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1
PERFORMANCE OF A PHYTOCAPPED LANDFILL IN A SEMI-
ARID CLIMATE
Kartik Venkatraman1, Nanjappa Ashwath1 & Ninghu Su2
1Centre for Plant and Water Science
Central Queensland University, Rockhampton, Queensland, Australia – 4702
2School of Earth and Environmental Science
James Cook University, Cairns, Queensland, Australia – 4870
k.venkatraman@cqu.edu.au
Abstract
Landfills have been the major repositories of urban wastes, and they will continue to
be built, so long as the humans live in communities. The costs of construction,
maintenance and remediation of landfills have escalated over the years and research is
therefore required to identify alternative techniques that will not only minimise the
costs, but also demonstrate increased environmental performance and community
benefits. This chapter discusses the alternative landfill capping technique known as
‘Phytocapping’ (establishment of perennial plants on a layer of soil placed over the
waste), which was trialed in Rockhampton, Australia. In this technique, trees were
used as ‘bio-pumps’ and ‘rainfall interceptors’ and soil cover as ‘storage’ of water.
Tree performance was measured based on their canopy rainfall interception and water
uptake potential. The rate of percolation of water was modelled using HYDRUS 1D
for two different scenarios (with and without vegetation) for the thick (1400 mm) and
thin (700 mm) covers respectively. Evidence and simulations incorporating 15 years
of meteorological data showed percolation rates of 16.7 mm yr-1 in thick cover and
23.8 mm yr-1 in thin cover, both of which are markedly lower than those expected
from a clay cap.
Keywords: HYDRUS 1D, landfill, percolation, phytocap, water balance, methane
Introduction
Landfill capping is a mandatory post closure procedure to isolate the deposited wastes
from external environment, mainly water (Vasudevan et al. 2003). Landfill capping
involves placing a barrier, which acts as a raincoat over filled landfill to minimise
percolation of rainfall or surface water into the waste (Scott et al. 2005). This is not
only expensive but also not viable for small and medium sized landfills in Australia.
In recent years, conventional capping systems made of compacted clay; Geosynthetic
Clay Liners (GCL) and HDPE have been used extensively in many countries.
Amongst these, the most popular practice in Australia has been the use of compacted
clay caps. A typical compacted clay cap recommend in Queensland is given below
(Fig 1)
2
Rubbish
Earthen cover (200-300 mm)
Low permeability clay (500 mm)
Sub-soil base (200-300 mm)
Topsoil/mulch (150 mm)
Waste
Figure 1: Various layers used in a typical clay capping system, permeability clay
(Ks = 10-8 m/s) (EPA 2005)
In Australia, the caps constructed on landfills should be sustainable for at least 30
years. Recent studies however show that clay caps have shorter life span (Vasudevan
et al. 2003) and fail to prevent percolation of water due to cracking (Khire et al. 1997,
Melchoir 1997). Furthermore, clay caps do not allow optimal interaction of methane
with oxygen, which is a must for methane oxidation (Abichou et al. 2004).
A new technology called ‘Phytocapping’ (Venkatraman and Ashwath 2007) was
trialed at Lakes Creek Landfill, Rockhampton. In brief, phytocaps have two major
components, viz. the trees that act as ‘bio-pumps’ and ‘rain interceptors’ and the soil
that acts as ‘storage’ (Fig 2). The research which was conducted over three years
(2005 to 2007) examined various aspects of the performance of the phytocap,
including soil water storage and tree performance along with the modelling for site
water balance using HYDRUS 1D.
Figure 2: The Phytocapping concept
Water sources in a landfill include the waste itself, soil cover and precipitation
(Bengtsson et al. 1994). It is essential to assess the effectiveness of the phytocaps with
respect to the amount of water that percolates into the waste. For these reasons
numerous water transport modelling softwares have been developed to predict
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percolation rates based on Richard’s equation and Water Balance method (Albright et
al. 2002, Williams 2005). The models using Richard’s equation have shown higher
accuracy (Albright et al. 2002) and proven to be better than those based on water
balance method due to their ability to describe water flow in any direction (Jirka
Simunek: pers. Comm.).
The performance of landfill caps have been evaluated either by qualitative
(groundwater monitoring, leachate collection etc.), indirect quantitative methods
(empherical methods, mass balance methods and unsaturated flow process methods
using Richard’s equation) or direct quantitative techniques that are based on
measurements using lysimeters (Albright et al. 2002). However, lysimeters are very
expensive to construct (Albright et al. 2002) and hence soil moisture measurements
(qualitative) and HYDRUS 1D (indirect quantitative methods) were used to estimate
percolation in this research.
A variety of models have been used to measure percolation rates in different
scenarios. Several modelling softwares have been developed and compared for their
accuracy and robustness (Ho et al. 2004). A few comparative studies have been
undertaken on hydrologic performance evaluations of landfills cover (Chai and Miura
2002, Ho et al. 2004) with most measuring seepage production (Dho et al. 2002, Ham
2002). These models include UNSAT-H, HYDRUS 1D, HYDRUS 2D, Simulation of
Heat and Water (SHAW), Vadose/W, Soil Water Balance and Infiltration Model
(SWIM), and The Hydrologic Evaluation of Landfill Performance (HELP), TOUGH-
2, MACRO and LEACHM (Fayer et al. 1992, Fayer and Gee 1997, Khire et al. 1999,
Scanlon et al. 2002, Benson et al. 2004, Albright et al. 2002, Johnson et al. 2001,
Ross 1998). Amongst these models, HYDRUS 1D, HYDRUS 2D, UNSAT-H and
Vadose/W are used most frequently (Benson 2004). Erosion Productivity Impact
Model (EPIC) (Williams 2005) uses the water balance method and has been
extensively used in agriculture, but was found less robust than the models using
Richards’s equation and more accurate than HELP (Hauser and Gimon 2001). The
performance of HELP was compared with that of Vadose/W (Chammas et al. 1999)
and UNSAT-H (Khire et al. 1997) in Alternative Cover Assessment Program (ACAP)
and the Alternative Landfill Cover Demonstration (ALCD) project (Khire et al. 1997).
It was found that percolation rates were over-predicted by HELP in comparison with
UNSAT-H (Khire et al. 1997). Hauser and Gimon (2001) compared HELP, HYDRUS
and UNSAT-H and found that UNSAT-H and HYDRUS were more robust and
accurate than HELP for phytocaps. Another study by Scanlon et al. (2002) compared
HELP, HYDRUS, SHAW (Albright et al. 2002), Vadose/W, SWIM (Dwyer 2003)
and UNSAT-H to model site water-balance from covers in semi-arid Texas, New
Mexico, and Idaho, USA, over a period ranging from one to three years. Scanlon et al.
(2002) concluded that models employing Richard’s equation predicted water balance
more accurately than the HELP model. Other models such as MACRO (Johnson et al.
2001) were not as robust as HYDRUS and TOUGH-2 (Albright et al. 2002), and
Vadose/W did not effectively predict drainage (Albright et al. 2002, Benson et al.
2004). A few efficient models like LEACHM, Model for Effluent Disposal using
Land Irrigation (MEDLI) (Tillman and Surapaneni 2002) and WATLOAD have not
been used in landfill studies to date. Amongst all the above models, it appears that the
UNSAT-H and HYDRUS predicted drainage effectively (Hauser and Gimon 2001,
Benson et al. 2004, Albright et al. 2002, Scanlon et al. 2002). Comparison of various
models in terms of their use, and various parameters used is shown in Table 1.
4
5
Table 1: Comparison of different models used in predicting site water balance.
Model
Acronym
Name
Developed by
Application
Plant Growth
Transpiration
Solute
Transport
Water Retention
Method
Reference
EPIC
Erosion-Productivity Impact
Calculator
Texas A & M
Agriculture
Yes
Yes
No
Water Balance
Williams (2005)
HELP
The Hydrologic Evaluation of
Landfill Performance
U.S. Army Engineer Waterways
Experiment Station
Landfills
No
No
Yes
Water Balance
Scanlon et al (2002)
TOUGH-2
Transport Of Unsaturated
Groundwater and Heat
Nuclear Waste
No
No
Yes
Richards Equation
Albright et al (2002)
MACRO
Swedish University of Agr. Sc.
Soil water Balance
Yes
Yes
Yes
Richards Equation
Johnson et al (2001)
UNSAT-H
U. S. Dept of Energy
Landfills
Yes
Yes
No
Richards Equation
Albright et al (2002)
HYDRUS 1D/2D
P.C Progress SRO and Simunek et al
(2005)
Landfills
Yes
Yes
Yes
Richards Equation
Albright et al (2002)
LEACHM
The Leaching Estimation And
Chemistry Model
Flinders University
Agriculture
Yes
Yes
Yes
Richards Equation
Albright et al (2002)
SWIM
Soil Water Balance and Infiltration
Model
Ross, 1998
Landfills
Yes
Yes
Yes
Richards Equation
Dwyer (2003)
MEDLI
Model for effluent disposal using
land irrigation
DNR & DPI, Forestry
Piggeries, Sewage
Treatment Plants
Yes
Yes
Yes
Tillman and Surapaneni 2002
WATLOAD
CSIRO
Vegetation Management,
Effluent Disposal
Yes
Yes
Yes
Myers et al. 1999
STOMP
Subsurface Transport Over Multiple
Phase
U. S. Dept of Energy
Nuclear Waste
Yes
Yes
Yes
Oostrom et al. 2004
Vadose/W
Geo-slope
Agriculture
Yes
Yes
No
Benson et al (2004)
SHAW
Simulation of Heat and Water
USDA Agr. Rech Centre
Landfills
Yes
Yes
No
Richards Equation
Albright et al (2002)
6
With all the contradictions and inaccuracies in the models used so far, the Subsurface
Transport over Multiple Phase (STOMP) (Oostrom et al. 2004) which takes into account
gaseous, aqueous and solid phase in one single model was also trialled during the study.
Due its complexity, however, HYDRUS 1D a model that can simulate water, heat and
solute movement in the saturated zone (Simunek et al. 2005) was used. HYDRUS 1D
has been extensively used in site water balance studies and is continuing to become
popular amongst hydrologists and environmental engineers. This software is a finite
element solution to Richard’s equation for one dimensional flow in variably saturated
media (Simunek et al. 2005).
Materials and Methods
Details of establishing the Phytocapping trial are provided in Venkatraman and
Ashwath (2007). Firstly an experimental site of 5000 m2 was selected for the trial
consisting of two soil thickness (thick cap: 1400 mm soil; thin cap: 700 mm soil),
replicated twice with 21 tree species (18 seedlings of each) (Fig 3). The experimental
site was mulched with shredded green waste (100 mm deep), and the plants were drip
irrigated. Various plant (plant growth, transpiration, canopy interception etc.) and soil
parameters (soil compaction, hydraulic conductivity) were monitored over two and a
half years, and the site water balance was predicted using HYDRUS 1D (Benson 2002,
Albright et al. 2004). Climate data was acquired from both the Bureau of Meteorology
(BOM) and the on-site weather station. The species were initially observed for their
growth and survival in the landfill environment followed by a study on canopy
interception, transpiration rate, biomass, root depth, mineral composition of plant and
soil hydraulic properties, but only the results of modelling are presented in this chapter.
Figure 3: Establishment of the Phytocapping trial at the Lakes Creek landfill, Rockhampton. above:
Placement of thin and thick soil caps over the waste, below: seedlings of 21 tree species were established
on each of these soil treatments.
Modelling
HYDRUS 1D uses soil hydraulic parameters and various tree parameters such as
transpiration rate and root depth, including climate data (rainfall and evaporation).
Water balance was water predicted for two scenarios, without vegetation and with
vegetation. Various plant and soil parameters required for the modelling were measured
during the study. Canopy rainfall interception was measured for 50 rainfall events over
7
two years. Transpiration was determined using Thermal Dissipation Probes (TDP) and
Dynagauges. Root depth was measured during biomass estimation by excavation
method. Soil hydraulic parameters were taken from the studies conducted by Dr Ian
Phillips (Griffith University) and mulch hydraulic parameters were obtained from
Findeling et al. (2007). Precipitation and evaporation data were obtained from the BOM
and the weather station located at the landfill site. Final simulations were completed
using the average values obtained for the selected ten tree species grown in the
phytocapping system.
Before running the model, canopy interception (32%) was deducted from the actual
rainfall data for the experimental the site. Irrigation values were added to the rainfall
data, and the rate of soil evaporation was taken as 50% of that of un-vegetated site
(worst case scenario), as the soil evaporation under agroforestry (Albright et al. 2002)
systems will be much less than that under a tree canopy (reduced by 23% - 40%;
Wallace et al. 2000, Jackson and Wallace 2000). Merta et al. (2006) found that the soil
evaporation under agricultural crops was considerably low under high LAI. For
example, the soil evaporation was 50% at a leaf area index (LAI) of 1.5 in comparison
with 5% for denser crops (LAI>3). Based on these data, soil evaporation was taken as
50% of that reported by the BOM. LAI recorded in 19 different species during the
study ranged 1.9 to 2.5. Based on these data, soil evaporation was taken as 50% of that
reported by the BOM as the worst case scenario.
HYDRUS 1D simulated surface soil storage and drainage of the phytocaps. The site
water balance was simulated for 15 years (1992 to 2006). Results from simulations
using 15 years of data are presented in the current chapter. Since the established species
grew at different rates, with some species growing slower than the others, transpiration
data of only the 10 best performed tree species (Dendrocalamus latiflorus, Casuarina
cunninghamiana, Acacia mangium, Hibiscus tiliaceaus, Eucalyptus grandis, Syzigium
australis, Acacia harpophylla var hillii, Ficus recemosa, Ficus mocrocarpa and
Eucalyptus raveretiana) in terms of canopy rainfall interception and transpiration were
used in the simulation. These selected species can be used in the Central Queensland
Region, as they perform well in a landfill environment and resilient to drought and fire
both of which are very common in the landfills.
Results and Discussion
Scenario 1: Percolation simulated for the thick cover (1400 mm) and the thin cover
(700 mm) without vegetation was 133.3 mm yr-1 and mm 153 mm yr-1 respectively (Fig
4). This difference in percolation rates between the two covers was expected, as the soil
depth plays a vital role in retaining maximum amount of water (Warren et al. 1996).
The thick cover could hold moisture up to 660 mm in comparison with 350 mm by the
thin cover (Fig 4). Surface runoff predicted for this site was infinitesimally small in both
covers (ranging from 0.20 mm to 4 mm) (Fig 4) due to flat surface and most
importantly, due to the presence of 100 mm of mulch layer and a thick layer of litter fall
under some tree species.
8
Thin Cap Thick Cap
Figure 4: Simulated storage capacity of soil (top), cumulative runoff (middle) and percolation of water
(bottom) in thin (left) and thick (right) covers, respectively in the absence of vegetation (cumulative of 15
years data; 1992 to 2006).
Scenario 2: In this scenario, percolation was simulated using the same parameters as in
scenario 1, but an additional component, vegetation was introduced. An average
transpiration of 1.5 mm day-1 was used, which was the average measured value from the
top 10 selected tree species grown on the experimental site. The average rain intercepted
by the 10 tree species was 32% and therefore, the incident rain was reduced by 32% and
this corrected rainfall was used in the simulation. The HYDRUS 1D simulation for the
vegetated site showed a percolation of 16.7 mm yr-1 for the thick cover and 23.8 mm yr-
1 for the thin cover (Fig 5). The 15 year rainfall data also included very dry periods and
very wet periods (300 mm rain in three consecutive days in 2003). The percolation for
vegetated covers was 8 to 10 times less than that simulated for non-vegetated sites. This
clearly demonstrates the role played by the vegetation in phytocapping. Benson et al.
(2004) has demonstrated the significance of vegetation in site water balance,
particularly their role in soil moisture depletion and the relationships between the root
depth and the soil moisture depletion. The soil storage capacity of the phytocaps was
reduced from 350 mm (scenario 1, without vegetation) to 320 mm (scenario 2; with
vegetation) in thin cover and from 660 mm to 570 mm in thick cover (Fig 4). This is
due to influence of tree roots on soil structure (Glinski and Lipeic 1990), bulk density
(Kalman et al. 1996) and pore size (Johnson et al. 2003) which in turn affects the water
retention property of the soil (Auge et al. 2001), as does the spatial variability (Shouse
9
et al. 1995). Roots of trees can perforate tough soil layers thereby creating macropores
(Lal et al. 1979, Glinski and Lipeic 1990) and allowing free water movement. The
surface runoff decreased in the thick cover but slightly increased in thin cover (Fig 4
and 5) in scenario 2 compared to scenario 1. The decrease in surface runoff can be
attributed to the increased water uptake by tree (Freebairn et al. 1986) thus creating
more space for water storage. The slight increase in surface runoff in thin cap may be
due to soil saturation (Liang and Xie 2001) and when rainfall rate exceeds the
infiltration capacity (Fiedler et al. 2002).
Thin Cap Thick Cap
Figure 5: Simulated storage capacity, runoff and percolation in the thick and thin covers in the presence
of vegetation (15 years data 1992 to 2006).
Results from the above simulation suggest that phytocaps are very effective in reducing
percolation of water into waste. In this simulation, establishment of 10 selected tree
species using 1400 mm layer of unconsolidated soil will allow a percolation of 251 mm
in 15 years. This is equivalent to 16.7 mm yr-1. This value is significantly lower than the
percolation rate expected for a clay cover (c. 10%; Geoff Thompson; pers. Comm.).
Comparing figures 4 and 5, it is clearly evident that the reduced percolation was due to
the presence of deep rooted tree species.
10
A comparison of the above results with those reported for the ACAP sites (Benson et al.
2002) suggest that the percolation estimated for the phytocaps in this research are within
the values reported for alternative covers (12 to128 mm yr-1) in the USA (Benson 2002).
For example, the percolation rates at the ACAP site at Omaha, Nebraska, is comparable
to Rockhampton site in terms of rainfall (760 mm yr-1), and the measured percolation
(using lysimeters) at Omaha was 60 mm yr-1. A comparison of the features of this site
and the Rockhampton site is shown in Table 2.
Table 2: Comparison of Rockhampton and Omaha site with regards to percolation of water.
Parameters
Rockhampton, Qld, Australia
Omaha, NE, USA
Rainfall
780 mm yr-1
760 mm yr-1
Soil thickness
1400 or 700 mm
1100 mm
Species grown
Trees
Grasses
Drainage
16.7 – 23.8 mm yr-1
60 mm yr-1
The rainfall pattern at the Rockhampton site matches with the rainfall distribution at the
Omaha site. The simulated percolation rate at Rockhampton ranges between 16.7 mm
yr-1 to 23.8 mm yr-1 for thick and thin covers respectively. This rate therefore is much
lower than the percolation rates reported for the Omaha site. The currently simulated
percolation rates are also significantly lower than those expected for clay cover (78 mm
at c. 10% of the rainfall; Geoff Thompson; pers. Comm.).
Conclusion
Percolation rates estimated using HYDRUS 1D were 16.7 to 23.8 mm yr-1 for thick and
thin covers respectively. The predicted percolation rate for the Rockhampton site is
much lower than that expected from well constructed and maintained clay capped
landfill (which is equivalent to 78 mm in Rockhampton; at 10% of the incident rain).
This shows the better or equivalent ability of the phytocapping system to limit entry of
water into the landfill (c. 50% of clay cap). The reduced cost of establishing phytocaps
on landfills (compared to clay caps) show possibly the superiority of phytocaps over
clay caps. Economical evaluation of phytocaps against clay caps is underway and will be
reported shortly.
Evidences show that the thick and thin phytocovers perform well in maintaining a low
percolation rate (less than 10% of the received rainfall). However, in events of heavy
rainfall, thick cover has better moisture retaining capacity than thin cover. These trends
and the consistency of the results amongst the thick and thin phytocovers clearly support
the recommendation of the phytocapping technique for landfill remediation in many
parts of Australia, especially in the drier regions of Queensland.
11
Acknowledgements
This research was funded by the Rockhampton Regional Council (RCC) via Phytolink
Australia Pty Ltd., and proudly supported by the Queensland Government’s Growing
the Smart State PhD Funding Program, and may be used to assist public policy
development.
We are grateful to Mr. Craig Dunglison (RRC), Mr. Richard Yeates (Phytolink
Australia Pty Ltd), Professor David Midmore, Central Queensland University (CQU),
Dr Ram Dalal, Department of Natural Resources and Water, Dr. Jirka Simunek
(University of California Riverside), Dr. Bill Albright (Desert Research Institute,
Nevada, Mr. Roshan Subedi (CQU) and many others who assisted with this project in
various ways.
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