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LandSim 2.5
groundwater risk assessment toolfor landfill design
www.environment-agency.gov.uk
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Environment Agency LandSim 2.5
IntroductionThe EC Groundwater Directive (80/68/EEC)(implemented
via Regulation 15 of the WasteManagement Licensing Regulations 1994
and theGroundwater Regulations 1998) requires priorinvestigation
for activities involving the discharge oflisted substances to
groundwater. Technicalprecautions must be taken to prevent the
dischargeof List I substances to groundwater and to ensurethat List
II substances do not cause pollution. Therecently adopted Landfill
Directive (99/31/EC)requires pollution to be prevented during the
entirelife-cycle of the landfill. Risk assessment tools
fordetermining the necessary technical precautions toprotect
groundwater at landfills must therefore beable to take account of
the inevitable future failureand degradation of active engineering
andmanagement control systems (for more informationsee Reference
1).
The LandSim software model has been developed forthe Environment
Agency to provide probabilisticquantitative risk assessments of the
performance ofspecific landfill sites in relation to
groundwaterprotection. Since 1996 it has been used as a
relativelysimple tool to evaluate leakage of leachate
fromlandfills, attenuation in the unsaturated zone, anddilution and
contaminant transport in the saturatedzone. LandSim allows landfill
operators and regulatorsto consider the environmental performance
ofdifferent liners (e.g. compacted clay versusHDPE/clay) and
leachate collection systems, and totake account of the large
variety of geological andhydrogeological regimes.
LandSim uses the Monte Carlo simulation techniqueto create
parameters for use in the model calculationsby random selection
from a pre-defined range ofpossible input values (probability
density functions).This process is repeated many times to give a
rangeof output values. This probabilistic methodologyallows
improved quantification of uncertainty withinthe geological
environment, the performance ofspecific landfill lining systems,
and leachate chemistry(see Reference 2).
LandSim 2.5 performance in thelong termFollowing implementation
of the Landfill Directive(LFD) it is necessary to be able to model
variations inthe performance of lining systems over the
entirelifetime of a landfill. Modern liner materials (e.g.
highdensity polyethylene (HDPE) geomembranes) areexpected to
function effectively for hundreds of yearsbefore degradation leads
to increased leachateleakage. However, since the timescale for
stabilisation
of many wastes is believed to exceed this time period,the
flushing of contaminants from landfills is likely tocontinue after
the liner has failed. LandSim 2.5extends the capability of the
existing model (v.2.02)by considering changes in the integrity
ofengineering and other active management controlmeasures
throughout the period (centuries) thatlandfills have the potential
to pollute.
The sophisticated approach to simulating changes inleachate
quality over time that was introduced for theLFD waste acceptance
criteria negotiations has beenincluded in LandSim 2.5.
Program outlineLandSim 2.5 is coded in the C++
programminglanguage. In common with earlier versions, it hasbeen
designed to enable groundwater riskassessments to be carried out by
experiencedprofessionals with a good background in
landfillengineering, hydrogeology and risk assessment butwithout
the need for an in-depth knowledge ofprogramming. A cartoon (Figure
1) guides you alongthe path of contaminant transport from
theengineered landfill to a groundwater receptor.
At each stage along the pathway, you are asked toenter data that
enables LandSim 2.5 to determine therate of contaminant migration.
Parameters areassigned to the model using pop-up dialogue
boxes(Figure 2) accessed by clicking on active areas of thecartoon,
or from drop-down menus, or through adata entry wizard. To prevent
unnecessary data entry,the dialogue boxes are customised to the
chosenlandfill design.
LandSim 2.5 is compatible with data files prepared inearlier
versions, although additional information mustbe entered.
Figure 1. LandSim 2.5 conceptual model
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Environment Agency LandSim 2.5
LandSim 2.5 conceptual modelLandSim 2.5 continues to be based on
the same siteconceptual model as previous versions (Figure 1),which
must be appropriate for the landfill beingassessed (for guidance on
conceptual models, seeReference 3). The landfill is assumed to be
above thewater-table. The receptor should not be too far fromthe
site as the model output becomes less precisewith increasing
distance. Landfills located on highlysensitive aquifers are likely
to require the use of moresophisticated flow and contaminant
transport models(see Reference 4).
Model run optionsRisk assessments using LandSim 2.5 should
beconducted in two stages.
1. Hydraulics: Evaluate whether the drainage systemcan maintain
a leachate head below the permittedmaximum. This is a hydraulics
model, constructed andrun in the same way as in LandSim 2.
Contaminantconcentrations are not predicted at this stage.
2. Predict the impact on groundwater quality from thelandfill:
Contaminant concentrations are calculated atspecified receptors
over time. LandSim 2.5 takesaccount of degradation of the synthetic
componentsof cap and basal lining systems and includes a newsource
term model. The conceptual models for capand liner degradation are
shown in Figures 3 and 4.
Main changes introduced forLandSim 2.5: degradation of FML
(flexible membrane liner)
component of caps and liners;
loss of active operational/institutional control;
contaminant-specific declining source term; new leachate
concentration ranges; biodegradation allowed in all pathways;
attenuation in mineral component of liners; option to input Darcy
flux.Degradation of engineeringThe inevitable physical and chemical
breakdown ofthe FML components of caps and liners in the longterm
will cause increased infiltration and leakage ofleachate. The
conceptual model for FML degradationis shown in Figures 3 and
4.
Figure 2. Data entry box (leachate concentrations)
LandSim 2.5 includes all of the functionality ofearlier versions
such as:
modelling of multiple phases and landfill sites; receptor for
each phase plus the combined effect of all
phases on a user-defined receptor;
many different lining systems included; contaminant
concentrations at the base of the unsaturated
zone and in groundwater;
retardation in unsaturated zone and aquifer; biodegradation of
organic contaminants; consideration of background water quality;
longitudinal dispersion in all pathways; flexibility in defining
probability density functions (PDFs)
for input parameters;
Laplace transform technique used to solve theadvectiondiffusion
contaminant transport equation;
requirement to justify input parameters; ability to save and
export results.
Figure 3. Conceptual model of cap degradation
Loss of operational/institutional controlWhile an operator or
regulator has control of a landfillsite it should be maintained in
accordance withpermit conditions. This period of managementcontrol
typically applies to the duration of landfillingand the period of
financial provision following closureof the site. At some point in
the future managementcontrol will cease, if, for instance, the
operator is no
LandSim 2.5 includes all of thefunctionality of earlier
versionssuch as:
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Environment Agency LandSim 2.5
longer in business and the period of financialprovision has
expired. Until the end of managementcontrol, the leachate head is
fixed at the permittedlevel. After this time, leachate accumulation
(leachatehead) is determined by the balance betweeninfiltration
through the cap and leakage through thebasal liner.
List I substances - compliance withthe Groundwater
DirectiveContaminant concentrations can be calculatedimmediately
prior to entry into groundwater, that isat the base of the
unsaturated zone. Since thedischarge of List I substances to
groundwater isprohibited, this feature is essential to
assesscompliance with the Groundwater Directive.
Background groundwater qualityBackground groundwater quality can
be consideredby LandSim. This may affect the degree of
additionalcontaminant loading that is acceptable.
Multiple sitesMultiple sites can be modelled to reflect the
commonsituation of new facilities being extensions to
existingsites. The cumulative impact on groundwater can bepredicted
rather than just the impact of the new site.Each individual site or
phase can have differentinfiltration values, cell geometries,
lining systems,leachate quality and unsaturated zone properties
(e.g.thickness).
Plan view plume generatorAn indication of the likely development
of anycontaminant plume can be viewed as a colour-codedcontour
plan.
Probability density functionsThere is considerable flexibility
in the type ofdistribution used for most input parameters.
Saving and export of resultsThe results of model runs can be
saved for laterreloading, and some results can be exported for
usein other modelling packages: leakage values,concentrations at
the base of the liner system andconcentrations at the base of the
unsaturated zone.
User interfaceThe size of phases and receptor location can be
easilychanged. All inputs must be explicitly justified, whilethe
output and printed record are locked togetherwith a
filename/timestamp system for audit purposes.
LandSim 2.5 model outputsThese can be divided into three
sections: hydraulics,travel times and contaminant concentrations
atspecified locations and times (Table 1).
Figure 4. Conceptual model of liner degradation
Contaminant-specific source termThe new declining source term
for non-volatilesubstances reflects the physical and
chemicalcharacteristics of individual contaminants. Thisapproach is
simulated using the kappa value, which isrelated to the
contaminantwaste partitioningcoefficient.
Research has shown that volatile organic compoundsare removed
from landfills via gas extraction systems,as well as partitioning
into leachate; this is reflected inthe variation of leachate
quality over time.
Leachate managementThe volume of leachate being re-circulated
can bespecified. The amount of leachate needing treatmentand
re-circulation or off-site disposal will becalculated.
RetardationPartition coefficients (Kd) are used for the
retardationof inorganic contaminants. Organic contaminants
arenormally modelled using partition coefficients toorganic matter
(Koc) rather than soils. Partitioncoefficients must be appropriate
for the system(leachate and soil/rock) being considered.
BiodegradationLandSim allows the biodegradation of
organiccompounds. A decay rate (half-life) can be specifiedfor each
pathway (liner, unsaturated zone, verticalpathway, aquifer). This
function must be used withcaution, and only where it is realistic
that theconditions are likely to be favourable
forbiodegradation.
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Environment Agency LandSim 2.5
Table 1. Outputs from LandSim 2.5
Hardware and softwarerequirementsTo run LandSim 2.5, you need an
IBM compatiblePC with a processor speed of at least 800 MHz and128
MB of RAM. The software has been developed torun under Windows
2000. Users will need 30 MB ofhard disk space for installation and
for temporary filesduring simulations.
Guidance manual and technicalsupportAn addendum to the LandSim 2
guidance manual(R&D Publication 120) has been produced
forLandSim 2.5. A technical report that contains adetailed
explanation of the evolution of LandSim 2.5is available from
www.landsim.com (Reference 5).For problems running the program or
for technicalenquiries, Help files are available within
LandSimeither from the main menu or through buttons ineach input
window. Full technical support forLandSim is offered by Golder
Associates (UK) Limitedthrough a help desk.
Obtaining LandSim 2.5LandSim 2.5 can be purchased from
GolderAssociates:Tel: 0115 9456 544Fax: 0115 9456 540E-mail:
[email protected]
A free demonstration version of LandSim 2.5 can bedownloaded
from www.landsim.com and the Agencywebsite
www.environment-agency.gov.uk.
Project managerThe Environment Agencys Project Manager
forLandSim 2.5 was Dr Hugh Potter, NationalGroundwater &
Contaminated Land Centre.
Further informationNGWCLC Project GW/03/09 was funded by
theAgencys National Groundwater & ContaminatedLand Centre and
the Landfill Directive Project. Furtherinformation can be obtained
from the R&DManagement Coordinator (Waste Programme): Tel:0117
9142 783; Fax: 0117 9142 673.
References1. Environment Agency (2003) Hydrogeological
RiskAssessments for Landfills and the Derivation ofGroundwater
Control and Trigger Levels. ReportLFTGN01.
2. Environment Agency (2001) Guidance onAssigning Values to
Uncertain Parameters inSubsurface Contaminant Fate and
TransportModelling. National Groundwater & ContaminatedLand
Centre Report NC/99/38/3.
3. Environment Agency (2001) Guide to GoodPractice for the
Development of Conceptual Modelsand the Selection and Application
of MathematicalModels of Contaminant Transport Processes in
theSubsurface. National Groundwater & ContaminatedLand Centre
Report NC/99/38/2.
4. Environment Agency (2001) Guidance on theAssessment and
Interrogation of Subsurface AnalyticalContaminant Fate and
Transport Models. NationalGroundwater & Contaminated Land
Centre ReportNC/99/38/1.
5. Golder Associates (2003) The Development ofLandSim 2.5.
National Groundwater & ContaminatedLand Centre Report
GW/03/09.
Property Output
Hydraulics Leachate head: Can the site operate within its
licence conditions? Is the
drainage system adequate?
Flow to leachate treatment plant and any surface breakout
Leakage through the liner and dilution in the aquifer
Travel times Time for peak concentrations at specified
receptors
Contaminant concentrations (over time) In the source
leachate
At the base of the unsaturated zone
At the monitoring well for each phase (5 metres
downgradient)
At the compliance point for the whole site
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