Copyright © ISWA 2005
Waste Manage Res 2005: 23: 113–125
Printed in UK – all right reserved
Waste Management & Research
ISSN 0734–242X
Experimental landfill caps for semi-arid and
arid climates
The United States EPA Subtitle D municipal solid waste landfill requirements specify that the permeability of a cap to a landfill be no greater than the permeability of the underliner. In
recent years the concept of the evapotranspirative (ET) cap has
been developed in which the cap is designed to store all rain
infiltration and re-evapotranspire it during dry weather. Concern at the long period required for landfilled municipal solid
waste to decompose and stabilize in arid and semi-arid climates
has led to an extension of the concept of the ET cap. With the
infiltrate–stabilize–evapotranspire (ISE) cap, rain infiltration
during wet weather is permitted to enter the underlying waste,
thus accelerating the decomposition and stabilization process. Excess infiltration is then removed from both waste and
cap by evaporation during dry weather. The paper describes
the construction and operation of two sets of experimental
ISE caps, one in a winter rainfall semi-arid climate, and the
other in a summer rainfall semi-arid climate. Observation of
the rainfall, soil evaporation and amount of water stored in
the caps has allowed water balances to be constructed for caps
of various thicknesses. These observations show that the ISE
concept is viable. In the limit, when there is insufficient rainfall to infiltrate the waste, an ISE cap operates as an ET cap.
Geoffrey E. Blight
Andries B. Fourie
University of the Witwatersrand, South Africa
Keywords: Landfill cap, semi-arid and arid climates, infiltration,
stabilization, evaporation, water balance, wmr 760–9
Corresponding author: G. E. Blight, University of the
Witwatersrand, Private Bag 3, WITS 2052, South Africa.
Tel: +27 11 476 8759; fax: +27 11 476 8759;
e-mail: blight@civil.wits.ac.za
DOI: 10.1177/0734242X05052458
Received 14 May 2004; accepted in revised form 20 December 2004
Introduction
The soil cover layer or cap of a landfill serves to isolate the
landfilled waste from its surface environment, preventing
exposure of the waste by, for example, sheet or gully erosion
caused by wind and water, or burrows dug by animals, and at
the same time controlling infiltration of rainfall. The cap
should also allow landfill gas to percolate out of the waste and
oxidize as it passes through the soil (e.g. Figueroa 1993, Bergman et al. 1993) to disperse relatively harmlessly into the surrounding air.
The influence of climate on landfilling practice
As landfill technology originated in the countries of northern Europe and North America, where rainfall is relatively
high, (see, e.g. the short history of sanitary landfilling given
by Vesilind et al. 2002) most well-respected landfilling regulations are based on the expectation that once rain infiltration has passed through the cap of a landfill, it will continue
to migrate downwards through the waste until it reaches the
landfill liner, where it will accumulate to be removed and
treated as leachate. For example the United States EPA Subtitle D requirement for a cap to a municipal waste landfill
(Geotechnical News 1993) is that the cap permeability must
be less than or equal to that of the bottom liner or subsoil,
but not more than 1 1 10–5 cm s–1(8.6 mm day–1). If this is
achieved, virtually all infiltration into the waste will be prevented.
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113
G.E. Blight, A.B. Fourie
Fig. 1: Average monthly rainfall (R) and A-pan evaporation (EA) for
Cape Town and Johannesburg, South Africa.
Over much of the earth’s land surface, however, the climate is semi-arid to arid, with seasonal rainfall and with
potential annual evaporation or evapotranspiration exceeding rainfall. As examples, Figure 1 shows the monthly average rainfall (R) and American Standard evaporation pan (A
pan) evaporation for two South African cities, Cape Town
and Johannesburg, both of which are located in semi-arid climatic zones. Cape Town has cool wet winters and warm dry
summers and Johannesburg warm wet summers and cool dry
winters. Cape Town has a short winter surplus of rain over
evaporation, whereas Johannesburg has a perennial water
deficit. (Note that because this research took place in the
southern hemisphere, the winter solstice occurs on 21 June.)
In such a climate, there is the potential for infiltration into a
landfill during the wet season to be completely re-evaporated
during the ensuing dry season, so that net accumulation of
water in a landfill over a period of years could be zero, or
could fluctuate about zero.
Dry tomb versus bioreactor
An efficient, impervious landfill cap may not be as desirable
as it may seem, however, as a supply of moisture is necessary
for the decomposition and ultimate stabilization of the waste.
The USEPA (1991) appears to have adopted a ‘dry tomb’
approach to the storage of municipal solid waste in landfills.
However, a dry tomb landfill will, by definition, never completely decompose and stabilize, and alternative ‘wet’
approaches may be more advantageous. For example, Lee &
Jones-Lee (1993) have pointed out the considerable advantages over the dry tomb system, accruing from a ‘wet cell’
approach in which degradation is accelerated by the addition
of water to the waste and the re-circulation of leachate. Many
114
other authors such as Barlaz et al. (1990) and Baldwin et al.
(1998) have also emphasized the importance of moisture in
decomposing and ultimately stabilizing waste. The role of
moisture is very well summed up by El-Fadel and Al-Rashed
(1998), as follows: ‘Moisture content appears to be the variable that is associated with the greatest effect on biodegradation processes because it provides an aqueous environment
that facilitates the transport of nutrients and microbes within
the landfill. This transport mechanism dilutes the concentration of inhibitors and enhances micro-organisms’ access to
their substrates and hence improves gas generation and stabilization.’
The advantages of encouraging biological degradation in
landfills, as opposed to the dry tomb approach, now appears
to be generally accepted in the USA. To quote a recent text
book (Vesilind et al. 2002): “The rate of stabilization in ‘dry’
landfills may require many years, thereby extending the acid
formation and methane fermentation phases of waste stabilization over long periods of time…… In contrast, leachate recirculation may be used…… This option offers more rapid
development of active anaerobic microbial populations and
increases reaction rates ….. The time required for stabilization of the readily available organic constituents can be compressed to…… two to three years rather than the 15- to 20year period” [expected for a dry tomb landfill].
That the short-comings of the USEPA subtitle D
approach to landfill caps are also now well appreciated in the
United States, is shown by the creation of the Alternative
Landfill Cover Demonstration (ALCD), which is currently
testing innovative semi-pervious covers, using accepted EPA
cover designs as baselines (Desert Research Institute 2004).
Water content of incoming waste
It must be emphasized that incoming waste may already have
a relatively high water content even in areas that are arid or
semi-arid. Most of the incoming water resides in the organic
or putrescible fraction for which the water content may be
two to three times the air-dry mass. This fraction is also
highly compressible and when the waste is landfilled, the
water inherent in the putrescible fraction of a particular layer
maybe expressed by the increasing overburden and may issue
from the base of the landfill as ‘squeezate’. If squeezate itself,
rather than rain infiltration-induced leachate is likely to
pose a pollution problem, it must be dealt with accordingly.
The South African land-filling regulations (Department of
Water Affairs and Forestry 1998) require a system of finger
drains to be provided, under a landfill, primarily to intercept
squeezate, even if infiltration-induced leachate is not expected
to be generated in the longer term.
Table 1 compares the composition of incoming waste in
the USA and the UK with that of waste from Cape Town,
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Experimental landfill caps for semi-arid and arid climates
Table 1: Composition of waste from USA and UK, compared with waste from Cape Town, Johannesburg, Gaborone (Botswana), Lima (Peru), Delhi
(India) and Abu Dhabi (United Arab Emirates).
% Composition by undried mass
Component
Ash, soil, building rubble, other
Paper and cardboard
USA
UK
Cape Town
Johannesburg
8–14
18
39*
1**
34–44
29
19
23
Delhi
Abu Dhabi
3
16
41
16
12
14
9
6
13
5
7
1
10
6
4
1+
+
8–10
7
7
Metals
11–13
8
2+
Organics, putrescibles
Lima
+
Plastic
Glass
Gaborone
+
4–9
10
1
12–22
25
32
8
6
3
1
45
68
56
47
* Building rubble landfilled, includes sand cover. ** Building rubble not landfilled, cover soil not included.
Johannesburg, Gaborone, Lima, Delhi and Abu Dhabi
(Rushbrook & Finnecy 1988, Rathje & Murphy 1993, Abu
Qdais et al. 1997, Bolaane & Ali 2004). The high putrescible
percentage of waste from the desert areas of Gaborone, Lima
and Abu Dhabi will be noted. The percentage of putrescrible
waste for Cape Town appears relatively low because of the
large percentage of building rubble that is landfilled. That for
Johannesburg, in contrast, appears high because building rubble is not landfilled. The water content of Cape Town waste
as a mass percentage of air-dry solids varies from 18 to 37%,
while for Johannesburg it varies from 30 to 50%. The average
field storage capacity for landfilled waste in Cape Town is
150%, and that for Johannesburg is 130% (Blight et al. 1992).
Stabilization in arid and semi-arid conditions
In arid and semi-arid climatic zones, however, there is no
superabundance of rainfall to exclude from the waste, nor is
there sufficient rainfall intentionally to operate a wet system.
It seems logical, therefore, to maximize the waste stabilizing
effect of the rain that does fall, by allowing it to be absorbed
by the landfill cap, to penetrate in a controlled way into the
waste, and then to be re-evaporated after seasonally or periodically accelerating the process of stabilization of the waste.
Even with an annual or periodic infusion of rain infiltration,
stabilization times will be very long. Rohrs et al. (2001) have
estimated, for example, that at least 1.6 pore volumes of infiltration must pass through landfilled waste before chloride
concentrations in the leachate will reach acceptable levels of
about 100 mg L–1, and the landfill can be regarded as having
reached stability. Even with an annual effluence of leachate
of 200 mm, this would take 300 to 500 years to occur. Further
evidence that dry landfills stabilize very slowly was given by
Morris et al. (2001) who showed that methane emissions from
landfills in a semi-arid climate decrease relatively rapidly
with time after a cell has been capped. The data showed a
decrease from 45 g m–2 per day immediately after capping, to
25 g m–2 per day after a year, to 18 g m–2 per day after 20 years.
The low emission rates were ascribed to the lack of available
+
+
12
8
9
49
Scavenged before landfilling.
water in the landfilled waste which retarded the decomposition and hence stabilization processes.
Evaporation from a landfill surface
There is a common misconception that if water penetrates a
landfill to deeper than 1 m or so, the water will move beyond
the influence of evaporative forces and will continue to migrate
downwards. This has been disproved in a number of experiments on landfills in semi arid climates. Blight et al. (1992)
presented water content profiles measured at the ends of the
wet and dry seasons in landfills situated in both Cape Town
and Johannesburg. The Cape Town landfill was temporarily
capped with 300 mm of clean beach sand, while the Johannesburg landfill was temporarily capped with 300 mm of a
pervious silty sand residual from the decomposition of granite.
The water content measurements showed that in Cape Town,
waste dried seasonally to a depth of 7.5 m (the full depth of
the waste). In Johannesburg similar profiles showed seasonal
drying to a depth of 16 m. In a separate experiment, Roussev
(1995, quoted by Blight, 1997) constructed two pairs of identical lysimeters in a landfill in Johannesburg. Each lysimeter
measured 4.5 m square in plan, one pair of lysimeters was 3 m
deep, the other 5.5 m deep. The four sides and the base of
each lysimeter were sealed by means of sheets of geomembrane
welded to form an impervious box. Each lysimeter was equipped
with a drainage layer at the base and a 100-mm-diameter
observation well and was filled with compacted waste. The
top surfaces of two of the lysimeters (3 m and 5.5 m deep)
were then sealed with a geomembrane, while the surfaces of
the other two lysimeters were left open to the atmosphere.
All four lysimeters were brought to their water storage capacity by irrigating their surfaces until leachate appeared in the
observation wells. The leachate was then pumped out by lowering a submersible pump down each observation well until
no more leachate collected. At this stage the waste in the
lysimeters was at its water storage capacity. The lysimeters
were then left for a period of 4 months and measured quantities of water were then slowly added at the surface until lea-
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115
G.E. Blight, A.B. Fourie
chate again appeared in each observation well. The difference between the water added to the open-topped and closedtopped lysimeters was then taken to be the evaporative water
loss from the open-topped lysimeters. The experiment was
then repeated over a period of 6 months. It was found that the
3-m-deep lysimeter lost an average of 0.17 mm m–1 per day,
whereas the 5.5-m-deep one lost 0.22 mm m–1 per day. This
was taken as a demonstration that evaporative losses can
occur from waste at least to depths of 5.5 m and supported the
evidence of the earlier water content profiles.
Further evidence that little or no leachate ever exits the
base of a landfill in a semi-arid climate was advanced by Fourie et al. (1999). They investigated six long-established unlined
landfills in South Africa that are situated in both semi-arid
and marginally humid (water surplus) climatic zones. The
results of soil sampling around the landfills showed that very
little leachate had exited any of them, even those located in
marginally humid climates.
Infiltrate–stabilize–evapotranspire
This paper will describe two large-scale field experiments,
one in Cape Town and one in Johannesburg which were
designed to investigate the behaviour of what will be called
‘infiltrate–stabilize–evapotranspire’, or ISE landfill covers or
caps. The function of an ISE cap is to absorb the scarce annual
rainfall, allowing part of the water to penetrate into the waste,
thus accelerating the decomposition process and hence the
compression of the waste, then abating the infiltration by
allowing it to combine with the waste and re-evaporate to the
surface. Ideally, no leachate would exit from the base of the
waste body, but depending on annual climatic variations,
there might be a small leachate flow from time to time.
There is evidence (e.g. Blight 1997, 2003) that all, or
almost all annual infiltration into a landfill (or any other
soil-covered surface, for that matter) will be removed by evapotranspiration by the end of that year, and that it is only in
exceptionally wet years that there will be surplus water available to infiltrate deeply into a landfill. Annual rainfall in
semi-arid regions can be highly variable. In Johannesburg,
for example, recorded annual rainfall has varied from 508 to
928 mm with a mean of 732 mm and a standard deviation of
117 mm. If annual evapotranspiration from the surface of a
landfill were a constant 732 mm and if all rainfall infiltrated,
it would only occur once in about 25 years that there would
be a surplus of infiltration over evapotranspiration of
200 mm or more. It follows that stabilization of the waste in
the terms described earlier would actually take considerably
longer than the 300 to 500 years mentioned previously.
Thus the aim of an ISE cap differs somewhat from that of
an ‘evaporative’ or ‘evapotranspirative’ (ET) cap (e.g. Zornberg & Caldwell 1998, Hauser et al. 2001, Dwyer 2001, Ben-
116
son et al. 2002) in which the aim is to store infiltrating rain
in the cap without allowing penetration into the waste and
then allowing it to evaporate. An ISE cap allows and even
encourages deep penetration of the waste by each seasonal
rainfall, followed by upward flow and evaporation at the surface during the ensuing dry season. Clearly, if there is insufficient rainfall to exceed the water storage capacity of the cap,
the ISE cap will function as an ET cap.
Field tests of ISE caps under summer and winter
rainfall conditions
To explore the concept of the ISE cap, two large-scale test
facilities have been set up, both in semi-arid regions, but one
in a winter rainfall area with a dry summer (Cape Town) and
the other in a summer rainfall area with a dry winter (Johannesburg). Figure 1 shows the long-term average atmospheric
water balances for the two semi-arid regions. Temperatures are
moderate at both locations. Cape Town has minimum temperatures of 10°C in winter and maxima of 35°C in summer,
whereas Johannesburg experiences –5°C in winter and 35°C
in summer. Cape Town’s latitude is 342 South at sea level and
Johannesburg’s 2623South at an altitude of 1700 m above sea
level. Both sets of caps use local soils and were intended mainly
to be monolithic (i.e. constructed of a single uniform soil).
The objectives of the tests were as follows:
• to record rainfall and evaporation from the cap surfaces
(by energy balance methods; Blight 1997, 2002);
• to sample and record variations in pore water stored by
capillarity in caps of various thicknesses throughout the
seasons; and thus
• to construct a water balance for each experimental cap; and
• to observe any other aspects of cap performance (e.g. surface cracking and wind and water erosion); and hence
• to assess the potential for practical implementation of the
ISE cap, including required thickness for the combination
of local soil properties and climate.
At the Simmer and Jack landfill near Johannesburg, five
large test caps were constructed, four of which are monolithic with thicknesses varying from 500 to 1500 mm and
one that was constructed according to the South African
“Minimum Requirements” (1998). A similar facility was
constructed at the Coastal Park landfill near Cape Town.
The plan and longitudinal section of the experimental caps
are shown in Figure 2. Construction of these facilities was
completed by mid 2001. The cap experiment at Coastal Park
was constructed on top of an experimental section of the
landfill (Blight et al. 2001) whereas at Simmer and Jack, the
experiment was sited next to, but not on the landfill.
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Experimental landfill caps for semi-arid and arid climates
Fig. 2: Layout of experimental landfill caps in Cape Town and Johannesburg. The ferricrete erosion protection layer was only present on the Cape
Town experiment.
The geomembrane lining mentioned in Figure 2 (plan)
defines the base of each trial cap. Any infiltration in excess
of the water storage capacity of the sand will be intercepted
and trapped by the geomembrane where it can be detected
and measured, but will still be available to evaporate out
through the sand. This gives an indication of the quantity of
water available to infiltrate the underlying waste. The first
cap (no. 1) on the left of Figure 2 consists of a 300 mm clay
layer covered by 200 mm of sand. This satisfies the Minimum
Requirements’ specification for a GLB- landfill (i.e. a large
landfill receiving general waste and located in a B-(semiarid) climate) and allows the behaviour of a cap incorporating a clay layer to be studied. Thicknesses of sand cap increasing from 0.5 to 1.5 m in thicknesses of 0.5, 0.75, 1.0 and
1.5 m were provided in covers 2 to 5. Each cap area is separated from its neighbours by a 0.5 m high clay bund within
which the geomembrane forms an impervious basin. The
possible accumulation of excess free water within each basin
can be detected and measured by means of four perforated
vertical standpipes built into each basin.
For a number of reasons, monitoring of the Simmer and
Jack caps was not initially done at regular intervals and regular observation only started in October 2002. The Coastal
Park caps have been monitored regularly since July 2001.
Considerations in constructing the Coastal Park
experiment
Because clay is not easily obtainable in the vicinity of Coastal
Park, the experimental caps were originally intended to enable a satisfactory alternative to the minimum requirements
(MR) cap to be proposed, using dune sand instead of clayey
soil. An analysis of rainfall at Coastal Park for the years 2001/2
showed that 75% of annual precipitation falls in events of
19 mm day–1 or less. Thus if the surface layer has a permeability of 20 mm day–1 or more, even a cap fully complying with
the MR would allow much of the rainfall to be absorbed by
the surface layer and then, slowly, by the clay layer. As the
average annual rainfall at Coastal Park amounts to about
520 mm, a quantity of perhaps 400 mm per year can be
expected to be absorbed by the MR cap and some to pass
through into the waste. In contrast, a cap consisting entirely
of sand in contact with the waste may allow more infiltration
to penetrate into the waste. Both types of cap will allow infiltration to be drawn up by capillarity and to re-evaporate from
the surface. Thus both types of cap act as ISE caps, absorbing
rainfall, accelerating stabilization of the waste and then
allowing the surplus water to re-evaporate at the surface.
The experiment at Cape Town was originally constructed
without the ferricrete layer, but gales off the South Atlantic
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G.E. Blight, A.B. Fourie
Fig. 3: Particle size analyses and suction-water content relationships for soils used in Coastal Park, Cape Town, experimental caps.
during October and November 2001 eroded the surface so
severely that in January 2002 the 100 mm thick ferricrete
layer was added as an erosion protection layer. Up to 300 mm
of sand was wind-eroded off the caps and the clay of the MR
cover was almost completely exposed. During repair, the
sand thicknesses were restored to their original values. The
ferricrete performed very well in terms of erosion protection
during the dry windy months of September to November
2002 and 2003, and no noticeable wind or water erosion has
occurred since it was placed.
Based on the performance of cap no. 1, it was decided to
construct a sixth cap at Coastal Park (cap no. 6) consisting of
0.3 m of compacted clayey silt overlying a 0.5 m minimum
thickness of sand, over the waste. When the clayey silt of the
GLB- cover was exposed by wind erosion in very dry, hot
weather, its surface was found to have cracked, but the cracks
were narrow (about 0.5 mm wide) and closely spaced (about
300 mm), hence it was likely they would readily close and
seal when re-wetted. No erosion of the clayey silt surface
occurred when it was exposed to wind erosion, and volunteer
vegetation appears to establish itself readily on the clay surface. Figure 2 shows a section through the clayey silt cap. The
expected behaviour of the exposed clayey silt layer during dry
weather has been confirmed by observation of cap no. 6.
Considerations in constructing the Simmer and Jack
experiment
Two soils were available at Simmer and Jack, one was a silty
sand residual from the decomposition of quartzites, and the
118
other a more clayey material residual from the decomposition
of diabase. The decomposed quartzite was used as the counterpart of the dune sand used at Coastal Park, whereas the
residual diabase was used to construct cap no. 1, the MR cap.
No counterpart of cap no. 6 was constructed at Simmer and
Jack, and because Johannesburg is 600 km inland, the fierce
sea gales are absent and it was not necessary to protect the
cap surfaces against wind erosion. Unfortunately, the experimental caps at Simmer and Jack were not constructed as carefully as they should have been, and the residual quartzite was
contaminated by pockets of residual diabase.
The cap surfaces at Cape Town and Johannesburg were
not vegetated, as it was decided to see if, and what type of
volunteer vegetation would establish itself. By the end of
2003 only a sparse scattering of grasses had invaded each set
of caps. Thus any losses of infiltrated water from the trial caps
have resulted from evaporation from the soil, with negligible
assistance by transpiration.
Properties of soils used in the experimental
caps
Figure 3 shows the gradings of the sand, clayey silt and ferricrete used in the Coastal Park cap experiment.
Coastal Park
The sand is a typical dune/beach sand. Eighty five per cent of
particles lie between 0.1 and 1 mm and the grading spans
almost the whole range of sand-size particles. The permeabil-
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Experimental landfill caps for semi-arid and arid climates
Fig. 4: Particle size analyses and suction-water content relationships for soils used in Simmer and Jack, Johannesburg, experimental caps.
ity of the saturated sand (measured by falling head test in the
laboratory) is 2000 mm h–1 (48 m day–1, or 5 1 10–2 cm s–1).
Hence all rainfall should infiltrate a sand surface and there
should be minimal run-off.
The clayey silt is residual from a weathered greywacke and
ranges from gravel-sized particles to clay. The clay content is
only 10%, the liquid limit is 30%, the plasticity index is 8%
and the linear shrinkage 3%. The clayey silt meets the South
African MRs for landfill caps. The in-situ permeability has
been measured as 20 mm h–1 by means of ring infiltrometer
tests. The low linear shrinkage explains the limited cracking
that occurred, and the gravel content accounts for its erosion
resistance. Because of the lower permeability, run-off can be
expected from an exposed clay surface.
The ferricrete has almost 70% of gravel size particles, 25%
in the sand and silt size range and 5% of clay. The index properties have not been measured, but the in-situ permeability
measured by infiltration tests is only 10 mm h–1. Thus the
thin ferricrete layer promotes runoff and impedes infiltration.
Figure 3 also shows the suction-water content curves for
the three soils. When draining from its water storage capacity the sand releases water at low suctions of 20 to 30 kPa
down to a water content of about 2%. It is only then that
increasing suction is required to extract further water. The
suction-water content curve for the ferricrete is very similar
in form to that for the sand, except that water is released at
low suctions down to a water content of 5%. The suctionwater content curve for the clayey silt shows that water is
held against suction from a water content of about 16%. At a
suction of 800 kPa about 9% of moisture is retained. The
suction-water content tests were cycled to simulate the
effects of repeated wetting and drying of the soils. As shown
in Figure 3, there is very little hysteresis between wetting and
drying for the sand, the ferricrete and the clayey silt.
Simmer and Jack
Figure 4 is the counterpart of Figure 3, for the experiment at
Simmer and Jack. The residual quartzite has 50% of its particles in the sand sized range, with 25% of gravel, 20% of silt
and 5% of clay size particles. The diabase, on the other hand,
consists of aggregates of finer particles with 40% in the sand
size range, 10% in the gravel and 50% in the silt and clay size
ranges. The index properties of the residual quartzite are: liquid limit: 21%, plasticity index: 4%, linear shrinkage 2%. For
the residual diabase they are: liquid limit 33%, plasticity
index 14%, linear shrinkage 8%. In-situ permeabilities measured in ring infiltrometer tests were 35 mm h–1 for the residual quartzite and 6 mm h–1 for the residual diabase. Thus a
residual quartzite surface can be expected to promote infiltration, whereas the diabase should encourage run-off.
The suction-water content curves are also shown in Figure 4
and also show little hysteresis between wetting and drying cycles.
Numerical modelling of an ISE cap
Prior to constructing the experimental caps, in order to explore
the viability of the ISE concept, the suitability of a soil having
a suction-water content curve lying between those for the residual quartzite and the ferricrete was investigated by carrying out
a number of numerical simulations using the method originated
by Ross (1990). The corresponding variation of permeability
with water content was derived using the method of Green &
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G.E. Blight, A.B. Fourie
Table 2: Predicted outflows from base of simulated ISE caps.
Cap thickness (mm)
300
600
900
1500
Predicted outflow (mm)
24.6
13.3
10.9
8.5
Corey (1971). The modelling procedure and the assumptions
it includes have been fully described by Fourie (2000).
A rainfall event with a 50-year recurrence interval, a 24-h
duration and a 6 mm h–1 intensity was modelled. After the
rainfall, the simulation continued for 50 days, during which
the only imposed boundary condition was potential evapotranspiration, in order to allow downward moisture movement to cease and drying of the profile to progress. For a 600mm-thick monolithic cap, the outflow from the base of the
cap was 11 mm, or 8% of total rainfall. Another 50-year
recurrence interval event was stimulated, having a duration
of 2 days but an intensity of 3 mm h–1. The outflow from the
base of the cover in this case was 3.4 mm, illustrating the
effect of not much exceeding the water storage capacity of
the soil, thus allowing evaporation to be more effective.
A more detailed series of simulations was then performed,
using measured daily climatic data for Johannesburg for the
first 3 months of 2000, which was a particularly wet period.
During these 3 months the total rainfall was 581 mm, which
is about 85% of the annual average. Thus, this represents an
extreme test of the suitability of an ISE cap in the climate of
Johannesburg. Four different cap thicknesses were simulated,
with the soil being the same as used in the 50-year recurrence interval simulations. The predicted outflow from the
bases of the caps is summarized in Table 2.
Even for a thin (300 mm) cap, the outflow from the base
was modest (4.2% of total rainfall). It is also worth noting
that the decrease in outflow with increasing cover thickness
was not linear; for example, compare the outflows for thicknesses of 900 and 1500 mm, where a 66% increase in thickness resulted in a 22% decrease in outflow. This is because of
the dynamic nature of the problem being modelled, where
soil water is continually changing its direction of movement.
As a 500- to 600-mm-thick cap is likely to be the thinnest
acceptable as a cap capable of resisting the ravages of erosion
for a century or more, it appeared that the concept of the ISE
cap was well worth exploring, although in many years, the caps
would, as a result of a lack of rain, act as ET rather than ISE
caps, allowing very little water into the waste. However, this
behaviour is a function of climate, rather than cap design.
Performance of experimental caps
Rainfall infiltration and water storage
Figure 5 shows a selection of water content profiles measured
at Coastal Park. Figures 5a and 5b represent the wettest and
120
driest conditions before placing the protective ferricrete
layer, whereas Figures 5c and 5d represent the wettest and
driest conditions after placing the ferricrete.
On 31 July 2001, free water was present on the basal
geomembranes of all of the trial caps, showing that if the
caps (minus the geomembranes) had overlain waste, water
would have been available to migrate downward into the
waste. By 9 November 2001, the free water on the basal
geomembranes had evaporated from all the caps, the maximum water content in the sand was 15% and the maximum
water stored in the sand was 45 mm per 200 mm depth. Considering all of the water content profiles and the suctionwater content relationship for the sand (Figure 3b), 15%
water content appears to represent the water storage capacity
of the sand. On 31 July 2001, there appears to have been at
least 30 mm of water per 200 mm of sand available for infiltration into the waste, whereas all of this excess water was
removed by evaporation during the ensuing 3 months to 9
November 2001. The experimental caps were therefore functioning as ISE caps. At that time the Coastal Park landfill
was capped temporarily with a layer of sand varying in thickness from 300 to 500 mm yet the leachate outflow from the
landfill base was only 5 mm year–1. Thus even the 300 to
500 mm of sand was acting efficiently as an ISE cap.
Once the ferricrete protective layer was placed, the entire
character of the caps was changed, with increased runoff
and decreased infiltration. In their wettest state (Figure 5c;
15 August 2002) the maximum water content in the sand
was only 7% and the sand in all caps was well below its water
storage capacity. The caps, including cap no. 6, now acted
efficiently as ET caps, there being insufficient infiltration
through the ferricrete and clay surface layers to allow them
to act as ISE caps.
Figure 6 shows the wettest and driest conditions encountered up to the beginning of 2004 in the experimental caps
constructed at Simmer and Jack. Even under the wettest condition of March 2004, no free water was encountered on any
of the geomembrane liners. Comparing the water contents
for the wettest condition (Figure 6a) with the suction-water
content curves shown in Figure 4b, it will be seen that caps
no. 1, 2, 3 and 4 must have been very close to their water storage capacity in March 2004, but the storage capacity was not
exceeded. In other words, caps no. 1, 2, 3 and 4 were about to
change their function from ET caps to ISE caps. There was
clearly a wave of moisture moving down through cap no. 5, but
the cap as a whole had not yet reached its water storage capacity.
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Experimental landfill caps for semi-arid and arid climates
Fig. 5: Water content-depth profiles for Coastal Park experimental caps: Wettest condition (sand at surface); Driest condition (sand at surface);
Wettest condition (ferricrete at surface); Driest condition (ferricrete at surface).
Water balance at Coastal Park
Figures 7a and 7b show the water balances for the caps at
Coastal Park. The water balance can be expressed as
R – RO = I = 4S + E
(1)
where R is the rainfall; RO is the runoff; I is the infiltration;
4S is the change in soil water storage; and E is the evaporation from surface.
Figure 7a shows the cumulative rainfall 5R, evaporation
rates found by solar energy balance measurements on the
1
landfill surface, E B, and the storage in the original five covers,
S, expressed in millimetres of water. As expected, the storage
in the covers increased during the wet season, probably
reaching a maximum in mid-September, and then declined
during the dry season. Note that the storage of cap no. 1 (the
MR cap) was consistently just less than that of cap no. 5
(1500 mm of sand) throughout the period of observation.
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121
G.E. Blight, A.B. Fourie
Fig. 6: Water content-depth profiles for Simmer and Jack experimental caps: Wettest condition; Driest condition.
The wind erosion damage was repaired in mid-January 2002,
by which time the storage in the covers had been depleted by
almost half as a result of evaporation. When the rain started
again in April, it had very little effect on the storage, showing
that infiltration had been very much reduced by the presence
of the ferricrete layer.
Referring to the water balance equation, in periods when
there is no rain, namely mid-September 2001 to early January 2002, one can get a measure of actual average evaporation from the cover surface E from the relationship:
E = –4S
(1a)
where –4S is the decrease in storage over the rainless period.
For the period from September 2001 to January 2002, the
actual average rate of evaporation E for the five caps was
about 0.7 mm day–1. The evaporation rate measured on the
landfill surface (covered temporarily with 300 mm of sand)
for this period averaged about 2.8 mm day–1. The reason for
the discrepancy is that water was not available in the cover
layers to evaporate as they were isolated from the waste by
geomembranes. Water could, however, be drawn up from the
waste within the landfill to provide much higher evaporation
rates from the remaining landfill surface. Later in 2002, midFebruary to mid-April, the actual evaporation rate from the
caps reduced to only 0.2 mm day–1.
Although the rainfall analysis suggested that there should
have been little or no runoff from a sand surface, the storage
122
figures for the moisture profiles show that there must have been
substantial runoff during very wet periods. If the solar energy
1
balance rates of evaporation ( E B) are assumed to operate during rainy periods, the water balance equation can be written:
1
RO = R – (4S + E B)
(1b)
and the average runoff can be determined for any period.
1. For the wet period during July 2001, for cap no. 5
RO = 246 – (180 – 135 + 0.7 1 20) = 187 mm and
RO/R = 187/246 = 76%
as shown in Figure 7a, this was a period of exceptionally
concentrated rainfall.
2. For the very wet period from mid-August to early September 2001, for cap no. 5:
RO = 146 – (178 – 172 + 3.5 1 25) = 52 mm and
RO/R = 52/146 = 36%.
3. For the wet period from early April to mid-August 2002
(after the ferricrete had been placed), for cap no. 5:
RO = 265 – (102 – 86 + 1.5 1 127) = 90 mm and
RO/R = 58/265 = 22%.
Thus even though the ferricrete was relatively impervious, it
still admitted a considerable portion of the rain to the underlying sand. Hence runoff can be an appreciable term in the
water balance, even with a highly permeable sand layer
exposed at the surface. Cap no. 6 (see Figure 2c) was con-
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Experimental landfill caps for semi-arid and arid climates
Fig. 7: Water balance data for Coastal Park experimental caps: July 2001 to September 2002; October 2002 to December 2003.
structed early in October 2002, and the storage in this cover
is recorded in Figure 7b from 10 October 2002, onwards. The
storage in cap no. 6 was consistently higher than that of the
other caps, showing that rainfall was both infiltrating and
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123
G.E. Blight, A.B. Fourie
Fig. 8: Water balance data for Simmer and Jack experimental caps: October 2002 to March 2004.
being efficiently stored in the clay layer. The very low actual
1
evaporation rates ( E ) noted for the dry months of 2001/2002
were repeated in 2002/2003.
Considering the data for storage (S) in Figures 7a and 7b,
it appears that a cap thickness of 750 to 1000 mm (caps no. 3
and 4) would be adequate to act as an ISE cap while still providing adequate separation of the waste from its surrounding
environment. However, careful attention must obviously be
paid to the likely annual loss of thickness of the cap by water
and wind erosion.
Water balance at Simmer and Jack
The water balance for the caps constructed at Simmer and
Jack is shown in Figure 8. As was the case with the caps at
Coastal Park, actual evaporation rates from the experimental
1
caps ( E ) were considerably less than rates of evaporation
from the landfill surface, determined by solar energy balance
(upper diagram in Figure 8). Because of the relatively permeable nature of the residual quartzite, it had been assumed, in
the early stages of the experiment, that no run-off was occurring and that infiltration was complete. When, after the 5month wet spell from mid-October 2003 to mid-March
2004, it was found that no free water had accumulated on
any of the geomembranes underlying the caps, it was at first
thought that run-off was the reason for this. However, as the
calculations on Figure 8 show, run-off either did not occur
(negative figures for RO) or was very small (19 mm for cap
no. 1 and 2 mm for cap no. 4). It then became apparent that
evaporation from the surface must have almost been keeping
pace with rainfall. For this 5-month period, evaporation
1
measured by solar radiation balance (5 E B) amounted to
124
420 mm, whereas rainfall (5R) was 485 mm. The difference
1
(5R – 5 E B) was absorbed by the cap layers, and any discrepancies in the figures can be explained by differences between
actual evaporation and evaporation predicted by solar energy
balance. Figure 8 also shows that a cap thickness of 750 to
1000 mm (caps no. 3 and 4) would probably be adequate to
function as on ISE cap.
Concluding discussion
1. Evapotranspirative (ET) landfill caps are designed to
exclude infiltration into the landfilled waste by maximizing run-off and storing any infiltration within the soil layers of the cap pending its re-evaporation or transpiration.
Any infiltration into the waste body is regarded as a leakage, and therefore a short-coming of the cap.
2. Recognizing that decomposition and stabilization of the
waste requires water to be available to promote bacteriological activity, and knowing that stabilization of the waste
is likely to be extremely slow if the water content of the
waste is not supplemented, the ISE cap is designed to
encourage limited infiltration into the waste during the
wet season. This is aimed at promoting decomposition and
stabilization, and then allowing any excess infiltration to
be re-evaporated during the ensuing dry season.
3. The experimental caps constructed at Cape Town and
Johannesburg have demonstrated that the ISE concept
will work in semi-arid climates both with winter and summer rainfall. They have also demonstrated that in drier
years, an ISE cap will function as an ET cap, storing all
infiltration into the cap and attenuating infiltration before
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Experimental landfill caps for semi-arid and arid climates
it reaches the waste. In wetter years, or during exceptionally wet periods, the ISE cap will function as designed.
4. The erosion protection layer applied to the Cape Town
experimental caps showed how the behaviour of an ISE or
ET cap can be modified very simply by applying a thin less
pervious layer to the surface, thus reducing infiltration.
5. It also appears from the water balance data that cap thicknesses of 750 to 1000 mm can be adequate for ISE caps to
perform their function of infiltration regulators. A thicker
cap may be required if the effects of long-term erosion are
a concern. However, a thinner cap will allow more infiltration into the waste, and therefore a shorter period for stabilization, and better functioning as on ISE cap. In fact, in
a semi-arid climate, the major problem is not one of
excluding water from the waste, but one of getting sufficient water to enter the waste, while simultaneously providing a durable physical separation between the waste
and the environment.
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