 Feet of the Grande Ronde Basalt 12
Attachment 1...Major Faults and Anticlinal Folds Surrounding
the Lewiston Basin, MAP
Attachment 2...Proposed Lewiston Basin Sole Source Aquifer and
Project Review Areas, MAP
TABLES
Table 1 Lewiston Basin Drinking Water Consumption 16
Table 2... .Alternative Source Economic Summary 17
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RESOURCE DOCUMENT FOR CONSIDERATION OF TIE
LEWISTON BASIN AQUIFER AS AS SOLE SOURCE AQUIFER
Sole Source Aquifer Program
The Safe Drinking Water Act, Public Law 93—523, was signed into law on
December 16, 1974.! This act provides the statutory basis for designation
of sole source aquifers by the Environmental Protection Agency. Section
1424(e) of the 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 defines a sole or principal source aquifer as one which supplies at
least 50 percent of the drinking water consumed in the area overlying the
aquifer.2 EPA guidelines also stipulate that designated sole source aquifer
areas have no available alternative source or combination of sources which
could physically, legally, and economically supply all those who depend upon
the aquifer for drinking water.2
Petition
On December 27, 1987, the Region 10 Office of the Environmental
Protection Agency (EPA) received a petition from the Asotin County Public
Utility District (PUD) requesting that EPA designate the "Russell Aquifer" as
a sole source aquifer.3 The PUD provided additional information through a
revised petition which was received by EPA on February 1, 1988.3
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The "Russell Aquifer" was defined as the upper 800 feet of the
Grande Ronde Formation within the Lewiston Basin by a hydrogeological
report published in 1980.4 For the purposes of this report, EPA has
combined the Grande Ronde Formation with other water-bearing rocks of the
Lewiston Basin and will refer to the aquifer system as the Lewiston Basin
Aquifer.
The Asotin County PUD petitioned EPA for designation of its water supply
as a sole source aquifer (SSA) under Section 1424(e) of the Safe Drinking
Water Act for a number of reasons. First, the PUD's action was designed to
"heighten public awareness and further concerns for protecting the
aquifer".2 Second, the PUD stated that SSA designation would provide "the
District and other groundwater users in the area a tool to protect the
aquifer from potential sources of contamination".3 Finally, the PUD
mentioned EPA project review authority and the possibility of future financial
assistance for aquifer protection efforts as reasons for submitting their
petition.
Purpose
This document summarizes available information about the ground-water
resources of the Lewiston Basin, Washington and Idaho, and will provide the
technical basis for an EPA decision regarding sole source aquifer
designation. Those interested in more detailed information may consult the
references listed at the end of the report.
GENERAL DESCRIPTION OF THE LEiSTON BASIN
Geographical Qverviex
The Lewiston Basin covers approximately 400 square miles of southeastern
Washington and western north-central Idaho.* The structural and topographic
downwarp lies near the eastern margin of a more than 50,000 square mile area,
underlain by layered basalt, known as the Columbia River Plateau
(Fig. l).5>o The asymmetrically shaped basin consists mostly of a basalt
plateau surface, deeply dissected by permanent and seasonal drainages, which
slopes gently northward.7 Along the northern margin of the basin and near
its structural center lies a pronounced triangular shaped lowland at the
confluence of the Snake and Clearwater Rivers.7 The cities of Lewiston,
Idaho and Clarkston, Washington are built upon a series of gravel terraces on
this lowland area where the two rivers meet.8 Elevations within the basin
range from about 800 feet along the Snake River to over 5000 feet along the
southern margin of the area. A description of the hydrogeologic boundaries of
the basin occurs later in this report, as does a map (Attachment 2)
delineating those boundaries.
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121
119°
1 17°
-491
-48°
WASHINGTON
Seattle
50
100 MILES
— 44
50 100 150 KILOMETERS
FIGURE 1
Areal Extent of the Columbia River Basalt
(Modified from Drost and Whiteman, 1986)
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A semi-arid continental climate prevails across the lower elevations of
the Lewiston Basin. Temperature records from the Lewiston-Nez Perce County
Airport (elevation 1436 feet) show an average January temperature of 31
degrees (Fahrenheit), an average July temperature of 74 degrees, and an
annual average of 52 degrees.4 Precipitation averages about 13 inches each
year in the Lewiston-Clarkston area whereas higher elevation areas average
close to 25 inches of precipitation annually.4 Summers in the
Lewiston-Clarkston area tend to be quite hot and dry. Temperatures often
exceed 100 degrees during July and August while the humidity averages 25
percent during those months.4
Population
The cities of Lewiston and Clarkston, located near the confluence of the
Clearwater and Snake Rivers, account for most of the Lewiston Basin's
population. A breakdown of the area's population by political subdivision
appears below.
POPULATION WITHIN THE LEWISTON BASIN
Political Subdivision
Nez Perce County, Idaho:
Lewiston
Lapwai
Unincorporated
Total
Asotin County, Washington:
Asotin
Clarkston
Unincorporated
Total
Garfield County, Washington:
Unincorporated
Estimated Population*
28,050
1,045
1,005
30,100
1,020
6,730
8.550
16,300
100
Lewiston Basin Total
46,500
* City and town populations are from the Idaho Blue Book, 1987-88 and the 1987
Washington State Yearbook. Rural population figures were provided by the
petitioner.
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Geology
The rocks of the Lewiston Basin fall into three general groupings:
pre-Tertiary rocks, Miocene basalts of the Columbia River Group, and
unconsolidated sediments. Pre-Tertiary rocks crop out extensively south,
east, and north of the basin but are almost completely covered by basalt
within the basin itself. Basalts of the Columbia River Group dominate the
landscape of the Lewiston Basin and also contain most of its ground water.
Pliocene to Holocene sediments, consisting mostly of gravels, cover the basalt
in low-lying areas of the basin along the Snake and Clearwater Rivers and some
tributary streams.
PRE-TERTIARY ROCKS
The only mapped exposure of pre-Tertiary rocks within the Lewiston Basin
occurs along the Snake River in Township 33N, Range 47E, just northwest of the
basin bounding Limekiln Fault. At this locality, a wide variety of Triassic
through Cretaceous metamorphic rocks, all mapped as one unit, crop out over a
roughly three square mile area which has over 3000 feet of relief.7 This
same map unit crops out extensively in Hells Canyon, to the south, and in
parts of the Clearwater Plateau east of the basin.7'9 North of the basin,
Jurassic through Cretaceous intrusive granitic rocks outcrop extensively at
Bald Butte.7 Numerous investigators, beginning in 1897, have concluded that
these pre-Tertiary "basement" rocks exhibit over 4000 feet of relief north,
south, and east of the basin.7,9,10,11 NO wells within the Lewiston Basin
have fully penetrated the basalt and reached the pre-Tertiary surface but
ground-water studies east of the basin indicate that the pre-Tertiary rocks do
not act as aquifers.12
COLUMBIA RIVER BASALT
Miocene basalts of the Columbia River Group cover much of the Pacific
Northwest, including the Lewiston Basin, as shown by Figure 1. These rocks,
referred to as flood basalts, originated as lava which flowed freely to the
surface from long, narrow, northwest-trending vents or fissures.13 The
low-viscosity molten rock spread easily, cooling to form thin but areally
extensive basalt beds.
Dozens of these individual flow units, whose thickness ranges from a few
inches to about 300 feet, are stacked upon one another to form the Columbia
River Basalt Group.11 The total thickness of the basalt flows exceeds 4,000
feet and may reach 9,000 to 15,000 feet in the central part of the area shown
in Figure 1.5,14 within the Lewiston Basin, the Columbia River Basalt Group
is over 3,000 feet thick, and has been stratigraphically subdivided into four
formations as shown in Figure 2.
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Formation
Thickness (feet)
a
3
o
O
>
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The earliest of the Columbia River Basalt flows, labeled the Imnaha
Formation, occurs only in northeastern Oregon and a'djacent parts of Washington
and Idaho.11 Apparent age determinations by whole-rock potassium-argon
(K-Ar) radiometric analyses suggest that the Imnaha lavas were extruded about
16.5 million years agoJ1 Younger basalts cover the Imnaha Formation over
most of the basin; mapped exposures occur only along the southeastern and
northwestern margins near basin boundary faults.7 The thickness of the
Imnaha varies greatly due to the local relief of the pre-basalt surface with
areas of ponded lava reaching 400 feet.7*11 The unit contains noticeably
coarser grains than the overlyinq Grande Ronde Formation, and has weathered to
form slopes where it outcrops.7*T1 No wells in the Lewiston Basin have been
drilled to the Imnaha Formation, because adequate supplies of ground-water are
available in the overlying Grande Ronde Formation.
The Grande Ronde Formation accounts for by far the greatest volume of
basalt throughout the Columbia River Plateau, including the Lewiston Basin.
Estimated thicknesses vary from 2800 feet in the southern Lewiston Basin to
2,000 feet in the northern basin.9 Individual basalt flows vary in
thickness from a few feet to over 150 feet, and sedimentary deposits between
flows are uncommon.4,7
The hard, dark gray to black colored rock, composed mostly of microscopic
crystals of iron, calcium, and magnesium rich alumnosi1icate minerals,
weathers along fractures to form prominent cliffs throughout the Columbia
River Plateau. In the Lewiston Basin, The Grande Ronde Basalt crops out
mostly where surface drainages have cut deep and narrow canyons into, but not
completely through, the formation.7
The Grande Ronde Formation produces most of the ground-water consumed in
the area because of its great thickness, extensive lateral continuity, and
lack of fine-grained sedimentary interbeds. Water level maps emphasize the
dominance of the Grande Ronde Formation within the Lewiston Basin by showing
generalized contours for the Grande Ronde Formation only, and not other
units.14 Ground water in the much thinner overlying basalts occurs in a
series of laterally discontinuous areas as a result of the many canyons which
have downcut into the Grande Ronde Formation.
A strati graphic horizon of weathered basalt and clay forms the top of the
Grande Ronde Formation. This interbed, which has an apparent absolute age of
about 14.5 million years, occurs continuously across the basin.4.7.11 The
weathered basalt (saprolite) and clay surface marks the top of the most
prolific aquifer in the basin. Water can percolate through the saprolitic
horizon, however, and so the Grande Ronde Formation of the Lewiston Basin
cannot be considered hydrogeologically separate from the overlying
water-bearing rocks.
Thick and areally extensive basalt flows of Wanapum age pinch-out west of
the Lewiston Basin.5,9 Within the basin itself, flows of the Wanapum
Formation occur sporadically.4'9 These patterns of occurrence suggest that
the basalt surface in the Lewiston Basin area was beginning to deform during
Wanapum time.4>9 Fine grained sedimentary beds between flow units are
common.4 Partly for this reason, wells completed in the Wanapum yield much
smaller quantities of water than those completed in the Grande Ronde Formation.
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Basalts of the Saddle Mountains Formation overlie an extensive fluvial
deposit, the Sweetwater Interbed, which thins away from the structural center
of the basin.9 The Saddle Mountains flows originated within the Lewiston
Basin and occur as two distinct physical types: (1) stratiform basalts, thin
areally extensive flows separated by sedimentary interbeds, and
(2) channelform basalts, interpreted to represent lava-filled canyons of the
ancestral Snake River.9,11 The massive channelform basalts are not tapped
for ground water. The stratiform basalts, like those of the Wanapum
Formation, yield much smaller quantities of ground water than the Grande Ronde
Formation.
BASALT DEFORMATION
Deformation of the entire Columbia River Basalt Group has produced the
present structural configuration of the Lewiston Basin. Deformation began
after deposition of the Grande Ronde Formation but became most intense after
deposition of the entire Columbia River Basalt Group during the period of
mountain building which produced the Blue Mountains and the Cascade
Range.4'8-9. Assuming that the top of the Grande Ronde Formation was
roughly level when formed, deformation has produced about 5000 feet of relief
in the basin since extrusion of the Grande Ronde Basalt ceased.7
UNCONSOLIDATED SEDIMENTS
Unconsolidated sedimentary material, composed predominately of gravel
with some sand and silt, covers much of the approximately 20 square mile
triangular lowland at the confluence of the Snake and Clearwater River.7.8
These sediments, which have been divided into seven stratigraphic units, are
interpreted to represent recent alluvium and ancestral channels of the
Clearwater, Salmon, and Snake Rivers.8 Driller's logs from 23 wells in the
Lewiston-Clarkston area record overburden depths of up to 190 feet although
typical thicknesses range between 20 and 100 feet.4 The unconsolidated
sediments do transmit water to the underlying basalt and, where adequate
recharge exists, form water table aquifers atop the basalt aquifer
system.4'15 However, few wells produce from the unconsolidated
sediments and no published information characterizes the water table
aquifers.15
Ground Water Hovement
Basalts of the Columbia River Group, and especially those of the Grande
Ronde Formation, store and transmit most of the ground water in the Lewiston
Basin. The basalt itself is dense and impermeable to water. However, the
basalt flows are fractured throughout as illustrated by Figure 3. Most ground
water moves laterally along flow tops (composed of vesicular and broken basalt
formed by rapid cooling at the top of the flow), but some water also moves
between flow tops through the entablature and colonnade (Fig. 3). Very thin
basalt flows may consist only of a flow top and an intensely fractured base
which forms a good hydrologic connection with the underlying flow top.6 The
center portions of thick flows, although not impermeable, may restrict
vertical ground-water movement enough to act as confining beds.5.6.14
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FIGURE 3
Idealized Basalt Flow Sequence and Basalt Intraf low Structures
250 feet
Flow top
Flow center
Flow base
Inter bed
flow top
Flow center
Flow base
Flow top
FLOW TOP
ENTABLATURE
(fanning columns)
COLANNADE
(Blocky joints)
(Platy joints)
(Vesicular base)
100 feet
Water moves nost easily along basalt flow tops as illustrated by the thicker
arrows snowing direction of water movement. The flow thickness of 100 feet
represents a typical flow, but actual flow thicknesses within the Lewiston
Basin range from a few feet to over 150 feet.
(from Whiteman, 1986)
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10
Sedimentary deposits between basalt flows, called interbeds, vary greatly
In their thickness and ability to transmit water. Ground water moves easily
through coarse-grained interbeds but hardly at all through fine-grained
units. Fine-grained interbeds, which act as confining units, occur commonly
in the Wanapum and Saddle Mountains Formations but rarely between flows of the
Grande Ronde Basalt.4 About two-thirds of selected well logs (mostly
sample descriptions by well drillers) from the Lewiston Basin document
fine-grained interbeds somewhere in the stratigraphic section; reported
thicknesses in individual beds range from 1 to 175 feet but are mostly less
than 30 feet.4
On a grand scale, ground water moves from the higher elevation areas of
the Lewiston Basin, mostly through the basalt flow tops, towards the lower
elevation areas of the basin. Generalized water level contours for the Grande
Ronde Basalt range from over 5000 feet near the crest of the Blue Mountains to
less than 800 feet along the Snake River.14 However, geologic features
within the basin act to intercept, direct, or dramatically slow the ground
water as it moves through the gently sloping basalt beds. Where canyons have
cut deeply into the basalt, ground water discharges as springs in the canyon
walls. Springs such as these provide the baseflow for the permanent streams
of Asotin, George, and Pintler Creeks.4 All permanent and a number of
seasonal drainages within the basin completely intercept ground water moving
laterally through the Wanapum and Saddle Mountains Formations.? In
contrast, no canyons entirely intercept ground water moving laterally through
the thick Grande Ronde Formation, although Asotin Creek does intercept ground
water moving through the upper 400 feet of the 2,000 to 2,800 foot thick
unit.4>7»9 Dikes, which are long and narrow northwest trending volcanic
vent relics, impede the flow of ground water since the solid basalt-filled
fissures cut vertically through the stratigraphic section.6.13 A pump test
in 1979 identified a hydrogeologic barrier, interpreted as a dike, located
between two water supply wells which produce from the Grande Ronde Formation
southwest of Clarkston.^
The rate of ground-water movement within the basin has only been
estimated for the Grande Ronde Formation beneath the Lewiston-Clarkston area.
An average ground-water velocity of 0.84 feet per day was calculated by using
a hydraulic conductivity value of 134 feet per day (derived from
transmissivity as determined by a pump test), a gradient of 3.3 feet per mile
(based upon water level data), and an estimated average porosity of
10 percent.4 However, assuming the same values for porosity and hydraulic
conductivity, but increasing the gradient to 2 degrees (184 feet per mile),
which roughly represents the regional dip of the basalt along the southern
flank of the basin, yields an average ground-water velocity of nearly 47 feet
per day.
RECHARGE
Recharge to the Lewiston Basin aquifer system occurs principally from
streamflow infiltration. Streamflow infiltration to the basalt aquifers
occurs mostly where rivers and creeks flow over basalt flow tops, which
happens where basalt beds dip more steeply than the surface drainage
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11
gradient.4 These areas- have been delineated for the upper 800 feet of the
Grande Ronde Formation (Fig. 4). Although no streamflow data have been
obtained to measure the amount of recharge, water level records of deep wells
in the Lewiston-Clarkston area suggest excellent hydrologic communication
between surface water sources and the Grande Ronde Formation. Deep wells in
the Lewiston-Clarkston area show no long term declines and, in fact, water
levels rose and stabilized at a higher level in response to the filling of
the Lower Granite Reservoir.4 Also, detailed measurements during aquifer
tests show that water levels in some wells respond to changes in river
stage.4
The Wanapum Basalts dip beneath the Clearwater and Snake Rivers in a
manner which would favor recharge from the rivers. However, declining water
levels over the past 20 years in wells near likely recharge areas suggest that
streamflow less effectively recharges the Wanapum Basalts.4
Precipitation easily recharges the basalt and unconsolidated sediment
lying at or near the surface. At lower elevations, however, scant
precipitation and high evaporation rates probably allow recharge via
precipitation on a sporadic basis only. Precipitation may significantly
recharge the basalt aquifers at higher elevations, but the hydrogeologic
impediments mentioned earlier (canyons and dikes) may prevent much of that
ground water from reaching the water supply wells in the Lewiston-Clarkston
area.4 Also, if most recharge to the Grande Ronde Formation occurred in
the higher elevation areas of the basin, static water levels would be
considerably higher rather than at the approximate elevation of the river
recharge areas.4
Excess irrigation water, which recharges water table aquifers in the
unconsolidated sediments, also partly recharges aquifers of the Wanapum and
Saddle Mountains Formations in parts of the Lewiston-Clarkston area.4'1^
Predominately lateral flow through the upper basalt and fine-grained interbeds
combine to prevent most excess irrigation water from percolating to the Grande
Ronde Basalt before reaching a discharge point.
DISCHARGE
Shallow ground water discharges mostly as springs along deeply incised
surface drainages, whereas production wells tap much of the deeper ground
water in the Lewiston-Clarkston area. Cohen and Ralston (1980) have
identified areas where ground water from the Grande Ronde Formation discharged
naturally to the Snake River before construction of Lower Granite Dam
(Fig. 4). However, since the filling of the Lower Granite Reservoir (February
of 1975), the static water levels in wells near the river have been below the
elevation of the reservoir surface.4 Thus, it seems that at least part of
the natural discharge area now acts as a recharge area.4
Rates of production (artificial discharge) from wells within the Grande
Ronde Formation are considerably higher than those from the overlying basalt
and unconsolidated sediments. The Asotin County PUD wells produce at rates of
1345 to 4220 gallons per minute (gpm) from the top 800 feet of the Grande
Ronde Basalt.3 In contrast, wells completed in the Wanapum and Saddle
Mountains Formations average 10 to 30 gpm.4
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12
Areas of Ground Water and Surface Water Interconnection
for the Upper 800 Feet of the Grande Ronde Basalt
Washington! Idaho
Natural Recharge Area
Natural Discharge Area
FIGURE 4
Available Information suggests that the natural discharge area became a
recharge area after filling of the Lower Granite Reservoir in 1975. All
shaded areas are now interpreted to represent recharge areas for the
upper 800 feet of the Grande Ronde Basalt.
(Modified from Cohen and Ralston, 1980)
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13
Lewiston Basin Boundaries
Major faults and anticlinal folds form most of the hydrogeological
boundaries of the proposed Lewiston Basin sole source aquifer area. Faults
act as ground-water barriers by offsetting highly permeable flow tops. 6
Also, the pulverized rock in the fault zone weathers to form a clay-rich plane
of low permeability.4'6 Tight folds, caused by stress intense enough to
severely deform the basalt, but not so strong as to offset the beds, crush and
compact the flow tops so that they transmit water much more slowly. Major
anticlinal folds, where the strata dip downward from the fold axis, also act
as regional ground-water divides. The major faults and anticlinal folds which
bound the Lewiston Basin are shown on Attachment 1. The basin boundaries,
delineated on Attachment 2, follow these faults and anticlinal folds except in
the southwestern part of the basin. The southwestern boundary has been drawn
along a major topographic divide in the Blue Mountains which acts as a
regional ground-water divide. No water budget studies for the basin, which
would serve to check the hydrogeological significance of the boundaries, have
been published.
Streamflow Source and Project Review Areas
Rivers and creeks flow across the structural barriers which act as
boundaries for the ground-water basin. Federal financially assisted projects
located in the drainage basins of surface streams which recharge ground water
within the Lewiston Basin may constitute significant sources of contamination
to the aquifer. While the entire Streamflow source area includes all of the
Snake River drainage upstream from Silcott, Washington, only a portion of the
Streamflow source area immediately adjacent to the ground-water basin has been
delineated for project review purposes (see Attachment 2).
Hater Quality
Public water supply wells in the Lewiston-Clarkston area produce
excellent quality water from the Grande Ronde Formation. The water typically
contains fewer than 350 parts per million (ppm) total dissolved solids (TDS),
and requires no treatment before drinking.3,4 jne chemistry of water
withdrawn from the Grand Ronde Formation appears typical for ground water
from the Columbia River Basalt, and also strongly resembles chemical analyses
of surface water from the Snake and Clearwater Rivers.4-16 The only
published report of contaminated ground water in the area occurred in shallow
alluvial sediments of Lapwai Creek, probably from septic tank and drain field
usage.'2
Temperature readings of water from public water supply wells (all
completed in the Grande Ronde Formation) range from 60 to 74 degrees
(Fahrenheit). This corresponds with the typical geothermal gradient of 2
degrees per 100 feet for the Columbia River Basalts.16 Temperature data for
shallower wells has not been published but probably range closer to the mean
annual surface temperature of 52 degrees.
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14
Potential For Contamination
Aquifer units within the Lewiston Basin are vulnerable to contamination
for one or more of the following reasons: (1) they occur at or near the
surface, where precipitation, excess irrigation, and other artificial recharge
can introduce contaminants to the subsurface; (2) they are extensively
recharged by surface waters; (3) they are hydrologically connected to
near-surface aquifers, either naturally or by well bores.
The most valuable portion of the Lewiston Basin aquifer system from a
drinking water standpoint, the upper 800 feet of the Grande Ronde Formation,
is most vulnerable to contamination from surface water recharge. Therefore,
any project or activity which would threaten the water quality of a possible
surface water recharge area (Fig. 4) would also pose a threat to the
principal source of drinking water within the Lewiston Basin. The Grande
Ronde Basalt aquifer could also suffer if the overlying water-bearing strata
became contaminated. In the event of such an occurrence, improperly cased
wellbores would provide the most likely conduits. Natural seepage of
contaminated ground water from overlying formations presents a real but less
likely threat because of (1) the predominately lateral ground-water flow
through overlying basalts and (2) areas of thick fine-grained sedimentary
interbeds which inhibit percolation.
Aquifers in the unconsolidated sediments and upper basalt units are
susceptible to contamination from surface activity since they lie close to the
surface in the most populous portions of the Lewiston Basin. Possible sources
of contamination include improper storage or handling of hazardous materials,
septic tank effluent, storm runoff, pesticides, and chemical fertilizers.
Although the shallow aquifer units serve far fewer people than the Grande
Ronde Formation, they do serve as the sole source of drinking water for some
households.3 Also, they are hydrologically connected to the Grande Ronde
Basalt (although poorly in many areas). But most importantly, they discharge
to surface waters which, in turn, recharge the Grande Ronde Formation.
Hater Supply Systems
The Lewiston-Clarkston area accounts for most of the drinking water
consumed in the Lewiston Basin. The city of Lewiston uses water withdrawn
from the Clearwater River for most of its needs but depends upon wells which
produce from the Grande Ronde Formation for about 17 percent of its
consumption.2 All other public water purveyors in the Lewiston Basin depend
entirely upon wells which produce from the Grande Ronde Formation.3 Private
users, such as food processors, who depend upon large volumes of high quality
water derive their supplies exclusively from wells completed in the Grande
Ronde Basalt.3 Individual households which need only small quantities of
ground-water utilize the basalts and sediments which overly the Grande Ronde
Formation.3-4 A summary of drinking water consumed within the Lewiston
Basin, prepared as part of the sole source aquifer petition submitted by the
Asotin County PUD, appears as Table 1. This table shows that ground water
supplies about 68 percent of the water consumed within the basin, which is
well above the 50 percent required for sole source designation.2
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15
Alternative Sources
Surface water supplies capable of serving the Lewiston Basin are
physically and legally available, but using the available surface water would
be economically infeasible. The main water purveyors for the area, the city
of Lewiston and the Asotin County PUD, have water rights which would allow
them to legally withdraw enough surface water to supply the whole area. In
fact, the Asotin County PUD alone has legal access to approximately 43 million
gallons per day from the Snake River - enough to supply the peak water usage
for the entire basin.3 However, public and private water purveyors have not
fully utilized surface water sources because the Grande Ronde Basalt provides
higher quality water at a far lower cost. Surface water from the Snake and
Clearwater Rivers requires filtration and disinfection before municipal use.
Also, surface water treatment, storage, transmission and distribution
facilities cost considerably more to build and operate than systems using high
quality ground water.
In order to be considered "economically feasible", an alternative water
source must not cost the typical household more than 0.4 to 0.6 percent of the
average annual household income for the area.2 Cost estimates generated by
the Asotin County PUD indicate that the cost of using surface water render
that alternative source economically infeasible for all the public water
purveyors in the basin (Table 2). The cost of replacing individual homeowner
wells with a surface water supply would be considerably higher.
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TABLE 1
Lewiston Basin Drinking Water Consumption
Grande Ronde Fonration
Other Ground Water
Surface Water
TOTAL
PUBLIC UTILITY
DISTRICT NO. 1
OF ASOTIN CO.
16MGO (100)
0 0
0 0
16 (100)
CITY OF
LEWISTON
26 (17)
0 0
12 (83)
14.6 (100)
OTHER ,
PUBLIC SUPPLIES
3.3 (100)
0 0
0 0
3.3 (100)
b
INDUSTRIAL
3.9 (100)
0 0
0 0
3.9 (100)
PRIVATE
0.3 (601
(EST.I
0.1 (20)
(EST.)
0.1 C20I
(EST.I
0.6 (100)
(EST.)
TOTAL
USAGE
26.0
0.1
12.1
38.2
ttOF
TOTAL USE
68
<1
32
too
OTHER PUBLIC SUPPLIES
LAPWAI CITY 0.3 MGD
LOID* 1.9
ASOTIN CITY 0.8
PORT OF WILMA 0.3
INDUSTRIAL USES (culinary and high quality process requirement)
POTLATCH INC. 0.4 MGD
TWIN CITY FOODS 2.8
OMARK IND. 0.7
3.9 MGD
3.3 MGD
NOTES:
I. All of the usage figures given are peak day flows in million gallons per day. MGD. (average day usage is approximately 30% ol the value shown)
2. Numbers shown in parentheses.!), indicate the percent ot supply provided by the source.
Lewislon Orchards Irrigation District
(From the Asotin County PUD Petition, 1988)
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TABLE 2
Sunmary of the economic burden if public water purveyors within the Lewiston Basin
were to depend entirely upon surface water sources of drinking water
water Uteri
rub II
III Mr
of Al
Clly
c Utility
lit No. 1
olln County
nf Uwlaton
Source
Petitioned
Aquifer
Petitioned
Aquifer
Potent lei
Source
Treat
Petitioned
or Surfece
Supply
Treat
Petitioned
or Surfece
Supply
Source
Rttgulreatentl
16 MCD
Treatment
Plant
2.5 MCO
Treatftcnt
Plant
Additional
Fact II ty
Coat
$9,000,000
52,500,000
Added
Annual
Coat
$900,000
S250.000
Added
Added
Coat
9250,000
$ 70,000 '
Current
Cu»toe*ra
5,189
Additional
Current Coat Per
Avcrefe Cuetuoer
Annual Ulln
Charge Source
$115 $111
Total
Projected
With llouaehold
Source 190?
$528 $19,676
Projected
Current Percent
Of With
For Ueter Source
i.ix i.n
5.0)1
$182
1 U
City of Lcwlaton
City of l-apwal
Lcwlaton Orchard
Irrigation Dlitrlct
(Culinary Sytten)
City of Asrtln
Treated
Surface
Water
Petitioned Treat .) MCD
Aquifer Petitioned Treatment
or Surface Plant
Supply
Petitioned Treat 1.9 HCD
Aquifer Petitioned Treat Mot
or Surface Plant
Supply
Petitioned Treat .8 MCD
Aqulfer PetItloned Treatpent
or Surface Plant
Supply
8 $00,000 t 50,000 | 14,000 100 I 40 $21) 827J
$2.100.000 $210,000 $ il.OOO 5,200 8U4 f SI $1W
11,000,000 $100,00 I 28,000 364 $21* $350 S)tt
$28,713
$2»,7IJ
$29.676
nnil Ibrketlni KinaeeBeni lb(eiln« Survey ami Buy In» Power, IVcceter 198) Cdlllun.
EPA considers any alternative water supply which would cost the typical customer
more than 0.5 percent of the average household income for the area to be
economically infeasible.^
(From the Asotin County PUD petition, 1988)
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18
An aquifer or aquifer system must supply 50 percent or more of the
drinking water for an area in order to receive designation as a sole source
aquifer.2 Additionally, EPA presently requires that no alternative sources
exist which could feasibly serve the population dependent upon the aquifer.2
Ground water supplies about 68 percent of the drinking water for the
Lewiston Basin. Adequate supplies of surface water are physically and legally
available, but are considered economically infeasible. Therefore, the
Lewiston Basin Aquifer meets the criteria for designation as a sole source
aquifer under Section 1424(e) of the Safe Drinking Water Act.
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19
REFERENCES
1. Safe Drinking Water Act, Public Law 93-523.42 U.S.C. 300 et. seq.
2. Environmental Protection Agency, 1987, Sole Source Aquifer Designation
Petitioner Guidance: Office of Ground Water Protection, 30 pages.
3. Sole Source Aquifer Petition Received by the U.S. Environmental
Protection Agency, Seattle, Washington, December 27, 1987. Revised
Petition Received by EPA on February 1, 1988.
4. Cohen, P.L. and Ralston, D., 1980, Reconnaissance Study of the "Russell"
Basalt Aquifer in the Lewiston Basin of Idaho and Washington: Idaho Water
Resources Research Institute, Moscow, Idaho, 164 pages.
5. Drost, B.W. and Whiteman, K.J., 1986, Surficial Geology, Structure, and
Thickness of Selected Geohydrologic Units in the Columbia Plateau,
Washington: U.S. Geological Survey Water-Resources Investigations Report
84-4326, 11 Sheets.
6. Newcomb, R.C., 1961, Storage of Ground Water Behind Subsurface Dams in
the Columbia River Basalt, Washington, Oregon, and Idaho: U.S. Geological
Survey Professional Paper 383-A, 15 pages.
7. Swanson, D.A., Wright, T.L., Camp, V.E., Gardner, J.N., Helz, R.T.,
Price, S.M., Reidel, S.P., and Ross, M.E., 1980, Reconnaissance Geologic
Map of the Columbia River Basalt Group, Pullman and Walla Walla
Quadrangles, Southeast Washington and Adjacent Idaho: U.S. Geological
Survey Map 1-1139, 2 sheets.
8. Webster, G.D., Kuhns, M.J.P., and Waggoner, G.L., 1982, Late Cenozoic
Gravels in Hells Canyon and the Lewiston Basin, Washington and Idaho, in
Bill Bonnichsen and R.M. Breckenridge, editors, Cenozoic Geology of
Idaho: Idaho Bureau of Mines and Geology Bulletin 26, pages 669-683.
9. Bond, J.G., 1963, Geology of the Clearwater Embayment: Idaho Bureau of
Mines and Geology Pamphlet 128, 83 pages.
10. Russell, I.C., 1897, A Reconnaissance in Southeastern Washington: U.S.
Geological Survey Water Supply Paper, volume 4, pages 1-76.
11. Swanson, D.A., Wright, T.L., Hooper, P.R., and Bentley, R.D., 1979,
Revisions in Stratigraphic Nomenclature of the Columbia River Basalt
Group: U.S. Geological Survey Bulletin 1457-G, 59 pages.
12. Castelin, P.M., 1976, A Reconnaissance of the Water Resources of the
Clearwater Plateau, Nez Perce, Lewis, and Northern Idaho Counties, Idaho:
Idaho Department of Water Resources Bulletin 41, 46 pages.
-------
20
13.
14.
15.
16.
Swanson, D.A., Wright,
and Estimated Rates of
Basalt on the Columbia
pages 877-905.
T.L., and Helz, R.T., 1975, Linear Vent Systems
Magma Production and Eruption for the Yakima
Plateau: American Journal of Science, volume 275,
Whiteman, K.J. 1986, Water Levels in Three Basalt Hydrogeologic Units
Underlying the Columbia Plateau, Washington and Oregon, 1984: U.S.
Geological Survey Water Resources Investigations Report 86-4046, 4 sheets
Personal Communication with Dale Ralston, Professor of Geology at the
University of Idaho, on April 13, 1988.
Newcomb, R.C., 1972, Quality of the Ground Water in the Basalt of the
Columbia River Group, Washington, Oregon, and Idaho: U.S. Geological
Survey Nater Supply Paper 1999-N, 71 pages.
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ATTACHMENT 1
MAJOR FAULTS AND ANTICLINAL FOLDS
SURROUNDING THE LEWISTON BASIN
-------
LOCATION OF FAULTS AND ANTICLINAL FOLDS
BORDERING THE LEWISTON BASIN
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ATTACHMENT 2
PROPOSED LENISTON BASIN
SOLE SOURCE AQUIFER AND PROJECT REVIEW AREAS
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