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TECHNICAL REPORT WC/99/17 Overseas Geology Series DFID Project No. R6532 Human health risk in relation to landfill leachate quality B A Klinck and M E Stuart

BGS International®

British Geological Survey Keyworth

Nottingham NG12 5GG United Kingdom


TECHNICAL REPORT WC/99/17 Overseas Geology Series DFID Project No. R6532 Human health risk in relation to landfill leachate quality B A Klinck & M E Stuart

This report is produced under a project funded by the UK Department for International Development (DFID) as part of the UK provision of technical assistance to developing countries. The views expressed are not necessarily those of DFID.

DFID classification: Subsector: Water and sanitation Theme: Increase protection of water resources, water quality and aquatic systems Project title: Human risk in relation to landfill leachate quality Project reference: R6532

Bibliographic reference: Klinck B A & Stuart M E (1999). Human risk in relation to landfill leachate quality BGS Technical Report WC/99/17

Keywords: landfill, leachate, groundwater contamination, risk assessment, Thailand, Mexico

Front cover illustration: Groundwater sampling from a shallow, hand dug well: Mérida, Mexico

© NERC 1999 Keyworth, Nottingham, British Geological Survey, 1999

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WC/99/17 Version 1, Revision 1 i

CONTENTS

LIST OF FIGURES ii

LIST OF TABLES ii

LIST OF BOXES iii

EXECUTIVE SUMMARY iv

1 INTRODUCTION 1

1.1 Scope of Report 1

1.2 Project Background 2

1.3 Objectives 5

1.4 Collaborating Institutions 5

2 LEACHATE CHEMISTRY AND GROUNDWATER IMPACT 7

2.1 Study Sites 7

2.2 Landfill Leachate Chemistry 8

2.3 Impact on Groundwater 11

3 LANDFILL IMPACT ASSESSMENT 15

3.1 Simple Analytical Models 15

3.2 LandSim 19

3.3 The HELP Model 23

4 HEALTH RISK ASSESSMENT 29

4.1 Risk Assessment Model 29

4.2 Toxic and Carcinogenic Risks 30

4.3 Microbiological Contamination 33

5 RELEVANCE OF PROJECT FINDINGS TO OTHER LANDFILL

SETTINGS 35

5.1 Summary of Findings and Discussion 35

5.2 A Generic Approach to Landfill Leachate Impact Assessment 37

5.3 Risk Management Options 38

6 REFERENCES 42

WC/99/17 Version 1, Revision 1 ii

LIST OF FIGURES

Figure 2.1 Stiff plots showing seasonal variation in leachate composition from Mérida 10

Figure 2.2 Oxidation of ammonium to nitrate during recharge flushing, Mae Hia, Chiang Mai, Thailand 12

Figure 3.1 Sketch map of the Tha Muang landfill site 17

Figure 3.2 Concentration with distance away from the landfill 19

Figure 3.3 The Mae Hia landfill site 22

Figure 3.4 Modelled output for LandSim for chloride at Well 12 at the Mae Hia site 23

Figure 3.5 Location map Mérida municipal waste site showing the shallow sampling points 25

Figure 3.6 TOC plume from the Mérida landfill 27

Figure 3.7 Relationship between leachate flow (flujo de lix.) and precipitation during the period of simulation 27

Figure 4.1 A Source-Pathway-Receptor conceptual model for environmental exposure adjacent to a landfill 30

Figure 4.2 Growth of thermotolerant coliforms in a leachate-clean groundwater mixture 34

Figure 5.1 Flow diagram of stages in completing a health risk assessment 39

LIST OF TABLES

Table 2.1 Summary of study sites 7

Table 2.2 Summary of selected incidents of groundwater pollution from waste disposal in newly industrialised countries 11

Table 4.1 Sensitivity analysis for groundwater consumption risk assessment at the Mae Hia site 33

Table 5.1 Data requirements checklist for risk assessment 38

WC/99/17 Version 1, Revision 1 iii

LIST OF BOXES

Impact of waste burning 4

Waste composition 9

Mobilisation by rainfall 13

Formation of trihalomethanes in contaminated groundwater 14

Advection dispersion equation 15

Toxic and carcinogenic risks 32

WC/99/17 Version 1, Revision 1 iv

EXECUTIVE SUMMARY

This report summarises the findings and conclusions of the research project R6523 ‘Human

Risk in Relation to Landfill Leachate Quality’ aimed at assessing the risks to human health

associated with the impact of landfill leachate on vulnerable groundwater systems. The study

was funded by the Department for International Development (DFID) under its Knowledge

and Research programme (KAR) which is focused on increasing protection of groundwater

resources, water quality and aquatic ecosystems. The overall purpose of the project was to

encourage healthy and improved practices in waste disposal. Within this aim were the

following specific objectives:

• To design and implement a sampling and analysis programme for leachate and

contaminated groundwater at contrasting case study sites;

• To produce a catalogue of leachate quality documented in terms of waste type, climate

and presence of hazardous compounds to enable hydrogeologists to carry out productive

modelling of contaminant migration in groundwater;

• To identify commonly occurring components of leachate which pose a human health risk

and carry out risk assessments for selected case study sites.

Three contrasting landfills have been studied and are described in this report. The study has

demonstrated that site geology is one of the main factors controlling the impact of landfill

leachate on aquifers.

Aquifer properties and groundwater flow regimes also play a critical role. It is evident from

the detailed sampling carried out around the Mérida landfill in Mexico that the impact on the

aquifer is not evident beyond a few hundred metres from the site and leachate-linked

contaminants are reduced to background levels by dilution in the highly transmissive

limestone aquifer.

At the Mai Hia landfill, Thailand, a colluvial aquifer situation where aquifer transmissivity is

reasonably high, a similar situation would be expected. However, considering the results of

the detailed sampling carried out, it can be seen that the contamination is more persistent, the

leachate groundwater plume extending for over 1000 metres from the site. Not only that, but

due to redox processes within the aquifer, toxic levels of manganese and unacceptable nitrate

concentrations occur which are well in excess of those found in the original leachate.

WC/99/17 Version 1, Revision 1 v

The alluvial site studied at Tha Muang, Thailand, seems to fall somewhere in between in

terms of aquifer impact. This is thought to be due to the practice of waste burning. The

result is an overall reduction in organic loading on the aquifer combined with attenuation of

contamination due to the physical nature of the silty aquifer.

In evaluating the human health impact of leachate contamination of groundwater, the

pathway considered has been direct ingestion of contaminated water. At all three case study

sites, faecal coliform contamination of the aquifer was detected as a primary impact, and

coliforms were also incubated from some of the leachate samples. In terms of risk

management bacterial contamination is generally easy to deal with, either by boiling or

disinfecting the groundwater.

The current study has demonstrated that inefficient chlorination of groundwater with a high

organic loading, possibly leachate derived, can give rise to the production of trihalomethane

compounds (THMs), some of which are recognised as being both toxic and carcinogenic.

Although this finding may be of only minor or no concern in a well-managed end-of-pipe

treatment system, it may constitute a hazard in other situations.

Dissolved organic carbon (DOC) is the main component of all of the leachates studied and is

directly attributable to the organic content of the waste. However, it has proved to be very

difficult to identify specific organic compounds responsible for the high DOC. Therefore, in

order to carry out a risk assessment bis(2-ethylhexyl)phthalate (DEHP), a plasticiser, was

chosen as an environmental contaminant which is directly linked to leaching plastics in

landfill waste. In calculating carcinogenic risk, a high DEHP value was used from the Mae

Hia site that produced an increased risk of cancer of 5 in 1,000,000. The United States

Environmental Protection Agency (USEPA) considers an increased risk of 1 in 1,000,000 as

significant and therefore there is an unacceptable risk of cancer on that basis. Possibly more

insidious is our lack of knowledge concerning the other possibly toxic components of the

DOC and the effect that they might have at very low concentrations on health risk estimates.

This is an area that needs further study.

Toxic heavy metals remain in the waste or at the waste-rock interface as a result of redox-

controlled precipitation reactions. This fixing of heavy metals dramatically reduces the risk

of direct toxic effects due to ingestion of leachate-contaminated groundwater. However, once

the leachate leaves the site the situation changes. The leachate is generally a strongly

reducing liquid formed under methanogenic conditions and on coming into contact with

WC/99/17 Version 1, Revision 1 vi

aquifer materials has the ability to reduce sorbed heavy metals in the aquifer matrix. The

most important reactions are the reduction of iron and manganese to more soluble species and

hence one sees an increase in the concentration of these components under favourable

conditions close to a landfill. The impact of this process has been demonstrated to lead to a

serious toxic risk.

There have been difficulties in applying the methodologies tested during this study and the

overriding contributing factor to this has been lack of knowledge to constrain problem

definition. The contaminant transport models used are sensitive to parameter selection, for

instance the values of hydraulic conductivity and hydraulic gradient. To adequately define

these variables requires a well-constructed monitoring network in order to perform hydrailic

tests and measure water levels. This level of monitoring sophistication was not encountered

at any of the case study sites. The risk assessment calculations are sensitive to choice of

exposure factors and one of the most difficult values to obtain for case study sites was an

estimate of daily water consumption. This lack of knowledge present throughout the

assessment process, and the requirement to estimate parameters from limited knowledge

means that the final outcome can be over-conservative and might flag up the need for

inappropriate and expensive risk management measures. An attempt has to be made to

produce a balanced risk assessment while at the same time adopting the precautionary

approach in implementing risk management.

Several options are explored to manage the risk to human health from landfill leachate and

include: removal of the source term, leachate plume management, and the waste reduction.

These options can be applied either singly or combined to optimise benefit.

WC/99/17 Version 1, Revision 1 1

1 INTRODUCTION

1.1 Scope of Report

This report summarises the findings and conclusions of the research project R6523 ‘Human

Risk in Relation to Landfill Leachate Quality’. This was aimed at assessing the risks to

human health associated with the impact of landfill leachate on vulnerable groundwater

systems. The study was funded by the Department for International Development (DFID)

under its Knowledge and Research programme (KaR) which is focused on increasing

protection of groundwater resources, water quality and aquatic ecosystems.

New data were collected from sites in Jordan, México and Thailand and were supplemented

with data from project R5565 ‘A Groundwater Hazard Assessment Scheme for Solid Waste

Disposal’ and unpublished BGS data holdings. Professor Somjai Karnchanawong of the

Department of Environmental Engineering, Chiang Mai University, Thailand, provided

additional archived data. Additional monitoring data for the site in Mérida, México, was

provided by Ing. Roger Gonzalez of the Universidad Autónoma de Yucatán, México.

The principal objectives of the project are given in Section 1.3 below. The potential impact

of landfill leachate on groundwater is discussed in Section 2 and illustrated using case studies

from the project. Section 3 describes the principles of risk assessment and Section 4 contains

the application of commercial models. The report presents a series of guidelines to be used in

assessing the impact of waste disposal sites in Section 5.

Associated project report and papers

Stuart, M.E. and Klinck, B.A. 1998 A catalogue of leachate quality from selected landfills

from newly industrialised countries. British Geological Survey Technical Report WC/98/49.

Karnchanawong, S., Klinck, B.A., and Stuart, M.E. 1999 The Mae Hia Landfill, Chiang Mai,

Thailand; the post-closure groundwater contamination legacy. In: Asnachinda, P.,

Lerdthusnee, S. Eds. Water Resources Management in Intermontane Basins, Chiang Mai,

Thailand.

Stuart, M.E., Klinck, B.A., and Gooddy, D.C. 1999 Trihalomethane formation potential: a

tool for detecting non-specific organic contamination from landfills, and the health risks


associated with chlorination. In: Christensen, T.H., Cossu, R., Stegman, R. Eds. Proceedings

of Sardinia 99: 7th International Landfill Symposium. Cagliari, Italy

Klinck, Ben 1999 Sustainable landfill: A developing world perspective. Earthwise, Issue 13,

p15.

Klinck, B.A., Stuart, M.E., Ramnarong, V., Buapeng, S. and Sinpool-Anant, S. 1999. Human

risk in relation to landfill leachate quality: Case studies from Thailand. British Geological

Survey Technical Report WC/99/15

Klinck, B.A., Stuart, M.E. and González H.,R. 1999. Human risk in relation to landfill

leachate quality: The Mérida municipal landfill, Mexico. British Geological Survey Technical

Report WC/97/34.

Stuart, M.E., Klinck, B.A, Tufaha, R. and Lafi, M. 1999. The Amman municipal landfill,

Jordan: Implications for groundwater quality. British Geological Survey Technical Report

WC/99/16.

1.2 Project Background

In many newly industrialised countries urbanisation is proceeding at an unprecedented rate.

Such development is often unbalanced with much of the disposable municipal expenditure

devoted to high profile, visible, infrastructure with waste disposal and waste management

coming well down the list of priorities in terms of allocation of funding. In the developing

world the prevailing method for the disposal of municipal and domestic refuse is usually open

dumping, often coupled with waste burning and with minimal effort directed towards sanitary

land filling practice, e.g. the use of daily cover. Site selection is generally based on

geographical rather than geological and hydrogeological considerations, i.e. the closer the site

to the source of the waste the better in terms of logistics. It is not uncommon therefore to find

waste disposal sites within municipal boundaries and surrounded by residential areas. Clearly

such sites pose a serious health risk not just in terms of degradation of groundwater quality

but also due to the related problems associated with proximity to litter, feral animals,

scavenging birds, vermin and airborne contamination arising from mobilisation of fine

particulate matter.


Cointreau (1982) lists four main health hazards associated with wastes typical of newly

industrialised countries:

1) Human faecal matter: mainly attributable to the use of disposable nappies, but also to the

widespread practice of discharging untreated household septage to landfill;

2) Industrial waste: poor control at the waste reception area, lack of knowledge of what

constitutes a dangerous waste, and a lack of understanding of the environmental impact of

the disposal of such wastes means that hazardous wastes frequently enter sites;

3) Decomposition products from the waste: susceptible to dissolution by infiltrating water

percolating through the waste, giving rise to leachate which can contaminate

groundwater. Christensen et al. (1994) have identified the following principal groups

contained in leachate:

• inorganic macro components: calcium, magnesium, sodium, potassium, ammonium,

iron, manganese, chloride, sulphate and bicarbonate.

• heavy metals: cadmium, chromium, copper, lead, nickel and zinc in trace amounts.

• dissolved organic matter expressed as chemical oxygen demand or total organic

carbon and including methane and volatile fatty acids.

• anthropogenic organic compounds derived from household and industrial wastes,

including aromatic hydrocarbons and phthalate esters.

4) Smoke from continuous burning of waste: creates extensive pollution in many cities, and

burning landfills are not an uncommon sight in newly industrialised countries. There is

some concern that toxic/carcinogenic compounds may be produced during open waste

burning. The low temperatures lead to much greater concentrations of such compounds

than from solid waste incineration (Bergström and Björner, 1992). The following classes

of compound have been studied:

• polycyclic aromatic hydrocarbons (PAH). The principal PAH isomers produced are

phenanthrene, fluoranthene and pyrene (Ruokojärvi et al., 1995). In practice, all of

these compounds have very limited water solubility and are likely to be immobilised in

the burnt residue.


• dioxins. Polychlorinated dioxins (PCDD) and furans (PCDF) were analysed in soil

samples from a closed site at Marka, Amman, Jordan, where waste had been burnt over

a long period (Alawi et al, 1996). Total concentrations of PCDD and PCDF of

between 0.685 and 112 µg/kg dry weight in overlying soil were found, considerably

exceeding limits for re-use of the site for activities such as housing.

Impact of waste burning

Waste burning fundamentally changes the waste composition and has the effect of inhibiting the acetogenic stage by removing some of the putrescible feedstock and reducing the moisture content of the waste and hence the overall leachate loading and heavy metal mobility. A reduction in the amount of carbon dioxide produced due to biodegradation, coupled with lower concentrations of bicarbonate in the leachate, is to be expected. This is illustrated by comparison of the leachate quality from two sites in Thailand ,Tha Muang and Kanchanaburi. The original waste composition is assumed to be broadly similar based on a similar population density and level of development, but waste is burnt at Tha Muang.

Leachate quality at Thailand study sites

Average concentration Component

Tha Muang Kanchanaburi

Chloride (mg/l) 2050 1500

Alkalinity (mg/l) 750 4000

Dissolved organic carbon (mg/l) 60 550

Redox potential (mV) 210 70

Nitrate (mg/l) 7 0

Manganese (mg/l) 0.4 7.9

Volatile fatty acids (µg/l) 0 90

The chloride concentrations at both sites are reasonably similar but at Tha Muang alkalinity and dissolved organic carbon are considerably lower. The relatively high redox potential and positive nitrate concentration together with low manganese concentration and absence of VFAs indicate aerobic conditions. In contrast, the leachate at Kanchanaburi has high alkalinity, moderate concentrations of VFAs and manganese, and low redox potential and no nitrate. It is considered to be transitional between the acetogenic and methanogenic phases.

Concentrations of polycyclic aromatic hydrocarbons (PAH) and polychlorinated hydrocarbons are known to increase as a result of waste burning. In practice many of these compounds have very limited water solubility and are likely to be immobilised to a large extent due to sorption onto the waste and superficial soils. Only very low concentrations of fluoranthene, the most soluble PAH, were detected in leachates during the present study.

In terms of a waste minimisation strategy waste burning is an attractive option, with reduction of waste volume and a reduced carbon loading in the leachate. It may also have benefits in terms of reduction of vermin and pathogens in the waste. However, the low temperatures associated with open waste burning in themselves pose a hazard, since toxic compounds such as PAH and dioxins may be formed in the smoke to the detriment of nearby inhabitants.


1.3 Objectives

The overall purpose of the project was to encourage healthy and improved practices in waste

disposal. Within this aim were the following specific objectives.

• To design and implement a sampling and analysis programme for leachate and

contaminated groundwater at contrasting case study sites;

• To produce a catalogue of leachate quality documented in terms of waste type, climate

and presence of hazardous compounds to enable hydrogeologists to carry out productive

modelling of contaminant migration in groundwater;

• To identify commonly occurring components of leachate, which pose a human health risk

and carry out risk assessments for selected case study sites;

• To evaluate options of landfill management to mitigate human health risk;

• To disseminate project findings to key workers in the collaborating countries through

interactive workshops. To this end three workshops were held, one in each of the

collaborating countries. Forty participants, representing 12 organisations including

academic institutions, government departments and agencies and one municipality

attended the workshop held at the Department of Mineral Resources in Thailand. Fifteen

participants from the water Authority of Jordan attended the two-day workshop in

Amman, Jordan A workshop held in Mérida, Mexico was held in conjunction with the

University and attracted 11 post-graduate students studying for a higher degree in

Environmental Engineering

1.4 Collaborating Institutions

The project was carried out jointly by staff from the following institutions:


Dr Ben Klinck, Project leader

Mrs Marianne Stuart


Water Authority of Jordan, Laboratories and Water Monitoring Department

Dr Raja Gedeon, Director

Eng Mohamed Lafi, Project leader

Eng Randa Tufaha

Mr Zeyad Quasmeh

Mr Loai Allan

Department of Mineral Resources, Thailand

Dr Vachi Ramnarong, Project leader

Ms Somkid Buapeng

Mr Suchai Sinpool-Anant

Chiang Mai University, Department of Environmental Engineering, Thailand

Somjai Karnchanawong

Autonomous University of Yucatán Faculty of Engineering, México

Ing Roger Gonzáles Herrera, Project leader

Javier Frias Tuyin

Victor Coronado Pereza.


2 LEACHATE CHEMISTRY AND GROUNDWATER IMPACT

2.1 Study Sites

Leachate quality and waste composition data was collected from a number of sites

representative of landfills in newly industrialised countries. These are summarised in Table

2.1. At sites marked with an asterisk, groundwater samples were also collected from wells

close to the landfill.

Table 2.1 Summary of study sites

Country Climate type Major cities Sites Rural area Sites

Semi-arid México City El Bordo

Mérida Mérida*

México

Tropical-humid

León León

Amman Ruseifa* Jordan Semi-arid

Irbid and Jerash

Al Akaider*

Tropical-humid

Bangkok On-Nooch

Nong Khaem

Lat Krabang

Kamphang Sein

Kanchanaburi province

Kanchanaburi

Tha Muang*

Thailand

Semi-tropical-humid

Chiang Mai Mae Hia*

San Sai

Lamphun

Indonesia Tropical-humid

Bandung Leuwigadja

Sukamiskin

Malaysia Tropical-humid

Kuala Lumpur

Penang

* Denotes collection of groundwater samples


2.2 Landfill Leachate Chemistry

Leachate quality varies throughout the operational life of the landfill and long after its

closure. During the early stages of waste degradation and leachate generation the composition

is acidic and high in volatile fatty acids (the acetogenic phase). This acid leachate may

dissolve other components of the wastes, such as heavy metals. The leachate also contains

high concentrations of ammoniacal nitrogen and has both a high organic carbon concentration

and a biochemical oxygen demand (BOD).

As degradation of the waste progresses conditions in the landfill become more anaerobic and

the strongly reducing methanogenic phase is initiated. The majority of the remaining organic

compounds are high molecular weight humic acids and the leachates are characterised by

relatively low BOD values. Ammoniacal nitrogen generally remains at high concentrations

in the leachate, but falling redox potential immobilises many metals as sulphides in the waste

(Pohland et al., 1993; Belevi and Baccini, 1992).

There are strong seasonal variations in both the quantity and quality of leachate generated.

Differences in leachate chloride concentration from the same site indicate the variation in the

volume of leachate being generated, since chloride is a conservative anion not affected by

biodegradation or decay. Changes in other major ion concentrations may result from pH or

redox changes in the leachate and interactions with the waste matrix. This is illustrated by a

Stiff diagram (Tonjes et al., 1995) of leachate quality from Mérida, México (Figure 2.1).

Leachate collected at the end of the dry season (April) has a very high chloride concentration.

This diminishes as the rainy season progresses and the leachate is diluted (August–

September-November). Decomposition of the organic content of the waste is also

accelerated due to the high moisture content resulting in a greater proportion of bicarbonate

formed as a by product of bacterial respiration.


Waste composition

Ultimately it is the waste composition which influences the chemistry of the leachategenerated. In the developing world municipal solid wastes tend to have a very highcontent of putrescible materials compared to a typical developed city in the westernworld (Klinck et al., 1995). This is clearly shown by comparison of waste compositionfor Amman and typical values for the United Kingdom. At all sites investigated duringthe current project the waste composition was overwhelmingly dominated by the organiccomponent The widespread practice of informal recycling in developing countries mayexplain to some extent this very high organic matter content. The resulting waste densityis between two and five times higher than industrialised countries and, with typicalmoisture contents well in excess of 30%, the waste is generally at field capacity and anyinfiltration produces leachate.

61.9%

23.9%

3.6%2.3%3.5%3.7%1.1%

Organic

Paper/Card

Plastic

Leather

Metal

Glass

Wood

Waste composition for Amman, Jordan

24%

8%

8%

5%

11%

9%4%

31%

Organic

Paper and card

Plastics

Metals

Textiles

Glass

Ash

Unclassified

Typical waste composition for United Kingdom


2.2.1 Hazardous Components of Leachate

Phthalate esters and other plasticisers, such as adipates, are leached from plastic products,

mainly PVC, under landfill conditions (Mersiowsky and Stegmann, 1997). These are now

ubiquitous in the environment and are commonly reported in fresh waters and industrial

discharges. These compounds are microbially degraded, either aerobically or under

methanogenic conditions, to carbon dioxide. However, in the acetogenic phase degradation

has been shown to be slow (Ejlertsson and Svensson, 1997). Of concern in landfill leachate

is the presence of bis(2-ethylhexyl) phthalate which has been shown to be carcinogenic in

laboratory animal experiments and has been detected in most of the leachates examined

during this study.

In leachates with a high volatile fatty acid (VFA) content the pH is generally less than 7 and

heavy metal concentrations can be high. To some extent, metal content is a function of the

Figure 2.1 Stiff plots showing seasonal variation in leachate composition from Mérida


waste stream composition. For example, wastes produced during the manufacture of leather

have high concentrations of chromium which is found in leachates from Bandung, Indonesia;

Bangkok, Thailand; and León, México. Manganese and zinc are also generally high in

acetogenic leachates.

Sewage sludge is a commonly present hazardous component of waste. The presence of

faecal coliforms, faecal Streptococcus, Clostridium and Salmonella has been demonstrated in

sludges from Mérida, México (Peniche A. et al., 1993). Indeed, high concentrations of faecal

coliforms were detected in all leachate samples analysed during the present project.

2.3 Impact on Groundwater

Contamination of groundwater by landfill leachate associated with the landfilling of wastes is

considered to be of major environmental concern. Table 2.2 shows that the most commonly

documented evidence of groundwater pollution in newly industrialised countries is increased

concentrations of the major ions sodium, chloride and bicarbonate together with ammonium

and iron. However, it is not known to what extent inadequate analytical facilities may limit

this list.

2.3.1 Implications of Redox Changes

The presence or absence of a number of species can be used as indicators of redox conditions.

The presence of nitrate usually indicates oxidising conditions. Once dissolved oxygen is

consumed by degradation of organic matter, nitrate is lost by denitrification to nitrogen gas

and disappears. This is followed progressively by reduction of insoluble Mn(IV) species to

soluble Mn(II), reduction of insoluble Fe (III) to soluble Fe(II), reduction of sulphate to

sulphide and finally by reduction of carbon dioxide to methane.

These redox changes can have important implications for groundwater quality. In Mae Hia,

Thailand, conditions in the groundwater plume during the dry season appear to be mainly

anaerobic with ammonia present and manganese being widely detected at unacceptable

concentrations. In the rainy season, the chemistry becomes more complex and nitrate

replaces ammonia as a consequence of oxidising groundwater recharge, but manganese still

persists. Figure 2.2 illustrates this pattern of redox changes.


Table 2.2 Summary of selected incidents of groundwater pollution from waste disposal in newly industrialised countries

Country Disposal method

Waste type Groundwater contamination indicators

Reference

Argentina Unlined fill with burial

Municipal Cl, HCO3, Cl/HCO3, Zn (Martinez et al. 1993)

Brazil Sanitary landfill Industrial Municipal

Na, Cl, NH4, (Vendrame and Pinho 1997)

Greece Sanitary landfill Municipal SEC, hardness, Cl, P, metals, NH4, NO3

(Loizidou and Kapetanios 1993)

India Open dumping in low-lying areas

Municipal TDS, Cl, SO4, Fe, NH4, COD

(Olaniya and Saxena 1977)

Romania Old quarry Municipal Industrial Medical

Na, Cl, Cr, Ni, Cu, CN, NH4,

(Mocanu et al. 1997)

Ukraine Solid wastes Municipal Industrial

Bioindicators –suppression of microbiological activity

(Magmedov and Yakovleva 1997)

0

2

4

6

8

10

89 90 91

Date

Con

cent

ratio

n (m

g/l)

---

-

Nitrate-N

Ammonium-N

Recharge flush

Figure 2.2 Oxidation of ammonium to nitrate due to recharge flushing, Mae Hia, Chiang Mai, Thailand


2.3.2 Organic Loading

There have been relatively few studies of the impact of organic compounds leached from

municipal waste sites to groundwater. Reinhard et al. (1984) found that the majority of the

compounds originated from decomposing plant material, including aliphatic and aromatic

acids, phenols and terpenes. Minor constituents were both chlorinated and non-chlorinated

hydrocarbons, nitrogen-containing compounds, alkyl phenol polyethoxalates and alkyl

phosphates. Albaiges et al. (1986) used a series of C4 to C7 carboxylic acids with

predominance of even carbon numbered compounds found in the leachate as indicators of

organic pollution from leachate.

Very few specific organic components were detected at any of the present study sites,

probably reflecting the original leachate chemistry. Compounds detected were:

• plasticisers, commonly the phthalate esters, with a single instance of dioctyl adipate

Mobilisation by rainfall

At the disposal site in Mae Hia, Thailand increased concentrations of contaminants can bedetected in groundwater after periods of high rainfall. This pulse of recharge has a highconcentration of contaminants but remains oxidising. It can be sequentially detected in shallowdug wells moving away from the landfill as the rainy season progresses. The velocity of thispulse shows that groundwater moves slowly in the alluvial sediments beneath the adjacent villageat about 1-2m/day.

0

500

1000

1500

2000

Jan-89 Apr-89 Jul-89 Oct-89 Jan-90

Date

Con

duct

ivit

y (µ

S/c

m)

0

100

200

300

400

500

Rai

nfal

l (m

m)

Rainfall

Conductivity

Conductivity peak in a dug well close to the landfill at Mae Hia, Thailand


• diesel or lubricating oil probably from well pump installations and disposed engine oil

• an isolated instance of chloroaniline, an industrial chemical

The non-specific organic loading from leachate, as indicated by measurements of dissolved

organic carbon, may pose a greater hazard due to the formation of trihalomethanes during

water disinfection with chlorine (Stuart et al. 1999).

Formation of trihalomethanes in contaminated groundwater

Dissolved organic material can react with halogens (fluorine, chlorine and bromine)during water chlorination for potable supply to form trihalomethanes (THMs). Foreffective disinfection an excess of chlorine over the sample consumption is needed andthis free chlorine can react with organic compounds present in the water during storageor distribution of treated water. There is concern that the use of abstracted water with anenhanced organic load may lead to increased THM production.

The THM compounds most commonly formed are chloroform, bromodichloromethane,chlorodibromomethane, and bromoform. Bromide, often present in raw water, fromeither natural or anthropogenic sources has an important effect on speciation if present insignificant concentration and results in a high bromine incorporation into the THMs.The figure shows the rapid rate of formation of THMs in a sample from a dug well in theleachate plume at Mae Hia, Thailand.

0

500

1000

1500

0 50 100 150 200 250 300

Time (H ours)

Con

cent

rati

on (

µg/

l)

Chloroform DichlorobromomethaneDibromochloromethane BromoformTotal

Trihalomethane formation in groundwater from Mae Hia, Thailand


3 LANDFILL IMPACT ASSESSMENT

During the present study three approaches have been used to model the impact on

groundwater quality of landfill leachate depending on the prevailing hydrogeological

situation. They are 1) simple analytical models, 2) a probabilistic risk model developed for

the UK Environment Agency called LandSim, (Golder Associates, 1996) and 3) the

Hydrologic Evaluation of Landfill Performance (HELP) Model, (Schroeder et al., 1994a;

Schroeder et al., 1994b).

3.1 Simple Analytical Models

When a conservative contaminant travels through a porous aquifer medium its movement is

governed by the advection dispersion equation (ADE).

Advection Dispersion Equation

The ADE can be expressed as follows:

t

C

x

Cv

x

CD xL ∂

∂=∂∂−

∂∂

2

2

where: DL is longitudinal hydrodynamic dispersion coefficient

vx is the average linear velocity in the x-direction

C is the mass per unit volume of solute

The longitudinal hydrodynamic dispersion can be thought of as a parameter

that accounts for the mixing of solute due to mechanical effects in the direction

of flow and diffusion around the grains in the aquifer matrix. The

hydrodynamic dispersion is defined mathematically as:

DL = vxα + η D*

where: α is the dispersivity

D* is the diffusion coefficient


The ADE is also used to estimate the transport and attenuation of bacteria and contaminants

that are reversibly adsorbed and result in retardation in contaminant transport rate. The ADE

assumes that the porous medium is homogeneous, isotropic and saturated with fluid. An

analytical solution, suitable for the situation of a landfill releasing leachate into an aquifer,

with the following initial and boundary conditions

C(x, 0) = 0 x ≥ 0

C(0, t) = C0 t ≥ 0

C(4,t) = 0 t ≥ 0

is given by (Fetter 1993) as:

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ +⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎟⎠

⎞⎜⎜⎝

⎛ −=

tD

tvLerfc

D

Lv

tD

tvLerfc

CC

L

x

L

x

L

x

2exp

220

where: C0 is the initial concentration,

L is the distance from the point of injection to the point of measurement.

This equation is readily manipulated in a spreadsheet and can be used as a first pass at

estimating landfill leachate impact.

3.1.1 A Case Study from Thailand: the Tha Muang Site.

3.1.1.1 Site Setting and History of Development

The disposal site serves the town of Tha Muang, which has a population generating about 3.5

tonnes of waste per day. The waste is deposited in a walled compound about 100 metres

square to the southeast of the town. Within the waste site are two lagoons that are collectors

for leachate during the wet season. The site has been in use for about 30 years and the

practice has been to incinerate the waste by open burning and periodically remove the ash to

another site.


A sketch map of the site is shown in Figure 3.1. To the north and south of the dump are

irrigated agricultural fields. To the west is an area of housing, each house surrounded by a

large garden, the property adjacent to well H is the nearest. To the south are various farms

and residential buildings served by hand dug wells and abstraction is by belt driven

centrifugal pumps powered by diesel engines.

Figure 3.1 Sketch Map of the Tha Muang Landfill Site.


3.1.1.2 Geology and Hydrogeology

Tha Muang is situated on the north bank of the Mae Klong River that drains an extensive

aquifer basin formed in unconsolidated Pleistocene boulder gravels, sandy gravels, sands and

silts. Away from the river the aquifer is concealed beneath more recent silts and clays.

Based on unpublished well test data from the Department of Mineral Resources, Thailand,

the hydraulic conductivity of the aquifer ranges from 10-4 to 10-5 m/s, values typical of a

mixed gravel, silty sand aquifer. Water level is three to four metres below ground level.

3.1.1.3 Leachate Impact Assessment

For the purposes of the modelling we are trying to predict the impact of the landfill on well C

which is about 25 metres from the landfill (Fig. 3.1). The immediate problem is the absence

of aquifer data to parameterise the model. The following estimates are reasonable based on

our local knowledge:

Hydraulic conductivity = 1e-5 m/s (based on well test data from the surrounding area)

Porosity = 0.25 (based on the presence of a silty sand lithology, Freeze and Cherry (1979)

Hydraulic gradient = 0.02 (estimated from topographic map).

Figure 3.2 is a graphical representation of the model output illustrating the change in

concentration away from the landfill. The model indicates that at a distance of 25 metres

from the landfill the concentration of a non-attenuating solute will be about 0.49 times the

initial concentration. The initial leachate chloride concentration was determined to be about

1000 mg/l, indicating that the concentration at 25 metres should be about 490 mg/l. The

measured concentration was between 460 and 480 mg/l. The concentration rapidly drops

away, and at a distance of about 100 metres, in well A, it is negligible. The background

chloride is between 78 and 160 mg/l, the concentration in well A varies from 78 to 115 mg/l,

i.e. background values. The rapid decline in leachate impact away from the landfill is

attributed to dilution of the leachate plume within the aquifer. This at first glance is a very

good result; however, the model is very sensitive to the value of hydraulic conductivity used

in the calculation and such simple analytical models need to be used with extreme caution.


3.2 LandSim

LandSim synthesises geological, hydrogeological and climate data to arrive at estimates of

the volume of leachate produced by a particular site, and the impact upon surface and

groundwater receptors in terms of predicted groundwater quality. It has been developed to

take into account the uncertainties associated with the geological and hydrogeological

characterisation of a site and also uncertainties in leachate composition, i.e. in the source

term. LandSim is modular in format and follows the classical source, pathway, receptor

scenario analysis to arrive at a quantified risk. The model is probabilistic and uses Monte

Carlo simulation to select randomly from a pre-defined range of input values to create

parameter values for use in calculation. The results are presented as probability plots and

performance can be quoted at a given confidence level.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100

Distance from source(m)

C/C

o

Figure 3.2 Concentration with distance away from the landfill

Well C

Well A


3.2.1 A Case Study from Thailand: the Mae Hia Site


Chiang Mai is the second largest city in Thailand with a population in excess of 160,000.

The city relied on open dumping for waste disposal at the Mae Hia site for over 30 years until

it was closed in May 1989 due to the unacceptable environmental impact on shallow

groundwater wells in the vicinity of the site. Mae Hia landfill site lies about 10 kilometres

southwest of the city of Chiang Mai and covers an area of approximately 21 ha, of which 12

ha have been used for waste dumping. Figure 3.3 is a detailed sketch map of the site and

shows the location of the sampling wells used in the study. Approximately 100 houses exist

in the vicinity of the site while the area to the south is used for various agricultural activities

including rice growing and pig farming.

During the initial operational phase of the site daily cover was used but, as the waste arising

from Chiang Mai increased from 15 t/d in 1958 to 150 t/d in 1989, open dumping became the

preferred option in order to increase the useful life of the site. The waste thickness varies

from two to five metres and at the southeast end of the site a bunded leachate lagoon has been

formed which overflows during the rainy season.


According to data presented by Margane et al. (1998) the Mae Hia Landfill Site is sited on a

sequence of Quaternary colluvial deposits which are derived from the high ground on the

western side of the Chiang Mai Basin. These deposits consist of impersistent sand and gravel

layers interbedded with clayey units. Locally they rest on preserved remnants of the “High

Terrace Deposits” which consist of thick sand and gravel beds. Based on a compilation of

hydraulic data from tested wells in the colluvial deposits, (Margane et al. 1998), the hydraulic

conductivity is log normally distributed with a mean value of 1.4x10-5 m/s and a maximum

value of 2x10-4 m/s, consistent with a sandy gravel aquifer.

The ten-year mean annual rainfall of the Chiang Mai Basin (1987-1997) is 1115mm and the

mean annual evapotranspiration is 1855mm. Recharge, calculated from monthly rainfall and

evaporation data, is 212±112 mm/a. In the absence of daily data, this is likely to be an

underestimate.


In the vicinity of the landfill site, water supply was originally from shallow wells dug into the

colluvial aquifer. Water depth ranges between 0.5 and 9.55 metres depending on the well

location and the season. In general, water level response to rainfall is fast in most wells

indicating that infiltration is quite rapid (Karnchanawong et al. 1999). Stable isotope studies

by Buapeng et al. (n.d.) indicate that recharge of the younger terrace deposits is relatively fast

and derives from rainfall on the higher ground at an altitude of 600 to 800 metres. The

groundwater flow direction has been estimated from the water level monitoring data and in

general flows from west to east. The average hydraulic gradient has been estimated to vary

between 0.008 and 0.009 although it is steeper in the vicinity of the landfill and is probably

topographically controlled.


The input data into the LandSim model takes the form of probabilistic distributions rather

than single variable values. The underlying mathematical model for contaminant transport is

similar to the one presented earlier, but allows for two-dimensional transport and also takes

into account attenuation. Figure 3.4 is the modelled LandSim output for a point three

hundred metres down gradient of the leachate pond at Mae Hia and represents modelled

impact on well 12. The data are presented as a reverse cumulative probability plot and

indicates that at 95% probability the chloride concentration will be 644 mg/l or less. This

figure compares quite well with the value of 773 mg/l determined by chemical analysis of

water from well 12. However, the model predicted a concentration of 191 mg/l ammonium at

the point of impact whereas the field value was about 0.06 mg/l. The problem is that the

model cannot cope with reactive transport and Karnchanawong et al. (1999) have

demonstrated that there is a redox change along the plume which causes ammonia to be

consumed, nitrate to be generated and increased manganese to be taken into solution. In

terms of human health it is this secondarily liberated manganese which constitutes the toxic

health risk.

WA

ST

E L

AN

DF

ILL

SIT

E

AB

AN

DO

NE

DC

OM

PO

ST

ING

PL

AN

T

LE

AC

HA

TE

PO

ND

cem

eter

y

297

296

303 30

2

301

300.

0

302

300

298298290

300

301

300

299

298

302

303

303

304

308

309

305

306307

308

308

307

306

308

309310

305

304303

301

301

303

302

299

298

297

308

23

24

12

3

4

5

67

8

9

10

11

12

13

14

15 1617

18

19

20

21

22

2526

27

28

29

30 31

32

33 34

35

36

37

38

3940

E

K

41

28 K

Ro

ad

Ch

an

ne

l

Sm

all

stre

am

Po

nd

Co

nto

ur

(at

1 m

etr

ein

terv

als

)

CM

U w

ell

with

sam

ple

nu

mb

er

BG

S w

ell

Fa

rm

Tem

ple

Pa

dd

y fie

ld

Fo

rest

N

0100

200

metr

es

BA

N B

OH

BA

NP

AC

HE

E

Fig

ure

3.3

The

Mae

Hia

Lan

dfill

Sit

e

WC/99/17 22 Version 1, Revision 1


Figure 3.4 Modelled output from LandSim for chloride in Well 12 at the Mae

Hia Site.

3.3 The HELP Model

This is a quasi 2-D model used to predict the movement of water across, into, through and out

of landfills. Sites consisting of various combinations of vegetation, cover soils, engineering

and capping can be modelled for different climatic settings. The model facilitates rapid

estimation of the amount of leachate generated and is applicable to open or closed sites. Data

input consists of weather, soil and design data and uses solution techniques that account for

the effects of storage, runoff, infiltration, evapotranspiration, vertical drainage and leakage

from the landfill. Although the model was primarily envisaged as a design tool to predict

water balances in landfills and cover systems it also provides useful information to

hydrogeologists on source term volumes of leachate leakage which can be used in dilution

calculations. This is a very basic level of impact assessment, but in some cases proves to be

of particular use. A case study is presented for a karstic terrain in southern México.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900 1000

Concentration (mg/l)

Pro

bab

ility


3.3.1 A Case Study from México: The Mérida Municipal Landfill, Yucatán


The state of Yucatán, México has a population of over 1.5 million and generates about 970

tonnes of solid waste per day. The waste is produced mainly in the principal tourist centres

on the Caribbean coast, and the state capital Mérida. When compared to the national daily

total of over eighty one thousand tonnes this quantity seems an insignificant amount (~1.2%).

However, in the Yucatán, where 100% of the water supply is groundwater derived from

karstic limestone, the uncontrolled dumping of mixed industrial and domestic waste, possibly

hazardous to the aquifer, is of serious concern.

The Mérida Municipal Waste Site is situated adjacent to the Anillo Periferico (outer ring

road) on the northwestern side of the city, and approximately 1.5 kilometres west of the

University’s Engineering Faculty, Figure 3.5. It occupies an area of about 38 hectares and

has been in operation for 18 years presently serving a population of 556,819 people. It

accepts a mixture of commercial, industrial (including hazardous) and domestic wastes.

On the northeast side of the site there is an oxidation lagoon which consists of four

interconnected cells accepting septic tank effluents. The bases of these lagoons are unlined

and in direct contact with the limestone bedrock. Originally the lagoons discharged onto reed

beds, but these have died off due to negligence and the impact of the highly mineralised

overflow from the silted up lagoons.

Gonzáles Herrera (1996) provides a detailed account of the operational practices at the site

and the disposition of the waste types. The waste is dumped in the open air in a zone known

as ‘El Cerro’ (the hill) a platform of waste up to six metres high where heaps of mixed waste

may be observed. As well as the main dump, selected areas are reserved for specific waste

categories. For example near the entrance to the site is an area dedicated to the disposal of

used tyres, one for slaughter house wastes and another for rotten eggs and egg shells. The

daily operation is usually carried out on an area of about one hectare where the garbage

trucks discharge their waste more or less systematically. The garbage is then sorted by the

community of informally organised scavengers who recover glass, metal, plastic and card,

which are then sold on for re-cycling. Throughout the day the sifted waste is bulldozed to the

main dump area and compacted. Daily cover is not used and the site was the haunt of

vultures and feral dogs.

C1

D

D2

I5

M1

N1

P1

P2

P4

P6

P3

UW

1

UW

2

UW

3

UW

4

UW

5

UW

6

Fig

ure

3.5

Loc

atio

n m

ap M

erid

a M

unic

ipal

Was

te S

ite

show

ing

the

shal

low

sam

plin

g po

ints

Rin

gR

oad

Uni

vers

ity C

ampu

s




The site geology is characterised by a sub-horizontal sequence of Pliocene and Miocene

karstic calcarenties and bioclastic limestones forming a regional platform, six to seven metres

above sea level. Northwest-southeast trending fractures promote the development of

dissolution features. The limestones are covered with a thin rendzina type soil up to 20

centimetres thick in the vicinity of the site.

The phreatic surface is at around 5.45 metres below ground level with an annual fluctuation

of ±50-70 mm and regional groundwater flow direction to the northwest. The hydraulic

gradient across the site has been estimated as 7.96 10-5

by Sanchez and Pinto (1989) and as

7 10-6

by Brewerton (1993).

Tests conducted by Sanchez and Pinto (1989) indicate hydraulic conductivities ranging from

9.3 10-5

m/s for calcarenite to 3.4 10-8 m/s for partially recrystallised, well cemented

limestone; porosities range from 40 to 50%. Brewerton (1993) working on core samples

demonstrated porosity variations from 35% to 55% and hydraulic conductivities ranging from

3.5 10-6

to 1.6 10-5 m/s.

The climate is considered to be humid tropical, with a mean annual rainfall of around 1000

mm, most of which falls between May and October. Rapid recharge, estimated to be about

100 mm per year (British Geological Survey, 1995), is assumed to occur in the region and is

responsible for leachate generation in the landfill.


The Mérida site has a relatively good monitoring network in place consisting of purpose-

constructed monitoring wells and shallow hand dug wells used for groundwater abstraction.

Based on the groundwater quality monitoring data it is evident that a contaminant plume is

moving away from the landfill towards the northwest due to the regional groundwater flow,

Figure 3.6. It is interesting to note that the impact of the landfill is not detectable beyond

about one kilometre from the site. The HELP model was implemented for the site and has

been reported in detail by Ku Cardenas (1998). The model was modified from the original to

take into account the temporal changes in construction of the fill and changes in waste

density due to settlement.

Using this approach it was estimated that approximately 35,293 m3 of leachate was produced

between March 1993 and December 1995. Figure 3.7 is a graphic demonstrating the landfill

response to rainfall events and the consequent leachate production.


Figure 3.7 Relationship between leachate flow (flujo de lix.) and precipitation during

the period of simulation.

BASUREROMUNICIPAL

Figure 3.6: TOC plume from theMerida Landfill

GROUNDWATER FLOW DIRECTION

DZITYA


Using these figures the average annual leachate infiltration rate is approximately 145 mm/a at

an average concentration of about 4857 mg/l chloride. To calculate the dilution factor beneath

the landfill the following equation is used:

wI

vDF

.

.1

δ+=

where: DF is the dilution factor,

δ is the flowing thickness of the aquifer, taken as 20 metres.

I is the leachate infiltration rate, i.e. 145 mm/a

w is the width of the active landfill site perpendicular to the flow direction, taken as

350 metres

v is the groundwater velocity.

The parameterisation of the velocity is very difficult in this karstic terrain. A simple

calculation of the Darcy velocity is a serious underestimate as it does not take into account

fracture flow. Derived values of the order of 6x10-4 m/d are inconsistent with well tests that

show almost instantaneous recoveries following prodigious abstraction rates. This suggests

that very high fracture porosity coupled with high matrix porosity are the governing controls.

Ward et al. (1985) describing the hydrogeology of the Xel Ha area on the Caribbean coast

calculated an average discharge rate of 8.6x109 l/a/km of coastline that equates to a discharge

velocity of about 24 m/d. Depending on the fracture connectivity, localised flows could be of

the order of hundreds of metres per day. Applying these values to the above equation, one

arrives at a dilution factor of about 1700. Taking the mean chloride concentration of the

leachate as 4857 mg/l, means that the concentration in the aquifer due to dilution will be

about 3 mg/l above background. Figure 3.5 indicates that some hydrodynamic dispersion of

the contaminant takes place but that background concentrations of chloride are reached

within a very short distance from the landfill boundary. Groundwater monitoring has

confirmed that the impact of the landfill is limited in terms of chloride.


4 HEALTH RISK ASSESSMENT

4.1 Risk Assessment Model

According to the US National Academy of Sciences, risk assessment is the process of

characterising the adverse health effects of human exposures to environmental hazards. For

the purpose of health risk assessments in the current study the Risk*Assistant model,

developed on behalf of the US Environmental Protection Agency (USEPA), has been used.

A risk assessment is subdivided into three stages, essentially following the classical source –

pathway – receptor model. They are:

1. Hazard identification,

2. Exposure assessment,

3. Dose – response assessment.

Hazard and risk are frequently confused: they are not synonymous. At the most basic level

hazard equals danger, and in the risk assessment context a hazard exists if a potential exists to

cause harm. Conversely, risk is the likelihood of an adverse event occurring in response to a

hazardous situation.

The preliminary step in exposure assessment is the construction of a conceptual model that

represents the exposure pathways. The conceptual model shown in Figure 4.1 is an attempt

to identify the principal exposure pathways associated with living close to a landfill.

Pathways are both direct, e.g. the ingestion of contaminated dust, or indirect, e.g. the

ingestion of contaminated groundwater.

The dose assessment is achieved by estimating total environmental exposure to a particular

hazardous compound identified in the source. Compounds deriving from landfill leachate

either constitute a toxic hazard or a carcinogenic hazard.


Figure 4.1 A Source – Pathway – Receptor Conceptual Model for environmental exposure adjacent to a landfill.

4.2 Toxic and Carcinogenic Risks

The general practice is to assume that a toxic chemical has a threshold below which toxic

effects do not occur. Toxic hazard estimates are expressed relative to a reference dose

concentration. The reference dose is an exposure that can occur over a prolonged period

without ill effect. Risk estimates are based on a comparison of actual exposure to this

reference dose for the particular chemical involved.

Carcinogenic compounds differ from systemic toxic compounds in that there is no lower limit

for the existence of cancer risk.

4.2.1 Case study from Thailand: Mae Hia Landfill Site

For the purposes of this study it is the groundwater pathway exposure scenario for a typical

Thai population living close to the Mae Hia landfill site that is of interest.


To illustrate the possible health problems associated with the groundwater contamination,

two pollutants are considered (WHO, 1993): manganese, with evidence of neurotoxicity in

miners, and a carcinogen, bis(2-ethylhexyl)phthalate (DEHP) which has been shown to

produce liver tumours in laboratory animals. To assess the risk, a hazard quotient (H.Q.) has

been calculated for manganese. As defined by the USEPA, it is the ratio of the average daily

dose to the reference dose. HQs greater than one indicate that there is a toxic risk. For well

12 (see Figure 3.3) with manganese at a concentration of 7 mg/l, the H.Q. is 2.75. A DEHP

concentration of 7 µg/l per litre in well 5 produces an increased risk of cancer of 5 in

1,000,000.

TOXIC RISKS

• Are defined for non-carcinogenic exposures

• Think in terms of a Hazard Quotient (H.Q.)

• H.Q. = ADD/Rfd

Where Rfd is the reference dose, and the ADD (the Average Daily Dose) is:

Total Potential Dose

Body Weight x Exposure Time

CARCINOGENIC RISKS

• Are statements of probability

• Individual excess risk is an estimate of the probability that an individual will get

cancer from an exposure, not the probability of dying from it

• It is calculated from Risk = Slope Factor (SF) x Lifetime average daily dose

(LADD). The LADD is the average daily dose averaged over a lifetime.

The SF and Rfd are compound specific and may be obtained from the USEPA database

IRIS (see http://www.epa.gov/ngispgm3/iris).


Although risk assessment involves the application of seemingly trivial mathematical

equations, problems arise in their parameterisation, and detailed knowledge of exposure

factors is required in order to make the calculations useful. The above risk assessment was

based on the following exposure factors:

• Exposure duration. The exposure duration can be estimated by taking the difference in

time from the inception of the landfill site to the present time, assuming that the

population has remained static, in this case 40 years.

• Body weight. Body-weight data were obtained from the Provincial Public Health Office

in Chiang Mai. For men the average body weight is 58 kg and for women it is 50 kg.

• Life expectancy. According to the Provincial Health Office life expectancy was 60 years

for both men and women during the last decade. However, deaths from AIDS has

reduced the average life expectancy of males to 50.20 years while females still experience

an expectancy of 60.48 years.

• Water ingestion rate. This factor is a little more difficult to quantify but a low estimate,

based on bottled water consumption, is 3 litres per day. The Exposure Factors Handbook,

(USEPA 1996), reviews water intake in detail and suggests a mean intake rate of 6 litres

per day for active adults in temperate climates increasing to 11 litres per day in a hot

climate.

It is evident that both uncertainty and variability in factor parameterisation exist. Uncertainty

refers to a lack of knowledge about specific factors whereas variability refers to factor

heterogeneity attributable to natural random processes (USEPA, 1997). A way of dealing

with the parameterisation uncertainty problem is to perform a sensitivity analysis, i.e. an

interactive process of changing an exposure factor within a range that encompasses the

known variability to observe the effect on the dose and hence risk. The results of the process

are illustrated in Table 4.1 for groundwater consumption, the least constrained parameter in

the above analysis. The body weight and life expectancy values used were average values for

a woman, and the exposure duration was taken as the time elapsed since inception of the


landfill. The analysis is therefore a worse case scenario; however the results demonstrate the

sensitivity to the groundwater consumption parameter.

Table 4.1 Sensitivity analysis for groundwater consumption risk assessment at the Mae

Hia site.

Water Consumption (l) Hazard Quotient (Mn) Carcinogenic Risk (DEHP)

3 2.897 4:1,000,000

6 5.794 7:1,000,000

11 10.622 1:100,000

The results also demonstrate that there is the potential for unacceptable toxic and

carcinogenic risk from groundwater consumption at this site. The assessment can be used to

highlight this fact to the responsible regulators and operators. A further important

observation is that although the Mae Hia site was closed down in 1986 it continues to pose a

health risk. In actual fact much of the local population is now on a water main, but the well

water is still used in some of the poorer households for drinking and cooking.

4.3 Microbiological Contamination

Cameron and McDonald (1977) found high levels of microbiological activity in landfill

leachate even in the absence of detectable coliform bacteria. Other studies of the microbial

ecology of landfill leachate contaminated groundwater have tended to focus on

biotransformation reactions (e.g. Beeman and Suflita (1987) and Ludvigsen et al. (1997))

rather than the pathogenic risk to groundwater consumers. Due to the difficulty of detecting

low concentrations of pathogenic bacteria and viruses, coliform bacteria are used to

determine faecal contamination of water supplies. Thermotolerant coliforms (faecal) were

detected in the groundwater plume at all the sites studied in this project and were found at

very high concentrations in many wells. It is thought that their presence is, at least partly,

derived from the disposal of septic tank effluents at landfill sites.

The theory that coliform bacteria can multiply rapidly where leachate enters an oxygenated

groundwater system was tested in México on leachates from the Mérida site. It was found


that when leachate was diluted with bacteria-free groundwater there was an increase in the

number of thermotolerant coliforms and the bacteria were able to survive for up to two weeks

under laboratory conditions, Figure 4.2. The limiting parameter was probably lack of

nutrients. This result means that the karstic groundwater system beneath the Mérida landfill

is particularly vulnerable to bacterial contamination given the zero filter efficiency of the

limestone, fast travel times and high dilutions.

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

25-Jan-98

30-Jan-98

4-Feb-98

9-Feb-98

14-Feb-98

19-Feb-98

24-Feb-98

Date

No/

100m

l

Figure 4.2 Growth of thermotolerant coliforms in a leachate-clean groundwater mixture.


5 RELEVANCE OF PROJECT FINDINGS TO OTHER LANDFILL SETTINGS

5.1 Summary of Findings and Discussion

Three contrasting landfills have been studied and are described in this report. They were

selected on the basis of size, geology, hydrogeology, climatic setting and waste type. The

study has demonstrated that site geology is one of the main controlling factors on the impact

of landfill leachate on aquifers. The aquifer types represented by the study were:

1) Alluvium: Tha Muang, Thailand

2) Colluvium: Mae Hia, Chiang Mai, Thailand

3) Fractured Limestone: Mérida, México

Aquifer properties and groundwater flow regimes also play a critical role. In an earlier study

Klinck (1996), using a variety of aquifer vulnerability assessment schemes, showed that the

Mérida aquifer was extremely vulnerable to pollution from the landfill operation. However,

it is evident from the detailed sampling carried out around the landfill that the impact on the

aquifer is not evident beyond a few hundred metres from the site and leachate-linked

contaminants are reduced to background levels by dilution.

In the case of the colluvial aquifer situation at Mai Hia landfill, where aquifer transmissivity

is reasonably high, a similar situation would be expected. Indeed, the evidence suggests that

the aquifer is rapidly flushed by the annual recharge event. But again, looking to the results

of the detailed sampling carried out, we see that the contamination is more persistent, the

leachate groundwater plume extending for over 1000 metres. Moreover, due to redox

processes within the aquifer, toxic levels of manganese and unacceptable nitrate

concentrations can occur which are well in excess of those found in the original leachate.

The alluvial site seems to fall somewhere in between in terms of aquifer impact. This is

thought to be due to the practice of waste burning. The result is an overall reduction in

organic loading on the aquifer combined with attenuation of contamination due to the

presence of a higher silt content, capable of sorbing metals and ammonia.


In evaluating the human health impact of leachate contamination of groundwater, the

pathway considered has been direct ingestion of contaminated water. At all three case study

sites faecal coliform contamination of the aquifer was detected as a primary impact and

coliforms were also incubated from some of the leachate samples. There is compelling

evidence from the dilution/incubation studies conducted in México that leachate can be a

source of pathogenic contamination down-gradient from the site. Generally, however, it is

difficult to differentiate between pathogenic contamination arising from leachate and that

arising from poorly constructed household septic tank systems.

In terms of risk management, bacterial contamination is generally easy to deal with, either by

boiling or disinfecting the groundwater. The current study has demonstrated that inefficient

chlorination of groundwater with a high organic loading, possibly leachate derived, can give

rise to the production of trihalomethane compounds (THMs), some of which are recognised

as being both toxic and carcinogenic. Although this finding may be of only minor or no

concern in a well-managed end-of-pipe treatment system, it may constitute a hazard in other

situations. For example, in the village of Dzitya, close to the Mérida landfill, chlorine tablets

in perforated bottles were left hanging in wells as a means of chlorination. This situation

provided the right conditions for prolonged chlorination and possibly the formation of THMs

in the stagnant well storage.

Dissolved organic carbon (DOC) is the main component of all of the leachates studied and is

directly attributable to the organic content of the waste. However, it has proved to be very

difficult to identify specific organic compounds responsible for the high DOC. Therefore, in

order to carry out a risk assessment, DEHP, a plasticiser, was chosen as an environmental

contaminant which is directly linked to leaching plastics in landfill waste. In calculating

carcinogenic risk a high DEHP value was used from the Mae Hia site that produced an

increased risk of cancer of 5 in 1,000,000. The USEPA consider an increased risk of 1 in

1,000,000 as significant and therefore there is an unacceptable risk of cancer on that basis.

However, in terms of risk management it might be decided that 5 in 100,000 is the cut off

limit. The definition of the limit might depend exclusively on the demands placed on medical

facilities for example. Possibly more insidious is our lack of knowledge concerning the other

components of the DOC and the effect that they might have at very low concentrations on

health risk estimates. This is an area that needs further study.


Toxic heavy metals are often cited as being a major concern in landfill leachates: the reality is

more prosaic. Research by Yanful et al. (1988) has shown that, unlike conservative species

such as chloride, heavy metals remain in the waste or at the waste–rock interface as a result

of redox controlled precipitation reactions. Pohland et al. (1993) determined that metal

mobility is also controlled by physical sorptive mechanisms and that landfills have an

inherent in situ capacity for minimising the mobility of toxic heavy metals. This fixing of

heavy metals dramatically reduces the risk of direct toxic effects due to ingestion of leachate

contaminated groundwater. However, once the leachate leaves the site the situation changes.

The leachate is generally a strongly reducing liquid formed under methanogenic conditions

and on coming into contact with aquifer materials has the ability to reduce sorbed heavy

metals in the aquifer matrix. The most important reactions are the reduction of iron and

manganese to more soluble species and hence one sees an increase in the concentration of

these components under favourable conditions close to a landfill. This is the case at the Mae

Hia site in Thailand where the toxic effect of the leachate contamination is due to this process

of reduction of manganese present in the aquifer matrix. The impact of this process has been

demonstrated to lead to a serious toxic risk.

5.2 A Generic Approach to Landfill Leachate Impact Assessment

The principal objective of this report has been to provide case study examples of risk

assessment and appropriate techniques of impact assessment. It is evident that there have

been difficulties in applying the methodologies tested during this study and the overriding

contributing factor to this has been lack of knowledge to constrain problem definition. This

lack of knowledge is present throughout the assessment process and the final outcome may be

an over-conservative assessment that will flag up the need for inappropriate and expensive

risk management measures. An attempt has to be made to produce a balanced risk

assessment while at the same time adopting the precautionary approach in implementing risk

management. It is suggested that the following checklist of data requirements (Table 5.1)

might be used in designing a risk assessment.

Figure 5.1 is a flow diagram illustrating a methodology to conduct a landfill impact

assessment and human health risk assessment once the data have been collected. The flow

chart illustrates that there are two ways to arrive at the risk assessment. The easiest one only

requires a chemical analysis at the point of compliance plus a constrained exposure scenario.


The second, more complicated approach first requires an assessment of concentration at the

point of compliance using a suitable mathematical model and then input of that result to the

risk assessment model.

Table 5.1 Data requirements checklist for risk assessment

Geological Data geological map, borehole logs, lithological information

Hydrogeological Data water level data, hydraulic gradient, aquifer parameters, e.g. porosity and hydraulic conductivity, climate data

Demographic Data life expectancy, body mass

Hydrochemical Data Leachate source term, groundwater quality

Exposure Data Exposure duration, frequency of exposure, exposure pathways, dose calculations

5.3 Risk Management Options

In dealing with an unacceptable landfill leachate impact there are limited management

options available. They basically come down to the following approaches:

1) Removal of the source term.

Source term removal is possible by:

a) removing the waste to a more suitable site. This could be a very expensive option

depending on the size of the landfill and the volume of waste deposited.

b) Reduce the amount of leachate being generated. This option is technically more feasible

and requires the landfill to be capped with a suitable, low permeability cover in order to

reduce infiltration and hence leachate production. There are knock-on engineering effects

from this approach which include the need for a landfill gas management system, and a

system to manage surface runoff from the cap.


Figure 5.1 Flow diagram of stages in completing a health risk assessment

Collate GeologicalData

CollateHydrogeological Data

Collate DemographicData

Conceptual Site Model

GroundwaterChemistry Dose Assessment

(Risk*Assistant)

Landfill ImpactAssessment

HELP Model Probabilistic ModelDeterministic

Model

Risk ManagementRequired?

Exposure PathwayAnalysis


2) Leachate plume management.

This could be achieved by defining the limit of unacceptable leachate impact through a

groundwater monitoring network and the definition of a hazard zone around the landfill using

appropriate impact and risk assessment models. This zone would then define where

groundwater abstraction and consumption would be unsafe. This option is relatively cheap as

long as the DOC loading is well characterised. This management approach would also aid in

optimising piped water supply planning.

3) Waste reduction.

Recycling and composting are often encouraged by some municipalities in order to reduce

the amount of waste going to landfill and incineration. These technologies in conjunction

with waste minimisation measures are seen as the sustainable option and have been placed in

the following hierarchy:

• waste reduction

• re-use

• recycling, composting and energy recovery

• disposal to landfill and incineration with no energy recovery.

At first glance, activity is at the lower end of the waste hierarchy in the newly industrialised

countries. Waste disposal, usually, is a low priority area for investment, generally lacking in

infrastructure development compared to the high visibility engineering projects which tend to

receive a larger share of the available financial resources. Paradoxically, landfills and wastes

are a rich source of re-usable materials and most developing country landfills support active,

scavenger communities. This culture of re-use usually begins at the waste source. Waste

collectors recover high value items such as intact glass and plastic bottles before the delivery

to the landfill. At the landfill scavengers collect other recyclable materials including glass,

paper, card, plastic and scrap metals. The overall benefits of such activity are not well

quantified, but indications are that between 1% and 5% of the disposed waste is recovered

and incomes from such activity often exceed local minimum wages.


Developing country wastes typically contain between 25 and 70% of putrescible material. An

activity, which could reduce the organic waste content and consequently DOC loading in

leachate, is composting. Taken together recycling and composting are effective methods of

waste minimisation and consequently mitigate landfill impact.


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