TECHNICAL GUIDANCE FOR
CORRECTIVE MEASURES -
SUBSURFACE GAS
Prepared For:
Land"Disposal Branch
Office of Solid Waste
U.S. Environmental Protection Agency
Washington, D.C. 2,Q460
Prepared By:
SCS Engineers
11260 Roger Bacon Drive
Reston, VA 22090
March 28, 1985
File No. 28286-12
LC 89
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DISCLAIMER
This report was furnished to the Environmental Protection Agency by the
SCS Corporation, Reston, VA, in fulfillment of a contract. The opinions,
findings, and conclusions expressed are those of the authors and not
necessarily those of the Environmental Protection Agency or the cooperating
agencies. Mention of company or product names is not to be considered as
an endorsement by the Environmental Protection Agency.
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TABLE OF CONTENTS
Page
SECTION 1 - INTRODUCTION 1-1'
1.1 PURPOSE 1-1
1.2 REGULATORY/STATUTORY REQUIREMENTS 1-2
1.3 DEFINITION OF A RELEASE 1-4
1.3.1 Constituents of Concern 1-4
1.3.2 Indicator Constituents 1-9
1.4 TYPES OF SOLID WASTE MANAGEMENT UNITS 1-16
1.4.1 Landfills 1-22
1.4.2 Sites Closed as Landfills 1-22
1.4.3 Underground Tanks 1-23
SECTION 2 - GENERATION AND MIGRATION OF SUBSURFACE 2.1
GASES
2.1 GAS GENERATION 2-1
2.1.1 Biological Decomposition 2-1
2.1.2 Chemical Decomposition 2-4
2.1.3 Physical Decomposition 2-5
2.2 GAS MIGRATION... 2-7
2.2.1 Natural Barriers 2-8
2.2.2 Design Barriers 2-9
SECTION 3 - IDENTIFYING RELEASES 3-1
3.1 APPROACH 3-1
3.2 PRELIMINARY ASSESSMENTS 3-4
3.2.1 Checklist Section I 3-8
3.2.2 Checklist Section II 3-9
3.2.3 Checklist Section III 3-10
3.2.4 Checklist Section IV and V 3-11
3.2.5 Checklist Evaluation 3-12
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TABLE OF CONTENTS (Continued)
3.3 SITE INVESTIGATIONS 3-14
3.3.1 Monitoring or Sampling Inside 3-14
Structures
3.3.2 Subsurface Monitoring or Sampling 3-15
3.3.3 Methane 3-16
3.3.4 Other Indicator Constituents 3-19
3.3.5 Factors Not Related to Monitoring 3-21
3.3.6 Evaluation 3-23
SECTION 4 - REMEDIAL INVESTIGATIONS AND HEALTH 4-1
ASSESSMENTS
4.1 EMERGENCY SITUATIONS 4-1
4.1.1 Criteria 4-1
4.1.2 Identification 4-2
4.1.3 Fast-Track Corrective Actions 4-4
4.2 ROUTINE SITUATIONS 4-6
4.2.1 Predictive Models 4-7
4.2.1.1 Data Requirements 4-8
4.2.1.2 Example Application 4-10
4.2.2 Use of Experts 4-21
SECTION 5 - CORRECTIVE MEASURES 5-1
5.1 MONITORING PROGRAMS 5-1
5.1.1 Methane 5-1
5.1.1.1 General 5-1
5.1.1.2 Equipment 5-4
5.1.1.3 Probe Pressure 5-4
5.1.1.4 Methane Monitoring 5-7
5.1.1.5 Interpretation 5-9
5.1.1.6 Record Keeping 5-11
5.1.2 Indicator Constituents of Concern 5-12
5.1.2.1 General 5-12
5.1.2.2 Equipment 5-13
5.1.2.3 Preparation of Sample Traps 5-16
5.1.2.4 VOC Monitoring 5-17
5.1.2.5 Handling and Record Keeping 5-22
5.1.2.6 Interpretation 5-22
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TABLE OF CONTENTS (Continued)
5.2 CONTROL SYSTEMS 5-23
5.2.1 Passive Systems 5-23
5.2.2 Active Systems 5-26
5.2.2.2 Perimeter Extraction Trench 5-28
5.2.2.3 Perimeter Well Injection 5-28
5.2.2.4 Perimeter Injection Trench 5-30
5.2.2.5 On-Site Extraction Well 5-30
5.2.2.6 Subslab Gravel Bed Injection 5-32
or Extraction
LIST OF FIGURES
Figure 1-1 Subsurface gas generation/migration 1-20
in a landfill
Figure 1-2 Subsurface gas generation/migration 1-21
from tanks and units closed as landfills
Figure 3-1 Typical Deep Subsurface Gas Monitoring....3-18
Wells
Figure 4-1 Example Landfill 4-11
Figure 4-2 Uncorrected Methane Migration Distance....4-13
Figure 4-3 Correction Factors for Landfill Depth 4-15
Below Grade
Figure 4-4 Correction Factors for Soil Surface 4-16
Venting Condition Around Landfill
Figure 4-5 Example Landfill Methane Contours 4-19
Figure 5-1 VOC Sampling Assembly 5-14
Figure 5-2 Gas Migration Control Vents 5-25
LIST OF TABLES
Table 1-1 Concentration Limits To Define a Release 1-6
Table 1-2 Constituents of Concern 1-7
Table 1-3 Compound Descriptions 1-10
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TABLE OF CONTENTS (Continued)
List of Tables (Continued)
Table 1-4 Indicator Constituents of Concern 1-18
Table 3-1 Preliminary Assessment Checklist 3-5
Table 3-2 Subsurface Sampling Techniques 3-17
Table 4-1 Actions to Take in Gas Emergency 4-5
Situations in Buildings
Table 4-2 Methane Migration Distance Tabulating 4-18
Form
Table 5-1 Field Data Form for Subsurface Gas 5-2
Monitoring
Table 5-2 YOC Field Data Form 5-18
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SECTION 1
INTRODUCTION
1.1 PURPOSE
This manual is intended to provide technical guidance for EPA
and authorized State personnel in assessing the needs for
corrective action, reviewing permit applications, and writing
permits for hazardous waste facilities. Its primary function
is to assist in the assessment of the potential for subsur-
face gas generation and migration from these facilities. In
addition, it provides a framework to States, EPA, and facil-
ity owners or operators to identify whether subsurface gas is
migrating or will likely migrate beyond the facility boundary
or into on-site structures at concentrations that are threats
to human health and the environment. If there is known or
probable migration of this kind, then technically sound cor-
rective action will be required.
The discussion in this section addresses current regulatory/
statutory requirements pertaining to subsurface gases, the
definition of a release, and the types of solid waste manage-
ment units at which a release can occur. Section 2 is an
overview on factors that impact subsurface gas generation and
migration. Section 3 provides methods to identify a subsur-
face gas release by the presence of specific hazardous waste
1-1
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constituents. This section summarizes the above into a
checklist for the permit writer, providing evaluation of the
presence or potential for a release based on the type of
unit, existing site data, the potential for subsurface gas
release, existence of control or systems, and other factors.
The checklist assists the permit writer in identifying sites
that require actions, emergency response, Remedial
Investigation/Health Assessment, or corrective measures.
Section 4 describes criteria for emergency situations and
related actions and describes procedures for Remedial
Investigations. Section 5 describes approaches to corrective
measures such as monitoring, treatment, and control systems
that could be implemented for subsurface gas releases.
1.2 REGULATORY/STATUTORY REQUIREMENTS
Releases of subsurface gases from hazardous waste facilities
can result in threats to human health and the environment.
Potential threats include accumulations of explosive or
flammable gases, development of oxygen-deficient atmospheres
in enclosed areas, possible acute or chronic health effects,
and impacts on local plant and animal communities. The
current regulatory/statutory requirements pertaining to these
threats are summarized below.
Concentration limits for subsurface gases released from a
solid waste management facility (applicable to all RCRA Sub-
1-2
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title D facilities) are established in 40 CFR Part
257.3-8(-a). The levels for explosive gas concentrations
are not to exceed:
1. The lower explosive limit; i.e., 5 percent methane,
at the property boundary, and
2. 25 percent of the lower explosive limit, i.e., 1.25
percent methane, in facility structures.
These limits were designed to address methane generation and
migration from landfills and open dumps and do not apply to
hazardous waste (Subtitle C) or deep well injection
facilities subject to regulation under 40 CFR Part 146 under
the Safe Drinking Water Act.
Hazardous waste facilities must minimize subsurface gas
releases and are required, under 40 CFR Part 264.31, to:
"be designed, constructed, maintained, and operated to
minimize the possibility of a fire, explosion, or any
,unplanned sudden or non-sudden releases of hazardous
waste or hazardous waste constituents to air, soil, or
surface water which could threaten human health or the
environment."
The 1984 Amendments to RCRA prov-ide additional authority for
corrective action at facilities for which'permits are being
1-3
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sought and for facilities with interim status under Section
3005(e). The amendments address: (1) continuing releases at
permitted facilities; (2) corrective action beyond facility
boundaries; (3) financial responsibility for corrective
action; and (4) interim status corrective action orders.
EPA is to require corrective action in response to a release
of hazardous waste from any solid waste management unit
(SWMU) to the environment regardless of when the waste was
managed. Its authority encompasses releases to all media,
e.g. air, surface water, and ground water, and specifically
includes releases to the unsaturated soil zone. This applies
to all SWMU's, including former or existing nonregulated
units, existing regulated units, and new units. While the
regulatory requirements cover all SWMU's, this document will
focus exclusively on those units exhibiting a potential for
subsurface gas releases that are a threat to human health and
environment.
1.3 DEFINITION OF A RELEASE
1.3.1 Constituents _ojf_ Conc er_n
A subsurface gas release from a SWMU is defined as having
occurred when the concentration of a "constituent of concern"
exceeds a specified level when measured in the unsaturated
soil at the property boundary or within any structure on the
1-4
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hazardous waste facility. In the context of this document, a
constituent of concern is (1) methane or (2) a compound,
specified in 40 CFR Part 261 or in Appendix VIII, with a
vapor pressure at 20°C that exceeds 0.1 mm Hg.
The components of a subsurface gas release will vary in
concentration depending on the relative amounts of
constituents volatilized by biological decomposition,
chemical reactions, and physical degradation. By far, the
predominant subsurface gas of concern from landfill SWMU's is
methane. Methane is included in the grouping of constituents
of concern due to its volatility, explosive properties, and
frequency of detection in subsurface gases. The methane
concentration limits specified to identify a subsurface gas
release are those limits defined for explosive gases in 40
CFR Part 257.
The other constituents of concern (besides methane) are
specified as toxic, corrosive, ignitable, or reactive in 40
CFR Part 261 or in Appendix VIII and are commonly termed
volatile hazardous waste compounds because of their
relatively high vapor pressures. A worker exposure level
established by OSHA for each of these hazardous compounds
will serve as the concentration limit to define a subsurface
release. In summary, the concentration limits for
identifying releases are shown in Table 1-1. Table 1-2
presents a specific listing of the constituents of concern by
1-5
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TABLE 1-1. CONCENTRATION LIMITS TO DEFINE A RELEASE
Constituent
_0f_ _C o n c e r n
Methane
Methane
Compounds for
which an exposure
level has been
established,
recommended or
adopted.
Other compounds.
Concentration Limit
(equalto or greater than)
5% i n ai r
1.25% in air
PEL or alternative
concentration*
To be established
Applicable
Location__
Soil at facili
ty property
boundary
Within any
structure on
the facility
Soil at facili
ty property
boundary or
within any
structure on
the facility.
Same as above
* A PEL is the Permissible Exposure Limit as established by
OSHA, in 29 CFR 1910.1 and is the 8-hour, work-shift,
time-weighted average (TWA) level. For other constituents of
concern for which a PEL has not been established by OSHA, the
NIOSH recommended level is to be used. NIOSH recommended levels
may be found in NIOSH Publication No. 78-210, September 1978. If
no NIOSH recommended level is published, the current Threshold
Limit Value - Time Weighted Average (TLV-TWA) adopted by the
American Conference of Governmental Industrial Hygienists (ACGIH)
is to be used.
1-6
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TABLE 1-2. CONSTITUENTS OF CONCERN
Compound Name
Maleic anhydride
2-Methy laz lr id ine
Methyl bromide
Methy 1 ch 1 or ide
Methane
Methyl ethyl ketone
Methylene chloride
Methyl cydrazine
Methyl isocyanate
Naphtnc . t, n e
N icxe i cc.r oony 1
Nitric ox i de
Nitrogen dioxide
Phenol
Phosgene
Phosph ine
Pyr id i ne
T etrachl or o benzene
1,1,2,2-Tetrachl oroethane
1 , 1, 1,2-Tetrachloroe thane
Tetrachloroethylene
Tetraethyl lead
To 1 uene
Tr i bromome thane
Tr ichlorobenzene
1,1,1-Trichloroe thane
1, 1,2-Trichloroe thane
Tr ich loroethy lene (TCE)
1 ,2,3-Tr ichloropropane
Vinyl ch lor ' de
N/E - Not Established
N/A - Not Ava i 1 able
Synonym
2, 5-Furaned lone
Propylene amine
Bromomethane
Ch loromethane
--
2-Butanone
D ich 1 oromethane
--
Wh >te tar
Nitrogen monoxide
--
Car bo lie ac id
--
Hydrogen phosph ide
--
--
Acetylene tetrach 1 or ide
--
Perch loroethy lene
--
Methy 1 benzene
Br omof orm
--
Methyl chloroform
Vinyl trichloride
Ethylene trichloride
Al ly 1 tr ichlor ide
Ch loroethy lene
Chemical
Abstract
System
Number
108-31-6
75-55-8
74-83-9
74-87-3
74-82-8
78-93-3
75-09-2
60-34-4
624-83-9
9 1-20-3
13463-39-3
10102-43-9
10102-40-0
108-95-2
75-44-5
7803-51-2
1 10-86-1
12408-10-5
79-34-5
630-20-6
127-18-4
78-00-2
108-88-3
75-25-2
12002-48-1
71-55-6
79-00-5
79-01-6
96-18-4
75-01-4
EPA
Hazardous
Waste
Number
U147
U194
U029
U045
None
U159
U080
P068
P064
UI65
U073
P076
P078
U188
P095
P096
UI96
U207
U209
U208
U2IO
PI 10
U220
U225
N/A
U226
U227
U228
N/A
U043
PEL
(ppm)
0.25
2
5
100
N/E
200
500
0.2
0.02
10
0,001
25
5
5
0.1
0.3
5
N/E
5
N/t
100
5
200
0.5
N/E
350
10
100
50
1
Vapor Pressure
(mm HG)
f 20 degrees C
0.2
N/A
> 760
> 760
> 760
70
354
36
346
< 1
321
> 760
720
0.4
> 760
3 760
18
< 1
8
8
10
0.2
22
5
< 1
100
18.8
58
3
> 760
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TABLE 1-2 (Continued)
i
00
Compound Name
Aceta 1 dehy de
Acetone
Aceton i tr i 1 e
Aero 1 e 1 n
Aery 1 on i tr H e
Al ly 1 alcohol
Ally! chlor ide
Benzene
Benz 1 eh lor ide
B i s (ch loromethy 1 ) ether
Carbon d Isu 1 f Ide
Carbon tetr ach 1 or i de
Chloroacetaldehyde
Ch 1 or obenzene
2-Chloro ethyl vinyl ether
Ch loroform
1,2- 0 i br omoethane
D i bromomethane
,2-Dichloro benzene
, 3-D ich lor obenzene
, 4-D Ich loro benzene
1 Ich lorod i f 1 uoromethane
, t -0 Ich loroethane
,2-0 ich lor oe thane
, 1-D Ich loroethy 1 ene
,2-Dlchloroethylene
,2-Dichloropropane
,1- D imethy 1 hydraz ine
Dimethyl sulfate
2,4- Din itrotol uene
Epichlorohydr in
Ethy lene Im i ne
Ethylene Oxide
F 1 uorene
Forma 1 dehyde
Hydraz ine
Hydrocyanic acid
Hexach 1 or oe thane
Hydrogen su 1 f ide
1 odomethane
Isobutyl alcohol
Synonym
Acet ic a 1 dehyde
2-Propanone
Cyanomethane
Aery lie a 1 dehyde
Propenon Itr 1 le
2-Pr opano 1
3-Chloropropene
Benso 1
Alpha-chlorotoluene
Syn-dlchlorodlmethylether
Carbon b 1 su 1 f 1 de
Tetr ach 1 or ome thane
2-Chloroethanol
Pheny 1 ch 1 or Ide
--
Tr ich 1 or ome thane
Ethylene dlbromide
Methylene bromide
o-D Ich lor obenzene
m-D Ich loro benzene
p-Dich loro benzene
Freon 12
Ethylfdene dlchlorlde
Ethylene dlchlorlde
Vinyl idene chloride
Acetylene chloride
Propylene dlchlorlde
--
Methy 1 su 1 fate
--
l-Chloro-2,3-epoxypropane
Az ir id 1 ne
1,2- Epoxy ethane
--
Methylene oxide
D lami ne
Hydrogen cyanide
Perch! or oe thane
Hydroso 1 f ur Ic acid
Methyl Iodide
1 sobutanol
Chemi ca 1
Abstract »
System
Number
75-07-0
67-64-1
75-05-8
107-02-8
107-13-1
107-18-6
107-05-1
71-43-2
100-44-7
542-88-1
75-15-0
56-23-5
107-20-0
108-90-7
1 10-75-8
67-66-3
106-93-4
75-09^-2
95-50-1
25321-22-6
106-46-7
75-71-8
75-34-3
107-06-02
75-35-4
540-59-0
78-57-5
540-73-8
77-78-1
121-14-2
106-89-8
151-56-4
75-21-8
86-73-7
50-00-0
302-01-2
74-90-8
67-75-1
7783-06-4
74-88-4
78-83-1
EPA
Hazardous
Waste
Number
UOOI
U002
U003
P003
U009
P005
N/A
UOI9
P028
P016
P022
U211
P023
U037
U042
U044
U067
U068
U070
U071
U072
U075
U076
U077
U078
U079
U083
U098
UI03
UI05
U04 1
U054
U1 15
P056
UI22
U133
P063
U131
U135
U138
UNO
PEL
(ppm)
200
1000
40
O.I
2
2
50
1
1
N/E
20
10
1
75
50
10
20
N/E
50
N/E
75
1000
100
10
5
200
75
0.5
1
185
2
0.5
50
0.1
3
1
10
1
10
5
50
Vapor Pressure
(mm HG>
» 20 degrees C
750
266
73
214
83
17
295
80
0.9
N/A
300
91
100
9
N/A
160
1 1
25
1
2.5
0.4
> 760
182
62
N/A
220
4 1
103
0.5
1
13
N/A
> 760
760
1
10
620
0.2
> 760
375
9
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compound name, synonym, EPA Hazardous Waste number, Chemical
Abstract System (CAS) number, OSHA Permissible Exposure Limit
(PEL), and vapor pressure. Table 1-3 provides further
information about constituents of concern. It includes
general descriptions of waste types that likely contain
specific constituents of concern.
1.3.2 Indicator Constituents
The presence of individual constituents of concern in
subsurface gases is often site-specific and dependent on
factors discussed in the next section. Although variations
in subsurface gas composition have been observed from
different sites and different locations on the same site,
some compounds .are detected more frequently than others.
Many of the compounds in Table 1-2 have not been reported in
samples from subsurface gas monitoring, are not stored,
treated, or disposed of in significant quantities at
subsurface SWMU's, or are not likely to migrate and exceed
PEL's due to physical or chemical properties. Thus
monitoring for each of the constituents of concern will not
be required to determine if a release has occurred. Instead,
a screening procedure may be used to monitor for those
compounds that the facility owner/operator or permit writer
expects are migrating from the SWMU and pose a threat to
human health and the environment.
1-9
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TABLE 1-3. GENERIC COMPOUND DESCRIPTIONS
Compound Name
Synonym
EPA
Hazardous
Haste
Number
DescrIptIon
i
H-1
O
AcetaIdehyde
Acetone
AcetonItrI Ie
Aero IeIn
Aery Ion ItrI Ie
A My I alcohol
Ally) chlor Ide
Benzene
Benzy I chI orIde
Bls(chloromethyI ) ether
Carbon dIsuIf Ide
Acet Ic a I deliyde
2-propanone
Cyanomethane
Aery lie a Idehyde
PropenonItrIle
2-propenol
3-Chloropropene
l-chlorotoluene
D IchlorodImethyI ether
Carbon bIsuIfIde
UOOI
U002
U003
P003
U009
P005
N/A
UOI9
P028
POI6
P022
Distillation bottoms and by products from production
of acetaldhyde from ethylene; chemical family Is
a I dehyde.
Spent non-haIopenated solvents; spent cleaning or splnnln
solvent; spent paint, laquor, or varnish remover; chemlca
fami Iy Is ketone.
By-products from the hydrocarbon extraction of butadiene;
by-products from the production of certain vegetable oils;
chemical family Is nitrites.
By-products from the manufacture and waste treatment of
various pharmaceutlea Is, herbicides, and polymers, expeclal'
polyurethane and polyester resins. Chemical family Is
a Idehyde .
By-products from the manufacture and waste treatment of
acrylic polymers and various semI conduct I ve polymers.
Chemical famllly Is nltrlle.
By-products from the manufacture and treatment of pharma-
ceutlcals, herbicides, pIast Ic I zers, and glycerol.
Chemical family Is ester.
By-products from the manufacture and treatment of
epIchlorohydrIn; certain synthetic Pharmaceuticals, and
adheslves. Chemical family Is halogenated hydrocarbon.
Chemical by-products from manufacture and waste treatment
of the following chemical Intermediates: ethyI benzene,
dodecyclobenzene, and nitrobenzene. Chemical family
Is aromatic hydrocarbons.
By-products from the manufacture and waste treatment of
photographic chemicals, quarternay (ammonium) agents, perfum
and Pharmaceuticals. Chemical famllly Is halogenated aromat
hydrocarbons.
Spent Ion-exchange resins and laboratory reagents.
f am My Is ether .
Chemleal
Manufacturing by-products and/or sludge from the treatment
of vlscone rayon, cellophane, and veterinary medicines.
Chemical family Is sulflde.
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Compound Name
Synonym
EPA
Hazardous
Haste
Number
Oescr I pt Ion
Carbon tetrachI orIde
Chloroacetaldehyde
ChIorobenzene
2-chloro ethyl vinyl ether
Ch1 or oform
1-2 DIbromomethane
DIbromomethane
I,2-dlchlorobenzene
I,3-d Ichlorobenzene
DIchlorodIfIuoromethane
I ,I-dIchloroethane
I,2-dichloroethane
j l-dIchloroethylene
TetrachIoromevhane
2-chloroethanol
PhenyI chlor Ide
Tr1chIoromethane
Ethylene dlbromlde
Methylene dlbromlde
o-d IchIorobenzene
m-dIchIorobenzene
Freon 12
EthylIdene chloride
Ethylene dlchlorlde
Vinyl Idene chloride
U2I I
P023
U037
U042
U044
U067
U068
U070
U07I
U075
U076
U077
Spent refrigerant and metal degreaslng solvents. By-produi
from the manufacture and waste treatment of various semi-
conductors. Chemlca! family Is halogenated hydrocarbon.
Various spent fungicides.
products of acetaldehyde.
Possible distillation by-
Chemical family Is aldehyde.
Possible distillation by-products of phenol, chloronltro
benzene, and aniline. Spent pesticide Intermediates.
Chemical family Is halogenated aromatic hydrocarbons.
Spent organic laboratory reagents,
I s ether.
ChemlcaI fami Iy
Various spent refrigerants and propellants. By-products
from the manufacture of Insecticides and fluorocarbon plastic
Spent analytical laboratory reagents. Chemical family Is
halogenated hydrocarbon.
ChemlcaI fami Iy Is
ChemlcaI fami Iy Is
U078
Spent organic Intermediate.
halogenated hydrocarbons.
Spent organic Intermediate.
halogenated hydrocarbons.
Spent organic solvent, by-product from the manufacture
of various dyes. Insecticides, and metal polishes.
Chemical famllly Is halogenated aromatic hydrocarbons.
By-products from the manufacture of Insecticides. Chemical
family Is halogenated aromatic hydrocarbons.
Spent refrigerant. Chemical family Is halogentaed
hydrocarbon.
Spent organic solvent. Chemical family Is
halogenated hydrocarbon.
Spent organic solvent. By-product from manufacture of
vinyl chloride, soaps, chelatlng agents, degreasers, end
anti-knock gasoline. Chemical family is halogenated
hydrocar bon.
Spent organic solvents. By-product from the manufacture of
perfumes, lacquers, and thermoplastics. Chemical family
Is halogenated hydrocarbon.
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Compound Name
Synonym
EPA
Hazardous
Waste
Number
Descr f pf Ion
I,2-dIchloroethylene
I,2-d Ichloropropane
1,1-dimethyl hydrazlne
Dimethyl sulfate
2,4-Dlnitrotoluene
Ep ichIorohydr i n
^_, Ethytene (mine
INJ
Ethy lene ox ide
FIuor ine
Forma Idehyde
Hexachloroethane
Hydraz i ne
Hydrocyan ic ac id
Acetylene chloride
Propylene dlchlorlde
Methyl sulfate
Az (r Idene
1,2-epoxy ethane
MethyIene oxIde
Perchloroethane
D i am Ine
Hydrogen cyanide
U079
U083
U098
UI03
UI05
I -chIoro-2,3-epoxy propane U04I
P054
Ul 15
P056
UI22
U031
UI33
P063
Spent organic solvent. By-product from the manufacture o
perfumes, lacquers, and thermoplastics. Chemical family
Is halogenated hydrocarbons.
Spent organic solvent. Gasoline component. By-products
the manufacture of carbon tetrachI or Ide, and various gums
waxes, and resins. Chemical family Is halogenated hydroca
By-products from the manufacture of various Jet and
rocker r els. Chem?C3l family is amine.
By-product of the manufacture for various amine and pheno
based compounds. Chemical family is allkyl sulfate.
By-products of the manufacture of varlouls dyes and
explosives. Chemical family is aromatic hydrocarbon.
By-products of the manufacture of various epoxy and
phenoxy resins. Chemical family Is epoxlde.
By-products of the manufacturing In the following
Industries: Pharmaceuticals, Ion exchange and protective
coatings, end surfactants. Chemical family Is imfnes.
By-products from the manufacture of several glycols,
surfactants and rocket propellent. Chemical family is
ox (rane.
By-product from the manufacture of rocket fuels and
various fIuorocarbons. Chemical family Is halogen.
Spent laboratory reagents. By-products from manufacture of
fertilizers, dyes, embalming fluids, disinfectants, and
germicides. Chemical family is aldehyde.
Spent solvent and organic laboratory reagent. Chemical
family Is halogenated hydrocarbon.
By-products from the manufacture of agricultural chemicals,
rocket fuels, and metal plating solutions. Chemical
fam fIy Is amine.
By-products from the manufacture of acrylates, cyanide salts
dyes, end chelating agents. Chemical family Is nltrile.
-------
Compound Name
Hydrogen sulf ide
Iodomethane
IsobutyI a IcohoI
Male !c anhydr ide
2-methylaz ir id ine
MethyI bromIde
Methy. oh!or irfe
Methane
Methyl ethyl ketone
Methylene chloride
Methyl hydrazine
Synonym
Hydrosul f ur Ic acid
Methyl Iodide
I sobutanol
2,5-Furanedione
Propylene am Ine
Bromomethane
OH : - GiV)F-Th;rne
2-butanone
Dlchloromethane
EPA
Hazardous
Haste
Number
DescrIpt Ion
UI35 Spent laboratory reagent. "By-product from the manufacture
of elemental sulfur, sulfurlc acid, and hydrochloric acid.
Chemical family Is sulfide.
UI38 Spent organic laboratory reagent. By-products from
manufacture of various medicines. Chemical family Is
hydroqenated hydrocarbon.
UNO Spent organic laboratory reagent. Spent paint solvent.
Chemical family is alcohol.
(Jl 4 7 By-products from the manufacture of polyester and alkyd
resins, pesticides, and paper. Chemical family is furan.
UI94 Spent organic laboratory reagent. Chemical family
Is am Ine.
U029 Spent organic laboratory reagent. By-product from the
manufacture of various agricultural disinfectants.
U045 Spoilt organic solvent. By-product from the manufacture
of herbicides and gasoline additives. Chemical family
is halogenated hydrocarbon.
None Natural gas. Chemical family Is hydrocarbon.
UI59 Spent solvent. Spent organic laboratory reagent. Spent
paint and wax remover. Chemical family is ketone.
U080 Spent solvent. Spent depreaser. Spent paint remover.
Chemical family Is haloqenated hydrocarbon.
P068 Spent solvent. Spent or contaminated rocket fuel.
Chemical family is amlne.
Methyl isocyanate
Naphtha Iene
N icke I car bony I
Wh Ite tar
P064
UI65
P073
Spent chemical Intermediate. Chemical family Is nltrile.
Spent solvent. By-product from the manufacture of
lubricants, fungicides, and explosives. Chemical
family Is aromatic hydrocarbon.
By-product from the manufacture of various nickel coating
Chemical family Is carbonyl compound.
-------
Compound Name
Synonym
EPA
Hazardous
Haste
Number
DescrIptIon
Carbolie acId
Nitric oxide Nitrogen monoxide
Nitrogen dioxide
Phenol
Phosgene
Phosph ine
Pyr Id Ine
Tetrachlorobenzene
I, I,2,2-tetrachloroethane Acetylene tetrachI orIde
II,I,2-tst- ;!->.loroethan?
Hydrogen Phosphide
Tetrachloroethylene
Tetraethyl lead
To I uene
Tr I bromomethane
Perchloroethylene
Methyl benzene
Bromoform
P076 Spent chemical Intermediate. Chemical family Is oxide.
P078 By-products from the manufacture of nitric acid.
Chemical family Is oxide.
UI88 Spent organic laboratory reagent. By-product from the
manufacture of various dyes, pharmaceut IcoIs, and
phenol-based compounds. Spent solvent. Chemical family
I s phenol.
P095 Spent organic solvent. By-product from the manufacture
of various Isocyanates, carbonates, chIoroformaIes,
pesticides, and herbicides. Chemical family Is ketone.
P096 Spent organic Intermediate. Spent doping agent for solid
state electronic components. Chemical family Is phosphide.
UI96 Spent solvent. By-product from manufacture of various
vitamins, drugs, dyes, and fungicides. Chemical family
is am Ine.
U207 Spent solvents. Found in dielectric fluids and electrical
Insulation. Chemical family Is halogenated aromatic
hydrocarbon.
U209 Spent solvent. Spent degreaser. Spent organic laboratory
reagent. By-product In manufacture of various Insecticides
11208 Spent solvent. Spent degreaser. Spent organic laboratory
reageni. By-product in manufacture of various insect icioes
and photographic tf i I ms Chemical family is halogenated
hydrocarbon.
U210 Spent dry cleaning solvent. Spent degreaser. By-product of
manufacture of fIuorocarbons. Chemical family is
halogenated hydrocarbon.
PI 10 Used as a fuel additive. Chemical family is metal
substituted hydrocarbon.
U220 Spent solvent. Fuel additive. By-products from manufacture
of resins, coatings, dyes, and explosives. Chemical family
is aromatic hydrocarbon.
U225 Spent solvent. Organic Intermediate. Chemical family
Is halogenated hydrocarbon.
-------
TADLE 1-3 (Continued)
Compound Name
Synonym
EPA
Hazardous
Waste
Number
DescrIptIon
Tr ichIorobenzene
I,I,I-trichI oroethane
I,1,2-trichloroethane
TrIchloroethylene
1,2,3 TrIchloropropane
Vinyl chI or ide
Methyl chloroform
Vinyl chloride
Ethylene trichloride
At lyI trIchlor Ide
Chloroethylene
N/A
U226
U227
U228
N/A
U043
Spent solvent. By-products from manufacture of dyes,
dielectric fluids, lubricants, and Insecticides. Chemica
family is halotjenated aromatic hydrocarbon.
Spent solvent. Spent metal degreaser.
Is halogenated hydrocarbon.
ChemicaI family
Spent solvent. Spent organic laboratory reagent.
Chemical family is halogenated hydrocarbon.
Spent solvent. Spent refrigerant. Spent decreaser. Spent
chemical Intermediate. Chemical family is halogenated
hydrocarbon.
Spent solvent. Spent degreaser.
genated hydrocarbon.
Chemical family Is halo-
By-product from the manufacture of polyvinyl chloride
end related polymers. Chemical family is halogenated
hydrocarbons.
By products are defined as those compound(s), reacted or unreacted,
found present in a non-product stream exiting the manufacturing process.
N/A : Not Available
-------
A group of 16 indicator constituents of concern has been
selected and is shown in Table 1-4. Monitoring for these 16
constituents may be used as a screening approach to the
identification of a subsurface gas release. Table 1-4
includes methane, a compound without an OSHA PEL, and the
other compounds that are primarily chlorinated hydrocarbons
with PEL's in the range of 1 to 1,000 parts per million
(ppm). These constituents were selected as indicators due to
their presence in past subsurface gas releases from hazardous
waste landfills that had received a wide variety of wastes.
Other compounds could be added to the indicator list
depending on the type of SWMU, available site-specific data
on construction, operations, monitoring, and spills, and
inspections performed at the site. For example, if an
underground storage tank contains acetaldehyde, it could be
added to the indicator list for this specific SWMU. To be
added to the indicator list, the compound should (1) be a
constituent of concern (listed on Table 1-2); (2) be expected
to migrate beyond the facility boundary or into on-site
structures at concentrations exceeding the specified levels;
and (3) pose a threat to human health and the environment.
1.4 TYPES OF SOLID WASTE MANAGEMENT UNITS
This document addresses SWMU's (1) at which only solid waste
is or was managed, and that are located at treatment, stor-
1-16
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TABLE 1-4. INDICATOR CONSTITUENTS OF CONCERN
Compound (Molecular Weight)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Benzene (78)
Chloroform (119)
Dichlorodifluromethane (121)
1,1-Dichloroethane (99)
1,2-Dichloroethane (99)
1,1-Dichloroethylene (97)
1,2-Dichloroethylene (97)
Dichloromethane (85)
Methane (16)
1,1,2,2-Tetrachloroethane (168)
Tetrachloroethylene (166)
Toluene (92)
1,1,1-Trichloroethane (133)
1,1,2-Trichloroethane (133)
Trichloroethylene
Vinyl Chloride (62)
RCRA
Synonym Appendix VIII
Trichloromethane
Freon 12
Ethyl idene Chloride
Ethylene Dichloride
Vinylidene Chloride
Methyl ene Chloride
Perchl oroethyl ene
Methyl Benzene
Methyl Chloroform
Vinyl Trichloride
TCE
Chloroethylene
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Vapor Pressure
(mm Hg at 20°C)
75
160
>760
182
62
490
220
350
>760
8
14
22
100
19
58
>760
PEL
(ppm)
1
50
1,000
100
10
5
200
500
*
5
100
200
350
10
100
1
* No PEL established. Levels for methane are not to exceed 5 percent at the property boundary or 1.25 percent
in facility structures.
-------
age, or disposal facilities seeking a RCRA Permit; and (2)
that exhibit a potential for subsurface gas migration of
methane or hazardous constituents at concentrations that pose
a threat to human health or the environment. The applicable
types of SWMU's are principally underground and are discuss-
ed below.
Underground units, (those designed for storage, treatment, or
disposal of waste below grade and not open to the atmosphere)
are the only units that have significant potential for sub-
surface gas releases. Gases generated in these units may
migrate upwards or horizontally and can attain concentrations
of concern to human health and the environment. For example,
the methane in landfill gas can be explosive and is known to
migrate through soils and along confining barriers such as
ground-water tables, clay layers, synthetic liners, and com-
pacted covers. Since gases in these units are generated well
below the surface, horizontal migration prior to surface
venting is possible.
Some SWMU's are essentially surface operations that extend
below grade into shallow soils. Although gases may be gen-
erated in these units and migrate into the unsaturated soil
zone, the potential for horizontal migration is low and a re-
lease is unlikely. Shallow subsurface gases will escape into
the atmosphere as surface emissions unless prevented by barr-
iers; e.g., paving, campaction, or installation of covers for
1-18
-------
closure. If gases are prevented from venting to the atmos-
phere, lateral migration may occur. Generally, this lateral
migration is limited to the extent of the barrier. There-
fore, while pathways do exist for subsurface gas releases
beyond a property boundary, the potential for releases from
shallow SWMU's is usually insignificant.
Although depth (underground versus shallow) is one of several
considerations for determining release potential, the type of
SWMU establishes potential migration pathways and the waste
characteristics create the driving forces for subsurface
gas movement. Figures 1-1 and 1-2 illustrate some potential
path- ways from a few example SWMU's. It should be noted
that for most SWMU's, it is unlikely that a release would
ever occur because gases generated in the units would be more
likely to vent to the atmosphere than to concentrate in the
unsaturated soil. Therefore, the types of SWMU's of concern
for subsurface gas releases are listed below.
0 Landfills
Sites closed as landfills
Underground tanks
1-1:
-------
ro
o
IB UNSATURATED
SOIL
Figure 1-1. Subsurface gas generation/migration in a landfill.
-------
UNDERGROUND TANK
PAVING
id
VAPOR
VOLATILE
HAZARDOUS
LIQUIDS
SURFACE IMPOUNDMENT CLOSED AS LANDFILL
HAZARDOUS
LIQUIDS/SLUDGES
UNSATURATED
SOIL
Figure 1-2. Subsurface gas generation/migration from tanks and units closed as landfills
-------
1.4.1 Landfills
Landfills are the most likely SWMIJ's to generate subsurface
gases resulting in a release. The underground deposition of
decomposable solid waste with or without hazardous constitu-
ents provides a large source of gas and a driving force that
can carry other gases venting to the atmosphere and/or migra-
ting horizontally as a subsurface gas. The closure of land-
fills with impermeable caps is one means of retarding these
landfill gases from escaping as surface emissions. In these
instances, a large percentage of those gases migrate later-
ally, possibly causing significant accumulations in facility
structures or at property boundaries.
1.4.2 Sites Closed As Landfills
Inactive SWMU's that have been closed as landfills may gen-
erate subsurface gases. These sites include closed surface
impoundments or waste piles containing decomposable or vola-
tile wastes with in-place impermeable covers. Similar to
landfills, gases generated in sites closed as landfills may
migrate laterally, possibly causing significant accumula-
tions. However, closed surface impoundments and waste piles
generally contain small quantities of decomposable and vola-
tile wastes and are at shallow depths. Thus, significant gas
migration and subsequent subsurface gas releases are unlike-
ly.
1-22
-------
1.4.3 Unjj_e_rg_rpunid Tanks
Underground tanks will not normally generate subsurface
gases. Only if the tank is leaking is there an opportunity
for a gas to migrate into the unsaturated zone. Since vola-
tile liquids are often stored in underground tanks, the
potential for a subsurface gas release exists. However,
anticipated gas leaks under these conditions would be small
and probably insignificant.
In summary, landfills, sites closed as landfills, and under-
ground tanks are the only SWMU's with characteristics condu-
cive to subsurface gas migration problems. That is, the
depth of waste deposition or storage is adequate to allow
subsurface gas migration and a source of subsurface gas is
present (generated by decomposition of organic wastes or
volatilization).
Other types of SWMU's, such as land treatment units, active
surface impoundments, and injection wells, do not exhibit a
potential for gas migration and are not applicable to sub-
surface gas releases. For instance, in land treatment units
the production of methane gas is absent and wastes are in-
corporated into the soil surface to allow free venting of any
generated gases to the atmosphere. Where gases are generated
within an active surface impoundment, they likely will vent
1-23
-------
across the liquid surface to the atmosphere rather than migrate
through impoundment walls. While it is possible that gases could
be generated from leaking leachate or volatile liquids from the
unit, significant subsurface gas migration and accumulation are
unl ikely.
1-24
-------
SECTION 2
GENERATION AND MIGRATION OF SUBSURFACE GASES
The generation and migration of subsurface gases are two distinct
processes that contribute to the potential for a release from an
SWMU. While gas generation centers primarily on waste types, and
the type of SWMU, gas migration concerns pathways and barriers for
movement through the surrounding soils. This section reviews
factors that influence and promote subsurface gas generation and
migration of those SWMU's where releases may occur.
2.1 GAS GENERATION
2.1.IB i o1og i c a1 Decpmpos i t i o n
Gas generation and accumulation in SWMU's occurs by biological,
chemical, and physical decomposition of the disposed or stored
wastes. The type of SWMU relates to the extent of biological
decomposition and subsequent gas generation from a given waste.
For instance, biological decomposition is significant in most
landfills and sites closed as landfills due to anaerobic microbial
degradation of organic wastes. However, biological decomposition
is not a gas generating process in underground tanks where
volatile liquids are stored.
Waste types will significantly influence the rate of gas
generation via biological decomposition and the ultimate amount of
2-1
-------
gas produced at an SWMU. Generally, the amount of gas generated
in a landfill is directly proportional to the amount of organic
matter present. In a landfill, the types of organic wastes can be
divided into rapid and slow decomposables. A high percentage of
rapid decomposables (such as food waste, sewage sludges, and
garden waste) will result in gas generation shortly after burial
and high initial yields. Slow decomposables include paper,
cardboard, wood, leather, some textiles, and several other
organic components. Inorganics and inerts such as plastics,
man-made textiles, glass, ceramics, metals, ash, and rock do not
contribute to biological gas production. At sites closed as
landfills, waste types that undergo biological decomposition might
include bulk organic wastes, food processing sludges, treatment
plant sludges, and composting wastes.
In addition to waste type, how the wastes are managed (stored,
treated, or disposed) and the specific waste characteristics will
both impact rates of gas generation from SWMU's. Other factors
important to biological production include:
waste characteristics,
unit design,
unit operations.
Waste characteristics can enhance or inhibit the rate of microbial
activity. For instance, high moisture content (in the as-receiv-
ed waste or through percolation, or both) will provide optimal
conditions to enhance anaerobic decomposition. Other enhancement
2-2
-------
characteristics include adequate buffer capacity and neutral pH,
sufficient nutrients (nitrogen and phosphorous), and mesophilic
temperatures. Inhibiting characteristics include the presence of
high or low pH, sulfur, and soluble metals, perhaps due to the
disposal of nonhazardous industrial wastes.
Unit design at landfills and sites closed as landfills can promote
gas generation. Waste depth, fill configuration, and cover soils
are important. Specifically, deep landfills are better gas
producers than shallow ones. They have a proportionately larger
anaerobic zone, better insulation and compaction, and a better
opportunity to confine generated gases. Fill configuration and
landfill location impact gas production. Deep landfills, such as
trench fills or canyon fills, trap gas along confining sidewalls
and bottom bedrock or ground water. Conversely, mound or shallow
landfills have large surface areas through which gases can vent.
Daily, interim, and final cover soils can retain gases of
biological decomposition within the landfill. Tight cover soils
(e.g., clays) impede vertical migration of gases better than
permeable soils.
Unit operations can have a significant impact on potential gas
generation. Methods and procedures used to segregate and isolate
inert wastes, to prevent moisture infiltration, to compact and
increase the density of the waste, and to minimize or prevent
mixing of waste types can affect resultant releases of subsurface
gases. Biological decomposition can be inhibited by containers
2-3
-------
and containment devices, such as drums, tanks, liners, soil berm
dividers, and daily covers. When organic material is landfilled
under anaerobic conditions, it is converted by microbial action
primarily into carbon dioxide and methane. Hydrogen, ammonia,
aromatic hydrocarbons, halogenated organics, and hydrogen sulfide
are also present as gases but usually in trace amounts. Due to
the large volumes of decomposable wastes deposited in landfills,
landfill gas yields can be high. The primary gases of concern
resulting from biological decomposition are methane (due to its
explosive properties) and other volatile organics that may be
present in amounts that pose a threat to human health or the
environment. Other volatile organics that are frequently detected
in landfill gases were presented in Table 1-4.
2.1.2 C hem i c a 1 necompj) s i tj_p_n_
Chemical decomposition of waste is not a significant gas-generat-
ing process in most SWMU's. However, gas production by chemical
reaction can result from the disposal or storage of incompatible
wastes. In landfills, sites closed as landfills, and underground
tanks, gas production from chemical reactions that pose a threat
to human health and environment is not expected under normal unit
operations. Waste types can influence the potential for chemical
reactions to occur. Reactive or ignitable wastes can produce
explosive or heat-producing reactions, resulting in rapid
production of gases, increased pressures, and increased tempera-
tures. Volatile liquids stored in underground tanks may have a
2-4
-------
significant potential for gas production by chemical reaction.
However, most waste types deposited in landfills and sites closed
as landfills are not amenable to significant chemical reactions
and regulations require special handling for reactive wastes to
minimize these problems.
In addition to waste type, how the wastes are managed will affect
the potential for significant chemical reactions and subsequent
gas production. In particular, the proper design and operation
(e.g., pressure-relief valves and leak detection systems) of
underground tanks will prevent the occurrence of chemical
decomposition by reaction. In a landfill under acidic conditions,
a strong oxidizing agent can react with organic wastes to produce
carbon dioxide and ammonia. Other chemical reactions that might
occur in SWMU's are difficult to predict. As-a result, the
primary gases of concern from chemical decomposition may include
all of the constituents of concern listed in Table 1-2.
2.1.3 Physical _Jtecpmj)_o_sj_tj_o_n
The type of SWMU relates to the potential for physical
decomposition and subsequent gas generation from a given waste.
In the context of this document, physical decomposition includes
volatilization and combustion. Volatilization of compounds is
most likely in underground tanks containing volatile liquids where
a liquid and vapor-phase equilibrium is established in the tank.
Volatilization occurs to a lesser extent in landfills and sites
2-5
-------
closed as landfills, primarily during waste application or other
exposure to air. Sublimation is the direct conversion of solids
to gases without a liquid phase. This occurs in landfills but
generation via this mechanism is considered insignificant.
Combustion processes can actively contribute to subsurface gas
generation. For example, underground fires are encountered at
landfills. Combustion processes resulting in subsurface gas
emissions are not expected in sites closed as landfills (that
were previously surface impoundments or waste piles) or
underground tanks.
Waste types will influence the potential for physical decompo-
sition to occur via volatilization and combustion. Volatility is
is perhaps the most important parameter affecting subsurface gas
generation from liquids stored in underground tanks. The vola-
tilization (or vaporization) of a compound is dependent on its
vapor pressure and concentration (relative to the surrounding
media) and the temperature, pressures, and porosity of the wastes
and surrounding soils. Theoretically, gases generated from liquid
wastes will continue to be generated until a liquid and vapor-
phase equilibrium is established (i.e., the rate of vaporization
equals the rate of condensation). The greater a compound's vapor
pressure, the greater will be its ability to volatalize.
Compounds with very low vapor pressures (generally less than 0.1
mm Hg) tend to remain in the liquid phase.
2-6
-------
Combustion requires a combustible waste type in addition to a
source of oxygen. Typical waste types deposited in landfills
include paper, wood, and flammable liquids. Typical wastes in
underground tanks include flammable liquids such as solvents,
waste oil, and other fuels.
Other factors that impact physical decomposition primarily result
from waste management operations. Unit operations can prevent
volatilization by providing maintenance of underground tanks
(pressure- relief valves and leak detection systems). At
landfills, operations to inhibit underground combustion center on
the prevention and control of fires. In a landfill, combustion
can convert wastes to by-products such as carbon dioxide, carbon
monoxide, and trace toxic components. In addition, the increased
temperatures from combustion may enhance chemical reaction rates
and biological decomposition, and create greater driving forces
for gas migration. Volatility is compound-specific, based on
physical properties, temperature, and pressure. Thus, the primary
gases of concern from physical decomposition include a wide range
of volatile constituents, including all those listed in Table
1-2.
2.2 GAS MIGRATION
Subsurface gas migration towards facility structures or property
boundaries concerns potential pathways and barriers for movement
through the surrounding soils. Barriers affecting migration are
2-7
-------
influenced by hydrologic and geologic site conditions, soil
properties, and unit design and operation features.
2.2.1 Natural barri ers
Gas migration can be impeded or prevented by hydrologic barriers
such as surface water, ground water, and saturated soils.
Subsurface gases that come in contact with these conditions will
tend to migrate towards the pathway of least resistance, usually
through a porous soil. As an uncommon example, if a landfill or
site closed as a landfill was surrounded (along all sidewalls and
bottom) by water, gas migration beyond the confining barrier would
not be expected. In most cases, however, ground water and
saturated soils only partially surround a unit (usually along the
bottom). Thus lateral or vertical migration can occur.
Gas migration can also be impeded or prevented by geologic
barriers such as unfractured rock or soil with low permeability.
Soil permeability is perhaps the most important natural barrier to
gas migration. Permeability is a function of soil type. Clayey
gravels and sand and organic clays will restrict gas flow at SWMU
sites. Conversely, clean gravels and sands have high
permeability, allowing subsurface gases to freely migrate. Soil
permeability can be impacted by climatic conditions such as
precipitation or freezing. Both tend to reduce the porosity of
surface soils preventing upward gas migration.
2-8
-------
_2_._2._2___De_s_i_g_n___B_a r r i e r s
Landfills and sites closed as landfills may be designed with caps
and liners to prevent moisture infiltration and leachate
percolation to ground water. If subsurface gases are generated
from these units, these same caps and liners act as barriers to
gas migration. Generally, caps promote lateral gas migration
since upward movement to the surface is restricted. Effective
liners restrict lateral migration into the surrounding unsaturated
soils. Similar to liners, slurry walls are used to border
landfill units and can restrict gas movement. Caps and liners are
not typically designed with underground tank units. However,
these tanks are often placed into soils with clay backfill during
installation, followed by paving on the surface. Thus, any
escaping gases from a leaking underground tank__may migrate
laterally or along the least resistant pathways adjacent to the
units.
Control systems for subsurface gas are designed to prevent
migration into structures or beyond property boundaries. Control
systems may be passive or active. Two basic types of systems are
barriers and vents or extractive systems. Barriers prevent
migration by providing an impermeable area between the gas source
and the area to be protected. Barriers include synthetic
membranes and air injection systems. Venting systems provide an
easy path for the gas to vent through before migrating to areas
needing protection. Vent systems include passive vent trenches
and active extraction systems. The latter also include flares to
burn the gases in order to control odors.
2-9
-------
Control systems for subsurface gas are almost exclusively
associated with disposal sites for municipal-type waste rather
than for hazardous waste. Thus they will be rare at hazardous
waste facilities and likely only present where municipal waste is
codisposed with hazardous wastes or where a sanitary landfill is
operated on the same site.
2-10
-------
SECTION 3
IDENTIFYING RELEASES
3.1 APPROACH
The preceding sections have evaluated landfills, sites closed as
landfills, and underground tanks in terms of the potential for
subsurface migration of methane and other constituents of concern.
Since few SWMU's are presently monitored for gases and since moni-
toring programs are difficult and often unnecessary to establish,
a Preliminary Assessment (PA) is appropriate to determine if
specific units will generate sufficient amounts of methane or
other constituents to cause a subsurface gas release. Certain
units will not generate gases, or will generate only minor amounts
so that further evaluation will not be required. To make these
determinations, information can be collected 'from construction
documents, permit and inspection reports, and records of waste
disposal, unit design and operation, and past documentation of
accidents, spills, and releases.
As a first step, each SWMU at the facility should be identified
and located by the owner or operator on a topographic base map.
This would include former, existing or new SWMU's. For example,
the location and aerial extent of each SWMU should be confirmed by
historical records, aerial photographs, or geophysical surveys.
The depth, dimensions, and capacity of underground structures,
deposited wastes, and on-site buildings, should be described to
assist the permit writer during the PA.
3-1
-------
Existing construction, operation, and monitoring records can be
used to determine if hazardous constituents have been managed at
the SWMU. Records of waste disposal, unit design and operation,
and documentation of accidents, spills, and releases should be
included during review by EPA to determine if a potential exists
for a subsurface gas release from the SWMU.
Waste disposal records include waste type, quantities managed,
location of wastes and the date of waste disposal. This
information might include waste receipts, waste composition
surveys, and records of special wastes such as municipal-type
refuse, bulk liquids, sludges, contaminated soils, industrial
process wastes, or inert materials. For underground tanks, this
information may address liquid waste compositions, quantities, and
physical properties.
Unit design and operation records provide background information
on site-specific construction methods. Engineering design plans,
inspection records, operation logs,- regulatory inspection and
enforcement logs, damage or nuisance litigation, and routine
monitoring such as ground-water sampling. For landfills and
surface impoundments closed as landfills, these data may include
to the presence and thickness of a liner, ground-water levels,
waste moisture contents, type and amount of daily cover, records
of subsurface fires, in-place leachate and/or gas collection
systems, etc.. For underground tanks, construction and monitoring
records may provide historical information on tank integrity,
indicating if a leak has occurred.
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Past documentation of accidents, spills, and releases can be
helpful if Federal, State, or local authorities were notified.
This information provides a historical perspective on problems,
corrective actions, and controls initiated.
Further information may be obtained by performing a site
investigation (SI), including field observations and interviews
with facility owners or operators. The Si's are technical
observations of SWMU's to detect evidence of a potential release.
Although not extensive, the SI is conducted in the vicinity of
each unit for indications of a release, including settlement,
erosion, or cracking of covers, stressed or dead vegetation,
contamination of surface waters, odors, elevated temperatures in
control or monitoring wells or active venting of gases or smoke.
The Si's might address the condition of monitoring, containment,
or control systems and any obvious structural defects in tanks,
liners, etc.. Observations should be made to check overflow/alarm
shut-off systems, subsurface leak detection systems, secondary
containment structures (e.g., concrete pads, curbs, or dikes), and
other safety triggers for early detection of potential releases.
If monitoring, control, or leak detection systems are not
in-place at the SWMU, Si's may be limited to primarily visual
aspects and reviews of site conditions to evaluate if a release is
suspected.
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3.2 PRELIMINARY ASSESSMENTS
A checklist for identifying actual or potential subsurface gas
releases during a Preliminary Assessment (PA) is shown .in Table
3-1. The checklist is designed to allow the permit writer to
review the PA data and to determine whether subsurface gas is
migrating or will migrate beyond the facility boundary or into
on-site or off-site structures at concentrations that are a threat
to human health and the environment.
The permit writer will be concerned with two types of subsurface
gas migration: (1) methane (CH4) and (2) gases containing the
other constituents of concern, listed in Tables 1-2 and 1-3.
These other constituents of concern must be evaluated only if the
facility owner/operator or permit writer believes they are present
in amounts that may pose a threat to human health or the
envi ronment.
The PA involves a desk-top evaluation of a facility to identify a
release or potential exposure from future releases. Information
used by the permit writer includes the Part A and Part 8 permit
applications. Personal knowledge of the site is important.
Additional information may be available from the owner/operator or
from other sources.
The PA Checklist guides the permit writer through the evaluation.
The checklist is divided into five sections. The first three are
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TABLE 3-1 PRELIMINARY ASSESSMENT CHECKLIST
_ ___Resp o n s e *
Criteria "T N U N/A
I. Unit Characteristics
Unit is a landfill that contains
organic or volatile wastes.
Unit is a surface impoundment or
waste pile that has been closed as
a landfill and covered, with organic
or volatile wastes.
Unit is an underground tank that
contains volatile wastes.
If answers to all above items are No,
no further evaluation is needed.
II. Boundary Conditions
Unit is entirely surrounded within
the property boundary by surface
water, or impervious rock with no
facility structures located inside
the surrounding boundary or on top
of unit.
Unit has effective gas migration con-
trol system(s) protecting both on-site
and off-site structures.
If either answer above is Yes, no further
evaluation is needed.
III. Quantitative Measurement of Release
The methane level is LEL at property
boundary or 25% LEL in facility
structures.
Constituents listed in Table 1-4
measured at PEL at property boun-
dary or in facility structures.
If answers to _b_p_t_n_ of above are No and
permit writer is confident of reported
results, no further evaluation is needed.
If eithe_r answer above is Yes, a Site In-
vestigatTon should be conducted.
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Table 3-1 (Continued)
Respons_e_*
Criteria Y N _ U N_/A_
IV. Other Evidence of Release
t History of methane-related fires
and explosions.
Leak detected and confirmed in
underground tank.
Complaints on file with local/State
health or fire authorities of odors
seeping into basements and subsurface
areas.
V. Migration Distance
Property boundary or structures
on - or off-site less than 800 ft.
at a landfill or landfill-type unit.
Structures on - or off-site less
than 300 feet from an underground
tank unit.
*Y = Yes
N = No
U = Uncertain
N/A = Not applicable
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used sequentially to eliminate facilities with virtually no
possibility of a release or those with good monitoring data that
indicate no release. These sections will eliminate many
facilities from further evaluation because these facilities do not
contain landfills or underground tanks. However, most landfill or
underground tank sites will not be eliminated from further
evaluation in the first three sections, but will be subjected to
Sections IV and V as part of the PA.
Sections IV and V of the checklist help assess the majority of
landfill and underground tank sites. These are sites with little
or no subsurface gas monitoring data. Section IV assesses
indirect evidence of a subsurface gas release and Section V is
concerned with the distance from landfill units or underground
tanks to the property boundary and structuresT"
If a site "passes" the PA Checklist, it is considered to have no
current releases and the potential for exposure from future
releases is small. Sites that fail the PA Checklist should be
subjected to a Site Investigation to obtain monitoring data and
other site-specific information for a more detailed evaluation.
Pass/fail criteria are presented in Section 3.2.5.
The use of predictive models was not included in the PA Checklist.
Several models of subsurface gas (primarily methane) migration
have been developed. A simplified version was included in the in-
structions to the Open Pump Inventory. Even the simplified ver-
sion requires data such as the age of the landfill, depth of fill,
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and soil types around the fill. This informatioin is not readily
available for closed units. Thus predictive models were
considered too burdensome on both the permit writer and the
owner/operator at this point in the site assessment. The question
of current and potential migration can be better answered by use
of the PA Checklist plus a later Site Investigation involving
actual monitoring. Predictive models may be applicable in a
later, more sophisticated evaluation when detailed, site-specific
information is available.
3.2.1 CheckJ_1st_Sect:ion I
The checklist is generally self-explanatory. However, suggestions
for its use and sources of information are presented in this and
subsequent sections.
Section I identifies the types of units of concern for subsurface
gas releases: landfills and units closed as landfills containing
organic or volatile wastes, and underground tanks with volatile
wastes. Examples of wastes that are organic are coal and peat
processing wastes; incinerator, composting, and resource recovery
residues; septic tank pumpings; wood; sewage sludges; and
municipal-type solid waste. Volatile wastes are defined as those
compounds that have a vapor pressure at 20°C that exceeds 0.1 mm
Hg. Tables 1-2 and 1-3 may be used to identify the types of waste
likely to contain constituents of concern.
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Units that meet the criteria in Section I of the checklist have
potentials to generate subsurface gas. Other types of units will
not be evaluated unless the permit writer observes evidence of
migration by other means. As an example, no further evaluation
would be necessary at a demolition landfill accepting inert
materials that did not in the past or does not presently accept
organic waste.
Sources of information for Section I include the waste type and
waste characterization information in the Part A and Part B
permit applications. Little information about colocated or
codisposed nonhazardous organic waste will be easily available.
Contact may have to be made with1 the owner/operator regarding
this. Both current and past operations are of interest. In
addition, contact could be made with the State's solid waste
permitting office, county planning office, or similar sources to
identify or confirm the presence or absence of disposal areas at
the sites in question.
3.2.2 Checklist Section II
Section II identifies situations where natural or manmade control
systems effectively Control migration of subsurface gas. Natural
barriers completely surrounding the units will be almost
nonexistent. Although many municipal landfills have gas migration
controls, the controls often protect specific portions of the
perimeter from gas movement and do not control the entire site.
It is unlikely that closed sites have controls.
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If a site is reported to have a control system completely around
it, the permit writer must be confident that it is operating
effectively. This can be shown by site monitoring data, operating
manuals, and confirmed by contact with local fire and solid waste
offices. Very few, if any, sites that fail Section I will pass
Section II and be released from further evaluation.
3.2.3 Checklist Section III
Subsurface gas monitoring data may be available from the
facility; however, that is not likely. Direct quantitative
measurements can be made during the SI through the simple field
sampling methods described later in this guidance. However, when
any of the indicator constituents of concern is measured at or
above the levels specified in Section II of the checklist, a
release has occurred and the permit writer should initiate a SI as
soon as possible to fully define the release and to assess the
need for emergency action. See Section 4 of this manual related
to emergency situations.
An owner or operator may submit monitoring data indicating no
release. The permit writer must be confident that the monitoring
was done properly and completely before passing a site. Note that
monitoring data must be available for both methane and the
indicator constituents shown in Table 1-4.
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Monitoring procedures should be fully documented. Information
needed is indicated later in the section describing SI field
sampling methods. Some items include:
Locations and depths of sampling
Methods used including sketches or photos
Instruments used
Date and atmospheric pressure
Analytical methods and laboratory used, if any.
Monitoring subsurface gas is a specialized field. The permit
writer should review submitted data with someone experienced in
the area. Sources include solid waste management offices in major
cities or at State offices or consultants experienced in the
topic.
3.2.4 Checklist Sections IV and V
These sections include the evaluation of indirect evidence of
releases or potential exposure from future releases. Sources of
information will vary widely. The owner or operator can be asked
about the history of methane-related incidents, leaks from
underground tanks, and odor complaints. The responses should be
verified through contacts with State or local government
personnel. The owners or operators should be able to provide
appropriate contacts. Possible sources of information include:
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Methane-related fires or explosions
- local fire department
local solid waste management office
- local emergency response or disaster teams
Odor complaints
- generally same as above
0 Underground tank leaks
- EPA Regional or State inspectors or enforce-
ment personnel
- local fire departments or emergency response
teams
The distances noted in Section V may be available on the maps
submitted with Part A or Part B permit applications. However,
closed units or active sanitary landfills may not be shown.
Likewise all on-site and off-site structures may not be shown.
The owner or operator should be contacted for the necessary
information. For purposes of the checklist, the term structure
should be used in the broad sense of any enclosed building that
personnel enter even infrequently.
3.2. 5 ChC k s t ^j on
Sites will either pass the PA Checklist evaluation and not be
subjected to further evaluation of subsurface gas migration or
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they will fail and a Site Investigation will be conducted. Sites
can fail either in Section III of the PA Checklist if a release is
shown or in Sections IV and V. If a site is subjected to
evaluation on criteria in Sections IV and V of the checklist it
should be failed and a SI conducted if either of the following
occur:
One or more Yes responses in Section IV or V
0 Two or more Uncertain responses in Sections IV
and V combined.
Personal knowledge of the facility will be important during the
PA. Responses of the permit writer on the PA Checklist should be
tempered with experience with the site. If there is any
uncertainty, the U response should be made rather than the N
response in Section IV and V. Examples are tfie distance criteria
in Section V. A structure may be more than 300 ft. from an
underground storage tank. However, if the surface between the
tank and the structure is paved,, migration to the building is
possible and a U response is appropriate.
The decision criteria are strict but appropriate. Injury or death
from a methane explosion can come quickly and with virtually no
warning. Often there is little or no concern about subsurface
gas until there is a tragic accident. The effort required for a
SI is well spent if there is _any_ uncertainty in order to identify
releases or potential releases so that they may be controlled or
prevented.
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3.3 SITE INVESTIGATIONS
Site Investigations (Si's) should be conducted at sites that fail
the PA Checklist criteria, either in Section III or Sections IV
and V. The most important results of a SI will be the monitoring
and sampling for subsurface gases in the soil and in on-site
structures. Other information to be collected during a SI include
soil characteristics, presence and effectiveness of any gas
control systems, and the proximity and construction of buildings
on - and off-site.
For simple field tests conducted during the SI, the locations of
probes and timing of samples is important. Sampling locations and
probe depths should be based on soil types, structure locations,
vegetative stress patterns, and the waste typ'e~.
_3._3_.JL __ ^AJJPJJ-PJL-P-ri Sampling Inside Structures
To test for subsurface gases inside a facility structure, sampling
conditions favorable to detecting gases are desired.
Specifically, monitoring should be conducted after the building
has been closed overnight or for a weekend, and when the soil
surface has been wet or frozen for several days. Sampling should
be performed in confined areas where gas may accumulate, such as
basements, crawl spaces, near floor cracks, attics, around
subsurface utility connections, and in untrapped drain lines. Gas
recovery and gas control equipment need not be sampled. The re-
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suits, location, date, barometric pressure and time for each
sample should be recorded. It might be desirable to repeat the
tests at a later date or under different climatic conditions to
verify the readings, particularly if the facility is considered
high priority based on structure locations and migration
distances. Monitoring results should be tabulated and plotted on
a map of the facility.
3.3.2 Subsurface Monitori ng or_ Sam_p_1J_n_g
Subsurface monitoring should begin around the units identified in
Section 1. Initial monitoring should be done as near the edge of
the unit as possible, but not in the unit. If gases are detected,
monitoring should be done further from the unit to identify
general migration paths. The exact location and depth of the
monitoring points should take into account any gas permeable
seams, such as dry sand or gravel, alignment with an off-site
structure, proximity of the waste deposit, areas where there is
dead or unhealthy vegetation that m-ight be due to gas migration,
and areas where underground construction might have created a
natural path for gas flow (utility lines).
In soils that are of uniform depth, subsurface gas probes or
sampling points should be at least three feet below the ground
surface. Where dry sand, gravel, or more gas permeable soil
strata might interconnect the waste deposit and the property
boundary, multiple sampling points should be used, with the upper-
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most three feet deep and additional ones in the deeper permeable
soil layers. General techniques for installing monitoring and
sampling probes are provided in Table 3-2 and Figure 3-1.
3.3.3 Methane
Simple field sampling methods for methane have been described
previously and in 40 CFR Part 257.3-8(a). Methane field sampling
can be performed with combustible gas meters, or by volumetric
sampling and subsequent analysis by gas chromatography. A
combustible gas meter will provide a reliable determination of
methane concentrations. Reported experience indicates 0 to 100
percent LEL detection to be accurate with hot-wire catalytic
combustion principal instruments. Since subsurface gas release
determinations only require readings of metha'ffe concentrations up
to LEL (5 percent methane), this single scale is sufficient.
However, many users prefer instruments with the capability of
determining both the 0 to 100 percent LEL and the percent methane
present when the concentration exceeds 100 percent LEL (5 percent
methane). Dual scale instruments are available for this
application. Typically, the 0 to 100 percent gas scale uses a
thermal conductivity sensor. The carbon dioxide in landfill-
generated gas is reported to interfere with the thermal
conductivity sensor, so the readings above 100 percent LEL, while
useful, cannot be assumed to be accurate. Some of the single
scale 0 to 100 percent LEL instruments can also be fitted with air
dilution tubes or valves to allow readings of the percent gas when
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TABLE 3-2
SUBSURFACE SAMPLING TECHNIQUES
SHALLOW (Up to 6 ft. deep)
Select locations as described in text.
Penetrate soil to desired depth. A steel rod 1/2 to 3/4 in. dia.
and a heavy hammer are sufficient. A bar punch is better for num-
erous holes. It is a small, hand operated pile driver with a slid-
ing weight on the top. Hand augers may also be used.
Insert plastic tubing to bottom of hole. Tubing may be weighted or
attached to a small diameter stick to assure that it gets to the
bottom of the hole. Tubing should be perforated along bottom few
inches to assure gas flow.
Close top of hole around tubing.
Attach meter or sampling pump and evacuate hole of air-diluted gases
before recording gas concentrations or taking samples.
When using a meter, begin with the most sensitive range (0 - 100
percent LEL for methane). If meter is pegged, change to the next
least sensitive range to determine actual gas concentration.
t Tubing shall be marked, sealed, and protected if sampling will be
done later.
If at all possible, monitoring should be repeated a day or two after
probe installation to verify readings.
DEEP (More Than 6 ft. deep)
Same general procedures as above.
Use portable power augers or truck-mounted augers.
For permanent monitoring points, use rigid PVC tubing and the
general construction techniques shown in Figure 3-1.
CAUTION
When using hand powered equipment, stop if any unusually high
resistance is met - it could be a gas pipe or an electrical cable.
Before using powered equipment, confirm that there are no
underground utilities in the locations selected.
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FIGURE 3-1
TYPICAL DEEP SUBSURFACE GAS MONITORING WELLS
PC,
1/2" DIA. SCH. 40
PVC PIPE
1/8" DIA.
PERFORATIONS
FIBERGLASS
SCREENING TO
BE WRAPPED
AROUND 8 TAPED
TO TUBE.
MONITORING PROBE DETAIL
1/2" DIA. SCH. 40
PVC PIPE
SOIL BACKFILL
g ( i i l i * ' '
flE-r-2' BENTONITE
" II PLUG
e-SOIL BACKFILL
t PEA GRAVEL
SOIL BACKFILL
'-2' BENTONITE
PLUG
SOIL BACKFILL
MONITORING
PROBE
2' PEA GRAVEL
SOIL BACKFILL
'-2' BENTONITE
PLUG
SOIL BACKFILL
2' PEA GRAVEL
3-18
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the concentration is above the LEL. Instructions on the use and
calibration of these instruments should be obtained from the
manufacturer.
When sampling with a combustible gas meter, samples should be
withdrawn slowly with the hand bulb until a high or constant
reading is obtained. For small diameter holes in tight soils, a
spike reading may be obtained initially and the concentration will
then drop to a low value. This is caused by exhausting the gas in
the hole. The spike reading should be recorded and checked by
allowing the gas to return for a few minutes and taking another
reading. The location or probe number, the time and date, and the
results should be recorded.
For volumetric sampling, subsurface gas can be- collected in an
evacuated flask or bottle with a vacuum pump. The volume of air
in probes or gas lines should be purged prior to obtaining the
sample. Generally, a 150 ml sample is appropriate. This sample
is then sent to the laboratory for analysis of methane by volume.
3_.3.4 _0ther Ijidicator Constituents
The other indicator constituents listed in Table 1-4 can be
measured by several methods, all of which require more extensive
sampling procedures, equipment, and instrumentation than
combustible gas meters for methane. There are two basic methods
for collection of subsurface gas samples. In one, a gas sample is
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obtained in a flask, bottle, bag, or other suitable container; in
the other, gases or compounds are removed from the air and
concentrated by passage through an absorbing or adsorbing medium.
The first method involves the collection of volumetric samples,
usually a liter or so depending on concentration and desired
detection limits. This type of sampling is convenient and easily
performed in the field. Evacuated stainless steel cylinders are
frequently used for sample collection. However, sample loss or
decay can occur with various containers (such as plastic bags) and
prompt laboratory analysis is required. To detect the indicator
constituents at levels fitting the definition of a release, the
volumetric sample collection method is acceptable, coupled with
the appropriate analytical methods. This type of sampling is less
reliable when compound concentrations fluctuate with time, or are
low, or when the gas sample is a complex mixture of many
compounds.
Because low concentrations of a wide variety of volatile organics
are frequently observed in subsurface gases, many compounds can be
more efficiently collected and simultaneously analyzed by
continuous sampling and collection onto a solid adsorbent. The
solid adsorbent concentration method uses a vacuum pump and
adsorbent traps to collect and concentrate volatile organics.
Specific adsorbents (such as activated charcoal or silica gel)
can be selected according to compound volatility and the type of
organics sampled (such as chlorinated hydrocarbons or pesticides).
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Flow rates through the adsorbent traps and sample volumes must be
selected prior to sampling, and then carefully measured during
sampling with calibrated meters. After sampling is completed, the
traps are sealed and shipped to a laboratory for analysis.
Several analytical methods are available for both volumetric and
concentrated trap samples. For quality control requirements, EPA
has approved Method 1624 for volatile organics in 40 CFR Part 136.
This method is amenable to the indicator constituents of concern.
Portable detection meters are not recommended for monitoring for
constituents other than methane. These instruments must be
recalibrated for each different constituent.
Factors Not
During a SI no release or even no migrating gas at all may be
found. However, there is the possibility of future releases
resulting in exposure of the public. SI inspections should
include factors related to the potential for releases and
migration. These factors are generally related to site location
and soils, subsurface gas control systems, and construction of
on - and off-site buildings.
Site location, soils, and surface and ground water influence the
potential for subsurface gas migration. Tight, uniform soils such
as clays at least to the depth of the unit are good barriers.
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Sand and gravel lenses below a less permeable cover are excellent
conduits for gas migration. However, sandy soil will likely
encourage venting of gas to the atmosphere with little chance of
horizontal migration. Observation of soil types at the site is
important. Other soil information from the Part B permit
application will also be helpful in assessing future migration
potential.
Water is a barrier to gas migration. Subsurface gas does not
penetrate ground water and surface water is usually a good
barrier. An exception is a pearched body of surface water with
unsaturated soil below it. Thus if there is a lake or perennial
stream between the unit and any structure, future migration and
exposure is unlikely. High ground water restricts migration to
the unsaturated zone. This focuses interest i_n soils to only that
zone. High water tables also allow for the use of effective but
relatively inexpensive trenches as gas control devices.
Some sites may have gas migration controls. If properly designed
and located, they can greatly reduce the possibility of future
releases, even if the controls do not completely surround the
unit. Control systems are usually constructed to protect existing
structures. If future building is possible, a system may have to
be expanded.
Control systems nay be either passive or active. Passive systems
require no energy and little maintenance. They are vents to the
atmosphere or barriers. Assuming proper design and construction,
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the only passive system not considered effective is a series of
vent wells. Active systems withdraw gas through wells or develop
pressure curtains by injecting air into the soil. Assessment of
effectiveness should be done by personnel experienced with gas
migration control. Control systems are described more fully in
Section 5.
Exposure to subsurface gas may be affected by the type of
construction used for buildings on or off the site. SI
inspections should at least determine the general construction
techniques used. Subsurface gas accumulates in basements, crawl
spaces, and other confined areas where there is easy passage for
the gas from the soil into the building. Buildings built with
slab-on-grade or with well ventilated crawl spaces are less likely
to have gas accumulations than other types; however, cracks in
floors and gaps around utility penetrations can provide good
conduits.
3.3.6 Evaluation
SI monitoring data may identify a release or even an emergency
situation (see Section 4). This will likely lead to permit
conditions to correct the situation or to more detailed studies to
prove that risk of future exposure is low.
Monitoring data may indicate that no indicator constituents are
present or they are at concentrations less than the criteria for a
release. In these situations, future releases are possible. SI
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investigations should assess the likelihood of future releases and
potential exposures of the public. Provisions of RCRA regulations
should be kept in mind related to reopening permits and the
10-year life of permits. If a site poses a threat of future
release and exposure of the public before regulatory corrections
can be applied, the site should be failed and more detailed
Remedial Investigations conducted.
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SECTION 4
REMEDIAL INVESTIGATIONS
4.1 EMERGENCY SITUATIONS
4.1.1 Criteria
An emergency situation with regard to subsurface gas exists when
gas has accumulated in a structure to the degree that an imminent
hazard to human health is present. For methane, an emergency
situation exists when the gas is found at concentrations equal to
or above its lower explosive limit (LEL). The LEL for methane is
five percent in air. Any concentration of methane between 5 and
15 percent is explosive. Above 15 percent in air, the methane is
flammable. Both situations should be considered as emergencies.
The criteria for a methane-related emergency situation should be
applied to structures both on and off-site. Likewise both
occupied and non-occupied structures should be included.
Emergency situations may also exist due to the presence of other
subsurface gases. For subsurface gas constituents of concern
other than methane, an emergency situation exists if:
o a constituent of concern is present in a structure
at concentrations greater than its PEL (or other ex-
posure limit as discussed in Section 2), and
o the structure is occupied routinely (five or more days
per week) by the same personnel for eight hours or more
each day of occupancy.
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This definition includes structures both on-site and off-site.
Obviously, dwellings on-site and off-site are included. Other
on-site structures that would be included are offices, scale
houses, laboratories, maintenance garages, and other buildings
with essentially continuous occupancy during an eight hour work
shift. Buildings such as locker facilities, lunch rooms, and
break areas that are only occupied for short periods are not
included.
4.1.2 Identification
Potential and actual emergency situations can be identified
through reviews of historical information and gas monitoring data.
A potential emergency situation exists if there have been any re-
ported explosions or fires (inside or outside "of a structure)
likely caused by subsurface methane. Also illnesses or deaths
verified as related to exposure to gases within a building on- or
off-site would indicate a potential emergency situation. Know-
ledge of the above will be extremely rare. Thus identification
of actual emergency situations will rely almost entirely on moni-
toring data.
Historical monitoring data for subsurface gas within occupied
buildings may be available, but is also unlikely. Probably only
sites with a history of subsurface gas migration will have
monitored inside buildings.
4-2
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The possibility of an emergency situation due to subsurface gas is
extremely low. Unless there is strong evidence, other than moni-
toring data, site visits only to monitor for subsurface gas are
not warranted. Evidence to trigger a visit to monitor for sub-
surface gas emergency situations include:
Municipal-type refuse is or was disposed in a
landfill within the property boundary or
t An underground tank containing constituents of
concern is known to be leaking.
Emergency situations will likely be identified only through moni-
toring by EPA or State regulatory personnel. This monitoring
shall be done using the instruments identified in Section 3.
Monitoring should be done in areas within structures where sub-
surface gas is most likely to enter and accumulate. These areas
include basements, crawl spaces, and other enclosed areas such as
storage rooms, closets and other areas with no windows. Gas con-
centrations should be checked near cracks or joints in founda-
tions or floors and around pipes and other utilities that enter
the building through a foundation wall or through the floor
Atmospheric pressure can influence the migration of subsurface gas
into structures. When monitoring is being done, the barometric
pressure should be recorded. The best time to sample is when the
pressure is low. If possible, schedule visits for days with low
pressure. If that is not possible and gas concentrations are just
4-3
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below the criteria for an emergency situation, schedule a return
visit for a time of lower atmospheric pressure.
Off-site buildings probably cannot be monitored for subsurface
gas. Permission may be obtained to monitor in buildings owned by
the facility owner and possibly publically-owned buildings near
the site. Monitoring inside privately-owned buildings should not
be attempted unless there is a great likelihood of an emergency
situation. If occupied off-site buildings are extremely close to
the property boundary (less than 100 ft) and subsurface gas at the
boundary is above the LEL, contact with local fire or health
officials should be made and interior monitoring coordinated
through them.
4.1.3 Fast-Track Corrective Actions
If monitoring data indicate an emergency situation related to
subsurface gas, steps should be taken promptly to reduce the
hazard whether it is related to human health or to the explosive
potential of methane. A checklist of steps recommended in case of
a gas emergency is shown in Table 4-1. The steps should be com-
pleted generally in the order shown; however, variations may be
appropriate due to site-specific circumstances.
Corrective actions for subsurface gas releases are highly site-
specific. In general, they involve either the venting of gas to
the atmosphere where it is diluted or incinerated, or the pre-
4-4
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TABLE 4-1
Actions to Take in Gas Emergency
Situations in Buildings
1. Evacuate the building.
2. Advise fire department.
3. Ventilate by opening windows and doors.
4. If methane is present shut off utilities - gas, electricity,
telephone.
5. Monitor to identify source if possible. Source could be
leaking natural gas appliance or supply system. Contact
local gas utility and fire department for monitoring and
sampling assistance.
6. Take samples and have analyzed to identify source.
7. Control source, repair leak, etc., if source is not subsur-
face gas. Continue to monitor until safe levels return.
8. If source is subsurface gas, call in experienced and quali-
fied experts to locate sources and develop corrective
actions.
4-5
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vention of migration through the use of barriers. Specific cor-
rective actions should be identified by experts at the site. Some
common actions are presented in Section 5.
4.2 ROUTINE SITUATIONS
Routine situations are those that are not emergencies but which
are indefinite as to the potential for future releases and the
resultant public exposure and health impacts. Either regulatory
agencies or owners and operators may decide that more detailed
data collection and analyses are appropriate. These efforts could
be used to support or refute decisions related to permit condit-
ions or the denial of a permit. Efforts related to such decisions
will be referred to as Remedial Investigations (RIs).
Owners and operators may want to conduct an RI to show that even
though a subsurface gas release has occurred, exposure to the
public is unlikely and thus the potential risk to human health and
the environment is not substantial. Similarly they may wish to
propose a corrective action that is different from that required
by the EPA or State. This latter situation is similar to a re-
quest for an Alternate Concentration Limit for ground-water pro-
tection.
Regulatory agencies may wish to initiate an RI to defend decis-
ions imposing permit conditions even though no release of subsur-
face gas was detected during an SI. Such an RI would be focused
on establishing that a substantial potential risk exists of ex-
posure to subsurface gas if a release occurs in the future.
4-6
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Two general approaches are available for the conduct and evalua-
tion of an RI related to an actual or potential subsurface gas
release. One approach is the use of predictive models for sub-
surface gas migration coupled with site-specific information con-
cerning the potentially exposed population. The other approach is
the use of an independent consultant experienced in control of
subsurface gas migration. This approach might be considered a
form of arbitration and both the regulators and the regulated or-
ganization should agree to honor the conclusion of the consul-
tant. The consultant firm may use predictive models, however, it
may also use more extensive monitoring and sampling than was done
in the SI and make more thorough soils analyses and evaluation of
other factors such as the abilities of buildings to prevent the
intrusion of subsurface gas.
4.2.1 Predictive Models
Models have been developed to predict the migration of subsurface
gas. Essentially all of them have been used to predict migration
of landfill gas from open dumps and sanitary landfills where
municipal refuse has been disposed. They are also applicable to
the assessment of migration control system effectiveness.
A generalized model was developed for use in the Open Dump Inven-
tory Manual and was presented in Chapter 2(a) of that manual. It
was developed to predict the distance to which landfill gas would
likely migrate from an open dump. Its data requirements and de-
4-7
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gree of accuracy are appropriate to landfill or landfill-type
sites subjected to an RI. The model was not designed for appli-
cation to a leaking tank situation, and its use for underground
tanks is discouraged.
The model yields a subsurface gas migration contour map. It
predicts the distance that gas will migrate from each unit based
on the unit's age, depth, soil characteristics, and other fac-
tros. These distances for two concentrations of methane (LEL or 5
percent methane and 25 percent of LEL or 1.25 percent methane) can
be plotted on a facility map. The resulting contours enclose
areas where methane could be expected at these concentrations. If
on-site structures are within the 1.25 percent methane contour or
the 5 percent contour extends beyond the property boundary, a re-
lease of subsurface gas as defined in Section! is possible. In
such situations continued monitoring and/or corrective measures
should be implemented.
4.2.1.1 Data Requirements
Data requirements for use of a predictive model are likely more
extensive than will be found in Part A and Part B permit applica-
tions and information collected during an SI. This is especially
true of sanitary landfills and other nonhazardous disposal units
located at the facility. Information requirements are listed be-
low:
4-8
-------
Map of the facility showing all landfill units
or units closed as landfills that contain organic
or potentially volatile wastes. The map should in-
clude land use at least 1/4 mile outside the prop-
erty boundaries and all structures within that area
including those on-site. A sample map is shown in
the example explained later.
Information about the degree to which the surface
between the landfill units and structures is imper-
vious to gas (see example)
For each of the above units the following is needed
area! extent of each unit
topography including elevation "contours at
least 1/4 mile beyond property boundaries
average elevation of the bottom of solid
waste in each unit
years in which solid waste was first put in-
to each unit
average elevation of a gas impermeable barrier
below the solid waste (usually ground water or
bedrock).
t« information on soils surrounding each unit at
least to the depth of the bottom of the solid
waste (descriptive soil names and classifica-
tions are included in the example).
4-9
-------
Accurate information on subsurface conditions win be the most
difficult to obtain. Surface features can be observed directly
and maps are usually available including those used in Part A and
Part B permit applications. The age of each unit may be a matter
of record; however, for older units, knowledge of local govern-
ment personnel should confirm owners' statements. The depth of
fill can be estimated by comparing contour maps of the area,
before and after filling. Depth to ground water or bedrock may be
available from on-sits or nearby water wells including ground-
water monitoring wells. Soils classifications may be available
from boring logs of ground-water monitoring wells. However they
may describe the soils only on two sides of the site. Other bor-
ing logs may be available from local water well or construction
drilling firms or local offices of the Soil Conservation Service
In some instances, soil boring may have to be-made.
4.2.1.2 Example Application
The predictive model is best explained through the use of an
example situation at a hypothetical landfill. The example and the
migration model and associated graphs were taken from the Open
Dump Inventory Manual. The example landfill is shown in Figure
4-1. Both a plan view and two cross sections of the landfill are
shown. They include most of the physical data needed for the
model.
Other information includes the age of the landfill since it first
received waste and soils information. In Figure 4-1, soils are
4-10
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»*.
I »r.
ATMkMI.
Figure 4-1
Examole Landfill
4-11
-------
shown as either sand or clay and are assumed to be uniform from
the surface to the ground-water level. Site-specific soil data
should be available. Soils may be described by names or via a
classification system. A common system is the USCS Classifica-
tion. Soil names, USCS Classification, and related graph use in
the model are shown below.
Soil Name USCS Classification Graph Use
Clean (no fines)
gravels and sands GW, GP, SW, SP Sand
Silty gravels and sands,
silt, silty and sandy
loan, organic silts GW, SM, ML, OL, MH Interpolate
Clayey gravels and
sands, lean, fat and
organic clays GC, SC, CL, CH, OH Clay
The model includes three basic steps. The first is the estima-
tion of migration distances related to the landfill's age and type
of soil being evaluated. The next two steps apply correction fac-
tors related to the depth of the landfill and to the degree to
which surface venting is likely.
Figure 4-2 is the graph for the uncorrected migration distance.
To use the graph the age of the landfill and the soil type must
4-12
-------
be known. Enter the graph at the site's age. Continue up to the
curve describing the soil type and concentration of interest.
Contours (migration distances) should be drawn for both 5 percent
and 1.25 percent methane. If the soil type is between day and
sand, interpolate between the curves. If in doubt about soil
types use the sand curves as the worst case (greatest migration
distance). Read the uncorrected migration distances (5 percent
and 1.25 percent) on the vertical axis. For the example landfill
in Figure 4-1, the uncorrected 5 percent methane migration
distances for a 10-year old landfill would be (Figure 4-2):
Section A-A: East side, 10 years, sand = 165'
West side, 10 years, sand = 165'
Section B-B: South side, 10 years, sand = 165'
North side, 10 years, clay = 130'
The corresponding uncorrected distances for tfie 1.25 percent
methane migration would be:
Section A-A: East side, 10 years, sand = 255'
West side, 10 years, sand = 255'
Section B-B: South side, 10 years, sand = 255'
North side, 10 years, clay = 200'
The next step is the selection of a correction factor related to
the depth of the landfill. The deeper the waste, the greater is
the potential migration distnce. A graph of depth correction
factors is shown in Figure 4-3. Enter Figure 4-3 with the unit's
age along the horizontal axis. Move upward to the appropriate
depth curve. The correction factor is found on the vertical axis
and ranges from - 0.5 to + 2.2. Note that the curve for a land-
4-13
-------
300
I
200
I
o
too
AND-
.26*
CLAV-1.2KK
CAND-bX
CLAV
fl
10 12
ITE AOC-ViARI
14
II
20
Figure 4-2
Unconnected Methane Migration Distance
-------
fill 25 ft deep is a straight line and yields a correction factor
of 1.0. For units of different depths, interpolate between the
curves, The depth corrective multipliers for the example site
would be:
Section A-A: East side, 10 years, 20' deep = 1.0
West side, 10 years, 20' deep = 1.0
Section B-B: South side, 10 years, 10' deep = 0.95
North side, 10.years, 50' deep =1.4
The corrective factors for the surrounding soil venting conditions
are next obtained using the graph in Figure 4-4. This graph is
based on the assumption that the surrounding surficial soil is
impervious 100 percent of the time. Thus the value read from the
graph must be adjusted, based on the percentage of time the
surrounding surficial soil is saturated or frozen and the
percentage of land along the path of gas migration from which gas
venting to the atmosphere is blocked all year (asphalt or concrete
roads or parking lots, shallow perched ground water, surface water
bodies not interconnected to ground water). The totally
impervious corrective factors on the vertical axis of Figure 4-4
are only used when the landfill is entirely surrounded at all
times by these conditions. An adjusted corrective factor is
obtained by entering the chart with site age and obtaining the
totally impervious corrective factor for the appropriate depth and
soil type and then entering this value in the following equation:
Adjusted corrective factor = [{Impervious corrective factor
from Figure 4-4) - 1] x (% of impervious time or area] + 1
If both time and area adjustments are necessary, the percentages
are additive. Estimates to the nearest 20 percent are sufficient.
4-14
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- - ' .. i .''.'".:
26 --------..- - - -;;; ; ; ; ;
20 : :
2 , * :
-f - . . - «
i "v v 4-
^ 5 ,«' ...«:-''
tn * ill
I ,:.;}{ fr t
1 « ' .... 26
' '
- - - **- - 1
- ' - - - - -
_jl ........ 1 .... ^ -
-------
- - - - - - - - -
. . _ . ... ._ _ _ ..
, , taa* O*«P
' * * _1
s ' !r
i
«
- - - - - - - - - - - - -
------ 1 ,,.
- p ( ] DO veer "
- ..-.--. - -
1' DEEP '...''.'...I.'. . . ........ . , ". . . . . :
.... - . -
- . . - - - - .
- ..... _
- -...--.
10
12
tO
Figure 4-3
Correction Factors for Landfill Depth Below Grade
-------
I
I
en
IMPERVIOUS-BO* DEEP
IMPEHVIOUB-IOO* DEEP
TTTTnnrrrrnwffl
IMPEHVIOUS-IOO' DEEP
IMPEnviOMB-2B' DEEP
10 12
GlfE AQE-VEAHS
M ia
Figure 4-4
Correction Factors for Soil Surface Venting Condition Around Landfill
-------
When free venting conditons are prevalent most of the year, simply
use 1.0 (no correction). For depths less than 25 feet deep, use
the 25 foot value. For the example site, the adjusted corrective
factors for frozen or wet soil conditions 50 percent of the year
are:
Section A-A: East side (ignore
narrow road, sand,
20' deep, 10 years old) = (2.1-1) (.50) +1 = 1.55
West side (sand, 20'
deep, 10 years old) = (2.1-1) (.50) +1 = 1.55
Section B-B: South side (sand, 10'
deep, 10 years old) = (2.1-1) (.50) +1 = 1.55
North side (clay, 50'
deep, 10 years old) = (1.4-1) (.50) +1 = 1.2
For ease of calculation, the above data are entered into a table
similar to Table 4-2. Table 4-2 includes the data from the
example problem. The corrected distances of probable gas
migration resulting in 5 and 1.25 percent concentrations in each
direction from the landfill are obtained by multiplying across the
table for each side of the landfill. These values can then be
plotted on the scale plan as contours of the 5 and 1.25 percent
methane concentrations or simply compared to the distnces from the
waste deposit to structures of concern.
The corrected distances from Table 4-2 are plotted on the map in
Figure 4-5 and the resulting contours sketched. Mote that surface
water along the western and southern boundaries are barriers to
gas migration and override the distances in Table 4-2.
4-17
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-pi
I
Landfill Methane Uioorrected
Side Concentration Distance
E 5%
1.25%
W 5%
1.25%
S 5%
1.25%
N 5%
1.25%
165'
255*
165'
255 '
165
255
130'
200 '
X
X
X
X
X
X
X
X
Correction
for Depth
1.0
1.0
1.0
1.0
0.95
0.95
1.4
1.4
X
X
X
X
X
X
X
X
Correction Corrected
for Venting Distance
1.55
1.55
1.55
1.55
1.55
1.55
1.2
1.2
- 256'
- 395
- 256* *
«= 3951 *
- 243* *
= 375 ' *
* 218
« 336'
(225* max.)
(225* max.)
(225* max.)
(225* max.)
. . . . t .......
* IJhen these distances are plotted on the landfill sketch, they exceed the distance to the creek,
which acts as a barrier to the gas migration, Ihus the distance to the creek is the naxinun
migration distance.
Table 4-2
Methane Migration Distance Tabulating Form
-------
SI-
SI
Figure i\-*>
Example Landfill Methane Contours
4-19
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The contour for 5 percent methane extends beyond the property
boundary. A release of subsurface gas is possible even if it has
not yet occurred. The 1.25 percent methane contour encloses
on-site buildings but does not reach off-site structures. Thus
there is a possibility of gas accumulation in on-site structures.
Both contours indicate that a release of subsurface gas (as
defined in Section 1) is possible. This should trigger concern
for potential risk to human health.
A situation shown in Figure 4-5 should lead to some type of
corrective action as described in Section 5. However, the owner
or operator may want to demonstrate that the risk to human health
is not substantial. The fact that off-site structures are beyond
both the 5 and 1.25 percent contours indicates that problems in
these buildings are not likely. On-site buidTings are close to
the 5 percent contour. Factors such as type of construction,
quality of sealing cracks in floors, or a gas monitoring program
may be used to demonstrate that the risk is not substantial or
that any potential problem will be identified before damage
occurs. Thus site-specific factors may be used to confirm or
refute the apparent results of the predictive model.
Essentially, any site can be made safe from releases of subsurface
gases. Monitoring programs, construction techniques, and gas
control features can be used singly or in combination as
warranted. The corrective actions that will provide advanced
warning or will prevent future releases are described in Section 5.
4-20
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4.2.2 Use of Experts
The predictive model may yield ambiguous results or either the EPA
or the owner may know of factors not considered in the model that
either raise or lower the potential for a release and for a
substantial risk to human health. In such situations, experts in
the area of subsurface gas generation and control should be used.
Experts may also be used in lieu of the predictive model. Experts
may be used by either the regulatory agency or the owner, or both.
If possible, an agreement to abide by the expert's findings should
be developed. If not, the assessment of the expert's findings by
the regulators will usually be binding on the owner.
Experts in subsurface gas generation and control are relatively
rare. Few are employed by the EPA or States and only a few major
disposal firms will have experts on staff. Most gas experts are
employed by a small number of environmental consulting firms or by
natural gas utilities; however, the latter are less familiar with
gases generated at waste disposal sites and their control.
Identification of qualified consulting firms can be made by
contacting the Governmental Refuse Collection and Disposal
Association (GRCDA). GRCDA is located in Silver Spring, MD, and
the telephone number is (301) 585-2898.
Gas experts can conduct detailed subsurface exploration and
monitoring for the presence of gas. Information obtained through
this field work can verify the extent of a landfill (both area!
and vertical extent), identify soil and other conditions specific
4-21
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to the site that will encourage or impede migration, and identify
gas concentrations at various depth and in directions related to
occupied structures. Predictive models more sophisticated than
the one described above may be used or the expert may use past
experience coupled with site conditions to assess the presence or
potential for a release.
Conditions that would reduce the consequences of a release should
also be identified. These include the design and quality of
construction of buildings. These impact on the likelihood that
gas would enter a building if it was present in the adjacent soil.
Experts can also recommend systems that will provide adequate
protection. These may include relatively simple and inexpensive
monitoring systems. As appropriate, venting or barrier systems
for gas migration control can be identified. "The experts used
should also be experienced in both the design and operation of
control systems. Protective systems may be necessary as part of
permit conditions and should be designed by experienced experts
and the designs be reviewed by equally qualified personnel. This
review is particularly important for designs submitted by an
owner.
4-22
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SECTION 5
CORRECTIVE MEASURES
5.1 MONITORING PROGRAMS
After the PA/SI is complete, the owner/operator may be required to
establish a gas monitoring program for the site. A monitoring
program consists of the design of sampling probe types and site
locations, well construction and probe installation, development
of a written protocol for field sampling procedures, maintenance
of the monitoring wells, data interpretation, and reporting
requirements. For purposes of this guidance, a general protocol
for subsurface gas field monitoring is described below. This
protocol addresses field data forms, equipment, monitoring and
sampling procedures, and record keeping for methane and the in-
dicator constituents of concern.
5.1.1 Methane
5.1.1.1 General
An example data form for methane field monitoring is shown in
Table 5-1. Data requirements specified include:
General background data. These include date and time,
monitoring personnel, barometric pressure and atmos-
pheric temperatures, general weather conditions, and
the types of instruments used.
Ground-water data. Most monitoring probes are expected
to be dry since they will be installed above the water
table. However, precipitation or a rising ground-water
5-1
-------
Date:
T ime:
a.m. p.m.
Ihstruuients Used:
t Water Level:
Monitoring Personnel:
Barometric Pressure Reading:
Atmospheric Temperature:
in. tig
in. II20
Probe Pressure:
Methane:
"F
Barometric Pressure:
Atmospheric Temperature:
General Weather Conditions:
en
i
r\j
Well No.
1
Ground Water
QDry
DWel
Probe
No.
A
B
C
Probe Pressure (jn.H;>0) Methane
Gage Absolute % LEL % Gas
Notes
QDry
Q Wet
Q Dry
QWet
A
B
C
A
B
C
Table 5-1
Field Data Form for Subsurface Gas Monitoring
-------
table can cause soil saturation and probe clogging. The
condition of the probe should be marked as either wet or
dry.
Probe pressure. The pressure or vacuum is read from a
pressure gauge. The reading then can be corrected to
"absolute" pressure in conjunction with barometric pres-
sure and temperature. Pressure is usually reported as
in. of H20 and ranges from very slightly negative to
about 5 in. H20 positive.
Methane content. Through the use of a combustible gas
meter, methane content of the subsurface gas can be
measured in terms of percent of LEL or percent volume in
air.
Completion of the data form should be routine." The date and time
of sampling should be consistent. That is, if field monitoring
is performed at weekly intervals, use the same day of the week.
For monthly intervals, field monitoring should be performed on
about the same day of the month. Similarly, field monitoring
should be performed consistently at the same time of day, prefer-
ably in early afternoon when subsurface pressures are most likely
to be positive.
For the barometric pressure reading, a sturdy field barometer
should be used. The current direction of movement (i.e., rising,
falling, or steady) should be recorded. For each monitoring day,
5-3
-------
a phone call should be placed to the National Weather Service to
obtain a current reading as a check of the field barometer's
accuracy. The barometric pressure reading obtained in the field
is recorded on the data form in inches of Hg. If desired, it can
be converted to inches of water by multiplying by 13.4. The
general weather conditions (i.e., wind conditions, snow or rain,
cloud cover, etc.) and instruments used should also be noted on
the form.
5.1.1.2 Equipment
The following equipment and other materials should be taken into
the field for each subsurface gas or facility structure monitoring
round: watch, thermometer (°F or °C), barometer (inches Hg),
looseleaf binder notebook, blank data forms, copy of protocol,
clipboard and pen, pressure measuring devices-,-and combustible gas
meter. The gauges used are differential pressure gauges with a 0
to 0.5 inch range or a 0 to 5 inch range. Other pressure reading
accessories might include spare tubing, a mounting board for
leveling, and a peristalic hand pump.
The combustible gas meter should be calibrated for methane, with
percent LEL and percent gas ranges. Other accessories might
include spare batteries, spare aspirator, water trap, extra
tubing, spare filaments and cotton filters, and calibration gas.
5.1.1.3 Probe pressure
The purpose of recording probe pressure is to determine whether
subsurface gases are at a higher or lower pressure than the
5-4
-------
atmosphere. Often, subsurface pressures lag behind changes in
atmospheric pressure, particularly for deep probes, and/or probes
that are separated from the ground surface by relatively imperm-
eable strata.
When subsurface gases are found to have positive gauge pres-
sures they can easily be extracted from the subsurface, and an
accurate methane content reading can be recorded. However, with
negative pressure (i.e., vacuum) relative to the atmosphere, the
tendency is for atmospheric gas to move into the probe. The
result is that an accurate reading of subsurface gas methane
content cannot be made. Thus, a methane reading should be taken
only when probe pressures are zero or positive.
To determine probe pressure, the following steps should be
employed:
Zero the gauge in the vertical position using the zero
adjustment screw located in the plastic cover at the
bottom of the gauge.
Connect flexible tubing from the "high" (positive) port of
the gauge (range of 0 to 0.5 in. water) to the test
probe. Connections must be air-tight, and clamps should
be used if necessary. If the pointer moves to the left,
remove the tubing from the high port to the low port.
For test probes exceeding +_ 0.5 in. water, reconnect the
tubing on the same port to the 0 to 5.0 in. water range
gauge.
5-5
-------
Before recording the reading, push the tubing further on
to the test probe after a period of time. If there is no
sudden increase in pressure, this indicates the test
probe and connective tubing are both free of foreign
material that might clog these openings, and could
otherwise give false readings. Record the pressure
reading on the data form.
If the pressure increases perceptively as the tubing is
slid further onto the test probe, the test probe is like-
ly clogged with foreign material, or filled with water
(i.e., either condensate or ground water). This indicates
that the gas between the tube blockage and the pressure
indicator becomes compressed as the tube is pushed further
on. The test probe and connective tubing should be
separated. The end of the test probe "should be inspected
for water, debris, or other foreign material. If these
are present, they should be removed.
t If the spurting pressure response remains, but no foreign
material appears to be present, the test probe is probably
filled with ground water or condensate. A peri static
hand pump should be used for approximately 10 to 15
seconds to force air into the probe and flush ground
water or condensate through the bottom perforations.
After this has been performed, the pressure indicator
tubing should again be attached to the probe, and the
procedures above repeated. If a valid reading can be
obtained, it should be recorded.
5-6
-------
Differential pressure gauges are fairly accurate devices,
requiring little maintenance. For quality control purposes,
checks should be made of their readings from time to time. It is
suggested that instruments used regularly in the field be returned
to the manufacturer every six months. This will allow accuracy
checks to be made, and appropriate repairs and recalibration to be
performed.
5.1.1.4 Methane monitoring
The determination of methane content is the most significant
parameter of the monitoring program. Methane is colorless, odor-
less, explosive, and an excellent indicator of subsurface gas
migration.
Combustible gas meters are available from several manufacturers.
Generally, filaments inside the instrument aVTow measurement of
combustible gases on either the percent LEL or percent gas (volume
in air) scales. The scales are typically printed on the meter's
face with a switch used to select the desired range.
Besides the instrument itself, inlet and outlet tubes are usually
provided. The inlet tube allows connection between the meter and
the gas probe being monitored. The outlet probe usually has a
hand-held aspirator on it. This creates a vacuum for gases to be
drawn through the meter.
To determine methane content, the manufacturer's instructions
should be followed and the following general steps should be
employed:
5-7
-------
t Set the on/off switch to the on position.
Zero the meter on both ranges alternately by
following the manufacturer's instructions until
the needle remains on zero when atmospheric air
is drawn through the meter.
Connect inlet tube to test probe. The connection
between the instrument and test probe must be com-
pletely air-tight. Clamps should be used to ensure
air-tight conditions at connection of 2 tubes.
Set the "range" switch to the 0 to 5 percent volume
in air scale. Squeeze the aspirator bulb to draw gas
from the test probe into the instrument. When the
needle stabilizes, record the methane reading on the
data form. If the needle goes off the 0 to 5 percent
volume in air scale, set the range swftch to the 0 to
100 percent volume in air scale and take the reading.
If the aspirator remains deflated or does not inflate
within 2 seconds after squeezing, disconnect the inlet
probe from the test probe. If the aspirator bulb then
inflates, the test probe may be clogged and/or saturat-
ed with water. Check the water level and repeat the
procedure described earlier for pressure monitoring.
Before each monitoring round, a calibration gas should be used to
check the accuracy of the instrument. If recalibration or other
repairs are required, the instrument should be returned to the
manufacturer. Regardless of comparisons between calibration gas
5-8
-------
gas and actual readings, the instrument should be returned to the
manufacturer every six months for routine maintenance and repair.
5.1.1.5 Interpretation
To interpret gauge pressure readings and barometer conditions
the following combination of possible field readings are cited:
t Gauge Pressure Positive, Barometer Falling. Under
these circumstances, subsurface pressures are higher
than atmospheric conditions. Monitoring personnel
should get an accurate reading of any methane. This
condition increases the potential for methane migration.
0 Gauge Pressure Positive, Barometer Rising. This con-
dition is unusual and it will exist for only a short
time. Normally, when atmospheric barometric readings
are rising, subsurface pressures lag 'behind, causing
negative subsurface gauge pressures. As long as gauge
pressure is positive, accurate subsurface methane read-
ings are likely.
Gauge Pressure Positive, Barometer Steady. Under these
circumstances, subsurface pressures are higher than
atmospheric. With barometric pressures steady, subsur-
face absolute pressures are likely to decrease to atmos-
pheric levels. Field personnel should be able to take
accurate subsurface methane readings.
Gauge Pressure Negative. When subsurface gas is at a
lower pressure than atmospheric, air will tend to be
5-9
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drawn into the subsurface, diluting gases there.
Thus accurate sampling cannot be made when subsur-
face gas is negative with respect to atmospheric
pressure.
0 Gauge Pressure 0. If this is an accurate reading and
is being measured on the 0 to 0.5 in. range of the
meter it should be a temporary condition. Methane
meter readings should be attempted. If subsurface
gauge pressure remains 0 over several days, the probe
is likely clogged with condensate or moisture. A peri-
static hand pump or wire should be used to unclog the
tubing.
To interpret methane content readings, the following combinations
of instrument readings allow evaluation of subsurface gas
migration potential:
0 Percent LEL, 0 Percent Gas. If these readings are
taken in conjunction with a positive subsurface pres-
sure reading they are an indication that subsurface soils
are truly "clean" of any combustible gases.
1 to 25 Percent LEL, 1 to 1.25 Percent Gas. These read-
ings indicate that some migration of combustible gases
from a SWMU may be occurring, but at sufficiently low
levels that a release is not likely. However, an in-
crease in the methane monitoring frequency may be appro-
priate.
5-10
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25 to 100 Percent LEL, 1.25 to 5 Percent Gas. These
levels of combustible gas indicate that a release has
occurred if measured within any structure on the
facility. They are reason for concern, and contingency
activities should be implemented. These include
increased frequency of methane monitoring, ventilation,
or other emergency response measures.
More than 100 Percent LEL, 5 to 100 Percent.Gas. This
indicates an emergency condition inside a structure.
See Section 4.1 related to emergencies. If methane at
these levels is found in the soil at the property line,
a release has occurred. Some corrective actions should
be implemented in the near future unless the likelihood
of human exposure is remote, e.g., no occupied buildings
within 1/4 mile. If these concentrations are found be-
tween a unit and the property boundary, gas migration is
occurring. Some corrective actions should be planned
and monitoring frequency increased.
5.1.1.6 Record Keeping
The field data form for field gas monitoring included earlier as
Table 5-1 should be completed each time that monitoring activities
for methane and subsurface pressures are performed in the field.
These forms should be collected in a single loose!eaf binder
notebook. Other observations about well conditions, weather
conditions, and contingency actions taken should also be recorded
in the appropriate spaces for "Notes".
5-11
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Field monitoring data should be evaluated by the facility owner or
operator in a timely manner. If a release has occured, EPA per-
sonnel should be notified and appropriate field data forms should
be transmitted.
Chronological logs should be kept for each monitoring well and
each probe in the well. Review of this historical data will in-
dicate trends of migration and will aid in predicting if and where
a release may happen.
5.1.2 Indicator Constituents of Concern
5.1.2.1 General
The monitoring procedures described below are for sampling of
vapor-phase (volatile) organics present in subsurface gas at a
facility boundary or in ambient air within a facility structure.
Several methods of sample collection are acceptable depending on
site conditions, concentrations, and method of analysis. Anal-
ytical methods are not presented in this guidance. Development of
a monitoring program for the indicator constituents of concern,
field sampling, and subsequent laboratory analysis should be con-
ducted by qualified personnel.
To monitor for the volatile organic constituents (VOC's) similar
procedures performed during methane monitoring are appropriate.
For example, probe pressures should be monitored and if negative,
collection of VOC samples should not be performed. If probe pres-
sures are positive, a VOC sample may be obtained.
5-12
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5.1.2.2 Equipment
The following equipment is typically used for collection and
concentration of VOC's onto adsorbent traps. Alternatively,
volumetric samples can be obtained from the well probes or from
within structures. Figure 5-1 illustrates a VOC sampling assem-
bly appropriate for monitoring.
A portable field sampling pump is necessary for VOC
sample collection. The pump should be capable of
operating on 110 volts AC or 12 volts DC. This por-
table vacuum pump should maintain constant flow for
the duration of the sampling and be explosion-proof.
An adsorbent trap appropriate to the VOC being monitor-
ed should be selected. Trap materials, sizes, and adsor-
bent resins are variables that qualified personnel must
determine to fit site conditions. Typically, sample
traps are glass or stainless steel.
A glass-tube, variable-area flowmeter (rotameter) can be
used to accurately set and maintain the gas flow rate
through the sample trap. The rotameter is available
through a variety of manufacturers. It should be capable
of accurately measuring flows up to 30 to 50 ml per
minute and include a standard needle-type metering valve.
The rotameter is used during the calibration procedures to
set the metering valve at the desired sampling flow rate.
5-13
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Gas Probe
Simol 119 ?robe
Toggle
Shut -off
N/l
/N
/alve
Re
tameter
Togg
Shut-
N/^
L/N
Valv
e
off
e \
Trap
\
Teflon
Putnp
Tub ino
F
iuflble
owneler
Figure 5-1
VOC Sampling Assembly
5-14
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Throughout the sampling period, the rotameter serves as a
check that the desired flow rate is maintained.
A gas chromatography-type bubble flowmeter can be used
to accurately calibrate the sampling assembly. While
the sampling pump is intended to deliver constant flow
rates from the gas probe, pressure variations may occur
throughout the sampling assembly. During the initial
calibration procedures, the bubble flowmeter is used to
determine the gas flow rate through a calibration trap.
During actual sampling, several readings are taken from
the bubble flowmeter to accurately measure the flow rate
through the sample trap. A 10-ml bubble flowmeter is
typical for the volatile organic gas samples.
0 Toggle-type shut-off valves can be placed upstream and
downstream of the rotameter. These sfiut-off valves are
used to test for a complete gas seal along the sampling
assembly prior to sampling and to stop gas flow to the
sample trap after the desired sampling volume has been
attained.
A spare parts box is recommended to supply the necessary
replacement fittings, extra bubble flowmeter, miscellan-
eous tools, and special plumbing required. Assorted fit-
tings and tools, such as screwdrivers, adjustable and
open-end wrenches, tubing cutters, Teflon tape, etc.,
should be available.
5-15
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Teflon tubing (1/8 in. OD) is typical for small flow
rates. The tubing is used to make connection between
the gas probe, the rotameter, and the sample trap.
Larger tubing (1/4 in. OD) may be necessary between
the sample trap, the pump, and the bubble flowmeter.
5.1.2.3 Preparation of Sample Traps
The sampling traps must be prepared in the laboratory prior to
field collection. This includes proper packing, sealed fittings,
and "bake-out" of extraneous organic compounds. Specifically,
virgin adsorbent resin should be extracted in a Soxhlet apparatus
for a minimum of 18 hours prior to use. The sorbent should then
be dried in a vacuum oven and subsequently sieved to provide a
desired packing fraction. This fraction is used to pack the traps
which are then conditioned with an inert gas flow for an appro-
priate time. Afterwards, they should be capped and stored.
More than a single sample should be taken at each field sampling
site to ensure that a sample will be available for analysis.
Sampling with adsorbent resins results in a "one-time" analysis
opportunity on the laboratory instrument. When a sample is
desorbed and run through the gas chromatography/mass spectrometer
(GC/MS), that sample cannot be reanalyzed. At a minimum,
duplicate samples must always be taken for each sampling site (gas
probe). The purpose of these duplicate field samples are two-
fold: (1) if a sample is lost, either during shipping or
laboratory analysis, then the field duplicate serves as an
5-16
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identical back-up sample, and (2) for quality assurance purposes,
duplicate pairs should be periodically analyzed by GC/MS to
establish the precision of the sampling technique.
The size of the sample depends on the type of sample trap and the
site being sampled. Often landfill and subsurface gas streams
yield complex matrices of volatile organic compounds. Highly
concentrated gas streams can saturate the adsorbent resin, and
breakthrough occurs during sample collection. Thus, when samp-
ling a previously unsampled site, or a site of high variability,
samples should be taken at different volumes to establish the
optimum sample volume for an individual site.
A permanent log of field sampling must be maintained throughout
the collection period. A recommended format "For data recording is
shown in Table 5-2.
5.1.2.4 VOC Monitoring
To accurately sample the indicator constituents of concern, the
following general steps should be employed for collecting and
concentrating the VOC's.
Libel and set aside a sample trap to be used for cali-
bration purposes only. Only one calibration trap is
necessary per sampling round. This calibration trap is
used to set the desired flow rate of gas through the
sampling assembly at every site (gas probe).
5-17
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Table 5-2
VOC Field Data Form
Date:
Weather:
Barometric Pressure:
Air Temperature (°F):_
Sampl ing Personnel :_
Trap
Number
Time of
Day
(hr :min)
Elapsed
Time
for 10 ml
(sec.)
Fl ow
Rate
(ml /min)
Rota-
Meter
Reading
(ml /min)
Total
Sample
Time
(min :sec )
-_
Sample
Vol ume
(ml)
Sar
Well
No.
nple Sit
Probe
No.
:e
Depth
(ft)
i
I
5-18
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Prior to sampling, determine the volume and number of
each type of of sample to be taken. Set aside the
necessary number of traps, including traps for dupli-
cate samples. In addition, set aside a sample trap
and label it "field blank". For every 25 sample traps,
two field blank traps should be set aside. Handle all
of the traps in a similar manner. Use a Ziplock plastic
bag to store all traps as samples are collected.
The purpose of the field blank is to ensure that no
sample contamination occurs during sample shipment and
handling. The field blanks are analyzed in the labora-
tory, but will not have subsurface gas passed through
them.
Leak test the gas sampling using the following steps:
- Ensure that all connections are free of dirt and
moisutre.
- Install the calibration trap. Use appropriate fit-
tings for all connections.
- Close the toggle valve on the rotameter's inlet side
(nearest the gas probe) and turn on the sampling pump.
- Check to see if flow is detected by the bubble flow-
meter when the needle control valve of the rotameter
is fully opened.
- If flow is detected, check the entire gas sampling
system for leaks. Replace Teflon tape or fittings at
suspected leaks.
5-19
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- As an additional check, test the second toggle valve
(on the rotameter's outlet side) in the same manner.
- If a no-flow condition is detected using the bubble
flowmeter, the system is free of leaks. Leak test the
gas sampling assembly at each monitoring well.
The gas flow rate can be calibrated as follows:
- Close only the second toggle valve (on the rotameter's
outlet side)- The first toggle valve remains open.
- Install the calibration trap and turn the sampling
pump on.
- Open the second toggle valve to allow subsurface gas
through the sampling assembly.
- Adjust the flow using the needle control valve on the
rotameter so that a desired flow rate is established
from bubble flowmeter readings. As~a minimum, tripli-
cate readings of the bubble flowmeter (start time at
0 ml and end time at 10 ml) should be used to establish
the flow rate.
- Close the second toggle valve (rotameter's outlet side),
shut off the sampling pump, close first toggle valve,
and remove calibration trap.
- Do not change setting of the needle control valve on
the rotameter while the series of volatile organic
samples is being collected, but do calibrate the
sampling assembly for each probe.
Sample collection should be completed by following these
steps:
5-20
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- Select a sample trap, note the identification number
in the field notebook, and install the trap in the
sampling assembly. Determine and record the required
sample volume.
- With the first toggle valve open and the second toggle
valve closed, turn on the sampling assembly. Open the
second toggle valve and start the stopwatch (one of two)
to measure elapsed sampling time.
- Time and calculate the flow through the bubble flow-
meter using the second stopwatch. Calculate the time
to obtain the required sample volume (if different
from the previous calibration test).
- Monitor the gas flow through the rotameter by visually
inspecting the scale. Close the second toggle valve
and stop the stopwatch at the time needed to obtain the
required sample volume. Shut off the pump.
- Close the first toggle valve. Remove and securely cap
the sample trap. Place trap in Ziplock bag.
- Reset the elapsed time clock/stopwatch. Repeat the
above steps for all samples.
All traps and sampling devices should be subject to
similar handling, shipping, and storage conditions. A
chain-of- custody from sample trap preparation through
GC/MS analysis and ultimate "bake-out" must be established
and strictly enforced. The chain-of-custody should be as
streamlined as possible. That is, the samples should not
5-21
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change hands frequently, should not be shipped using
unknown conditions, and should be received and analyzed in
an expeditious manner.
5.1.2.5 Handling and Record Keeping
The VOC samples collected using sorbent traps must be capped with
proper fittings. Each sample should be labeled appropriately with
sample number, sample location, date, etc., and immediately stored
in a cool, dry container. Care should be taken with regard to the
integrity of the airtight seal and the storage location to avoid
direct contact with heated surfaces. If the ambient temperature
is expected to rise above 80°F (26.5°C) during shipment or
storage, the samples should be placed in a container that can keep
the samples below that temperature.
Sample analysis should be performed as soon as possible following
collection. All analyses should be completed within seven days
following receipt of samples in the laboratory -
5.1.2.6 Interpretation
To interpret the analytical results from VOC sampling, the
laboratory should convert compound-specific concentrations to
volume parts per million (Vppm). Levels detected can then be
compared with compound PEL's (in Table 1-2 or 1-4) to determine if
a release has occurred. As for methane, if a release is
documented, EPA personnel should be notified and appropriate field
and laboratory data forms should be transmitted.
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5.2 CONTROL SYSTEMS
If a release has occurred, a subsurface gas control system may be
required to prevent potentially harmful migration into facility
structures or beyond a facility boundary. Selection or evalu-
ation of a system to control subsurface gases must consider
several site-specific characteristics including:
o Landfill, ground water, and bedrock depth.
o Age, composition, and moisture content of the waste.
o Distance to property boundary and facility structures.
o Type and location of proposed on- and off-site develop-
ments.
o Soil characteristics.
o Characteristics of cover material and final surface
treatment.
Control systems can be either passive or active. In some in-
stances, passive control systems such as interceptor trenches or
barriers can provide adequate control for migrating subsurface
gases. However, in general, when a facility surface or adjacent
property is developed for public use, passive control methods are
used as backup for an active subsurface gas control system.
5.2.1 Passive Systems
Two types of passive systems are commonly used to control gas
migration from landfills and can be employed at other sites closed
as landfills or possibly for underground tanks.
5-23
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Passive vents provide a permeable, low resistance pathway to the
surface for gases moving laterally from an SWMU. The intercepted
gases are vented safely into the atmosphere.
Installation of passive vents can be by excavation of a trench,
subsequently backfilled with gravel or crushed stone. An alter-
native is a series of well points, essentially performing the same
venting function. However, well points are not always an effec-
tive control system. Passive vents are typically installed around
landfill perimeters to protect adjacent areas as shown in Figure
5-2.
A passive trench barrier shields or isolates areas from subsurface
gas migration by installation of an impermeable material. A
passive trench barrier promotes gas venting thorough the more
permeable soil adjacent to the trench by blocking the lateral
movement of gas and forcing it to move upward to the atmosphere.
This system is created by excavating a trench and backfilling it
with either an impervious synthetic liner or compacted clay soil.
The advantages of passive vents and trench barriers include:
1. Operation and maintenance costs are low and initial
capital costs are generally lower when compared to
active gas control systems.
2. The effectiveness of passive control system is not de-
pendent on power supply and mechanical dependability.
5-24
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Plan View
BUILDINGS
PASSIVE
TRENCH VENT
REFUSE
Section View
LANDFILL GAS VENTED"
TO. ATMOSPHERE
y^t
&£%S&8-3
> 00~ V^V'O.
nC .'. ' \?L'»^-S?--:?>.LV!?l
VENT PIPE
LANDFILL
GAS FLOW
PASSIVE
TRENCH VENT
£&$.$>$&
.x-j IMPERVIOUS LAYER OR GROUND WATER TABLE
Figure 5-2
Gas Migration Control Vents
5-25
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3. Passive systems do not pull surface air into the waste
mass, posing a potential combustion hazard. Rather they
only collect and vent migrating subsurface gases.
4. Construction methods are relatively simple.
5. A properly designed and installed passive system can
provide effective gas control for many years.
Some disadvantages of passive systems include:
1. Due to construction equipment limitations, passive trench
barriers become prohibitively expensive at depths greater
than 30 feet.
2. Passive systems alleviate migration but do not serve as
control devices unless they extend to the ground water
table, to another impervious zone, or below the greatest
depth of migrating gas.
3. Passive systems may become covered over at the surface.
This restricts their effectiveness and does not provide
a permanent pathway to the surface.
5.2.2 Active Systems
Active control systems include extraction well or trench systems
and air injection well or trench systems. Perimeter extraction or
injection well and trench systems are used to control 1 off-site
gas migration. Well extraction and subslab extraction or
injection systems are designed to protect on-site facilities.
5-26
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5.2.2.1 Perimeter Extraction Wells
These systems consist of a series of vertical wells installed
along the perimeter of the SWMU. Extraction wells are connected
by a common horizonal header pipe to a suction blower. Extracted
subsurface gas is subsequently vented, flared, or incinerated.
Perimeter extraction well systems are typically installed in sites
20 feet or deeper where there is little or no distance between the
limits of deposited refuse and the property line or off-site
facility to be protected.
The electrically driven suction blower for the perimeter system
creates a negative pressure in the extraction well which is
extended as a negative zone of influence into the waste around
each well. Wells are spaced such that their zones of influence
overlap. Gas generated or migrating into an area of influence is
drawn to the wells from the soil surrounding the SWMU.
To verify system effectiveness, monitoring wells are typically
installed along the property line or between the wells and the
area requiring protection. Data collected from the monitoring
wells are used to adjust extraction rates to obtain the desired
control.
Venting collected gases may pose odor and/or air quality problems;
therefore a flare may be required to combust collected gases. The
cost for perimeter extraction well systems is dependent on depth
5-27
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and density of the waste mass, and any flaring requirements for
recovered gases.
5.2.2.2 Perimeter Extraction Trench.
These systems consist of a gravel-filled intercept trench
containing perforated pipe. The perforated pipe is connected by
laterals to a collection header and suction blower. Collected
gases are vented or flared. Perimeter trench systems are
installed in natural soils, and are applicable to shallow SWMU's,
20 feet deep or less. The suction blower creates a negative
pressure in the intercept trench which has an influence zone
extending toward the deposited wastes. Gas migrating into this
zone is drawn into the perforated pipe, and subsequently vented or
flared at the blower station. The gravel trench is sealed at the
surface; it extends vertically from the ground"surface down to the
refuse depth or to ground water, and laterally along the SWMU
perimeter. Laterals connecting perforated pipe to the collection
header contain valves to allow adjustment of flow. As with other
control systems, monitoring wells to verify system performance are
installed between the intercept trench and the property line or
other location requiring protection.
5.2.2.3 Perimeter Well Injection.
These systems consist of vertical wells installed in natural soils
between deposited wastes and the property line or area requiring
protection. Perimeter well injection systems are typically
selected for SWMU's 20 feet deep or greater and having available
5-28
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undisturbed soil between the limits of wastes and the area
requiring protection. Injection wells are connected by a common
header pipe to a blower. Air is forced through header lines and
wells into the soils surrounding the landfill site, therby
creating a zone of positive air pressure around each well.
Injection wells are spaced such that their zones of influence
overlap, creating an air curtain along the perimeter- Gas that
migrates toward this barrier is blocked by high positive pressures
produced by the induction blower.
Monitoring wells are installed between the injection wells and the
property line or area being protected. Adjustment of wells to
evenly distribute forced air is based on test data from the
monitoring wells.
Well injection systems provide gas control for deep landfills.
However, they do require some undisturbed ground located between
the limits of the wastes and the area requiring protection. Since
this type of system is installed in natural ground, there are no
problems with differential settlement. Forcing air into the
system eliminates condensate and odor problems associated with
extraction systems.
Reduced permeabilities in natural soils relative to the deposited
wastes limit the influence area for an air injection well. More
wells may be required to protect a given perimeter distance than
5-29
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would be required for the equivalent extraction well system
located in refuse. However, air injection systems do not require
a vent or flare for odor control.
5.2.2.4 Perimeter Injection Trench
These systems are similar in design to the extraction trench
described above. However, air is forced into the system creating
a positive air curtain barrier in the gravel trench. Air
injection trenches are installed in natural soils around shallow
SWMU's (20 feet deep or less), having some distance available
between the refuse limits and the area requiring protection.
Air pumped into the injection trench creates a positive pressure.
This pressure blocks gas migration resulting from convective
forces, and dilutes gas movement resulting from diffusive flow.
5.2.2.5 On-Site Extraction Well.
Control of subsurface gas around structures located directly on
the SWMU requires systems installed as an integral part of the
structure. These systems must be designed to accomodate the
differential settlement often experienced with landfilled wastes.
The on-site extraction well system consists of vertical extraction
wells placed in refuse that are connected to a suction blower.
Collected gas is vented or flared. This system is recommended
when buildings are founded on piles. However, it is also
applicable for buildings on floating foundations.
5-30
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These well systems are typically installed in deep landfills with
high generation rates. Extraction wells are located around
proposed on-site structures (or the area designed for protec-
tion). The blower creates a negative pressure in the wells.
Wells are spaced such that the area of influence of each well
overlaps, providing a continuous draw on gases generated or
migrating into the controlled area. The extraction rate is. set to
be slightly higher than the gas generation rate (if possible) to
prevent overpulling and subsequent air infiltration in the waste
mass.
The well and header systems must be designed with flexibility to
accommodate the differential settlement that can occur at landfill
SWMU's. Subslab monitoring probes are used to verify system
performance and to aid in well adjustments. A~neutral or slightly
negative pressure is maintained at the ground surface to preclude
gas venting or excess air movement into the waste mass. A subslab
or sandwiched membrane liner is typically used as backup to
provide protection during extraction system downtime. Automatic
sensors can be installed inside the structures to sound alarms or
actuate fans if gas concentrations reach levels indicating a
release.
Deep well extraction systems, when combined with a backup passive
or alarm system provide effective subsurface gas protection for
on-site structures. Since these systems are located within the
deposited wastes, the associated problems of settlement, conden-
5-31
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sate removal, odor, and maintenance exist. Access for maintenance
and adjustment of wells must be readily available.
5.2.2.6 Subslab Gravel Bed Injection or Extraction
These systems consist of a network of perforated horizontal
collection pipes installed in a gravel bed beneath the floor slabs
of on-site structures. The gravel bed system is not recommended
for pile-supported structures. Collection pipes are connected by
header pipes to a blower where extracted gas is vented or flared.
For the injection system, air is pumped to the pipes in the gravel
layer.
Gravel bed systems are typically installed under structures
located atop shallow fills with low gas generation rates. The
suction blower creates a zone of negative pressure in the gravel
bed. Gases migrating into this zone of influence are collected,
and either vented to the atmosphere or flared. Monitoring and
backup membrane systems are installed similar to those for the
deep well extraction systems described above. Because subsurface
gases may contain high concentrations of methane and be present in
the subslab gravel layer, the concentration gradient necessary for
diffusion will still exist. A low-permeability membrane, such as
a chlorinated polyethylene material, is necessary to insure
adequate protection of the structure.
In the injection system, air is forced into the gravel bed
providing a zone of higher pressure and dilution. In some in-
5-32
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stances, a backup membrane may not be necessary in this type of
control system.
Gravel bed systems have the disadvantage of limited access for
repair. Settlement may pose problems depending on the age, depth,
and composition of the deposited wastes.
In summary, both passive and active systems can be used
effectively for control of gas migrating off-site to adjacent
properties, or as an integral part of a facility to be constructed
directly on the SWMU. Active systems involve mechanized equip-
ment. This fact, coupled with the dynamic nature of gas genera-
tion and migration, requires that regular maintenance and
monitoring be scheduled for these control systems.
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