Guidance for Design,
Installation and Operation of
Soil Venting Systems
Working together for
a cleaner tomorrow
Wisconsin Department of Natural Resources
Emergency and Remedial Response Section
July 1993
PDBL-SW185-9S
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Prepared by:
Wisconsin Department of Natural Resources
Emergency and Remedial Response Section
P.O. Box 7921
Madison, WI 53707
PUBL-SW185-93
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Errata sheet for the Guidance for Design, Installation and Operation of Soil
Venting Systems, through February 7, 1994.
Additional information, changes, clarification and errata include the
following:
• All wells, including air extraction and injection wells need to be
abandoned after the project is complete. This information is included
on Page 2 under Wjs. Admin. Code NR 141 but was inadvertently left out
of Subsection 5.4.
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1
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e«id«nc» for Soil Tratiaf Syttmt
P«g«
Table of Contents
1 . 0 Introduction . ......
1.1 Purpose ..... ....." ...... ..... • • ..... . 1
1.2 Scope of Soil Venting and Bioventing." ...... . ..... • • • 1
1.3 Permitting, DNR Regulations and Related Guidance ..... ' ' ' ' o
1.3.1 LUST, ERP and Superfund Program Requirements, ' '.'.'.'.'' I
1.3.2 Bureau of Air Management. .... ..... *
1.3.3 Bureau of Wastewater. ... ''''••'*•• ..... 2
1.3.4 Department of Industry, Labor and Human Relations.' .' .' .' .' 4
£*? -Technical Considerations and Sfi-» Charart- .^^.^^ ,
2.1 Theory. .... ...... - **•*• ....... 3
2.2 Site Characterization. ....'.' ......... ....... 5
2.2.1 Contaminant Characterization •••••,.... ..... 8
2.2.2 Geological Factors ....... .*.'.".'"' ........
2.2.2.1 Horizontal Permeability. ..... * " ' ,!!
2.2.2.2 Stratification and Vertical ' ......
Permeability. ...
2.2.3 Hydrogeologic Factors. . . ............. J°
2.3 Other Site-Specific Factors. ......'.'.'.** ........
Treatabilifry or Pilot Ta.'t.^C| ....
3.1 Laboratory Treatability Tests ' ........ 12
3.2 Pilot Tests .......... ......... • • • ..... 12
3.2.1 Purpose of a Pilot'lest.' .*.'.'""-"""*;'" ...... • 12
3.2.2 Conducting a Pilot Test. ... ...... •••..... 12
3.2.3 Analytical Monitoring Methods for Pilot Tests ' ...... II
3.2.3.1 Sites With Petroleum Product .......
Contamination. . .
3.2.3.2 Sites With Non-Petroleum' ""*''""".'*'.•*
Contamination.
3.2.4 Reporting Results From Pilot Tests ':••••. ...... "
3 . 3 Alternative to a Pilot Test ..... ' ....... 16
.......... . • ..... ... 18
of
4.1 Well Placement and Air-Flow Modeling
. Achievable Air-Flow Ra^es, "and^ir^ission " '
4.3 Well or Trench ' Des igA. ' ". \ [ [ \ [ [ ] ........ ...... 24
4.3.1 Vertical Extraction Wells. . . ...... ........ 27
4.3.2 Horizontally Screened System DesigA. ' ! ......... ,n
4.3.3 Gas Probes ..... ' ' ...... ..JO
4.4 Manifold and Instrumentation! ;;•'•••.••••*•••.••••. 31
4.5 Water Trap ............ ...... ••-..... 31
4.6 Blower (or Vacuum Extractor) Type and Size *•*•••. ..... 35
4.7 Emission Control Devices. . ' ....... " '
4.8 Air Injection. ....... ....... " ..... " * ' ..... 36
4.8.1 Passive Vents ..... ...'."." ...... ...... • ,?
4.8.2 Forced Injection ..... ... ..... <; ....... »o
4.8.3 In Situ Air Sparging. ..."!.".'! ..... ....... H
4.9 Other Design Considerations ..... «•••....... jy
4.9.1 Surface Seal .......... •.•-««•.•...... jy
4.9.2 Stagnation Zones. ......'.I....'. [ , ', ' ' ' ' ' f?
4.9.3 Vacuum- Enhanced Product Recovery. . . . . . ....... 41
4.9.4 Groundwater Extraction from Air -Extract ion 'wells;.' .".'!*" 42
4.9.5 Enhanced Biodegradation of Petroleum Compounds in
SoilV ................ ....... ...:.. 42
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Guidance for Soli V«otin* Symttut Fag* ii
4.10 Soil Venting System Design Report. ......' 43
5.0 Operation of a Soil Venting System. ..-....•....' 46
5.1 Overview. 46
5.2 As-Built Submittal ,. , . 43
5.3 Reporting ., 49
5.4 Case Close Out 50
6.0 References 53
Tables,.
Table 1-1 Guidance Documents Related to Soil Venting 3
Figures.
Figure 4-1 Vacuum at a Distance From a Single
Air-Extract ion Well 20,. 21
Figure 4-2 Typical Air-Extraction Well Design with
Above Grade Manifold 28
Figure 4-3 Wellhead Details with a Buried Manifold 33
Figure 4-4 Combined Performance Curve for Three Types of Blowers. . 37
Figure 4-5 Typical Air-Flow Patterns 40
Attachments.
Attachment 1
Guidance on air sampling and emission monitoring for LUST
soil and groundwater remediation projects with a synopsis of
air regulations.
Acknowledgments. In addition to many DNR employees, the following
individuals also reviewed and commented on this document:
Kelton Barr, C.P.G. - Geraghty & Miller, Inc.
Chi-Yuan Fan, P.E. - USEPA Risk Reduction Engineering Laboratory.
David Kill, P.E. - Recovery Equipment Supply Inc.
Michael C. Parley - Vapex Environmental Technologies, Inc.
Tom R. Peargin, R.G. - Chevron Research and Technology Company.
This document may not represent the views of all reviewers. The DNR thanks
the reviewers for donating their time and input.
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soil Vmtiac
Acronyms,
CPVC
DNR
DRO
EPA
ERF
ERR
FID
GAG
GC
gpm
GRO
Kh/Kv
LUST
FID
PVC
PVOC
scfm
TPH
VOC
Chlorinated polyvinyl chloride. Material commonly used for
pipe.
Wisconsin Department of Natural Resources.
Diesel Range Organics
U.S. Environmental Protection Agency.
Environmental Repair Program of the DNR.
Emergency and Remedial Response Section of the DNR Bureau of
Solid and Hazardous Waste Management.
Flame lonization Detector.
Granular Activated Carbon.
Gas Chromatograph
Gallons per minute.
Gasoline Range Organics
Ratio of horizontal permeability to vertical permeability.
Leaking Underground Storage Tank Program of the DNR.
Fhotoionization Detector.
Polyvinyl Chloride. Material commonly used for pipe, well
casing, and well screens.
Petroleum Volatile Organic Compounds.
Standard Cubic Feet per Minute.
Total Petroleum Hydrocarbons. As used in this document TPH
refers to tests for gasoline range organics and diesel ranee
organics. °
Volatile Organic Compound.
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Guid«ne. lex Sell VmUag Syttmt
1.0 Introduction.
This guidance document is intended to aid environmental professionals in
designing soil venting systems for soil contaminated with volatile organic
compounds (VOCs). It provides information to Department of Natural
Resources (DNR) staff for efficient and consistent oversight and review.
This document should be read with the existing DNR Guidance for Conducting
Environmental Response Actions, specifically Chapter 7 (Site Investigation)
and when available, Chapter 8 (Remedy Selection).
1.1 Purpose. .
This document is a guide to using soil venting as a remediation technology.
Soil venting is a technology that uses air to extract volatile contaminants
from contaminated soils. The technology is also known as soil vapor
extraction, in situ volatilization, in situ vapor extraction, in situ air
stripping, enhanced volatilization, in situ soil ventilation, and vacuum
extraction. The term bioventing has been applied to soil venting projects
when biodegradation is a significant part of the remediation process and/or
biodegradation is enhanced with nutrient addition.
Soil venting is a multi-disciplinary process. The designer should have a
working knowledge of geology and basic engineering to design an optimal
system. A basic knowledge of chemistry is also necessary to develop a
quality sampling and monitoring plan.
This document is intended as general guidance. Because each site has
unique characteristics, it may be necessary for system designers to deviate
from the guidance. The DNR acknowledges that systems will, deviate from
this guidance when site-specific conditions warrant. Vilheii deviations
occur, designers should document these differences in their work plan to
facilitate DNR review. For additional information on the DNR's permitting
and regulatory requirements, please refer to Subsection 1.3 in this
document.
This document discusses the basics of soil venting system design. Refer to
the publications listed in Section 6 for more detailed discussions of soil
venting systems. A more complete list of articles is included in the
reference and the bibliography sections of the U.S. Environmental
Protection Agency (EPA) Soil Vapor Extraction Technology, Reference
Handbook (1991(a)).
1.2 Scope of Soil Venting and Bioventing.
Soil venting generally works well with gasoline and some common solvents
such as trichloroethene and tetrachloroethene. Remediating heavier
hydrocarbons (jet fuel, kerosene, and diesel oil) with a soil venting
system may be possible, but the rate of remediation is very slow compared
to more volatile compounds. Enhanced biodegradation takes place through
oxygen delivery during soil venting (Hinchee and Miller, 1990, Miller,
1990). Unusual site conditions, such as an inability to excavate, are
necessary to make soil venting technology the best alternative for the
heavier hydrocarbons. Soil venting is not appropriate for contaminants
that do not volatilize or aerobically biodegrade.
Soil venting may be used with other cleanup technologies, such as steam
stripping, groundwater extraction, product recovery, air sparging
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for Boil Vcatia* Symtmm Pag. 2
(saturated zone) .and heated air injection (unsaturated zone).
Soil venting is an effective technology to prevent vapor accumulation in
buildings, and soil is also remediated in the process (Knieper, 1988).
Using soil venting to remove vapors from a building can be considered an
emergency or interim remedial measure. In such cases, a pilot test is not
necessary if the operator has received Bureau of Air Management approval.
1.3 Permitting, DNR Regulations and Related Guidance.
I
Refer to Table 1-1 for more information on permitting and related guidance
documents.
1.3.1 LUST, ERF, and Superfund Program Requirements.
Submittal Contents . Recommended Leaking Underground Storage Tank (LUST) ,
Environmental Repair Program (ERP) and Superfund program submittal contents
are listed in Subsections 3.2.4, 4.10, 5.2, and 5.3.
Wjs. Admin. Code MR 141. Air- extraction well designs and gas probes do not
need DNR's preapproval under Chapter NR 141. Designers must submit boring
logs and well construction diagrams in accordance with NR 141.23 after well
installation. Designers must also abandon wells and gas probes in
accordance with NR 141.25 after project completion.
Wastes
with DNR guidance on investigative wastes.
1.3.2 Bureau of Air Management.
Designers should handle drill cuttings in accordance
Wig. Admin. Code 406. 445. and Aio, Soil venting systems must comply with
any state emissions standards. Chapter NR 445, Wisconsin Administrative
Code, sets hazardous air emission standards for atmospheric pollution
sources. 'Chapter 406 sets requirements for air permits and Chapter NR 419
includes additional requirements . .Air emission limits, reporting, methods
of monitoring, and a summary of air regulations for petroleum sites are
discussed in Attachment 1. See Chapter NR 445 for a complete listing of
compound- specific limits for other sites. The total volatile organic
compound (VOC) limit in NR 419.07 (4) (b) takes precedence over the hourly
limits for individual compounds in Chapter NR 445. Designers may need a
permit from the Bureau of Air Management prior to using control or air
treatment devices. A pilot test may be necessary to comply with Bureau of
Mr Management requirements.
Note: If an air permit is necessary, the application for the permit should
be submitted early to reduce or prevent project delays. An air permit
takes a minimum of two to three months after a COMPLETE application is
submitted.
Form 4400-120 n Designers must complete Form 4400-120 and receive DNR
approval prior to operating a soil venting system at a LUST site. Data
from a pilot test may be necessary to complete the form.
1.3.3 Bureau of Wastewater.
Water Disposal . Groundwater pumped from air-extraction wells and the
accumulated water in a water trap must be disposed of in accordance with
state and/or local permits. Local municipalities regulate discharges to
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Cuid«nc» £ot Sou Vmtlag Sritm*
Table 1-1
Guidance Documents Related to Soil Venting
Guidance
Documents i
Air Emissions
Drilling, Well
Construction,
and Abandonment
Vapor Well
Labeling and
Color Coding
Condensate
Disposal
Investigative
Wastes
I^M^M^MM
Electrical
Safety
NR 406,
NR 419,
NR 445
NR 141
ILHR 10
Various
DNR Rules
Various
DNR Rules
Various
DILHR
Rules
April 5, 1991
Memo for LUST
Sites2
None for Other
Sites
None Specific to
Soil Venting
Systems
None
Guidance for
Treatment of
Groundwater and
Other Aqueous
Waste Streams
""^"^""^^""^"•^•^•••"••^•^™
January 14, 1993
Memo3
DILHR UST/AST
Program Letter
10, May 25, 19934
DNR
District
Air
Management
Staff
DNR
District
ERR Staff
DILHR
nt
£
n.cj.e.(.ence
Sections
Subsections.
1.3.2,
3.2.3, 4.2,
4.7 and
5-3
Subsections
1.3.1, 4.3
and 5.4
Subsection
DNR
District
Wastevrater
Staff or
Local POTW
DNR
District
ERR Section
1.3.4
Subsections
1.3.3, 4.5
and 4.10
DILHR Staff
and/or
Local
Building
[nspectors
Subsections
1.3.1 and
4.3
Subsections
3.2.2, 4.4
and 4.6
Notes:
(1) Guidance
document
(2) Included
(3)
(4)
D°CUmentS refers to
documents other than this
as Attachment One.
Jnteriffl Guidelines foe the
of
Guidance titled Design Criteria for Process Equipment Buildings
Associated with Environmental Remediation of UST/AST Sites
«S !£ed £ Attf ^t Two to the Guidance on Design, Installation
and Operation of Groundwater Extraction and Product-Recovery
Systems. *
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GuicUno for Soil VnUng Sy«t«M •
sanitary sewers. A Wisconsin Pollutant Discharge Elimination System
(WPDES) permit is necessary for storm sewer or surface water discharge.
See Guidance for Treatment Systems for Groundwater and Other Aqueous Waste
Streams for a further discussion of permit requirements.
1.3.4 Department of Industry, Labor and Human Relations.
TLHR 10. ILHR 10.41 covers color coding for flush mount well covers for
groundwater monitoring wells and vapor wells.
Electrical Safety. See DILHR's Design Criteria for Process Equipment
Buildings Associated with Environmental Remediation of VST/AST Sites, which
is included as Attachment 2 to Guidance- on Design, Installation and
Operation of Groundwater Extraction and Product Recovery Systems,
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Cuidnc* tor Soil V«ntinj Sytt .
Pag* 5
Cpnsideraflons and Site Chm-««.«-»rf
2.1 Theory.
an»« dJSC^Ss1ion of the the<»7 and dynamics of soil venting.
Jon ventin8 • *»*»**>&;*» encouraged to review polished literature on
soil venting. See Section 6 for a list of selected references.
ll\\ ^lnSv, removesvVOCs fro» "ils by creating an airstream through the
soil that enhances the volatilization of the VOCs and act-s as a carrier
008- S0il Venting alS° ^^ ~"bic ^ode-datio
Actors? af follows :aCti°n/deStrUCti0n *" C°ntrolled ^ a T «f
Air-flow Rate. The rate of air flow controls the advective
transport of the VOCs from the subsurface. Soil permeability
is a major subsurface physical limitation associated with air-
flow rate. Other important factors include the number of air-
extraction wells, extraction well placement, size and type of
blower, the amount of vacuum applied, and the depth of the
water, table. Subsection 4.2 discusses air-flew rates to a
well. Shan et al. (1992) and Baehr et al. (1989) discuss
.f^5£?\jf 1£ fl°W t0 an air-e*<=raction «ell. Johnson et
al. (1990) also discusses a method to estimate the air- flow
rate based on permeability that is useable for initial design
estimates . °
Air-flow rates are less critical to the biodegradation process
rJ!c Y^^S Astern achieves the most volatilization at high
rates of air flow; a bioventing system may operate at a much
slower air-flow rate, possibly as much as an order of magnitude
less (see biodegradability below) . B'"-«iae
«Sr?° "T"^?1" Th^ remediation rate in highly permeable
soils is primarily controlled by the rate of advection.
If the unsaturated zone is heterogeneous (I.e. fine-grained
soils mixed with coarse-grained soils) , the extraction is
dependent on the diffusion rate of the contJiminants from the
fine grained soil matrix into the coarse -grained soil matrix.
The extraction rate at sites with fractured clay till or
fractured consolidated deposits is also dependent on the
diffusion rate because the VOCs diffuse out of the soil or rock
matrix into the fractures, where advective flow extracts the
SoU Moisture. The air- flow rate through soil may decrease if
soil moisture occupies void spaces, making them unavailable to
advective air flow. High moisture in the capillary fringe zone
reduces the effective porosity to air flow near the water
table, and may retard the extraction of the contaminants from
the capillary fringe. The highest levels of soil contamination
at a site are often near or within the capillary fringe. This
occurs because the contaminants often collect at the top of the
water table. For these reasons, the zone that is most
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Ouidrac. for Soil Vwtiag Sy>t«M ' *•»• 6
difficult: to remediate (because of air flow patterns) is also
the zone that often has the highest contamination. Pumping
groundwater to drop the water table may expose more
contaminants to the air flow that were formerly submerged. See
Subsection 4.9.4 and Guidance for Design, Installation and
Operation of Groundwater Extraction and Product-Recovery
Systems for a further discussion of groundwater extraction.
Soil moisture is necessary for maximizing biodegradation. A
moderate level of soil moisture is necessary to maintain
viable, aerobic bioactivity. Some practitioners propose that
soil moisture should be in the range of 40 to 60 percent of
field capacity, others propose that moisture should be between
50 to 75 percent of field capacity. If soil moisture, drops
significantly below this range, the activity and even the
microbial population density can drop significantly.
The vacuum that is applied in the subsurface lifts
the water table. The effects of upwelling are greatest near
the extraction wells, where the vacuum levels are the highest.
If contaminants are submerged (below the water table) by
upwelling, the effectiveness of soil venting is reduced.
Stagnation Zones. Subsurface structures and/or multiple air-
extraction wells can result in stagnation zones. These; zones
are areas that have no or minimal air flow through the soil.
•The effectiveness of a soil venting system is minimal in these
zones. Stagnation zones most often occur at locations in
between two or more air -extract ion wells that operate at a
relatively constant rate, but it can also occur if subsurface
structures block the air flow. The reduced air -flow rate
through these zones reduces the contaminant extraction rate
from these zones. These zones may also be zones of anaerobic
conditions (see biodegradability below) .
Vapor Pressure. Vapor pressure is a critical factor in
assessing the ability of a soil venting system to volatilize
the contaminant in the soil. Generally, the higher the vapor
pressure, the more likely a soil venting system will extract
the contaminant from the spil. The vapor pressure of the
contaminants are highly temperature .dependent; higher
temperatures increase the vapor pressure and the rate of
volatilization. A vapor pressure of 1.0 mm Hg at subsurface
conditions- is the cutoff for soil venting (Appendix B in the
USEPA Reference, 1991 (a)).
Henry's Law Constant and Solubility. The rate that the
contaminants are released from the natural soil moisture (pore
water) are dependent on the Henry's Law Constant for each
compound within the contaminant matrix. The Henry's Law
Constant is the ratio of the concentration of a compound in air
to the concentration of the compound in water at equilibrium.
The Henry's Law Constant for a compound is a measure of the
rate that the compound will be released from the moist soil
into the soil air. A low Henry's Law Constant indicates that
the compound at equilibrium in an air and water mixture is
largely held within the water phase. It is, therefore, not
readily volatilized into the extracting air stream, resulting
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Guldtace tee Sell ftntiac Sy«t«.
in a very slow rate of extraction.
Solubility in water is another factor in the extraction rate
for a specific compound. A significant amount of a highly
soluble compound (acetone, alcohols, etc.) dissolves in the
soil moisture, retarding the rate of volatilization. If the
soil venting system has a large enough air- flow rate to dry out
the soils, then solubility is a less critical factor.
fraoult's Law, A mixture of gasoline and heavier hydrocarbon
compounds (such as diesel or lubrication oil) can be slow to
remediate, because the volatile compounds may be trapped in the
heavier, relatively nonvolatile compound matrix. In this case,
the effectiveness of a soil venting system depends upon the
rate of molecular diffusion of the volatile compounds out of
the nonvolatile hydrocarbon matrix. Even a highly volatile
substance like gasoline weathers and becomes much less volatile
as the highly volatile compounds are removed from the mixture.
The extraction rate of the less volatile compounds is often the
controlling factor in closing out sites with soil venting
systems. Some sites are not suitable for soil venting systems
because it may not be technically feasible for a system to meet
cleanup criteria for some compounds. See Subsection 2.2.1 and
the above discussion of vapor pressure and Henry's Law
Constant. The vapor pressure and the Henry's Law Constant
should be assessed for unusual or unique mixtures of
contaminants where soil venting has a limited history. Bench
scale tests may also be useful in unusual conditions (EPA, 1991
Adsorption, Adsorption of contaminants on the soil slows the
rate of extraction. Soils that have a high surface area (fine-
grained soils) or high total organic carbon content have a much
greater ability to adsorb contaminants than soils with a small
surface area (coarse-grained soils). Therefore, coarse-grained
soils release contaminants at a faster rate thstn fine-grained
soils or soils with a high total-organic compound.
BjodeRradation, Petroleum hydrocarbons biodegrade at a higher
rate under aerobic conditions than anaerobic conditions. The
rate of biodegradation is generally controlled by four factors:
oxygen, food (the petroleum product) for the microbes,
moisture, and nutrients. The limiting factor under non-venting
conditions is usually oxygen. A very slow air- flow rate is
usually sufficient to provide enough oxygen to the bacteria.
Aerobic biodegradation is not significantly inhibited until
oxygen levels drop below 5 percent. When a soil venting system
is active, the limiting factors generally will either be a lack
of. moisture or lack of nutrients .
It is possible that portions of the soil witMn a soil venting
regime are not frequently replenished by oxygen. If this
occurs, these zones will be largely stagnant and only
anaerobically active with accumulating fermentation products ,
such as methane. It is possible that anaerobic conditions
could exist in a system, even if there are high oxygen levels
in the extracted air. This occurs because some of the
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GuicUnc* for Soli V«ntin* Sy«t«M , *•»• 8
extracted air could have passed through "clean" soil'.
• ' Generally the halogenated compounds biodegrade at a much slower
rate than petroleum fuels under aerobic conditions.
For specific details'on blodegradation, as part of a soil
venting system, please refer to Subsections 4.9.5 and 5.1.
2.2 Site Characterization.
The following is a summary of technology-specific aspects of a site
characterization and should be used with Chapter 7 in the Guidance for
Conducting Environmental Response Actions.
Soil venting as a remediation technology depends on the flow of a fluid (in
this case air) through the unsaturated soil. For this reason,
environmental professionals need to characterize the geological conditions
of the site in sufficient detail so that they can design a soil venting
system that is appropriate for the site conditions. An inadequate site
characterization may result in a venting system that has large stagnation
zones, excessive groundwater extraction during times of high water table,
significant short circuiting, or a system that will not work at all.
The* following subsections identify the significant site characteristics
that should be defined or estimated when considering a soil venting
technology.
2.2.1 Contaminant Characterization.
Characterize the site for contaminant types in order to prepare a
monitoring plan that will comply with criteria set by the Bureau of Air .
Management. Characterizing the contaminants is also necessary to evaluate
the feasibility of successfully remediating the site with a soil venting
system. Contaminant volatility should be • identified and characterized so
designers can estimate the total mass of contaminants to evaluate the size,
cost, and life of the project and to determine if there is a need for air
emission controls. Air standards are established by the Bureau of Air
Management and are found in Subsection 1.3.2 and in Table 1-1.
During the investigation, assess the components of the product lost and its
degradation products.
Example: Halogenated solvents will often degrade to compounds that
are more toxic than the original product that was released.
Tetrachloroethane will transform to trichloroethane, then to
dichloroethane, and finally to vinyl chloride, which.is a known
carcinogen (-Fetter, 1988).
For emission estimates at sites with petroleum contamination, the two
parameters that need to be assessed in soil are total benzene and total
VOCs (see Attachment 1). If a laboratory test is used to quantify the
total VOCs for petroleum products, use an analytical test for TPH that* also
quantifies compounds that are not identified in a normal VOC scan (propane,
butane, pentane, etc.) . Do not use a sum of benzene, toluene, ethylbenzene
and xylene. The Bureau of Air Management may require an air permit and
needs to know what the potential contaminants at the site are and estimated
quantities for each.
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Guidance for Soil V«ntini Sy>t«M ! *„. 9
An assessment of the vapor pressure and Henry's Law Constant for gasoline
contamination is not necessary because of the large number of venting
systems demonstrating that gasoline is readily removed from the subsurface.
Gasoline is a mixture of more than 100 compounds.
Some compounds with less than six carbon atoms in the molecule (C6) have
very high vapor pressures and readily volatilize; some heavier hydrocarbons
.with greater than nine carbon atoms per molecule (C9) volatilize very
slowly. Most of the highly volatile compounds are quickly extracted by a
soil venting system. Rainwater, et al. (1988) demonstratisd in column
studies that when greater than 50 percent of the pentatie is removed, only
10 percent of the xylene is removed. DiGiulio et al. (1990) estimated that
40 percent of gasoline contamination may still remain when off gas
concentrations have fallen to 1 percent of the initial concentration. The
lower volatility compounds in gasoline (>C9) that are less; readily
extracted may prevent a soil venting system from meeting site-specific
cleanup standards that are based on TPH.
Soil samples collected from soil borings should be field screened for VOC
measurements. Field screening could consist.of headspa,ce analysis by PID
or FID; headspace analysis by field GC; or headspace analysis by the Lab in
a Bag Method (Robbins et al. 1989).
2.2.2 Geological Factors.
This Subsection discusses soil description, horizontal permeability,
stratification, vertical permeability, hydrogeology, and ether site-
specific considerations. '
To design an effective soil venting system, it is necessary to sufficiently
characterize the site geology to evaluate any preferred zones of air flow
An experienced scientist or engineer should classify the borings in detail.
To describe the soil column, the soil description should include the
following:
Approximate percentages of major, and minor grain-size
constituents. Note: Terms such as "and," "some," "little,"
"trace," etc. are acceptable if defined in percentages they
.represent; .
• Color and Munsell color;
Geologic origin;
Description of moisture content (dry, moist, wet);
Any visual presence of secondary permeability;
Voids or layering; .
Pertinent field observations such as odor;
Description and notation of any product smearing evidence.
Since depth of smearing is evidence of past aquifer water-level
variations, note the depths carefully.
Any other pertinent observations.
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Coldraca tor Soil Vatlas Sftttmf P*** 10
2.2.2.1 Horizontal Permeability.
The horizontal permeability of the unsaturated zone is a key factor in
designing a soil venting system, and to some degree, in estimating the life
of the project. The rate of contaminant extraction by volatilization and
advection is proportional to the rate of air flow. At a given operating
vacuum, a soil venting system installed in a highly permeable soil will
allow a high air-flow rate through the soil, whereas installation in a low-
permeable soil will result in a lower achievable air-flow rate.
« There are two common ways to estimate what air flow is achievable from a
soil venting system: a pilot test, or a permeability estimate of the soil.
See Section 3.0 for pilot test information.
2.2.2.2 Stratification and Vertical Permeability.
It is important to evaluate the presence of stratified soils at a site
during the site characterization. Stratified soils are soils that have
been deposited in layers that are typically horizontal. Stratification can
channel the air flow through the relatively coarse-grained horizontal soil
layers and restrict vertical flow through the relatively fine-grained
horizontal layers. The horizontal component of flow is increased relative
to the vertical component, thus the horizontal zone of influence of an air-
extraction well is increased. Stratification at a site can easily be
identified by an inspection of soil boring logs from the site.
Stratification exhibits characteristics similar to a high Kh/Kv ratio on a
. macro scale.
The Kh/Kv ratio is generally controlled by the natural depositional
environment of the soils. Horizontal channeling of the air flow patterns
is caused by a high ratio of horizontal permeability (Kh) to vertical
permeability (Kv). Eolian silt deposits (loess) may have a Kh/Kv ratio of
100 or more. Glaciofluvial (or outwash) deposits commonly have a Kh/Kv
ratio of 3 to 10. Manmade fill typically has a Kh/Kv ratio near 1. On a
macro scale, glacial till may have a Kh/Kv ratio that is less than one due
to vertical fracturing of the till.
There are two ways to estimate the Kh/Kv ratio: using pilot test data; and
identifying the depositional environment and making assumptions for typical
characteristics for different depositional environments. The method
proposed by Shan et al. (1992) can be used to estimate the Kh/Kv ratio from
field pilot tests (Subsection 3.2). The reference also includes figures
that portray streamline flow patterns for different Kh/Kv ratios.
System designers can evaluate the effects caused by stratification or the
Kh/Kv ratio to adjust well placement to site-specific conditions as
discussed in Subsection 4.1.
The volatilization of VOCs from sites with stratified soils is often
inhibited by poor air-flow rates through the finer-grained soil layers.
The diffusion rate of the VOCs from fine-grained layers into the coarser-
grained layers controls the extraction rate, as the coarse grained layers
act like short circuiting pathways for the advective air flow, and the air
flow passes through the fine grained soils very slowly.
2.2.3 Hydrogeologic Factors.
Certain hydrpgeologic factors will affect the design of a soil venting
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Galdtaet tor Soil ttatiat 8r«t«u '
system. The location of the screened portion of the air-extract ion wells
is determined by the soil geology, surface conditions, and the depth and
seasonal fluctuation of the water table. The seasonable-high and the
seasonal-low water table should be estimated during the remedial
investigation. Since investigations often span periods of only two-to-five
months, it is generally necessary to estimate seasonal variations.
i
2.3 Other Site-Specific Factors.
There are many other site-specific factors that affect the design and
performance of soil venting systems. A brief discussion of some factors
include the following:
Surface Seal. A surface seal, such as a pavement layer, is
often recommended in the literature. A surface seal channels
air flow horizontally and restricts vertical air flow from the
ground surface near the extraction well(s). Surface seals are
difficult to construct properly, see Subsection 4.9.1.
Artificial Conduits, Backfilled trenches in soils can act as
short circuiting paths for the air flow. Trenched sites with
relatively impermeable native soils are most affected because
the backfill in the trench may be much more permeable than the
natural soil. Designers should indicate utility trenches
(sewers, water mains, electricity lines, etc.) on maps with
soil venting system design plans.
Air-Flow Obstructions^ Building basements are typical air-flow
obstructions which may change the subsurface air flow patterns.
In these cases, designers should note buildings with basements
in the reports, especially if the floor of a basement is near
or below the capillary fringe. Underground storage tanks are
also obstructions to air flow and designers should also
indicate their locations on maps with the soil venting system
design.
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Cuid«nc« for Soil Tcntlag 8rat
3.0 Treatabllltv or Pilot Testing.
• . 3.1 Laboratory Treatability Tests.
** '• •
Laboratory treatability tests are useful for sites with mixed wastes that
have unusual characteristics.. Generally, because of past successes with
common solvents and-highly volatile petroleum products on a national basis,
these compounds do not warrant laboratory treatability studies for
volatility. See EPA Interim Guidance 1991(b) for guidance on treatability
testing.
At sites with aerobic-degradable contaminants and substances that are toxic
to microbes, such as leaded gasoline contamination or foundry sand,
biodegradation treatability testing may be needed. Other site-specific
factors may also warrant biodegradation testing.
3.2 Pilot Tests.
A pilot test is preferred over a laboratory grain-size test to estimate the
possible air-flow rate from a proposed soil venting system. A pilot test
is the only method that directly measures all pertinent site
characteristics and geologic heterogeneities as an inherent part of the
test procedure.
A pilot test is a short-term test that typically is smaller in scale than a
full-scale remediation system. Generally, a pilot test at a LUST site or
small ERP site is conducted for no longer than one day. Some practitioners
and the EPA may recommend long-term testing for certain situations, such as
CERCLA treatability studies (EPA, 1991(b)). NR 419.07 (3) exempts pilot
tests of negative pressure venting systems from emission limits if the rate
of air extraction does not exceed 100 scfm, and the test does not exceed
eight hours at a site. The pilot test is not exempt from notification and
emission limits if it is conducted, longer than eight hours or exceeds 100
scfm.
3.2.1 Purpose of a Pilot Test.
The purpose of a soil venting system pilot study is to determine design
parameters prior to and for construction of a full-scale soil venting
system. For these purposes, a short-term pilot test with a small blower is
usually sufficient.
Key parameters include the following:
The air-flow rate that is achievable from a soil Venting system
extraction well configuration under a given vacuum rate.
• • The measurable vacuum at a .distance from the air-extraction
well (zone of vacuum influence).
A quantitative estimate of the VOC emissions that initially
occurs with a soil venting system.
3.2.2 Conducting a Pilot Test.
• . •
A pilot test should be conducted under conditions that are typical at the
site. For example, misleading pilot test results could occur if a pilot
test is conducted during or shortly after a rain storm. The temporary
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Baidtnet for Soil ftntlac feitm
wetting front in the soil column created by infiltrated or pooled water may
create a temporary surface seal to air flow. In this example, a temporary
seal would suggest that the zone of influence is much greater than it
really is. Misleading results could also occur if there is a significant
ambient barometric pressure change during the test; specifically, if vacuum
readings in distant gas probes are taken for designing well placement.
The following equipment is needed to conduct a pilot test:
Air -Extract ion Wells. Designers should install one to three
air-extraction wells at the site for the pilot test. Construct
these wells according to the criteria for permanent full-scale
soil venting system use. See Subsection 4 ..3 for construction
details.
A water-table well may be used if there are no air -extract ion
wells constructed at the site for testing. However, the
existing water-table wells should have a filter pack and
screen-slot size that is appropriate for soil venting. If the
slot size and filter pack are too fine, the vacuum measured in
the extraction well will be too high and will' not reflect a
realistic vacuum for a given air- extraction rate.
It is important to choose a well with known construction
details if a water-table well is used, because water-table
wells typically have less than 5 feet of screen exposed to the
unsaturated zone. It is also important to operate the pilot
test in a manner that does not significantly lift the water
table during the test. Lifting the water table by the vacuum
more than half way up 5 feet of unsaturated screen greatly
limits the use of the data for estimating achievable air flow
per foot of well screen. It is highly recommended that the
consultant use a small -diameter pump to lower the groundwater
to assure accurate pilot test data. See Subsection 4.9.3 for a
discussion of matching drawdown to the applied vacuum.
Portable Blower (or Vacuum Extractor^ . A small blower should
be used to pull air from the air -extraction well(s) during the
pilot test. The blower can be almost any size. Since pilot
tests are exempt from the air emission limits - provided the
test is conducted at less than 100 scfm - a large blower may
not be useful in high-permeable soils. Blowers should be
equipped with a discharge stack. A muffler (or silencer) on
the exhaust and a dilution (or bleed) valve on the blower inlet
are also recommended. Designers should use blowers with an
explosion-proof motor and switch. In most eases, regenerative
blowers are used for pilot tests, however, a high vacuum blower
may be necessary at sites with low-permeable soils.
Extraction Well Sample Port and Instrumep^atf-f «ni The basic
instrumentation needed on a pilot test is an air -flow meter,
vacuum gauge, and thermometer. See Subsection 4.4 for a
further discussion of instrumentation. A sjample port is also
needed to collect .air samples . It may be most convenient to
install all instrumentation and the sample port on a single
temporarily- ins tailed pipe between the blower and the
extraction well. A section of 2 -inch diameter or smaller pipe
.is recommended for this purpose if an averaging pitot tube or
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CuidUmc* for Soil V«ntln* Systw* • **«• 1*
regular pitot tube is used. See Subsection 4.4 for a
discussion of sizing a pipe to a pitot tube..
Note: The temperature of the air stream at the wellhead may be
a qualitative indication of the residence time of the air in
the subsurface.• If a pilot test is conducted in mid-summer and
the extracted air is significantly warmer than the natural
grouridwater temperature, the air has a low residence time in
the soil. The converse is also true -- unusually cold wellhead
temperatures in winter also indicates a low residence time.
Sample Collection Equipment or Instruments. See Attachment 1
for a discussion of typical sampling equipment. Attachment 1
is designed for petroleum sites. However, field instruments,
portable gas chromatographs, and carbon'tubes (or other
adsorptive media) are useable at other sites. The equipment
used must be appropriate for the site contaminants. See
Subsection 3.2.3 for a further discussion of equipment
parameters.
A combustible gas meter may be needed at sites with ignitable
contaminants to ensure that the off gas measured at the stack
is below the lower explosive limit.
Zone of Influence Instrumentation. The vacuum in the soil at a
distance from the air-extraction well can be measured at
existing water-table wells, other air-extraction wells, or with
temporary gas probes that are normally used for soil gas
surveys. Some designers also install permanent gas probes as
discussed in Subsection 4.3.3. Since water-table monitoring
wells generally have less than 5 feet of exposed screen above
the water table, measuring the vacuum in water-table wells
provides a vacuum reading that is essentially measured at the
warer table, provided that the well casing couplings are air
tight. Air-extraction wells generally have longer screens and
measure an average vacuum over the entire screened interval.
Because there are significant vertical pressure gradients under
active venting, it is IMPORTANT to use vacuum monitoring points
that are equal in depth (or as close as possible), unless a
three-dimensional model is used that corrects vertical
gradient.
To measure the vacuum in a well, fit an air-tight cap with a
hose barb to the well and use an. inclined manometer, vertical
manometer, or magnehelic gauge. Vacuum measurements should be
to two digits of accuracy (e.g., 0.01 to 0.99, 1.0 to 9.9, and
10 to 99). If the vacuum is very low, use an inclined
manometer or other device that can accurately measure to 0.01
inch of water column. Vacuum measurements should be taken
after the vacuum in the subsurface has stabilized. A minimum
of two measurements, at different times, at each data point
should be taken to assure that the vacuum has stabilized.
Generally, in coarse-grained soils the vacuum measurements are
reasonably stable after a half hour. Subsection 4.1 describes .
how to use this data to evaluate well placement. . Note: If
designers use manometers instead of magnehelic gauges, they are
available with oil instead of water. This may be an advantage
in'freezing weather. Oil manometers are calibrated to the .
-------
Outamci tar Soil Vmtiag Sr»tm* i „
rag* IS
'
density of the oil and cannot be used with water. Some air-
flow modeling methods require barometric pressure monitoring
during the pilot test to correct for atmospheric pressure
changes.
Some sites are sufficiently, .simple so three -dimensional vacuum measurements
are not needed, but sites with complex stratified soil may need three-
dimensional vacuum measurements to fully understand th« air flow patterns
Probes that are normally used for soil -gas surveys can be used instead of'
wells to measure the vacuum at specific discreet depth intervals.
If multiple air-extraction wells are available for testing, test each well
by extracting air from it during the pilot test. Test wells that are most
likely to be used in a full-scale system. If the vacuum stabilizes at a
distance from the well in a reasonable period of time, multiple air-
extraction well tests can be used for zone of influence measurements If
it takes more than two hours for an air-extraction well to stabilize only
a few wells can be tested during the eight hour air emission exemption
period.
If .the air-extraction well is screened into the water table, measure the
depth to the water table -- both before and IMMEDIATELY after the pilot
te^*TJ^en if the Wel1 is B0t screened into the water table, inspect the
well IMMEDIATELY after the pilot test for water accumulation in the bottom
ttow during £%£? *" ""* tO — — «" «*— *•** available
Some consultants operate the pilot test at two or more air-flow rates
during the pilot test to gather information for air modeling. The method
proposed by Clarke et al. (1993) and Wilson et al. (1992) - to scale up
from a pilot test to a full-scale system - requires flow and vacuum
measurements at three or more different flow rates. Note: If the method
r™! d %l /* al- (1"3) ^ WilS°n 6t al- (1992> :Ls used' ^e DNR
recommends flow and vacuum measurements at four or more different flow
3.2.3 Analytical Monitoring Methods for Pilot Tests.
Use the same analytical methods during the pilot test a» would be used in a
full-scale remediation. Frequency of sampling is not specified for a pilot
test but a minimum of two gas samples should be collected for analysis
If a field portable instrument is used, take samples every half hour or
every hour. J
Do not take the first sample until after approximately 100 to 300 cubic
feet of air has been evacuated from the soils adjacent to tiie well air-
extraction well. This initial purge of air is needed to thoroughly
evacuate the air that has been in and near the air -extraction well and
filter pack. If 100 cubic feet was not produced within 30 minutes because
of low permeable soils, sampling after 30 minutes is acceptable.
3.2.3.1 Sites With Petroleum Product Contamination.
During the pilot test, assess both total VOCs and benzene (see
Attachment 1).
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Ould«nc» *or Boil V«ttn» Symttmt • *"** 16
3.2.3.2 Sites With Non-Petroleum Contamination.
Assess the known and suspected contaminants and any biodegradation products
of the'contaminants during the pilot test. Any other non-natural gases or
vapors that may be in the subsurface from on-site and possible off-site
sources should also be assessed.
Example: There is a tetrachloroethene loss at a manufacturing
. facility, and there is an UST containing gasoline 200 feet from the
tetrachloroethene spill site. Even though there is no known gasoline
loss, the pilot test at the tetrachloroethene spill site should also
test for benzene and/or petroleum hydrocarbons in this case because
vapor phase migration may occur over significant distances (Mendoza
and McAlary, 1990). Besides gasoline constituents and
tetrachloroethene, samples should be analyzed for trichloroethene,
1,2-dichloroethene, and vinyl chloride because these compounds are
degradation products of tetrachloroethene and are expected to be
present.
3.2.4 .Reporting Results From Pilot Tests.
The results of a pilot test can be included in the site investigation
report, the design report or as a separate report. The report from a pilot
test should include the following:
Discussion.
Figures.
A description of the test and final conclusions. The text
should include dates, weather (ambient temperature, wind,
etc.), and any other pertinent field observations from the
pilot test. The barometric pressure and whether climbing or
falling may also be listed.
A site map drawn to scale (horizontal accuracy to +/- one
foot). The map should indicate:
— Locations of air-extraction wells and vacuum measuring
points;
— Suspected and/or known source location(s) (if differing
contaminants types are present ait a site, the locations
should identify the contaminant types);
— zone of soil contamination (if three-dimensional data is
available; multiple maps may be used);
— Paved areas, buildings, and structures that may act as a
surface seal or an infiltration barrier;
— Buried utility trenches that, may act as zones of higher
permeability;
- Scale, north arrow, title block, site name, key. or
legend, and date(s) of pilot test;
— Any other pertinent site information that may affect a
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Cuidmc* /or Soil Vatinf Syttmu
Tables.
permanent: soil venting system on the site, such as
overhead power lines (they may conflict with future
drilling activities).
A graph representing subsurface vacuum at a distance from the
extraction well.,is recommended if there ar« three or more data
points in addition to the air-extraction well,. The distance
scale should be on the horizontal axis and th« vacuum should be
plotted on the vertical axis. The graph may be plotted on
normal graph paper or on semilog paper with the vacuum on the
log scale. Note: The DNR recommends semilog graphs for this
purpose. The graph should identify which elata points were
used. A line or curve predicting the vacuvun at a distance from
the air-extraction well should be drawn on thts graph. The line
or curve may or may not intersect the air-extraction well due
to partial penetration effects and possible: extraction well
inefficiency. Note: If the screened intervals between
different monitoring points vary significantly, the graph may
not provide a smooth curve because there are significant
vertical pressure gradients under active extraction.
A water-table map of the site for the day of the pilot test.
A cross section showing screened intervals, geological-units
contour lines of vacuum readings, and vacuum measuring points.
If sufficient data points are available, a map of measured
vacuums and contours of the vacuum in the soil during the pilot
test may be included. This map is only recommended if the
full-scale remediation system will use a single air-extraction
well.
Tabulated flow rates, vacuum distribution, soil gas
temperatures, times of readings, ambient barometric pressure
(if taken), and the ambient temperature.
Water levels in all wells.
Appendices.
A complete description of the field equipment and field
procedures that were used.
Sampling methods and procedures.
•
Analytical methods, analytical results, and lab reports. The
analytical results should be quantified in uiasi; per volume
units, such as pounds per cubic foot or milligrams per cubic
meter of contaminants in air.
Boring logs and well-construction' diagrams for air-extraction
wells. If groundwater monitoring wells are used for measuring
vacuum, the screened interval of the monitoring wells.should be
listed in a table and/or the well construction diagrams should
be included in an appendix. Any vacuum measuring points that
are in fill should be identified as such.
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Guid«nc» for Soil Vantins Sftttmrn • ***• 18
Engineering calculations. Clearly state all assumptions.
Legible, hand written calculations are acceptable. Include the
initials or name of the author and the person who performed a
quality- control check of the calculations. List references for
any formulas that were used.
5 s
Any other pertinent field data.
Some pilot test reports also include a conceptual or detailed design of a
full-scale soil venting system. If a pilot test report includes a detailed
design, see Subsection 4.10 for recommended submittal contents.
3.3 Alternative to a Pilot Test.
Another way to estimate the air flow available from a soil venting system
is by estimating the permeability of the soil based on a grain-size
analysis. This method should only be used if all of the following
conditions exist:
• The unsaturated zone of the site is a single relatively
homogenous geologic unit.
• The volume of contaminated soil is very small.
• The total mass of contamination is relatively small.
• ' The Bureau of Air Management approves of the soil remediation
without conducting a pilot test.
The best reason for using this -method is the low cost of a grain-size
analysis relative to a pilot test. The following are disadvantages of
using this method:
• The effects of geologic heterogeneities are exaggerated by
using only a small sample(s) to characterize a site. Sampling
location selection can inadvertently bias the results.
• Layered geologic conditions cannot be evaluated by using a
grain- size analysis to estimate intrinsic permeability because
of the variations in permeability.
• Air emissions cannot be estimated.
• The calculated permeability assumes dry soil. If there is
significant .soil moisture, the permeability to air flow could
be less than estimated.
To calculate the air flow available by the grain- size analysis method,
first estimate the hydraulic conductivity by using a mathematical analysis
of the grain, size (Shepherd, (1989)., Hasch and Denny, 1966 or by the Hazen
method in Freeze arid Cherry, (1979) and Fetter (1988)). Note: The Hazen
Method is only valid when. 0.1 < Dio < 3.0 mm. Then calculate the intrinsic
permeability of the soil from the hydraulic conductivity. Note: At 15
degrees Celsius, the conversion factor is approximately 1 darcy -8.5 E-4
cm/sec based on data from Fetter (1988), page 84. Finally, estimate the
air -extract ion rate (Johnson et al. , 1990, see figures 4 and 5) per unit
length of extraction-well screen.
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Guidtne* for Soil Vtatiat Syntmu p • 19
Design and Installation of a Soil Venting System.
The soil venting system components are described in this section, beginning
with a Discussion of well placement. .The discussion of design parameters
then follows the same route as the flow of air: from well design, to
manifold, to water trap, and the blower (or vacuum extractor). Subsections
4.7 through 4.9 discuss other equipment that may or may not be used at a
site. This Section concludes with a discussion of the information that
should be submitted to the DNR.
4.1 Well Placement and Air- Flow Modeling.
The key design variables with soil venting are the number of air -extract ion
wells and the flow rate from each well. There is no equation to determine
these parameters. In the literature, well spacing generally ranges from 20
to 50 feet. .
A capture zone for a well can be mathematically determined for groundwater
plume capture (given the gradient, extent of contamination, pumping rate
and aquifer transmisivity) . Soil venting systems do not have a single
mathematical solution to use for determining well placement. Some
mathematical models exist that are excellent tools for estimating well
spacing, however, the users of these models should be sufficiently skilled
to know if and when model assumptions are valid.
Some system designers use a model that estimates the number of pore volumes
that are needed to clean up a site. An air-flow rate that is based on pore
volumes is then selected. If a method based on pore volumes is proposed
the volume of air that enters the well(s) through the ground surface nea^
the well should be assessed using a method that evaluates three-dimensional
air flow, such as the method described by Shan et al (1992) or by a
similar method. Other models are complex two- or three-dimensional models
of air flow patterns. Some models use pilot test data to determine site-
specific parameters, such as Kh/Kv ratio, intrinsic permeability, etc
Some of the mathematical models (both analytical and computer) used for
modeling air flow through soil are based on horizontal flow only and do not
take into account vertical recharge through the ground surface. Models
that use limited assumptions, such as horizontal and not vertical air flow
are good tools for rough estimates, but are not useful for determining an '
exact distance for well spacing. Designers should assess the key
assumptions in an air-flow model prior to its use-. Professional judgement
is necessary in interpreting model results.
The DNR does not endorse any models and does not require* air modeling for
the system design. If a model is used, include the key assumptions and
results of the model in an appendix to the design report:.
The zone of influence is the area from which an extraction well can
effectively draw air. Figure 4-1 is based on the mathematical formulas in
Shan et al. (1992); it simulates the vacuum that would be measured in
water-table wells at different distances from a single air-extraction well
There are four different graphs simulating Kh/Kv ratios of 0.67, 1, 3 and
10. As demonstrated in Figure 4-1 there is no clear cut "radius" of ' -
influence; the effectiveness gradually decreases with distsmce In theory
the vacuum extends significant distances beyond the point «4iere it- can be
measured by field measuring devices. Even though in theory there is a
vacuum at these great distances, in reality, the vacuum is so low that
there is essentially.no induced air movement through the sell. The
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GulcUnc* tor Soil V«ntin* Syibaa*
P««» 20
Figure 4-1
Vacuum at a distance from a single extraction well
c
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3
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o
Vacuum With On* Air Extraction Wall
r 100-:
60 :
40-
20-
6-
4-
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fr-:
0.6 :
0.4 :
0.2-
0.1 ".
0.06 :
0.04-
0.02-
0.01
.006
.004
.002
.001 •
.
V :
\. 1
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.
\ I
N. . • :
X.
: N. j
^\^
' 20.0 Depth to Woter Table (feet) x. ]
20.0 Depth to Base of Screen (feat) x.
12.5 Depth to TOD of Screen (feet) \
: 17.5 Depth of Predicted Vacuum (feet) \
; 7.5 Length of Screen (feet) N.
50.0 Air Flow Rate (scfm) xss^
0.67 kh/kv (or kx/kz) Ratio >v
' 1.00E+01 Horizontal Permeability (Darcies) ^
1.5QE+01 Vertical Permeability (Darcies) i i i i
D -20 40 60 80 100
Figure 4-1A
Distance From Air Extraction Well
Vacuum With One Air Extraction Well
o
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O
[-100 j
60 :
40-
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0.6 :
0.4
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.006
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.002
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•
.
•\ . j
>. :
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• ^\ •
^v.
• 20.0 Depth to Water Table (feet) ^ — '
: 20.0 Depth to Base of Screen (feet) ^s.
12.5 Depth to Too of Screen (feet) \
: 17.5 Depth of Predicted Vacuum (feet) \.
• 7.5 Length of Screen (feet) , ..
• 50.0 Air Flow Rate (scfm) \.
1.00 kh/kv (or kx/kz) Ratio ^s.
1.00E+01 Horizontal Permeability (Darcies) \
1.0QE+01 -Vertical Permeability (Darcies) i i i i
0 20 40 60 80 100
Figure 4-1B
Distance From Air Extraction Well
-------
Guidance tax Soil Tooting Sy>tea>
Face 21
Rgure 4-1 continued
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Vacuum With One Air Extraction Well
Figure 4-1C
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.004-
.002-
001
'• ' . '
{
v - i
^\
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: "\^^ .
; ^"^^^^ j
^^\^
. ^J
;
20.0 Depth to Water Table (feet)
20.0 Depth to Base of Screen (feet)
12.5 Depth to Top of Screen (feet)
17.5 Depth of Predicted Vacuum (feet) :
7.5 Length of Screen (feet) :
50.0 Air Flow Rate (sefm) :
3.00 kh/kv (or kx/kz) Ratio
1.00E+01 Horizontal Permeability (Darcies)
3.33E+00 Vertical Permeability (Darcies) i ,
0 20 40 60 80 10
0
Distance From Air Extraction Well
Vacuum With One Air Extraction Well
60 :
40-
20-
— 1-9-
6:
4-
2-
0.6 :
0.4
0.2
0.06 :
0.04 :
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p Q\
.006':
.004-
.002-
Uei-
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^^^
• — — -^^J
•
20.0 Depth to Water Table (feet) '
20.0 Depth to Base of Screen (feet)
12.5 Depth to Top of Screen '(feet)
17.5 Depth of Predicted Vacuum (feet) :
7.5 Length of Screen (feet) :
50.0 Air Flow Rate (scfm) -
10.00 kh/kv (or kx/kz) Ratio
1.00E+01 Horizontal Permeability (Darcies')
1.0QE+00 Vertical Permeability (Darcies) , , , ,
20 40 60
Distance From Air Extraction Well
80
. 100
Figure 4-1D
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Guidance for Soil V«otin* Sytttm* > p«g« 22
numerical example below is based.on the graph in Figure 4-IB where the
Kh/Kv ratio is one.
Example: The stabilized (steady state) vacuum and the distances
from the vacuum measuring points to a single air-extraction
well in a uniform sand are as follows:
Measuring Point Vacuum Distance
(Inches of (feet)
Water Column)
W-l 54 NA
MW-2 5.2 10
Mtf-1 1.6 20
. MW-5 0.25 40
MW-4 0.10 50
In this case, the vacuum decreases by 3.6 inches of water
column from 10 feet to 20 feet, and it decreases by 0.15 inches
of water column from 40 to 50 feet. Assuming that the rate of
horizontal air flow is directly proportional to the horizontal
difference in pressure head, the air velocity through the soil
at 40 to 50 feet from the air-extraction well is only 4.2
percent of the velocity at 10 to 20 feet (0.15 / 3.6 - 0.042 or
4.2 percent).
As the air velocity through soil decreases at greater distances
from the well, the system's ability to volatilize and remove
VOCs by advection is reduced at a distance. In this example,
the effectiveness of the system is only marginal at distances
beyond 50 feet even though there is measurable vacuum to 75
feet and immeasurable vacuum beyond.
Since there is a significant vertical pressure gradient, it is
VERY IMPORTANT that all vacuum measuring points are equivalent
in depth when using vacuum versus distance data to evaluate
well spacing, unless a three-dimensional model is used that
corrects for vertical gradient.
Use professional judgement to estimate the well spacing that is needed in
each specific situation. Take the following items into account when
. assessing optimal well .placement:
Some areas of a site usually have much higher levels of soil
contamination than others. It may be appropriate to use a
closer well spacing in these areas to increase the rate of
remediation. - •
• Generally, there is a tradeoff between time, efficiency, and
cost. Closer well spacing speeds the cleanup, but increases
costs for wells, analytical testing and blower capacity. If
the total cost of wells is significant, a longer cleanup time
with fewer wells, spaced farther apart may be more appropriate.
Relatively close well spacing is needed in low permeable soil
because the rate of air flow from each well is very low, and
therefore the rate of contaminant extraction on a pounds-per-
time basis is also very slow per well. In high permeable soil,
-------
Gaidtaem for Sell tontine Sy«t«M • p.g. 23
wells can be placed farther apart because higher air flow per
well can result in a greater rate of contaminant extraction per
well.
If the Kh/Kv ratio is very high due to the depositional
environment of the soil, or if there is a high quality surface
seal, the air-flow pattern will have a preferred horizontal
orientation. In this case, wells can be placed farther apart
because there is less vertical recharge near the air-extraction
wells. '
In a heterogenous, mixed lithology site, the «one of influence
in the more permeable layers is augmented by overlying layers
of silts and clays, which allows increased well spacing. The
silts and clays, however, take longer to clean up because
extracting contaminants from these soils is limited by the rate
of diffusion.
At sites with a very shallow water table, a significant
proportion of the air that enters the air-extraction well(s) is
from.the ground surface near the well. In these cases,
relatively close well spacing may be necessary.
To remediate contaminants that have a low vapor pressure
through volatilization, relatively rapid air-flow rates through
the soil are necessary. In this .case, relatively close well
spacing may be appropriate. Enhancing biodegradation, however
does not usually require a high air-flow rate.
At sites where geologic conditions at depth are sufficiently
uniform, a single set of wells at the same depth may be
sufficient. Sites that are significantly stratified or that
have other geologic heterogeneities in the site soils may have
a very high rate of contaminant removal initially, but the
removal rate will decline rapidly after the coarse-grained
layers are remediated. Late in a project the rate of
extraction is controlled by the rate of VOC diffusion out of
the.fine-grained soils. At a site with these conditions,
tailor the design for the natural geologic conditions.
Example: A site has two distinct, fairly thick geologic
units, a sand and gravel unit and a sllty sand unit.
Remediation of the silty sand unit is expected to be much
slower than the sand and gravel unit. lin this case,
fewer extraction wells screened in the sand and gravel,
and more extraction wells screened in the silty sand may
maximize the remediation rate of the silty sand. Air-
injection wells (Subsection 4.8) may also be needed in
complex geological conditions.
The spacing of air-extraction wells in a full-scale soil venting system is
determined by the desired air-flow rate through the impacted soil and the
desired rate of cleanup. Use professional judgement to weigh the costs
against the cleanup time when determining well placement. A higher air-
flow rate is needed for an increased rate of volatilization and advection.
The air-flow rate is less important if diffusion and biodegradation are
controlling factors in the remediation rate.
-------
Ouid«no» for Soil Vntlns Symttm* . *•»• 2*
r
Buscheck and Peargin (1991) suggest that the design radius of vacuum
influence at a gasoline"contaminated site be at the distance where the
vacuum .in the soil is 1 to 0.1 percent of the measured extraction well
vacuum." According to the Buscheck and Feargin method applied to the
numerical .example above, the well spacing (which is twice the design radius
of vacuum influence) should.-range from 20 to 65 feet.
Generally, the well spacing should range from 20 to 50 feet.. If proposed
well spacing is closer than 20 feet or farther than 50 feet, and the
Buscheck and Feargin (1991) method is not used, the spacing selected should
• be justified in the workplan submitted for the site.
4.2 Air Permeability, Achievable Air-Flow Rates, and Air-Emission Limits.
•Use the vacuum and air-flow rate measured from the air-extraction well
during the pilot test to estimate the vacuum and air-flow rate that are
achievable in a full-scale soil venting system design.
If pilot test results from multiple geologic strata are evaluated, it may
be appropriate to evaluate the achievable air-flow rate per foot of screen
and/or the intrinsic permeability of each geologic unit. If all the wells
in a final system design have equal screen lengths in the same lithologic
materials, the flow rate per well can be used instead of calculating the
flow per foot of screen.
In most cases, if the vacuum is loss than about 40 inches of water column
(one-tenth of an atmosphere), designers can assume that the rate of air
flow to vacuum is linear. Note: This assumption is invalid because air is
compressible, but the method is useful for estimating air flow under low
vacuum conditions.
Example: If the pilot test indicates that 72 scfm is -achievable from
a well under 9 inches of water column vacuum, the system is designed
to have 14 inches of water column at the well head. The flow rate
would be:
14
72 scfm * Q — approximately 112 scfm
Where the vacuum is greater than about 40' inches of water column, the rate
of air flow to vacuum is not directly proportional because air is a
compressible fluid. In this case, adjust the required vacuum to account
for the compressibility of air. A multiplier that approximates the
compressibility of air is appropriate.
Example: A pilot test indicates that 40 scfm are achievable under 3
inches of Hg. Note: 1 inch of Hg - 13.55 inches of water column.
The full-scale system is intended to provide 100 scfm from each air-
extraction well.
3 Inches Hg * ,„ S<1 m — 7.5 inches Hg uncorrected for compression
° 40 sctm
Assume that atmospheric pressure is 29.92 inches Hg.
-------
Guldtnci tot Sail Vtotlog Syttmu
7.5 Inches Hg * *9'*2 '3 inehas gf
* 29.92-7.5 inches Hg y'u
^
In this example, 9 inches of mercury vacuum is necessary to achieve
100 scfm per well. This empirically-derived approximation is not
very accurate at vacuums above 10 inches of Hg, but it is generally
usable in the vacuum range of most soil venting ssystems. A better,
more complicated correction factor that compensates for the laminar
to turbulent flow transition is described in Clarke et al. (1993) and
in Wilson et al. (1992). If the system is very large and/or will run
at high vacuums, use the correction method described by Wilson
instead of the simplistic method described in the example above.
The above correction factors assume that there is no water-table
upwelling. If there is significant upwelling, the screen length in
the unsaturated zone changes, and ,the estimate is not correct.
If the pilot test uses a 2-inch well and the full-scale system uses a 4-
inch well, approximately 15 percent more air flow will be extracted at the
same vacuum because of the larger well (Johnson et al., 1990).
The above means of estimating the achievable air-flow rate in a full-scale
soil venting system assumes that the soil intrinsic permeability and the
exposed length of screen remains the same over time. There are a number of
reasons that the air-flow rates in soil venting systems change with time
including the following:
Seasonal water-table fluctuations and Vacuum induced water-
table fluctuations change the amount of well screen available
for air flow.
Clay and silt soil types may dry out and crack while operating
a soil venting system, increasing air flow through secondary
permeability.
The effective porosity (to air flow) of the soil and thus air
permeability can increase as moisture is reduced in the soil by
the drying effects of the air flow.
The air permeability of the unsaturated zonis changes with
infiltration events because of fluctuating effective porosity
to air, changing the air flow to the well(A). A paved ground
surface can minimize this effect.
Pilot test data should be used because it is the best data available for
designing a soil venting system, even though it may not provide 100 percent
accurate results.
Determine a total desired air-flow rate for the site using the total
available well-screen length from all wells and the rate of air flow per
linear foot of screen. If all wells in a final system design have equal
screen lengths in the same lithologic materials, the flow rate per well can
be used instead of calculating the flow per foot of screen,. In general,
the total flow rate will be between 50 and 500 scfm for most petroleum
sites. Sites that have a larger area than typical petroleum sites may have
higher total flow rates. Some sites that have contamination in a very
limited area (diameter of 50 feet or less) may only require one air-
-------
Ould*nc« for Soil Venting Syfttm* . P«M 26
extraction well and less than 50 scfm.
To evaluate whether or not the vacuum will be too high, which may create an
unacceptable amount of upwelling, designers should determine the design
vacuum and air-flow rates. It is possible that the vacuum will lift the
water table above the zone with the highest level of contamination. In
this case, the high vacuum is counterproductive because the contamination
is submerged below the cleansing effects of the air flow. A, lower vacuum
and corresponding lower air-flow rate is more productive over time, unless
the groundwater table is lowered by pumping. Pumping groundwater is
discussed in Subsection 4.9.4.
The rate of contaminant extraction will decline with time during a full-
scale site remediation. The anticipated contaminant removal rate at start-
up is similar to the pilot test results. If the air-extraction wells are
not installed in the highest soil contamination zone, the contamination
extraction rates may actually climb for the first few days after full-scale
start-up as contaminants are drawn towards the well(s). Otherwise, the
pilot test results may provide the highest achieved contamination
extraction rates. For engineering design purposes, apply a safety factor
of 1.5 to the highest.levels of contaminants in the air during the pilot
. test to predict the highest levels of contaminants from a full-scale
system.
Compare the maximum air emissions at start-up to the total desired air-flow
rate using the emission limits and the achievable air-flow rate calculated
from the pilot test data (with safety factor). It is possible that air
emission control will be needed; no emission control is necessary or; the
initial flow rate from the system needs to be limited using a timer or a
dilution valve. Air emission control devices are discussed in greater
detail in Subsection 4.7.
The air emission limits in Wisconsin are based on the total mass of
contaminants emitted during a period of time. If no emission control is
anticipated, the contaminant extraction limits are the limiting factor
during start-up. The limits for petroleum sites are discussed in
Attachment 1. System designers can estimate the maximum air-flow rate at
start-up by dividing the total emission limit by .the concentration measured
during the pilot test. ' '
Example: A LUST site pilot test indicates that total VOCs are
1.0 E-3 pounds per cubic foot and benzene is 1.0 E-6 pounds per cubic
foot. Assume that the total VOCs are limited to 5.7 pounds per hour
and benzene is limited to 300 pounds per year. The maximum air-flow
rate for total VOCs is then estimated to be:
*
5.7 pounds per hr e ^nn . . . ^. * „ t_
1 E-3 pounds per foot? ' " 5'700 standard cubic feet Per hour
5,700 standard feet3 per hr «c *
..f. . , * <• 95 scxm
60 minutes per hr
Applying a safety factor of 1.5, the maximum becomes:
95 scfm
1.5
- 63 scfm
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Gatdtnci tar Soil Vmtiag Bfttim* ' $M. 2?
A similar calculation for benzene is:
- 300 pounds per yr
1.0 £-6 pounds per foot3 " 30°'°°°'°°° standar^ cubic feet per year
/
300,000,000 standard feet3 per yr
525,600 minutes per year " s ffl
Applying a safety factor of 1.5, the maximum becomes:
570 scfm
1.5
-380 scfm
In this example the limiting factor is total VOC«. The designer
(based on past experience) anticipates that the «ystem will have a
contaminant extraction rate at about one-third of the maximum after
two months. Therefore, the designer selects a blower much bigger
than 63 scfm. The flow rate at start-up is limited to approximately
63 scfm by using a dilution valve (Subsection 4.A).- If approximately
two or three months of sampling indicates that a hijjher air-flow rate
can be used while complying with the air rules, the system operator
increases the flow rate by adjusting the.dilution valve to increase
the contaminant extraction rate.
Note: Upon start-up, a dilution valve may be used t:o control total
VOC emissions, but not to control benzene emissions,
4.3 Well or Trench Design.
Vertical air-extraction wells at most sites are used to extract air from
the soil. In rare cases, a horizontal air-extraction system is warranted
over vertical wells. Conceptually, a trenched system is preferred if the
groundwater table is very shallow (less than about 10 feet) or if the soil
contamination is very shallow. Figure 4-2 shows a typical well design.
4.3.1 Vertical Extraction Wells.
The extraction wells should be constructed using 4.25-inch or larger
inside-diameter, hollow-stem augers. Any drilling methods; (other than
hollow-stem auger), such as mud or clear-water direct rotary, should not be
used if the method creates an excessive filter cake buildup on the bore-
hole wall. Refer to Subsection 1.3.1 for a discussion of regulatory
requirements on well construction.
The pipe and screen should be flush threaded schedule 40 EVC or CPVC.
Steel or other materials may also be used. The recommended diameter of the
screen and casing is 4 inches. Two-inch, 2.5-inch, and 3-inch diameter
screen and casing are also used on some systems.
The advantages of 4-inch diameter over 2-inch are:
i
The flow rate at a given vacuum is higher with a larger well
(Johnson et al., 1990). Doubling the well diameter increases
the air-flow rate by 15 percent. Two-inch diameter wells are
more restrictive to air flow.
-------
Oold«ac» for Soil Vntint Syst
Ff«« 28
Figure 4-2
Typical air extraction well design with above grade manifold
Threaded
locking
Sample port
Flow meter Vacuum
*^o Q' o ^Thermometer
Surface seal
8 to 10 inch boring
2 to 4 inch diameter well casing
Cement or bentonite/cement grout
Valve
Bentonite seal
Fine filter pack
Well screen
Coarse filter pack
Bottom plug
Seasonal low water table
-------
Guidtnm tee Sail tontine Sy«t«ms PM> 29
Little water is lifted up the well, if the up-hole air velocity
flow is limited to 1,000 feet per minute or less. This reduces
water production and accumulation. In many cases, a water trap
(Subsection 4.5) is not needed on a soil venting system if the
up-hole velocity is minimized. A 2-inch diameter, schedule 40
well casing will only deliver 23 scfm if the up-hole velocity
(at atmospheric pressure) is 1,000 feet per minute. A 4-inch
diameter, schedule 40 well will produce 88 scfm at 1,000 feet-
per-minute of up-hole velocity.
Packers may be used to seal off portions of a screen that
intersect relatively clean soils. There are packers and
similar devices available off the shelf for insertion in a 4-
inch diameter pipe.
If there is a sufficient water accumulation to block off a
significant portion of the screen, it may be necessary to pump
groundwater from the well. A greater variety of pumping
equipment is available that can be used inside 4-inch wells
rather than 2-inch wells.
If the diameter is less than 2 inches or greater than 4 inches, the
diameter should be justified in the work plan submitted for the site.
The Screen.length is a function of the water-table depth and the
contamination zone. In isotropic soil conditions, set the base of the
screen at the seasonal-low water table. Set the top of the screen at a
depth that will channel most or all of the air flow through the
contaminated soil and limit short circuiting of relatively clean air from
the ground surface. At some sites with unusual or complex geological
conditions, it may be appropriate to nest wells. In this case, set the
well-screen depth for each well for the specific purpose of that well (such
effects)"8 SCreetlS itl differinS geologic strata to reduce short circuiting
At petroleum sites, the soil near the capillary fringe may have the highest
levels of soil contamination because contaminants often collect at the top
of the water table. The air-flow rate through the soil is reduced near the
capillary fringe because of reduced effective porosity for air flow By
screening the wells near the water table, the rate of contaminant
extraction in these cases is maximized. This channels more air flow past
the high contamination zone. This may be an appropriate a.ction at this
type of site to reduce the air flow through the upper, relatively clean
soils. •
It is permissible to install additional screen length below the water table
to place a groundwater extraction pump. With a pump that has sufficient
capacity, this form of pump placement allows the system operator to dewater
the entire screen, if necessary. See Subsection 4.9.3 and 4.9.4 for a
discussion of groundwater extraction and free-product recovery. Designers
should place a plug at the base of the well screen in accordance with
NR 141.
The filter pack should be sized for the formation. Since air-extraction
wells are not developed, a filter pack that is coarser than a typical well
used for groundwater extraction, is usually acceptable. Size the screen-
slot size for the filter pack. Generally, a slotted pipe provides
-------
0uid«nc» tor Soil V«ntln» Sytttmm 30.
sufficient open area per linear foot of screen. The filter pack and well •
screen-slot size at and below the seasonal-high water table should be
determined based on groundwater extraction criteria. Please refer to the
Guidance for Design, Installation and Operation of Groundwater Extraction
and Product-Recovery Systems for groundwater extraction well design.
The top of the filter pack should be a short distance above the top of the
screen; generally 1 to 2 feet is appropriate. If a coarse-gravel pack is
used, a fine-filter pack that is 6 to 12 inches in height can be placed
above the coarse-gravel pack to limit the potential for grout or bentonite
entering the well screen.
A bentonite seal is often used to prevent grout from entering the screened
interval. In air-extraction wells, limit the bentonite seal to a short
thickness of 6 to 12 inches because bentonite can dry out in the
unsaturated zone. This may allow air to short circuit through the annular
space.
Cement grout or bentonite/cement grout in the annular space above the
bentonite seal should be vised. The grout seals the annular space up to the
ground surface or to the manifold, if a buried manifold is used. If the
grout is poured instead of tremied into place, use care to avoid displacing
or damaging the bentonite seal and upper-most portion of the filter-pack.
A tee fitting and not an elbow to connect the air-extraction well to the
manifold should be used. Using a tee fitting allows for the attachment of
a threaded cap to the top. The threaded cap provides access to the
interior of the well to take water-level measurements or to install pumps
or packers. • ' .
If the manifold is buried, the surface seal should be constructed in a
manner similar to that described in Chapter NR 141. An air and water-proof
manhole cover should also be used. Other fittings (valves, etc.) discussed
in Subsection 4.4 can also be installed under the manhole cover(s).
4.3.2 Horizontally Screened System Design.
Horizontally-screened systems are sometimes used at sites where groundwater
tables are shallow or where contamination is limited to shallow portions of
the soil column. A significant amount of care is necessary when designing
and installing an efficient horizontally-screened soil venting system.
Short circuiting of air .flow through the backfill above the screen or
perforated pipe is a common problem. Mixing a small amount of bentonite
into the spoils prior to back filling may reduce the permeability and short
circuiting problems.
A thorough hydrogeologic knowledge of the site is essential to design a
trench for an air-extraction system. Because vacuum induced water-table
upwelling and/or seasonal variations in the water table can flood the air
inlets in the perforated pipe or screen, the screen or perforated pipe
needs to be installed high enough to prevent flooding.
Generally, a horizontal system is installed with a backhoe. Dig the trench
and install a PVC-perforated pipe or screen in a pea gravel backfill.
Place the spoils that were removed during trenching over the gravel.
Placing plastic sheeting above the gravel and below the backfilled spoils
reduces vertical short circuiting of air through the trench backfill.
-------
Cuid«nc» tot Soil Vratlof ,
P«*» 31
Compact the spoils as much as possible to reduce the vertical permeability
f J! ^T10!^18 ?0t, Safe C° enter> comPact tl»e spoils by tamping the soil
with the backhoe bucket in very thin lifts.
If pavement is placed over the trench) plastic sheeting should be installed
under any gravel subgrade that is placed below the pavement. This will
limit vertical recharge from the subgrade to the backfill.
Installing a trench that is very long may increase the occurrence of short
circuiting. Since the construction of a trench may cause a short
° T1^,1?8 "^ f°r air fl°W' the lonSer the trench, the greater the
probability of inadvertently constructing a short circuit route in the
trench. •
Handle excess spoils that are not placed back in the excavation in
accordance with the DNR guidance on investigative wastes or solid and
hazardous waste regulations dependant on the volume of contaminated soil.
4.3.3 Gas Probes.
Permanent gas probes are vapor wells that are installed to assess
vacuum/pressure and subsurface vapor concentrations of VOCs and/or
biodegradation products. Temporary gas probes, such as those used for soil
gas surveys are also acceptable. The construction details;, materials
i^T' etC' a"not sPecifi«d in Chapter NR 141. However, gas probes
should have an annular seal and a surface seal constructed to Chapter NR
141 standards to prevent the gas probe from acting as a conduit for
an^°f a Sh°rt CirCUit route for air flow- I* * g« Probe is
0f1b*1T ^ seasonal-hiSh wat« table, then the pVobe is a
S ^ C0nstruct the •" P«*'e to Chapter NR 141
Purging a minimum of 3 to 5 volumes (of air) is appropriate when taking
samples for field instruments or laboratory analysis.
1 £°r " dUCUSSl'"1 °£ *>-*« «as probes tn predicted
4.4 Manifold and Instrumentation.
The manifold in soil venting systems is either installed above grade or it
is buried. When the area is used for activities that will not allow the
use of above-ground manifolds (parking lots, driveways , dispenser islands,
JC>)> Jhe"anlfold should be buried. Above-ground manifolds are suitable
when uninhibited access does not have to be maintained at a site.
If contaminant migration is minimal AND if the DNR project manager
approves, some systems may operate only during the warmer portions of the
year. In cases where the project must operate all year, the manifold
should be winterized (or capable of being winterized) at a later date
Generally, an above -ground manifold can be winterized with self regulating
heat tape and/or pipe insulation at any time. Above-ground systems
therefore, are not usually winterized until it is necessary. Buried
manifolds are not easily winterized, so these systems are usually insulated
or installed near or below the frost level. If the manJLfold is winterized
at a later date with heat tape, use CPVC pipe instead of PVC pipe to
provide higher strength in high temperatures.
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Ould*nc« for Soil Vntinc Sy«t««s P«S« 32
Generally, .manifolds should be constructed with 4-inch pipe. Systems have
been installed with manifolds as large as 24 inches in diameter, but these
large systems have centrifugal blowers that require a low manifold vacuum.
The designer should evaluate pipe friction in the system to ascertain that
the manifold will conduct the desired air-flow rate under either of the
following conditions:
If a 2-inch manifold pipe is used, the air-flow rate is over 50
scfm and any piping run is longer than 50 feet.
• If a 4-inch manifold pipe is used, the air-flow rate is over
300 scfm and any piping run is longer than 50 feet.
The manifold may accumulate condensation if the air velocity is lower than
a few thousand feet per minute. One method to avoid a condensation buildup
is to slope the manifold towards the air-extraction wells where it can
drain. Another satisfactory method with buried manifolds is. to use a
relatively small-diameter vertical pipe where the direction changes from
horizontal to vertical, allowing the airstream to carry condensation up
towards a water trap.. This method is satisfactory if the air flow in this
smaller pipe has an up-hole air velocity of 3,000 feet per minute or more.
A less satisfactory method is to maintain a high air velocity on the entire
manifold by using a small-diameter pipe. This alternative is less
satisfactory because pipe friction may be excessive, resulting in added
requirements for blower capacity and excessive electrical costs.
Designers should configure the manifold and place valves in such a way to
allow control and sample collection at each well. Above-ground systems may
have the sample ports and instrumentation for each well near the well
itself. The sample ports and instrumentation on buried manifold systems
may be located near the blower system where the manifold pipe exits the
ground. Figure 4-3 shows two different options for instrumentation
locations on a buried manifold. The option that places the instrumentation
nearest the well generally provides the best vacuum and temperature
information for the well, but is more likely to freeze up in winter on low
flow systems and systems with a shallow water table.
Construct the manifold with glued fittings, since slip fit joints may fail
with time. It is recommended that a steel wire or similar material is
installed in the trench with buried manifolds that have plastic pipe to
find the trench later with a metal detector. Note: This is unnecessary at
sites with reinforced concrete pavement, since the metal detector will only
"see" the rebar.
Install a flowmeter, a vacuum gauge or manometer, a thermometer, and valve
at locations where samples are collected. These devices are described as
follows: •
• Valves. Each well should be installed in a manner that allows
the well to be isolated from the rest of the system. PVC ball
valves or gate valves are generally used to isolate each well:
If the flow rate through the pipe is expected to be over 100
scfm, the valve should not be smaller in diameter than the
manifold pipe. In lower-flow systems, the valve may be smaller
to reduce costs.
A dilution or bleed valve is also needed on the manifold
immediately before the air enters the air filter or blower (if
-------
Gaidanet tat Soil Italia* Sr«t«M
Figure 4-3
Wellhead details with buried manifold
Ground surface Air and waterproof
/ /well cover
P««« 33
Surface
seal
/
Steel wire
Manifold pipe
Air extraction well
n O O
fi
Frost level
or below
Sample port and
instrumentation
Instrumentation located
near blower
Access
cap
Surface
seal
Air
extraction
well
Air and waterproof
well covers
i
r,O<
Sample port and
instrumentation
Ground surface
Frost level
or below
Instrumentation
at wellheads
Steel wire
Manifold pipe
-------
Guidinc* for Soil Voting Sy«t««« • P«c« 34
no filter is used). The dilution valve allows atmospheric air
into the blower, when opened, and relieves vacuum to reduce
overall air-extraction rates from the wells. Do not install
the dilution valve between the wells and the sample ports,
because the sample results would not represent extracted air
concentrations. ..A dilution valve is more energy efficient than
a throttle valve that restricts air flow because blowers
require the least amount of electrical power when.the pressure
differential across the blower is relatively low. In addition
to a dilution valve, install an automatic pressure relief valve
if the blower may overheat under a blocked flow condition. A
silencer on the inlet to the dilution valve may be needed in
some cases. If the dilution valve is opened to the atmosphere,
an air filter on the blower is needed, even if it is a
centrifugal blower.
An alternative to installing the dilution valve that opens to
atmosphere is to install a bypass valve to draw air from the
blower exhaust. This allows air to circulate from the exhaust
back to the blower inlet. This alternative does not need a
• , silencer on the intake which lowers equipment costs. A bypass
valve, however, does not allow the system operator to dilute
the airstream at the stack to reduce the concentration to below
the lower explosive limit.
Sample Port. The sample port design is specific to the sample
container and the field procedure for collecting samples. It
may have a septa fitting for direct syringe insertion, or it
may be as simple as a hose barb for a piece of plastic tubing.
The sample .ports may have to be fabricated for the specific
sampling devices.
Flow Meter. Averaging pitot tubes or regular pitot tubes are
generally used to measure air flow. Pitot tube manufacturers
specify that a number of transverse readings are collected at
different points within the air stream when pitot tubes are
used. Averaging pitot tubes are designed to only require a
single reading. In general, manufacturers recommend 10 or more
straight unobstructed pipe diameters upstream and five or more
diameters downstream of the pitot tube or averaging pitot tube.
(Example: A pitot tube on a 2-inch pipe requires 20 straight
'unobstructed inches upstream of the. pitot tube and 10 inches
downstream.) A minimum of approximately 1,000 feet per minute
of air velocity is needed to get accurate readings, therefore,
the pipe diameter may need to be reduced at the location within
the manifold where the flowmeter is installed.
Orifice plate meters are also acceptable if they are installed
in accordance with manufacturers specifications.
Hot wire anemometers are also used in soil venting systems, but
may be inaccurate if there are liquid water droplets in the air
stream. These devices must be classified as "intrinsically-
safe* when working with ignitable contaminants.
For a discussion of flow meters, see Ginesi and Grebe (1987).
• Vacuum. Measure the vacuum with a manometer, a magnehelic, or
-------
Ouldtnet tar Soil Vatlac SjriUtt p-s- 35
a vacuum gauge. Most soil venting systems operate at a low
enough vacuum that the measurements are read in inches of water
column. Higher vacuum units may use inches of mercury as
vacuum measurement units.. Note: 1 inch of Hg - 13.55 inches
of water column. Vacuum gauges should be to two digits of
accuracy.
Temperature, The temperature is usually read with a bimetal
dial-type thermometer that is installed through a hole in the
manifold pipe.
Relative Humidity or Dew Point Relative humidity or dew point
.measurements are not required, but may be beneficial when
evaluating moisture content for biodegradatioin or carbon
filters. Use & wet bulb thermometer or digital meter to
measure relative humidity or dew point.
4.5 Water Trap. "
A water trap (also called a separator tank or demister) may be necessary
In general, a water trap should be included in the design if the up-hole
air velocity in the air-extraction wells is greater than 1,000 feet per
minute or if a rotary lobe blower is used: If.in situ air sparging (see
Guidance on Design, Installation and Operation of In Situ Air Sparzine
Systems) is used, a water trap should be included because the sparEine
process can cause water to enter the air-extraction wells.
Water trap configurations include the following:
A vertical pipe, cap, and tee in a manifold that is capable of
holding less than 5 gallons;
A large tank in line with the manifold;
An engineered trap that uses a cyclone action to separate the
water droplets from the air stream.
I
It is necessary to address the water that accumulates in water traps If a
groundwater extraction system is also used at the site, the accumulated
water can be added to the pumped groundwater that is treated and/or
disposed of. If no groundwater extraction system is used at the site it
is necessary to arrange for proper disposal of the water.
4.6 Blower (or Vacuum Extractor) Type and Size.
I
The following are three common types of blowers for soil venting systems:
Centrifugal. Centrifugal blowers perform best in high flow,
low vacuum applications. Advantages of centrifugal blowers'
include low equipment cost, low electrical costs, and minimal
maintenance requirements. The main disadvantage is that they
cannot develop a high vacuum. These units nre only usable in
sand and gravel environments or in .trenched systems that have a
very high length of perforated pipe. Due to the small vacuum
they develop, long manifold systems may need large-diameter
manifold piping to reduce pipe friction.
Regenerative, Regenerative blowers develop hijjher vacuums than
-------
lor Soil V«ntln« Symttmt , *««• 36
centrifugal blowers (up to 8 inches of mercury) . These are the
most common blowers for smaller sand and gravel sites.
•"" Rotary Lobe. The rotary lobe blowers are capable of producing
very high vacuums (up to 15 inches of mercury is not uncommon) ,
which is the primary advantage of this blower type.
Disadvantages include higher cost, high electrical demands,
high noise levels, and frequent maintenance requirements. Soil
venting systems in silt or clay soils require the rotary lobe's
high -vacuums . ' •
Figure 4-4 shows performance curves for these three blower types. Each
curve on the figure is for comparison purposes only; larger and smaller
models are available for each blower type. It is apparent that the rotary
lobe units have a very high vacuum capability and are the best choice for
sites that need a high vacuum. It is also readily apparent that a
centrifugal unit is the best choice for sites that have high-flow rates
that are achievable with a low vacuum. Regenerative blowers have
characteristics between the rotary lobe and centrifugal units.
Other blower types, such as liquid ring, may also be used when conditions
warrant.
The type and size of the blower determines the 'electrical requirements.
Some of the rotary lobe units are large enough to require three-phase
power.
A discharge muffler should be used to reduce noise for larger soil venting
systems, especially systems that use rotary lobe blowers.
Size the circuit breaker for the motor to trip the circuit breaker if the
rotor is locked. The motor and all controls should be explosion-proof if
there is ANY POSSIBILITY of igniting the contaminants. Sensors should be
intrinsically-safe and controllers need to be in explosion-proof enclosures
or located in non-hazardous locations.
Rotary lobe blowers and other blower types that have close tolerance
clearances should be equipped with a particulate air filter. Regenerative
blowers may also need an air filter. A centrifugal blower is usually best
used without an air filter because the filter restricts air flow.
It is recommended that the discharge stack be constructed with CPVC or
other materials that retain strength at high temperatures on the higher
vacuum systems. The higher discharge temperatures on the high vacuum
systems may weaken FVC. Host blower manufacturers include methods for
estimating the discharge temperature from the blower. If the discharge
temperature reaches approximately 140 degrees 'fahrenheit (or higher) , PVC
may become too weak. In general, FVC is acceptable on all centrifugal
blower systems. A drain at the base of the stack is useful to drain any
accumulated moisture.
4.7 Emission Control Devices.
*>
If emissions exceed the table value of any contaminants listed in Table 3
of Chapter NR 445, then 95 percent contaminant removal or destruction
capability is required. Sources requiring air treatment devices that
exceed table values in Table 3 need DNR Bureau of Air Management permits.
The following are three types of air treatment devices for controlling
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ftiidaoc* for Soil Vmtlai Syittmt
P«8« 37
Figure 4-4
Performance curves for three types of blowers
Rotary lobe blower
Regenerative blower
"" "~ Centrifugal blower
160 200
Airflow - SCFM
I
240 280
Notes:
Centrifugal blower type shown is a New York model 2004A at 3500 rpm
Regenerative blower type shown is a Rotron model DR707
Rotary lobe blower type shown is a M-D Pneumatics model 3204 at 3000 rpm
nmcoh« °es* nc* endorse tnese b'ower manufacturers; performance
curves shown for discussion purposes only
-------
GolcUnc* for Soil Viotinc Sytttmf • ?•»• 38
emissions from petroleum projects:
Incineration. Incineration is most cost-effective with high
contaminant levels because the contaminants provide a
significant amount of fuel for the incineration process.
Catalytic Destruction. Catalytic units generally are used with
high contaminant levels, but lower contaminant levels compared
to the incineration units. If the contaminant level is too
high, the catalyst becomes too hot and burns out. Pilot test
data is necessary to assess if it is appropriate to use
catalytic destruction units.
• Granular Activated Carbon (GAG). GAG units are not used
frequently in Wisconsin compared to other states. At the
levels where GAG is most cost effective, the emission limits in
Wisconsin generally allow a direct discharge. If.carbon
filters are used, a device to dehumidify the air may be needed.
Other treatment devices, such as bio-filters or internal combustion engines
may also be acceptable to the DNR on a site-specific basis. This should be
discussed with the DNR project manager before purchasing and installing.
Greater blower-pressure capacity is needed with off-gas control systems
because of the flow restriction within the system. Manufacturers of off-
gas control equipment may provide pressure and flow requirements for
equipment.
4.8 Air Injection. ' ' .
Some projects use air injection to direct air flow through a specific part
of the contaminated soils. It is usually used to help create more flow
near the capillary fringe, which is often the hardest part of the soil
column to remediate.
In general, do not use air injection if the injected air temperature is
lower than the normal groundwater temperature. The colder air reduces the
volatility of the contaminants and also reduces the biodegradation rate.
4.8.1 Passive Vents. .
Some projects use passive air injection to help direct air flow through
contaminated zones. Passive air vents are venting wells that inject air
under atmospheric pressure without using a blower. The driving -force is
the induced vacuum in the -subsurface that* is created by the soil venting
system. Only a small percent of the air that is extracted from a soil
venting system is from passive injection. Air-flow rates in passive
injection wells typically cannot be accurately quantified because of the
low rate of air flow into the well. The wells can be constructed for the
purpose of air .injection, uncapped water-table monitoring wells, or air-
extraction wells that are valved off of the manifold and open to the
atmosphere.
Generally, passive injection is not very effective. However, converting
existing wells to passive injection may be appropriate if useable wells
already exist at the site. • .
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Gaidtnci tor Soil Vtettiac Syfttmt Pmg. 39
4.8.2 Forced Injection. ! •
In some cases, clean (contaminant-free) air is injected into the soil
through a series of wells or trenches.. Designers should justify the use of
forced injection in the workplan on a site-specific basis, -i
To assure that the injected air is extracted by the extraction system, the
injection rate should be no more than one quarter of the extraction rate.
If the proposed ratio of.injection to extraction is greater than 0.25
provide justification in the work plan. Air injection wells must inject
air at very low air-flow rates if they are within the izone of
contamination. Otherwise, they may force contamination outward to
uncontaminated areas or through the ground surface. If air injection is
proposed within the zone of contamination, air modeling is needed to
evaluate flow paths.
Air that is oxygen deficient should not be injected at sites with
aerobically degradable contaminants because it could slow the
biodegradation rate.
To assure that there is no positive pressure in the subsurface near
• basements or other structures where vapors may collect,, gas probes may be
needed with air injection on a site-specific basis.
It is possible to heat air to increase the volatility of the contaminants
If heat is added, the temperature must be low enough so that it does not
disinfect the soils, which adversely affects natural biLodugradation.
4.8.3 In Situ Air Sparging.
Air sparging is a form of forced air injection into the saturated zone If
air sparging is used, include a water trap in the design of the soil
venting system. The sparging process may cause excessive amounts of water
to enter the air-extraction wells. See Guidance on Design, Installation
and Operation of In Situ Air Sparging Systems for a detailed discussion of
air sparging.
4.9 Other Design Considerations.
4.9.1 Surface Seal.
A surface seal, such as a pavement layer,, is often recommended in the
literature. A surface seal directs air now horizontally and restricts
vertical air flow from the ground surface near the extraction well(s)
Sites that are highly stratified, or sites that have a high Kh/Kv ratio do
not need surface seals because the natural geologic conditions force the
air-flow patterns horizontally. Sites without a high Kh/Kv ratio or
stratification may benefit from a surface seal. Figure 4-5 indicates air-
flow patterns with a quality surface seal, a poorly constructed surface
seal and no surface seal.
If surface seals are used, it is important to construct them properly.
There is usually a gravel subgrade below pavement. Significant quantities
of air can flow horizontally through a highly permeable subgrade toward the
extraction well(s), even though the subgrade is less than a foot thick.
The propensity of the subgrade to act as a short circuiting route is
directly proportional to the ratio of horizontal permeability of the
subgrade to the vertical permeability of the underlying soils.
-------
Guidmc* tor Soli Vontiag Synttmm
P«S« 40
Figure 4-5
Typical air flow patterns
Ground surface
. •(*•
Water
Isotropic conditions
Stagnation zone
Stagnation zone
isotropic conditions with perfect surface seal
/• Stagnation zone ,1"
Isotropic conditions with gravel subgrade under surface seal
• \ . . . \
Heterogeneous conditions
Stagnation zone
Note: Not to scale, for conceptual discussion purposes only
Flow is three dimensional. Recharge also occurs in the dimension
perpendicular to the drawing.
-------
Guidaas* tor Sou Vmtiaf Syittas
P«8» 41
Surface seal modeling is described by Krishnawa et a] ri988^ Tn t-Ko
S^TISIS' S^ »***«£ tJSSJ*i.££i fl^d
suSacf 'sea? n * recharS* that occurs with . very high quality
finer eralne^ T? '"P61^ inf a11 a *»"* surface seal, especially in
firSiS ? s^ls« Because of the potential for horizontal short
circuiting immediately below the seal.
4.9.2 Stagnation Zones.
Stagnation zones are areas that have little or no air flow because two or
WS 6 UUi ^
in diffe"»t directions
of a
a1modi~i0n Wil1 Sene?ally occur a<= Blower rates in .stagnation zones than
at other areas of a site due to the low velocity of air flow through these
soils. Because stagnation zones are created by the location of onf air!
11 riativt "
Systeils with
wes mcint
f^i™ C i? ^ operation requires periodic changes in flow rates
from each well Designers should change flow rates from different wells
-
eren wells
r-s;
As discussed in Subsection 2.1, stagnation zones can also be anaerobic
zones where aerobic biodegradation is slowed due to Umited oxygen
4.9.3 Vacuum- Enhanced Product Recovery.
.
^
Lrer y £"***•"* ' ™* <*"*»<* for Design, iHstallatlo^d
Operation of Groundwater Extraction and Product -Uecoverr Systems provides a
a
aPFlied t0 ** ""very well should not be
is created by
Example: Assume that the submerged portion of th«s roe'ovaxy well is
iStET? t-H I1?"? C? 8roundwater flow» «* the unsaturLed screen
portion of the well is 100 percent efficient to air flow. A 10 gpm
pumping test indicates that the specific capacity of the recovery
well is 12 gpm per foot of drawdown. The hydraulic conductivity
determined from the pumping test is used to calculates the capture
zone. It is determined that 50 gpm is required to capture the plume
-------
Ooidmoa *or Soil V«ctin* Sy*tma **•• *2
The predicted drawdown at start-up of the recovery system is:
^-L 50 SPm -5 » 4.17 feet or 50 Inches of water column
12 gpm per ft of drawdown
Therefore, the maximum vacuum that should be applied to the well is
50 inches of water column.
If the specific capacity is unknown, it can be estimated. See
Attachment 3 to the Guidance on Design, Installation and Operation of
Groimdwater Extraction and Product-Recovery Systems for estimating
drawdown.
Volatilization of liquid product will take place because vacuum-enhanced
product recovery passes the air flow through a well that has liquid phase
product in it. This raises the possibility that the air emissions may be
quite high, possibly exceeding air emission limits. Designers should
evaluate the cost efficiency on a site-by-site basis to compare the costs
of air emission control with the advantages of vacuum-enhanced product
recovery.
Also see Subsection 4.9.4 in the Guidance on Design, Installation and
Operation of Groundwater Extraction and Product-Recovery Systems for a
discussion of well design.
4.9.4 Groundwater Extraction from Air-Extraction Wells.
In some situations, air-extraction wells may also be used for groundwater
extraction. In most cases, air-extraction wells are used for groundwater
extraction because they are in a convenient location and drilling costs are
reduced by using one well for two purposes. In other cases, the vacuum is
used to increase the yield of the well. Occasionally, the water is pumped
out of the well to counteract the effects of upwelling, and to lower the
groundwater table to expose this smeared zone to air flow (Johnson, et al.,
1992).
If a well serves these two purposes (groundwater and air extraction), it
must be designed for both purposes. Construct the lower portion of the
well that is used for groundwater extraction with a well screen and filter
pack sized for groundwater extraction. If the slot size or filter pack is
too large, the well may pump sand. See Guidance for Design, Installation
and Operation of Groundwater Extraction and Product-Recovery Systems.
If the formation is highly permeable, enormous quantities of groundwater
have to be extracted to significantly lower the water table. If the
primary purpose of groundwater extraction is to lower the water table to
expose contaminated soil to the air flow (and not to extract dissolved
phase contaminants), it may not be cost-effective to pump and treat
groundwater at some sites. In this situation, in situ air sparging or
other techniques may be preferable. See Guidance on Design, Installation
and Operation of In Situ Air Sparging Systems.
4.9.5 Enhanced Biodegradation of Petroleum Compounds in Soil.
Petroleum based contaminants readily biodegrade during operation o'f a soil
venting system. Biodegradation for petroleum projects is an important part
of the remediation process because a significant quantity of the
-------
GuiOtnet tot Sell Vmtiac SystM*
contaminants are destroyed by natural bacteria (Hinchee and Miller, 1990
and Miller, 1990). Generally, the degradation rate is much faster under
aerobic^ conditions than anaerobic conditions. The level of oxygen is
usually the limiting factor under static conditions, The venting system
provides oxygen when.using active venting, and moisture or nutrient supply
become the limiting factors,
To quantify biodegradation rates based on oxygen or carbon dioxide
emissions, it is necessary to measure background oxygen and/or carbon
dioxide in the soil. Ideally, the background measuring point is one or
more upgradient water-table well(s), and/or gas probe(«) that are located
in an uncontaminated part of the site which is/are not us«d for air
extraction. Measuring the ambient background levels of o:cygen and/or
carbon dioxide .in the soils is necessary whenever oxyg«n or carbon dioxide
samples are collected because the ambient levels may change seasonally
(Wood et. «!.. 1993; Solomon and Cerling, 1987). The change in the carbon
dioxide or oxygen levels, relative to background, is the value to use when
quantifying biodegradation rate. Attachment 1 includes! a sample method to
quantify the biodegradation rate based on carbon dioxide.
The advantage to using carbon dioxide measurements to measure
biodegradation is that carbon dioxide can be quantified with a high level
of precision at very low levels; It is difficult to precisely measure a
very small oxygen deficiency. Two disadvantages to measuring
biodegradation with carbon dioxide are that carbonates in the soil can
dissociate or precipitate, and carbonic acid can form which reduces the
accuracy of the estimate. Generally, the literature indicates that
practitioners prefer to use oxygen to quantify biodegradation, instead of
(or in addition) to carbon dioxide because oxygen is less affected by the
soil geochemical properties.
Also, see Subsection 5.1 for a discussion of oxygen breakthrough.
4.10 Soil Venting System Design Report.
In some cases, the design of a soil venting system is included in a
comprehensive report with the results of a pilot test. In other cases, the
design is submitted separately. A report that includes the design of a
soil venting system should include the following:
Pas* 43
Discussion.
A discussion of the system design and a description of
capabilities for remediating the soil at the site. Include a
brief discussion of the geological conditions at the site.
Describe the logic used to determine well placement and
spacing.
Details of the air-extraction well design include the screen
length and diameter, slot size, depths and specification of the
filter pack and seals, and the drilling method,. If multiple
well depths are needed, discuss the logic for determining, well-
.screen depths.
*
Justify a horizontally-trenched system if it: is proposed in the
design report.
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GultUoca for Boil Vntias Sytttmt p*** **
• Details of the manifold design including pipe type, diameter,
and a description of instrumentation for measuring flow and
vacuum. Indicate the depth of the manifold, if-it is buried.
f
Blower specifications including total anticipated air-flow
rate, vacuum levels, type and size of blower.
Discuss the Wisconsin air emission limits, anticipated flow
rates, pilot test results, and the possible need for air
emission control devices. If air emission control is proposed,
discuss the type of system and the status of any air permitting
requirements. The discussion should include an estimate of
total VOCs in the unsaturated zone.
• Discuss options for water disposal, if a water trap is
proposed.
The height of the stack.
Monitoring plan.
— Non-petroleum sites. There are no specific requirements
for non-petroleum sites. The designer should propose a
monitoring plan in the workplan,. In most cases,
reporting frequency and sampling frequency will be the
same as the one in Attachment 1. Sampling parameters,
methods, etc. are determined on a site-specific basis.
— Petroleum sites. Attachment 1 is a generic plan for
' petroleum sites. The designer should prepare a site-
specific plan based on Attachment 1. Deviations from
Attachment 1 should be identified and justified. If a
photoionization detector is used, see Robbins et al,
(1990).
Figures.
A map of proposed well locations drawn to scale. The map should
include the following:
— locations of proposed and existing air-extraction wells;
— locations of the manifold, instrumentation, and sample
port;
— location of blower and other equipment;
—• suspected and/or known source location(s) (if differing
contaminant types are present at a site, the locations
should identify the contaminant type);
— extent of soil contamination;
— paved areas, buildings, and structures that may act as a
surface seal or an infiltration barrier;
— buried utility trenches that may act as zones of higher
permeability;
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Guidance for Soil Venting Syst«u • p.,. 45
- scale, north arrow, title block, site name, and key or
legend; and
— any other pertinent.site information.
A current water-table map and a table of water levels.
Indicate the date of water-level measurements on the map.
A process flow diagram indicating piping network,
instrumentation and key components.
Appendices.
Engineering calculations for determining the well spacing and
zone o.f influence measurements from the pilot test, if any.
Clearly state any assumptions. Hand written (if legible)
calculations are acceptable. Include the initials or name of
the author and the quality control-checker. Include references
for any formulas used.
If an air-flow model is used, include the results of the model
and any assumptions that the model uses.
Engineering calculations predicting the total air-flow rate.
Include the performance curve that is provided by the
manufacturer of the blower. Note the manufacturer and model of
the blower. Note the rpm of the blower if it is belt driven.
A description of sampling procedures and analytical methods.
Form 4400-120 for LUST sites.
-------
Guid«nc« for Soil V«ntln« Sy*t«u FmR* *6
5.Q Operation of a Soil Venting^System.
5.1 Overview.
Operation of a soil venting system requires ongoing monitoring and system
adjustment to maximize performance.
Immediately after start-up, VOCs associated with the glue from the manifold
are discharged from the system. Samples for compound-specific testing
should not be collected until at least one or two hours after start-up to
allow the VOCs from the glue to be discharged from the system.
For safety purposes, air should not be discharged from the stack at or
above the lower explosive limit. The use of a dilution valve may be
• necessary at some sites during the pilot test or vtpon start-up.
Immediately after start-up of a soil venting system, a large mass of VOCs
are rapidly removed because the concentration of VOCs in the extracted air
is very high. During this.initial phase, air-flow advection through the
coarse-grained soils rapidly extracts VOCs from coarse-grained soil. If
there is a significant amount of stratification or other geologic
heterogeneities in the site soils, the extraction rate will rapidly decline
to a non-zero asymptotic rate of extraction. Buscheck and Peargin (1991)
and Johnson et al. (1990) have an excellent discussion about the reduced
extraction rate over time.
Small fluctuations in the extraction rate are normal with soil venting
systems.
A slow contaminant-extract ion rate may occur even if soil sample results
indicate there is a significant amount of contaminants remaining in the
soil. The slow extraction rate can be due to a number of factors:
• Fine-grained soil units or layers readily retain significant
quantities (relative to coarser-grained units) of contaminants.
Clay soils will commonly retain contaminants at concentrations
that are orders of magnitude higher than the coarser-grained
soils.
The fine-grained soil layers in stratified soils are generally
parallel to the direction of the air flow. Therefore, the
pressure gradient induced by the vacuum does not force the air
flow (and VOCs) through the fine-grained soil layers. Instead,
the air flow is around the fine grained layers. In many cases,
the VOCs diffuse very slowly out of the fine-grained soils into'
the coarser layers for advective transport to the extraction
well.
• Even if the geology is largely homogenous, the distribution of
the most highly contaminated soil in the unsaturated zone is
typically near the water table. The extraction of VOCs is
slowed in the most highly contaminated soil because the air-
• flow rate is relatively slow near the capillary fringe due to
the reduced effective porosity to air flow.
•
Remaining contaminants are relatively non-volatile. See
Subsection 2.1 for a discussion of vapor pressure and Raoult's
Law.
-------
Guldmet tar Soil Vntiat Syt
The following are reasons that extraction rates can increase significantly:
A new loss of product.
Higher air temperature raises the volatility of contaminants.
If there is a high air-flow rate and a low air residence time
in the soil, the ambient temperature in the warmer months can
increase the volatilization rate. Use temperature trends over
time at the wellheads to assess this effect.
Water-table fluctuations can expose additional contamination
that was previously, submerged.
After the system has operated for a few months to a few years, the
emissions fall to A very low level, relative to initial concentrations. At
this point, significant contaminant reduction at petroleum sites is due to
biodegradation. In these cases, the soil venting systems provide oxygen to
the bacteria. Aerobic biodegradation is not significantly inhibited until
oxygen levels have dropped below 5 percent. See Attachment 1 fot a sample
method for determining the biodegradation rate based on carbon dioxide.
It is possible that oxygen deficiency in stagnation zones could exist even
if extracted air is quite low in carbon dioxide and high in oxygen because
of dilution. If most of the air passes through relatively clean soil and
only a small amount of the extracted air passes through biologically active
contaminated zones, there could be oxygen deficient parts of the site that
go undetected because of oxygen breakthrough. Therefore, at larger sites
that have the potential for oxygen breakthrough, it may be prudent to
install gas probes near predicted stagnation zone locations to assess
oxygen and carbon dioxide. Gas probes in those locations way also be
useful for assessing contaminant concentrations or methane.
On a site-specific basis, if significant biodegradation ralres are necessary
to complete the cleanup (for a site with significant levels of aerobically
biodegradable, but relatively non-volatile compounds), an evaluation of
methane may also be needed to assess the presence of anaerobic zones.
Biodegradation requires a high moisture content in the soil. During colder
months the atmospheric dew point is likely to be lower than the soil
temperature. In this case air that is drawn through the ground can remove
significant amounts of moisture from the soil. When the atmospheric dew
point is higher than the soil temperature (which occurs occasionally during
the summer months) drying the soil with an excessive air-flow rate is less
likely.
Because biodegradation requires a fairly high moisture content, it is
•possible that using a slower air-extraction rate late in a project is more
productive than a high rate of air flow. A high air-flow rate may remove
too much moisture and inhibit bacteriological activity. Opening the
dilution valve to reduce the flow rate may be necessary to reduce the
drying effects of the air flow. Another option is to use a timer to
operate the system for only a few hours per day. If the blower is very
large, it may be practical to purchase and install a smaller blower because
of reduced electrical demand and/or reduced maintenance costs.
o
Stagnation zones that develop between the air-extraction wells in multi-
well systems inhibit the ability of a soil venting system to operate
efficiently throughout the entire site. Changing the flow rates from
-------
GuitUnc. for Soil V«otin* SystoM ' *•*• 48
different wells on a periodic basis improves overall system performance.
Some consultants use temporary or permanent gas probes to evaluate air
quality"within the subsurface at points other than the extraction wells.
Water-table wells can also be used for air sample collection. If the trend
of air samples from the probe(s) over time indicate that high levels of
VOCs and/or biodegradation products (carbon dioxide or methane) are
remaining, it is a clear indication that the part of the site where the
probe(s) is/are located is not being cleaned up. Either the probe(s)
is/are located in or near a stagnation zone, or something else is not
working correctly.
Some operators cycle soil venting systems by operating the system
intermittently. In the literature, there is no clear advantage or
disadvantage to cycling soil venting systems. If the consultant chooses to
cycle the1 system, the sampling plan should acknowledge that cycling causes
inconsistent contaminant-extraction rates over time. Increased sampling
frequency may be necessary to accurately evaluate the extraction rate.
5.2 As-Built Submittal.
After a soil venting system is constructed, the "as-built" information
should be included in a report. Since most of the information is in a
design report, a separate submittal is not always necessary. The "as-
built" information can be included in the first progress report after
start-up. The "as-built" submittal should include the following:
Any deviations from the specifications in the design report.
• A map of actual-well locations .drawn to scale. The map should
include the following:
— locations of existing airrextraction wells;
— the manifold, instrumentation, and sample port .locations;
— location of blower and other equipment;
— suspected and/or known source location(s) (if differing
contaminant types are present at a site, the contaminant
types should be identified per location);
— zone of soil contamination;
— paved areas, buildings, and structures that may act as a
surface seal or an infiltration barrier;
— buried utility trenches that may act as zones of higher
permeability;
— scale, north arrow, title block, site name, and key or
legend; and
— any other pertinent site information.
•
• A table with the air-flow rate, vacuum levels, and temperature
at all sampling locations at start-up.
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Guidance for Soil Vratin* Systn* ¥•&»
A table of water levels in all wells.
Air-extraction well construction diagrams.
Boring logs and any other documentation required by Chapter NR
141. . . ,
Any other pertinent information.
5.3 Reporting.
The reporting frequency for most sites are as follows:
Petroleum sites. As described in Attachment 1. The DNR
project manager may specify a different reporting schedule.
• Non petroleum sites. The reporting frequency will be
established on a site-specific basis by the DNR project
manager.
Progress reports should be sequentially numbered starting with the first
report after the remediation system start-up. In general,, the progress
reports do not need to be detailed documents. In most cases, only one or
two pages of text in a letter format with supporting tables and figures is
sufficient.
The progress reports should include the following information:
A brief discussion of the progress of the remediation system
including:
— Contaminant extraction totals to date in pounds or
gallons of contaminant(s) removed.
— System operation details; periods of shvit down, equipment
malfunctions, etc.
— Overall evaluation of the system effectiveness.
— Recommendations for future activities, if appropriate.
Graphs that include data through the life of the project are
very useful to evaluate•trends. Graphs may include:
— Total contaminant removal graph with time on the
horizontal axis and cumulative contaminant removal on the
vertical axis. The consultant may provide a graph with
this information on a per well basis foi: smaller systems
(four wells or less), but a graph on a per well basis
typically is not required unless requested by the DNR.
— Contaminant level time graph, with time on the horizontal
axis and mass per volume values on the vertical axis. A
graph on a per well basis is recommended for smaller
systems, but typically is not required for larger
systems.. i
• Tables that include data throughout the project are useful to
-------
OuicUnc. fox Soil Vantlas Sy«t««« Fag* SO
establish trends. Include the following tables:
— Field data and flow-rate measurements.
— Contaminant levels and extraction rates at each sampling
point. (This table can be combined with the field data
table- if space allows).
— -Table of water levels and product levels or thicknesses.
• If analytical data.is available from & laboratory, include the
lab reports.
• A discussion of sampling procedures, analytical procedures,
etc. is not required, but include a reference to the report
that lists the procedures.
• Any other pertinent information or data.
5.4 Case Close Out.
When to Consider a Site for the Close Out Process. The volatilization rate
on a pounds-per-day basis needs to be calculated prior to sampling before
terminating operation of a soil venting system. If the contaminants are
aerobically biodegradable, the sum of the current rate of both
volatilization and biodegradation — on a pound-per-day basis — should be
included in the mass removal calculation. It is premature to consider the
site for case close out if the mass-removal rate is significant, relative
to the remaining contamination mass.
Methods For Determining the Biodegradation Rate. To determine the
biodegradation rate, the DNR recommends the methods discussed in the
April 5, 1991 guidance on air monitoring for LUST sites, or other
scientifically-valid methods, such as a soil respiration test. Background
carbon dioxide levels, oxygen levels, or both are necessary to evaluate the
biodegradation rate. High carbon dioxide levels do not always mean a high
biodegradation rate.
Site-Specific DataNecessary to Consider Terminating Operation of a Soil
Venting System. The DNR will evaluate soil venting system termination on
the basis of confirmation borings. Soil samples need to be analyzed for
the appropriate contaminants, as follows: .
• For petroleum contaminated sites, soil samples for PVOCs and
GRO and/or DRO need to be collected as appropriate for the
site.
• For non-petroleum contaminated sites and sites that have a
mixture of petroleum and non-petroleum contamination, the
system operator must use sampling protocols that are
appropriate for the site. .The -system operator should consult
the DNR project manager to determine appropriate laboratory
methods.
Number of Soil Borings Per Site. The number of soil borings will-vary from
site to site. Generally, two soil borings are the minimum number to
determine a soil venting system operation termination. For larger, more
complex sites, approximately one spil boring for every three.air-extraction
-------
Guiifae* for Soil Vuntlat 8]rat
-------
for Soil V«otirn &y*ttm*
Fag* 52
contact the appropriate Emergency and Remedial Response Unit Leader in the
district .where the site is located.
-------
Gaidaiet for Soil tontlng Syttttu • r • ss
6.0 References .
Baehr, A.L. , Hoag, G.E., and Marley, M.C. 1989. Removing Volatile
Contaminants from the Unsaturated Zone by Inducing Advective Air-Phase
Transport. Journal of Contaminant- Hydrology, 4:1-26.
** . i
Buscheck, T.E. and Peargin T.R. , 1991. Summary of a Nation-Wide Vapor
Extraction System Performance Study. Proceedings of Petroleum Hydrocarbons
and Organic Chemicals in Ground Water: Prevention, Detection, and
Restoration. November, 1991. NWWA. Pages 205 to 219,,
Clarke, A.N. . Megehee, M.M. and Wilson, D.J., 1993. Soil Clean Up by In-
Situ Aeration. XII. Effect of Departures from Darcy'jj Law on Soil Vapor
ti°n' Sepaeation Science and Technology. Volume 28. Pages 1671 to
DiGiulio D.C., Cho J.S., Dupont R.R. , and Kemblowski M.W. 1990
Conducting field Tests for Evaluation of Soil Vacuum Extraction
Application. Proceedings of the Fourth National Outdoor Action Conference
on_ Aquifer Restoration, Groundvater Monitoring, and Geophysical Methods.
« W \nA * ftfly ( 1 7 9 U , '
EPA. 1991(a) . Soil Vapor Extraction Technology, Reference Handbook
February, 1991. EPA/540/2-91/003.
1991- Guide for Conducting Treatability Studies Under CERCLA-
° aCti°n' Interim Guidance. September 1991. EPA/540/2-'
IeSfrv,C'W' 1988" APPlied Bydrogeology, Second Edition. Merrill
Publishing Company. Columbus, Ohio.
Freeze R.A. and J.A. Cherry. 1979. Groundvater. Prentice Hall
Englewood Cliffs, NJ. '
Ginesi D and Grebe, G. , 1987. Flowmeters a Performance Review.
Chemical Engineering, June 22, 1887. Pages 102 to 118.
Hinchee, R.E. and R.N. Miller, Bioventing for In Situ Treatment of
Hydrocarbon Contamination. Hazardous Materials Control, Vol. 3, Number 5
Sept/Oct 1990, Pages 30 to 34.
Johnson, P.C., M.W. Kemblowski, and J.D. Colthart. 1990,, Quantitative
Analysis for the Cleanup of Hydrocarbon Contaminated Soils by In Situ
Venting. Groundvater, Volume 28, Number 3, May- June 1990, Pages 413 to
*frfc«r » . " • ,
Johnson, P.C., M.W. Kemblowski, and J.D. Colthart. 1988.! Practical
Screening. Models for Soil Venting Application. Proceedings of Petroleum
Hydrocarbons, and Organic Chemicals in Ground Water: Prevention, Detection
and Restoration. November, 1988. NWWA. Pages 521 to 346,,
Johnson, R.L., Bagby, W. , Matthew, P.. and Chien, C.T. 1992. Experimental
Examination of Integrated Soil Vapor Extraction Techniques. Proceedings of
Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention
Detection, and Restoration. November, 1992. NGWA. Pages 441 to 452.
Knieper, L.H. Pollution Engineering. August, 1988. Page 56.
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Guid*Bca tor Soil V«atin* Sy«t««m Fae* 5*
Krishnayya, A.V., M.J. O'Connor,. J.G. Agar, and R.D. King. 1988. Vapor
Extraction Systems, Factors Affecting Their Design and Performance.
Proceedings of Petroleum Hydrocarbons and Organic Chemicals in Ground
Water: 'Prevention, Detection, and Restoration. November, 1988. NWWA.
Pages 547 to 569. •
.*
Masch, F.D. and K.J. Denny. 1966. Grain Size Distribution and its effect
on the permeability of unconsolidated sand. Water Resources Research.
Volume 2, Number 4, pages 665 to 677.
Mendoza, C.A. and McAlary, T.A., 1990.. Modeling of Ground-Water
Contamination Caused by Organic Solvent Vapors. Groundwater, Volume 28,
Number 2, March-April 1990. Pages 199 to 206.
Miller, R.N., 1990. A Field Scale Investigation of Enhanced Petroleum
Hydrocarbon Biodegradation in the Vadose Zone Combining Soil Venting and an
Oxygen Source with Moisture and Nutrient Addition. Ph.D. thesis, Civil and
Environmental Engineering Department, Utah State University, Logan Utah.
Rainwater, K., Claborn, B.J., Parker, H.W., Wilkerson, D., and Zaman, M.R.
Large-Scale Laboratory Experiments for Forced Air Volatilization of
Hydrocarbon Liquids in Soil. Proceedings of Petroleum Hydrocarbons and
Organic Chemicals in Ground Water: Prevention, Detection, and Restoration.
November, 1988. ' NWWA. Pages 501 to 512.
Robbins, G.A., Bristol, R.D., and Roe, V.D. 1989. A field Screening
Method for Gasoline Contamination Using a Polyethylene Bag Sampling System.
Ground Water Monitoring Review. Fall, 1989. Pages 87 to 97.
Robbins, G.A., Deyo, E.G., Temple, M.R., Stuart, J.D., and Lacy, M.J.,
1990. Soil-Gas Surveying for Subsurface Gasoline Contamination Using Total
Organic Vapor Detection Instruments. Ground Water Monitoring Review.
Summer, 1990. Pages 122 to 131.
Shan, C., Falta, W.,. and Javendel, I., 1992. Analytical Solutions for
Steady State Gas Flow to a Soil Vapor Extraction Well. Water Resources
Research, Volume 28, Number 4, pages 1105 to 1120.
Shepherd, R.G. 1989. Correlations of Permeability and Grain Size. Ground
Water. Volume 27, Number 5, Pages 633 to 638.
•Solomon, D.K. and Cerling, T.E., 1987. The Annual Carbon Dioxide Cycle in
a Montane Soil: Observations, Modeling, and Implications for Weathering.
Water Resources Research, Volume 23, Number 12, pages 2257 to 2265.
Wilson, D.J, Clarke, A.N, and Megehee, M.M., 1992. Darcy's Law Limited in
Soil Venting Test. Environmental Protection, November, 1992, pages 26 to
28 and 54.
Wood, B.D., Keller, C.K., and Johnstone, D.L., 1993. In Situ Measurement
of Microbial Activity and Controls on Microbial C02 Production in the
Unsaturated Zone. Water Resources Research, Volume 29, Number 3, pages 647
to 659.
Wisconsin Administrative Code NR 141, Groundwater Monitoring Well-
Requirements.
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Guidanc* for Soil Vratin* Sjr«t«a» r pM. 55
Wisconsin Administrative Code NR 419, Control of Organic Compound
Emissions.
Wisconsin Administrative Code NR 445, .Control of Hazardous Pollutants.
Wisconsin DNR - Guidance on. Design, Installation and Operation of In Situ
Air Sparging Systems.
Wisconsin DNR - Guidance on Design, Installation and Operation of
Groundwater Extraction and Product-Recovery Systems.
Wisconsin DNR - Guidance, for Treatment Systems for Groundwater and Other
Aqueous Waste Streams for a further discussion of permit requirements.
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Attachment 1
Guidance on Air Sampling
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CORRESPONDE^CE/MEMORANDUM-
State of Wisconsin
DATE: April 5, 1991 File Code: 444Q
TO: District AM staff
District and Central Office LUST staff
FROM: Dan Johnston AM
George Mickelson \SW/3
SUBJECT: Guidance on air sampling and emission monitoring for LUST soil and
ground water remediation projects with a synopsis of air
regulations.
This memo is intended to (1) assist district staff in understanding air
SST JST±1S ^L!!!!^_!? 'Oil "d •»«- -t- remediation
' 8amn stu
y88 *an « ^ stu
LUST remedial technologies that produce air emissions. Existing and new
projects are subject to the regulations set forth in sec. KR 419.07 wis Adm
Code, some of the regulations in NR 419.07 are summarized below?
ho frequen=y and method» of monitoring described here are intended
be used for new soil venting systems and new ground water air stripping
' Se°~ 419'07' WiE- Adm- C°de' 8et8 f°rth re^ir^ents f or ?£ *
eth ««» «» «»"*ions from negative presnur* venting systems.
onlv £h frequency and methods of Campling discussed in this memo are a guide
only, the air management staff or the LUST hydrogeologiat has the authoritv to
specify additional samples based on site conditions. authority to
Means and methods of monitoring other remedial systems a± H site (landf armed
soil, etc.) are not discussed here, means and methods for monitoring theS
systems are approved by the air management staff on a site by site basis?
Is^nof a~X^^ »" P • t- -mo
Since existing remedial systems that are in place and have an established
monitoring plan may not use the same analytical methods or frequency described
here, it is up to the district staff to determine on a site specific basis if
monitoring modifications for existing projects should be made.
The frequency and type of monitoring described here is expected to be
sufficient for the vast majority of LUST sites. Air monitoring for soil and
ground water systems is conducted for two purposes, (1, compliance with air
management rules, and (2) for measuring progress and performance of the
remediation system. For compliance with air regulations, the air management
?«f iTrn?:01?^ different "Coring P^n, for performance monitoring, the
district LUST staff may also specify a different monitoring plan. .
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Except: for requirements set forth in the rule, sites that may not be
applicable to the monitoring methods and frequency described here include the
following:
Nonfuel wastes, such as halogenated solvents.
have different emission limits that need to be
specific basis.
These materials
assessed on a site
A large remediation system that is operating near the benzene
emission limit may require more frequent stack sampling for
compliance purposes.
A soil venting system with 4 or more wells may not need the
frequency of wellhead samples described here, less frequent
wellhead sampling may be appropriate.
Less frequent sampling may be appropriate for very small systems
that are well below the emission limits.
A synopsis of pertinent air regulations is as follows:
The maximum amount of benzene that can be emitted per year is 300
pounds without a permit and without installing lowest achievable
emission technology. There is no hourly or daily limit [NR 445.04
(3) (a) and NR 445.04 Table 3, Group A] .
If a treatment system for benzene is used, the system requires no
less than 95% destruction of benzene to comply with the "Lowest
achievable emission rate limit" [NR 445.04 (3) (a)].
The maximum amount of VOCs that can be emitted to the ambient air
is 9 pounds per hour of total VOCs [NR 419.07 (4) (b)].
The maximum amount of VOCs that can be emitted per hour without an
air pollution control permit is 5.7 pounds total VOCs.
If the remediation will be completed within 3 months (after
startup) and if the total VOC emission is greater than 5.7 pounds
per hour, a permit is not needed [NR 406.04 (1) (m) ] . But the
system shall not emit more than 9 pounds of total VOCs per hour.
* Frequency of -testing for organic compounds shall be once each day
for the first 3 days of operation, weekly for the next three
weeks, and monthly thereafter. The benzene emissions shall be
tested once during the first 3 days of operation, once during the
third week of operation, and once every six months thereafter. [NR
419.07(6)]. "•
' . The method of testing for total VOC or benzene emissions shall be
approved by Air Management in advance of the tests.
. Pilot tests for soil venting systems are exempt from permit
requirements [NR 406.04 (1) (m) 3], notification requirements, and
hourly emission limits [NR 419.07 (3) (a) 4], provided, that the
pilot test is conducted for a maximum of 8 hours and the total .
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flow rate is a maximum of 100 standard cubic feet per minute.
Tests of longer duration or higher flow rates are not exempt.
* * "
Form 4400-120 (Application to treat or dispose! of petroleum
contaminated soil) is required for soil remediation systems, even
if no air management permit is necessary. This form is not
required for ground water remediation systems, such as air
strippers, but ground water remediation systems are required to
file an application form with the Bureau of Air Management.
The following sources are exempt from total VCiC limits, and
reporting requirements (for VOCs); treatment of potable water
supplies, crop irrigation systems, remedial actions under CERCLA,
and permitted sewage treatment plants [NR 419.07 (3)].
Applicability of the rules to these projects should be verified
with the air management staff on a site by site basis. These
projects are still required to comply with the benzene limitations
listed above and hazardous air rules in NR 445.
Some answers to frequently asked questions about the rules are as follows:
For determining compliance with Chapters NR 406.04(1)(m) and NR
419.07 of the air regulations, the sum of total emissions for all
remedial activities at a site are calculated. In a case where
there is; a soil venting system, an air stripper, and/or
landfarmed soils, the emissions from all remediation sources are
summed for determining compliance with air regulations. If some
emission points are on site (a soil venting system) and some off
site (soil disposal in landfills), the only sources to sum for a
site are on site sources.
Sources at a site that are not associated with soil and ground
water remediation activities are not included in the sum of all
sources mentioned above.
Two parameters are needed for measuring compliance with air rules,
benzene and total VOCs. BETX analysis is not needed for air
management analysis, however it may be required for a wastewater
discharge permit. The analytical requirementa for determining
compliance with air management rules do not exempt sampling for
compliance with other rules.
When VOCs are the only air pollutants emitted from the source,
then no air permit is required if total VOC emission is under 5.7
pounds per hour and total benzene emission is less than 300 pounds
per year. .
. , If total VOC emission is above 5.7 pounds per hour during any
single monitoring event, an air permit is required, unless the
remediation is completed within 3 months of startup.
If an air treatment system is in'use on a remediation system and
if the treatment system is not needed for compliance with air
regulations, the emission control system can be removed upon
approval from the air management staff.
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If there is no air permit required, the air management staff still
need to preapprove the remediation except for those sources listed
in MR 419.07 (3) (a). These sources include; treatment of potable
water supplies, crop irrigation systems, remedial actions under
CERCLA, and permitted sewage treatment plants.
The district LUST staff may need to preapprove -the remediation.
Air management must preapprove all soil and'water remediation
projects. The consultant should verify the need for preapproval of
a remediation workplan on a site by site basis with the district
LUST staff.
For purposes of calculating benzene emission, the year starts when
the first air emitting device at the site starts operation.
The parameters, recommended sampling locations, and recommended methods
include the following:
Benzene.
Ground water treatment units generally are sampled at the influent
and effluent on a regular basis for BTKX to comply with the
wastewater discharge permit. Calculate the benzene emission by
subtracting the effluent benzene from the influent benzene values,
multiply by the ground water mass flow rate.
For soil venting systems, samples are to be collected at the point
of emission (stack). If there is only'one air extraction well,
the benzene sample, may be collected at the extraction well instead
of the stack. A single sample can be collected or up to three
consecutive hourly samples can be collected and averaged. Benz«n«
is measured by either; a carbon adsorption tube or a tenex tub*
that is analyzed by a laboratory, or a portable gas chromatograph
(field GC), or another method approved by the air management
staff. Procedures are discussed below under total VOCs.
Total VOCs.
No specific methods of sampling total VOCs from ground water
treatment units are specified for all projects. It is possible
that total VOC samples will not be required by the air management
staff on smaller (less than 35 gallons per minute) air stripping
units. This is because the smaller units are unlikely to exceed
the 9.0 pound hourly limit.
- Calculations for the maximum total VOCs at. 35 gallons per
minute are as follows:
35 gallons / minute * 8.34 pounds / gallon * 60
•minutes / hour .« .12510 pounds per hour of ground
water.
9 pounds total VOCs / 12510 pounds water «- 0.000508 or
roughly 500 parts per million by weight(ppm).
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Since very few projects have a concentration of total
petroleum hydrocarbons (TPH) as high as 500 ppm during an
active ground water extraction project, samples for total
VOCs generally are not required on smaller (less than 35
gpm) air stripping units.
Systems larger than 35 gpm may need :j rounds of both
influent and effluent ground water TPH samples early in the
project to ascertain that the 9 pound per hour limits are
not exceeded. If sample results indicate that the 9 pound
per hour limit is being exceeded, th« air management staff
probably will have an increased future involvement in the
project. An alternative to sampling ground water is to
sample the air stream according to the methods described for
soil venting systems. The disadvantage of sampling the air
stream is the requirement for an accurate air flow
measurement to quantify a mass per time value.
For soil venting 'systems, the total VOC samples are to be
collected at the emission point (stack) for compliance purposes
and at each air extraction well for measuring system performance
If there is only one air extraction well, only I singX sample
needs to be collected. When sampling for air compliance purposes,
a single sample can be collected, or up to three consecutive
hourly samples can be collected and averaged. Methods to use to
measure total VOCs include; a laboratory analysis of a carbon^
adsorption tube, a field 6 C, a flame ionization detector (FID),
or a photoionization detector (PID), or another method approved by
the air management staff. Each of these methods has different
procedural requirements, these are as follows:
Laboratory analysis of a carbon adsorption tube or tenex
tube. NO procedures are specified, describe method of
sampling and analysis in workplan.
Field GC. A description of the devic«, Jinalytical
procedure, and a description of the standard should be
irind^in
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flame out is experienced, a dilution device OK a serial
dilution method of analysis described by Robbins (Ground
Water Monitoring Review, Fall 1990, Page 110 to 117) should
be used. Assume a molecular weight of 95 grams/mole when
converting' from parts per million to a mass per volume
(pounds per cubic foot or grams per cubic meter).
- PIDs. Only PIDs with a lamp with a 11.7 eV ionization
potential may be used. 10.2 eV and 10.6 eV lamps do not
detect all the compounds that are in gasoline. PIDs should
be calibrated to a standard recommended by the instrument
• manufacturer. PID readings are adversely affected by carbon
dioxide'in the air sample and they do not provide a linear
result with increasing concentrations of VOCs in air,
therefore, a serial dilution method is required (referenced
above under FIDs). The dilution method must be with ZERO
grade air, or air with no moisture. Otherwise changes in the
relative humidity of the ambient air will make correlation
with one -sampling day to another impossible. The dilution
must be done until the results from three dilutions plot
linearly and the final concentration is less than 125ppm.
Assume a molecular weight of 95 grams/mole when converting
from parts per million to a mass per volume.
- Other methods. If another method is proposed by a
consultant, the specific field and analytical procedures
need approval from both the air management staff (for
compliance purposes) and the district LUST staff (for
performance purposes).
Carbon dioxide fsoil venting svstema onlv>. Carbon dioxide is
only for measuring the rate of biodegradation, this parameter is
not measured for compliance with air management rules. The sample
points are each air extraction well and a background well.
Ideally the background well is an upgradient water table well that
is located in an uncontaminated part of the aquifer and is not
used for 'air extraction. The background well is strictly a
background location for measuring ambient levels of carbon dioxide
in the soils. Carbon dioxide can be measured by any convenient
means available (including Drager Tubes or similar devices). The
work plan should describe the sampling devices and the procedure
for sampling the background well. Purging a minimum of 3 to 5
well volumes (of air) from the background well is appropriate
prior to sampling.
A quantification of bioremediation rate is not required. If the
consultant desires to quantify the rate of biodegradation, a
sample procedure is as follow:
Assume that all excess CO2 above background its generated by
biodegradation. In this case, elevated CO2 means the
difference between the background CO2 level and the wellhead
CO2 level.. This also assumes that no carbon remains in the
soil as part of a biomass.
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Assume that all gasoline is methylcyclohexane (C7H14), this
compound has a molecular weight (98.2) that is similar to
unweathered gasoline (approximately 95). (Note: If site
specific product chemistry information is available, use
site specific data.)
For purposes of the following example, assume that the
elevated level.of carbon dioxide level is one percent. At
1% CO2, one cubic foot of air contains:
0.01 * (1 ft*3) * (28.3 l/ft*3) / (22.4 I/mole) -
0.0126 moles CO2.
Since there are 7 moles of carbon dioxide generated from
every mole of gasoline (assuming methylcyclohexane or
C7H14), for every cubic foot of air that has CO2 elevated by
1%, one cubic foot represents:
(0.0126 moles CO2) / (7 moles of CO2 per mole of
gasoline) « 0.0018 moles of gasoline
The weight of 0.0018 moles of gasoline is:
(0.0018 moles) * (98.2 grams/mole) * (0.0022
pounds/gram) «= 0.00039 pounds of gasoline.
Therefore, for every standard cubic foot of air that has CO2
elevated by 1%, 0.00039 pounds of gasoline is destroyed by
biodegradation. 5% CO2 elevation would represent
approximately:
(5) * (0.00039 pounds of gasoline at 1%) « 0.00195
pounds of gasoline per cubic foot of air.
Other measurements (soil venting systems onlvl. Other operational
measurements that should be determined at each sampling point
during each sampling event include; the air flow rate in cubic
feet per minute, the temperature, and the vacuum. Air flow rate
. can be measured with an averaging pitot tube, a pitot tube or an
. anemometer. Vacuum can be measured with a manometer or a
magnehelic. Temperature is measured by any convenient means
available. Relative humidity can be measured as a option, but is
not required.
Monitoring frequency is specified for soil, venting systems in the regulations
but frequency for other types of remediations is not specified in the
regulations. Recommended sampling frequency for all systems is as follows:
.Ground water treatment systems generally have a sampling frequency
specified in the waste water discharge permit. For this reason,
sampling frequency for air management purposes generally will be
as specified in the waste water discharge permit or monthly,
whichever is more frequent.
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8
A typical soil venting system could have the following monitoring
frequency, unless otherwise specified by the air management or
f IMST staff.
- For benzene, one test during startup (one test anytime
during the'first 3 days), one test during the third week,
and one test every 6 months thereafter. If the consultant
prefers more frequent monitoring for benzene, this should
not be discouraged.
- ' For VOCs, test daily for the first 3 days, weekly for the
next 3 weeks, and monthly thereafter. If analytical costs
are significant, wellhead (but not stack) samples can be
collected on a reduced frequency (such as quarterly) on
larger systems, if approved by the district LUST staff.
Only air management staff can approve reduced sampling
frequency on the stack.
- Carbon dioxide is tested quarterly after one quarter.
- Flow rate, vacuum, and temperature are measured every time
that other samples are taken.
Air management staff may allow air emissions testing to be discontinued (upon
written approval) if past sampling indicates that the remediation system(s)
are emitting contaminants at rates significantly below air management
regulations. In this case, district Lust staff may reduce sampling frequency
to a quarterly basis for total VOCs and discontinue sampling for benzene.
If changes to a remediation system are to be made after the first month that
may raise emissions by more than 50%, the monitoring frequency for total VOCs
should be weekly for 3 weeks and monthly thereafter. Changes that can be
expected to increase emissions from a soil venting, system include (but are not
limited to) the following; adding extraction wells, increasing air flow rate
by more than 50% by; adjusting a bleed or dilution valve, or by increasing
blower capacity. Changes that can be expected to increase emissions from air
strippers include (but are not limited to) increasing the water flow rate,
adding additional extraction wells, or reducing residence time in sparge
tanks.
Reporting. The recommended reporting frequency is monthly for the first 3
months and quarterly thereafter. During the first month of operation the
emission reports are to be submitted as soon as possible to the air management
staff. All other reports are submitted to both the air management staff and
the district LUST staff. Since the air management staff is only concerned
with air emissions and not other details associated with the. site remediation,
a separate report for air management may be appropriate that only describes
details pertinent to air emissions. -If the same report is submitted to both
air management .and LUST staff, it is recommended that the consultants discuss
pertinent air issues in the beginning of the report. .
airmemS
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