Natural and Enhanced Remediation Systems - Chapter 7 ppt

Suthersan, Suthan S. “Engineered Vegetative Landfill Covers”

Natural and Enhanced Remediation Systems

Edited by Suthan S. Suthersan
Boca Raton: CRC Press LLC, 2001

©2001 CRC Press LLC

CHAPTER

7
Engineered Vegetative LandÞll Covers

CONTENTS

7.1 Historical Perspective on Landfill Practices
7.2 The Role of Caps in the Containment of Wastes
7.3 Conventional Landfill Covers
7.4 Landfill Dynamics
7.5 Alternative Landfill Cover Technology
7.6 Phyto-Cover Technology
7.6.1 Benefits of Phyto-Covers over Traditional RCRA Caps
7.6.2 Enhancing

In Situ

Biodegradation
7.6.3 Gas Permeability

7.6.4 Ecological and Aesthetic Advantages

7.6.5 Maintenance, Economic, and Public Safety Advantages

7.7 Phyto-Cover Design
7.7.1 Vegetative Cover Soils
7.7.2 Nonsoil Amendment
7.7.3 Plants and Trees
7.8 Cover System Performance
7.8.1 Hydrologic Water Balance
7.8.2 Precipitation
7.8.3 Runoff
7.8.4 Potential Evapotranspiration — Measured Data
7.8.5 Potential Evapotranspiration — Empirical Data
7.8.6 Effective Evapotranspiration

7.8.7 Water Balance Model

7.9 Example Application
7.10 Summary of Phyto-Cover Water Balance
7.11 General Phyto-Cover Maintenance Activities
7.11.1 Site Inspections
7.11.2 Soil Moisture Monitoring

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7.11.2.1 Drainage Measurement
7.11.3 General Irrigation Guidelines
7.11.4 Tree Evaluation
7.11.4.1 Stem

7.11.4.2 Leaves
7.11.5 Agronomic Chemistry Sampling
7.11.6 Safety and Preventative Maintenance
7.11.7 Repairs and Maintenance
7.12 Operation and Maintenance (O&M) Schedule
7.12.1 Year 1 — Establishment
7.12.2 Years 2 and 3 — Active Maintenance
7.12.3 Year 4 — Passive Maintenance
7.13 Specific Operational Issues
7.13.1 Irrigation System Requirements
7.13.2 Tree Replacement
References

Maintaining and enhancing the closed landfill as a bioreactor requires modifi-
cation of design and operational criteria normally associated with traditional
landfill closure…

7.1 HISTORICAL PERSPECTIVE ON LANDFILL PRACTICES

The practice of using shallow earth excavations, or landfills, for disposal of liquid
and solid waste has a very long history. Landfill practices basically followed the
design philosophy of “out of sight, out of mind” in that a pit or trench was excavated
into the ground, waste was placed into the excavation, and, when it was full, the
excavation was covered with soil and abandoned. If thought was ever given to the
matter, it was likely assumed that the soil surrounding the waste effectively prevented
contaminant migration from the burial zone.
It was not until 1976, with the passage of the Resource Conservation and
Recovery Act (RCRA), and 1980, with the passage of Comprehensive Environmental
Response, Compensation, and Liability Act (CERCLA) that federal and state regu-
lations mandated much improved methods for disposal of waste in landfills. Today

there are a plethora of federal and state regulations controlling all aspects of landfill
disposal of municipal, radioactive, and hazardous waste. The problem in the U.S.,
however, is that hundreds of thousands of landfills were operated and then decom-
missioned prior to the requirements of current regulations. Many of these old landfills
now come under the closure requirements of RCRA or CERCLA, depending on the
agreements between the responsible parties. In 1989, U.S. Environmental Protection
Agency (USEPA) stated that there are 226,000 sanitary landfills in the U.S. requiring
evaluation for potential risks to human and environmental receptors.

1

Regardless of the corrective action imposed on these old sites, almost all of them
will require installation of a new cover as a final step in the closure process. The

©2001 CRC Press LLC

design of most landfill covers in the U.S. has been based on criteria developed by
EPA for use in closing either RCRA subtitle C (hazardous waste) or subtitle D
(municipal solid waste) landfills. Two major themes emerge in reviewing recent
work in landfill cover design:

2

1) there has been an overemphasis on regulatory
compliance, thus inhibiting innovative and creative design that looks at the entire
landfill system as a holistic biogeochemical environment, and 2) there are few
published data on field performance of constructed cover systems and their impacts
on the biogeochemistry of the groundwater within the footprint of the landfill.

7.2 THE ROLE OF CAPS IN THE CONTAINMENT OF WASTES

Because of the expense and risk associated with treating or removing large
volumes of landfill wastes, remediation usually relies upon containment, which
requires the construction of a suitable cover. Both regulators and the public usually
accept covers as part of the presumptive remedy for final landfill remediation;
therefore, covers are likely to be included in the optimal remedial actions for closure
of most landfills.
The intent of landfill remediation is to protect the public health and the environ-
ment. In keeping with this intent, a modern philosophy has evolved requiring con-
taminants in the waste to be isolated from receptors and contained within the landfill.
As a result, landfills have become warehouses in which wastes are stored for an
indefinite time, possibly centuries.
There are fundamental scientific and technical reasons for placing a cover on
landfill sites. Although regulations are often the most apparent influence governing
the selection and design of landfill covers today, these regulations were drafted
because of specific environmental concerns and were based upon scientific and
technical understandings. The three primary requirements for landfill covers are to:

•

Minimize infiltration:

water that percolates through the waste may dissolve
contaminants and form leachate, which can pollute both soil and groundwater as
it travels from the site.
•

Isolate wastes:

a cover over the wastes prevents direct contact with potential

receptors at the surface and prevents movement by wind or water.
•

Control landfill gas:

landfills may produce explosive or toxic gases, which, if
allowed to accumulate or to escape without control, can be hazardous.

Landfills have been covered by barriers for years, usually built with little regard
for the monetary and environmental costs associated with constructing and main-
taining them. A typical landfill cover design consists of a sequence of layered
materials to control landfill gas infiltration and promote internal lateral drainage.
The uppermost layer of a landfill cover consists of a vegetative soil layer to prevent
erosion, promote runoff, and insulate deeper layers from temperature changes. The
landfill cover is not a single element but a series of components functioning
together.

3

©2001 CRC Press LLC

Landfill covers are designed to minimize infiltration of rainfall and melting snow
into the landfill in order to minimize postclosure leachate production. This objective
is achieved by converting rainfall into surface runoff and infiltration into evapotrans-
piration and lateral drainage without compromising cover integrity. Secondary per-
formance objectives of landfill cover design include the following:

3

minimize post-

closure maintenance; return the site to beneficial use as soon as possible; make the
site aesthetically acceptable to adjacent property owners; accommodate post-closure
settlement of the waste; address gas and vapor issues; provide stability against
slumping, cracking, and slope failure; provide resistance to disruption by animals
and plants; and comply with landfill closure regulations.
The design features of a landfill cover are varied to affect changes in the overall
water balance within the landfill to meet primary landfill cover objectives. The design
adopted must take into account numerous other considerations, including costs, long
term maintenance implications, and construction risks. The relatively large areas
that landfill covers protect, and the thickness and number of individual layers within
them, make covers a cost-intensive component of landfill facility design.

7.3 CONVENTIONAL LANDFILL COVERS

Nearly all conventional landfill covers in current use incorporate a barrier within
the cover. The “impermeable” barrier layer is intended to prevent water from moving
downward in response to the force of gravity. In effect, these covers are designed
to oppose the forces of nature. Barrier-type covers commonly include five layers
above the waste (Figure 7.1).

1

The top layer consists of cover soil typically two feet
thick and supports a grass cover that provides erosion control. The barrier layer
consists of either a single low-permeability barrier or two or more barriers in
combination. The fourth layer is designed to remove landfill gases as they accumulate
underneath the barrier layer. The bottom layer is a foundation layer of variable
thickness and material; its purpose is to separate the waste from the cover and to
establish sufficient gradient to promote rapid and complete surface drainage from
the finished cover.

The barrier layer is the defining characteristic of conventional landfill covers. It
may be composed of compacted clay, a geomembrane, a clay blanket, or two or
more layers of materials in combination. A compacted clay layer is frequently
specified to have a maximum saturated hydraulic conductivity (K)

£

1

¥

10

–7

cm/sec.
In contrast, both the drainage and gas collection layers are constructed to enhance
flow and commonly contain washed and selectively sieved sand, gravel, or specially
designed synthetic materials.
The soil in the top layer of barrier-type covers is usually too thin or has inadequate
water holding capacity to store infiltrating precipitation during a large storm. These
covers rely on barrier layers and rapid drainage through lateral drainage layers to
prevent precipitation from reaching the waste. Barrier-type covers must accommo-
date specific site conditions, and supplemental components are sometimes added.
For example, gravel may be added to the surface soil in desert regions to control

©2001 CRC Press LLC

wind erosion, or a layer of cobbles may be used with the cover to discourage animal
burrowing into the waste.

7.4 LANDFILL DYNAMICS

Landfills that contain a large amount of organic, putrescible materials (such as
municipal solid waste) literally function as bioreactors. Most “landfill bioreacters” in
general contain anaerobic and/or facultative microorganisms. Landfill leachate is gen-
erated as a result of the percolation of water or other liquids through the waste and
also due to the accumulation of moisture generated as a result of microbial degradation
of waste. Leachate is a concentrated fluid containing a number of dissolved and
suspended materials, specifically, high concentrations of organic compounds (organic
acids, hydrocarbons, etc.) and certain inorganic compounds (ammonia, sulfates, dis-
solved metals, etc. characteristic of the parent waste materials) from which it is derived.
In addition, natural microbial activity in landfills also results in the generation of gases
such as methane, carbon dioxide, ammonia, and hydrogen sulfide, a fraction of which
will be dissolved in the leachate and may be introduced into the groundwater.
Numerous landfill investigation studies

4

have suggested that the stabilization of
waste proceeds in sequential and distinct phases. The rate and characteristics of
leachate produced and biogas generated from a landfill vary from one phase to
another and reflect the processes taking place inside the landfill. These changes are
depicted in Figure 7.2.

Figure 7.1

Typical single barrier cover system.
Waste
Foundation Layer

Protective
Cover Layer
Vegetative Layer
0'-6"
1'-18"
2'-30"
(min.)
Vegetation
Topsoil
Common Borrow Material
Geocomposite
(Textile-Net-Textile)
40-mil LLDPE

©2001 CRC Press LLC

The initial phase is associated with initial placement of waste and accumulation
of moisture within landfills. Favorable biochemical conditions are created for the
decomposition of waste. During the next phase, transformation from an aerobic to
anaerobic environment occurs, as evidenced by the depletion of oxygen trapped
within and introduced to landfill media and continuous consumption of nitrates and
sulfates. Subsequent phases involve the formation of organic acids and methane gas.
During maturation phase, the final state of landfill stabilization, available organic
carbon and nutrients become limiting, and microbial activity shifts to very low levels
of activity. Gas production dramatically drops and leachate strength remains constant
at much lower concentrations than in earlier phases.
Biochemical decomposition of putrescible solid waste is shown below by an
example (Equation 7.1). Typical landfill gas composition during peak activity as a
bioreactor is: 60% methane, 40% carbon dioxide, 5–10% other gases, and 0.3–1.0%
VOCs and non-monitored organic compounds. Gas generation rates during peak

activity typically fall within the ranges of 5–15 ft

3

per pound of refuse per year.

9

(7.1)
Due to very high gas pressures generated at the source areas within the landfill
(up to 4 atmospheres), migration of dissolved contaminants into the gaseous phase
could be a serious concern. Contaminants transferred into the gas phase could be
readsorbed in the waste above the water table, or dissolve in the moisture, condense
in the waste zone, or migrate away from the landfill. The potential for contaminant
migration from the dissolved phase into the landfill gas can be evaluated as shown
in Equation 7.2, and Figure 7.3.

Figure 7.2

Description of leachate and gas concentration changes during landÞll lifecycle.
CH OHO CH CO
6
10
5
242
33
Anaerobic
Bacteria
Æ+

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Under non-equilibrium conditions:
(7.2)
where
= transfer rate from gas to water
K=phase transfer coefficient
H=Henry’s Law Constant of VOC
C

g

=gas phase concentration of VOC
C

w

=water phase concentration of VOC
A=gas/liquid contact area
The progress toward final stabilization of any landfill and the organic waste in
it is subject to physical, chemical, and biological factors within the landfill environ-
ment, age and characteristics of the waste, operation and management controls
applied, as well as site-specific external conditions.
Although barrier layers are sometimes referred to as “impermeable” layers, in
practice this is seldom true. An extensive review of failures and failure mechanisms
for compacted soil covers in landfills was performed and emphasized that “…natural
physical and biological processes can be expected to cause [clay] barriers to fail in
the long term.”

5

Another study discussed a field test conducted in Germany in which,

Figure 7.3

Equilibrium mass transfer conditions of contaminants into landÞll gas.
dm
dt
KC HC A
gw
=-
()
dm
dt

©2001 CRC Press LLC

seven years after construction, percolation through the compacted clay was almost
200 mm/yr and increasing. Geomembrane barriers are also prone to leak.

6

Others
have traced most leaks in geomembranes to holes left by construction.

7,8

A modification of the typical barrier cover is the subtitle D cover (Figure 7.4)
that relies on compaction to create a layer of soil with reduced K value. Used
primarily for municipal landfills in dry regions, its use and components are specified

in subtitle D of RCRA (40 CFR, Part 258.60), hence the name. From the surface
downward, the cover includes an erosion control layer and a layer of compacted
soil. A major advantage of the subtitle D cover is that its construction cost is lower
than for an RCRA subtitle C cover. Even though it has gained regulatory and public
acceptance, the subtitle D cover cannot ensure long-term protections against infil-
tration of water into the waste, even in dry regions, because 1) the topsoil layer has
limited water holding capacity, 2) there is no drainage layer, 3) few roots can grow
in the barrier layer to remove water, and 4) soil freezing and root activity are likely
to increase the K value of the barrier soil layer over time.

Figure 7.4

Subtitle D cover for municipal solid waste landÞlls.
0.47 m
0.15 m
Waste
Soil Barrier
K ≤1 x 10-5 cm/sec
Topsoil
Precipitation
Foundation -
Gravel
Runoff

©2001 CRC Press LLC

7.5 ALTERNATIVE LANDFILL COVER TECHNOLOGY

Alternative covers to the RCRA subtitle C or D design include evapotranspiration
(ET) covers and capillary barriers. The ET cover uses no barrier or horizontal

drainage layers; it is designed to work with the forces of nature rather than attempting
to control them. An ET cover in its simplest form is a vegetated soil cover with a
sufficiently deep soil profile so that infiltrated water is stored until removal by
evaporative losses from the soil surface and by plant roots at depth in the profile. A
capillary barrier also relies on water removal by ET, but is designed such that water
storage near the surface is enhanced to promote the efficient removal of infiltrated
water by the ET process. Optimization of material types and thicknesses for capillary
barriers is critical to their effective performance. The use of sands or clays as the
fine-soil component in the capillary barrier has proven to be less effective in storing
water than silt loams. Capillary barriers can be thought of as enhanced ET covers
— alternative cover systems that work best in semi- and/or arid environments where
high ET rates and low precipitation make it possible to remove all infiltrated water
by ET. However, even in arid environments there are situations where ET covers
and capillary barriers can allow excessive percolation, particularly where the soil
used in the cover design has insufficient storage capacity to accommodate winter
snow melt events.

7.6 PHYTO-COVER TECHNOLOGY

The phyto-cover is the most popular application of the ET cover and is an
engineered agronomic system that harnesses the natural transpiration process of
plants to limit percolation to the groundwater. A phyto-cover relies on shallow- and
deep-rooted plants to create a thick root zone from which the plants can extract
available moisture. In effect, the plants serve as natural, solar-powered “pumps” to
withdraw soil moisture and either convert it into biomass or evaporate it through
their leaves. The withdrawal rate of the botanical pumps is limited by the available
energy (sunlight), rate of growth, and available soil moisture; withdrawal virtually
ceases during winter dormancy. Accordingly, the depth and composition of the root
zone must be sufficient to store accumulated water like a sponge and hold it until
the plants remove it. Properly designed, this “sponge and pump” water removal

system (Figure 7.5) can limit water from percolating below the root zone and can
be equally protective of groundwater as a RCRA cap. Thus, a phyto-cover serves
as a functional alternative to natural clay, geocomposite, or geosynthetic membrane
cap, yet offers several advantages over those technologies.
The effectiveness of poplars in maintaining low soil moisture levels was first
documented by data collected from a phyto-cover application in Iowa.

10

The phyto-
cover consistently maintained soil moisture levels substantially below the soil’s field
capacity (i.e., the amount of water that soil can retain without allowing percolation)
of 40–45%. Soil dryness was maintained by the trees’ prodigious water extracting
ability. The capacity of certain trees such as hybrid poplar and willow trees to extract
soil moisture has been demonstrated by monitoring data from landfill at many sites.

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The poplars are employed at this site not as a cover, but to treat collected landfill
leachate, which is applied to the poplars during the growing season. The total amount
of water extracted from the soil in one growing season by these two- and three-year-
old poplars was equivalent to about 62 inches of precipitation.

10

One of the most important design considerations for a phyto-cover is choosing
appropriate tree species and varieties. Selected trees must be capable of achieving the
desired treatment objective and adapt to the irrigation water, soils, and climate of the
site. Typically, achieving the highest possible rate of evapotranspiration is an important
goal. Critical site conditions for plant selection include soil chemistry, irrigation water

or groundwater chemistry, and adaptation to pests and diseases of the area. Any factor
that compromises tree health and growth will reduce performance. For example, hybrid
poplar clones that include either

trichocarpa

or

maximowiczii

parents are quite sus-
ceptible to Septoria canker if used in the U.S. midwest.

10

Especially for the commonly used Salicaceae, a number of different types of
plant materials may be used. These include stem cuttings, whips or poles, and bare
root or potted material. Use of larger or rooted plant material will result in more
rapid establishment and reduced weed competition, but plant material and planting
costs are much higher than for smaller material. Whips and poles are commonly
used for deep planting applications. Economics, especially planting costs, drive most
larger installations (>5 ha) toward short stem cuttings.
Certain varieties may result in a more valuable final wood product because of
straighter stems or better paper processing properties. Significant differences in
damage from voles has been observed among hybrid poplar trees at phytoremediation
sites. Salt tolerance is a very important selection criterion, as differences between
species and varieties can be significant. Only limited data for Salicaceae are currently
available to guide design, but a number of relevant research programs are ongoing.
For an increasing number of sites, use of non-native species is unacceptable for
local community groups and sometimes for regulators. Use of native material will

generally ensure resistance to local pests and disease, but may not afford the greatest
efficiency.

Figure 7.5

Conceptual phyto-cover diagram.
Infiltration
Evaporation
Precipitation
Poplars
Root
Depth
Evapo-transpiration
Surface Runoff
Topsoil Storage
Native Soil Storage
Daily Cover
Waste
10'-0"

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Once the tree system forms a complete canopy, spacing has little effect on
evapotranspiration or nutrient requirements. The impact of spacing on hydraulic and
nutrient loading is primarily an early establishment phase concern. Establishing
dense initial plantings with the intention of thinning may provide small increases in
early capacity, but thinning operations are often neglected and the resulting mature
tree stands are excessively dense. Enough space must be left between tree rows to
allow planned maintenance activities such as mowing or spraying.
The engineered phyto-cover system consists of densely planted, deep-rooted

trees and understory vegetation (perennial rye grass and clover). Photographs of
hybrid poplar tree stands of varying ages are shown in Figure 7.6. The water-holding
root zone (“sponge”) includes the existing topsoil and fill at the site (including
intermediate and daily cover soil) supplemented with additional soil or soil amend-
ments as dictated by design calculations. A phyto-cover will provide a protective,
living “skin” for a landfill that permanently heals the wound to the landscape
originally created by anthropogenic activities. This skin can equal the percolation-
blocking performance of a “rain coat” RCRA cap while being substantially more
cost effective and providing additional benefits. The final design of a phyto-cover
often includes provisions for monitoring soil moisture levels to ensure that perfor-
mance criteria are met.
Engineered phyto-cover systems have been applied to contain spilled petrochem-
icals and cover landfills, as well as buffers to remove nitrogen from industrial and
municipal wastewater. Sites where phyto-covers have been installed and recent
research and demonstration sites for phyto-cover systems include the following:

2,10-13

•A 15-acre construction debris landfill in Beaverton, OR was covered with trees
in 1990 as an alternative to excavation of the fill, the installation of a liner, and
then recovering with a geomembrane. The phyto-cover is serving to protect
groundwater cost-effectively. The owner has continued to expand the cover as new
areas are closed.

Figure 7.6

Phyto-covers: comparison of two-year-old and four-year-old growth of a phyto-
cover (courtesy of Licht, 1998).
Four-year Old Poplar Trees
Two-year Old Stand of Poplars

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• From 1992 to 1993, the Riverbend Landfill in McMinneville, OR planted a 17-
acre phyto-cover to manage landfill leachate water and soluble compounds. All
nutrient and water cycling results indicate the cap is achieving all regulatory
requirements for ammonia treatment and ground protection.
• From 1993 to 1995, a 15-acre perimeter buffer was planted to reduce infiltration
from upgradient runoff, grow a visual and sound barrier, and intercept downgra-
dient leachate seepage. Data collected at the site indicate the cap has been suc-
cessful in stopping all leachate.
• At the Grundy county Landfill in Grundy Center, IA, a two-acre cap and perimeter
buffer was planted from 1993 to 1994 to reduce leachate formation by installing
a phyto-cap over a completed subtitle D cap. The cap also provides a visual and
sound barrier, and intercepts downgradient leachate drainage.
•A three-acre poplar cap was planted in 1994 at the Bluestem 1 Landfill in Cedar
Rapids, IA over a pre-subtitle D cap. The cap serves to reduce leachate formation
vertically and intercepts downgradient leachate drainage. The data collected at
this site have been used in writing specifications for soil and compost cover
requirements and use of MSW waste as a planting media.
•A five-acre cap and perimeter boundary were planted in 1994 over a pre-subtitle
D cap at the Bluestem 2 Landfill in Marion, IA to reduce leachate formation.
Moisture management data from this cap have been used in subtitle D equivalence
comparison between a soil/clay cover and the “sponge and pump” concept for
deep rooted trees in four feet of rootable soil.
• In 1995, a ten-acre area was planted with poplar trees and a clover/grass understory
over a subtitle D cell filled with MSW and industrial waste. The Department of
Environment Quality and governor’s office were interested in future phytoclosures
for many funded pre-RCRA landfills in Virginia that have been abandoned and
are creating potential environmental risk. The trees are growing well and are being
maintained by the owner. A soil moisture measurement system using time domain

reflectometry (TDR) is used to monitor the impact of tree roots on vadose zone
water content. A drip irrigation system using collected storm water can control
the water stress during periods when moisture in the root zone has been exhausted.
• At a railroad RCRA site in Oneida, TN, a one-acre area impacted by coal and
creosote from past manufacturing activities was covered with poplar trees and
grass in 1997. The site soils were amended with compost and mineral fertilizer,
then trenched in the root zone. The trees and grass managed to accelerate biomass
growth with resulting water uptake and

in situ

constituent removal. The site
groundwater is monitored by a university research grant to measure groundwater
elevation and the containment of constituents. The concept is similar to landfill
capping where the phytosystem pumps water at high rates during the growing
season and minimizes groundwater movement during the dormant season.
• The Woodburn WasteWater Treatment Plant in Woodburn, OR has been a dem-
onstration site since 1995; a full-scale installation took place in 1998. This site is
the first full-scale tertiary treatment of secondary municipal wastewater effluent
and is being designed for no leakage through the root zone in the summer months.
• The Mid-Lakes Co-op site in Bonduel, WI used an aesthetically pleasing poplar
cover over a spill plume to contain pollutant migration, make use of all available
precipitation, protect public exposure, and remove constituents from the ground-
water . Closure requirements for this site included planting trees, monitoring the
depth to groundwater, and monitoring groundwater quality over a three-year-
period.

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• In Staten Island, NY a phyto-cover consisting of poplars, willows, paulowia, and
grasses is being used to prevent constituent migration and formation of leachate.

Enhancement of existing vegetation is expected to establish hydraulic control of
groundwater by reducing water infiltration through the landfill materials.
• Evidence collected at a closed landfill in Elmore, OH indicates naturally occurring
trees have created a treatment barrier for leachate seeps. An evaluation of on-site
box elder and osage orange trees yielded evidence of TCE uptake. An evaluation
of the existing cover for supplemental enhancement for additional groundwater
remediation and restoration was then conducted.
•A poplar tree phyto-cover was installed in 1996 at a landfill in Acme, NC. The
trees were planted in the most downgradient area of the landfill to stop leachate
migration. Groundwater constituent concentrations have dropped substantially in
the area of the poplar trees but not in areas where trees were not planted.
• From 1992 to 1993, over 2000 poplar trees were planted at a site in Anderson,
SC to be used for processing waste from mining ore material. The waste was used
to fill low areas over six acres of the site. Data collected at the site indicate
infilitration and leachate formation is being controlled.
• In 1991, a succession of trees (willow and black locust), legumes, and grasses
were planted to dewater slurry waste at a site in Baton Rouge, LA. The waste
material was in a slurry state from a depth of 6 inches to 30 feet below ground
surface. The planted vegetation reduced the hydrated state of the waste and the
occurrence of leachate through the impoundment.
•A process waste was placed as a slurry into an impoundment at a site in Texas
City, TX. Naturally occurring trees (osage, orange, and mulberry) and vegetation
have reduced the hydrated state of the top ten feet of the waste. Research on the
site has found that dewatering and net water removal are directly correlated to the
size of the trees.
• Ongoing research, funded by the USEPA Great Plain and Rocky Mountain Haz-
ardous Substance Research Center involves planting trees at CERCLA sites to
control erosion and leaching of zinc, arsenic, lead, and cadmium.
•A grass/soil cover system is one of five alternative covers being evaluated by
Sandia National Labs in NM as part of an alternative landfill cover demonstration

study. Similar phyto-cover systems are being considered as potential demonstra-
tion sites by USEPA ORD at sites in Tulsa, OK; Beatty, NV; and Hill Air Force
Base in CA.
•A phyto-cover system has been proposed and designed at the 95% completion
level for the F.E. Warren Air Force Base Superfund Site in Cheyenne, WY. This
site is currently being considered as a technology demonstration candidate by
USEPA-Region VIII.
• Pfitzer junipers have been used in a landfill cover field demonstration at Beltsville,
MD. The juniper phyto-cover was installed over a clay layer to add to the “robust”
cover development, but not as a replacement of the low-permeability layer. The
objective of the demonstration study was to document the influence of junipers
as water scavengers, yet maintain the water runoff performance of the low-
permeability cap. Compared to a reference soil, the “bioengineered” juniper cover
reduced infiltration; it was demonstrated that the mature plant system improved
the system’s resilience to failure.
• Research regarding the establishment of sufficient vegetation to provide adequate
biomass growth with resultant evapotranspiration is being conducted at Idaho
National Engineering Lab, Idaho Falls, ID. This research focuses on four plant

©2001 CRC Press LLC
species to deplete soil moisture, and the configuration of a capillary barrier and root
zone to prevent deep percolation during wet periods. The use of such phyto-covers
has been demonstrated to be applicable to landfill sites in the semi-arid west.
• In Ljubljana, Slovenia, a ten-acre cover was planted with poplar trees in
1993–1994 with the primary goal of protecting groundwater by reducing leachate
formation through municipal and industrial wastes. Installation of the cover has
greatly improved the aesthetics of the area and increased the value of the wildlife
habitat. The design concept is being considered as a model that will become
national policy.
• The author also knows of many other phyto-cover applications in MA, OH, MD,

NC, MI, PA, NY, NJ, and IL.

In 1998, USEPA began an effort to establish a design database and improve numer-
ical prediction methods for alternative landfill covers. The initial task of the Alternative
Cover Assessment Project (ACAP) was to catalog past and existing research efforts
into measurement of cover performance and to describe the current state of numerical
prediction methods. The primary criterion was to measure percolation directly. Several
research sites operated by branches of the federal government were included in this
study. These sites include the national laboratories at Hanford, Sandia, Los Alamos,
Savannah River, and Idaho Falls, and DOE locations at Monticello, UT, Nevada Test
Site, DoD locations at California, Hawaii, Colorado, Utah, and others.

7.6.1 BeneÞts of Phyto-Covers over Traditional RCRA Caps

In addition to satisfying the critical antileaching requirement, phyto-covers pro-
vide a number of significant pollution control, ecological, and economic benefits
when compared to traditional RCRA caps:

•A phyto-cover actually enhances natural biodegradation processes, instead of
interfering with them, as a RCRA cap would.
•A gas-permeable phyto-cover allows for passive venting of gaseous byproducts
of biodegradation and allows oxygen to move into the fill to facilitate additional
biodegradation.
•A phyto-cover provides a forest ecosystem and an attractive alternative to an
RCRA cap.
•A phyto-cover can be installed with less cost and less risk to public safety than
a RCRA cap and, once the cover is established, the system has a natural stability
that minimizes long-term maintenance requirements.

7.6.2 Enhancing

In Situ

Biodegradation

Installing a phyto-cover at a site has the potential to enhance biodegradation of
waste materials, including organic waste and contaminants, in the root zone. In
natural ecosystems, high concentrations of indigenous soil microorganisms are found
in association with plant roots, because the roots exude a wide variety of compounds
such as sugars, amino/acids, carbohydrates, and essential vitamins. These com-
pounds not only sustain the microbial consortia, which can degrade many organic
compounds directly, but also enhance and accelerate cometabolic degradation of

©2001 CRC Press LLC

other pollutants resistant to direct degradation. In addition, the plants themselves
will take up and metabolize or volatilize some of the organic contaminants in the
fill. Finally, exuded organic acids also help in sequestering and immobilizing any
metals present in the root zone. By contrast, a RCRA cap provides no stimulation
to natural biodegradation and would be expected to substantially change bio-
geochemical conditions in the fill by trapping the gaseous byproducts of biodegra-
dation (e.g., methane, carbon dioxide, and ammonia), thereby affecting factors
critical to natural attenuation mechanisms, such as pH and REDOX potential.
The main reason for the enhanced

in situ

biodegradation in landfills with phyto-
covers is the ability of the atmospheric oxygen to transfer into the landfill. The
primary mechanism transferring oxygen into the landfill is diffusion into the soil

from the atmosphere, based on an excellent summary shown below:

14

The exchange of gases between the soil and the atmosphere … is facilitated by two
mechanisms: mass flow and diffusion. Mass flow of air, which is due to pressure
differences between the atmosphere and the soil air, is less important than diffusion
in determining the total exchange that occurs. It is enhanced, however, by fluctuations
in soil moisture content. As water moves into the soil during a rain … air will be
forced out. Likewise, when soil water is lost by evaporation from the surface or is
taken up by plants, air is drawn into the soil. Mass flow is also modified slightly by
other factors such as temperature, barometric pressure, and wind movement. Most
of the gaseous interchange in soils occurs by diffusion.

The minimum rate of oxygen diffusion at the bottom of the root zone was
estimated to be 5

¥

10

–8

grams per centimeter, squared per minute, or 2340 pounds
per year per acre.

14

The maximum rate could be up to 9200 pounds per year per
acre. Over the surface of a 30-acre landfill, this translates into at least 70,000 pounds

of oxygen per year into the landfill, which facilitates stabilization of the waste. By
contrast, the single-barrier cap would admit only an estimated 75 pounds of oxygen
or about one tenth of one percent of the influx that could support the aerobic natural
attenuation mechanisms (Figures 7.7a and b).

15

7.6.3 Gas Permeability

Unlike RCRA caps, which are essentially impermeable to gases and therefore
require elaborate gas venting systems to deal with gases and vapors generated by
biodegradation of the fill, a phyto-cover is porous and permeable to gas. A phyto-
cover can thus eliminate the need for a gas collection system at many sites. Equally
important, a phyto-cover will allow oxygen to migrate into the fill, which will help
to support additional aerobic biodegradation and thereby hasten the completion of
the waste life cycle.

7.6.4 Ecological and Aesthetic Advantages

Both a phyto-cover and an RCRA cap are designed to be vegetated on the surface,
but vegetation on a phyto-cover has the appearance of a tree farm and, eventually,

©2001 CRC Press LLC

a forest, and serves the same ecological function as a forest while the RCRA cap is
covered with grass and, in order to protect the impermeable liner, must be prevented
from functioning like local natural ecosystems. Specifically, maintenance of the
integrity of the RCRA cap’s impermeable layer dictates that deep-rooted plant
species, such as trees and shrubs, not be allowed to colonize the site through natural
succession. Moreover, protection of the impermeable liner also requires that small

burrowing mammals, such as those normally associated with a meadow, must be
perpetually monitored for and eliminated when found. By contrast, the trees of a
phyto-cover provide nest sites for birds and other arboreal species and readily accept
in-fill by shrubs and native tree species, as deemed appropriate under site manage-
ment criteria. Because no animal is likely to excavate below the deep root zone, it
is not necessary to prevent native fauna from inhabiting the phyto-cover. Besides
offering a preferred natural ambiance, the phyto-cover forest would also serve the
community as a noise pollution buffer and assist incrementally with global climate
issues by fixing substantially more carbon dioxide from the atmosphere than a grass
RCRA cap.

Figure 7.7a

Biogeochemical conditions and mass balance for a presumptive remedy.

Figure 7.7b

Biogeochemical conditions and mass balance for a holistic remedy.

©2001 CRC Press LLC

7.6.5Maintenance, Economic, and Public Safety Advantages

The ongoing maintenance requirements for an established phyto-cover are min-
imal. Although relatively intensive monitoring for disease and pests is needed during
the three growing seasons that the trees need to become fully established, mainte-
nance activities thereafter are expected to be minimal because of the self-healing
nature of the phyto-cover. Like a natural forest, the phyto-cover is expected to be
resistant to wind and water erosion. Unlike a RCRA cap, which can suffer cracks,
rips, and tears due to factors such as differential settling or physical intrusion, the

phyto-cover maintains its integrity and actually heals itself with new root growth in
response to physical disturbances. Thinning of trees may be undertaken in the future
to avoid crowding as the trees reach their mature size. However, the trees cut in a
thinning operation represent a valuable forestry crop, so revenue from their sale
should compensate for the operation’s costs.
The lower economic cost of the phyto-cover compared to the RCRA cap is accom-
panied by lower noneconomic social costs in the form of safety risks. Studies have
shown that remedy implementation imposes risks of injury and death to site workers,
neighbors, and the public using transportation routes. These risks are more certain and
typically substantially greater in magnitude than risks to the public from exposure to
site contaminants. For example, assuming that bulk construction materials can be found
at an average distance of 15 miles (i.e., 30 miles round trip) and using the U.S. truck
fatality rate of 4.7

¥

10

–8

/mile, construction of RCRA “C” cap at a 30-acre landfill
site could lead to an estimate of transportation fatalities risk of 0.033.

16

This estimate
will be further increased if the nontruck driver fatalities estimate is combined with
this. Since phyto-covers require less site work and fewer truckloads of imported
material, such as borrow soil and gas collection layer sand, constructing a phyto-cover
instead of an RCRA cap would involve less risk of an accidental injury or fatality to

a construction worker and lower risks of traffic incidents associated with truckloads
of construction materials carried over local roads
Finally, unlike a RCRA cap, which locks the site into an “impermeable barrier”
strategy, the phyto-cover system can be operated in a flexible way to take into account
the results of ongoing performance monitoring data. For example, if performance
data show that native species perform as well as hybrid poplars in preventing
infiltration, then the natural transition to native species can be accelerated, to enhance
the ecological service provided by the area. By the same token, in the unlikely event
that performance data show that a part of the cover is not performing to expectation,
then a supplementary measure such as additional “sponge” or denser planting would
be available to improve performance.

7.7 PHYTO-COVER DESIGN

The typical components of an engineered phyto-cover system consist of vege-
tative cover soils (existing and supplementary), soil amendments, nonsoil amend-
ments, understory grasses and plants, and trees. An irrigation system is an optional
component to ensure sufficient water for tree growth in case of drought. Irrigated

©2001 CRC Press LLC

trees grow more rapidly, thus meeting closure objectives in less time; however, there
is often lack of a convenient water source or on-site operation and maintenance
personnel to make an irrigation system feasible at a site. The need for on-site
irrigation should be based upon the expected water consumption of the trees.

7.7.1 Vegetative Cover Soils

The existing cover soil at many sites is sufficient to support an adequate root
system for healthy tree growth. This is evidenced by the vigorous growth of trees

often seen at abandoned landfills (Figure 7.8); however, the ability to grow trees is
not evidence that percolation and leachate production are controlled. Typically,
natural stands of vegetation are not effective at controlling percolation. Therefore,
sufficient soil and nonsoil amendments may need to be added to meet the require-
ments for tree growth, and to achieve minimum land surface slopes to promote
surface drainage and to provide sufficient soil water holding capacity for storage to
function as an adequate “sponge.” The amount of soil and nonsoil amendments
necessary must be determined by site-specific information, often collected in the
later stages of design.
Any supplemental cover soil added to achieve the required grades, as well as
sufficient water storage capacity, will comprise common borrow soils. Supplemental
soil should be placed in 6-inch thick and loose lifts, and be lightly compacted to
achieve the minimum slope and thickness. This material is typically available from
several sources in the vicinity of most sites; the specific local source usually depends
upon availability during the construction period. The surficial lift of supplemental
soil and existing cover, depending upon which is exposed at the final grade surface,

Figure 7.8

Existing root penetration of a tree at a landÞll site.

©2001 CRC Press LLC

must be ripped in two directions following final grading to assure noncompaction
and to prepare the surface to receive the nonsoil amendments.

7.7.2 Nonsoil Amendment

The addition of nonsoil amendments will increase the water-holding capacity
and nutrient transfer properties of the common borrow soils. Typical nonsoil amend-

ments include compost, chipped wood, digested sewage biosolids, lime-stabilized
sludge, manure, and other organic biomass. The incorporation of this type of organic
matter into the existing and supplemented soils will greatly increase the tilth, fertility,
and water-holding capacity of the soil, and further reduce percolation through the
cover. Biosolids compost and lime-stabilized sludge are readily available through a
compost contractor. Typically a minimum 6-inch thick layer of organic amendments
needs to be applied to the soil surface after achieving final grade. This material is
spread evenly in a six-inch thick layer on the area to be planted with the engineered
phyto-cover system and is ripped into the surficial soils to a depth of 14 to 18 inches.
Ripping is performed in both an east-west and north-south orientation in order to
achieve a uniform mixing within the soil profile. Finally, the site is tilled in prepa-
ration for planting.
If the organic materials used for the nonsoil amendment have a high carbon to
nitrogen ratio, fertilizer is added along with organic amendments to aid in stabilizing
these amendments and to provide sufficient nutrients to the rooting plants. This
organic amendment is expected to supply all micronutrients required by the plants.
A mineral fertilizer is also added as needed, based on nutrient analyses of the applied
compost, to supplement the macronutrient reserves of nitrogen, phosphorous, and
potassium. All amendment addition, ripping, and tilling is completed prior to under-
story planting in the fall and before the trees are installed in the following
early spring.

7.7.3 Plants and Trees

The area to be planted will generally exhibit a minimum 2% or greater grade;
therefore, stabilization of the site cover material remains necessary to prevent ero-
sion. Understory planting will be established for early erosion control and water
uptake during the first year. Understory establishment is a combination of annual
and perennial grasses, such as varieties of rye, oats, wheat, barley, and fescue, applied
at a rate of 20 to 40 pounds per acre. This mixture of seed is designed to meet the

short- and long-term objectives of the understory. Annual species will be fast growing
to control near-term erosion; perennial grasses will be deep-rooted species selected
as the primary long-term understory for the site. The long-term effectiveness of the
overstory is dependent upon establishment and long-term maintenance of the under-
story, which understory depletes shallow soil moisture, turn causing tree roots to
grow deeper to meet water requirements. As discussed earlier, the success of a phyto-
cover is dependent upon establishment of deep-rooted trees to create a sufficient
sponge to store soil moisture in the dormant season.

©2001 CRC Press LLC

The trees normally selected for construction of a phyto-cover are hybrid poplars
of the variety

Deltoides



x



nigra

.

10,13

The candidate varieties, DN-21, DN-34, OP 367
and others, are planted based on demonstrated growth ability and hardiness in the

environment. The poplars are installed as either rooted plants or whips at a density
of approximately 1200 trees per acre.

15

The rows are located by measurement and
flagged, and the trees installed by a tractor and mechanical planter. These trees are
typically planted with an in-row spacing of 3 feet and a row spacing of 10 to 13
feet. They are planted in rows positioned along the land elevation contours, perpen-
dicular to slopes to aid in reducing sheet flow velocities and surface erosion.

7.8 COVER SYSTEM PERFORMANCE

The engineered phyto-cover system should be designed to meet the post-closure
and remediation objectives established for any landfill site as specified below:

• Minimize infiltration of precipitation through the cover system into the waste to
protect groundwater quality at the site.
• Resist surface soil erosion by wind and precipitation.
• Minimize long-term maintenance.
• Protect human health and the environment.
•Offer post-closure and future beneficial use.

The achievement of these objectives is outlined in this and subsequent sections.

7.8.1 Hydrologic Water Balance

The engineered phyto-cover system is designed to mature into a remedial system
that exceeds the hydrologic performance of more conventional cover systems. How-
ever, instead of acting as a constructed barrier layer, which diverts precipitation from

the cover area as surface runoff or internal drainage, this system intercepts and uses
the water for plant growth. In other words, the engineered phyto-cover functions as
a sponge and pump system, with the root zone acting as the sponge, and trees acting
as the solar-driven pumps. In contrast to restrictive permeability barrier design, the
engineered phyto-cover design involves the storage of free water in soil pores and
the extraction of stored water by the tree roots.
The effectiveness of engineered phyto-cover systems as landfill closure systems
has been demonstrated at sites in the U.S. At sites in various climates with engineered
and agronomically optimized growing conditions, rapidly growing poplar trees are
capable of transpiring all natural precipitation that infiltrates into a site. While the
performance of engineered phyto-cover systems has been demonstrated, a proven
tool to design phyto-covers is not available. Therefore, to support the design and
demonstrate the effective performance of phyto-cover systems, this section discusses
some fundamental scientific methods of water balance analysis.
As discussed previously, the phyto-cover system utilizes specially selected trees
with a grass understory to optimize evapotranspiration and achieve the equivalent

©2001 CRC Press LLC

performance of a conventional barrier cover system. This alternative landfill cover
system has been designed to minimize percolation to the waste by incorporating a
landfill soil cover with sufficient evapotranspirative and water holding capacity to
store precipitation temporarily in the nongrowing season for subsequent evapotrans-
piration by vegetation in the growing season. The two key design elements in
engineering a phyto-cover system are 1) determining the thickness and material
composition of the soil cover system required to provide sufficient water storage
capacity; and 2) incorporating a supportive phyto-cover system to access water stored
in the soil cover system for evapotranspiration to the atmosphere.
Moisture flow and moisture content in a landfill are extremely important to the
dynamic processes of decomposition and potential leachate generation. The funda-

mental means to assess the moisture conditions is through an evaluation of various
processes comprising a water mass balance. A water mass balance analysis is an
“accounting” procedure for tracking the moisture inputs to storage and the moisture
outputs that influence the potential flux of water through the cover into the waste.
The primary elements of a water mass balance include precipitation, surface runoff
(R/O), potential evapotranspiration (PET), infiltration (I), soil moisture storage (ST),
actual evapotranspiration (AET), and flux (or percolation) of water through the
system. The water shedding efficiency of a cap is then derived by calculating the
percentage of flux relative to total precipitation. The phyto-cover system design
concept involves maximizing efficiency by optimizing ET, runoff, and soil moisture
storage to minimize infiltration, flux, and potential leachate generation. The water
balance accounting for a phyto-cover can be summarized by the following equation
and Figure 7.9:
Percolation = Precipitation – Runoff – Evapotranspiration – Moisture Storage
(7.3)
The water mass balance processes within a landfill are typically evaluated using
the hydrologic evaluation of landfill performance (HELP) model, developed by the
Waterways Experiment Station.

17

The applicability of this model to design and
evaluation of an engineered phyto-cover system has been reviewed, and it has been
determined that the HELP model is inappropriate for this analysis because of several
computational deficiencies.

18,19

The HELP model was developed based on assump-
tions pertaining to water management through low permeability soil covers with

vegetative covers comprising short-rooted grasses. No opportunity exists for user
input of higher ET values more representative of plant species with significantly
higher potential water uptake than the short grasses assumed by the HELP model.
Therefore, the model significantly underestimates evapotranspiration from trees and
other deeply rooted vegetation that are key elements of a phyto-cover system. This
application limitation of the HELP model results in an overestimation of infiltration
and coincident underestimation of efficiency (overestimation of drainage) if the
model were to be applied to an evaluation of a phyto-cover system.
A detailed assessment of various computer models used for landfill cover designs
during the early phases of the alternative cover assessment program (ACAP) came
to similar conclusions.

11,12

Of the four codes tested, HELP was the most widely used

©2001 CRC Press LLC

for landfill design, and the most user-friendly and highly dependable. HELP predic-
tions consistently provided the highest estimates of drainage. Three concerns with
HELP were 1) a nonrealistic response of increased drainage as available water
capacity increased, 2) insensitivity of drainage to thickness of the cover surface
layer, and 3) consistent overprediction of drainage. EPIC was also relatively easy
to use, but consistently underpredicted drainage in comparison to other codes. The
study suggested that Richards’ equation-based codes (HYDRUS–2D, UNSAT–H)
were better able to capture the behavior of alternative landfill covers than simple
water balance codes such as HELP and EPIC.
Although the HELP model itself cannot accurately simulate the hydraulic effects
of an engineered phyto-cover system, the water balance method that is the funda-
mental principle applied within the HELP model has been employed to evaluate the

performance of vegetative cover systems.

20

These same scientific principles are
employed to design and evaluate the performance of an engineered phyto-cover
system with a new software tool called PHYTOSOLV.

15,21

In using the water balance
method, the first step is to acquire accurate precipitation records applicable to the
site and encompassing various extreme wet and dry periods. The second step is to
determine the quantity of surface water runoff and infiltration (which are functions

Figure 7.9

Diagram of a phyto-cover (modiÞed from Licht, 1998).
Depth of
Capillary
Zone Draw
Root
Depth
Storage
Infiltration
Soil Evaporation
Surface Evaporation
Leaf Transpiration
Surface Evaporation
Surface Cover

Interception
Canopy
Interception
Precipitation
Potential
Infiltration
1) Surface Litter or
Compost
2) Soil and Nonsoil
Amendments
3) Waste Rootable Upper
Layer (Contributes toward
Storage)
Thickness of the Arrow is
Proportional to the Volume of Water

©2001 CRC Press LLC

of the site soils, slope, and surface texture). Infiltration is computed as the difference
between precipitation rates for the site and surface-water runoff from the soil cover.
The third step is to apply PHYTOSOLV, assuming a variety of soil cover depths, to
generate a range of annual hydrologic water balances using daily precipitation data.
Finally, a supporting phyto-cover system is designed that would access infiltrated
soil water throughout the entire extent of root growth (the “sponge”), and the
necessary evapotranspiration rate (the “pump”) required to deplete soil moisture
during the growing season is computed. This iterative water balance analysis is used
to select the appropriate soil cover design to best achieve desired hydraulic perfor-
mance, thereby minimizing generation of leachate. The measure of performance
for the designed phyto-cover is compared to the water-shedding efficiency of tradi-
tional barrier cover systems. Presented below is a discussion of each of these steps

and the basis for the general engineered phyto-cover system design.

7.8.2Precipitation

Long-term precipitation data need to be assembled from the closest weather
station to evaluate local hydrologic conditions. There are no established regulatorily
approved procedures or protocols to evaluate the hydrologic performance of a phyto-
cover design. Therefore, long-term data are needed in order to characterize the long-
term precipitation trends and extremes. Typically, precipitation can vary widely from
site to site for a given year, season, or month. To demonstrate this variability, data
should be assembled summarizing average monthly and annual precipitation for
decades at weather stations near any given site. For example, during a long period
at a site in Maryland, the average annual precipitation varied from a minimum of
26.29 inches in 1965 to a maximum of 62.36 inches in 1996. Similar variability can
also be observed in monthly precipitation totals. To the extent practical, these
dynamics must be accounted for in the design of the phyto-cover system to demon-
strate adequate performance under extreme weather conditions. The application of
these data to evaluate the phyto-cover design assumes that daily precipitation totals
are the result of individual storm events.

7.8.3Runoff

Runoff from the designed phyto-cover is computed using the USDA Soil Con-
servation Service (SCS) curve number model.

22

The model computes direct runoff
from an individual storm event as a portion of total precipitation (Figure 7.10). The
method was developed from field studies by measuring runoff from various soil

cover, land slope, and soil type combinations. Curve numbers were developed based
upon each of the combined hydrologic effects of these factors and enable the model
to be applied to any area within the U.S. The curve number model is widely used
and is incorporated into the HELP model and other agronomic models to compute
rainfall runoff and other elements comprising a water balance. The major deficiency
in this model is that it underestimates runoff from small precipitation events. This
discrepancy results in overestimates of infiltration and the amount of water that must
be managed by the cover system.

3

Consequently, the resultant engineered

©2001 CRC Press LLC

phyto-cover is overdesigned and conservative: the engineered phyto-cover has the
ability to control more infiltration than it is designed to manage.

15

(7.4)
where
Q= runoff (in)
P=precipitation (in)
S=potential maximum retention after runoff begins (in)
I

a

= initial abstraction (in)

The initial abstraction is all water loss before runoff begins. It includes water
detained in surface depressions, as well as water intercepted by vegetation, evaporation,
and infiltration. The initial abstraction is highly variable but from data collected from
small agricultural watersheds, I

a

was approximated using the following equation:
I

a

= 0.2 S (7.5)
By eliminating I

a

as an independent parameter, this approximation allows use of
a combination of retention storage (S) and precipitation (P) to predict a unique runoff
amount. Substituting Equation 7.5 into Equation 7.4 gives
(7.6)

Figure 7.10

Runoff — SCS Method.
Direct Runoff (Q), inches
8
7
6
5

4
3
2
1
0
0123456789101112
Rainfall (P), inches
Curves on this sheet are for
the case I
s
= 0.2S, so that
Q =
(P - 0.2s)
2
P + 0.8S
Curve Number = 100
90
80
70
60
50
40
Q
PI
PI S
a
a
=
-
()

-
()
+
2
Q
PS
PS
=
-
()
+
02
08
2
.
.