Waste disposal has always been an important issue for human soci-eties (Kato, 1986). Solid wastes are disposed either on or below the land’s surface, resulting in potential sources of groundwater con-tamination. One of the most common waste disposal methods is landfilling. Landfilling is a controlled method for disposing of solid wastes on land, with the dual purpose of eliminating public health and environmental hazards, while minimizing nuisance, without contaminating surface or subsurface water resources. A municipal solid waste (MSW) landfill is not a benign repository of discarded material; it is a biochemically active unit, where toxic substances are leached or created from combinations of non-toxic precursors, and gradually released into the surrounding environ-ment over a period of decades (Papadopoulou et al., 2006). Biological, chemical and physical processes within landfills pro-mote the degradation of waste and result in the production of lea-chates and gases. Landfill leachate is one of the most recalcitrant wastes for bio-treatment and can be considered as a potential source of contamination to surface and groundwater ecosystems (Karaca & Özkaya, 2006; Mohajeri et al., 2011; Mor et al., 2006).
In modern landfills, waste is contained using a liner system. The primary purpose of the liner system, is to isolate the landfill’s contents from the environment and, therefore, to protect the soil
and groundwater from pollution originating in the landfill (Alslaibi et al., 2010). The greatest threat posed by modern landfills to groundwater, is leachate. Leachates consist of water and water-soluble compounds in refuse that accumulate as the water moves through the landfill. This water may originate from rainfall or from the waste itself (Hubé et al., 2011; Hughes et al., 2008). Leachates contain a host of toxic and carcinogenic chemi-cals, which may cause harm to both humans and the environment (Alslaibi et al., 2011; Laner et al., 2011; Singh et al., 2010).
Factors that affect leachate generation are climate (rainfall), topography (run-on/runoff), landfill cover, vegetation and type of waste (Jaber & Nassar, 2007). The main objective of this research
Quantification of leachate discharged to groundwater using the water balance method and the Hydrologic Evaluation of Landfill Performance (HELP) model
Tamer M Alslaibi1, Ismail Abustan1, Yunes K Mogheir2 and Samir Afifi3
AbstractLandfills are a source of groundwater pollution in Gaza Strip. This study focused on Deir Al Balah landfill, which is a unique sanitary landfill site in Gaza Strip (i.e. it has a lining system and a leachate recirculation system). The objective of this article is to assess the generated leachate quantity and percolation to the groundwater aquifer at a specific site, using the approaches of (i) the Hydrologic Evaluation of Landfill Performance model (HELP) and (ii) the water balance method (WBM). The results show that when using the HELP model, the average volume of leachate discharged from Deir Al Balah landfill during the period 1997 to 2007 was around, 6800 m3/year. Meanwhile, the average volume of leachate percolated through the clay layer was 550 m3/year, which represents around 8% of the generated leachate. Meanwhile, the WBM indicated that the average volume of leachate discharged from Deir Al Balah landfill during the same period was around 7660 m3/year—about half of which comes from the moisture content of the waste, while the remainder comes from the infiltration of precipitation and re-circulated leachate. Therefore, the estimated quantity of leachate to groundwater by these two methods was very close. However, compared with the measured leachate quantity, these results were overestimated and indicated a dangerous threat to the groundwater aquifer, as there was no separation between municipal, hazardous and industrial wastes, in the area.
KeywordsGroundwater pollution, HELP model, landfill, leachate quantity, water balance method (WBM)
1 School of Civil Engineering, Universiti Sains Malaysia, Penang, Malaysia
2 Environmental Engineering, Islamic University of Gaza, Gaza, Palestine
3 Environmental and Earth Sciences, Islamic University of Gaza, Gaza, Palestine
Corresponding author:Ismail Abustan, School of Civil Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia. Email: [email protected]
465462WMR31110.1177/0734242X12465462Waste Management & ResearchAlslaibi et al.2013
is to assess the generated leachate quantity and percolation to groundwater aquifers in a semi-arid site, such as Deir Al Balah landfill, using the Hydrologic Evaluation of Landfill Performance model (HELP) and the water balance method (WBM).
MethodsHELP Model
The HELP model (version 3.07) is the most widely used tool by the United States Environmental Protection Agency (US EPA) to predict leachate quantity and analyse water balance in landfill lining and capping systems. It is a quasi-two-dimensional hydro-logic model of water movement across, into, through and out-of landfills. HELP generates estimations of runoff amounts, evapo-transpiration, drainage, leachate production and leakage from lin-ers. The HELP model was developed to help hazardous waste landfill designers and regulators to evaluate the hydrologic per-formance of proposed landfill designs.
Although the HELP model usually overestimates the actual leachate quantity, a great number of studies were done for esti-mate of leachate quantity and potential percolation into the subsurface used the HELP model in recent years (Alslaibi, 2009; Bou-Zeid & El-Fadel, 2004; Qrenawi, 2006; Yalçin, 2002).
The model accepts weather, soil and design data, and uses solution techniques that account for the effects of surface storage, snowmelt, runoff, infiltration, evapotranspiration, vegetative growth, soil moisture storage, lateral subsurface drainage, lea-chate recirculation, unsaturated vertical drainage; and leakage through soil, geo-membrane or composite liners. Landfill sys-tems, including various combinations of vegetation, cover soils, waste cells, lateral drain layers, low permeability barrier soils and synthetic geo-membrane liners, may be modelled. The program was developed to conduct water balance analyses of landfills, cover systems and solid waste disposal and containment facilities (Schroeder et al., 1994). The primary purpose of the model is to
assist in the comparison between design alternatives, as judged by their water balances. The model, which is applicable to open, partially closed and fully closed sites, is a tool for both designers and permit writers (Schroeder et al., 1994).
The HELP model uses many process descriptions that were previously developed and reported in the literature and used in other hydrologic models (Alslaibi, 2009; Berger, 2000; Nyhan et al., 1997; Schroeder et al., 1994). For example, runoff model-ling is based on the Soil Conservation Service (SCS) curve num-ber method (Mack, 1995). Potential evapotranspiration is modelled using the modified Penman method (Schroeder et al., 1994). Evaporation of interception and surface water is based on the energy balance method, and interception is modelled by a method proposed by Horton (Berger et al., 1996). Vertical drain-age is modelled by Darcy’s law and saturated lateral drainage is modelled by an analytical approximation to the steady state solution of the Boussinesq equation (Yalçin & Demirer, 2002). Evaporation from soil, plant transpiration and vegetative growth were extracted and modelled using the methods included in the Simulator for Water Resources in Rural Basins (SWRRB) model (Qrenawi, 2006).
These processes are linked in a sequential order, starting at the surface with a surface water balance, then evapotranspiration from the soil’s profile and, finally, drainage and water routing, starting at the surface with infiltration, proceeding downward through the landfill profile, to the bottom. The solution procedure is applied daily, as it simulates the water routing throughout the simulation period (Schroeder et al., 1994). The model accepts weather, soil and design data, as shown in Table 1.
WBM
This method is simple and has been used to predict the generated leachate within landfills (São Mateus et al., 2011). The basic con-figuration of this method is that the landfill consists of a covered surface, a compacted waste compartment, and a lining system, as shown in Figure 1.
Table 1. Input data required by the Hydrologic Evaluation of Landfill Performance (HELP) model (PMO, 2008; GTZ, 2002).
Data type Parameter Unit Input value
Weather data Evaporative zone depth cm 60 Maximum leaf area index – 3.5 Relative humidity % Seasonally Average wind speed km/hr 10.92 Rainfall data mm Daily Temperature data °C Daily Solar radiation MJ/m2 DailyLandfill characteristics Landfill area Hectares 15 % of landfill where runoff is possible % – Runoff curve number – 81.3Soil and solid waste data Layer type and texture – – Layer thickness cm – Hydraulic conductivity cm/sec – Porosity, moisture content, field capacity and wilting point vol./vol. – Recycling ratio % 40
The water balance of the landfill was derived; making use of assumptions in instances where it is applicable that infiltration through the top of the waste pile is calculated using equation (1).
I P J R R AET Uon off s= + + − − ±
where:
I: Infiltration (mm/year)P: Precipitation (mm/year)J: Leachate recirculation (mm/year)Roff: Runoff (mm/year)Ron: Run-on (mm/year)AET: Actual evapotranspiration (mm/year)Us: Water content in soil cover (mm/year)
assuming that:
1. The final soil cover is existent and the moisture content of the daily thin layers of soil is assumed to be at field capacity, and is assumed to not contribute significantly to the total moisture content of the cells (Us=0)
2. The landfill has been designed so that water from outside the site does not enter (Ron = 0).
Therefore, infiltration (I) through the top part of the waste pile becomes:
I P J R AEToff= + − −
Where the change in waste water volume, due to external sources (PL), is computed as:
L gP I I= +
where Ig: is the water from the aquifers entering the landfill (mm/year).
Assuming that water entering the landfill from aquifers is neg-ligible (Ig = 0), the change in waste water volume, due to external sources (PL), is computed as:
LP I=
Then, the total leachate production is computed as:
L P U bL w= ± +
where b is water production by the biodegradation of waste (m3/year) and Uw is the water content in waste (at field capacity) (m3/year).
The water produced, due to the biodegradation of waste, is assumed to be very small and negligible (b = 0). Therefore:
L P UL w= ±
It is worth noting that water percolating through from the sur-face of a landfill tends to be absorbed by the waste until field capac-ity is reached. It is only when the infiltration of water exceeds this value that movement of water through the waste occurs; initially under unsaturated conditions or, if sufficient water is present, under saturated conditions. The WBM steps are summarized in Table 2.
Study area
Gaza Strip is situated on the south-west of coast of Palestine. It is bordered by Egypt to the south, the Negev desert to the east, and
Figure 1. Hydrologic balance of landfill. (Reprodued from Jagloo, 2002 with permission).AET: actual evapotranspiration; b: water production by biodegradation of waste; Ig: water from underground; J: leachate recirculation; L: leachate generated; Lc: collected leachate; LI: leachate infiltration in clay liner; P: precipitation; Roff: runoff; Ron: run-on; S: water in sludge; Uw: water content in wastes; Us: water content is soil cover; Wg: water consumed in the formation of landfill gas;Wv: water lost as water vapour.
Table 2. Steps of the water balance method.
Step 1 Input values for evapotranspiration (ET) and precipitation (P)
Step 2 Calculate runoff Roff = CRO × PStep 3 Calculate flux – movement of water Flux = P – Roff – AET If flux has a negative value (-ve up): water is
evaporating from wastes If flux has a positive value (+ve down): water is
infiltrating the wastesStep 4 Calculate STORE = AW + FluxStep 5 Determine AW: If STORE > maximum storage capacity
(FC), Then, AW = maximum storage capacity Otherwise, AW = STORE or AW = 0 (if STORE = 0)Step 6 Determine PERC IF STORE > maximum storage capacity PERC = STORE – maximum storage capacity Otherwise PERC = 0 Note: If PERC has a positive value (+ve): leachate
formed If PERC has a negative (-ve): moisture deficit
AET: actual evapotranspiration; PERC: percolated leachate
(1)
(2)
(3)
(4)
(5)
(6)
Alslaibi et al. 53
green line to the north. Following the Oslo agreement, three con-trolled landfills were constructed on Gaza Strip, namely the Gaza landfill, located in Gaza Governorate; Deir Al Balah landfill, located in Medal Area Governorate; and Rafah landfill, located in the Rafah Governorate. This research will concentrate on Deir Al Balah landfill, as presented in Figure 2.
The total area of the Gaza Governorates is 365 km2; it is 40 km long with an average width of 7–12 km. The estimated popu-lation is 1.5 million inhabitants, which means the area is highly populated.
Gaza Strip area is classified as semi-arid, as the average annual rainfall is about 351.4 mm/yr, whereas the average annual rainfall in Deir Al Balah is about 322 mm/yr (Alslaibi & Mogheir, 2007). The highest mean annual temperature is 30.8°C, while the lowest is 14.2°C. The average annual wind speed is about 10.92 km/hr. In a semi-arid region, like Gaza Strip, it is expected that the relative humidity is high in the summer (72%) and low in the winter (61%). This may be because the evapora-tion rate in the summer is higher than that in the winter and, hence, the relative humidity values are expected to be higher. Estimated annual solid waste generated in Gaza Strip is around 603,000 ton/year. Most of the generated solid waste amount is household waste and is buried in Gaza, Deir Al Balah and Rafah landfills, which were 450,000, 90,000 and 63,000 ton/ year respectively (Alslaibi, 2009). The generation rate of solid waste in Gaza strip is around 1.1 kg capita/day and the composition of municipal solid waste is organic matter (70%), paper (9%), plas-tic (8%), glass (5%), metals (3%) and others (3%) (Jaber & Nassar, 2007). In Gaza Strip there is no separation between municipal, hazardous and industrial waste collection system. Furthermore, there is no a special cell constructed and designed
in the landfill sites for the disposal of hazardous waste from hos-pitals, clinics and expired medical wastes (UNEP, 2003; Jaber & Nassar, 2007).
Deir Al Balah landfill site has an area of 60,000 m2, which translated to average dimensions of 400 m in length and 150 m in width, and an average height of 17 m. The landfill area consists of two leachate ponds, scale house, warehouse and screening plant. Although Deir Al Balah landfill is overloaded and the lifes-pan is expired, some random extensions were made to disposal of the waste generated from the heavily populated area. The nearest residential area is about 4 km away. The landfill receives about 90,000 tons of municipal, hazardous and industrial waste annu-ally, of which more than 60% is food waste (UNEP, 2003). Many types of activities in Gaza Strip could potentially generate haz-ardous waste, including hospitals, clinics and research laborato-ries, whereas industrial waste could arise from batteries factories, textile factories and photographic processing centres (Abdalqader, 2011). Deir Al Balah landfill is located 8 km from the shore line. The soil under the landfill is sandy and silty clay, and the depth of groundwater is around 60 m (Alslaibi, 2009; GTZ, 2002). The layout and the cross section of the Deir Al Balah landfill site are shown in Figure 3. The cross-section consists of six layers, which are a sandy cover layer, a waste layer, an aggregate layer, an asphalt layer, a base coarse layer and a clay layer. The lining sys-tem at Dear Al Balah landfill consists of an aggregate layer, an asphalt layer and a base coarse layer.
Leachate is collected by gravity through the drainage system into leachate ponds. Approximately 40% of the collected lea-chate is re-circulated by spraying over the top covering layer. The characteristics of the landfill leachate, which was operated in 1996, for pH, electrical conductivity (EC), NH4, chemical oxy-gen demand (COD), biological oxygen demand and total organic carbon were 8.3, 32,200 μ.s/cm, 3473 mg/l, 46,500 mg/l, 8000 mg/l and 15,600 mg/l respectively (Alslaibi, 2009).
Results and discussion
Leachate water quantity was quantified using HELP and WBM in Deir Al Balah landfill.
The HELP model was run using 11 years of daily climatic data for Deir Al Balah site (between 1997 and 2007). This period was chosen for model simulation because the measured data of lea-chate was available for this period to validate the reliability of the model to simulate measured data and calculate the percentage of error. After 2007 lack of measured data occurred owing to the absence of a leachate measuring device. The landfill was simu-lated using six layers (from the bottom to the top), namely a clay layer, a base coarse layer, an asphalt layer, an aggregate layer, a compacted solid waste layer and a soil cover layer (sandy soil), as shown in Figure 3. Approximately 40% of the collected leachate is recycled, via the soil cover layer and is used in the simulation.
Figure 4 presents the annual rates of precipitation and leachate volume generated at the asphalt layer, and percolated through the clay layer, at Deir Al Balah landfill, as estimated by the HELP
Figure 2. Landfill location of the study area.
54 Waste Management & Research 31(1)
model. The average annual leachate volume, generated at Deir Al Balah landfill, for the simulation period (1997–2007) was 6800 m3, which represents 35.2% of the total precipitation (322 mm × 60,000 m2) as shown in Table 3, while the average annual lea-chate volume percolated through the clay layer was 550 m3. This represents approximately 8% of the generated leachate.
Figure 5 shows the cumulative annual leachate volume gener-ated at the barrier layer and the cumulative quantity of percolated leachate through the clay layer during the study period of simula-tion. The cumulative annual leachate volume generated was
74,800 m3, while the cumulative annual leachate volume perco-lated through the clay layer was 6050 m3.
From Table 3 it can be observed that the major component of the water budget is the evapotranspiration with a yearly average of 186.37 mm and accounting for 57.87% of rainfall because the Gaza strip is classified as a semi-arid region. The average surface runoff accounts for 6.93%, while the remaining 35.2% are accounting for the average leakage/leachate discharged.
The WBM was used to estimate the quantity of leachate water during the same study period (1997–2007), as shown in Table 4.
Figure 3. Plan and cross section of Deir Al Balah landfill site. Reproduced from GTZ (German Technical Cooperation), 2002. Annual report of solid waste management – Gaza middle area – Landfill design with permission from GTZ.
Figure 4. Annual leachate volume generated and percolated at Deir Al Balah Landfill estimated by the HELP model.
Alslaibi et al. 55
The annual leachate volume, generated at the barrier layer (asphalt layer), is plotted in Figure 6 for Deir Al Balah landfill. The average annual leachate volume generated at Deir Al Balah landfill without recycling, but with 40% recycling over the simu-lation period (1997–2007), was very close and had values of 7360 and 7663 m3 respectively. The cumulative annual leachate volume was 73,345 m3, as shown in Figure 7.
The cumulative annual leachate quantity came from three sources, namely precipitation, and moisture content of both the waste and the re-circulated leachate. This classification of leachate sources for Deir Al Balah landfill is plotted in Figure 7. The figure clearly shows that about half of the leachate quantity comes from waste moisture content, while the remainder comes from the infiltration of precipitation and re-circulated leachate.
Figure 8 represents the estimated cumulative leachate quantity using the WBM with 40% of recycling and the HELP model, and the measured leachate quantity at the site. The two methods used offered close results during the study period. Missing measurements of the leachate quantities were identified during the first four years. Figure 8 shows that the measured leachate volume was 50% less than the estimated leachate volume, using the same study methods, between 2001 and 2004. However, during the last three years of the study period (i.e. 2005–2007) the estimated quantities, using the study methods and the measured leachate volume, were very close.
According to the results, there is a gap between the estimated and measured leachate volumes at Deir Al Balah site, as shown in Figure 8. This gap may be because:
1. The HELP model tends to overestimate the predicted quantity of leachates generated, as verified in the case study presented
2. There is a quantity of leachate that percolates through the lin-ing system to the groundwater. This was estimated using the HELP model, as shown in Figure 5
3. There is an error in the measured leachate volume. This error is caused by the absence of a leachate measuring device. Apparently, landfill administration at Deir Al Balah reverted to quantifying leachate volume using primitive techniques, such as a mathematical calculations, which were used when the measuring device shut down and it depended on the human observation of the quantity of leachate in the leachate pond, the number of pumping hours of leachate recirculates to the landfill and the pumping rate
4. There is an accumulated quantity of leachate that was absorbed inside the landfill to reach the stabilization stage.
Therefore, leachate volume data obtained from the landfill administration are lower than the actual amounts. However, the gap between measured leachate volume and estimated using the HELP model and the WBM decreased as the landfill reached its stabilization level in 2007 (end year of expected lifespan) and, therefore, the deference became irrelevant during this year. In addition, an increase in precipitation in years 2006 and 2007 may affect to increase the measured line of leachate.
Groundwater samples from 9 wells located downstream of landfill within 500 m radius circle area were collected during dry season in November 2008 and compared with the groundwater samples before landfill constructed to study the possible effect of leachate percolation into groundwater. Several physical and chemical parameters were tested in groundwater samples; these include pH, EC, NO3, Cl, NH4 and COD. The average concentra-tion of these parameters were 7.82, 2765, 80, 595, 7.34 and 291 mg/l, respectively, while the average concentration of pH, EC, NO3 and Cl before landfill constructed were 7.3, 1540, 45, 235,
Table 3. Average annual totals for years 1997 through 2007.
Figure 5. Cumulative annual leachate volume generated at Deir Al Balah landfill, estimated by the HELP model.
56 Waste Management & Research 31(1)
Tabl
e 4.
Wat
er b
alan
ce m
etho
d ca
lcul
atio
ns o
f Dei
r A
l Bal
ah la
ndfil
l.
Year
W (k
g) *
10
3C
W (k
g)M
C
(m3 )
V (m
3 )C
V (m
3 )V
c (m
3 )A
(m2 )
P
(mm
)R
(m
m)
EL1
(mm
)I (m
m)
I (m
3 )R
ec.
(m3 )
ET2
(mm
)ET
2 (m
3 )R
ec. n
et
(m3 )
TMC
(m
3 )AW
C
(m3 )
AWS
(m3 )
MD
(m
3 )C
L
(m3 )
L (m
3 )
1997
77,0
0077
,000
15,4
0096
,250
96,2
5010
5,87
535
,000
315
5320
457
1982
2025
428
14,9
680
15,4
0017
,382
12,3
20–
5062
5062
1998
88,0
0016
5,00
017
,600
110,
000
206,
250
226,
875
35,0
0021
737
141
3913
6439
7842
915
,008
034
,982
36,3
4626
,400
–99
4648
8419
9988
,000
253,
000
17,6
0011
0,00
031
6,25
034
7,87
535
,000
132
2286
2483
257
1940
914
,314
053
,946
54,7
7840
,480
–14
,298
4352
2000
95,0
0034
8,00
019
,000
118,
750
435,
000
478,
500
35,0
0025
543
166
4616
0778
8242
414
,832
073
,778
75,3
8455
,680
–19
,704
5407
2001
90,7
0043
8,70
018
,140
113,
375
548,
375
603,
213
35,0
0055
093
357
9934
6210
,718
428
14,9
680
93,5
2496
,986
70,1
92–
26,7
9470
9020
0288
,000
526,
700
17,6
0011
0,00
065
8,37
572
4,21
360
,000
391
6625
470
4218
13,8
1342
925
,727
011
4,58
611
8,80
484
,272
–34
,532
7738
2003
84,0
0061
0,70
016
,800
105,
000
763,
375
839,
713
60,0
0037
363
242
6740
2416
,767
409
24,5
390
135,
604
139,
628
97,7
12–
41,9
1673
8420
0482
,000
692,
700
16,4
0010
2,50
086
5,87
595
2,46
360
,000
317
5420
657
3419
19,4
4643
426
,030
015
6,02
815
9,44
811
0,83
2–
48,6
1666
9920
0591
,000
783,
700
18,2
0011
3,75
097
9,62
51,
077,
588
60,0
0034
659
225
6237
3422
,396
431
25,8
740
177,
648
181,
381
125,
392
–55
,989
7374
2006
105,
000
888,
700
21,0
0013
1,25
01,
110,
875
1,22
1,96
360
,000
245
4215
944
2646
25,1
3444
026
,420
020
2,38
120
5,02
714
2,19
2–
62,8
3568
4620
0799
,000
987,
700
19,8
0012
3,75
01,
234,
625
1,35
8,08
860
,000
410
7026
774
4431
28,4
9144
026
,372
2118
224,
827
229,
259
158,
032
–73
,345
10,5
09
A: a
rea
cove
red
by w
aste
(m2 )
; AW
C: a
ctua
l wat
er c
onte
nt o
f sol
id w
aste
; AW
S: a
mou
nt o
f wat
er th
at c
an b
e he
ld in
sol
id w
aste
; CL:
cum
ulat
ive
leac
hate
; CV:
cum
ulat
ive
volu
me
of w
aste
dep
osite
d (m
3 );
CW
: cum
ulat
ive
wei
ght o
f was
te d
epos
ited
(kg)
; EL 1
: eva
pora
tion
loss
in r
ain
days
(mm
); EL
2: e
vapo
ratio
n lo
ss th
roug
h th
e ye
ar (m
m);
I: in
filtr
atio
n (m
m);
I: in
filtr
atio
n (m
3 ); I
Rec
: inf
iltra
tion
from
reci
rcul
a-tio
n; L
: lea
chat
e; M
C: m
oist
ure
cont
ent o
f was
te (m
3 ) 2
0% b
y m
ass
and ρ H
2O =
100
0 kg
/m3 (
valu
e ob
tain
ed fr
om th
e si
te);
MD
: moi
stur
e de
ficit;
P: p
reci
pita
tion
(mm
); R
: run
off (
mm
); R
ec.:
reci
rcul
atio
n;
TMC
: tot
al m
oist
ure
cont
ent (
m3 )
; V: v
olum
e of
was
te d
epos
ited
(m3 )
ρw
aste
= 8
00 k
g/m
3 ; W
: was
te d
epos
ited
(kg)
; Vc:
vol
ume
of w
aste
dep
osite
d +
cove
r vo
lum
e (m
3 ).
respectively, whereas NH4 and COD were below the detection limit (Alslaibi, 2009). The results showed that most of the wells were contaminated, where the concentration of most physical and chemical parameters were above acceptable standard levels required by local and international standards for potable and irri-gation water. It is quite evident that landfill presents potential threats to the surrounding environment.
Statistical analysis of the results (for the methods used and the measured data) it is helpful to compare and evaluate the degree of relationship between the results of the two methods and the measured data. The F-test was used to describe this relation. Because the F distribution describes the probabilities of obtain-ing specified ratios of sample variances drawn from the same population, an F-test can be used to check the equality of the vari-ances we obtain in statistical sampling (Davis, 2002). This func-tion is used to determine whether two samples have different variances. The HELP model and the WBM show a low ratio vari-ance of 1.22 for estimating the cumulative annual leachates and the P-value was high (0.808), while the HELP model and the WBM show relatively high variances with the measured data of cumulative annual leachates of 1.54 and 1.89, where the P-values were relatively low (0.613 and 0.457 respectively), as shown in Table 5. The high variance between the two methods and meas-ured data refer to the error of the measured data.
Furthermore, the percentage of error along simulation period (1997–2007) between the HELP model and the WBM was 3.94% for the estimated cumulative annual leachates, while the percent-age of error between the two methods and the measured data were 29.44% for the WBM and 33.04% for the HELP model, as shown in Table 6, as the HELP model tends to overestimate the predicted quantity of leachates generated (Fatta et al., 1999). However, the percentage of error in the last year (2007) between the HELP model and the WBM, the WBM and measured data, and the HELP model and measured data were reduced to 1.98%, 4.77% and 6.85%, respectively, as shown in Table 6.
ErrorMeasured Calculated
Measured(%) =
−×100
Sensitivity analysis is another tool that studies the variation (uncertainty) in the output of a mathematical model, and can be apportioned qualitatively or quantitatively, to different sources of variation in the input of a model (Breierova & Choudhari, 1996). The aim of sensitivity analysis is to present the sensitive param-eters that influence the results of the simulation process. Figure 9 shows that using the HELP model and the WBM, the relationship between precipitation and generated annual leachate volume at Deir Al Balah landfill during the simulation (i.e. 1997 to 2007) was very close. Furthermore, Figure 9 shows that the behaviour of the generated quantity of annual leachate follows the same annual rate of precipitation trend.
Assessing the effect of other landfill components, such as the existence of a lining system, the rainfall level, the landfill area, the existence of a recirculation system and waste depth, on perco-lated leachate to groundwater aquifer, which is 60 m from the
Alslaibi et al. 57
bottom of the landfill, using the HELP model were done else-where (Alslaibi, 2011) by changing one parameter while other parameters are fixed. The results showed that the landfill compo-nents were ordered in priority according to their effects on perco-lated leachate through clay layer as follows: (i) existence of a lining system enhances the percolation reduction up to 87%; (ii) 30% reduction of rainfall level enhances percolation reduction up
to 50%; (iii) a 50% reduction of existing landfill area enhances percolation reduction up to 50%; and (iv) the absence of a recir-culation system slightly enhances percolation reduction up to 2.5% more than with the availability of recirculation system. The waste depth has no significant effect on the quantity of percolated leachate. Analysis suggests that changes in the lining system type, rainfall level, landfill area and recirculation ratio have the most significant effect on model outputs indicating that these parameters should be selected carefully when similar modelling studies are performed.
Conclusion
The HELP model and the WBM were used as tools to assess the generated leachate quantity and percolation to groundwater aqui-fers at a specific site. The application of the HELP model showed
Table 5. F-test between the Hydrologic Evaluation of Landfill Performance (HELP) model, water balance method (WBM) and measured data of leachate.
Method comparison F-statistic P-value
WBM vs HELP 1.22 0.808Measured vs WBM 1.89 0.457Measured vs HELP 1.54 0.613
Figure 6. Annual leachate volume generated at Deir Al Balah landfill–Estimated using the Water Balance Method.
Figure 7. Cumulative Annual Leachate Volume Generated at Deir Al Balah Landfill, estimated using the water balance method (WBM).
Figure 8. Cumulative annual leachate volume generated at Deir Al Balah landfill.
58 Waste Management & Research 31(1)
that the annual leachate from the landfill base was 6800 m3/year, which represents 35.2% of the total precipitation (322 mm), while the annual evapotranspiration and runoff represent 57.87% and 6.93% of the total precipitation respectively. The average annual leachate volume percolated through the clay layer was 550 m3, which represents approximately 8% of the generated lea-chate (6800 m3/year). Meanwhile, the WBM showed that the annual leachate from the landfill base was 7663 m3/year.
FundingThis research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
AcknowledgementsThe authors acknowledge the Universiti Sains Malaysia (USM) for its financial support under the USM and TWAS Fellowship scheme and the research grant provided by the University Sains Malaysia under the Research University (RU) Scheme and e-Science Fund by MOSTI (03-01-05-SF0502).
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