Tai lieu 3 ( Mô Hình Lan Truyền Ô Nhiễm Trong Đất)

Từ kết quả nghiên cứu đáy các bãi rác cho thấy hầu hết các bãi rác chưa được xây dựng đúng tiêu chuẩn. Hệ số thấm của nền đất dưới các bãi rác khoảng 106 đến 104 cms chưa đạt yêu cầu kỹ thuật. Hầu hết các bãi rác đều gây ô nhiễm môi trường nước xung quanh và vượt ngưỡng yêu cầu so với quy chuẩn nước thải của bãi chôn lấp chất thải Mô hình lan truyền bằng thực nghiệm và Geoslope đều cho thấy tầm quan trọng của lớp đáy bãi rác, với độ chặt lớn, hệ số thấm nhỏ có khả năng kìm hãm và ngăn chặn được các chất ô nhiễm. Tuy nhiên nước thấm qua đất dung trọng 1,55 (gcm3); 1,6 (gcm3); 1,65 (gcm3) có nồng độ COD, chì và cadimi vẫn vượt ngưỡng cho phép. Nước thấm qua đất có dung trọng 1,7 (gcm3), đạt 98% độ chặt tiêu chuẩn có nồng độ COD đạt tiêu chuẩn so với quy chuẩn nước thải của bãi chôn lấp chất thải, tuy nhiên vẫn vượt ngưỡng so với tiêu chuẩn nước mặt và nước tưới tiêu, gấp 410 lần. Nồng độ chì, đồng và kẽm đạt tiêu chuẩn cho nước sinh hoạt và tưới tiêu. Nồng độ cadimi vượt ngưỡng so với tiêu chuẩn cho nước sinh hoạt. Kết quả mô phỏng sự lan truyền chất ô nhiễm theo chiều sâu dưới đáy bãi rác bằng Geoslope cho thấy với nền đất được đầm chặt đạt hệ số nén K98, hệ số thấm đạt khoảng k = 109 cms: thì chất ô nhiễm không bị phát tán hoặc phát tán với độ sâu rất nhỏ dưới 10m

Research Paper

AN INVESTIGATION OF THE CONTAMINANT TRANSPORT FROM THE WASTE

DISPOSAL SITE, USING FEMLAB

MD TAUHID-UR-RAHMAN

Department of Civil and Environmental Engineering, Shahjalal University of Science and Technology,

Sylhet-3114, Bangladesh

Phone: +88-0821-71349 extn-169; Fax: +88-0821-715257; E-mail: liton2005@gmail.com

Received: 4 th April 2009; Revised: 15 th May 2009; Accepted: 28 th July 2009

Abstract: Untreated landfill leachate produced typically at the unmanaged waste

disposal site may pose substantial contamination hazards not only to the nearby

surface water but also to the groundwater system This study attempts to focus on the

potential risk arising with the spreading of contaminant generated at the solid waste

disposal site in the lake Kiyanja watershed which has usually been considered as the

key source of supplying water to the surrounding community of the Masindi district of

Uganda A numerical model was developed with the help of FEMLAB to study the

transport of the leached contaminants from the waste disposal site through the

subsurface soil The model shows that the contamination plume needs around 40

hours to infiltrate through the saturated groundwater after being released from the

source Further, it needs at least 183 hours to reach close to the nearby lake water

Although, the deep groundwater currently seems to be protected, but it needs

adequate attention while installing deep wells through the clay layers to make it

remained safe for the future It can however be mentioned that to protect and preserve

the groundwater resources, the waste disposal site should immediately be relocated

elsewhere

Keywords: Numerical model, groundwater-pollution, leachate, unmanaged solid waste

INTRODUCTION

Urban landfill, due to its relatively lower maintenance cost, has popularly been choosing for disposing finally the solid waste produced at any city dwellers, over the decades [1] Well maintained and scientifically designed landfill could significantly reduce the solid waste burden of any urban community However, unmanaged solid waste usually disposed in this landfill has increasingly been becoming as one of the sources of potential environmental concern in most of the developing countries [1,2] Most of the pollution hazards that can likely be taken place at any

V o l u m e 4 , N u m b e r 2 : 7 9 - 8 8 , M a y - A u g u s t , 2 0 0 9

© T2009 Department of Environmental Engineering

S e p u l u h N o p e m b e r I n s t i t u t e o f T e c h n o l o g y , S u r a b a y a

& Indonesian Society of Sanitary and Environmental Engineers, Jakarta

O p e n A c c e s s h t t p : / / w w w t r i s a n i t a o r g

uncontrolled waste disposal site are such as air pollution due to the spreading of bad odour, potential public health risk since this landfill can be a breeding host for the disease causing vectors and also the water pollution due to the subsurface transport of leachate [3-8]

The study site, Masindi District is located in the northernmost part of the western region of Uganda It is bordered by Luwero and Kiboga districts in the south, Hoima district and the Democratic Republic of Congo in the west, while in the north it is bordered by the districts of Gulu, Apac and Nebbi (Fig 1) It extends between 1°21′ to 2°24′ in the North and 31°18′ to 32°21′

in the East

Fig 1: Map showing the location of Masindi district along with that of the lake Kiyanja The District comprises four counties namely Kibanda, Buliisa, Bujenje and Buruli It has a total area of 9,326 sq km (8,458 sq km of land and 868 sq km of water bodies), much of which is gazetted as conservation area The population of this Masindi District has increased steadily over the years, from 155,511 in 1969 to 223,230 in 1980 and 260,796 in 1991, up to the present level

of 469,865 persons (National Census 2002) This district is largely rural, with more than 90% of the population living in rural areas and depending on arable land and livestock farming Fewer than 10% of the district population live in the urban areas of Masindi Town Council, Kigumba Town Council and Kijura (part of Masindi Town) Thus, the situation indicates a low level of urbanisation in the district The settlement patterns in the rural areas have also been greatly influenced by geographical factors of terrain, climate, vegetation, disease agents and the physical socio-economic infrastructure like roads, water supply and the different forms of land ownership The community water supply system of this town has never been well appreciated Due to the lack of well constructed water supply facilities, people have generally been collecting their water required for domestic as well as drinking purposes from the boreholes that were constructed around the community (Fig 2C)

Like other cities in developing countries, Masindi in Uganda doesn’t have any sanitary landfill

to cope with the ever increasing solid waste disposal demand As a consequence to this, solid waste has always been disposed near the household premises (Fig 2A, B) Leachate

MASINDI TOWN

LAKE KIYANJA

subsequently produced at this waste disposal site may easily get worst while it mixes with the surface runoff The subsurface Infiltration of this landfill leachate appears to be a potential source that may cause substantial pollution to the surface water as well as groundwater resources [7-9] Moreover, organic humic acid which forms in the intermediate stage can even cause severe problem for the aquatic lives due to the rapid oxygen depletion Physical and mathematical models capable of quantifying the different environmental consequences occurring continuously around us, have been practising by the environmental modellers since long [7-9] In addition, numerical analysis has long been applied as a tool in solving the intricate mathematical problems associated with any environmental study FEMLAB a multi-physics based numerical modeling package that can simply be utilized to simulate any physical progression related with any adverse environmental incident [10]

Fig 2: A typical solid waste disposal custom around the community premises (A); an unmanaged

solid waste disposal site (B); and boreholes used by the community people for collecting their demanded water (C) [10,11]

The objective of this present study is to highlight the pollution possibility for the groundwater

of the lake Kiyanja watershed of Masindi, Uganda, that may take place with the spreading of the highly organic contained leachate which has been generating continuously at the nearby solid waste disposal site

METHODOLOGY

A multi physics numerical modeling package FEMLAB was selected to capture the hydrological processes that can explain the possible transport of waste disposal contaminant transport in the subsurface layers For this, FEMLAB Chemical Module coupled with Earth Science Module had been utilised to solve the advective-dispersion equation For specifying the dimensions and physics of the flow domain Richard’s equation had been used In order to generate the geometry of flow domain, background grid had been formed

In developing the physical environment of the model, three sub domain layers including their dimensions and the physics of the flow domain were specified Various characteristics of the associated parameters were also assigned to the three different subdomains (Table 1, 2 and 3) These could be categorised as subdomain-1 representing the bottom clay layer, subdomain-2 that stands for the intermediate sandy clay layer and the subdomain-3 expressing the upper sandy- silt layer mixed with organic materials (Fig 3) The hydraulic conductivity for the bottom

layer (layer-1), intermediate layer (layer 2) and the upper layer (layer 3) were taken as 1*10-7 (m

s-1), 1*10-4 (m s-1) and 1*10-3 (m s-1) respectively based on the soil report of the Masindi district Table1: Parameters assigned to the subdomain layers

Position

X-0

Y-0

Position X-0

Y (-1.0)

Position X-0

Y (-1.4)

Table 2: Subdomian setting for the Richard’s equation (esvr)

Constitutive relation Brooks and Corey -

θs, θr 0.6, 0.005 Liquid fraction saturated and residual

Xf, Xp 4e-10, 1e-5 Compressibility of fluid and solid

Table 3: Subdomain setting for solute transport (flow and media)

Flow Field

Boundary conditions were specified to the different boundaries considered for the conceptual model (Fig 3) It can easily be seen that out of the 11 boundaries presented in Fig 4, only the no

4 and no 6 should be recognized as internal boundary, while the no 2 would be considered as the bottom boundary and the no.7 and no.8 should definitely be thought as top boundaries It can also be mentioned that only the boundary no 7 and no 3 have the inward flux, whereas the boundary no 9 have seepage face However, mixed condition existed at boundary no 10 could

be noticed However, rest of the boundaries except no 2 have zero flux Moreover, free drainage occurs at the bottom boundary as the selected inward flux and the saturated hydraulic conductivity might have balanced themselves

Fig 3: Conceptual model showing different sub domain layers [10]

Fig 4: Boundary conditions assigned to different layers

Considering steady state flow condition, the simulation was run to solve the Richard’s equation

An initial value of a contaminant influx of 1*10-6 (m s-1) with an assumed concentration, C (0.015 kg/m3) at the inward boundary was considered in this study At boundary no 2, the out flow was estimated as -6.88*10-12 (m s-1) This outflow value might be negligible in compare to that of the influx boundary The outflow at boundary no 10 was estimated as 5*10-9 (m s-1)

B-8 B-5

B-3

B-1

B-6

B-4

B-2

B-9 B-10 B-11 B-7

RESULTS AND DISCUSSION

The leachate that could have generated at the waste disposal site may get started to move

in different directions after being guided by the surface runoff This leachate contaminant could mainly reach to the subdomain-03 through the boundary no 7 and then can continue flowing through the unsaturated zone vertically After that it can advance further till it reaches the seepage boundary no 9 due to the existence of the differential water head there At this boundary, the contamination carried by the soil pore water could try to exit the subdomain as an out flux A part of influx could also have entered through the boundary no 3 in the form of either seepage or lateral infiltration

The simulation run for the solute transport model shows that after the first one hour of its being released from the waste disposal site, the contamination starts to move through the infiltration boundary no-7 (Fig 5) towards the unsaturated zone of the upper top layer Through this layer it can easily be transported along the vertical as well as horizontal direction after being adequately directed by the pore water velocity The visualization of this numerical model also shows that the contamination needs at least 180 hours to reach closer to the boundary no 10 After 183 hours (Fig 7), the pollutant just touches the out flux boundary and then tries to move out of that There hasn’t been observed any flow moving towards the bottom layer due to the presence of the impermeable clay layer It has also been observed that the water inside the saturated layer would only get contaminated if the pollutant would have passed 40 hours of its travel distance after being first released from the disposal site (Fig 6) This pollutant may continue

to move forward till it reaches the lake water This would surely contaminate the lake water as well as the other adjacent surface water sources The shallow and intermediate groundwater in the saturated layer-2 would already have contaminated by this pollutant

Fig 5: Pollutant infiltrates through the unsaturated layer just after 1 hour

Figure 6: Pollutant reaches in the saturated layer after 40 hours

Fig 7: Pollutant touches the exit boundary after 183 hours

However, the contaminant from the waste disposal site to the deep ground water lying beneath the impervious clay layer will not be able to reach immediately unless there have cracks

and fissures in that layer which may guide the pollutant infiltrating through that Considering the future protection, the landfill should immediately be stopped and attempts should also be made to shift it in another farthest location with providing sufficient provisions for the bottom lining, top covering, leachate collection system, gas collection system and time-to-time monitoring system

At the same time, the soil that had already been affected should be treated with soil remediation such as soil washing and biodegradation The shallow ground water extraction should also be stopped for a certain periods and the community should be supplied water either from the deep ground water aquifer or from the treated surface water sources Proper care in adequate sealing should also be taken while installing deep groundwater wells so that there wouldn’t be any leakage in the clay layer

It can also be stated that the overall Masindi area would only be sustainable if the present environmental degradation resulting from the waste disposal site would possibly be minimized as soon as possible However, if the area would be exploited further due to the rapidly progressing

of the urbanisation, then there may be a major risk for the soil and ground water being polluted eventually Moreover, to get rid of this risk, policy-makers, the urban planners, donor agency and

in fact the people who are living in this area should take collectively immediate decision to restore the groundwater resources of the that concerned area

CONCLUSION

A numerical model using the multiphysics FEMLAB has been explored in this present study

to forecast the vulnerability of the groundwater resources of the Masindi district It has been presented here that the shallow groundwater has already been polluted after getting mixed with the spreading of the contaminant plume from the waste disposal site The clay layer has been serving as a barrier for the deep groundwater However, the waste disposal site should immediately be handled due to its synchronized adverse impacts on the groundwater resources Such type of contaminant spreading understanding gained from the numerical model will obviously assist to select appropriate mitigation measures to save the groundwater in the concerned locality

Acknowledgement: The author would like to express his gratitude to Professor Roger Thunvik, Land and Water Resources Department, KTH, Stockholm, Sweden for his great help during the completion of the project assignment using FEMLAB for the Quantitative Hydrogeology course

References

1 Mwiganga, M and F Kansiime, 2005 The impact of Mpererwe landfill in Kampala–Uganda, on the

surrounding environment Physics and Chemistry of the Earth, 30:744–750

2 Heyer, K.U and R Stegmann, 2002 Landfill management: leachate generation, collection,

treatment and costs Available from: <http://www.Ifas-hamburg.com>

3 Landfills: Environmental Problems, http://www.landfill-site.com/html/landfills environmental_

probl.html Accessed on 10th March 2009

4 Landfills, http://www.iun.edu/~environw/landfills.html Accessed on 12th March 2009

5 Landfills: Hazardous to the Environment, http://www.zerowasteamerica.org/landfills.htm Accessed

on 15th march 2009

6 Kulabako R., Thunvik R, Mnalubega M and Soutter L, 2005, Hydrodynamic modelling of

subsurface flow and contaminant transport in a Peri-urban settlement in Kampala Land and water Resources Department, KTH, Stockholm, Sweden

7 Kulabako, R., M Nalubega Uganda and T Roger, 2004 Characterization of Peri-Urban

Anthropogenic Pollution in Kampala, Uganda 30th WEDC International Conference, Vientiane, Lao PDR, pp 474-482

8 Kulabako, R 2005 Analysis of the Impact of Anthropogenic Pollution on Shallow Groundwater In

Peri-Urban Kampala, PhD thesis, Land and water Resources Department, KTH, Stockholm, Sweden

9 Rahman, M.T., Buccheri, M., Baghbanan, A., Kizito, F., Kwarteng, O.A., and Ntiamoah, B 2005,

Development of a Conceptual model and Numerical Modelling of Contaminant Transport for the Lake of Kiyanja Cachment (Uganda-Africa), Report of Quantitative Hydrogeology Course, submitted to the Land and Water Resources Engg Dept of Royal Institute of Technology, KTH, Stockholm, Sweden

10 FEMLAB 3, 2004, Earth Science Module Library, 2004, COMSOL AB

11 Quantitative Hydrogeology, LWR, KTH, Stockholm, Sweden http://www.lwr.kth.se/Grundutbildning/

1B1635/QH/CourseHome/index.htm

[This page is intentionally left blank]