an alternative approach for ground gas risk assessment ppt card and wilson.pdf · an alternative...
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SCLF 2011SCLF 2011
An alternative approach for An alternative approach for ground gas risk assessmentground gas risk assessment
Dr Geoff CardDr Geoff CardGB Card and Partners LimitedGB Card and Partners Limited
Steve WilsonSteve WilsonEnvironmental Protection Environmental Protection
GroupGroup
IntroductionIntroduction
•• Experience from low risk sites shows gas Experience from low risk sites shows gas monitoring results can be misleadingmonitoring results can be misleading
•• Leads to disagreements and delays in planningLeads to disagreements and delays in planning•• Do we need to monitor on low risk sites?Do we need to monitor on low risk sites?•• Do we need to monitor on low risk sites?Do we need to monitor on low risk sites?•• Do we need to monitor where gas protection is Do we need to monitor where gas protection is
to be provided anyway?to be provided anyway?•• Always working with limited number of gas wellsAlways working with limited number of gas wells•• Application of principles to Scottish landfill siteApplication of principles to Scottish landfill site
Gas riskGas risk
•• Low risk sites Low risk sites –– majority?majority?
•• High risk sites High risk sites –– minority?minority?
•• Experience of Experience of anomolousanomolous results from gas results from gas
monitoring on low risk sites monitoring on low risk sites –– concentrations concentrations
and flow ratesand flow rates
•• Results not representative and cause problemsResults not representative and cause problems
Influences on flow rateInfluences on flow rate
Influences on flow rateInfluences on flow rate
Influences on concentrationInfluences on concentration
•• Degradation of organic matter in groundwater in Degradation of organic matter in groundwater in wellwell
•• Dissolution of gas into headspace from Dissolution of gas into headspace from groundwatergroundwatergroundwatergroundwater
•• Hydrocarbon contamination Hydrocarbon contamination –– but consider but consider ethanol in fuelsethanol in fuels
•• Disturbance due to installation of well Disturbance due to installation of well –– short short term gas concentrationsterm gas concentrations
•• Well leakage Well leakage –– oxygen ingressoxygen ingress
We want to avoid thisWe want to avoid this
Ground Ground gas gas –– still an issuestill an issueGround Ground gas gas –– still an issuestill an issue
•• Other Other incidents incidents as well as well as as LoscoeLoscoe
•• Sept 2010 Sept 2010 –– houses houses •• Sept 2010 Sept 2010 –– houses houses built over buried built over buried vegetation in Atlantavegetation in Atlanta
•• Methane detected in 3 Methane detected in 3 year old developmentyear old development
•• In water meters but not In water meters but not in houses in houses
New approachNew approach•• Considers source of gasConsiders source of gas
•• Considers current building constructionConsiders current building construction
•• Assess risk based on how much gas is likely to Assess risk based on how much gas is likely to •• Assess risk based on how much gas is likely to Assess risk based on how much gas is likely to be generatedbe generated
•• Removes need for monitoring wellsRemoves need for monitoring wells
•• Answers in days rather than weeks or monthsAnswers in days rather than weeks or months
Key aimKey aim•• Identify whether site is low or high riskIdentify whether site is low or high risk
•• Manage the risk in an appropriate mannerManage the risk in an appropriate manner
•• Avoid another Avoid another LoscoeLoscoe•• Avoid another Avoid another LoscoeLoscoe
•• Provide a more consistent assessment methodProvide a more consistent assessment method
•• Avoid disputes between regulators and Avoid disputes between regulators and consultantsconsultants
Outline of approachOutline of approach
•• Are there any sources of gas?Are there any sources of gas?
•• Are there any landfills nearby or below site?Are there any landfills nearby or below site?
•• Is radon protection already required?Is radon protection already required?•• Is radon protection already required?Is radon protection already required?
•• Allows for air tight building construction that is Allows for air tight building construction that is required for Building Regulationsrequired for Building Regulations
•• Considers organic carbon content of Made Considers organic carbon content of Made GroundGround
InformationInformation•• Robust desk study is vitalRobust desk study is vital
•• Site investigation to allow good visual Site investigation to allow good visual assessment of source assessment of source –– trial pits are besttrial pits are best
•• Forensic description Forensic description –– not as complex as it not as complex as it sounds!sounds!
•• TOC tests on soilsTOC tests on soils
Flow chartFlow chart
Theory to practiceTheory to practice
•• The method has been used successfully on a The method has been used successfully on a number of sitesnumber of sites
•• Used where disputes over classification of site Used where disputes over classification of site based on gas monitoring databased on gas monitoring databased on gas monitoring databased on gas monitoring data
•• Used other site investigation data to confirm Used other site investigation data to confirm classification of siteclassification of site
•• Saved time and money by avoiding need to Saved time and money by avoiding need to collect more gas datacollect more gas data
Thanks toThanks to
•• Hugh MallettHugh Mallett•• Richard BoyleRichard Boyle•• Karen ThorntonKaren Thornton•• Simon FirthSimon Firth•• Simon FirthSimon Firth•• Heidi HutchingsHeidi Hutchings•• Paul NathanialPaul Nathanial
•• For their time and effort reviewing the paperFor their time and effort reviewing the paper
Application of principles to a Application of principles to a Scottish Landfill SiteScottish Landfill Site
•• Used generation models together with simple Used generation models together with simple migration models to assess risk of gas migrationmigration models to assess risk of gas migration
•• Minimises need for further gas monitoringMinimises need for further gas monitoring•• Desk study and robust conceptual model are Desk study and robust conceptual model are •• Desk study and robust conceptual model are Desk study and robust conceptual model are
vital for gas risk assessmentvital for gas risk assessment
Desk Study
Conceptual modelConceptual model
Carbon dioxide only outside siteCarbon dioxide only outside site
Use conceptual Use conceptual model to identify model to identify credible credible pathways for pathways for pathways for pathways for migrationmigration
Identify critical Identify critical receptors and receptors and distance from distance from sourcesourcesourcesource
Analyse gas Analyse gas generation and generation and migration through migration through the groundthe groundthe groundthe groundAssess dilution Assess dilution within buildingswithin buildings
•• Calculations to Calculations to estimate rate of estimate rate of generationgeneration
•• Rate of flow Rate of flow horizontallyhorizontally
•• Rate of flow Rate of flow verticallyvertically
•• Dilution in buildingDilution in building•• Which is limiting Which is limiting •• Which is limiting Which is limiting
factor?factor?•• Estimate gas Estimate gas
concentration in concentration in buildingbuilding
BOD/COD >0.4 BOD/COD <0.4INCREASING DECREASING
ααααt = gas formation at time t (years)
A = mass of waste
2626
αt = Σ1.0846.A.Ci.ki.e-k.t
A = mass of waste (tonnes)
Ci = carbon content (kg/tonne) DOC
k = rate constant (year-1)
t = time elapsed (years)
Assessment FrameworkAssessment Framework
Copyright EPG Limited 2011
Multi level small diameter monitoring Multi level small diameter monitoring pointspoints
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•• DumbleDumble et al, 2006et al, 2006
A final thoughtA final thought
