NUREG/CR-5698

Comparing Monitoring

Strategies at the

Maricopa Environmental

Monitoring Site, Arizona

University of Arizona

U.S. Nuclear Regulatory Commission

Office of Nuclear Regulatory Research

Washington, DC 20555-0001
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NUREG/CR-5698

Comparing Monitoring

Strategies at the

Maricopa Environmental

Monitoring Site, Arizona

Manuscript Completed: June 1999

Date Published: July 1999

Prepared by

M. H. Young*

A. W. Warrick, P. J. Wierenga, L. L. Hofmann, S. A. Musil**

**Department of Soil, Water and Environmental Science

University of Arizona

Tucson, AZ 85721

T. J. Nicholson, NRC Project Manager

Prepared for

Division of Risk Analysis and Applications

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

NRC Job Code W6151

*Currently at School of Civil and Environmental Engineering

Georgia Institute of Technology

Atlanta, GA 30332-0512

ABSTRACT

The purpose of this document is to discuss the alternative

monitoring strategies used during field experiments at the

Maricopa Environmental Monitoring site, Maricopa, AZ.

The strategies used at Maricopa were selected so that they

could potentially be incorporated into monitoring programs

at Low Level Waste disposal facilities. Although the

evaluation of the strategies was mostly qualitative in nature,

they were supported by data collected during two,

field-scale infiltration experiments. The results of the field

experiments with respect to water movement and tracer

migration were presented in a companion NUREG report

(NUREG/CR-5694). The emphasis was on monitoring in

the vadose zone.

This document describes and compares four monitoring

strategies that were implemented at the Maricopa site. They

were designated as Monitoring Trenches, Monitoring

Islands, Borehole Monitoring, and Geophysical Monitoring.

The strengths and weaknesses of each strategy were

described with respect to installation, maintenance and

replacement of monitoring systems and instruments.

Monitoring Trenches and Islands provide excellent

opportunities for specific placement of monitoring

instruments, with the possibility of direct observation of

undisturbed soil material. Borehole Monitoring is more

flexible with respect to depth of installation than the other

three strategies. Maintenance of monitoring instruments in

the Monitoring Trenches and Borehole Monitoring is not

always possible because instruments are often backfilled in

place. Some techniques used to support the Geophysical

Monitoring strategy (e.g., electroresistive borehole

tomography) suffer from the same infeasibility. Instrument

maintenance in the Monitoring Islands is easier if the island

is not backfilled. Limiting access to undisturbed soil,

especially with respect to the Monitoring Trenches and

Borehole Monitoring strategies, will also make instrument

replacement more difficult. Portability of surface

geophysical instruments used during the infiltration

experiments removes several restrictions on maintenance

and replacement.

The document also presents the concept of primary

performance measures (e.g., water content, water tension

and solute concentration), each of which directly influences

water movement and contaminant migration from disposal

sites. The majority of commercially available monitoring

instruments measure secondary performance measures,

which are soil water conditions that are converted to

primary measures using calibration curves. Unfortunately,

each instrument has different operational limitations and

sensitivities, which depend on the soil water environment.

Therefore, it is recommended to use multiple instruments

whose data convert to the same primary performance

measures. This should improve the confidence that changes

in soil water conditions are real and not affected by the

monitoring systems themselves.

°i° iii NUREG/CR-5698

CONTENTS

A bstract .......................................................................................... ii

Executive Sum m ary ......................................................... ........................ ix

Forew ord ......................................................................................... xi

Acknowledgments .................................................................... xii

List of Abbreviations ................................................................................ xiii

I INTRODUCTION AND OBJECTIVES ........................................................ ........ I

1.1 M otivation ................................................................................. 1

1.2 Goals of the Field Studies ............................................................ 1

1.3 Brief Description of Field Studies ............................................................... I

1.4 Brief Description of M onitoring Strategies ....................................................... 3

2 ASPECTS OF UNSATURATED ZONE MONITORING AT DISPOSAL FACILITIES ......................... 7

2.1 Definitions of M onitoring ..................................................................... 7

2.2 Definition of Performance M easures ............................................................. 8

2.3 Goals of Subsurface M onitoring ................................................................ 8

2.4 Role of Monitoring in Performance Confirmation .................................................. 9

3 DESCRIPTION OF MONITORING STRATEGIES ..................................................... 11

3.1 Characteristics of Strategy and Collected Data .................................................... 11

3.1.1 M onitoring Trench Strategy ............................................................. 11

3.1.2 M onitoring Islands Strategy ............................................................. 11

3.1.3 Borehole M onitoring Strategy ........................................................... 14

3.1.4 Geophysical M onitoring Strategy ........................................................ 14

3.2 Strengths and Weaknesses with Respect to Monitoring Objectives and Data Analysis ..................... 19

3.2.1 M onitoring Trench Strategy ............................................................. 19

3.2.2 M onitoring Islands Strategy ............................................................. 19

3.2.3 Borehole M onitoring Strategy ........................................................... 20

3.2.4 Geophysical M onitoring Strategy ........................................................ 20

3.3 Strengths and Weaknesses with Respect to Installation, Maintenance and Replacement .................... 21

3.3.1 M onitoring Trench Strategy ............................................................. 21

3.3.1.1 Installation .................................................................... 21

3.3.1.2 M aintenance ................................................................... 22

3.3.1.3 Replacem ent ................................................................... 23

3.3.2 M onitoring Islands Strategy ............................................................. 23

3.3.2.1 Installation .................................................................... 23

3.3.2.2 M aintenance ................................................................... 25

3.3.2.3 Replacem ent ................................................................... 25

3.3.3 Borehole M onitoring Strategy ........................................................... 25

3.3.3.1 Installation .................................................................... 25

V NUREG/CR-5698
3.3.3.2 M aintenance .................... I .. ........................................... 26

3.3.3.3 Replacem ent ................................................................... 27

3.3.4 Geophysical M onitoring Strategy ........................................................ 27

3.3.4.1 Installation .................................................................... 27

3.3.4.2 M aintenance ................................................................... 28

3.3.4.3 Replacem ent ................................................................... 29

4 D ISCU SSION ................................................................................... 31

4.1 Use of Strategies to Fulfill Subgoals ............................................................ 31

4.2 Choosing different monitoring strategies ........................................................ 35

4.2.1 Natural site conditions/processes ......................................................... 35

4.2.2 Anthropogenic site conditions/processes ................................................... 38

4.3 Analysis of M onitoring Systems ............................................................... 40

5 SUM M ARY ..................................................................................... 43

6 REFERENCES .................................................................................. 45

NUREG/CR-5698 vi
Figures

1.3-1 Index m ap for M aricopa site . ................................................................... 2

1.4-1 M onitoring devices installed at buried trench transect . .............................................. 5

3.1-1 Cross-section (A) and map view (B) of monitoring island and instrument orientation ...................... 13

3.1-2 Schematic field plot, showing the location of major monitoring systems. Monitoring strategy

designation is listed in the legend; MTis monitoring trench, MI is monitoring islands,

BM is borehole monitoring, GM is geophysical monitoring. Note that EM-31 and EM-38

monitoring points (90 total) are not shown ....................................................... 16

3.1-3 Locations used for surface electromagnetic induction surveys and for ERT studies. Open circles

represent the EM points and closed circles are the ERT boreholes ..................................... 18

Vii vii NUREG/CR-5698
Tables

1.3-1 Summary of experimental conditions at the Maricopa site ............................................ 4

3.1-1 Summary of monitoring instruments and monitored environment in buried trench ........................ 12

3.1-2 Summary of monitoring instruments and monitored environment in monitoring island strategy ............. 15

3.1-3 Summary of monitoring instruments and monitored environment in borehole monitoring strategy ............ 15

3.1-4 Summary of monitoring instruments and monitored environment in geophysical monitoring strategy ........ 17

4.1-1 Breakdown of adequacy of monitoring strategies to fulfill subgoals. MT, MI, BM, GM correspond

to monitoring trench, monitoring island, borehole monitoring, and geophysical monitoring, respectively.

Check marks denote adequacy ................................................................ 32

4.2-1 Specific site conditions and processes: how they affect the choice of monitoring strategies. MT, MI,

BM, GM correspond to monitoring trench, monitoring island, borehole monitoring and geophysical

m onitoring, respectively . .................................................................... 36

NUREG/CR-5698 viii
Executive Summary

U.S. Nuclear Regulatory staff identified a need for research

to better assess unsaturated zone monitoring techniques and

strategies applicable to LLW disposal facilities. A field

work plan was developed and implemented at the Maricopa

Environmental Monitoring site (Maricopa, AZ) where two

field-scale infiltration experiments were conducted. A

companion NUREG (i.e., NUREG/CR-5694) presents the

results of the field experiments with respect to water

movement and tracer migration.

This document discusses the need for redundancy in the

monitoring of primary performance measures (e.g., water

content, water tension and solute concentration). Each of

these performance measures directly influences water

movement and contaminant migration from disposal sites.

Almost all monitoring instruments provide data on

secondary measures, which are converted to primary

measures using calibration curves. Using more than one

instrument to convert data to the same primary performance

measure, can compensate for limitations and sensitivities of

each instrument.

This document also describes and compares four monitoring

strategies that were implemented at the site. The strengths

and weaknesses of each strategy were described with

respect to installation, maintenance and replacement of

monitoring systems and instruments. The comparisons stem

directly from insights gained during the field experiments,

and from the work of others. The comparisons are

qualitative in nature, so that they are not influenced by the

specific characteristics of the Maricopa site. The four

strategies follow:

1. Monitoring Trenches consist of potentially long, wide

or narrow trenches, into which instruments can be

installed. Once opened, the trench provides direct

access to the soil or engineered material. The material

can be sampled and described easily. Instruments can

be precisely placed and visually inspected for

completion. Wiring and electrical connections can be

placed in conduit and protected from environmental

degradation. However, maintenance and replacement

of instruments in the trench are limited after the trench

is closed.

2. Monitoring Islands consist of large diameter boreholes

drilled vertically into the soil, allowing monitoring

instruments to be installed into undisturbed material.

Culverts are placed vertically in the these boreholes

for access and to prevent cave in of the soil. This

strategy permits monitoring of vertical transects of

soil water conditions, because the islands can be

installed to depths exceeding 10 m. However, the

horizontal spatial resolution is limited because

instruments must be installed close to the island wall.

Instrument maintenance and replacement are easier

than for the other strategies which rely on fixed,

subsurface instruments.

3. Borehole Monitoring consists of vertical and

horizontal boreholes used for monitoring. Using

portable probes (e.g., neutron probe), spatial and depth

resolution of data can be excellent. Fixed point

devices can be installed in the borehole, enhancing the

redundancy of monitoring for primary performance

measures. Maintenance and replacement of portable

probes is not an issue with this strategy, but is very

difficult for permanently-installed instruments.

4. Geophysical monitoring consists of a combination of

intrusive and non-intrusive techniques for measuring

bulk electrical properties of subsurface material. The

data cannot easily be converted to primary

performance measures, and could require a significant

amount of ground-truthing. However, portability of

non-intrusive instruments (e.g., EM-31 and EM-38)

permits rapid data collection. Intrusive techniques

used at the Maricopa site (e.g., electroresistive

tomography) can provide 2-D tomograms of electrical

properties, which were shown to change with water

content. Maintenance of ERT boreholes is not

practical, requiring complete replacement.

We identified 12 subgoals for the monitoring program, all

of which are compatible with the three major goals

described in Section 2. The subgoals are intended to better

define the important aspects of the monitoring program and

how or whether use of the monitoring strategy can address

ix ix NUREG/CR-5698
the subgoal. We also included a section that deals with site

conditions and processes and how they affect the choices of

possible monitoring strategies. We further subdivided these

into natural (e.g., precipitation, depth to water table) and

anthropogenic (e.g., presence of buildings) conditions. A

total of 12 site conditions were identified and opposing

conditions were defined (e.g., shallow/deep water table).

We then categorized each strategy as to whether the

condition supported its use, weakened its use, or made no

difference with respect to its use. When used in

combination with the subgoals, the choice which strategy

(or combination of strategies) to use becomes more clear.

NUREG/CR-5 698x X
FOREWORD

This technical report was prepared by the Department of.

Soil, Water and Environmental Science at The University of

Arizona (UAZ), under its research project with the

Radiation Protection, Environmental Risk and Waste

Management Branch in the Office of Nuclear Regulatory

Research (Contract No. NRC 04-95-046). The research

objectives were to: assess capabilities, limitations, and

usefulness of alternative techniques for monitoring moisture

movement and contaminant transport in the unsaturated

zone; provide technical bases for identifying and evaluating

appropriate techniques for unsaturated zone monitoring; and

test monitoring strategies and instrumentation on a variety

of field scales using actual water and tracer applications and

geometries. The research was requested by the Office of

Nuclear Material Safety and Safeguards to provide technical

information and bases for licensing reviews.

This report, NUREG/CR-5698, describes the principal

lessons learned from the field study to evaluate alternative

monitoring strategies and related instrumentation.

Specifically, four monitoring strategies were evaluated at

the Maricopa Environmental Monitoring site. The

evaluation of these strategies focused on their inherent

strengths and weaknesses with respect to installation,

maintenance, and replacement of monitoring systems that

support the four monitoring strategies.

Detailed research results and data from the field studies

conducted over a three-year period are provided in a

companion report, NUREG/CR-5694 (Young et al., 1999).

NUREG/CR-5694 provides the technical bases for

identifying and evaluating the four monitoring strategies

and related instrumentation.

Information in both NUREG/CR-5694 and this report may

be useful to those involved in designing or reviewing

monitoring programs for water and contaminant movement

at low-level radioactive waste and decommissioning

facilities. Two workshops were conducted by the UAZ

investigators: a "hands-on" technology transfer workshop

held at the Maricopa field site in February 1998; and a

"lessons learned" seminar held at NRC Headquarters

auditorium in July 1998. Agreement State regulators and

their contractors were notified of the workshops, and

attended along with scientists from other Federal agencies,

DOE national laboratories, universities and industry.

NUREG/CR-5698 is not a substitute for NRC regulations,

and compliance is not required. The approaches and/or

methods described in this NUREG/CR are provided for

information purposes only. Publication of this report does

not necessarily constitute NRC approval or agreement with

the information contained herein.

xi xi NUREG/CR-5698
Acknowledgments

This research was supported by the U.S. Nuclear Regulatory

Commission, under Contract No. 04-95-046. The authors

would like to thank peer reviewers Glendon Gee, Phil

Meyer, and Mark Rockhold of Pacific Northwest National

Laboratory for their insights, discussions and comments.

NUREG/CR-5698 i xii
List of Abbreviations

2-D Two dimensional

3-D Three dimensional

AC Alternating Current

DC Direct Current

E-W East-West

EC Electric conductivity

EC, Apparent Electrical Conductivity

EM Electromagnetic Induction

EM-31 Electromagnetic induction at intermediate depth (3 to 6 m)

EM-38 Electromagnetic induction at shallow depth (0.75 to 1.5 m)

EM-39 ERT borehole device

ERT Electroresistive Borehole Tomography

GPR Ground-penetrating Radar

HDS Heat Dissipation Sensors

LLW Low-level waste

MAC Maricopa Agricultural Center

MEM Maricopa Environmental Monitoring

N-S North-South

NRC U.S. Nuclear Regulatory Commission

PVC Polyvinyl Chloride

RFP Request for Proposal

SD Standard Deviation

SDMP Site Decommissioning Management Plan

TDR Time Domain Reflectometry

0" Volumetric water content

xiiiN°6 NUREG/CR-5698

NUREG Introduction and Objectives

1 INTRODUCTION AND OBJECTIVES

1.1 Motivation and contaminant transport in the unsaturated

zone of humid and arid areas.

In September, 1994, the U.S. Nuclear Regulatory

Commission (NRC) issued a Request for Proposal (RFP) to

assess unsaturated zone monitoring techniques and

strategies applicable to low-level radioactive waste (LLW)

disposal facilities. The RFP stemmed from a request by

NRC staff for additional information on monitoring systems

and strategies that could be used for evaluating future

disposal site applications. This research was later expanded

so that it would be relevant to sites located primarily in the

humid, eastern portion of the US.

Unsaturated zone monitoring to detect releases of

radionuclides is an important safety issue at LLW disposal

facilities. The unsaturated zone is a primary component that

isolates near surface waste from underlying ground-water

systems. Monitoring can be used to show that facilities are

operating safely during waste emplacement and after site

closure. Effective unsaturated zone monitoring requires

choosing instruments and installation procedures, and

integrating them into an overall program that incorporates:

1) the results of site characterization, 2) operational limits

for each device, 3) frequency of data collection, and 4) the

need for performance confirmation of the site. This

document reports on comparisons made of four monitoring

strategies that were used in a large field study at the

Maricopa Environmental Monitoring site (called the

Maricopa site). The results of the field studies are

summarized by Young et al. (1999).

1.2 Goals of the Field Studies

There were three broad goals for the field study portion of

this contract: 1) to construct a field site in which monitoring

strategies could be evaluated through a series of water flow

and solute transport experiments; 2) to evaluate the use of

several strategies for monitoring flow and transport in the

unsaturated zone at both arid and humid sites; and 3) to be

able to address the specific objectives as listed below:

1. Assess capabilities, limitations, and usefulness of

alternative techniques for monitoring water movement

2. Provide the technical basis for identifying and

evaluating appropriate techniques for unsaturated

zone monitoring at LLW sites.

3. Develop guidance on the design, installation, use,

and decommissioning of unsaturated zone

monitoring systems.

4. Examine the issue of whether and how

unsaturated zone monitoring systems may

compromise the performance of natural and

engineered barriers at LLW facilities and how to

eliminate or mitigate such compromises.

5. Test monitoring strategies and instrumentation

on a variety of field scales using actual water and

solute tracer application rates and geometries.

1.3 Brief Description of Field Studies

The study developed and tested monitoring strategies which

can be used for a variety of LLW conditions. Only the

subsurface transport pathway was considered. The original

field testing plan (Young et al., 1996) described some of the

supporting factors that were considered during the design

phase of this project.

Subsequent field activities were conducted at the Maricopa

Agricultural Center, Maricopa, AZ (Figure 1.3-1). The

Center, located about 42 km southwest of Phoenix, is in the

northwestern portion of Section 20, Township 4 South,

Range 4 East, in western Pinal County, Arizona. The

facility is owned and operated by The University of Arizona

and comprises 770 hectares. The region is characterized by

broad valleys surrounded by mountains of moderate height.

The mountains range in age from Precambrian (granite and

schist dominated) to Tertiary (andesite dominated) (Soil

Conservation Service, 1974). The valley floor is covered

with material eroded from these mountains, placed in thick

alluvial deposits up to several hundred feet thick. The

alluvial deposits exhibit characteristic depositional

I 1 NUREG/CR-5698
NUREG Introduction and Objectives

1-17

Phoenix area

Maricopa site

Scale - km

0 100

MEXICO

Figure 1.3-1. Index map for Maricopa site.

NUREG/CR-56982 2
NUREG Introduction and Objectives

variability with lenses of material ranging from gravelly to

clayey textures. At Field 115, where the research was

conducted, the fine-loamy soil is classified as a Casa Grande

soil (fine-loamy, mixed, hyperthermic Typic Natrargids)

(Post et al., 1988)..

The field research was conducted on a 50 m by 50 m plot

located within a 0.9 hectare field. Irrigation canals exist at

the southern and eastern boundaries of the site, a floodirrigated alfalfa field is located to the north and an access

road is located to the west. A support zone is located

between the irrigated plot and the access road, where

components of the irrigation system (e.g., solenoids, timers,

valves and piping) were installed. All basic utilities (e.g.,

AC power, water, land-line telephone) were made available

through either the agricultural-center infrastructure or onsite construction. This allowed AC power, for example, to

be installed throughout the plot for a variety of uses.

The irrigation system was designed specifically for

conducting flux-controlled infiltration experiments. It

consisted of self-cleaning precision drip emitters (0.6

gallon/hour, Netafim Techline, Fresno, CA), placed on a 0.3

m by 0.3 m grid over the 2500 mn area of the plot. Nearly

27000 emitters were used. A total of 164 drip lines were

cut to a length of 50 m, and then connected to manifolds on

either end of the plot. The system was divided into six,

nearly identical, irrigation stations. Water application was

measured using two precision flow meters (McCrometer,

Hemet, CA) placed in series immediately before entering

the individual stations. After installing and testing the

irrigation system, 0.8 mm thick Hypalon pond liner was

placed over the field plot, extending at least 5 m beyond the

irrigated area. The pond liner was used to eliminate

evaporation from the soil.

Two experiments were conducted at the site covering the

time frame from Spring 1997 through Summer 1998.

During Experiment 1, water at an average rate of 1.85

cm d' was applied to the soil for 23 days for a total

application of 44.4 cm. Bromide was added as a tracer for

the first 15 days. Redistribution was monitored for 69 days

before the experiment ended. During Experiment 2, water

at an average rate of 1.97 cm d' was applied for 33 days for

a total application of 64.8 cm. Salt (NaCI) was added as a

tracer for the first 7 days. Redistribution was monitored for

177 days before field experiments at the site ended on July

1, 1998. A complete discussion of results for the field

experiments is available (Young et al., 1999). Table 1.3-1

lists the basic boundary conditions for the full suite of field

experiments at the Maricopa site.

1.4 Brief Description of Monitoring

Strategies

The field plot incorporated a variety of monitoring

strategies, which form the basis for comparisons in this

research. They are designated as monitoring trench,

monitoring island, borehole monitoring, and geophysical

monitoring. Briefly, each monitoring strategy consists of

the following:

I Monitoring Trench - This strategy used an excavated

trench, into which monitoring instruments were

installed. A total of 13 instrument clusters were

chosen, which are located in a N-S transect, parallel to

the horizontal neutron probe access tubes, with offsets

of 2.5 and 5.0 m (Figure 1.4-1 is a schematic of the

trench). The majority of instruments were installed at

1.5 m depth, though a few were installed at 1.0 m and

closer to ground surface. Instrument wires, cables and

tubes were brought to ground surface, placed in

conduit and taken to data loggers and manifolds

placed at key locations on the irrigated plot.

2. Monitoring Islands - This strategy used two large

diameter boreholes (1.7 m diameter), into which

highway culverts (1.5 m diameter) were lowered to a

depth of 3 m. The annular spaces were then backfilled

using sieved soil. The bottoms of the culverts were

left open to the bare soil surface and the top was

closed with a plywood lid. Three vertical transects of

instruments were installed in each island. Two

transects were equipped with clusters of four

instruments installed at offsets of 50 cm to 300 cm

depth, and approximately 50 cm into undisturbed soil.

The third transect was instrumented at 100 cm offsets,

but the instruments were installed in the annular space

between the culvert and the undisturbed soil; this was

done to observe potential preferential flow through the

repacked soil material. A total of 18 instruments of

each type were installed in each island. All

3 3 NUREG/CR-5698
NUREG Introduction and Objectives

Table 1.3-1. Summary of experimental conditions at the Maricopa site.

Experiment 1 Experiment 2 Experiment

Totals

Water Water

Start Date: 4/28/97 12/3/97

End Date: 5/21/97 1/5/98

Duration: 24 days 34 days 58 days

Application Rate: 1.85 cm d-' 1.97 cm d-'

Depth Applied: 44.4 cm 64.8 cm 109.2 cm

Bromide Total conductivity

(salts)

Start Date: 4/28/97 12/3/97

End Date: 5/11/97 12/9/97

Duration: 14 days 7 days

Mean Concentration: 31.6 ppm 2.2 dS m-1

Depth Applied: 26 cm 14 cm

NUREG/CR-56984 4
NUREG Introduction and Objectives

Irrigated Area (50 m) Tensiometers

only -It "!

1.0m Om

1.5 m _ S'0 05 0

n n l- A 

Ground surface

0 Trench bottom

\ I

Instruments found at monitoring

Total Number of instruments in trench transect locations are listed in box.

Thermocouple P3n Psychrometer 

Tensiometer 19

TDR Probe 13

Heat Dissipation Sensor 13

Stainless Steel Solution Sampler 13

Figure 1.4-1. Monitoring instruments installed at buried trench transect.

5 NUREG/CR-5698
NUREG Introduction and Objectives

instruments were connected to data loggers or vacuum

systems using multiplexers and manifolds,

respectively.

3. Borehole Monitoring - This strategy relied on

conventional and unconventional borehole installation

and usage. Vertical boreholes were installed at regular

intervals throughout the plot to observe infiltration of

water into the soil using a variety of instruments. The

boreholes were installed by advancing a 10.2 cm

diameter auger to target depths of either 3 m or 15 m,

depending on the location and the use of the borehole.

A total of 41 access tubes were used for vertical

neutron probe monitoring, and 13 were used for

ground water monitoring; 27 tubes each were used for

permanently-installed tensiometers and soil solution

sampling; and 12 were used for electroresistive

borehole tomography (ERT), also used to support the

geophysical monitoring strategy. Three horizontal

access tubes were also installed (2 N-S and I E-W) for

neutron logging.

4. Geophysical Monitoring - This strategy relied on

surface and subsurface monitoring of bulk electrical

conductivity or resistivity. A total of 90 surface

locations were chosen on a grid and used for

electromagnetic induction surveys (e.g., EM-31 and

EM-38) of changes of bulk electrical conductivity

(ECJ. Subsurface monitoring was also done using

ERT, mentioned in the Borehole Monitoring

description. For the ERT methodology, 12 boreholes

were advanced to 15 m each, and equipped with

sources/detectors at 1 m vertical offsets. Together the

EM and ERT techniques provided data and

information on changes of bulk soil EC,, caused

primarily by changes in water content.

As can be inferred from the above descriptions, the

monitoring systems (which encompass access ports,

instruments and data collection systems) can be used to

support more than one strategy. For example, the ERT

system is part of the Geophysical Monitoring strategy, but

was installed in boreholes, so it can be considered an

overlap. Likewise, the horizontal access tubes used for

neutron probe logging were installed in monitoring

trenches, so this system overlaps the Monitoring Trench and

Borehole Monitoring strategies. These overlaps improved

the design flexibility of the overall field plot.

NUREG/CR-56986 6
NUREG Monitoring at Disposal Facilities

2 ASPECTS OF UNSATURATED ZONE MONITORING AT

DISPOSAL FACILITIES

2.1 Definitions of Monitoring

For the purposes of this document, monitoring means

"observing and making measurements to provide data to

evaluate the performance and characteristics of the disposal

site" (U.S. NRC, 1998a). Though this definition is

somewhat self-explanatory, the difficulty lies in the details

of designing a monitoring system to accommodate the

myriad of disposal site environments and designs, so that

the discrete observations can be used for evaluating the

performance of disposal sites. We are limiting the extent of

monitoring to unsaturated soil and geological material

immediately outside of the waste containment structure

itself, and on detecting and quantifying the rate and volume

of either water or contaminant movement, or both, released

from containment cells (e.g., a trench, concrete bunker,

landfill, etc.) to the undisturbed environment. Many aspects

of monitoring, which are discussed here, can be used for

observing changes in engineered material, but this is a

secondary benefit.

Throughout this document, we refer to phases of monitoring

programs, including preoperational, operational and

postoperational (or post-closure) phases. In these

references, we also make a distinction that site

characterization has been completed, and that the basic

environmental characteristics and processes have been.

defined well enough for the site to be licensed.

Preoperational monitoring refers to measurements made

prior to waste emplacement at the facility, including

monitoring to support site characterization (before

submitting license application). Operational monitoring

refers to measurements made from the beginning of facility

construction through the period when waste is no longer

accepted. Postoperational monitoring begins when the

facility no longer accepts waste material and continues until

the custodial care period ends. Much of the document

addresses operational and postoperational monitoring

phases.

Monitoring subsurface environments is achieved through

the implementation of conceptual frameworks and related

physical components. Thus, the monitoring program is a set

of monitoring strategies, including data collection intervals,

analytical methods and data analysis. Data collection

intervals are important aspects of both spatial and temporal

sampling; obviously, a larger number of sampling points

could improve the likelihood of detecting releases from the

containment unit, or other environmental changes which

could signify a potential future release. Likewise, sampling

at a higher temporal rate permits the quantification of

seasonal variability and other time-dependent behavior.

Methods and statistical procedures are available to evaluate

the data, and to make conclusions with respect to facility

performance. Aspects of conceptual frameworks for groundwater quality monitoring (e.g., Franke et al., 1997) can be

tailored for site-specific monitoring at LLW sites, especially

those that relate to the preparation of long-term monitoring

programs, subsequent review and evaluation of monitoring

goals, and modification of the monitoring program to address

changes in goals and objectives.

Embedded into the monitoring program are monitoring

strategies, which we define as a set of monitoring systems

that emphasize and support a specific concept or philosophy.

Numerous combinations of philosophies can be proposed,

depending on the site characteristics and the goals of the

monitoring program, and this document will not attempt to

generate an exhaustive list. However, emphasizing 1)

intrusive versus non-intrusive methods; 2) manual versus

automated data collection and 3) direct or indirect sampling,

or combinations thereof could be used for guiding the design

and implementation of the monitoring program. Judicious

choices of monitoring strategies must be made early in the

design process, because subsequent reviews of monitoring

goals could change the monitoring emphasis. Therefore, the

best combination of strategies would be one that addresses

potential dovetailing of monitoring goals during different

phases of site monitoring.

A monitoring system is defined as a system that collects the

output of sensors (Dictionary of Science and Technology,

1992). The monitoring system supports the monitoring

strategy. The monitoring system includes the sensor itself,

wiring or cabling, data loggers, and other components that

7 NUREG/CR-5698
NUREG Monitoring at Disposal Facilities

support the data collection. At the Maricopa site, significant

effort was spent on designing the ancillary components of

the monitoring system, such as electrical conduits, AC

power for the data loggers, etc.

A monitoring instrument is a device or sensor that collects

information about the site environs. It is only that portion

which measures some physical or electrical soil condition.

For example, the tensiometer is composed of a ceramic cup,

two lengths of plastic tubing, a rubber stopper and a

pressure transducer, which is then connected to a data

logger through electrical wiring. For the purposes of this

document, only the ceramic cup and plastic tubing are

considered part of the instrument; we include the pressure

transducer and wiring as part of the monitoring system.

2.2 Definition of Performance

Measures

We define performance measure as a primary soil water

parameter which directly relates to water movement and

contaminant migration from a disposal area. We designated

soil water tension, soil water content and pore water

concentration of some target constituent as performance

measures. Soil water tension influences the direction and

magnitude of the hydraulic gradient. Soil water content

influences the hydraulic conductivity and acts as the

primary medium which transports contaminants. Pore water

concentration represents the source of contamination;

numerical concentration levels are often incorporated into

environmental regulations, where exceedances could trigger

some level of enhanced monitoring or other corrective

action.

Primary performance measures are differentiated from

secondary measures by the use of instruments which use

indirect measures and calibration curves. Considering soil

water content, for example, the TDR and neutron probe

methods measure dielectric constant and hydrogen ion

concentration, respectively. These secondary parameters

are then converted to water content by way of calibration

curves, using the secondary parameter as independently

measured and the performance measure as the dependent

parameter. In this way, water content is obtained from the

TDR dielectric constant using, e.g., Ledieu's (1986)

equation (0, = bo + b1c'"), where 0,, is the volumetric water

content and c is the dielectric constant. Different forms of

calibration curves may be used for other instruments.

Throughout the Maricopa field work and this document, we

stressed the need to monitor each of the three performance

measures during the development of the field program. Each

of the four monitoring strategies incorporated this

philosophy, to the extent possible. Instruments in the

monitoring trench strategy, for example, measured soil water

content using TDR and the neutron probe; soil water tension

using tensiometers, HDS units, and thermocouple

psychrometers; and solution concentration using pore water

samplers and destructive soil sampling. Each of these

devices have specific ranges of operation and sensitivities

that span soil water conditions that we expected in the field

(i.e., from very dry to very wet). Field observations during

Experiments 1 and 2 would have been limited if we had

relied heavily on only one or two monitoring systems; our

field conclusions would have been limited also.

2.3 Goals of Subsurface Monitoring

We define three goals for subsurface monitoring programs

which can be applied at actual disposal sites. They are:

I. To provide early warning of releases of contaminants

(e.g., radionuclides) from disposal sites before they

reach the facility boundary (U.S. NRC, 1998b).

This goal, paraphrased from NRC regulation (10 CFR

61.53), applies to monitoring programs during facility

operation and after facility closure.

2. To design a system that reduces or eliminates active

maintenance, emphasizes protection of the facility

during potential future replacement of instruments, and.

allows for consistent monitoring of site conditions

throughout different monitoring phases

Monitoring programs should be designed to avoid the use of

monitoring strategies or systems that will require significant

maintenance and replacement actions throughout the lifetime

of the program. The need to disrupt engineered cover

material during instrument replacement introduces enhanced

risk of failure of the containment unit. The use of

instruments that require active maintenance (e.g., certain

NUREG/CR-56988 8
NUREG Monitoring at Disposal Facilities

types of tensiometers) could be problematic if the facility is

closed and technical staff need to come to the site often.

3. To use strategies that focus on redundant observations

of performance measures, reducing the dependency of

the program on a single monitoring system.

Describing complex, subsurface phenomena with

instruments that monitor only a few secondary parameters

increases the risk that instrument range or sensitivity will

reduce the effectiveness of the instrument and, thus, the

effectiveness of the monitoring program. For this reason,

we believe that the flexibility of the monitoring program is

enhanced by emphasizing redundancy in the measurement

of performance measures by using multiple instruments.

2.4 Role of Monitoring in Performance

Confirmation

Performance confirmation, for the purposes of this

document, is defined as the program of tests, experiments,

and analyses which are conducted to evaluate the accuracy

of the information used to determine facility compliance

(U.S. NRC, 1998c). Elements of performance confirmation,

originally designed for the high-level waste disposal

program, can be used for LLW sites as well.

Goal #1, listed in Section 2.3, is to monitor the subsurface

environment in such a way that releases are detected before

they reach the facility boundary. Releases to the subsurface

environment are based on changes in soil water conditions

(content or tension) or contaminant concentration as they

relate to baseline conditions at the site. The data supplied

by the monitoring program thus provides information about

the integrity of the disposal unit, and whether or not

elements of the disposal site are functioning as intended and

predicted. The program used for unsaturated zone

monitoring, during and after waste emplacement, must be

capable of detecting changes to subsurface conditions,

greater than those expected during the license review.

Therefore, the long-term monitoring program needs to be

designed so that it does not adversely affect the natural and

engineered elements of the disposal site, and so that it can

operate without the need for significant and active

maintenance, because this could affect the ability of the

program to perform adequately.

9 9 NUREG/CR-5698

NUREG Monitoring Strategies

3 DESCRIPTION OF MONITORING STRATEGIES

3.1 Characteristics of Strategy and

Collected Data'.

3.1.1 Monitoring Trench Strategy

The monitoring trench strategy uses either one or a series of

shallow trenches excavated from native soil or backfill

material. Two types of trenches can be envisioned: a wide

trench excavated with backhoe or other heavy equipment,

and a shallow trench excavated with ditching equipment.

The wide trenches allow unfettered access to the soil

profile, allowing site personnel to sample the material,

install monitoring instruments through the trench wall, or

simply visually observe the material. Narrow trenches

greatly restrict the ability of site personnel to view the soil

profile and install instruments in the trench wall, but they

can be very quickly excavated, and with relatively little site

disturbance.

With a the wide trench, layering and textural characteristics

of the profile are easy to view and describe. During trench

construction at the Maricopa site, a soil scientist described

the morphology of the profile, identifying layering, textural

discontinuities, the presence of calcic horizons, and soil

structure. Numerous grab samples were collected from the

bottom of the trench so that spatial variability of texture

along the N-S transect could be described. Nearly 70

undisturbed core samples were collected and analyzed for

soil hydraulic properties. The ability to "walk through" the

soil profile for upwards of 60 m (in our case) greatly

enhanced our understanding of the shallow soil profiles at

the Maricopa site. The trench was excavated to only 1.5 m,

very shallow when considering deep vadose zones in semiarid regions. Excavation could have been deeper, but a

practical limitation exists for the depth of excavation

because of wall stability and worker safety issues.

A wide trench permits monitoring instruments to be

installed laterally into undisturbed soil material or

engineered covers. During our experimental setup, 81

sensors were placed 1.5 m into undisturbed soil, and one

horizontal neutron probe access tube was placed onto the

trench floor (Table 3.1-1 and Figure 1.4-1). Of course,

additional sensors could have been used, and indeed many

more sampling points might be necessary to monitor water

flow at an actual disposal site. All electronic sensors (e.g.,

TDR, HDS, thermocouple psychrometers) were connected

to data acquisition equipment for remote data collection.

Thus, a large amount of data and information can be

collected without the presence of site personnel or the

potential problems of worker error.

Two narrow trenches were added to the sampling program,

into which we lowered horizontal neutron probe access

tubes. The ability to collect data along a single transect, at

spatial offsets of (in our case) 0.25 m, greatly offsets the

disadvantage of requiring on-site personnel during data

collection. We concluded that use of these horizontal

access tubes could benefit monitoring programs at actual

disposal sites.

3.1.2 Monitoring Islands Strategy

The monitoring island strategy, as used in this research,

utilized large diameter (1.53 m) highway culverts,

permitting access to undisturbed soil material for sampling

and surveillance. The culverts were advanced to only 3 m

depth, still relatively shallow in a deep vadose zone

environment. In practice, the depth of the island is limited

by the drilling equipment.

The island strategy has several unique characteristics. It

provides the site personnel with direct access to deep soil

material, much deeper than practical with the trench

strategy. Before the island material is lowered into the

borehole, the soil profile is exposed and can be described or

sampled in any direction radially from the borehole. Once

the island material is placed in the borehole and backfilled,

the choices of location and numbers of monitoring '

instruments are quite flexible and can be tailored for

specific goals of the monitoring program. The use of

vertical transects allows site personnel to observe and

quantify hydraulic gradients, and therefore the direction of

soil water movement, which is complex near the soil

surface. The strategy can be used to achieve a variety of

goals depending on disposal site characteristics. The

monitoring island concept has been used by other research

I1I 11 NUREGICR-5698
NUREG Monitoring Strategies

Table 3.1-1. Summary of monitoring devices and monitored environment in buried trench.

Device Fundamental Performance Number of Total number

measure measure points or length of points

Horizontal Hydrogen ion Water content 3 x 58 m - 699

neutron logging concentration

Time domain Soil bulk Water content 13 13

reflectometry dielectric

Tensiometers mV response Water tension 19 19

Heat dissipation Temperature Water tension 13 13

sensors change

Thermocouple Soil humidity Water tension 13 13

psychrometers

Solution Soil pore water Solute 13 13

samplers concentration

Temperature Voltage Soil temperature 10 10

thermocouples

Soil sampling Soil material Soil texture, 213 213

water content,

solute concentration

NUREG/CR-5698 12
NUREG Monitoring Strategies

North Monitoring Island

A4

West

side

B

Cross-section view

Annular

backfill

Soil Surface

-

Soil Surface

0.5 m x

1.0m m

1.5 m x

2.o m x

2.5m *x

3.0 m I

I

I

West

side

Monitoring

Instruments

East

Island

Device

• Locations

-- m

I I

Figure 3.1-1. Cross-section (A) and map view (B) of monitoring island and instrument orientation.

13 13 NUREGICR-5698
NUREG Monitoring Strategies

groups (e.g., Scanlon et al., 1997), though in a slightly

different form. Though only two islands were used at the

Maricopa site, disposal site monitoring programs could

benefit through the use of multiple islands, placed either

adjacent to or inside the disposal zone.

At the Maricopa site, we installed clusters of instruments in

three vertical transects, each offset by 90* and 50 cm depth

(Table 3.1-2 and Figure 3.1-1). Similar to the monitoring

trench, all electronic sensors were sampled using data

acquisition equipment, so the rate of data collection was

higher than would be possible if collected manually. Only

the solution samplers required on-site personnel when

active. All of the 15 samplers per island were connected to

a single vacuum manifold, greatly reducing staff effort

during sample collection. Textural samples were also

collected.

3.1.3 Borehole Monitoring Strategy

The borehole monitoring strategy - probably the oldest

strategy used for subsurface monitoring - involves the

excavation of small diameter boreholes with standard

drilling equipment (e.g., hollow-stem auger). During the

drilling activities, grab or core samples can be collected at a

rate specified by technical personnel. The depth of drilling

is limited by the type of drilling method and size of the

drilling rig; generally, the boreholes can be advanced to any

depth necessary. Thus, the borehole monitoring strategy

represents the only strategy which is not depth limited.

Monitoring programs have historically used this strategy

because of the need to monitor ground water quality at

disposal sites.

This strategy is flexible with respect to the number of

boreholes that can be advanced at a particular site, and the

method of advancement. Because drilling rigs are small and

mobile, they can be used for excavating boreholes in a wide

range of surface conditions, at spatial offsets that are quite

small (in the case of the Maricopa site, many of the

boreholes were spaced I meter apart). Surface conditions

are kept relatively undisturbed during the drilling process

because only a small volume of cuttings are returned to

ground surface. A wide variety of drilling methods are

available depending on the subsurface environment

encountered at a particular site (Driscoll, 1986), and the

restrictions on subsurface introduction of fluids.

Once the borehole is advanced, site personnel can design

the monitoring program to include either mobile or fixed

monitoring points. Both were used at the Maricopa site

(Table 3.1-3 and Figure 3.1-2). Mobile points include the

neutron probe or water level indicators. Probes are lowered

into the borehole, readings are taken, and the probe is

removed. In the case of the neutron probe, is held

stationary in the borehole during a reading, and then

advanced as needed. Other probes are raised or lowered for

continuous data collection, such as for the downhole EM-39

method. Therefore, the site personnel have complete

discretion on the vertical resolution of measurements

needed.

When the sampling points are fixed in space, such as the

deep tensiometers and dual-chamber solution samplers, the

flexibility with respect to vertical resolution is lost.

Moreover, a practical limitation exists on the number of

sampling points that can be fixed inside each borehole.

However, this disadvantage is offset by the fact that direct

measurements of performance measures can be taken.

Recent advances in subsurface monitoring have led to the

development of tensiometers and solution samplers which

can be installed to any depth. The neutron probe method,

though very useful, is sometimes disparaged because of the

need for onsite calibration, a difficult procedure to conduct

accurately. However, on site calibration is not critical where

the main interest is in water content changes. Data

collection using tensiometers is automated, so that site

personnel are not needed. These fixed point monitoring

instruments can supplement the neutron probe method, as

done during this research.

3.1.4 Geophysical Monitoring Strategy

The geophysical monitoring strategy involves the

measurement of bulk electrical properties of soil or

geological material, and the conversion of the data to water

content or salinity. Instruments can be categorized as

surface or subsurface. Both types were used at the

Maricopa site (Table 3.1-4 and Figure 3.1-3). The surface

instruments (e.g., EM-31 and EM-38) are used for obtaining

electrical conductivity readings to depths between ground

surface and approximately 4-6 m, depending on the device

NUREG/CR-5698 114
NUREG Monitoring Strategies

Table 3.1-2. Summary of monitoring devices and monitored environment in monitoring island strategy.

Device

Time domain

reflectometry

Tensiometers

Heat dissipation

sensors

Solution samplers

Soil sampling

Fundamental

measure

Soil bulk

dielectric

mV response

Temperature

change

Soil pore water

Soil material

Performance

measure

Water content

Water tension

Water tension

Solute concentration

Soil texture, water

content, solute -

concentration

Number of

points or length

30

30

30

30

12f

Total number

of points

30

30

30

30

12

t Represents only those samples collected before irrigation phases of either experiment.

Table 3.1-3. Summary of monitoring devices and monitored environment in borehole monitoring strategy.

Device

Vertical neutron

probe

Tensiometers

Solution samplers

Monitoring wells

Soil sampling

Fundamental

measure

Hydrogen ion

concentration

mV response

Soil pore water

Water level,

ground water

Soil material

Performance

measure

Water content

Water tension

Solute

concentration

Gradient,

solute concentration

Soil texture,

water content,

solute concentration

Number of

points or length

34 x 3 m

lox 11 m

27

27

13

159

Total number

of points

848

27

27

13

159

15 15 NIJREG/CR-5698
NUREG Monitoring Strategies

60 Extent of irrigated plot

50... .. oShallow neutron

E 40 L • • A• - probe tubes (BM)

" .. -0 Deep neutron

probe tubes (BM) S0 0- N Fully constructed

Z 20 0 0 00 6 0monitoring wells (BM)

10 Horizontal access 10 - 6 tubes (BM)

0. 'a. 71 ' 1 Monitoring islands (MI)

-10 0 10 20 30 40 50 60

Easting- m A Deep solution samplers

(5, 10, 3 m) (BM)

• v Deep tensiometers

(5, 10, 3 m) (BM)

. Borehole tomography

points (GM)

Figure 3.1-2. Schematic field plot, showing the location of major monitoring systems. Monitoring strategy designation

is listed in the legend; MT is monitoring trench, MI is monitoring islands, BM is borehole monitoring, and

GM is geophysical monitoring. Note that EM-31 and EM-38 monitoring points (90 total) are not shown.

NUREG/CR-5698 116
NUREG Monitoring Strategies

Table 3.1-4. Summary of monitoring devices and monitored environment in geophysical monitoring strategy.

Fundamental Performance Number of Total number

Device measure measure points or length of points

Change in water

EM-31 Bulk ECa content or - 90 180

salinity

Change in water

EM-38 Bulk EC. content or 90 180

salinity

Electroresistive Electrical Change in water

borehole resistivity content or 10 x 15 m n/at

tomography salinity

t Borehole tomography method provides 2-dimensional representations of electrical resistivity; therefore,

the number of discrete points cannot be defined.

17 17 NUREGICR-5698
NUREG Monitoring Strategies

70

60

50

E 40

z 20

10

0

-10

-0 0 2 0

-0 0 0 0 0

.0 0 0 0 0 0 0 0 30. 0 -0 0 0 ! 0* 0* *00

0 cp 00 0o*,00 43 0

-0 0 0 0 0 0 0 0 0 0

-0 0 0 0 0 0 0 0 0 0

-0 0 0 0 0 0 0 0 0 0

*0 0 0 0 , I , I , I , I , I , I ,

- Extent of

irrigated plot

- Extent of.

covered plot

-20 -10 0 10 20 30 40 50 60 70

Easting - m

Figure 3.1-3. Locations used for surface electromagnetic induction surveys and for ERT studies. Open circles represent

the EM points and closed circles are the ERT boreholes.

NUREG/CR-5698 118
NUREG Monitoring Strategies

and orientation. The EM instruments are carried from point

to point across the plot, and placed on the ground surface.

The EC value is then read manually or recorded digitally

with a data logger. The subsurface ERT device is combined

with the borehole monitoring strategy and measures "

electrical resistance. Electrical conductors were installed on

the outside of PVC pipe, and permanently installed inside

augered boreholes. Data collection requires site personnel

to run a series of computer programs that operate the field

equipment.

For EM measurements, field personnel are required to be

present during data collection. Because the instruments are

either placed on the ground or kept at hip level, they can be

used without disturbing surface soil or engineered material.

This allows the monitoring program to utilize this method

during post-closure time periods. The ERT method is

considerably more complicated to use, and requires highly

trained field personnel to collect and analyze the data.

However, the method provides 2-D or 3-D tomograms of

electrical resistance, as opposed to the neutron probe

method, which only provides one-dimensional

measurements. The tomograms can provide valuable

insights of layering and soil electrical properties, which is

related to water content.

3.2 Strengths and Weaknesses with

Respect to Monitoring Objectives

and Data Analysis

3.2.1 Monitoring Trench Strategy

water balance either adjacent to or above the waste material.

The water holding capacity, considered important for

assessing the potential for downward flow of water (Gee et

al., 1998), can be monitored appropriately and calculated.

If data from the trenches indicate that large volumes of

water are percolating below the root or evaporative zones,

then it is reasonable to assume that leachate has been

generated, and that some type of corrective action should be

taken. Another advantage of the trench concept is that

spatial variability of soil hydraulic properties and soil water

conditions can be assessed. Understanding the spatial

variability of these properties allows site personnel to assess

the significance of local changes in water content. r

It is important to note,- however, that the trench strategy is

very depth-limited. From a practical standpoint, the trench

cannot be excavated too deeply because of health and safety

requirements to protect site personnel which increases

costs; moreover, it would increase the likelihood of wall

collapse, thereby affecting the continuity of the soil material

and water flow. With this restriction in mind, the trench

strategy is useful for monitoring near-surface conditions,

limiting its functionality as a monitoring strategy. In arid

and semi-arid regions, where vadose zones are 10s to 100s

of meters thick, instruments installed in a trench may not be

capable of providing direct information on releases to the

subsurface. However, if the trench is used in combination

with other strategies, then it can provide important earlytime information on possible downward movement of

water.

3.2.2 Monitoring Islands Strategy

Goals Achieved Goals Achieved

* Observing surface and near-stirface conditions along

the length'of disposal cell or trench

* Determining whether the monitored conditions at one

portion of a disposal or containiiient cell may be

changing significantly versus another portion

An advantage of this strategy is that soil water conditions

are monitored along a potentially long horizontal transect.

The monitoring trenches could be constructed, for example,

to parallel the entire length of a disposal unit. This allows

site personnel to study changes in the near-surface soil

* Hydraulic gradient can be determined, improving flux

calculations

* Improves understanding of water flow throughout the

soil profile

* Soil and/or pore water in the disposal zone can be

sampled

An advantage of the.monitoring island strategy is that

instruments can be installed at different depths and at

different directions from the center of the island (e.g.,

instruments can be installed radially, like spokes on a

'19 NUREG/CR-5698
NUREG Monitoring Strategies

wheel). This allows much higher depth resolution for

quantifying deep percolation of water below the root and

evaporative zones. At the Maricopa site, instruments were

installed to 3 m depth, so that we were able to monitor the

zone where most of the rapid changes occurred. Depending

on the needs of the monitoring program, the islands can be

installed much deeper and with larger diameter material,

increasing the capacity of the strategy for monitoring nearsurface and deeper subsurface conditions. The islands can

be installed relatively close to one another. At the Maricopa

site, we installed two islands 5 m apart, with reasonable

confidence that the islands were independently monitoring

subsurface conditions. Furthermore, the islands can be

secured so that expensive equipment is stored safely. In

remote areas, or at sites where security is a primary concern,

this could be a distinct advantage.

Though the monitoring islands represent a very flexible

strategy, they provide poor spatial coverage of soil water

conditions. Thus, multiple islands will be needed if they are

used as the primary strategy for any monitoring program,

which increases costs and complexity. This may be

particularly important if the monitoring instruments are

remotely queried, and if the data loggers are independently

operated. We believe that the island strategy would be most

beneficial to a monitoring program when used in

combination with one or two other strategies, such as the

borehole or trench strategy. This would allow intensive

measurements to be taken at a few, key locations and less

intensive measurements elsewhere.

3.2.3 Borehole Monitoring Strategy

Goals Achieved

Deep unsaturated zone monitoring using different

instruments

Simultaneous monitoring of saturated and unsaturated

material

Enhanced ability to make redundant measurements in

same borehole

An advantage of the borehole monitoring strategy is that

new monitoring locations can be added to the program

relatively easily, thereby allowing the program to expand as

necessary. By contrast, adding a new monitoring trench is

much more difficult or may not be possible depending on

site geometry. Boreholes can be advanced directly below

disposal zones using angled or horizontal drilling rigs,

which are now commonly used and accessible. This allows

the site personnel to design a monitoring program which

focuses on measurements taken to confirm or disprove the

release of contaminants or leachate material from the

disposal cell, rather than on measuring ambient conditions

adjacent to the cell. Once the borehole is drilled, instrument

clusters can be added that include tensiometers and solution

samplers. Boreholes completed as access tubes create ports

that can be used for neutron probes, or other instruments

that are developed in the future and that can be lowered into

or pulled through the tube. During the field experiments at

the Maricopa site, we studied the use of EM-39 and

crosshole radar methods. Though insufficient data were

available to be included in this report, initial data analyses

indicated some promise, especially with the EM-39 method.

Other methods that become available could be used in the

access tubes.

The borehole monitoring strategy suffers from a number of

disadvantages. A limited number of instruments can be

installed in any single borehole, and the integrity of any

low-conductivity layer between sampling points is difficult

to confirm. For this reason, we chose to install only a single

device in each borehole, then backfill to the surface. This

increased the drilling cost, and the number of potential

pathways for focused flow. Another potential disadvantage

is that the loss of a single borehole could result in the loss of

numerous monitoring points, especially if the borehole is

completed as an access tube. If the monitoring program

relies solely on the borehole monitoring strategy, then the

loss of a few access tubes could leave large gaps of the

subsurface unmonitored. New access tubes can be installed

as replacements; however, it is sometimes difficult to

transfer time series data of water content readings from one

borehole to another. Uncertainty in the transferred readings

can be difficult to explain without a sufficiently long time

series of data.

3.2.4 Geophysical Monitoring Strategy

The geophysical monitoring strategy has a unique

advantage over the other strategies in that some of the

techniques are non-intrusive and others are intrusive. The

NUREG/CR-5698 20
NUREG Monitoring Strategies

non-intrusive instruments (EM-31 and EM-38) are very.,

portable, allowing the user to collect information over a

large area which otherwise would not be possible. Data

collection is very rapid and facilitated by using a portable

data logger. The portability of the instruments allows users

to collect data over wide areas in a very short amount of

time. It appears that the data can be correlated with some

confidence to soil water content, indicating that nonintrusive measurements can be used for monitoring

subsurface changes in water content (Kachanoski et al.,

1988). Data collected from the Maricopa project support

this conclusion (Young et al., 1997), but more research

needs to be done to understand the limitations of the

method. The data can be analyzed using time invariance

(Vachaud et al., 1985), so that only specific, time-invariant

locations at the site are measured, potentially reducing

personnel effort and cost.

The intrusive, ERT method can provide data at multiple

depths and locations adjacent to those used for borehole

monitoring. The field design used at the Maricopa site

consisted of 12 boreholes at 10 m lateral spacing, with

conductors at I m depth offsets. Resolution was

approximately 0.5 m, which is too low for detecting

preferential flow pathways, but clearly high enough for

monitoring field-scale water movement. The boreholes

were advanced to 15 m depth. The water table was clearly -

identified at 11.5 m, as were several electrically distinct

layers across the site. We showed very clearly during

Experiment I that wetting fronts were detectable using the

ERT method, and that the results could be confirmed using

the neutron probe method. Extensive data analysis can

yield 3-dimensional tomograms, provided that the borehole

spacing is designed appropriately. However, we did not

have sufficient data to quantify the lower detection limit of

water content change at the scale used during the field

experiments, so the use of this technique in soil with low

water holding capacity could not be verified.

Some disadvantages of the geophysical strategy were

identified. First, the EM technique is sensitive to external

electrical noise, especially in the presence of AC or DC

power or other metallic objects (e.g., electric fence, steel

highway culvert). A significant number of monitoring

points inside the irrigated plot were removed from the data

base because they were influenced by shallow electrical

lines that were installed on the field plot. Industrial areas

could also be affected by either power lines or metallic

objects. Second, the geophysical techniques provide

volume-averaged data. It is therefore very difficult to

identify small-scale spatial variability in water content or

water movement, especially at depth. The response

functions for each device are different depending on

orientation (see McNeill, 1992); thus, isolating specific

depths is complex and still a subject of research (Borchers

et al., 1997). Third, changes to EC could be indicative of

changes in either water content, salinity, or both. However,

using the EM technique to measure soil water content, by

itself, could be difficult without a significant amount of onsite calibration and ground-truthing.

3.3 Strengths and Weaknesses with

Respect to Installation,

Maintenance and Replacement

The strategies emphasized during this research have specific

characteristics that either help or hinder their use in longterm monitoring programs. Because the goals of

monitoring programs may change over time, the direction

and emphasis of the program needs to be adaptable. This

section provides a list of strengths and weaknesses of each

strategy with respect to installation, maintenance and

replacement. The focus of this section is on the individual

monitoring systems that support the strategies, rather than

the strategy itself. The list stems from experience obtained

during the field experiments at the Maricopa site. Technical

personnel at other sites may have different experiences.

3.3.1 Monitoring Trench Strategy

3.3.1.1 Installation

Strengths

Multiple instruments can be installed at the same

location

Monitoring clusters enhance the redundant measurements of

performance measures. The use of tensiometers and HDS

units permits direct comparisons of two instruments that

measure different soil water conditions; but the same.

performance measure.

21 NUREG/CR-5698
NUREG Monitoring Strategies

* Completion of access ports is visible

Directly observing the backfilling and sealing of the access

ports increases the confidence that preferential flow

pathways are not created during the installation of

monitoring instruments.

• Single manifolds can be used for many solution

samplers

Multiple soil solution samples can be obtained simultaneously, reducing personnel costs.

Horizontal neutron probe tubes can be installed when

the trench is open

The horizontal access tube can be used for collecting large

amounts of data at very small lateral offsets. The

installation procedure is simple and the presence of the

access tube could be very useful as new instruments become

available for field monitoring.

Weaknesses

* Excavation disrupts the soil surface and trench walls

This makes installation of instruments at specific distances

from the trench wall very difficult. Extra care can be used

during excavation to ensure that the trench walls are

smooth, but this may not be possible in field situations.

Replacing material to natural bulk density and layering

is very difficult

If the trench is backfilled (as done during this research), the

material should be replaced to approximately the same

layering and bulk density. Large differences between

undisturbed and disturbed materials could affect water

movement and the representativeness of the instrument

readings near the trench wall.

* Wire lengths for the electronic instruments have the

potential to be very long

Long lead lengths can introduce higher errors in measurements because of residual voltages and temperature affects.

This can be reduced by using low resistance (higher gauge)

leads or burying the wires deeper in the soil profile to

reduce diurnal fluctuations.

Adding more samplers to the monitoring clusters is

difficult after trench is closed

Closure of the trench removes accessibility to the

undisturbed soil, which is both an advantage and

disadvantage of the trench strategy. It is an advantage

because it provides physical protection to the instruments,

but a disadvantage because it limits the flexibility of the

strategy to expand the number of instruments for future

monitoring.

3.3.1.2 Maintenance

Strengths

* Headspace on tensiometers is easy to access

Measurement of the headspace and recharging of

tensiometers are required for use of these instruments.

Tensiometers used at Maricopa were protected by irrigation

valve boxes, but a more water-tight design should be used

for better protection.

Most instruments in the trench are protected from the

elements

Temperature fluctuations, which affect many instruments,

are minimized because of the depth of burial and thickness

of soil. The tensiometer headspace was (in our case) the

only exposed instrument. Data from the site improved the

theoretical framework for temperature corrections of

tensiometers (see Section 4.3 below and Warrick et al.,

1998a).

Weaknesses

Instruments cannot be easily removed for calibration

or maintenance

Removal of the instruments will result in disruption of the

trench backfill or surface soil material, possibly affecting

the representativeness of measurements.

NUREG/CR-5698 22
NUREG Monitoring Strategies

Electrical connections are below ground

The most significant cause of instrument failure at the

Maricopa site was corrosion of electrical connection

between sensors and wire leads. Because these are

completed below or at ground surface, special care must be

taken to ensure that all connections are fully

weatherproofed.

3.3.1.3 Replacement

Strengths:

Electrical connections can be made accessible for

replacement

The trench can be designed so that electrical connections are

accessible and replaceable. Monitoring instruments must be

designed, however, so that potted leads reach ground

surface before the first electrical connection.

* Pressure transducers on the tensiometers can be easily

replaced

Of the instruments used in the trench during the Maricopa

experiments, only the tensiometers required this secondary

measurement component. Pressure transducers, designed to

be located near ground surface, were replaced on all

tensiometers at the trench before Experiment 2 with little

difficulty.

Weaknesses

* Intrusive activity needed for instrument replacement

As stated above, the instruments can not be replaced

without disruption of the surface material. During the field

experiments, several instruments failed to provide reliable

data or samples, and were taken off line, resulting in data

gaps. Replacement would have been beneficial to the

experiments.

Fully decommissioning the trench would be very

difficult

Removing all instruments, including the conduit, wires and

cables, will be extremely difficult without fully excavating

the trench area. If the trench were installed either in or

adjacent to engineered cover materials, then disruption of

the barrier system could occur, increasing the risk for

recharge to occur.

Rewiring could be problematic if new instruments are

added

New conduit was required before Experiment 2 to store

wire leads used for replacement pressure transducers.

Depending on the size of the conduit and the wire bundles

used during initial field installation, existing conduits could

be sufficient, making installation and security of new

conduit unnecessary.

3.3.2 Monitoring Islands Strategy

3.3.2.1 Installation

Strengths

* Islands can act as hubs for monitoring systems

The island is a centralized area for electronic and other

support equipment. The need for electrical conduits and

other storage equipment are greatly reduced because

components are stored in a central location.

* Easy to incorporate redundancy of measurements

As stated above, we advocate the concept of collecting data

from instruments that reduce to one or two performance

measures. The monitoring islands encourage the

installation of multiple instruments in close proximity to

one another, so that side-by-side comparisons can be made

easily. This strength makes the island strategy more

adaptable to changing priorities of the monitoring program.

New instruments can be added at different depths and

through additional ports with little difficulty

Before the start of Experiment 1, six sampling ports each

were added to the monitoring islands in less than one day.

The ability to easily increase monitoring capabilities greatly

enhances the flexibility of the system when monitoring

goals are modified.

23 NUREG/CR-5698
NUREG Monitoring Strategies

• Instruments can be installed at any orientation through

the island wall

Monitoring instruments at the Maricopa site were oriented

parallel to the X and Y axes of the site coordinates. At

other sites (e.g., Scanlon et al., 1997), instruments were

oriented normal to the island wall. This flexibility improves

the targeting of specific layers (either natural or engineered)

adjacent to the island.

* Islands can be installed into deep soil material

Boreholes were drilled to approximately 3 m depth at the

Maricopa site, though the drilling rig used was capable of

advancing deeper than 20 m. Deeper monitoring can be

beneficial to monitoring systems in arid and semi-arid

environments, where vadose zones are often thicker.

* Wiring remains inside the island

As stated above, corrosion of wiring and connections was a

significant cause of data loss from devices in the monitoring

trench. We found no corrosion in the islands during the

field work, because no direct contact existed between soil

and wiring and because the relative humidity was lower,

with condensation of water reduced to nearly zero. The

accessibility of the wiring permits easier expansion when

new instruments are installed.

* Subsurface material is accessible

The immediate proximity of the island to undisturbed

subsurface material permits sampling for future textural or

chemical analyses, or both. This greatly simplifies the

sampling procedures if small amounts of material are

collected at infrequent time intervals, because surface

disturbance is obviated.

* Easy to make manifolds for multiple solution samplers

The close proximity of the solution samplers (in contrast to

those in the monitoring trench) facilitates the installation of

a central manifold for collecting numerous samples

simultaneously. This reduces staff cost by quickening the

sampling process.

Weaknesses

Requires intrusive activity with less common rig

equipment

Installation of the monitoring island culvert into undisturbed

soil is intrusive, and therefore requires the presence of

drilling equipment. Therefore, obvious limitations exist for

installation of monitoring islands after closure of the

disposal cell. In some cases where the island culvert is

backfilled within the engineered barrier or disposal area,

this weakness becomes insignificant because drilling is

unnecessary.

* Backfilling the annular space is difficult

Some difficulty was encountered during backfilling of the

annular space between the borehole wall and the outside of

the culvert material. The problems stemmed from what, we

believe, was a slight expansion of the soil material into the

borehole, reducing the effective borehole diameter. During

the backfilling process, we noticed that the sieved material

tended to bridge, and that the bulk density was probably not

consistent with the undisturbed material. (Bridging occurs

when one or two large particles of backfill become lodged

in the annular space, causing finer grained material to pile

up behind it. This can cause potentially large gaps in the

annular space to be unfilled.) The experimental results

indicated, however, that water flow through the annular

space was similar to that recorded in the undisturbed soil,

but that the dispersivity tended to be higher in the annular

material.

* Presence of metal close to soil material could affect

TDR or surface EM readings

Islands made of metallic material could influence the

readings of TDR and EM systems. Several EM data points

were removed from the data base because the instrument

was affected by the monitoring island. The use of concrete

or fiberglass material could reduce this problem.

0 Instruments need to be installed within a meter of the

island wall

The diameter of the monitoring island poses a practical and

physical limitation with respect to the distance that

NUREGICR-5698 24
NUREG Monitoring Strategies

instruments can be installed away from the island wall.

This is because the augers or drills, used to create the access

port, should be no longer than the diameter of the island.

Therefore, the islands are restricted to relatively localized

measurements, except where installation can be done in

sections, which are no longer than the monitoring island

diameter.

3.3.2.2 Maintenance

Streneths

Individual instruments can be removed and maintained

Unlike permanently-installed instruments that support other

strategies, those installed in monitoring islands are relatively

accessible and can be removed for maintenance and

calibration. This was helpful during the interim period

between Experiments I and 2, when a number of

tensiometers were removed and pressure tested. Other types

of routine maintenance are simplified because of the easy

access to instruments.

* Components are protected from the sun

Direct exposure to the sun, especially in the desert

southwest, led to some heat-related problems during the

field experiments. For example, the high heat caused a

deep-cell marine battery, used to operate the multiplexer for

the Dynamax TDR system, to lose its charge more quickly

than normal. This required the installation of a backup

battery charger to maintain adequate charge. Also,

temperature fluctuations inside the truck boxes that stored

the data loggers, caused significant diurnal fluctuations to

develop in the HDS units. Protection from the sun inside

the monitoring island reduces these impacts and related

extra maintenance.

* Connections less likely to corrode

As stated above, the lack of direct contact with either the

outside environment or soil material and the lower humidity

and subsequent lack of significant condensation of water

eliminated corrosion of soldered connections at the site. In

the long term, this greatly reduces the maintenance costs at

the site.

Weaknesses

0 Metal island themselves are susceptible to corrosion

Long-term monitoring programs that use metallic islands

may be affected by corrosion of the island material itself. If

the island strategy is utilized for relatively short-term

programs (e.g., 5 - 10 years), then this may not become a

significant issue. However, monitoring programs that last

for decades or longer probably should rely on different

culvert material, such as fiberglass or concrete.

3.3.2-3 Replacement

Strengths

* Instruments and samplers can be more easily replaced

As implied above, instruments and associated wiring can be

replaced relatively easily because of their close proximity to

the sampling port.

Weaknesses

0 The island material itself cannot be easily replaced

It would not be practical or feasible to replace the entire

monitoring island, should it become damaged and unsafe.

Therefore, an entire new facility would need to be

constructed, which could adversely impact engineered cover

material or the surrounding area. Similar to restrictions on

the monitoring trench strategy, design personnel should

consider installing .1onitoring islands before site closure.

3.3.3 Borehole Monitoring Strategy

33.3.1 Installation

Streneths

Technology for installing monitoring points is readily

available

Methods of drilling monitoring points have been used for,in

some cases, thousands of years (Driscoll, 1986). Newer

25 NUREG/CR-5698
NUREG Monitoring Strategies

methods (e.g., cone penetrometer method ) can also be used

depending on the purpose of the borehole.

Subsurface material can be sampled continuously

during drilling

Site characterization activities are greatly enhanced by

sampling subsurface material. For thick vadose zones,

installation of borehole monitoring points is the only option

available for characterizing soil/rock layering and other

properties. Though the ability to collect "undisturbed"

samples at depth is questionable, the material can be placed

fairly well within the geologic framework.

Borings can be completed below grade

Security against accidental or intentional damage to

boreholes may sometime require completion below grade.

The borehole monitoring strategy can be implemented so'

that completed points, including cables and wires, are

secured behind locked caps.

* Alternative material is available for access tubes

A wide variety of material is available for access tubes,

including PVC, stainless steel, and ABS. This flexibility

permits the designer to specifically choose access tube

construction for the intended usage.

Weaknesses

Instruments cannot always be placed precisely in deep

boreholes

Placement of monitoring points inside deep boreholes can

be affected by potential cave-in, rocks or other protrusions.

Moreover, deep boreholes may not be perfectly vertical,

causing the monitoring point or access tube to vary from the

final designed location.

Lateral distances between boreholes can be quite far

Some boreholes used for deep tensiometers were almost 60

m from the data logger. The long wiring needed for these

locations could lead to some measurement error.

* Incomplete backfilling of boreholes may lead to

conduits for water flow and contaminant transport

Of the more than 100 boreholes constructed to support site

experiments, only three were completed with grout, sand,

and bentonite. The annular space (between 2.05 and 3.22

cm) of the other boreholes were backfilled to ground

surface with sieved native material. Special care was taken

to avoid bridging in the annular space. Howeverjit isdifficult to ensure that the boreholes themselves are not

conduits for rapid downward water movement.

Large diameter boreholes could be susceptible to

higher measurement interference

The neutron probe is affected by the material immediately

outside of the access tube; larger diameter boreholes mean

that the neutron probe is more affected by the backfill

material than smaller diameter holes. It is therefore.

important to consider the counter issues of wanting a small

borehole to reduce the influence of backfill material on

measurements, but needing a larger diameter borehole to

facilitate backfilling.

Installation of boreholes can drag down contaminants

from shallow to deeper depths

It is possible that contaminants can be dragged downward

from shallow soils to deeper, uncontaminated soils. This

increases the possibility of cross-contamination, and thus,

false positive readings.

3.3.3.2 Maintenance

Strengths

• Portable instruments (i.e., neutron probe) can be

removed from the site

From a strictly instrumentation standpoint, portable

instruments that are pulled through the access tubes can be

brought back to a laboratory for calibration and repair

without altering or affecting the access tube in any way.'

Other instruments, such as the deep tensiometers, can be

easily disassembled, so that the pressure transducers can be

returned to the lab for calibration. Other system

components, such as the vacuum system for collecting

NUREG/CR-5698 26
NUREG Monitoring Strategies

solution samples, can also be brought to the site or removed

at the discretion of the field personnel.

Access tubes have no electronics or moving parts

The access tubes generally require little to no maintenance.

However, we noticed that water condensation appeared on

the walls of the access tube during Experiment 1 and

affected the neutron probe readings; so, we used Drierite' to

maintain dryness in the access tube. We also noticed that a

small number of o-rings, used to seal sections of the access

tubes, became brittle from exposure to the sun, so these

were replaced as needed.

3.3.3.3 Replacement

Strengths

Portable instruments can be replaced easily

Measurement instruments that are taken to the site and

found to be faulty can be brought back to the laboratory and

replaced. This would apply to the neutron meter, which

failed several times during Experiment 1, and to the

pressure transducers used on the deep tensiometers.

Weaknesses

Weaknesses , Borehole replacement require- redrilling

Some instruments, which are permanently left in the

boreholes (i.e., deep tensiometers and solution

samplers), cannot be maintained easily

During the field experiments, one-way check valves became

stuck, making it difficult for the sampler to maintain an

adequate vacuum for collection of samples. By pressurizing

and depressurizing the sampler, we were able to fix the

problem and continue sampling. However, if this "repair"

failed, then the sampler would have been disabled. Several

deep tensiometers provided suspect data because of

difficulty in sealing the rubber stopper to the tensiometer

cup. The lack of adequate sealing was manifested in

erroneously low water tension values. In several cases, we

were unable to repair the tensiometer cup, leading to ,

removal of the tensiometer from the sampling program.

Long-term monitoring programs that rely on these

instruments could also be affected.

Borehole maintenance can be difficult as the boreholes

become deeper

Deeper boreholes used for neutron probe access tubes (e.g.,

> 5 m) cannot be readily maintained if they become either

wet, or if the integrity of individual borehole sections is lost.

Thus, water leaking through these poorly sealed joints

(between borehole pipe sections) could lead to

abandonment as well.

Depending on the location of the original borehole, and the

proximity to engineered cover material, replacement may

not be an option. Loss of one or more of these sampling

units could therefore greatly affect the ability of the

monitoring program to detect potential releases from the

disposal area. The cost of drilling new boreholes must be

incorporated in the operational and maintenance budgets.

Stationary instruments probably cannot be replaced in

same borehole

Several of the boreholes at the Maricopa site were equipped

with instruments that cannot be replaced, including the ERT

boreholes, tensiometers and solution samplers. Loss of

instruments in these boreholes from the sampling grid led to

data loss during the experiments. Long-term monitoring

programs likewise would be affected by the loss of these

boreholes. Though replacement through additional drilling

can be done, it is difficult to transfer the new data to the

older time series, adding some uncertainty to the

observations.

3.3.4 Geophysical Monitoring Strategy

3.3.4.1 Installation

Strengths

Surface EM instruments are not permanently installed

27 27 NLJREG/CR-5698
NUREG Monitoring Strategies

No permanent installations are needed for the EM readings;

rather, only surveyed monitoring points are required.

During the site experiments, a total of 90 surveyed points

were chosen around and inside the irrigated plot, providing

a set of spatially distributed data. Therefore, no security

measures needed to be taken to protect on-site equipment,

other than markers used for future measurement points.

EM-39 can be used in the same boreholes as

monitoring wells and neutron probes

The EM-39 instrument, tested on a limited basis during

Experiment 1, can be used in the neutron probe access

tubes. [The data looked promising, but were not sufficient

to describe in these research documents.] No special

installation procedures were needed. The EM-39 is an

example of a cross-over instrument that can support the

borehole monitoring and geophysical monitoring strategies.

ERT monitoring points are permanently installed at

the site

Each of the 12 ERT monitoring points were permanently

installed at the site. Though this represents a disadvantage

with respect to maintenance or replacement, it provides data

from a stationary point in space, thereby removing potential

errors due to poor placement of portable EM instruments.

ERT data acquisition systems can be designed for

portability

The data acquisition system used for collecting tomography

data was contained in a small trailer and removed from the

site during down times. The portability of this electronic

equipment thus represents an advantage of the method,

because no specific actions need to be taken to secure or

otherwise maintain the equipment at the site.

Weaknesses

Duplicating exact placement of the EM instruments is

difficult over time

Because each data point collected with the EM instruments

required the unit to be (in our case) placed on the ground

surface, a potential exists for the instrument to be

incorrectly located, producing biased data. This potential

problem was minimized during the site experiments by

assigning specific personnel to EM data collection.

However, long-term monitoring programs will need to

emphasize accurate instrument placement to ensure

accuracy of data.

Some techniques may not work properly under all soil

conditions

Highly conductive subsurface materials reduce the

effectiveness of EM instruments because of higher

attenuation of electromagnetic waves. We attempted to use

the downhole ground-penetrating radar technique for

obtaining 2-D tomograms. However, the 10 m lateral offset

between access tubes was too wide, so the technique could

not be used. Though surface GPR has been used for

monitoring soil water content (e.g., van Overmeeren et al.,

1997), the downhole method is new and still under

development.

ERT requires substantial lengths of wiring when

boreholes are far apart

The ERT design used at the Maricopa site required 15-wire

cable from each of the 12 boreholes, all of which were

connected to a central processing unit immediately off the

irrigated plot. If the depth of measurement is at least 1.5

times the lateral offset of the couplets, as designed here,

monitoring large facilities would require potentially very

deep boreholes and long lead lengths. This leads to a more

complicated monitoring system and hence higher

installation costs.

3.3.4.2 Maintenance

Strengths

Portable EM instruments can be removed from the

field for servicing

Instruments can be maintained more easily in a laboratory

setting. Maintenance of data acquisition equipment used for

ERT can be maintained more easily because of its

portability.

NUREG/CR-5698 28
NUREG Monitoring Strategies

Weaknesses 3.3.4.3 Replacement

Portable instruments could provide different readings

with time if maintenance or calibration procedures are

not performed properly

For example, data collected from the EM instruments

required periodic zeroing and rechecking to ensure a

minimum of drifting. Though standard operating

procedures are available, different persons will carry out

these procedures to different degrees, thereby leading to

possible measurement biases.

ERT wiring is permanently left in the borehole

Maintenance of ERT wiring, sources and conductors is not

an option, using the design implemented at the Maricopa

site. Therefore, no actions can be taken to repair or

refurbish downhole components of the system, making it

more vulnerable to degradation during long-term

monitoring programs.

Strengths

Portable geophysical instruments can be replaced

easily and taken back to the site

This permits instrument replacement or upgrades as new

products become available.

Weaknesses

ERT sources and detectors cannot be replaced without

redrilling

Given the specific geometrical requirements of the ERT

method, long-term monitoring programs could become

greatly affected if even one borehole is damaged.

Therefore, it may be appropriate to use ERT methods in

combination with other strategies, so that long-term

monitoring programs do not rely specifically on vulnerable

monitoring systems.

29 29 NUREG/CR-5698

NUREG Discussion

4 DISCUSSION

4.1 Use of Strategies to Fulfill Subgoals

Subsurface monitoring (Section 2.3 above) is composed of a

number of subgoals. The monitoring strategies that we

discuss, when used together, provide assurances that the

goals of the monitoring program are achievable. However,

because the strategies emphasize the monitoring of specific

subsurface zones (e.g., deep or shallow soil), some of the

subgoals may be poorly addressed using only one or two

strategies. Table 4.1-1 provides a list of 11 subgoals of

subsurface monitoring, based loosely on the three major

goals for monitoring programs (Section 2.3). Check marks

placed in the adjacent columns indicate whether a specific

monitoring strategy is reasonably capable of satisfying that

specific subgoal. The bullets listed below provide an

explanation of why some strategies are better than others.

Collect sufficient data over a time period long enough

to quantify background variability

Quantification of environmental impacts is necessarily

based on changes from baseline conditions. Whether the

preoperational monitoring phase is used for establishing

baseline conditions, or whether the monitoring program is

maintained continuously from preoperational through

postoperational phases, baseline conditions must be

quantified. This reduces potential occurrences of false

negatives (data incorrectly indicate that no change from

baseline has occurred) as well as false positives (data

incorrectly indicate significant change has occurred).

Seasonality, background trends, natural variability should

be observable using each of the monitoring systems. A

variety of parametric and nonparametric statistical tests are

used for establishing baseline conditions (see, for example,

Gilbert, 1987). The baseline conditions include estimates of

mean, inherent (or random) variability, seasonality, and

long-term trends. Failure to understand each of these

aspects of time-varying data sets can lead to false positive

and false negative conclusions when compared to future

data sets. All four strategies are checked off because

baseline environmental conditions can be quantified using

each of the strategies.

Establish surface/near-surface conditions and

determine spatial variability along the length of a

disposal unit or trench.

Near-surface water storage has been considered an

important parameter for understanding when infiltrating

water breaks through engineered layers (Gee et al., 1998).

If we use the premise that water entering a disposal facility

from the surface is the predominant source of potential

leachate, then monitoring changes in water content below or

adjacent to the disposal area can be used to indicate the

possibility of contaminant movement. Enhanced

monitoring of near-surface soil or cover material could thus

be an important task for providing early warning of

potential releases. The need for near continuous

measurements along a trench around or above disposal

facilities is directly addressed using the monitoring trench

strategy, especially with the use of horizontal neutron probe

access tubes. Likewise, the ease with which EM-31 and

EM-38 data can be collected, some recent successes with

the strategy (Scanlon, et al., 1999), and the non-destructive

nature of the method makes it also well suited for this

purpose.

Establish hydraulic gradient in both shallow and

deeper soils

The direction of the hydraulic gradient is, ultimately, one of

the most important factors that govem potential water

movement into or out of disposal area. A lack of

monitoring devices installed in vertical transects at disposal

sites, makes the direction and magnitude of the hydraulic

gradient difficult, if not impossible, to determine.

Therefore, provisions need to be made for monitoring

subsurface environments vertically, as well as horizontally.

Considering the restrictions on excavation for monitoring

trenches, the MT strategy may not be adequate by itself for

site monitoring. Geophysical monitoring likely will not

provide accurate assessments of hydraulic gradients either,

though research is still ongoing. However, the use of

borehole monitoring, especially with the recent advances of

borehole tensiometry and the use of heat dissipation sensors,

greatly improves the ability to measure soil water tension in

deep or shallow environments. We found that the

monitoring islands provided excellent access to deeper soils

31 NUREG/CR-5698
NUREG Discussion

Table 4.1-1. Breakdown of adequacy of monitoring strategies to fulfill subgoals. MT, MI, BM, GM correspond to

monitoring trench, monitoring island, borehole monitoring and geophysical monitoring, respectively. Check

marks denote adequacy.

Subgoal MT MI BM GM

Collect sufficient data over a long-enough time period to quantify

background variability

Establish surface/near-surface conditions along the length of a disposal S V

unit or trench

Establish hydraulic gradient in both shallow and deeper soils V V

Quantify flux into and/or out of the disposal area 6/ VS

Collect sufficient quantities of reliable data to enable comparisons V V

with baseline conditions

Monitor deep soil conditions with both direct and indirect instruments V V

Quantify uncertainty in measurements and observations S V S /

Monitor water table for both water level and water quality V

Ensure that decommissioned monitoring systems do not affect the V V V

integrity of the containment unit

Emphasize consistency of data collection throughout different phases V V V V

of monitoring programs

Include non-intrusive activities during latter phases of the monitoring V V

programs

Consider the potential for retrofitting existing ports to accommodate V V

new instruments

NUREG/CR-5698 332
NUREG Discussion

(3 m in our case), improving the determination of gradients.

Therefore, if the facility is located in an area with a deep

water table, and if the licensee includes goals of monitoring

deep and shallow conditions, then using devices in islands

and boreholes could provide the necessary data.

Quantify flux into and/or out of the disposal area

Combining the previous two items (water storage and

hydraulic gradient) permits the quantification of flux. The

ability of the monitoring island and borehole monitoring

strategies to facilitate the collection of soil water content

and tension data in deep or shallow profiles make them well

suited when flux calculations are needed. Monitoring

trenches alone, though excellent for determining spatial

variability of conditions along a long transect, are not well

suited for measuring hydraulic gradients. The geophysical

monitoring strategy (as used during this research project)

relied on the collection of electrical conductivity and

resistivity, which cannot be converted to soil water tension

without the use of soil water retention curves; thus, this

strategy would not be well suited either. This strategy

would not be appropriate either.

Collect sufficient quantities of reliable data to enable

comparisons with baseline conditions

This goal obviously applies to all strategies. Without

sufficient amounts of data, either spatially or temporally, it

is not statistically possible to conclude whether baseline

conditions have changed. Therefore, all monitoring

strategies incorporated into a site monitoring program need

to be used long enough to allow comparisons to baseline

conditions. If baseline conditions are not established, then

findings with respect to environmental impacts are

ambiguous. Moreover, if sufficient data are not available

for comparisons with baseline conditions, then findings also

cannot be made. For example, if the user seeks to know the

minimum number of data points needed for running

comparisons on serially-correlated data, then he/she will

heed to have an acceptable autocorrelation coefficient and

accuracy known a priori, and a known lag distance in time

(see Gilbert, 1987, esp. Eqn. 4.20). This could require

several dozen data points depending on the accuracy needed

for the comparison. If the data are collected monthly, then

several years of data might be needed; if data are collected

quarterly, then required data collection could be longer.

Monitor deep soil conditions with both direct and

indirect instruments

Deep soil profiles, located in arid and semi-arid

environments can be up to several hundred meters thick.

Depth of excavation would be an obvious limitation for the

monitoring trench and island strategies under these

circumstances. Surface geophysical techniques also would

not be very effective, because they provide depth-averaged

values of resistivity in relatively shallow soil. The ERT

method, however, showed significant promise, especially

when the soil profile was initially dry as before Experiment

1. Borehole monitoring, with the advancement of

tensiometry and solution samplers (both dual-chamber or

improved single-chamber models), were found to be very

effective at monitoring changes in pore water pressure and

concentration, respectively. Therefore, borehole monitoring

will likely be the optimum strategy for sites located in arid

and semi-arid climates, especially if the monitoring program

emphasizes water movement in deep soil at several depths

below the disposal area.

Quantify uncertainty in measurements and

observations

Quantifying uncertainty in measurements and observations

at the disposal facility is particularly important when

predicting groundwater or soil water concentrations. In-situ

measurements of soil water conditions, and ex-situ

characterization of soil hydraulic and transport properties

needs to be sufficient so that 1) uncertainty analysis on the

data themselves can be performed, and 2) modeling water

flow and solute transport through the vadose zone can be

accomplished with uncertainty analysis. Recent

publications stress the need for including probability

distributions in flow and transport modeling (Meyer et al.,

1997). Therefore, because the measurements themselves

are often used as input for inverse modeling, or as

calibration data for the modeled parameter estimates,

probability distributions of the data (e.g, characteristics of

the distribution - mean, standard deviation, etc. - should be

quantified for specific locations). This subgoal is related to

others listed here, but is considered important enough for

specific mention.

Monitor water table for both water level and water

quality

33 33 NURLEG/CR-5698
NUREG Discussion

Many environmental regulations either mandate or

emphasize monitoring of groundwater quality for

quantifying environmental impacts. We include this

subgoal here for that reason. Historically, the borehole

monitoring strategy has been the only viable alternative

available for monitoring water table depth and ground water

quality, especially in deep water table environments. We

found that ERT worked very well for identifying the

position of the water table, but we did not use it for

monitoring water quality. If the monitoring program

emphasizes groundwater monitoring and if the water table is

very shallow, monitoring islands could be used to

complement the use of boreholes. Surface geophysical

monitoring would probably not be an accurate strategy for,

looking at groundwater quality, though research is still

being done on this subject.

Ensure that decommissioned monitoring systems do

not affect the integrity of the containment unit

Decommissioning of monitoring systems can affect the

integrity of engineered barriers, creating fast flow paths

toward the waste material. Care should be taken with

decommissioning monitoring systems used to support the

trench and island strategies, because of the large amounts of

electrical wire and multiple exit/entry ports that are

generally used. Attempts to fully decommission the

monitoring trenches at the Maricopa site, for example, will

require re-excavation of the backfill material. Depending

on the proximity of a buried trench with respect to the waste

material at a true disposal site, re-excavation could become

a significant issue. Therefore, if the monitoring program

calls out for the use of monitoring trenches and monitoring

islands, during the preoperational and operational phases,

with subsequent decommissioning of these features, special

precautions would need to be taken to ensure that

engineered barrier systems are unaffected. Boreholes have

traditionally been decommissioned using cement grout, so

use of this strategy in the overall monitoring program could

be advantageous in site characterization through operational

phases, if postoperational monitoring is minor. It may be

possible to use traditional methods of decommissioning

ERT boreholes, depending on the method of installation;

however, the more commonly used non-intrusive methods

require no decommissioning, so this method was not

checked in the table.

* Emphasize consistency of data collection throughout

different phases of monitoring programs

Regardless of the monitoring strategy, it is important to

emphasize consistency of data collection throughout the

monitoring program. This includes using similar time

intervals, monitoring locations and sensor types across each

distinct monitoring phase. Site characterization programs

implemented before licensing should be maintained during

preoperational programs, thereby providing longer time

series and higher confidence in the data. Decommissioning

monitoring devices or access tubes after site

characterization ends, and then implementing a different

operational monitoring program does not necessarily

improve the quality of the data set, because most of the

temporal variations need to be requantified. Monitoring

plans should indicate how the monitoring strategies will be

maintained when the facility closes, when the program

shifts from operational to postoperational phases.

Include non-intrusive activities during latter phases of

the monitoring programs

Non-intrusive activities refer specifically to surface

geophysical techniques, including EM-31 and EM-38, as

used during this research project. Ground-penetrating radar,

not used during the Maricopa experiments, has been shown

to improve site characterization (Kung and Lu, 1993) and

subsurface monitoring (van Overmeeren et al., 1997;

Hubbard et al., 1990), though the soil texture and presence

of strong lateral discontinuities in the soil horizons will

affect the depth of penetration of the radar waves. Other

methods that require intrusive activities, especially if this

involves potential borehole drilling or'excavation, could

cause more problems than they solve. Breaching the

engineered barrier during post-closure time periods, when it

exists in some level of static equilibrium, could create fast

flow paths or enhance erosion or settlement rates. However,

boreholes that are installed prior to postoperational periods,

or those installed so as to avoid potential damage to the

disposal area, can serve as access ports for the insertion of

portable instruments, such as neutron probes. Though the

boreholes are clearly intrusive by nature, they require low

maintenance by themselves and allow data to be collected

without any site disturbance. Given that the geophysical;

monitoring strategy includes monitoring devices which are

non-intrusive, we opted to reject the use of the MT and MI

NUTREG/CR-5698 334
NUREG Discussion

strategies and include the BM strategy for those situations

where non-intrusive monitoring is required.

Consider the potential for retrofitting existing ports to

accommodate new instruments

The monitoring island and borehole monitoring strategies

use access ports which can be retrofitted to accommodate

new instruments as they are developed. The neutron probe

access tubes should be resilient against environmental

damage, and the monitoring islands should be made of

material that is equally resilient (e.g., fiberglass). For

example, downhole ground-penetrating radar is currently

being tested for monitoring changes in water content, much

in the same way as ERT (Eppstein and Dougherty, 1997).

Other technologies that rely on non-stationary probes could

be developed in the future and found promising. The

existence of access tubes and ports will facilitate the use of

these new technologies. Installing new devices through

access tubes at the backfilled trench, as used during the

Maricopa experiments, would be difficult, unless the

horizontal access tube was used. Likewise, access tubes

were not used for the majority of monitoring activities in the

geophysical strategy; thus the buried trench and geophysical

strategies are not checked off on Table 4.1-1. Including

extra access tubes in monitoring plans, even if not used

extensively during site characterization and preoperational

monitoring phases, could be very advantageous in the

future.

4.2 Choosing different monitoring

strategies

The wide variety of environmental and anthropogenic

conditions that could be encountered at any single disposal

site precludes the prescription of monitoring strategies

without site specific information. In this section, we

attempt to list the major factors that could influence the

suitability of the four alternative strategies considered in

this document. For each factor, we define opposing

conditions (e.g., shallow water table to deep water table)

and categorized each strategy as whether the condition

supported its use, weakened its use, or made no difference

with respect to its use (Table 4.2-1). Moreover, it must be

noted that the use of these strategies, given specific site

conditions, relies strongly on the goals and subgoals of the

monitoring program. For example, if the monitoring goals

stress early warning of releases at the base of a disposal

unit, and the water table is shallow, then most geophysical

devices will not be able to resolve the small changes in soil

water content that could signal a release. Thus, the strategy

likely would not be well chosen.

Our conclusions and recommendations are based on our

experience with the strategies at the Maricopa site, and we

recognize that our experiences may differ from the

experiences of others. Nonetheless, we believe that the

conditions and factors represent a good range of what might

be expected in the field and how they affect the choice of

monitoring strategies. This section provides a brief

discussion of the factors and our conclusions on the use of

different strategies.

4.2.1 Natural site conditions/processes

* Consolidation of subsurface material

Consolidated, unsaturated subsurface material would make

trench excavation or large-diameter drilling very difficult.

Installation of the monitoring trench at the Maricopa site

likely would not have been feasible if the material was

consolidated, e.g., sandstone or fractured igneous rock.

Borehole installation through consolidated or

unconsolidated material is well known, so the material at

the site may have no bearing on the use of the BM strategy.

The choice of using instruments that support the GM

strategy also may be independent of the consolidation of the

subsurface material because excavation is not needed. If the

subsurface material is soil or other unconsolidated material,

then excavation can be done more easily, favoring the MT

and MI strategies.

Soil texture

Clayey soils are normally conductive, which tends to reduce

the precision of geophysical methods. Downhole GPR was

attempted prior to Experiment 2, but the soils were found to

be too conductive when borehole placement was 10 m apart.

The sandier soils at the Maricopa site were more conducive

for monitoring with geophysical methods, except in the

presence of very coarse sand and gravel. We found that the

low water holding capacity of these materials caused

35 35 NUREG/CR-5698
NUREG Discussion

Table 4.2-1. Specific site conditions and processes: how they affect the choice of monitoring strategies. MT, MI, BM,

GM correspond to monitoring trench, monitoring island, borehole monitoring and geophysical monitoring,

respectively.

Supports Neutral Weakens

Natural site conditions/factors

Type of subsurface material:

Consolidated BM, GM MT, MI

Unconsolidated MT, MI BM, GM

Soil texture:

Clayey MT, MI, BM GM

Sandy GM MT, MI, BM

Precipitation:

Heavy BM, GM MT, MI

Light MT, MI BM, GM

Depth to water table:

Deep MI, BM (unsat) MT, GM

Shallow BM (sat) MT, MI GM

Wetness of soil material:

Wet/Moist MT, MI BM GM

Dry BM MT, MI, GM

Anthropogenic site conditions/factors

Proximity to population centers:

Distant MT, MI BM, GM

Close BM, GM MT, MI

Existing buildings/plumbing:

Present BM MI MT, GM

Absent All

Facility status:

Operating MT, MI, BM GM

Closed GM BM MT, MI

NUREG/CR-5698 363
NUREG Discussion

Table 4.2-1., Specific site conditions and processes: how they affect the choice of monitoring strategies. MT, MI, BM,

GM correspond to monitoring trench, monitoring island, borehole monitoring and geophysical monitoring,

respectively. (Continued)

I Supports Neutral Weakens

Anthropogenic site conditions/factors

Accessibility of heavy equipment:

Accessible All

Inaccessible GM MT, MI, BM

Availability of AC power:

Available MT, MI BM, GM

Unavailable BM, GM MT, MI

Cellular telephone coverage:

In-range MT, MI BM, GM

Out-of-range BM, GM MT, MI

Lifespan of monitoring program:

Long-term GM, BM MT, MI

Short-term All

37 37 NtJREG/CR-5698
NUREG Discussion

smaller changes in soil water content during the water input

phase, reducing the ability of the devices to resolve changes

in bulk electrical conductivity and resistivity. Devices that

supported the other three strategies appeared to operate

without effects from the texture.

Precipitation

Precipitation rates are normally heavier at Eastern U.S. sites

than at Western U.S. sites. None of the monitoring

strategies is particularly well suited to locations

experiencing heavy precipitation, but the MT and MI

strategies in particular could be adversely affected. This is

because of the potential problem with preferential flow of

water through backfill material in the trench and island

material, not because of the potentially shallow water table.

Preferential flow of water could reduce the

representativeness of the data, leading to false conclusions

that soil water content has changed. This could be

especially problematic in the case of the MT strategy, if

constructed similar to that at the Maricopa site, where a

neutron probe access tube was installed directly in the base

of the trench. During the field infiltration experiments, we

observed near surface fast flow paths that were attributed to

bulk-soil properties, but not necessarily to backfill. Without

the presence of numerous instruments in the trench, we

would not have been able to make this conclusion. The MT

and MI strategies could be supported at sites experiencing

light precipitation where preferential flow may not be a

significant problem, or where significant excavation and

backfilling is occurring, but in such a way to be independent

of precipitation. We do not believe that the precipitation

rate affects the choice of either the BM or GM strategy to

any significant degree.

MT and GM strategies are normally limited to monitoring

near surface conditions (with the exception of the ERT

method), measurement of vertical gradients may not be

possible. If, for example, the subgoals of the monitoring

program require the assessment of spatial variability along

the trench, then the presence of a deep water table would

support the use of the MT strategy. For sites with a shallow

water table and an emphasis on groundwater quality, the

BM strategy is most useful. The MT and MI strategies are

both unaffected by a shallow water table, unless the base of

the excavation intersects the capillary fringe during the

year. In these cases, wiring from the monitoring

instruments could become submerged and potentially

damaged.

Wetness of soil material

Of particular interest during Experiment 2 was the apparent

lack of detection of the wetting fronts by the GM devices.

In each case of poor detection, changes in bulk electrical

conductivity and resistivity were too low for accurate

tracking of the wetting front. Therefore, if the soil is

initially moist, and the goal of the monitoring program is to

detect changes in soil water content as a result of infiltration

or leachate release, then relying on GM could become

problematic. MT and MI strategies in wet soils allow a

wider range of devices to be used, such as tensiometers and

solution samplers. In dry soil, especially when they are too

dry to use tensiometers and solution samplers, borehole

monitoring may be advantageous relative to the other

strategies.

4.2.2 Anthropogenic site conditions/processes

* Proximity to population centers

Depth to water table

We are not defining a specific depth as being deep or

shallow, but we look at depth as being relative to the ability

to install monitoring instruments and the goals of the

monitoring program. For example, the presence of deep

water tables may require one to monitor water content or

constituent concentrations in the unsaturated zone. The

presence of a deep water table supports the use of both MI

and BM strategies because of the ability of these strategies

to define vertical distributions of subsurface conditions in

the unsaturated zone for gradient calculations. Because the

The relative proximity of the disposal site to population

centers or transportation networks improves overall access

to the site. For sites that are close to these hubs (e.g.,

several SDMP sites), frequent travel for data collection

using BM and GM strategies may not result in substantially

higher costs to the operator of the monitoring program.

However, relying on BM and GM strategies for monitoring

a distant site without easy access could increase program

costs due to longer travel time for personnel. In these

circumstances, choosing reliable, low-maintenance

NUREG/CR-5698 38
NUREG Discussion

instruments (e.g., heat dissipation probes) could reduce the

number of visits to the site and hence reduce the cost.

Existing buildings/plumbing

Buildings and subsurface plumbing are more likely to be

present at SDMP sites than at facilities specifically designed

and constructed for housing waste material. Presence of

these structures can be problematic for the MT strategy

which relies on relatively long excavated cuts into the soil.

If the contaminated material is stabilized on site, then it

might be difficult to avoid some of these structures. Also,

geophysical monitoring is usually negatively affected by the

presence of metallic structures, thereby increasing

background noise and causing potential loss of data points.

This occurred at several locations at the Maricopa site

because of the presence of subsurface AC power lines. The

BM has a much smaller disturbed area (just the borehole

diameter and a small support area around the borehole), so

this may be the most useful monitoring strategy under these

conditions. For newer LLW facilities, this factor should not

significantly affect choice of monitoring strategy.

Facility status

While the facility is operating, personnel are present at the

site almost daily, enhancing security and maintenance of

monitoring equipment. Data can be evaluated at the site

and systems can be upgraded or repaired more easily. High

maintenance systems, such as tensiometers and (in our case)

TDR, required higher staff effort to ensure good data.

Remote downloading and analysis of data from off-site

locations may indicate when device output begins to drift;

however, we found that daily observations of instrument

status (e.g., manual measurements of tensiometer air space)

greatly improved the reliability of the measurements. Once

the facility is closed, and visits to the facility are less

frequent, those higher maintenance instruments should be

de-emphasized, especially if they are used in data

acquisition systems. The geophysical monitoring strategy

can be very Useful for non-intrusive data collection after site

closure because the instrument is taken to and removed

from the site after each data collection episode. Data and

instrument reliability can be evaluated quickly, if needed.

Accessibility of heavy equipment

While the facility is open and earthmoving equipment is

present onsite, installing trenches, monitoring islands, etc. is

relatively easy.. Monitoring programs can be expanded

through installation of new trenches or islands, or reduced

by decommissioning these features. Drilling equipment can

also be brought to the facility with fewer chances of

disrupting operations. After site closure, however, large

disturbances to the site could affect the integrity of the

disposal system. For this reason, use of MT, MI and BM

strategies is weakened for sites that are closed, where future

installation or trenches, islands or boreholes will be needed,

or where decommissioning of these features is pending.

Some cause for concern could be warranted if the facility

closure plan calls for a small support or buffer area, and the

monitoring islands or trenches are relied upon for the

majority of the data collection. Because many of the GM

devices are portable and nonintrusive, data collection can

continue without the need for equipment or large support

areas.

* Availability of AC power

The presence of AC power can be a significant advantage

for those monitoring programs that rely on high currentdrain instruments, such as TDR or vacuum pumps for

solution samplers. These devices are commonly used to

support the MT and MI strategies. We delivered AC power

to several locations at the Maricopa site to supply power to

vacuum pumps for solution samplers, to battery chargers,

TDR cable testers, and data loggers. [We eventually

removed the cable testers from AC power during water

content sampling, then placed them back onto AC power

during quiescent periods to recharge the batteries]. With the

level of data collection and data transfer used during the

field experiments, DC power alone with batteries recharged

using solar panels, likely would not have been a sufficient

power source. Installing high power-consuming monitoring

devices to support strategies at disposal sites may be

difficult without reliable AC power. Because many

downhole (e.g., neutron probe) and EM instruments operate

only on rechargeable batteries, the presence of AC power is

convenient but not required.

* Telephone coverage

Telephone links (cellular or landline) to data loggers allow

direct downloading of data and information to off-site

39 NUREG/CR-5698
NUREG Discussion

personal computers. Thus, because of the emphasis of

automated measurement systems to support the MT and MI

strategies, presence of data lines could be very economical.

Given that the BM and GM strategies traditionally require

on-site personnel for data collection, automated data

collection is not a significant issue, and hence neither is the

need for telephone coverage. However, the case for using

deep tensiometers or other electronic monitoring devices in

boreholes could be greatly strengthened if data lines are

present and instruments are connected to data loggers. If

the field site is not accessible by telecommunications, then it

may be more appropriate to focus on manual data

collection, such as with BM and GM strategies.

* Lifespan of monitoring program

We believe that long-term monitoring programs (e.g.,

greater than 10 years) can benefit from the installation of

access boreholes. As we stated above, future developments

in downhole instrumentation could improve the accuracy in

monitoring subsurface conditions, and the presence of

access tubes could facilitate the testing and use of these

instruments. The use of non-intrusive methods is also

strengthened by the need for long-term monitoring if the

goals of the monitoring program include the monitoring of

near-surface soil water conditions. Long-term monitoring

programs that rely heavily on the MT strategy could be

affected by failure of monitoring devices, their associated

wiring, and logging systems. During Experiments I and 2,

we experienced a number of instrument failures that

required either ongoing maintenance or replacement. In

several instances (e.g., trench tensiometers), replacement

would have required intrusive activity, which would have

been a potentially difficult task if excavation through

engineered barriers was needed. Though the MI strategy

also requires a significant amount of wiring, they are easily

replaced, thus prolonging the use of the monitoring systems.

Therefore, if the monitoring goals require long-term

monitoring of soil water conditions, then the program

should include access tubes, or a combination of access

tubes and the MI strategy. This will increase the flexibility

of the overall monitoring program.

4.3 Analysis of Monitoring Systems

Regardless of the number of subgoals that are identified for

a monitoring program and whether the monitoring strategies

will work within that framework, it will likely require a

combination of strategies in order to adequately survey

changes in site conditions, especially if the facility is located

in a geologically or structurally complex area. Moreover,

the difficult task of identifying the number of devices, their

locations and the sampling frequency for each instrument is

complicated by the fact that each instrument needs to be

matched to the site conditions expected for long-term postclosure periods.

This question of coverage cannot be answered without site

specific information. However, we know that the

monitoring program must provide data which shows that the

disposal site is, or is not, performing acceptably. Therefore,

the monitoring system is designed and operated so that

potential releases are detected (i.e., undetected releases are

minimized), whether to the saturated or unsaturated zone.

However, objective analyses for evaluating monitoring

systems are lacking in the literature, and this lack of

methodologies introduces the potential for subjectivity in

both the design and review of monitoring systems.

One way to remove this subjectivity would be to analyze the

ability of the monitoring program to detect releases in a

repeatable and objective manner. Several approaches to

monitoring systems analysis have been proposed in the

literature, using a variety of numerical approaches. In the

work by Warrick et al. (1998b), we sought to develop a

simplified procedure that could be added to existing and

often-used numerical models that predict future subsurface

conditions. The result of our analysis is a value which

indicates the likelihood that a release from the disposal unit

will be detected by the monitoring system, which we termed

"monitoring efficiency." Larger values indicate that a

larger percentage of releases will be detected, and smaller

values mean that fewer releases will be detected. Thus,

larger releases and more monitoring points Would generally

lead toward higher likelihood of detection. The analytical

procedure optimizes the countervailing need to maintain a

cost-effective monitoring program, while improving the site

coverage.

Briefly, the detection method uses the following steps:

1. Develop a numerical model to predict possible release

scenarios to the soil and/or groundwater. Determine

NUREG/CR-5698 40
NUREG Discussion

the center and standard deviation (SD) of the release.

2. Take the domain of interest, and divide it into pixels.

Each pixel is assigned a probability p(x,z) that a spill

will occur, and then normalized according to the size

of the pixel; the sum of all p(x,z) values equals unity.

The probability values come from the results of the

numerical modeling.

3. Overlay the proposed monitoring scheme onto the

domain of pixels, and assign coordinates to them. For

example, a tensiometer would be assigned a single x, z

coordinate; a neutron probe access tube can be

represented by a series of x,z points.

4. Choose a spill with user-defined characteristics and

place it in the center of a pixel.

5. Given the size and orientation of the spill, determine

whether the spill intersects a device location. If the

answer is yes, then hij = p(x,z), where ij are the pixel

coordinates; if the answer is no, then hij = 0.

6. Repeat the steps for all pixels, and calculate the

probability of detection as the sum of hj for all values

of i,j.

The procedure was tested using a number of theoretical

examples, as well as the monitoring points installed at 1.5 m

depth at the Maricopa site. By rerunning the algorithm

several times for different-sized releases, an X-Y scatterplot

was developed that showed how release size affected the

likelihood of detection. We did not implement the

algorithm using specific water movement data from the

experiments at Maricopa, because the constant flux

boundary condition would have provided ambiguous results

(i.e., all the monitoring strategies would have recorded

100% detection because the release encompassed the entire

monitored region). However, the results using the

theoretical examples clearly show the dependence on a

number of factors which can be either implemented or

measured in the field. The methodology can be used as a

planning tool to optimize the locations and numbers of

devices at the site, during current and future monitoring

programs, if the goals and objectives of the program change

with time.

41 41 NUREG/CR-5698

NUREG Summary

5 SUMMARY

Three overall goals were identified for the development and

implementation of monitoring programs. First, the

monitoring program must be capable of detecting releases

from the disposal site before they reach the facility

boundary. Second, the monitoring program should

emphasize strategies and systems that are as maintenance

free as possible, especially during the long-term phase.

Three, the monitoring program should stress the use of

redundant monitoring systems, so that primary performance

measures are obtained using more than one system.

Four strategies for monitoring the soil water conditions in

the unsaturated zone were defined, tested in the field, and

compared to each other with respect to their characteristics

and ability to achieve the goals of the monitoring program

during post-closure time periods. The four strategies

included: 1) monitoring trenches - wide or narrow trenches,

into which instruments can be installed along a transect; 2)

monitoring islands - large diameter boreholes drilled

vertically into the soil, allowing monitoring instruments to

be installed into undisturbed material; 3) borehole

monitoring - vertical and horizontal boreholes used for

monitoring; and 4) geophysical monitoring - intrusive and

non-intrusive methods for measuring bulk electrical

properties of soil.

Quantitative comparisons of the strategies were not made,

because of the wide variability of potential monitoring

programs at actual disposal sites, and the limited

applicability that our comparisons would have when applied

to other sites. Rather we looked at the strengths and

weaknesses of each strategy with respect to installation,

maintenance and replacement. These could be essential

considerations when choosing the strategy(ies) for another

particular site, depending on the specific design criteria and

ambient environmental conditions. Below is a summary of

our comparisons:

1. Monitoring trenches - With a potentially long length,

and open access to direct installation of instruments,

trenches permit long distances to be monitored, though

only at shallow depths. The soil material can be

directly sampled and observed while the trench is

open, but once closed (backfilled), access is lost.

Monitoring instruments can be precisely placed along

the transect. However, maintenance and replacement

are difficult because of the need to potentially disrupt

the subsurface soil material during these activities.

The trenches strategy can be combined easily with

other strategies that stress more intensive

measurements over shorter distances.

2. Monitoring islands - This strategy permits excellent

monitoring of vertical transects of soil water

conditions, because the islands can be potentially

installed to depths exceeding 10 m. Using the general

design from the Maricopa site experiments,

instruments can be installed at any depth and any

radial orientation from the island. Soil samples can be

collected at any time because the island is not

permanently sealed. However, the island strategy,

when used alone, is limited with respect to spatial

resolution unless multiple islands are used. Instrument

maintenance and replacement are easier than other

strategies, given their proximity to the island walls and

the general accessibility of the islands themselves.

3. Borehole monitoring - This traditional monitoring

strategy has distinct advantages over other strategies

because no practical depth limitation exists for

installation; monitoring wells and access tubes can be

drilled to almost any depth. A wide variety of

completion designs can be implemented, including the

use of fixed-point instruments. Soil samples can be

collected continuously during drilling, but, generally,

not after the borehole is completed, Some limitations

exist in terms of the number of devices that can be

placed in each borehole, reducing the flexibility during

program design. Maintenance and replacement of

permanently-installed devices is not practical,

requiring abandonment and redrilling. Expanding the

number of monitoring points requires additional

drilling, but this can often be done with minimal site

disturbance.

4. Geophysical monitoring - Though measurement of

bulk electrical properties cannot easily be converted to

primary performance measures, the combination of

43 43 NUREG/CR-5698
NUREG Summary

intrusive and non-intrusive techniques provides

flexibility in long-term program design. The EM-31

and -38 instruments are portable, easy to use and

require no installation other than site surveying.

Research has shown that the method can be used for

profiling water contents in shallow soils, though we

were unable to confirm this in the initially wetted soils

during Experiment 2. The borehole tomography

method appeared to effectively measure wetting front

migration during Experiment 1; however, it was

difficult to ascertain the lower limit of water content

change for soils with low water holding capacity and

whether the method would be limited by this. The

ERT method is potentially limited by an inability to

maintain or replace the sources/conductors in the

borehole during long time periods. New monitoring

points can be installed, but this could affect the rather

specific geometric arrangements required for the

boreholes.

Section 4 (Discussion), divided into monitoring program

subgoals and site conditions and processes, summarizes the

general implications of these factors on the choices of

monitoring strategies. We identified 12 subgoals, all of

which are compatible with the three major goals described

in Section 2. They are intended to help define the important

aspects of the monitoring program. Each subgoal is briefly

explained, and we then describe whether the monitoring

strategy would be useful in satisfying or addressing the

subgoal. Many times, only one or two strategies would be

useful and in some cases, all four strategies would be useful.

For example, if a goal of the monitoring program is to

quantify and monitor hydraulic gradients in deep soils, then

the monitoring island and borehole monitoring strategies

would help to satisfy those goals. Monitoring trenches are

very shallow, and are not well suited for multiple

instruments installed in vertical transects; geophysical

monitoring instruments (as used in this study) are not

designed for measuring hydraulic gradients, because 1) they

have poor resolution in the vertical dimension, and 2)

electrical conductivity is affected by water content and not

water tension. Other examples similar to this are discussed.

The second subsection in Section 4 deals with site

conditions and processes and how they affect the choices of

possible monitoring strategies. We further subdivided these

into natural (e.g., precipitation, depth to water table) and

anthropogenic (e.g., presence of buildings) conditions. A

total of 12 site conditions were identified and opposing

conditions were defined (e.g., shallow/deep water table).

We then categorized each strategy as to whether the

condition supported its use, weakened its use, or made no

difference with respect to its use within an overall

monitoring program. When used in combination with the

subgoals, the choice which strategy (or combination of

strategies) to use becomes more clear.

The controlled flux experiments conducted at the Maricopa

site provided a large set of data and information that can be

used for directly comparing numerous monitoring systems,

which were chosen to support four monitoring strategies,

and for studying other aspects of monitoring systems that

are not directly related to the field experiments. Three such

studies were undertaken, ranging from analysis of

monitoring programs to correction of individual monitoring

instruments. Other future studies will be initiated using the

existing database.

NUREG/CR-5698 44
NUREG References

6 REFERENCES

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NUREG/CR-5698 46
NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER

(2-89) (Assigned by NRC, Add Vol, Supp., Rev.

NRCM 1102, and Addendum Numbers, IN any.) 3201,3202 BIBLIOGRAPHIC DATA SHEET

(See instructions on the reverse)

2. TITLE AND SUBTITLE NUREG/CR-5698

Comparing Monitoring Strategies at the Maricopa Environmental Monitoring Site, Arizona 3. DATE REPORT PUBLISHED

MONTH J YEAR

July 1999

4. FIN OR GRANT NUMBER

JCN W6151

5. AUTHOR(S) 6. TYPE OF REPORT

M.H. Young, A.W. Warrick, P.J. Wierenga, L.L. Hofmann and S.A. Musil Technical

7. PERIOD COVERED (Inclusive Dates)

04/95 - 02/99

B. PERFORMING ORGANIZATION - NAME AND ADDRESS (If NRC, provide Division, Office or Region, U.S. Nuclear Regulatory Commission, and mailing address; if contractor,

provide name and mailing address.)

Department of Soil, Water and Environmental Science

University of Arizona

Tucson, AZ 85721

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (if NRC, type Same as above*; ffcontractor, provide NRC Division, Office or Region, U.S. Nuclear Regulatory Commission,

and mailing address.)

Division of Risk Analysis and Applications

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

10. SUPPLEMENTARY NOTES

T.J. Nicholson, NRC Project Manager

11. ABSTRACT (200 words or less)

The purpose of this document is to discuss the alternative monitoring strategie s used during field experiments at the Maricopa

Environmental Monitoring site, Maricopa, AZ. The strategies selected could pot entially be incorporated into monitoring

programs at low-level radioactive waste disposal facilities. The four monitori ng strategies include: Monitoring Trenches,

Monitoring Islands, Borehole Monitoring, and Geophysical Monitoring. Strengths and weaknesses of each strategy were

described with respect to installation, maintenance and replacement of monitori ng systems and instruments. Evaluation of the

strategies was mostly qualitative in nature, but were supported by data collect ed during two, field-scale infiltration experiments

in the vadose zone. Each of the strategies possess benefits and drawbacks, req uiring site specific analyses of site and

environmental conditions during monitoring program design. The document also p resents the concept of primary performance

measures (e.g., water content, water tension and solute concentration), each of which directly influences water movement and

contaminant migration from disposal sites, and discusses the need to accurately convert field observations to these primary

measures. Using multiple instruments whose data convert to the same primary pe rformance measure, could improve the

confidence that changes in soil water conditions are real and not affected by t he monitoring systems themselves.

12. KEY WORDS/DESCRIPTORS (Ust words or phrases that will assist researchers In locating the report.) 13. AVAILABIUTY STATEMENT

Hydrology unlimited

Borehole Monitoring 14. SECURITY CLASSIFICATION

Geophysical Monitoring (Ibis Page)

Monitoring Islands unclassified

Monitoring Programs (TUhis Report)

Monitoring Strategies unclassified

Monitoring Trenches

Performance Measures 15. NUMBER OF PAGES

Subsurface Monitoring

Unsaturated Zone 16. PRICE

NRC FORM 335 (2-89)


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NUREG/CR-5698 - COMPARING MONITORING STRATEGIES AT THE

MARICOPA ENVIRONMENTAL MONITORING SITE, ARIZONA

JULY 1999

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