United States
Environmental Protection

    How to Design Environmentally Safe Covers
     Including additional design guidance for arid regions

                  How to Design Environmentally Safe Covers
              Including additional design guidance for arid regions
1.1   INTRODUCTION                                              9
2.3   EROSION	20
5.0   REFERENCES                                                29

                COVER THICKNESS

Figure 1.  Typical Evapotranspiration cover profile                                    12
Figure 2.  Typical soil-plant-atmosphere water potential variation                       13
Figure 3.  United States Climate Map	14
Figure 4.  Climate's demand for water vs. supply of water Albuquerque, NM (1998)	15
Figure 5.  Climate's demand for water vs. supply of water Liver more, CA                 16
Figure 6.  Relation between moisture retention parameter and soil texture class           18
Figure 7.  Desiccation cracking in clay barrier layer_19
Figure 8.  Types of Water Erosion That May Occur on a Cover System	20
Figure 9.  Infiltration / Erosion vs. Cover Slope                                        21
Figure 10. Relation between moisture retention parameter &soil texture class            23
Figure 11. Recommended Cover Profile                                              27
Cover Photo: An Evapotranspiration cover system at the Rocky Mountain Arsenal site near
Denver, Colorado. September 2010.

                       How to Design Environmentally Safe Covers
                  Including additional design guidance for arid regions

The purpose of this guidance is to help tribal  public works  managers, and  environmental
managers, and tribal  leadership determine an appropriate final cover profile for closing  small
landfills and open dump sites on tribal lands. This document includes updated information from
previous guidance documents for closing small landfills and open dump sites within arid regions
on tribal lands.

This document presents design and operation considerations for the closure of small landfills and
open dump sites, that while not recognized as landfills under the federal regulations in 40 CFR
Part 258, should consider additional environmental safeguards.  This document further provides
guidance and a recommended design methodology for a final cover profile to close  open dumps
and  solid waste landfills that meet the requirements set forth  in  40 CFR 258.l(f).   More
specifically, this applies to  small municipal solid waste landfills and open dumps that satisfy
criteria  summarized  in the  Decision Tree contained in Section 1.0 of this Guidance.  This
document is intended for facilities on trust land within Indian Country.

ASTM is the acronym for the  American Society for Testing and Materials. An  ASTM testing
method defines the way a test is performed and the precision of the result.
Biodegradation  is the  chemical dissolution of organic waste  material by bacteria  or  other
biological means.
Biointrusion is the intrusion into the underlying waste through the final cover profile by flora
and/or fauna.
Bulk Density is defined as the mass of soil divided by the total volume it occupies.
CFR is the  acronym for the Code of Federal Regulations, the codification of the general and
permanent rules published in the Federal Register by the  federal government. 40 CFR 258
covers criteria for municipal solid waste landfills.
Desiccation Cracking is defined as the polygonal-shaped cracking that develops in  clay or mud
which has dried out.
ET Cover or evapotranspiration cover is a cover profile composed of soil with native vegetation.
Evapotranspiration (ET) is a term that represents the removal of water from a soil profile by the
combination  of evaporation from the  soil  surface  and transpiration by  the available  plant
Flux is the percolation rate through a cover profile.
Geomembrane  is a  kind of geosynthetic material  made up of an impermeable membrane.
Geomembranes  are  made  of  various  materials;  most commonly  low-density polyethylene
(LDPE), high-density polyethylene (HOPE), or polyvinyl chloride (PVC).

Hydraulic Conductivity is a property of vascular plants,  soil or rock, which describes the ease
with which water can  move through pore  spaces  or  fractures. It depends  on the intrinsic
permeability of the material and on the degree of saturation. Saturated hydraulic conductivity
describes water movement through saturated media.
Infiltration is the process by which water on the ground surface enters the soil.
Landfill is a site for the disposal of waste materials by burial and is the oldest form of waste
Matric Potential is also referred to as soil suction. It is that component of the *water potential of
plants  and soils that is  due to  capillary  forces. Thus  the  water potential of cell walls  and
intercellular spaces is largely due to matric potential.
Municipal Solid Waste (MSW) commonly known as trash or garbage, is a waste type consisting
of everyday items we consume and  discard. It predominantly includes food wastes, yard wastes,
containers and  product packaging,  and other miscellaneous inorganic wastes from residential,
commercial, institutional, and industrial sources.
Open Dump An uncovered  site used for disposal of waste without environmental  controls. It
includes any facility or site where solid waste is disposed  of which is not a sanitary landfill
which meets the criteria promulgated under section 4004 of the Solid Waste Disposal Act (42
U.S.C. 6944) and which is not a facility for disposal of hazardous waste.
Percolation is water that infiltrates into and through a cover profile into the underlying.  The rate
of water percolating through the cover is referred to as the cover's flux.
Permeability is the property of a porous material to permit a liquid or gas to pass through.
Permeability is  often interchangeably used with the term saturated hydraulic conductivity.
Plant Transpiration  is a process similar to evaporation. It is a part of the water cycle, and it is
the loss of water vapor from parts of plants (similar to sweating), especially in leaves but also in
stems, flowers and roots.
Potential Evapotranspiration (PET) is a  measure of the ability  of the atmosphere to remove
water from  the surface through the processes of evaporation  and transpiration assuming no
control on water supply. It is commonly referred to as the climate's demand for water.
Prescriptive Cover is a cover profile that is specifically described by  regulatory  statue.  For
federal application it is described in 40 CFR Part 258. It may also be described in state regions.
RCRA is the acronym for the Resource Conservation and Recovery  Act. RCRA was enacted in
1976 and is the  public law  that creates the framework for the proper management  of non-
hazardous and hazardous solid waste.
Resistive Cover is a cover profile  that is designed to minimize percolation by restricting or
=resisting' the movement of water through it by the inclusion  of a  barrier layer with  a  low
saturated hydraulic conductivity.
Sole Source Aquifer is a sole or principal source aquifer as one which supplies at least fifty
percent (50%) of the drinking water consumed in the area overlying the aquifer. These areas can
have no alternative drinking water source(s) which could physically, legally, and economically
supply all those who  depend upon the aquifer for drinking water. For convenience, all designated
sole or principal source aquifers are referred to as "sole source aquifers" (SSA).

Store and Release Cover, also referred to as an ET Cover, is cover profile that is designed to
store any infiltrated water within its profile until that water can be removed via ET.

Use the Decision Tree on page 8 to determine if the Closure Design Guidance in Section 4 (and
the Appendices) applies to each site under consideration. Note that this design guidance applies
only to arid sites where annual precipitation is less than 25 inches.

For sites that do not meet the criteria in the Decision Tree, a prescriptive cover summarized in
Section 2.2 may be an option. An alternative cover such as the recommended cover profile may
still be an option; however it should be evaluated based on site-specific conditions.

This guidance is intended for municipal solid waste landfills and open dump sites that are not
specifically covered  by 40  CFR Part 258 and are located  on Indian Country (may include all
Federal Lands).   A recommended cover profile for managed landfills or open dumps that meet
the criteria summarized in the following Decision Tree is described in Section 4.

DECISION TREE: Simplified Cover Design Guidance for Closure of
        Small Landfills/Open Dumps In Indian Country
Coordinate Closure
with applicable
State/Local Authority

Determine if hazardous
waste (HW) is present.
If some HW is present,
remove to HW Landfill if
practical. If significant
HW is present, refer to
40 CFR Parts 260 -265
for closure.
Follow Applicable
Guidance in effect in
1991 or prior and
applicable Best
Management Practices
Follow applicable
Tribal and/or
, No



Located within
Country ir

the boundary of Indian
the United States
Waste is Municipal Solid Waste and/or
Construction Debris

Site received Waste after 09/09/1991

Average Annual Precipitation < 25 inches
Qualify as Small Quantity Generator
Site received < 20 tons of municipal
waste per day
Location poses threat to local
population (e.g. located within or close
to residences or public facilities such as
schools, offices, or hospitals)
Location poses threat to groundwater
(e.g. located above sole source aquifer
with a depth to groundwater < 100 feet
*~H Location poses threat to surface water


This Guidance is intended to assist and simplify the design and construction of a final cover
system for closure of managed small landfills and open dumps that pose limited risk to human
health  and the  environment.   Limited risk  is summarized in Section 1.2 (below) while the
recommended criteria intended to limit the use of this Guidance is outlined in the Decision Tree
on page 8. This Guidance recommends the use of an Evapotranspiration (ET) Cover for sites
that meet  this limited risk criterion.  The ET Cover concept is described in Section 2.1 with a
recommended cover provided in Section 4.0.  If the criteria outlined in the Decision Tree are not
satisfied  and unacceptable  risks  are present  at  the  site,  that  site  may warrant  further
considerations including the possible use of a prescriptive cover as detailed in 40 CFR Part 258
and summarized in Section 2.2.  The use of a prescriptive cover at a higher risk site does not
mean that a prescriptive cover is a  superior cover; only that the use of an alternative cover
system at such a site may not be acceptable under federal regulations.

Design steps for the ET cover profile are described in Section 2.5.  Other design considerations
beyond minimization of water intrusion through the cover  and  establishment of vegetation
include mitigating soil loss  due to erosion and controlling surface  water.  These issues are
discussed in Section 2.3 and 2.4, respectively.

Design steps  for  a prescriptive  cover system can be found in EPA Publication EPA/625/R-
94/008: Design, Operation,  and Closure  of Municipal  Solid Waste Landfills.   Prescriptive
requirements are described in 40 CFR Part 258.

A good design is critical to the successful deployment of a final cover system. Just as important
to the long-term success is the quality of the construction of this  cover  system.  Construction
considerations are described in Section 3.0.
Controlling risk is a primary goal for environmental remediation efforts.  Landfill and large open
dump closures are no exception.  Risk, as it relates to solid waste disposal sites, is defined as the
actual or potential threat of adverse effects on human health and the environment by  effluents,
emissions,  wastes,  resource  depletion,  etc.,  arising from  the  landfill's encapsulated waste.
Within  the  realm  of  environmental  risk,   this  process  involves  a  multi-staged  analysis
characterized by a  ^isk assessment", ^4sk  characterization" and ^isk management". A risk
assessment  is  -the evaluation  of scientific information  on  the hazardous properties  of
environmental agents, the dose-response relationship, and the extent of human exposure to those
agents".  In  essence a  pollutant is identified and its possible  effects on those exposed are
described. The result of this analysis -4s a statement regarding the probability that populations or
individuals  so exposed will be  harmed  and  to  what  degree,"  also known  as  a risk
characterization. Once risk has been assessed and characterized, -political, social, economic and
engineering  implications together  with risk-related  information"  are gathered -4n  order  to
develop, analyze  and  compare  management options and  select the appropriate  managerial
response to a potential chronic health hazard". This process is called risk management. Together
these steps comprise the scientific approach to risk.

The number of variables to be considered are generally site-specific and too numerous to list all.
However,  some  common  variables  to   consider  when  assessing  the  site's  human  and
environmental health risks include:
    >  Proximity to shallow sole source aquifers: considerations should include whether the site
       is directly above a  sole source aquifer; the depth to groundwater; and the potential  of
       leachate from the site (quantity and quality) that could adversely affect groundwater.
    >  Proximity to residences and public facilities:  Is there the possibility of growth of nearby
       communities toward the  site or the possibility of future construction on the site? Risks
       associated with this  issue include  the  potential  for  methane migration into nearby
    >  Type  of waste:  Considerations should include whether the  site  contains  municipal  or
       hazardous waste; are  there any medical  or other wastes such as asbestos; is  the waste
       biodegradable - byproducts of biodegradation include carbon dioxide  and methane gas as
       well as potential differential settlement. Obviously hazardous waste will create more risk
       than waste such as construction debris like broken concrete and wood.
    >  The size and  shape of the trench, pit, or excavated area can  determine the potential for
       future  differential  settlement.   Differential  settlement  can  result  in  the creation  of
       significant surface cracks that allow water flow down through the  cover and into the
       underlying waste while also allowing the upward escape of methane gas.   Additionally,
       differential settlement may create surface depressions or low spots that allow for ponding
       of surface water and thus increased percolation into the waste. This in turn can lead to
       increased methane generation along with the potential for increased leachate production
       and thus an increased potential groundwater contamination.
    >  Site location:  considerations include whether the  site  is  susceptible  to  flooding;
       earthquakes; is  the site susceptible to substantial erosion due to its shape,  slope, etc.; is
       the site subject to intrusion by burrowing animals and plant roots  that may harm  the
       effectiveness of the cover system or bring waste to the surface?
    >  Site climate:  considerations can include does the  site  have  excessive  precipitation; is
       there significant snow; are there high intensity storm  events that can create  excessive
    >  Soil type:  considerations  can include - does the underlying soil beneath the waste have a
       low permeability such as silt or clay or a high permeability such as sand or gravel; does
       the cover soil have adequate storage capacity; is the cover soil capable of maintaining a
       quality stand of native vegetation?
    >  Waste placement:  considerations  should include - was the waste compacted as  it was
       placed; was the waste placed dry or wet; was the waste placed in a  manner that  would
       lead to future differential  settlement?

2 0
(fafssiJ   \^\J W IJI% I^JLJskJlUlllJ
A final cover system for a solid waste landfill is intended to isolate the underlying waste and thus
reduce its inherent risk to human health and the environment.  The final cover may be composed
of a single monolithic soil layer or multiple layers acting as a system.  The cover profile should
be designed to address each of the site-specific potential release vectors from the landfill or open
dump. A release vector is a potential pathway for harmful encapsulated waste to be released into
the environment. Examples include:
       a. Flux: Water that infiltrates into and through a  cover profile to the underlying waste is
          referred to as  percolation. Percolation rate is referred to as flux.  Excessive flux can
          lead to significant leachate generation that can potentially escape  a landfill  and carry
          harmful contaminants to groundwater, polluting drinking water supplies.
       b. Erosion:    Excessive  erosion can  remove  the  protective  cover  or reduce  its
          effectiveness to isolate the underlying waste.  Some erosion can  be planned for and
          taken into account in the cover design.  However,  excessive erosion, particularly rill
          and gully formation  should be avoided.
       c.  Surface Water:  Surface water must be controlled  to minimize erosion and potential
          release of harmful contaminants.  Surface water controls can include controlling the
          speed, direction, and volume of water. They can also control surface water  generated
          upstream of the site from running onto the site  and/or running off of the site.
       d. Biointrusion:  Excessive or unwanted intrusion into the underlying waste through the
          final  cover profile by flora and/or fauna can  be a release vector. A layer of rock such
          as cobble can  be added as part of the cover, or the cover can simply be made thicker.
       e. Gas:    Carbon dioxide  and methane  are   the  primary  gases  created  during
          biodegradation of buried municipal solid waste.  Inclusion of a gas collection layer is
          a common solution for this issue.  A major concern related to methane generation and
          transport  is the potential accumulation of gas in nearby structures. Methane gas can
          accumulate within an enclosed structure  and create an explosive threat.  If structures
          exist near a landfill or large  open  dump, it is a good idea to monitor for methane
          within these structures.
Two types of covers are discussed in this guidance. The first type of cover is referred to as a
=store and release' cover.  That is,  the  cover  is designed to store infiltrated water within its
profile until it can be released to the atmosphere via evapotranspiration (ET). The combination
of evaporation from the cover soil surface or through plant transpiration is termed ET.  These are
alternative earthen covers designed to take advantage of site-specific conditions such as climates
where the demand  for water  referred to as  potential evapotranspiration (PET)  significantly
exceeds  the supply  of water (precipitation).   This  is further discussed in Section 2.1.1.  The
second type of cover, referred to as a =resistive'  cover,  attempts to block or resist the downward
movement of water typically with low permeable soil barrier layers and/or geosynthetic materials
such as  high  density polyethylene membranes. These =resistive'-type barriers  are considered
prescriptive covers as detailed in 40CFR258.   The prescriptive  cover  is further  discussed in
section 2.2.

The ET Cover also referred to as  a JStore and Release' Cover is an excellent cover system if
cover is
water ui
                                . \   T            t
                        n   ...    Transpiration T
                        Precipitation       '
                                 Topsoil/Surface Treatment
                                Figure 1. Typical ET cover profile
The ET cover concept relies on the cover soil to act like a sponge. Infiltrated water is held in this
=sponge' until it can be removed via ET. Previous research has shown that a simple ET cover can
be very effective  at minimizing  percolation and erosion,  particularly in dry environments
(Hauser et al. 1994, Hakonson et al. 1994, Nyhan et al.  1997,  Khire et al.  1997, Dwyer 2001,
Dwyer 2003, Albright et al 2004, Nyhan 2005).

ET provides the mechanism to remove stored water from the cover soil layer.  Water can move
upward because  of evaporation drying  the upper portion of the cover  soil layer.  Evaporation
from the surface will decrease the water content and thus increasing the matric potential of the
soil, (soil suction) resulting in an upward matric potential gradient and inducing upward flow.

Plant transpiration also relies upon matric potential gradients to remove water from the cover soil
layer. Figure 2 shows the large matric potential  difference between the soil and atmosphere. In
dry environments, the total potential difference between soil moisture and atmospheric humidity
can  be up to  1000  atmospheres (bars) (Hillel  1998).  The larger the  soil-plant-atmospheric
potential gradient, the more effective an ET cover system can be.
                        Air (up to-1000 bar)
                                                    Roots (-3 bar)
                       Soil Water (-0.3 bar)
                 Figure 2. Typical Soil-Plant-Atmosphere Water Potential Variation (Hillel 1998)

Sites well suited for an ET Cover include climates where the demand for water or potential
evapotranspiration (PET) is significantly greater than the supply of water or precipitation. One
of the criteria included in the Decision Tree in Section 1.0 applies to climates with 25-in of
annual precipitation or less (Figure 3).
                  Precipitation:  Annual Climatology (1971-2000)
       Precipitation (in.)
        niS-20 O36-40
      1? Q28-32
    11- 16 QJ: ::f.
       •-.: -on
40-50 BIOO-IZO
50-60 QI20-140
                 Figure 3. United States Climate Map (Natural Resources Conservation Service
However, more important than the annual precipitation is the ratio of the demand for water or
PET to the climate supply of water.  As long as this ratio is significantly large enough, an ET
Cover should be able to minimize flux.  Typical climates that extend across much of the United
States where the majority of precipitation occurs during the summer months when PET is at its
peak while precipitation during the winter months is lower also while PET is lower are generally
suited for store and release covers given the average PET is greater than precipitation for each
month of the year.  An acceptable ratio  of PET: precipitation should be based on site-specific
conditions and risk.

An example of a typical environment well  suited for an ET Cover is Albuquerque, NM.  The
PET far exceeds the precipitation for each month of the year (Figure 4) and thus a well designed
ET Cover should provide adequate storage capacity to minimize flux.
                                                                Precipitation (in.) n PETnn.)
        Figure 4. Climate's demand for water (PET) vs. supply of water (precipitation) Albuquerque, NM (1998)

An example of a non-typical  climate also well suited for an ET Cover  includes Livermore,
California, just east of San Francisco. The weather in this area is different than many areas of
the country.  It is hot and dry  in the summer with much of the vegetation dormant during this
period due to lack of precipitation.  The area receives most of its precipitation during the winter
months while the PET for  the area  is at its lowest levels.  Despite this  weather pattern,  the
average PET exceeds the precipitation for the area for every month of the year (Figure 5).
                                                                D Precipitation (in)  n PET (in)
                    Figure 5. Climate's demand for water vs. supply of water Livermore, CA

Emphasis during the design and construction of a cover system is placed  on achieving the
designated performance goals.  Two primary performance goals for a cover system include
minimization of flux  through the cover  and minimization  of  soil  loss  due  to erosion.
Performance goals can also include controlling the release of gas and/or biointrusion. For an ET
Cover to successfully minimize flux and erosion, a moderate density in the cover soil should be
achieved.   Covers placed too loosely will allow infiltrated water to quickly move down and
potentially through the soil profile not allowing the cover to store the water until the moisture is
removed via ET. Placing the cover too densely subjects the profile to preferential flow resulting
from desiccation cracking described in section 2.2.1  and can adversely affect the establishment
of vegetation. Therefore, the cover soil should be placed with moderate compaction that closely
mimics the natural or Jn-situ' density of the soil in its undisturbed state.  Generally, this is about
90% of the maximum  dry density using  the standard proctor method (ASTM D698).  Other
considerations should include limiting the amount of salts in the soil that can adversely affect the
vegetation establishment as well as increase the erodibility of the soil (Appendix B).  Adequate
plant  nutrients are also important, but cover soil can be amended to increase available plant
A prescriptive cover also referred to as a =iesistive cover' is designed to limit percolation by
incorporating a barrier layer within its profile.  The barrier layer is typically a clay layer with a
low saturated hydraulic conductivity or a geomembrane or a combination of the two.  The intent
of the low permeable barrier layer is to =iesist' the movement of water into and thus through it.
Federal prescriptive standards for this profile  are described  in 40 CFR Part 258 (see  below).
Many states have the authority to regulate the closure of landfills and as such have their own
unique prescriptive  standards for the cover profile or may  have directly adopted  the federal
Regulations for the final cover of a RCRA Subtitle -0" facility are prescriptive.  Specifically,
these minimum prescriptive requirements for the final cover system include:
   1)  have a permeability or saturated hydraulic conductivity less than or equal  to that of the
       bottom liner or natural subsoils present, or no greater  than 1 x 10"5 cm/sec, whichever is
   2)  minimize infiltration through the closed Municipal Solid Waste Landfill (MSWL) by the
       use  of an infiltration layer containing a minimum 45  cm (18-in) of earthen material  [40
   3)  minimize erosion of the final  cover by the use of an erosion layer containing a minimum
       15 cm (6-in) of earthen material that is  capable of sustaining  native  plant growth

These requirements are summarized in a cover profile shown in Figure 6.
                                           ll      "
             Barrier Layer -/
                                                                     ' 5 cm
5 cm
                           Figure 6. RCRA Subtitle D Minimum Cover Profile
Many state and local regulatory authorities have modified the minimum cover requirements set
forth in 40CFR258; generally making the requirements more stringent. Some have altered the
barrier soil layer requirements by requiring a lower saturated hydraulic conductivity requirement,
for example requiring a maximum of 1 x 10"6 cm/sec instead of 1  x 10"5 cm/sec.  Others have
required a composite barrier layer  comprised of both  a  clay barrier layer and a synthetic
geomembrane.  Local environmental regulations should be consulted for minimum  cover profile

As previously discussed, the emphasis of the prescriptive cover profile is to minimize flux by
resisting the movement of water vertically downward.  This is achieved via a low saturated
hydraulic conductivity in  the barrier layer.  There are some  drawbacks to using the minimum
profile described in 40CFR258.  To achieve the lower saturated hydraulic conductivity in the
barrier layer, the soil is typically installed in a moist condition (generally wet of the optimum
moisture content per ASTM D698) and heavily  compacted to achieve a higher density.  This
allows for the clay particles to be remolded and lower the initial or =as-built' saturated hydraulic
conductivity of the soil layer. Experience has shown these clay barrier layers to be vulnerable to
such things as near-surface desiccation cracking (Figure 7), especially in  drier environments
(Suter et al. 1993, Dwyer 2003).
                          Figure 7. Desiccation Cracking in Clay Barrier Layer

Desiccation cracking can provide easy pathways for water migration downward and defeats the
purpose of trying  to install a relatively impermeable  (low saturated  hydraulic conductivity)
barrier layer.  An EPA design guidance document (EPA 1991) for final landfill covers states: "In
arid regions,  a barrier layer composed of clay (natural soil) and a geomembrane is not very
effective. Since the soil is compacted ,wet of optimum ',  the layer will dry and crack".  Further,
some soil textures cannot meet the low saturated hydraulic conductivity without amendments
(e.g. mixed with bentonite). These amendments significantly increase the cost of construction.

The minimization of flux through a cover is often the primary concern during the design process
of a final cover system.  However, the minimization of erosion has proved to be of significant
concern especially in dry environments.  Annual soil loss should be limited to 2 tons/acre/year
(EPA 1991).  Perhaps more important, the formation of rills and gullies (Figure 8) in the cover
system should be avoided.  The recommended cover profile described in section 4.0 utilizes a
gravel/soil admixture for the surface of the cover to minimize erosion and mitigate the formation
of rills/gullies. Refer to Appendix A for a recommended design methodology for the upper layer
of the cover to minimize erosion.

A cover system's susceptibility to erosion is a function  of a number of factors, including slope
angle and length,  surface  soil characteristics,  rainfall  intensity and  duration,  and vegetation
(Figure 8).   Vegetation will assist  to  minimize  erosion, however in dry climates; native
vegetation is commonly sparse  and unable to  form a continuous blanket  to completely limit
erosion. Consequently, each cover design should address how to assist vegetation in minimizing
both short and long-term erosion.
             i  ^i
   Raindrop Erosion

       Sheet Erosion

            Rill Erosi
                    Figure 8. Types of Water Erosion That May Occur on a Cover System
Cover top slopes are generally influenced by the topography of the site.  Engineering concerns
such as erosion, settlement, shedding surface water, and final aesthetics all play a role in the
determination of the final cover top slope(s). Side slopes should be minimized and must meet
applicable slope stability requirements.
The determination of the final cover slope can be a balancing act between maximizing slope to
increase  runoff  and thus  decrease  infiltration  or  minimize  slope to  decrease erosion.

Unfortunately, erosion and infiltration are inversely related when compared against slope and
slope length (Figure 9).
                                        Cover Slope

                            Figure 9. Infiltration / Erosion vs. Cover Slope
Control of surface water runoff at a landfill disposal facility is necessary in order to minimize the
potential  for environmental damage to ground and surface waters by direct and indirect effects.
Direct surface water contamination can result from solid waste and other dissolved or suspended
contaminants carried by  surface runoff.  Uncontrolled  surface runoff can also contribute to
leachate and gas generation, thereby increasing the potential for both surface- and ground water
contamination. Surface water courses should be diverted from the landfill and there should be no
uncontrolled hydraulic connection between the landfill and standing or flowing surface water.
A set of recommended design steps for a cover system are described in this section. The
following steps should be formally or informally considered when designing a cover system to
meet  determined performance and/or risk objectives  at each  site.   These steps  are  briefly
described below.
Note: Throughout this document, a cover system is referred to instead of merely a cover because
it is very important that the design of a final cover be designed as a system rather than merely as
a group of individual components comprising a cover.
    1.  Determine the regulatory drivers for closure of each specific site.

   2.  Determine the  design life of the cover system to be deployed based on the applicable
       regulations and encapsulated waste. Subtitle D municipal solid waste landfills as defined

   by RCRA typically require a minimum 30-year post-closure monitoring period to ensure
   the cover system is working as intended.  Sites that are not considered municipal  solid
   waste landfills under RCRA should determine the length of closure based on site-specific
   parameters. See the Decision Tree in Section 1.0 for additional guidance.

3.  Determine performance objectives of the  cover system. Review,  assess,  and determine
   additional data needs, and design documentation to support the final design. Performance
   objectives can include, but are not limited to:

       a.  Risk. The cover system must be designed  to control  risks associated with each
          specific site.

       b.  The cover flux (moisture  that  has  moved vertically down  through the  cover
          profile) must be less than that which will produce an adverse risk to groundwater.
          Generally, flux should be minimized.

       c.  Erosion of the cover system. As a minimum, all cover systems should be designed
          so that the calculated sheet erosion rate does not exceed 2 tons/acre/year (EPA
          1991). Erosion effects due to both  wind and water should  be  taken into account.
          Refer to Appendix A.

       d.  Gas emissions should be taken into account  and be controlled where applicable.

       e.  Control biointrusion if warranted. Biointrusion in a landfill cover system refers to
          the flora and fauna interactions or intrusion into the cover system. Uncontrolled
          biointrusion may increase contaminant release from a  closed site via  such things
          as burrowing animals and/or insects and  root intrusion from plants or grasses
          whereby contaminants can be brought  to the  surface or allow for increased flux
          and thus increased potential for groundwater contamination.

       f  Access control. A closure system may require limited access to the  site. A typical
          control is the installation of a fence around the site with a locked gate.

       g.  Aesthetic considerations. Closed sites may require the cover system to be
          aesthetically appealing to nearby communities.

       h.  Future use considerations. A cover  system should take into account the potential
          future use of the site.

4.  Determine site-specific issues that will affect the design of the cover system—these relate
   to those  identified in step 3, as well as differential settlement, subgrade  considerations,
   extent of subsurface contamination, size, slopes,  seismic, adjacent facilities,  existing
   complications such as underground utilities, and surface water  management issues.

5.  Determine the cover type to be deployed  whether it is a prescriptive cover  such as the
   summarized in  section 2.2 or an alternative  cover  such as an ET Cover.  This document
   emphasizes the use of an ET Cover.  The ET cover is a  monolithic soil layer  that has
   adequate soil-water storage capacity to  retain  any  infiltrated water from the determined
   design precipitation event(s) until it can be removed via ET.
6.  Identify an acceptable borrow soil to be used in the  cover system.

       a.  For  an ET Cover, the soil  should have adequate water  storage  capacity  while
          having the capability to support native vegetation (refer to  Appendix B).  The


       borrow soil can be excavated soil from a nearby area or purchased off-site and
       transported  on-site.  Nearby,  native soils  are ideal for use in an ET Cover for
       economic reasons and because local vegetation is adapted to them. Refer  to
       Appendix C to assist in  determining whether these  soils have adequate water
       storage capacity.   Loams tend to  have  good  storage capacity and generally
       minimize the potential for desiccation cracking that can enable preferential flow
       (Figure 10).
             0.30 -
          Si  °-20
                   SAND    SANDY    LOAM
      Figure 10. Relation between moisture retention parameter & soil texture class (Schroeder et al. 1994)
   b.  For a Prescriptive Cover similar to that shown in Figure 6, the topsoil properties
       should provide an effective rooting medium for native vegetation.  However, the
       key to the  barrier layer soil properties  are  such that the saturated  hydraulic
       conductivity is less than the regulatory mandated minimum. To achieve the low
       permeability criterion,  the soil must have limited  rock  content and  have an
       adequate volume  of fines - preferably  clay.  Generally, the  higher  the  clay
       content, the lower the soil's saturated hydraulic conductivity.   Furthermore, the
       soil barrier layer is typically placed =wet of its  optimum' moisture content (ASTM
       D698) and heavily compacted in an effort to remold the soil and ultimately lower
       the initial saturated hydraulic conductivity of the layer.  Refer to section 2.2.1 for
       pitfalls related to this construction process and soil texture.
If laboratory testing of the soil is to be performed  for design analysis,  the following
considerations for this testing are offered (Refer to Appendix C):

   •   If the potential cover soil is to be modeled,  it is common to  test for  the soil's
       hydraulic properties.  It is recommended to test the borrow soil at a  remolded
       density similar to the intended installation density.  That is, any laboratory testing
       of the soil to be used in the cover system should be tested at a density  similar to
       the intended constructed density of the cover.  A soil's hydraulic properties are

          sensitive to the soil's density and thus any modeling performed should be done
          based on hydraulic properties derived at the intended soil density.

       •  Other soil properties that may be tested for include strength characteristics (i.e., if
          slope stability is a concern with steep side  slopes)  or particle size analysis if
          erosion is a concern.
7.  Cover soil placement density.  The density of the cover soil should meet the performance
   requirements.   If low  permeability is required,  such as the soil barrier  layer of a
   prescriptive cover, the soil  density will be higher and thus heavy compaction is generally
   warranted.  The upper topsoil  layer in Figure 6, is generally placed without compaction.
   If it becomes  too dense during the construction process, it is advised that it be disced
   prior to seeding to enhance the vegetation establishment. The recommended density for
   an ET Cover referred to as  the goal density is intended to mimic the natural conditions of
   the borrow soil  in an  undisturbed setting  (i.e. in-situ  density)  because soils  tend to
   migrate toward this density in the long-term.  Furthermore, the density needs to be dense
   enough to slow water infiltration to the point where ET can remove the water while not
   adversely affecting the water storage capacity of the cover profile.  It is recommended to
   determine this in-situ density of the undisturbed borrow soils to be used and set this as the
   goal density for placement  of the cover soil.  A tolerance of+/- 5 pounds per cubic foot is
   common for this constructed density.
8.  Determine the cover profile required layers and depth.  For a prescriptive cover, the
   minimum requirements  are  shown  in Figure 6.  For an  ET Cover,  determine  the
   minimum required depth of cover soil required to minimize flux (Appendix C). For a site
   that meets the criteria outlined in Section 1.0 that utilizes the recommended cover profile
   described in Section 4.0, this step is assumed to be unnecessary.  That is, the cover soil
   has an adequate quantify of fines and thus has adequate storage capacity for the relatively
   dry environment and low risk site. However, a technique is described in Appendix C that
   allows for a technical justification for the depth of cover to be deployed to minimize flux
   should this analysis be warranted.
9.  Perform an analysis to  predict soil loss due to both surface  water and wind erosion as
   required. Any predicted soil loss due to erosion is to be added to the overall  cover depth
   in addition to the minimum depth estimated in previous steps.  Refer to Appendix A.
10. Determine other layers  or enhancements  to the cover system  as required based on
   performance and/or risk assessment(s) performed. These may include a bio-barrier,  gas
   control layer, subgrade structural support layer, or a lateral drainage layer.
11. Determine the vegetation mix to be utilized on the cover system.
12. Evaluate the available field data of similar climatic  and soil textural classifications to
   determine whether the design is feasible.  Compile this data as supporting documentation
   in the final  design report to be submitted to regulators for final approval and permitting.
   This field data will provide short-term data that will justify that the design will perform
   as intended.
13. Evaluate  applicable natural analogs  for all parts  of the cover  systems such as  the
   hydraulic storage capacity, as well as for such things as  biointrusion, climate scenarios,
   erosion control,  and vegetation.  In this  case,  natural analog  can be defined as an


       occurrence  of materials or  processes which  resemble those expected in the proposed
       geological  waste repository.   Natural  analogs  can help  evaluate  the long-term
       performance of a cover system.

    14. Determine the installation requirements, to ensure performance  of the cover delivers that
       desired per the design.  This  will be included in the construction documents  (design
       drawings and specifications).

    15. Determine the method to be used to ensure that acceptable materials and construction
       methods are used to build the cover system.   This should be included in the  Quality
       Assurance (QA) documentation.  Refer to Appendix D.

    16. Determine maintenance monitoring criteria  (if any),  methods and frequencies to be
       performed to ensure that cover systems are not degrading. This will be included in the
       cover system maintenance plan.
    17. After the various design considerations have been evaluated and applicable  options
       selected, the cover system must be engineered to produce the final cover system details.
       Besides selecting the required cover profile described above, the appropriate soils must
       be used or amended.  Erosion must be minimized. Surface water run-on must be avoided
       while surface runoff must be controlled. Cover slopes should  be designed to minimize
       erosion while shedding  surface  water.  For  steeper slopes,  stability issues may be of
The quality  of construction  and materials used is  critical to achieving design objectives and
criteria.  The borrow  source used  for  cover  soil  should be well characterized  prior to its
acceptance for adequate volume, quality  of material, and uniformity of material.  A description
of some of the important issues related to cover soil is located in Appendix B.

An  important aspect involved with the construction of a soil cover system is that the soils are
placed at a uniform density. This will help limit preferential flow through the cover. Preferential
flow cannot be avoided, but necessary precautions should be employed to ensure it is minimized.
Cover  soils  should be placed within an acceptable density range.  When possible, cover soil
(water storage layer and  surface treatment layers only) should come from the same borrow
source.  If there is not adequate borrow soil available from a single source, then soils imported
from multiple borrow sources should all  be well characterized and  placed within respective
specified ranges.
The upper rock/soil admixture layer should be placed in a loose state without compaction. If this
soil layer becomes compacted or is determined to be too density after its installation, but prior to
seeding, it should be loosened prior to seeding. Care should be taken  to ensure the rock to soil
ratio is maintained. The  final slope and slope tolerances described  in the design should be
maintained.  Positive drainage should be maintained at all  times during installation of the cover

Quality Assurance (QA) of both the construction process and materials used is critical to the
successful installation of a cover system. Paramount to this success is the assignment of a person
or team to  ensure that  adequate  QA is applied  to the project.   The  QA engineer should
understand the design and the ultimate goal for success of the project and help ensure that the
constructed product meets the goal.

A QA Plan specific for the project should be written. This should include any QA testing to be
performed, success or failure criteria for the testing, frequency of testing, equipment or materials
to be used in testing along with applicable ASTM standards.  The QA Plan should identify what
documentation is  to be  completed during the construction  process and  what  individuals or
companies should retain the documented QA process.
Refer to Appendix D for recommendations of information to be included in the QA Plan
including examples of QA testing/frequency/criteria.

This guidance document serves to provide technical assistance in the closure of sites that have
been determined to  meet acceptable risk and  regulatory requirements  by applying  a well
designed cover system over them.  Figure 11 describes a recommended cover profile for sites
that satisfy the criteria outlined in Section 1.0.

           6". min
        3-0", mm
Native Vegetation

 Surface Treatment -
 Rock/Soil Admixture
                                                        Cover Soil
                                                        Existing Interim
                                                        Cover (free of
                                                        Waste Material
                            Figure 11. Recommended Cover Profile
The basic recommended cover is an ET Cover. The elements of this cover system are described

   1. Vegetation.  Native vegetation should be established on the cover.  Plant cover reduces
      the harmful effects of surface erosion resulting from both runoff and wind.  It provides
      for the removal of infiltrated water through transpiration.

2.  Surface treatment layer. An admixture composed of soil and rock is designed to resist
   erosion due to both surface water runoff and wind.  The admixture also  enhances the
   vegetation establishment.  The  soil is  to be composed of quality topsoil  capable of
   sustaining native vegetation. This layer should not be compacted to allow for better plant
   establishment and initial growth.   A typical mix is 25% rock  (nominally  %" to  2"
   diameter) to  75% topsoil by volume.  The mixture should be  well mixed  prior to final
   placement. The rock should be a quality durable rock. Refer to Appendix A for a design
3.  Cover soil. This layer is composed of quality soil with adequate water storage capacity.
   The soil  should possess adequate levels of plant-essential nutrients  to encourage the
   establishment and  productivity of non-woody indigenous plants (primarily grasses and
   forbs).  The amount of slat in the soil should also be limited (Appendix B).  Amendments
   may be required to achieve this.  The soil is to serve as a rooting medium and provide for
   storage of infiltrated water until removed via ET.   If there is substantial depth  to  an
   interim cover system that contains quality soil and is free of waste and debris, this soil
   depth  can be included  in the  final cover depth.  The  interim cover should be  well
   characterized prior to its inclusion in the final cover  profile. All  soil  in the final cover
   profile should be placed  as dry as possible while maintaining dust control and no wetter
   than the optimum moisture content for the given  soil.  This will allow for an increased
   initial  soil storage capacity and mitigate the potential for  desiccation cracking.  Refer to
   Appendix B  for soil quality issues and  Appendix C  for storage capacity and minimum
   cover thickness issues.
4.  Subgrade. The subgrade soil for a final cover system should be heavily compacted to
   provide a stable foundation for the final  cover profile. The upper foot  of the subgrade is
   also recommended to be dry of its optimum moisture content (ASTM D698).  This will
   help maintain the initial storage capacity of the cover system.

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The cover system design should address the potential for short-term erosion (i.e., before a good
stand of vegetation is established), and make  use  of temporary erosion-control  measures as
necessary.   The design should  also  address  long-term erosion after  vegetation has been
established. Erosion can be damaging not only to the cover system but also to areas into which
eroded soil  is deposited.   Erosion can  spread waste and contaminants.  Furthermore, it is
important that constructed erosion-control measures be properly installed and maintained.
The timing  for  completion of cover  system construction  can impact the potential  for early
vegetation establishment and thus affect the severity  of erosion.  Ideally the conclusion of cover
construction  should  be scheduled to  allow vegetation to become  established as  soon as
practicable and before the end of the growing season, if at  all possible.  Short-term erosion
control materials may be needed to protect the  surface layer until vegetation is adequately
established.  The design specifications should be written to ensure  optimization of vegetation
establishment based on that site's climate seeding and/or planting of native vegetation.
The construction contractor is often made responsible for maintaining temporary erosion control
measures and repairing damage due to erosion during and shortly after construction. However,
the general contractor may  only have limited expertise in soil erosion control. Furthermore, the
contractor may not be privy to design  decisions that affect the potential for severe short-term
erosion. Thus, caution should be exercised in placing responsibility upon the contractor, who
may be ill-equipped  to make  informed decisions about appropriate erosion control measures.
The design engineer  should consider the potential for and consequences of short-term erosion
and be proactive in  specifying appropriate control  measures  (e.g.,  silt fences, rolled erosion
control materials, sediment traps, hay bales, etc.) in the construction documents.
The  Natural  Resources   Conservation  Service  (NRCS)   (2000)   makes   the  following
recommendations to limit short-term erosion during construction:

    •   cover disturbed soils as soon as possible with vegetation or other materials (e.g. mulch) to
       reduce erosion potential;

    •   divert water from disturbed areas;
    •   control concentrated flow and runoff to reduce the volume and velocity of water and
       prevent formation of rills and gullies;

    •   minimize the length and steepness of slopes (e.g., use benches);
    •   prevent off-site sediment transport;

    •   inspect and maintain any structural control measures;

    •   where wind erosion is a concern, plan and install windbreaks;
    •   avoid soil compaction by  restricting the use of trucks and heavy  equipment to limited
       areas after seeding of the cover system; and

    •   scarify or disc the upper 6-inches (15  cm) of cover  soil that may have been compacted
       during construction activities prior to vegetating or placing sod.

Long-term erosion is an important consideration in the design of the cover's surface layer. In
spite of the admittedly approximate nature of predictive equations for erosion control, most cover
systems will require an analysis of long-term and, sometimes, short-term erosion. Typical design
criteria are as follows:

   •   The design sheet and rill erosion rate should not be exceeded. Although it is advisable to
       select allowable rates  of soil erosion  on a project-specific basis, all covers should be
       designed so that the sheet erosion rates not exceed 2 tons/acre/year (EPA,  1991). This
       maximum allowable rate is a result of both wind and runoff erosi on rates.

   •   Using the sheet and rill erosion rate from this calculation, the thickness of cover soil at
       the end of the design life should be calculated to verify that there is adequate thickness
       remaining and that sheet and rill  erosion has not progressed through the cover soil and
       into the underlying layers. There should  also be sufficient soil thickness to support
       vegetation and provide for adequate water storage capacity.

   •   The surface layer should resist gully formation under the tractive forces of runoff from
       the site-specific design storm(s).

   •   Wind erosion should also be evaluated.
Some of the more common methods for calculating sheet and rill erosion, gully formation, and
wind erosion are briefly described below.

The Natural Resources Conservation Service (NRCS) in 7 CFR Part 610  describes erosion
measures suggested for  highly  erodible soils.   Specifically,  subpart 610.11  sets forth  the
equations and rules for utilizing the equations that are used to predict soil erosion due to water
and wind. Section 301  of the Federal Agriculture Improvement  and  Reform Act of 1996
(FAIRA) and the Food  Security Act,  as amended, 16 U.S.C. 3801-3813 specified that  the
Secretary would publish the universal  soil loss equation  (USLE) and wind erosion equation
(WEQ) used by the Department  within  60 days of the enactment of FAIRA. This subpart sets
forth the equations, definition of factors, and  provides the rules under which NRCS will utilize
the USLE, the revised universal soil loss equation (RUSLE), and the WEQ.

RUSLE represents a revision of the USLE technology in how the factor values in the equation
are determined. RUSLE is explained in the U.S. Department of Agriculture Handbook 703,
predicting  Soil Erosion by Water: A Guide to  Conservation Planning with the Revised
Universal Soil Loss Equation (RUSLE).'' The RUSLE is expressed as:
       As - RexK xLSxC x Pc                            Equation A. 1


             AS = average annual soil loss by sheet and rill erosion in tons per acre caused by
                   sheet and rill erosion;

             Rg = rainfall energy/erosivity factor (dimensionless) and is a measure of rainfall
                   energy and intensity rather than just rainfall amount;

             K  = soil credibility factor (dimensionless), is a measure of the relative resistance
                   of a soil to detachment and transport by water, and varies based on seasonal
                   temperature and rainfall (adjusts  it bi-monthly for the effects of freezing and
                   thawing, and soil moisture);

             LS = slope length and steepness factor (dimensionless). It accounts for the effect
                   of length and steepness of slope  on erosion based on the relationship of rill
                   to interrill erosion;

             *-  = vegetative cover and management factor (dimensionless) and is the ratio of
                   soil   loss from land  cropped  under  the  specified  conditions to the
                   corresponding loss  from clean-tilled, continuous fallow. Estimates the soil
                   loss ratio at one-half month intervals throughout the year, accounting for the
                   individual effects  of prior land use, crop  canopy,  surface cover, surface
                   roughness, and soil  moisture.; and

             PC  = conservation support practice factor (dimensionless) and is the ratio  of soil
                   loss with a  specific support practice  (such  as  cross-slope  farming,  strip
                   cropping, buffer strips, and terraces) to the  corresponding  soil loss  with
                   uphill and downhill tillage.

Input values for RUSLE are developed using site-specific information and the database that is
part of the RUSLE computer program. A free Windows based version of RUSLE, Version 2 can
be downloaded  from http://www.ars. usda.gov/Research/docs.htm?docid=6010    Using As
computed from Equation A.1, the thickness of cover soil at the end of the cover system design
life can be calculated to verify that there is sufficient cover soil remaining.
The WEPP erosion model computes soil loss along a slope and sediment yield at the end of a
hillslope (USDA 1995).  Interrill  and rill erosion processes are considered. Interrill  erosion is
described as a process of soil detachment by raindrop impact, transport by  shallow sheet flow,
and sediment delivery to rill channels.  Sediment delivery rate to rill flow areas is assumed to be
proportional to the product of rainfall intensity and interrill runoff rate. Rill  erosion is described
as a function of the flow's ability to detach sediment, sediment transport capacity,  and the
existing sediment load in the flow.

The appropriate scales for application are tens of meters for hillslope profiles, and up to hundreds
of meters for small watersheds. For scales greater than 100 meters, a watershed representation is
necessary to prevent erosion predictions from becoming excessively large.

Overland flow  processes are conceptualized as a mixture of broad sheet flow occurring  in
interrill areas and concentrated  flow in rill  areas. Broad sheet flow  on an  idealized  surface is
assumed for overland  flow  routing  and  hydrograph development.  Overland flow  routing


procedures include both an analytical solution to the kinematic wave equations and regression
equations derived from the kinematic approximation for a range of slope steepness and lengths,
friction factors (surface roughness coefficients), soil textural classes, and rainfall distributions.
Because the  solution to the kinematic wave  equations is restricted to an upper  boundary
condition  of  zero depth, the routing process for strip cropping (cascading planes) uses the
concept of the equivalent plane. Once the peak runoff rate and the duration of runoff have been
determined from the  overland flow  routing,  or by  solving the  regression equations to
approximate the peak runoff and duration, steady-state conditions are assumed at the peak runoff
rate for erosion calculations. Runoff duration is calculated so as to maintain conservation of mass
for total runoff volume.

The erosion equations are normalized to the discharge of water and flow shear stress at the end
of a uniform slope and are then used to calculate sediment detachment, transport, and deposition
at all points along the hillslope profile. Net detachment in a rill segment is considered to occur
when hydraulic  shear  stress of flow exceeds  the  critical shear stress of the  soil and when
sediment load in the rill is less than sediment transport capacity. Net deposition in a  rill segment
occurs whenever the existing sediment load in the flow exceeds the sediment transport capacity.

In watershed  applications, detachment of soil in a channel is predicted to occur if the channel
flow shear stress exceeds a critical value and the sediment load  in the flow is below the sediment
transport capacity. Deposition is predicted to occur if channel  sediment load is above the flow
sediment transport  capacity. Flow  shear  stress in channels is computed  using regression
equations that approximate the spatially-varied flow equations.  Channel erosion to a  nonerodible
layer and subsequent channel widening can  also be simulated. Deposition within and sediment
discharge  from  impoundments is  modeled using conservation of mass  and overflow  rate

The WEPP model was developed in the 1980's when an increasing need for improved erosion
prediction technology was recognized by the major research and action agencies of the United
States  Department of  Agriculture  and Interior,  including the Agricultural  Research Service
(ARS), Natural  Resource Conservation Service (NRCS), Forest  Service (FS), and Bureau of
Land  Management  (BLM).  In 1985, these agencies  embarked on a 10-year research and
development effort to replace the Revised Universal Soil Loss Equation. Some of the differences
between the WEPP model and the RUSLE are as follows:

   •   The RUSLE  equation  is based on undisturbed  agricultural and rangeland top  soil
       conditions, whereas any  kind of soil  can be  described with WEPP. Thus, WEPP is well
       suited to describe a landfill cover, which is a disturbed condition.

   •   The WEPP model is capable of predicting erosion and  deposition  in more complex
       situations, such as when berms are involved. WEPP can predict the erosion on a cover as
       well as the deposition in berm  channels  in the watershed mode. The WEPP model's
       ability to determine runoff and channel flow can also aid in determining stability issues
       with berms, such  as overtopping. RUSLE can only predict the upland  erosion between

   •   RUSLE  can only predict average annual upland erosion. WEPP's  climate  generator
       includes stochastically generated events. This is  an important point in arid environments
       where there are very few precipitation events annually, but when they occur, they are
       often torrential events that have major  impacts on the site. Thus, a  landfill in an arid


       climate is unlikely to fail in an average year, whereas, it is very likely to fail in a year
       when a major storm event has occurred.  WEPP can predict the impacts from a major
       storm event, but RUSLE cannot.
The windows based version of the WEPP software is available along with additional information
regarding      the      WEPP      model,      software,     and     documentation      at:
The concentration of runoff under many circumstances encourages the formation of rills, which,
if unchecked, grow into gullies (Figure A-l). This can be the most severe type of erosion of
cover systems soils at landfill and waste remediation sites.
                Figure A-l. Gully Formation Measured Over Six-Feet Deep in Albuquerque, NM

The dynamics of gully  formation are complex and not completely understood.  Gully growth
patterns are cyclic,  steady, or spasmodic and  can  result  in the  formation of continuous or
discontinuous  channels.   Gully advance rates  have been obtained by  periodic  surveys,
measurements to steel  reference stakes or concrete-filled auger holes,  examination  of gully
changes from small-scale maps, or from aerial photographs. Studies are producing quantitative
information and some procedures that combine empirically- and physically-based methods have
been advanced. Vanoni  (1975) presented six methods used for prediction of gully growth and/or
gully head advance. They all follow some type of multiplicative or power law and are replete
with empirical constants that  are generally site specific. McCuen  (1998) updated and further

described gully erosion prediction equations with the observation that five factors underlie the
relevant variables  of the process: land use, watershed size, gully size, soil type, and runoff
momentum. Having investigated the relevant factors, however,  McCuen found that none of the
equations treat all terms. Better methods of evaluating gully formation that are more physically
based are needed. Consequently, all covers should be designed to mitigate gully formation.

The potential for gully development in vegetated soil surface layers has been assessed at landfill
sites using the tractive force method described by  Temple et al. (1987) and DOE (1989) and
developed for channel flow. The tractive force method (Temple et al., 1987; DOE,  1989) can be
used to calculate the allowable shear stress of a vegetated surface layer as:
       ra = TQ6 x C| > Q.9kPa                         Equation A.2

                    Tab = allowable shear stress for the surface layer with bare soil (kPa); and

                    C0 = void ratio correction factor (dimensionless).

Temple et al. (1987) and DOE (1989) provide graphs for both Tab  and ce  values.

The allowable shear stress (Ti°0  (Equation A.3) must be equal to or greater than the effective
shear stress (Tie)  applied to the surface layer by the flowing water:
                                               Equation A.3
                    Yw = unit weight of water (kN/m3);

                    D = flow depth (m);

                    •->  = slope inclination (dimensionless);

                    ^F = vegetal cover factor (dimensionless);

                    n = Manning's roughness coefficient for the considered vegetative cover
                    (dimensionless); and

                    ns = Manning's roughness coefficient for the bare soil (dimensionless).

Guidance on the selection of values for the vegetal cover factor and the Manning's coefficients is
provided by Temple et al. (1987) and DOE (1989).
The depth of flow can be calculated using the Manning's equation (Equation A.4) (DOE, 1989):

       D =
            5°5 '                             Equation A.4

                    *?  = peak rate of runoff (mP3P/s/m) from the Rational Formula (and
                       incorporating the flow concentration factor);

                    n  = Manning's roughness coefficient for the considered vegetative cover

                    s  = slope inclination (dimensionless);
Wind erosion physically removes the lighter, less dense soil constituents such as organic matter,
clays, and silts (Figure A-2). Thus it removes the most fertile part of the soil and lowers soil
productivity (Lyles,  1975).  During the 1930's, a prolonged drought culminated in dust storms
and soil destruction  of disastrous proportions.   The Wind Erosion Equation (WEQ) (Equation
A.5) for predicting soil loss due to wind erosion is:
       E = f(I K C L V)
Figure A-2. Wind Erosion in Arid Climate

                        Equation A.5

                 £ = the estimation of average annual soil loss in tons per acre.
                 f  indicates the equation includes functional relationships that are not straight-line
                     mathematical calculations.
                 I  = the soil erodibility index.
                 K = the ridge roughness factor.
                 C = the climatic factor. All climatic factor values are expressed as a percentage of the
                     value established at Garden City, Kansas. Garden City, Kansas was the location
                     of early research in the WEQ and established the standard for climatic factors
                     against which the other locations are measured.
                 L = the unsheltered  distance across an erodible field, measured along the prevailing
                     wind erosion direction.
                 V = the vegetative cover factor.
The erosion analysis tools described above, as well as similar others, are all best suited for
farmlands or uniform watersheds  with frequent  and average  rainfall.  They are much less
applicable to desert or dry climates where infrequent storms are the rule.  The models are also
better suited for finer grained soils like clay and silt and less so for coarser loams. They are best
suited for larger areas and less accurate for smaller areas.  They all state that they can deal with
minor rill development but none can deal with gully  formation other than the tractive force
method that estimates the potential for  gully erosion.  In arid and semi-arid climates, soil loss
due to gully erosion can be orders of magnitude greater than sheet flow erosion.   In  order to
overcome these shortcomings Anderson and Dwyer (Dwyer et al 1999) developed  a gravel/soil
admixture designed to mitigate soil loss on landfill covers.  This design was later  modified by
Dwyer (et al 2007).

Gravel/soil  admixtures provide  excellent  means to  minimize  erosion  while allowing  for
vegetation  establishment without a significant reduction in  evaporation (Waugh  et al  1994,
Dwyer 2003).  Erosion (Ligotke 1994) and water balance  studies (Waugh  1994)  suggest that
moderate amounts of gravel mixed into the  cover topsoil will control both  water and wind
erosion.  As wind and water pass over the landfill cover surface, some winnowing of fines from
the admixture is expected, creating a vegetated erosion-resistant surface sometimes referred to as
a -desert pavement". Figure A-3  shows the results of wind erosion in northwestern New Mexico
on the Navajo  Reservation where  a prescriptive landfill cover had been installed.  The local
native soils were generally a coarser loam material, but to comply with prescriptive regulations -
a soil was imported  that contained a  significant amount of fines (silt  and clay).  This soil was
installed to meet the low saturated  hydraulic conductivity requirement for a prescriptive barrier
layer.  Severe winds eroded the newly installed fine cover soils leaving behind some desiccated
clay and minimal fines  stabilized by sparse vegetation roots.  Of the two-feet of soil originally
installed as the  cover, less than a foot remained after a year.

        Figure A-3. Landfill Cover Located on the Navajo Reservation that experienced significant Wind Erosion

The design of a gravel admixture layer should be based on the need to protect the soil cover from
water and wind erosion.  A gravel admixture can effectively protect a cover from long-term wind
erosion.  The protection from water erosion will depend on the depth, velocity, and duration of
water flowing across the landfill  cover. These flow values can be established from the physical
properties of the cover (slope, convex or concave  grading, slope uniformity, and length of flow
paths) and the intensity of the  precipitation water (precipitation rates, infiltration  vs.  runoff
relationships, snowmelt and off-site flows).
Erosion is greatly  affected by rainfall intensity.   As the  intensity increases, the velocity of
subsequent run-off also increases. Thus the erosive energy of the flowing water increases as the
square of the velocity.  Consequently, the  amount of erosion can increase significantly as  the
rainfall intensity increases.  Anderson and Stormont (1997) estimated that a single 6-hr, 100-year
storm produces more than ten times the annual average erosion quantity.  In response to intense
rainfall, erosion does not occur as a uniform lowering of the surface, but by the formation of rills
that turn  into  gullies.  When run-off is channeled into the developing gullies, the velocity
increases, and thus erosion increases (Figure A-l).  For a  cover surface, gully formation is
particularly problematic because it can compromise the function of the cover system to isolate
the underlying waste.

                    0.1     0.2       0.4          1.0           2.0
                                   Size of Parti cl es  (mm)

   Figure A-4. Relationship Between Erosion Mechanism (Air or Water), Particle Size and Fluid Velocity (Garrels, 1951 as
                                 referenced by Mitchell, 1993).

Erodibility of soils increases as particle size decreases (Figure A-4). Clay particles, while small,
can possess cohesive strength that resists erosion until they become nearly saturated whereby
their cohesion approaches zero.  Silts are generally the most erosive soils.  Surface soils have
been modified by the addition of larger particles,  e.g. gravel, to increase their resistance to
erosion (Ligotke 1994, Waugh et al 1994, Dwyer et al 1999, Dwyer 2003, Dwyer et al 2007).  As
the finer  portions of the soil are removed by erosive forces, the larger particles remain behind
and form an -armored" surface sometimes referred to as a -d-esert pavement". This surface is
much more stable and resistant to surface erosion due to both surface water run-off and wind
An approach to  design  a  soil/gravel  admixture  to serve  as a surface -a-rmor"  or  -desert
pavement" (Figure A-5) was developed that combines analytical and empirical relationships in a
step-by-step process (Dwyer et al, 1999, Dwyer et al 2007).

                     b-JJr  JT
                  •  *w. ' •*»
                j?       ><
              r^-x -•--   _»
                Figure A-5. Gravel recently installed on Superfund closure in Farmington, NM
The following steps are involved in the design methodology:
   1.  Estimate the design rainfall event;
   2.  Predict run-off for the given slope characteristics, including slope angle and length;
   3.  Estimate the channel (gully) geometry in response to estimated run-off;
   4.  Calculate the particle size that will be displaced by the channel; velocity;
   5.  Determine the depth of scouring and remaining armored layer.

Use a design rainfall return period that makes sense for the site and the waste encapsulated.  A
common design event used is a 100-year return period.  Local hydrology requirements should be

The national method" is one of the simplest and best-known analysis methods routinely applied
in urban hydrology for smaller areas.  The rational method (Equation A.6) is based on the
assumption that rainfall occurs uniformly over the watershed and  at a constant intensity for
duration equal to the time of concentration. This method is commonly used for areas under 40
hectares in size.

The peak rate of runoff, (Q) in cfs (runoff is actually in acre-inches/hour but is rounded to equate
to cfs), is given by the following expression:
                                                             Equation A.6

                           C = Run-off coefficient (dimensionless)
                           I = Rainfall intensity (in/hr)
                           A = Surface area that contributes to run-off (acres)

The appropriate value for =I' in this case where erosional processes are being evaluated is the
peak intensity.  The duration of the peak rainfall intensity  is often derived from the -time of
concentration," which  represents the  time for run-off from the  most remote portion of the
contributory watershed to exit that watershed.  This time of duration is dependent on the slope
angle and length and the surface described by the value =C'.  Common values for =C' are listed in
table Al.
Vegetation and
Slope Conditions
Flat. 0-5% slope
Rolling, 5-10% slope
Hilly, slope
Flat, 0-5% slope
Rolling, 5-10% slope
Hilly, 10-30% slope
Flat. 0-5% slope
Rolling, 5-10% slope
Soil Texture
Open sandy
Clay and silty
Tight clay
The assumed contributory area is found from the following figure A-5.

                              1      I       I
                              I      I       I
                              \    I     /
                          Figure A-5. Contributory Area for Gully Formation

The contributory area on a landfill can generally be assumed to be the slope length multiplied by
the width that contributes to the formation of gullies, that is, the lateral gully spacing. The slope
width is assumed to about one quarter that of the slope length based on professional experience
and consultation with experts. Consequently, =A' is equal to
The channel geometry shown in figure A-6 is that assumed for the gully formation.
                                 Figure A-6. Channel Geometry
The geometry of the channel that forms is based on regression equations developed from analysis
of a large number of channels (Simons, Li & Assoc., 1982). The channel width is given by:
                                                     Equation A.7

              b = width of flow (ft)
              Qm = mean annual flow (cfs)
              M  = percentage of silts and clays in soils

The mean annual flow (Qm) is assumed to be between 10 to 20% of the peak rate of run-off (Q)
(Dwyeretal, 1999).
For the given discharge point of geometry, the  hydraulic depth (dh) defined as the flow cross-
sectional area divided by the width of water surface is half of the gully depth (d).
For flows at the critical slope:
       fa = 0.5 x F°-< x F-^ x Q°<*              Equation A.8
              F = width to depth ration = b/dh
              ^"t  = Froude Number -1.0
These equations can be solved simultaneously to yield the channel width and depth for a given
peak flow rate and percentage of silt and clay.  With the channel dimensions, the velocity in the
channel can be found.
A  1 1  4
The incipient  particle  size is the particle that  is on the brink of movement  at the assumed
conditions.  Any increase in the erosional forces acting on the particle, due to an increase in
velocity or slope, for example, will cause its movement.  This incipient particle  size (Dc) can be
calculated using the Shield's Equation:
       D  = T/
        c   IFS(YS - y)                               Equation A.9
              T = total average shear stress (pcf)
              Fs = Shield's dimensionless shear stress = 0.047
              ys = specific weight of soil (pcf)
              y = water density = 62.4 pcf

The total average shear stress is given by:
       T = y x dh x 5                                  Equation A.10

             S = slope (ft/ft)

The incipient particle size defines the maximum size of particle that will be eroded for a given
set of conditions.  The material larger than the incipient particle size will not be displaced or
eroded and can form an armoring that will protect the channel from further erosion from similar
or lesser storm events.
The depth of scour (Ys) (Figure A-7) to establish  an armor layer is given by (Pemberton and
Lara, 1984):
                                                     Equation A. 11
             Ys = scour depth
             Ya = armor layer thickness
             PC = decimal fraction of material coarser than the incipient particle size.

                               'Original Surface
                                               New Surface
                              Figure A-7. Armor Layer Development
Other considerations that should be included in the rock/soil admixture design include:

   •   Rock mixed into the soil/rock admixture on the top slope  and side  slope should be

   •   The hydraulic properties of interstitial soil would match the underlying water storage soil

   51. The interstitial  soil  would  be live topsoil  with favorable  fertility,  microbiology,
       propagules, and nominal phytotoxicity.


Excessive soil salts can prevent the establishment of vegetation, as well as, precipitate out on the
surface creating a surface crust that reduces or prevents the infiltration of water.   Saline soils are
susceptible to concentrated surface water flow and thus gully erosion. It is understood that a primary
goal of a cover system is to limit flux through the cover into the underlying waste.  However,
infiltration of water into the cover system is required to maintain the integrity of the cover's
vegetation.  Vegetation  is  essential to ensure  the long term  integrity  of the  cover system  by
stabilizing the soil and minimizing erosion while removing moisture via transpiration.  The lack of
vegetation and/or surface crust increases runoff that can lead to increased erosion as seen in figure

                          Figure B-l. Excessive Gully Erosion on Shallow Slope
Soluble salts in a cover soil can go into solution following a precipitation event or series of events.
As the soil dries, moisture is moved upward by matric potential gradients where the salts in solution
precipitate out at or near the ground surface as the water evaporates.  These precipitated salts, in
conjunction with the existing salts present in the upper soil layer, promote the formation of a brittle
surface crust (Figure B-2).  Soil  water salinity can  negatively affect  soil physical properties by
promoting the binding of fine mineral particles into larger aggregates. This process may promote the
formation of surface crusts.  Surface crusts are  essentially impermeable to water when dry.   The
reduced permeability promotes higher surface runoff volumes due to decreased water infiltration into
the landfill cover soil.  In turn, the higher surface runoff volumes increase the erosion of the landfill

                             Figure B-2. Surface Crack in Brittle Cover Soil

Infiltration in the cover soil is compromised by salt-induced soil dispersion. High  salt contents
induce dispersion of soil particles and the dispersed particles plug pores within the soil surface
by two means. First, dispersed soil particles plug underlying pores in the soil thereby constricting
avenues (channels  and pores) for water and roots to move through  the  soil.   Secondly,  soil
structure promoting favorable water  infiltration is disrupted  because of this dispersion  and a
cement-like surface layer is formed when the  soil  dries. The hardened upper layer,  or surface
crust, further restricts water infiltration and plant establishment on the cover soil.
As described above, excessive salt concentrations of soil in the rooting medium can adversely
affect vegetation that in turn increases erosion.  Soil dispersion disrupts natural soil structure and
hardens the soil and blocks water infiltration.  Under these  conditions,  it is difficult for plants to
get established and grow.  Saline soils are a problem because high salt concentrations prevent
plant roots from effectively utilizing soil water. Plant roots absorb water from the soil through
the process of osmosis. Osmosis is the process whereby water is moved from an area of lower
salt (higher water) concentration to an area  of higher salt (lower water) concentration. The salt
concentration inside a normal plant cell (approximately 1.5%) is relatively high compared to
normally dilute salt concentrations in soil water. Therefore, under -^ormal" soil water salinity
levels water will move into root cells from the surrounding  soil. However, under high saline soil
conditions, the concentration of salts  in the  soil water can  rise above 1.5% and may inhibit the
movement of water from the surrounding soil to the plant roots.   High salt concentrations in the
soil can in fact cause water to move out of plant roots, thereby dehydrating the plant. High salt
contents in the soil may also induce nutrient deficiencies in existing plants since plants take up
nutrients from the surrounding soil via water intake.
Cover soil can be characterized by the following agronomic characteristic ranges that include:
pH,  electrical  conductivity  (EC),  sodium absorption ratio  (SAR),  exchangeable sodium
percentage  (ESP),  calcium carbonate equivalent, cation exchange  capacity (CEC), percent
organic matter, nitrogen (N), phosphorous (P), and potassium (K).


Electrical conductivity (EC) estimates the amount of total dissolved salts (TDS), or the total
amount of dissolved ions in the water. EC is measured in micro Siemens/cm or micromhos per
centimeter (l|iS/cm =  1 |imho/cm). The Sodium Adsorption Ratio (SAR) is the proportion of
sodium (Na) ions compared to the concentration of calcium (Ca) plus magnesium (Mg).  An
SAR value of 15 or greater indicates an excess of  sodium will be adsorbed by the soil  clay
particles.  Excess sodium can cause soil to be hard and cloddy when dry, to crust badly, and to
take water  very slowly.  Cation Exchange  Capacity (CEC) is  a calculated  value that is an
estimate of the soils ability to attract, retain, and exchange  cation elements.  It is reported in
millequivalents per 100 grams of soil (meq/lOOg).  The exchangeable sodium percentage (ESP)
refers to the concentration of sodium ions on cation exchange (CEC) sites.  An ESP of more than
15 percent is considered the threshold value for a soil  classified as sodic. This means that sodium
occupies more than 15  percent of the soil's CEC. Be aware that sensitive plants may show injury
or poor growth at even lower levels of sodium.  Table B-l summarizes the tests that evaluate the
salt content in soils with recommended limits for each test.
                          Table B-l. Soil Requirements to Limit excess Salts

Less than
Less than
Less than
Less than
8 |jS/cm
The aforementioned discussion on salt content in soils centered on soluble salts.  Less soluble
salts such as calcium carbonate (CaCO3) are also harmful to cover soils in excess. Figure B-3
reveals the difference in vegetation establishment on  cover soils based  on CaCO3 content.
CaCOs is prevalent in soils in the southwestern United States.

     Soils with Higher than 10% CaCO3 by Weight
Soils with Lower than 10% CaCO3 Content by
           Figure B-3. Negative Impact of High Salt Content on Vegetation on a Cover System (Dwyer 2003)

CaCOs is a salt that can be formed by the reaction of carbon dioxide (an acid-forming oxide),
and  calcium  oxide (a  base-forming oxide).   Carbon dioxide produced  by  root  (and soil
microorganism) respiration, in the presence of water, forms H2CO3  (carbonic acid)  (Birkland
1974).  This  formation tends to be most active in the upper soil where biological activity  is
highest.  Calcium cations from weathering of primary minerals, or from windblown dust, or even
entering the soil in rainwater, tend to stay dissociated in the upper soil where pH tends to be
lower and water tends to be more abundant (Birkland 1974; Jones and  Suarez 1985; Monger and
Gallegos 2000).  As  soil  solutions pass to greater depth  in the soil, increased pH and less
abundant water drive  the equilibrium toward precipitation of CaCOs (Birkeland 1974; Harden
1991; Pal et. al. 2000; Monger and Gallegos 2000). As this process continues over time, CaCOs
accumulates in the lower soil.  Soil that contains CaCOs is called calcareous soil.  Secondary
accumulations of CaCOs in the subsoil are referred to as calcic horizons.  They may exist either
as cemented layers, accretions, or concentrated horizons in lower soil profiles.  These features
are often colloquially  but incorrectly termed caliche.  Caliche (a geologic feature) forms on  or
very near the surface of soil in arid and  semiarid regions), typically as a result of capillary rise
and evaporation of CaCOs-charged ground water.
Calcic soil horizons, by comparison,  are a phenomenon of downward leaching.  To a certain
extent, the depth to calcic soil horizons depends on the amount of rainfall. Typically,  as rainfall
increases, so  too  does the  depth to a calcic soil  horizon.  When annual rainfall exceeds 100
centimeters (-39 inches), calcic soil horizons disappear from the soil profile (Blatt et al. 1980).
One of the primary means by which CaCOs affects plant growth is by inhibiting the ability  of
plants to absorb nutrients from the soil. CaCOs affects plant uptake of both macronutrients (e.g.,
nitrogen and phosphorus) and micronutrients (e.g., zinc and boron).
The macronutrient most affected by  the presence of CaCOs is phosphorus.  Phosphorous  is
absorbed by plants in two forms: H2P(V, and HPO42". Of these, H2PO4" is most readily available
to plants, whereas plants do not readily absorb HPO42".  In fact, McGeorge (1933) considered the
monovalent form the  only  form  of phosphorus that influenced plant  growth and nutrition.   In

order for phosphorus to be absorbed by the root, the solution or film around the root must have a
pH of 7.6, which is more difficult to attain in higher pH soil (McGeorge 1933). The abundance
of these forms of phosphorous available to plants depends upon the pH of the soil (McGeorge
1933).  In low pH (acidic) soil H2P(V is most abundant, whereas HPC>42" is most abundant in
high pH (alkaline) soil.  The presence of H^PCV is greatly reduced in calcareous soil with pH
between 8.0 and 8.5 (McGeorge 1933;  Sharma et al. 2001).  In addition,  phosphorus can react
with CaCOs in soil to form calcium carbonate phosphate (McGeorge 1933; Dominguez 2001), a
form unavailable to plants.
The  uptake of micronutrients by plants is also affected by the presence of  CaCOs.  The
micronutrients whose absorption by plants is most affected by the presence of CaCOs are boron,
zinc, iron, copper, and  manganese (Brady and Weil 1994; Jones and Woltz 1996; Abdal et al.
2000).   Reactions  with CaCOs, water, and  carbon  dioxide  in  soil  can transform these
micronutrients into forms unavailable for plants (Muramoto et al.  1991; Wang and Tzou 1995;
Jones and Woltz 1996). One of the most common  micronutrient deficiencies in plants is boron
(Brady and Weil 1994). In calcareous soil, boron is fixed or bound by soil colloids (Brady and
Weil 1994; Rahmatullah  et al. 1998).  For example, a study on sunflowers found that,  as soil
concentrations of CaCOs increased, the dry weight of sunflower shoots decreased, and correlated
with decreasing concentrations of boron in the plant tissue (Rahmatullah et al. 1998).
Concentrated CaCOs in soil  also  increases  the potential for crusting, thereby reducing water
infiltration and inhibiting root penetration (West et al. 1988; Abdal et al. 2000; Dominguez et al.
2001; Sharma et al. 2001).  In other words,  physical  changes of the soil caused by  higher
concentrations ofCaCOs can cause reductions in plant production.
In addition to inhibiting plant growth, increasing CaCOs concentrations in soil have also been
linked  to  decreases in soil  microfauna  populations  (Sharma  et  al.  2001).   The affected
microfauna include  fungi, bacteria and actinomycetes, and azotobacter (Sharma et al.  2001).
Microfauna  are  critical to  the conversion  of soil nitrogen into forms  available to  plants.
Mycorrhizal associations (a symbiotic relationship between the root and fungi) can be critical for
plants to increase uptake and harvesting of nutrients, especially phosphorus, and water.
The maximum allowable calcium content levels for cover soil in the rooting zone (upper 3-feet
[91 cm]) should be less  than 10% by weight (Dwyer 2003).
Just as important to limit the amount of salts in a cover soil is that the soil used have adequate
nutrients to allow  for a quality stand  of native vegetation.   Adequate soil nutrients must be
available to adequately establish native  vegetation on  the  cover  surface.   The  parameters
considered for acceptable nutrients for a given borrow soil are: cation exchange capacity (CEC),
percent organic matter, nitrogen (N), phosphorus (P), and potassium (K+).  The following soil
nutrients values are required in the upper 3-feet (91  cm)  of all cover soil installed.  Table B-2
summarizes the recommended tests to be  performed on soils  to  determine the appropriate
nutrients levels with their recommended acceptable ranges.

                           Table B-2. Soil Nutrient Requirements for Covers
Percent organic matter
Greater than 15
Greater than 2% (g/g)
Greater than 6 parts per million
4 to 7ppm
61 to 120 ppm
The disadvantages of a low CEC obviously include the limited availability of mineral nutrients to
the plant and the soil's inefficient ability to hold applied nutrients.
Organic matter makes up only a small part of the soil. Even in small amounts, organic matter is
very important.  Soil  organic  matter has several parts: The living microbes in the soil (like
bacteria and fungi), which break down very rapidly  when  they die.  Partially decayed plant
material and microbes, for instance, plant material you mix in or manure.  The stable material
formed from  decomposed plants and microbes. This material is called humus, and is broken
down very slowly.
Organic matter affects both chemical and physical properties of the soil:
       Chemical Effects: Organic matter releases many plant nutrients as it is broken down in
       the soil, including nitrogen (N), phosphorus (P) and sulfur (S). It  is also one of two
       sources of CEC in the soil. (Clay is the other major source.) CEC represents the sites in
       the soil that can hold positively charged nutrients like calcium (Ca++), magnesium (Mg+)
       and potassium  (K+).  If CEC is  increased, the soil can hold more nutrients and release
       them for plant growth. To increase CEC, you have to increase organic matter.
       Physical Effects: Organic matter loosens the soil,  which increases the amount  of pore
       space. This  has several important effects. The density  of the soil goes down (it becomes
       less compacted) and the soil structure improves. This  means that the sand, silt and clay
       particles in  the soil stick together, forming aggregates or crumbs. Because there  is more
       pore space, the soil is able to hold more water  and more air. Plants grown on healthy soils
       won't be as stressed by drought  or excess water. Water also flows into the  soil from the
       surface more quickly. With less compaction, it is also easier for plant roots to grow
       through the  soil.
There are many ways to add organic matter  to soils.  Compost and manure may  add larger
amounts of organic matter. Compost is very  similar in composition to soil organic matter. It
breaks down slowly in the soil and is very good at improving the physical condition  of the soil.
Manure may break down quickly, releasing nutrients for plant growth, but it may take longer to
improve the soil using this material.  Whatever matter is chosen to amend the soil, it should meet
the environmental standards of the site.

A major consideration when selecting plants for a site is provided in Executive Order  13148,
which promotes the use of native  species on revegetated sites.  EPA defines native plants as
plants that have evolved over thousands of years in a specific region and have adapted to the
geography, hydrology, and climate (see http://www.epa.gov/greenacres/). Native plants found in
the surrounding natural areas have the best chance of success, require the least maintenance, and
are the most cost-effective in the long term. Ideally, revegetation of a  site will  create  natural
conditions that encourage re-population by native animal species and are consistent with the
surrounding land.   Using non-native plants located close to native  plant environments could
displace the native plants; therefore, it  is important to check the invasive nature of the proposed
plants (Executive Order 13112). Plant  succession must be considered; for example, the original
species planted may not  survive but may attract local wildlife to the area that will disperse the
seed and aid in the overall revegetation of the site.

A key element in the stability and performance of an ET  cover system is vegetation.  Native
grasses are desired on landfill and open dump covers because they stabilize the surface soil and
reduce erosion, transpire stored soil-water, and have relatively shallow thin roots that generally
do not result in preferential flow paths (EPA 1991).
Landfill cover vegetation goals (Waugh et al. 2002):

    •   are well adapted to the engineered soil habitat,
    •   are capable of high transpiration rates,
    •   limit soil erosion, and
    •   are structurally and functionally resilient.

Diverse mixtures of native and naturalized plants will maximize water removal and remain more
resilient given variable and unpredictable changes in the environment resulting from pathogen
and  pest  outbreaks,  disturbances  (overgrazing,   fire, etc.), and climatic  fluctuations. Local
indigenous ecotypes that have been  selected over thousands of years are usually best adapted. In
contrast, the exotic grass  plantings common on engineered covers  are genetically and structurally
rigid, are more vulnerable to disturbance or  eradication by single  factors,  and  will  require
continual maintenance (Mattson et al. 2004).

Selection of plant species is an important consideration in the design of a vegetated surface layer.
The vegetation serves several functions (Mattson et al. 2004):

    •   Plant leaves intercept some of the rain  before it impacts the  surface layer, thereby
       reducing the energy of the water and the potential for erosion.

    •   Plant vegetation also helps dissipate wind energy.

    •   The shallow root system of plants enhances the surface layer resistance to water and wind

    •   Plants promote ET of water, which increases the available water storage capacity of the
       cover soils and decreases drainage from these soils.

    •   A well-vegetated  surface layer is generally considered  more natural and esthetically
       pleasing than an unvegetated surface layer.

In selecting the appropriate vegetation for  a  site, the following general recommendations are
    •   Locally-adapted,  low-growing  grasses  and  shrubs  that  are  herbaceous  or  woody
       perennials should be selected.
    •   The plants should survive drought and temperature extremes.
    •   The plants should contain roots that will penetrate deep enough to remove moisture from
       beneath the surface but not so deep as to disrupt the drainage layer, hydraulic barrier, or
       gas collection layer.
    •   The plants should be capable of thriving with minimal addition of nutrients.
    •   The plant population should be sufficiently diverse to provide erosion protection under a
       variety of conditions.
    •   The plants should not be an attractant to burrowing wildlife.
    •   The vegetative cover should be capable of surviving and functioning with  little or no
       maintenance  (e.g.,  without  irrigation  other than  for  initial  plant  establishment,
       fertilization, and mowing).

           APPENDIX C

Determine the minimum required depth of cover soil for an ET Cover required to minimize
flux.  For a site that meets the criteria outlined in Section 1.0 that utilizes the recommended
cover profile described in Section 4.0.  this  step is assumed to be unnecessary.  That is, the
cover soil has an adequate quantify of fines (greater than 20%  pass the number 200 sieve)
and thus has  adequate storage capacity for the relatively dry environment and low risk site.
The following steps may be performed if this is not the case.
       a.  Identify the  design infiltration event(s).  This  can be dependent on design life.
          Utilizing average climatic conditions is often used. Other possibilities include the
          simulation of  extreme  or  stress  conditions  such  as wet years  or extreme
          precipitation  events.
       b.  A first-order estimate of required cover  thickness can  be determined from
          estimates  of the water-holding or storage capacity of the soil and the amount of
          infiltrated water that has to be  stored. The design strategy for an ET cover system
          is to ensure the storage capacity  is sufficient to store the -worst-case" infiltration
          quantity resulting from the design precipitation event until it can be removed via
          ET. The maximum water content a soil  can hold after all drainage downward
          resulting  from gravitational forces is referred  to  as its  field capacity.  Field
          capacity is often arbitrarily reported as  the water  content at about 330  cm of
          matric potential head (Jury et  al.  1991). Below field capacity, the  hydraulic
          conductivity  is  often assumed  to  be so low that  gravity drainage becomes
          negligible and the soil moisture is held in  place by suction or matric potential. The
          storage capacity of a  soil layer is thus calculated by multiplying its field capacity
          by the soil layer thickness. This assumes a consistent field capacity. However, not
          all of this  stored water can be removed via transpiration (by plants). Vegetation is
          generally  assumed to reduce the soil moisture content to the permanent wilting
          point,  which is typically defined as the water content at  15,000  cm  of matric
          potential head (Cassel and Nielsen  1986). Evaporation from the soil surface can
          further reduce the soil moisture below the wilting point to the residual saturation,
          which is the  water content ranging from  below  15,000 cm to an infinite  matric
          potential.  If  water is only removed by plants, Stormont and Morris (1998)
          reported that the net storage  capacity, also referred to as the available  water-
          holding capacity, of a soil layer can be approximated by:
                 NSC = (FC- PWP) b                      Equation C.I
                        NSC = net storage capacity
                        FC = field capacity
                        PWP = permanent wilting point
                        b = soil layer thickness
                 For example, the water content at field capacity from a representative soil
                 sample  is estimated to be  16%   while the  permanent wilting point  is
                 assumed to  be about 6%. Thus the net storage capacity for this  soil  is
                 about 10% of its thickness.


       It is important to note that the use of field capacity and permanent wilting
       point here is arbitrary and ignores other factors that affect the amount of
       moisture retained in a soil layer (e.g., Jury et al. 1991, Cassel and Nielsen
       1986). Nevertheless, these are simple and commonly used  concepts and
       are applicable for approximating the water storage capacity of a soil layer.
Model the cover system given desired vegetation characteristics and determined
climatic conditions. Model the cover profile for a deeper than desired depth. If a
unit gradient bottom boundary condition is used, place it below any significant
transient soil-moisture activity. Determine the minimum depth required based on
the Dwyer Pont of Diminishing Returns Method (Dwyer et al. 2007).
A more detailed method to determine the minimum cover soil depth required to
minimize flux utilizes an accepted unsaturated flow software package based on
the Richards' Equation (ITRC 2003). The Dwyer Point of Diminishing Returns
Method (Dwyer et al. 2007) should be utilized. This method simply determines
the cover depth at which flux has been minimized. That is, the cover soil depth
where an additional increment of soil will no longer decrease flux (Figure C-l) is
determined to be the point of diminishing return for soil  depth. A  cover  profile
should be modeled with  the  expected  cover and waste  layers included.  The
monolithic  soil-water storage from the ET cover should be modeled at a depth
greater than the minimum expected depth. If a capillary barrier is introduced into
the cover profile resulting from the addition of a bio-barrier  or other underlying
coarse soil layer, multiple model runs will be required to determine the minimum
cover soil required for storage capacity to minimize flux.  The effect  of the
capillary barrier on the storage capacity of the cover profile may be ignored
resulting in an added factor of safety (FS) in the cover's water storage capacity.
The model output of predicted  percolation  at various points within the cover
profile is then plotted against the cover depth. Generally, in arid  and  semiarid
climates, the point of diminishing returns is when the estimated flux approaches
zero  or actually produces a negative flux (upward movement of moisture).  The
cover soil depth that produces the minimum flux or -point of diminishing returns"
is the  minimum depth required for water storage capacity only.

                             Annual Flux
                     Cover Depth where flux has
                     been minimized
               Figure C-l. Cover depth vs. annual percolation
Perform  sensitivity modeling of the cover profile as required during the final
design process.   Examples  of model  sensitivities include  variances  in soil
properties, optional layer additions, and climatic and vegetation variations. These
sensitivity analyses may increase the thickness of the cover  soil layer  or help
determine the best choice(s) for optional layers such as bio-barriers and drainage


Quality Assurance (QA) is a planned system of activities that provides the owner / operator and
permitting agency confidence that the facility was constructed as specified in the design. QA is
generally the responsibility of the owner and often designated to the design engineer(s) or other
designated  party.  Quality  control  (QC) is a planned system of inspections used to directly
monitor and control the quality of the construction process and materials used. QC is generally
the responsibility of the contractor building the facility. Recommendations and considerations to
be included in a QA Plan and in the QA process are included below.  It is understood that the
size and complexity of the project will dictate the extent of the QA process.

A written QA Plan should precede any field construction activities.    This plan should consider
standards recommended in the  EPA Technical Guidance Document -Quality Assurance and
Quality Control for Waste Containment Facilities" (EPA, 1994).
A copy of the site-specific plans and specifications, QA plan, and QA documentation reports
should be  retained  at the facility  by the  QA engineer.  The plans,  specifications, and QA
documents  can  be  the  chief means for the  facility owner/operator  to  demonstrate to the
regulatory agency that QA objectives for a project have been met.
The QA plan  should include a detailed description of all QA activities  to be used during
materials inspection and construction to  manage the installed quality of the covers and associated
facilities. The QA plan should be tailored to  each specific project with  all important guidelines
and standards integrated into the project  plans and specifications.
An important factor in assessing the quality of a cover system installation is the degree to which
key personnel involved in the process are qualified to perform the required tasks. QA personnel
must be familiar with:

    1.     the proj ect' s design including plans and specifications;

   2.     proj ect lay out;
   3.     materials to be used;

   4.     drainage control features;
   5.     soil borrow materials;

   6.     construction procedures, complications, schedule, and equipment;
   7.     material placement techniques and requirements;

   8.     equipment to be used and its capabilities;  and
   9.     site specific complications/concerns.

Key individuals involved in the QA process during the construction of cover systems and their
minimum recommended qualifications are listed in Table D.I.
                Table D.I.  Minimum Personnel Qualifications (EPA, 1994)

   Design Engineer(s)
Minimum Qualifications

Registered Professional Engineer with
adequate job-specific experience.
   QA Personnel/Inspectors) - Designated
The individual(s) designated by the
appropriate authority with knowledge of
the project, and its plans,  specifications,
and QA documents. Employed separately
from the contractor(s) building the facility.
   QA Engineer -Designated Representative
   QA Certifying Engineer. Representative or
   Designated Individual(s)
   QC Personnel
The individual in charge of the daily QA
process. Employed separately from the
contractor(s) building the facility.
Adequate experience and technical
knowledge of cover system design and
construction process and requirements.

Individual with intimate knowledge of the
project, and its plans, specifications,  and
QA documents. Employed separately from
the contractor(s) building the facility.

Employed by the general  contractor,
installation contractor, or earthwork
contractor involved in the waste
containment facilities; appropriately
   QC Officer
The individual specifically designated by
the general contractor, manufacturer or
fabricator in charge of quality control

In addition to insuring the correct installation of the cover system, another major intent of the
QA process is to provide documentation of the construction process.

Daily reporting and documentation procedures  should be required.  The QA engineer should
prepare daily written inspection reports that are to be included in the final QA documentation.
The daily reports should include information about the work accomplished, tests performed and
observations made, along with descriptions of the adequacy of the work completed.

A daily written summary is to be prepared by the QA engineer.  These reports provide a
chronological framework for identifying and recording all other reports and aids in tracking what
activities/tasks were completed and by whom. At a minimum, the daily  summary reports should
include the following:
   •   Date, project name, location, waste containment unit under construction, personnel
       involved in major activities, and other relevant identification information.
   •   Description of weather conditions, including temperature, cloud cover, and precipitation.
   •   Summaries of any meetings held and actions recommended or taken.
   •   Specific work units and locations of construction under way during that particular day.
   •   Equipment and personnel being utilized in each work task, including subcontractors.
   •   Identification of areas or units of work being inspected.
   •   Unique identifying sheet number of geomembranes for cross-referencing and document
   •   Description of off-site materials received, including any quality control data provided by
       the supplier.
   •   Calibrations or recalibrations of test equipment, including actions taken as a result of
   •   Decisions made regarding approval (or disapproval) of units of material or of work
       and/or corrective actions to be taken in instances of substandard or suspect quality.
   •   Inspection  data sheets and/or problem reporting and corrective measures used to
       substantiate any QA decisions described in the previous item.
   •   Signature of the QA engineer.
   •   Any other pertinent information.

All observations, results of field tests, and results of laboratory tests performed on- or off-site
should be recorded on a data sheet. Recorded observations and test results can take the form of
notes, charts, sketches, or photographs, or a combination of these.
At a minimum, the inspection data sheets should include the following information:
    •  Description or title of the inspection activity.
    •  Location of the inspection activity or location from which the sample was taken.
    •  Type of inspection activity and procedure used (reference to standard method when
      appropriate or specific method described in QA plan).
    •  Unique identifying geomembrane sheet number for cross-referencing and document
    •  Recorded observation or test data.
    •  Results of the inspection activity (pass/fail); comparison with specification requirements.
    •  In addition to the individual preparing the data sheet, identification of all personnel
      involved in the inspection.
    •  Signature of the QA inspector and review signature by the QA engineer.

A problem is defined as material or  workmanship that does not meet the requirements of the
plans, specifications, or QA plan for a project or any obvious defect in material or workmanship
(even if there is conformance with plans, specifications, and the QA plan). At a minimum,
problem identification and corrective measures reports contain the following information:
    •  Location of the problem.
    •  Description of the problem (in  sufficient detail and with supporting sketches or
      photographic information where appropriate) to adequately describe the problem.
    •  Unique identifying geomembrane sheet number for cross-referencing and document
    •  Probable cause for the problem.
    •  How and when the problem was identified (reference to inspection data sheet or daily
      summary report by inspector).
    •  Where relevant, estimation of how long the problem existed.
    •  Any disagreement noted by the inspector between him/her-self and contractor about
      whether or not a problem existed or the cause of the problem.
    •  Suggested corrective measure(s).

   •   Documentation of correction, if corrective action was taken and completed prior to
       fmalization of the problem, and completed corrective measures report (reference to
       inspection data sheet, where applicable).

   •   Where applicable, outline of suggested methods to prevent similar problems in the future.

   •   Signature of the QA inspector and review signature of QA engineer.

D.l.3.5            DRAWINGS OF RECORD
Drawings of record (better known  as -a-s-built" drawings) should be prepared to document the
actual lines, grades, and conditions  of each component of the covers/facilities.  For the cover soil
components, the record drawings  should include  survey data that identifies lower and upper
elevations of a particular component (layer), the plan dimensions  of the  component,  and
locations of all destructive and nondestructive test sampling sites.

Upon completion of the project, the QA engineer should  prepare a final documentation and
certification report.  This report is to include all daily inspection reports, the daily QA engineer's
summary reports, inspection data sheets, problem identification and corrective measures reports,
other  documentation  (such as quality control data provided  by manufacturers or fabricators,
laboratory test results, photographs, as-built drawings, internal QA memoranda or reports with
data  interpretation  or analyses), and design  changes  made by the  design engineer during
construction. The document should be certified to be correct by the QA certifying engineer.

D.l.3.7            DOCUMENT CONTROL
The  QA  documents  should be  maintained under  a document  control  procedure.   Any
modifications to the documents should be reported to and agreed upon by all parties involved.
An indexing procedure should be  developed for conveniently replacing  pages  in the QA plan
when modifications became necessary; the replacement pages detail the revision status.

D.l.3.8            STORAGE  OF RECORDS
During construction, the QA engineer  should be responsible for all QA documents including
copies of the design criteria,  specifications, plan revisions,  and  originals of all  data sheets and
reports. Duplicate records  should be kept at a  separate location  to  prevent the loss of this
valuable information if the originals were inadvertently destroyed.

D.1.4       MEETINGS
Pre-designated meetings included a pre-bid meeting held prior to bidding of the contract. Also, a
pre-construction meeting that can  be held in  conjunction with a resolution  meeting after the
contract has been awarded, but prior to the start of construction activities.

D.l.4.1      PRE-BID MEETING
The intent of this meeting is to discuss the QA plan and to resolve differences of opinion among
the various concerned parties before the project was let for bidding.  Holding the pre-bid meeting
before formal  construction  bids  are  prepared  can allow  the companies bidding  on  the
construction to better understand the level of QA required on the project.   Also, if the bidders
identify problems with the QA plan, they can be corrected early on in the process.
The  objectives of the resolution  meeting  are to establish  lines of communication, review
construction plans and specifications, emphasize the  critical aspects of the project needed to
achieve proper quality, begin planning and coordination of tasks, and identify potential factors
that  might cause difficulties or delays in construction.  The meeting should be  attended by
appropriate personnel including the project's design  engineer, representatives of the general
contractor and major subcontractors, the QA engineer, and the QA certifying engineer.
The resolution meeting can cover the following activities:

   •  An individual should be assigned to take minutes.

   •  Individuals can be introduced to one another along with their project responsibilities (or
       potential responsibilities) can be identified.

   •  Copies of the project plans and specifications should be made available for group

   •  The QA plan can be distributed.

   •  Copies of any special permit restrictions that are relevant to construction or QA should be

   •  The plans and specifications should be described, along with unique design features (so
       the contractors would understand the rationale behind the general design), potential
       construction problems, and allow for questions from any of the parties concerning the

   •  The QA plan should be reviewed and discussed, with the QA engineer and QA certifying
       engineer outlining their expectations and identifying the most critical components of their
       proj ect parti ci pati on.

   •  Procedures for Manufacturing Quality Control (MQC) and Construction Quality  Control
       (CQC) proposed by installers and contractors should be reviewed and discussed.

   •  Corrective actions to resolve potential construction problems should be discussed.

   •  Procedures for documentation and distribution of documents should be discussed.

   •  Each organization's responsibility, authority, and lines of communication should be

   •  Suggested modifications to the QA plan that would improve quality management on the
       project should be solicited.

    •   Climatic variables (e.g., precipitation, wind, temperature) that might affect the
       construction schedule should be discussed.

Familiarizing all project participants with inspection and testing procedures and the criteria for
pass/fail  decisions (including the resolution of test  data  outliers) is a key objective of this
meeting.   Additionally,  it is  imperative  that  all  parties understand  the  key problems QA
personnel have identified and that each party fully understands their roles and responsibilities
and the procedures regarding problem resolution.
The pre-construction meeting can be held in conjunction with the resolution meeting if desired.
The meetings should be scheduled after the general construction contracts have been awarded
and the major subcontractors and material  suppliers have been established.  The purpose of the
pre-construction meeting is to review the details of the QA plan, to ensure that the responsibility
and authority of each individual  is clearly understood, to reach agreement on the established
procedures to resolve construction problems, and  to establish a foundation of cooperation in
quality management.  The pre-construction meeting should be attended by the design engineer,
representatives of the general contractor and major subcontractors, the QA engineer, and the QA
certifying engineer.
The pre-construction meeting should include the following activities:

   •   Assignment of an individual to take meeting minutes.

   •   Introduce parties and identify their responsibilities and authority.

   •   Distribute the QA plan, identify any revisions made after the resolution meeting, and
       answer any questions about the Q A plan, procedures, or related documentation.

   •   Discuss lines of project communication.

   •   Discuss reporting procedures, distribution of documents, the schedule for routine project
       meetings, and resolution of construction problems.

   •   Review site requirements and logistics, including safety procedures.

   •   Review the project design, discuss the most critical construction aspects, and discuss
       scheduling and sequencing issues.

   •   Discuss MQC procedures to be employed by the fabricators contracted to the general

   •   Discuss CQC procedures to be employed by  the installer or contractor.

   •   Compile a list of action items requiring resolution and assign responsibilities for these
D.l.4.4             PROGRESS MEETINGS
Weekly progress meetings should be held at the job site.  At times, additional progress meetings
can be  called at the discretion of the Construction Quality Assurance engineer.   Meeting


attendees should be those involved in the specific issues being discussed. These meetings should
be helpful  in  maintaining lines of communication,  resolving problems  soon  after they are
developed,  identifying action items, and improving overall quality  management.   The  QA
engineer or his/her designated representative should be present at all meetings.
All samples should be identified and described in the QA plan.  Whenever a sample is taken, a
chain of custody record should be made for that  sample. If the sample is transferred to another
individual or laboratory, records of the transfer should be established so that chain of custody can
be traced.  The purpose for the records  of sample custody is to assist in  tracing the cause of
anomalous test results or other testing problems, and to minimize the potential  for accidental
sample loss.

Weather can play a significant factor on construction activities and material placement during
cover installation. The contractor or installer is responsible for complying with the contract plans
and specifications (along with the MQC/CQC plans for the  various components of the cover
system).  Specifications should include restrictions on weather conditions for certain construction
activities such as soil placement where dry soil placement is critical. The contractor  or installer
is responsible for insuring that these weather restrictions are observed during construction.
Unexpected work stoppages can result from a variety of causes.  The QA engineer should be
careful during any work stoppages to determine: (1) whether in-place materials were covered and
protected from damage, (2) whether partially covered materials were adequately protected, and
(3) whether manufactured materials were properly stored and properly or adequately protected
from the elements.  In essence, the cessation of construction during work stoppages does not
mean that QA inspection and documentation temporarily ceases.

EPA Region 9 Waste Management Division contact information:
                     Office Manager
            U.S. Environmental Protection Agency
        Office of Pollution Prevention and Solid Waste
                   75 Hawthorne Street
                     Mail Code WST-7
                 San Francisco, CA 94105

               Email: baker.michelle@epa.gov

 Webpage: http://www.epa.gov/region9/waste/tribal/index.html
EPA Region 9, Waste Management Division Office Receptionist