TSD

000009

---

NATURAL

RESOURCE

TECHNOLOGY

POND

D

CLOSURE ALTERNATIVES REPORT

HOTSONVTLLE POWER

STATION

CRAWFORD

COUNTY

ILLINOIS

Project No:

1954

Prepared

For:

Ameren

Services

One

Ameren

Plaza

1901

Choutcau Avenue

St. Louis,

Missouri

Prepared

By:

Natural

Resource

Technology,

Inc.

23713

W.

Paul

Road,

Suite

D

Pewaukee.Wl

53072

April

27,2009

€^.

A

C-^^-^L^

EricJ.Tuichac,PE^

Senior

Engineer

6ruceR.

Hensel'r'GY

/f

Principal

Hydrogeologist

23713

W.

Paul

Rd.,

Ste.

D

•

Pewaukee,

Wl

53072

•

Phone: 262.523.9000

•

Fax:

262.523.9001

•

www.naturalrt.com

TSD 000010

---

TABLE

OF

CONTENTS

1

INTRODUCTION..............................................................................................1-1

1.1

Background............................................................................................1-1

1.2

Closure

Objectives

and

Approach

..............................................................1-2

2

SITE

CONDITIONS...........................................................................................2-1

2.1

Distribution of

Coal

Ash

Fill

......................................................................2-1

2.2

Hydrogeology

and

Groundwater

Quality

.....................................................2-1

2.2.1

Hydrogeology

..............................................................................2-1

2.2.2

Groundwater

Quality.....................................................................

2-2

2.3

Potential

Groundwater

Receptors

..............................................................2-3

3

IDENTIFICATION

AND

SCREENING OF CLOSURE

ALTERNATIVES..........................3-1

3.1

Overview...............................................................................................3-1

3.2

Screening

Criteria

...................................................................................3-1

3.3

Groundwater

Management

Alternatives......................................................

3-2

3.3.1

Overview

....................................................................................3-2

3.3.2

Site

Monitoring

with

No

Groundwater

Collection

...............................3-3

3.3.3

Groundwater

Collection

Trench

......................................................3-3

3.3.4

Containment

Using

a

Low-Permeability

Vertical

Barrier.......................3-4

'"\

3.3.5

Ash Stabilization

'

..........................................................................3-4

.//

3.3.6

Ash

Removal

and

Disposal.............................................................3-5

3.3.7

Pond

D

Reconstruction..................................................................3-6

3.4

Final

Cover Alternatives

...........................................................................3-6

3.5

Surface Water

Management

Alternatives....................................................3-7

4

SELECTED

CLOSURE ALTERNATIVES

................................................................4-1

4.1

Overview...............................................................................................4-1

4.2

Total Estimated Preliminary

Costs for

Selected

Alternatives...........................4-1

5

REFERENCES..................................................................................................5-1

pond

d

closure

alternatives

report.doc

NATURAL

i

RESOURCE

TECHNOLOGY

TSD

000011

l

---

TABLE

FIGURES

Figure

1-1

Figure

1-2

Figure

2-1

Figure

2-2

Figure

2-3

Figure

2-4

Figure

2-5

Figure 2-6

Figure

2-7

Figure

2-8

Figure

2-9

Figure

2-10

Figure

2-11

Figure

2-12

Figure

2-13

Figure

2-14

Figure

2-15

Figure

2-16

Figure 2-17

Figure

2-18a-d

Figure

2-19a-d

TABLES

Table 2-1

Table

2-2

Table

2-3

Table

2-4

Table

2-5

Table

2-6

Table

3-1

Table

3-2

Table

3-3

Site

Location

Map

Site

Plan

Geologic

Cross

Sections

Third

Quarter

2003

Upper

Migration

Zone

Flow

Contours

Fourth

Quarter

2003

Upper

Migration

Zone

Flow

Contours

First

Quarter

2004

Upper

Migration

Zone

Flow

Contours

Second Quarter

2004

Upper

Migration

Zone

Flow

Contours

February

12,

2007

Upper

Migration

Zone

Flow

Contours

May

13,

2007

Upper

Migration

Zone

Flow

Contours

July

2,

2007

Upper

Migration

Zone

Flow

Contours

October

2,

2007

Upper

Migration

Zone

Flow

Contours

Third

Quarter

2003

Deep

Alluvial

Aquifer

Flow

Fourth

Quarter

2003

Deep

Alluvial

Aquifer Flow

First

Quarter

2004

Deep

Alluvial

Aquifer

Flow

Second

Quarter

2004

Deep

Alluvial

Aquifer Flow

February

12,

2007

Deep

Alluvial

Aquifer

Flow

May

13.

2007

Deep

Alluvial

Aquifer

Flow

July

2,

2007

Deep

Alluvial

Aquifer Flow

October

2.

2007

Deep

Alluvial

Aquifer

Flow

Box-whisker

plot

showing

parameter

of

concern

concentrations

in

the

upper

migration

zone

from

2002

through

2008

Box-whisker

plot

showing

parameter

of

concern concentrations

in

the

deep

alluvial

aquifer

from

2002

through

2008

Selected

Closure

Alternatives

Soil

Boring

and

Discrete

Groundwater

Sampling

Data

Monitoring

Well

Locations,

Elevations,

Depth

to

Bedrock,

and

Screened

Formation

Monitoring

Well

Completion

Details

Monitoring

Well

Slug

Test

Results

Monitoring

Well

Programs,

Monitored

Aquifers,

and

Positions

Relative

to

Pond

D

Groundwater

Monitoring

Results:

Pond

D

Monitoring

Wells

Closure Alternatives

Screening

Summary

Areal

Extent

and

Volumes

of

Unsaturated

and

Saturated

Ash

in

Pond

D

Final

Cover

Alternatives

Material

Balance Analysis

pond

d

closure

alternatives

report.doc

NATURAL

RESOURCE

TECHNOLOGY

TSD000012

---

TABLE

APPENDICES

Appendix

A:

Site

Investigation

Appendices

A-1:

Soil

Boring

Logs

A-2:

Monitoring

Well

Completion Reports

and

Abandonment

Log

A-3:

Slug

Test Data

A-4:

Groundwater

Sampling

SOP

(AEG)

Appendix

B:

Alternative Cost

Summary

Sheets

Appendix

C:

Potable

Well

Survey

pond

d

closure

alternatives

report.doc

ill

NATURAL

RESOURCE

TECHNOLOGY

TSD000013

---

1

INTRODUCTION

1.1

Background

Ameren

Energy

Generating

(AEG)

operates

the

Hutsonville

Power

Station

in

Crawford

County

Illinois

(Figure

1-1).

The

power

station

is

located

on

the

west

bank

of

the Wabash

River,

one

mile

north

of

the

City

of

Hutsonville

(SW 'A,

Section

17,

Township

8N, Range

11W).

The

coal

fired

power

plant

has

been

in

operation

since

the

1940's.

There

are currently two

units

operating

at

the

plant,

completed

in

1953

(unit 3)

and

1954

(unit

4),

with

a

combined

generating capacity

of

164

MW.

Fly

ash from

the operating

units

is

collected

by

an

electrostatic

precipitator

and

sluiced

to

a

12-acre

lined

ash

impoundment

(Pond

A,

Figure

1-2),

which

was

constructed

in

1984.

Bottom

ash

is

sluiced

to

a

separate

pond

and

eventually

recycled.

Sluice

water

from

Pond

A

is

routed

through

a

4.2-acre

lined

interim

pond

(Pond

B,

constructed

in

2000)

before

discharge

to

the

Wabash

River

via

NPDES-pennitted

outfall

#002

(IL0000175). Sluice

water

from the

bottom

ash

pond

is

routed through

a

1.7-acre

drainage

collection

pond

(Pond

C,

constructed

in

2000)

and

Pond

B

before

discharge

to

the

Wabash

River

via

the

same outfall.

)

The site

also

has

a

22-acre

unlined

ash

impoundment

(Pond

D),

which

was

constructed

in 1968.

This

impoundment was

the

primary

ash

management

unit

prior

to

construction

of

Pond

A,

and

was

used

as

a

secondary

settling

pond

until

it

was

removed

from service

in

2000.

On

occasion,

precipitation

and

flood

backwater

can accumulate

in

the

impoundment

and

cause

ponded

conditions

in

low

areas.

Groundwater

quality

has

been

monitored

at

this

facility

since

1984.

Concentrations

of

boron

and sulfate

at

several

monitoring

wells

exceed

the

Illinois Class

I

groundwater

quality

standards

(Title

35,

Part

620,

Illinois

Administrative

Code,

or

35

IAC

620).

Boron

and

sulfate

are

indicator

parameters

for coal

ash

leachate.

In

1999,

Ameren

retained Science

&

Technology

Management, Inc.

(STMI) and

Natural

Resource

Technology

Inc.

(NRT)

to

perform

a hydrogeologic

assessment.

The

hydrogeologic

assessment

identified

a

correlation

between

shallow

groundwater

quality

(elevated

boron

and sulfate

concentrations

in

groundwater)

and

potential

leachate

sources,

namely

the

former

ash

laydown

area

(which

was

excavated

prior

to

construction

of

Ponds

B

and

C)

and

Pond

D.

Boron

and

sulfate

are

migrating

east

towards

the

Wabash

River;

however,

there

are

no

groundwater

supply

wells

in the

shallow

sediments

between

Pond

D

and

the

Wabash

River.

pond

d

closure

alternatives

report.doc

NATURAL

1-1

RESOURCE

TECHNOLOGY

TSD

000014

---

INTRODUCTION

,-..,

Groundwater

quality

data

from

monitoring

wells

in

the

deep

alluvial

aquifer,

as

well as

periodic

samples

\

_./"

from

the

plant

production

wells show

that

boron

and sulfate

concentrations

in this

deeper aquifer

are

lower

than Illinois

Class

I

groundwater

quality

1.2

Closure

Objectives

and

Approach

While

Pond

D

has

been

dewatered, Ameren

desires

to

close

the

impoundment

so as

to

prevent

off-site

groundwater

impacts

and

construct

a

final

cover

system

to minimize

infiltration.

The

goal

of

these

actions

is

to

close

the

impoundment

in

a

manner

protective

of

human

health

and

the

environment.

Site-

specific

considerations

for

establishing

appropriate

closure

objectives

include

a

risk

assessment

confirming

that

groundwater

discharge

to

the

Wabash

River

from Pond

D

is

not

harming

human

health

or

the

environment

(AECOM,

2009).

A

variety of groundwater management

and

final

cover

alternatives

for

closure of

Pond

D

have

been

identified

and

screened

based

on

factors such

as

technical

feasibility

and

cost.

Tables

3-1,

3-2,

and

3-3

summarize

the

closure

alternatives

evaluated

and

screening

process,

which

is

described

in

Section

3.

0

pond

d

closure alternatives

report.doc

1-2

NATURAL

RESOURCE

TECHNOLOGY

TSD000015

---

~\

r

2

SITE CONDITIONS

Hydrogeology

and

groundwater

quality

were

characterized

in

the

1999

hydrogeologic

assessment.

Additional

field

investigation

was

performed

in

2001

and

2004

to

upgrade

the

monitoring

well

system

surrounding

Pond

D,

characterize

the

deep

alluvial

aquifer,

and to

collect

detailed

information

specific

to

the

alternatives

assessment

(Appendix

A).

Data

from

these

sources

were

used

to

develop

the

description

of

current

site

conditions

presented

here.

Figure

1-2

shows

the

locations

of

soil

borings

and

monitoring

wells

used in

site

investigations

and

monitoring.

Tables

2-1

through

2-4

present

information

pertaining

to

soil

borings

and

groundwater

monitoring

wells

from

which

samples

were

collected.

2.1

Distribution of

Coal Ash

Fill

Ash at

the

Hutsonville

Power

Station

has

been

managed

in

Ponds

A

and

D.

In

addition,

ash

was

placed

in

a

laydown

area

between

the

southern

portions

of

Ponds

A

and

D.

In 2000,

all

ash

in

the

laydown

area

(

^

was excavated,

and

the

interim

pond

(Pond

B)

and

drainage

collection

pond

(Pond

C)

were

constructed

in

that

location.

Four

direct-push

probe borings

(GP20

through

GP23)

advanced

through

Pond

D

during

the

1999

hydrogeologic

assessment

indicated ash thickness

ranging

from about

12

feet

at

the

north

end

of

the

impoundment

to

31

feet in the

central

portion

of

the

impoundment

(Figure

2-1,

Section

C-C').

Ash

in

the

central

and

southern

portions

of

Pond

D

extended

as

much as

16

feet

below

the

normal

water

table

2.2

Hydrogeology

and

Groundwater

Quality

2.2.1

Hydrogeology

The

impoundments are

underlain

by

two water-bearing

units

separated

by

materials

that

have low

hydraulic

conductivity

(shale

bedrock

or

silts

and

clays).

The

upland portion

of

the

power

plant

property

and

the

western

portion

of

Pond

D,

are

underlain

by

a

thin

(less

than

20

feet

thick)

layer

of

sand-rich

soil,

which

is

underlain

by

Pennsylvanian-age

sandstone

and

then

shale

(Figure

2-1,

Cross-Section

A-A').

The

•

lowland

portion

of

the

site

and

eastern

portion of

Pond

D

are

underlain

by

90

feet

of

alluvium

in the

pond

d

closure

alternatives

report.doc

NATURAL

2-1

RESOURCE

TECHNOLOGY

TSD000016

---

SITE

CONDITIONS

Wabash

River

bedrock

valley.

The

upper

20

feet

of

alluvium

is

fine-grained,

primarily

composed

of

silt

and

day

with

thin

sand

lenses,

while

the

lower

70

feet

is

coarse-grained,

consisting

of

sand and

gravel.

Every

boring

drilled

into

the

alluvium

encountered

the

fine-grained

deposits,

and

on-site

borings

that

extended

to

the

underlying

sand

and

gravel (SB101,

SB102.

MW7D,

MW14, MW115,

and

MW121)

encountered

19

to

25

feet

of

these

deposits

(Figure

2-1,

Cross-Section

B-B').

Pennsylvanian-age

shale

underlies

the

alluvium.

The

shallow

upland

sand

and

sandstone,

and

sand

lenses

in the

fine-grained

alluvium, are

referred

to as

the

upper

migration

zone,

and

constitute

the

uppermost

aquifer

at

this

site.

There

are

13

monitoring

wells

screened

in

this

aquifer

(Table 2-5);

six

of

these wells

are

monitored

for

Pond

D,

and

four

of

these

are

downgradient

of

Pond

D.

The

coarse-grained

alluvium

is

referred

to as

the

deep

alluvial

aquifer.

This

aquifer

is

not

present

beneath

most of

the

site,

including

the

power

plant,

Ponds

A,

B,

and

C,

and

the

northern

and

western

portions

of

Pond

D.

There

are

five

monitoring

wells

screened

in

the

deep

alluvial

aquifer, all

of

which

are

monitored

for

Pond

D

(Table

2-5).

The

shale

underlying

the

upland

sandstone

and

the silts

and

clays

of

the

fine

grained

alluvium separate

the

upper

migration

zone

from

the

deep

alluvial

aquifer.

Groundwater

flow

was

mapped

for four consecutive

quarters during

which

complete

sample

sets

were

available.

Unfortunately,

depth

to

water

readings

for

all

of

the

upper

migration

zone

and

one

of

the

deep

alluvial

aquifer

maps were

not

collected

on

the

same

day

during

this

period.

While

this

discrepancy

did

not

appear

to

change

map

depictions

of

the

overall

direction

of

groundwater

flow,

it

affected

relative

readings

between

wells.

Therefore,

a

second

set

of

drawings

was

produced

using

data

collected

after

the

plant

initiated

same-date measurements

beginning

in

2006.

All

maps

(Figures

2-2

through

2-17)

show

that

groundwater

flow

in

the

upper

migration

zone

and

the

deep

alluvial

aquifer

is

eastward

toward

the

Wabash

River.

2.2.2

Groundwater

Quality

The

1999

Hydrogeologic

Assessment

identified

boron,

sulfate,

manganese,

and

TDS

as

parameters

of

concern

(POCs)

because

their

concentrations

in

groundwater

near

Pond

D

exceeded

Illinois

Class

I

groundwater

quality

standards. Boron

and

sulfate

are

indicator

parameters of

coal

ash

leachate,

and

are

the

primary

POCs-

Manganese

is

ubiquitous

in

soils,

may

have

higher

concentrations

in soil

than

in

coal

ash,

and

is

highly

sensitive

to redox

conditions;

therefore,

it

is

not

a

reliable

indicator

of

coal ash

leachate.

High

TDS

may

be

observed

at

sites

where coal

ash

leachate

migration

occurs,

because

high

TDS

concentrations

reflect

elevated

concentrations

of

soluble

ash

constituents

such

as

calcium,

potassium,

pond

d

closure

alternatives

report.doc

NATURAL

2-2

RESOURCE

TECHNOLOGY

TSD000017

---

SITE

CONDITIONS

sodium,

and

sulfate;

however,

other

natural

and

anthropogenic

sources

can

cause

high

TDS

_

'

concentrations,

therefore

it

is

not

a

reliable

indicator

of

coal

ash

leachate

impacts.

Pond

D

monitoring

wells

MW6, MW7, MW8,

and MW11R

have boron

and

sulfate

concentrations

higher

than

Class

I

standards;

these

wells

are screened

in

the

upper

migration

zone.

Groundwater

monitoring

results

are

presented

in

Figures

2-18a

and

b,

and Table

2-6a

and

b.

Groundwater

within the

deep

alluvial

aquifer

complies

with

Class

1

groundwater

quality

standards and reflects

only

nominal

impacts

from

Pond

D

in

only

one

of

the

five

wells.

The

lack

of

significant

groundwater

impacts

in

the deep

alluvial

aquifer

after

more

than

40

years

of

Pond

D

operation

provides

further

evidence

that

the

silts

and

shales

separating

the

upper

migration

zone

from

the

deep

alluvial

aquifer

are an

effective

confining

2.3

Potential

Groundwater

Receptors

There

are no groundwater

supply

wells,

other

than

the

two

plant

wells,

between

Pond

D

and

the

Wabash

River,

which

is

the

ultimate

receptor of

groundwater

impacted

by

leachate

from Pond

D

(Appendix

C).

The

plant

wells and

four

irrigation

wells

that

are

south

of

Pond

D

are

completed

in the deep

alluvial

aquifer.

/

As documented

previously, groundwater

in

the

upper migration zone

downgradient

of

Pond

D

has

elevated boron

and

sulfate

concentrations

and

therefore

represents

an

exposure

pathway;

however,

this

formation

is

not

utilized

for

water

supply

in the

vicinity

of

Pond

D.

The

deep

alluvial

aquifer

is

utilized

as

a drinking

water

supply

by the

city

ofHutsonville,

approximately

1

mile

to

the

south.

However,

groundwater

flow

in this

aquifer

is

toward

the

Wabash

River

(Figures

2-10

through

2-16).

As

a

result,

there

are

no

potable

water

supply

wells,

other than

the

two

plant

wells,

situated

between

Pond

D

and

the

discharge

point

for

groundwater

(the

Wabash

River).

The

plant

wells

have low

boron

and

sulfate

concentrations

and

do

not

show

evidence

of

impacts

from Pond

D.

pond

d

closure alternatives

report.doc

2-3

NATURAL

RESOURCE

TECHNOLOGY

TSD000018

---

^

3

IDENTIFICATION

AND

U

3.1

Overview

Several

closure

alternatives

were

identified

for

Pond

D

and

evaluated

to

determine whether

or

not

they

would effectively and

efficiently

meet

the

closure

objectives, specifically:

•

Prevent off-site

migration

of

impacted groundwater;

•

Minimize infiltration

of

rain

and

snowmelt

to

the

coal

ash

within

Pond

D;

and

•

Protect

human

health

and

the

environment.

Alternatives

that

potentially

meet

the

closure

objectives

are

presented

below

and

summarized

in

Table

3-1.

These alternatives

are

divided

into

two

distinct

categories:

Groundwater Management

and

Final

Cover

Alternatives.

|

i

Additionally, since

surface

water

management

is

a

necessary component

of any

final

cover

design,

surface

water

management

alternatives

were

developed

and

evaluated

for

incorporation

into

the

final

cover

alternatives.

3.2

Screening

Criteria

Screening

criteria

for

assessing

groundwater

management,

final

cover,

and

surface

water

management

alternatives

consist

of

the

following:

•

Construction

/

Implementation

Feasibility:

Construction

feasibility

refers to

the

ability

to

build

the

system

given

site-specific

conditions.

Implementation

feasibility

refers

to

the

ability

of

this

alternative to

meet

technical

factors,

such

as

appropriateness

or

suitability,

and

availability

of

the

technology

given

site-specific

constraints,

geographic

location;

and

administrative

factors,

such

as

local

and

state

permitting

requirements

and

regulatory

reviews for

approval.

•

Effectiveness: Effectiveness refers

to

the

ability

of

the

alternative to achieve

the

three

closure

objectives.

pond

d

closure

alternatives

report.doc

3-1

NATURAL

RESOURCE

TECHNOLOGY

TSD

000019

---

IDENTIFICATION

AND SCREENING

OF

CLOSURE

ALTERNATIVES

^,,

•

Cost:

Costs

for

the

purpose of

initial screening

refer

to relative

cost

ranges

for

each

of

j

i

the

alternatives,

and include

utilization

of

available

published

cost

data

from

similar

v—

projects,

vendor

data,

and

engineering

judgment.

As

such,

costs

are

for

general

comparative purposes,

and

are

not

used

singly

as

a

screening

tool

unless substantial

cost

differentials

would

immediately preclude

the

technology

from

further

consideration.

Construction

/

implementation

feasibility

and effectiveness

were

significant

criteria for

screening.

If an

alternative

failed these

criteria,

then

it

was

not considered

further.

Therefore,

the

criteria

of

cost

was

secondary

unless

substantial

concerns

were

identified

that

would

clearly

eliminate

the

alternative

(e.g.,

same

feasibility

and

effectiveness

with

significantly

higher

costs).

Comments

on

the

screening

criteria for each

closure alternative

are

provided

with

the

description

of

each

alternative below

and

summarized

on

Table

3-1.

Rough

cost

summaries

for

each

of

the

alternatives

are

provided

in

Appendix

B.

Table

3-2

provides

a

summary

of

the

areal

extent

and

volumes

of

ash in

Pond

D

used

for

quantity

estimation

in

the

rough

cost

summaries.

Table

3-3

provides

a

material

balance

analysis

for

each

of

the final

cover

alternatives

that

explains

how each

source of

fill

available

on

site will be

utilized

within

the

final

cover

alternative.

3.3

Groundwater Management

Alternatives

v-.'7

3.3.1

Overview

As noted

in

AECOM,

2009,

groundwater

migration

from Pond

D

to

the

Wabash

River

does

not

pose

a

threat

to

human

health

and

the

environment.

Further,

impacted

groundwater

is

localized

and

limited

to

the

pond

area

itself and

a

narrow

band

of

shallow

groundwater

immediately

south

of

the

property.

Accordingly,

the

goal

of

the

groundwater management

alternatives

is

to

prevent

southward

off-site

migration

of

impacted

groundwater

in

the

upper

migration

zone.

The

following

groundwater management

alternatives

were

evaluated:

•

Site

monitoring

with

no

groundwater

collection;

•

Groundwater

collection

trench;

and

•

Containment

using a

low-permeability

vertical

barrier.

In

addition,

the

following

source

control

measures are

grouped

with the

groundwater

management

alternatives

because

they

have

a

similar

objective

of

preventing

off-site

migration:

pond

d

closure

alternatives

report.doc

3-2

NATURAL

RESOURCE

TECHNOLOGY

TSD

000020

---

IDENTIFICATION

AND SCREENING OF

CLOSURE

ALTERNATIVES

9

Ash

stabilization;

•[

,

•

Ash

removal

and

disposal, recycling

at

an

off-site

facility,

or

beneficial

reuse;

and

•

Ash

impoundment reconstruction.

As

noted

in

the

discussion

that

follows,

the

source

control

measures

were

eliminated

during

the

screening

process

because

they

are

technologically

infeasible

and/or

economically unviable.

3.3.2

Site

Monitoring

with

No

Groundwater

Collection

This

alternative

represents

a

no-action

alternative.

Establishing

a

groundwater

monitoring

program

will

be

required

as

a

component

of

each

Groundwater

Management

Alternative

discussed

below;

therefore,

costs for

site

monitoring

have not

been

separately

evaluated.

Groundwater

modeling

performed

separately

from

this

evaluation

(NRT,

2009)

suggests

that

groundwater

quality

at

the south

property

boundary

may

achieve

compliance

with

Class

I

groundwater

quality

within

a

period

of

about

17

years

after closure

of

Pond

D.

This

alternative does

not

achieve

the

objective

of

preventing

off-site

migration

of

impacted

groundwater.

Therefore

the

no-action component

of

this

alternative

was

not

carried

forward,

although,

as

presented

above,

the

groundwater monitoring

component

is

a

necessary

part

of any

groundwater

management

alternative.

")

3.3.3

Groundwater

Collection

Trench

This

alternative consists

of

a

collection trench

south

of

Pond

D.

The

collection trench

would contain

a

perforated

horizontal

pipe

surrounded

by

gravel

bedding.

A

geotextile

would

be

placed

along

the

trench

walls

to

filter

out

surrounding

soils.

The

horizontal

pipe

would have

a relatively

shallow

pitch

to

sumps

placed

along

the

alignment

of

the

trench at

a

spacing

determined

by

site-specific

hydrogeologic

conditions.

Pumps

would

be

placed

in the

sumps

to

extract

groundwater from

the

trench.

Extracted

groundwater

would

be

directly

discharged

to

the

interim

pond

(Pond

B)

for

management

and

eventual

discharge

to

the

Wabash

River

via

the

existing

NPDES

permit.

This

alternative

was

carried

forward because

it

is

capable

of

achieving

the

closure objective

of

preventing

off-site,

southward

migration of

impacted

groundwater

in the

upper

migration

zone.

pond

d

closure

alternatives

report.doc

3-3

NATURAL

RESOURCE

TECHNOLOGY

TSD

000021

---

IDENTJF1CA

T10N

AND

SCREENING OF CL

OSURE

AL

TERN

A

77

VES

3.3.4

Containment

Using

a

Low-Permeability

Vertical

Barrier

;__^

This

Groundwater

Management

Alternative

would

prevent

off-site

migration

of

impacted

groundwater

by

installing

a

low-permeability

vertical

barrier

through

the

upper

migration

zone.

Construction

of

a

vertical

barrier would

require

keying

into

a low-permeability

geologic

formation,

such

as

shale

bedrock

or clay.

Two

basic

barrier

configurations

were

considered:

•

Partially

Encapsulating

Barrier:

this

type

of barrier

would

be installed

along

the

east

and

south

(downgradient)

sides

of

Pond

D.

The barrier

would

be

completed

with

an

interior

hydraulic

gradient

control system

utilizing

groundwater

collection

trenches

upgradient

of

the

barrier

or

extraction

wells

within

the

impoundment.

The

hydraulic

gradient

control

system

would

prevent

hydraulic

mounding

by

maintaining

an

inward

gradient.

•

Fully

Encapsulating

Wall:

This

type

of

barrier

would

surround

the

entire

perimeter

of

Pond

D

to

fully

encapsulate

the

saturated

ash

zone

and

deflect

upgradient

groundwater

flow

around

Pond

D.

Internal

hydraulic

controls would

be

required

to

manage

groundwater

fluctuations

that

could

potentially

compromise

containment

integrity.

However,

since

this

type of

barrier

would

deflect

upgradient

groundwater

flow,

a

significantly

lower

volume of groundwater

compared

to

the

partially

encapsulating

barrier

would need

to

be

extracted

to

maintain

an

inward

gradient.

Several

vertical

barrier

technologies

are

available,

including

sheet

piling with sealed

interlocks,

cement-

/

»

bentonite

or

soil-cement

slurry,

and

jet

grouting.

Each

of

these

technologies

has the

capability

to

create

a

\

/

barrier

with

hydraulic

conductivity

approaching

1

x

10'7

centimeters

per

second

(cm/s)

with

proper

design

and

construction

quality

control

/

assurance.

However,

without

a

competent low-permeability

formation

in which

to key

the

barrier, proper

containment

cannot

be

achieved.

Accordingly,

this

alternative

was

not

considered.

3.3.5

Ash

Stabilization

Ash

stabilization