Introduction Experience from low risk sites shows gas monitoring results can be misleading Leads to disagreements and delays in planning Do we need to

Transcription

1 SCLF 2011 An alternative approach for ground gas risk assessment Dr Geoff Card GB Card and Partners Limited Steve Wilson Environmental Protection Group

2 Introduction Experience from low risk sites shows gas monitoring results can be misleading Leads to disagreements and delays in planning Do we need to monitor on low risk sites? Do we need to monitor where gas protection is to be provided anyway? Always working with limited number of gas wells Application of principles to Scottish landfill site

3 Gas risk Low risk sites majority? High risk sites minority? Experience of anomolous results from gas monitoring on low risk sites concentrations and flow rates Results not representative and cause problems

4 Influences on flow rate

5 Influences on flow rate

6 Influences on concentration Degradation of organic matter in groundwater in well Dissolution of gas into headspace from groundwater Hydrocarbon contamination but consider ethanol in fuels Disturbance due to installation of well short term gas concentrations Well leakage oxygen ingress

7 We want to avoid this

8 Ground gas still an issue Other incidents as well as Loscoe Sept 2010 houses built over buried vegetation in Atlanta Methane detected in 3 year old development In water meters but not in houses

9 New approach Considers source of gas Considers current building construction Assess risk based on how much gas is likely to be generated Removes need for monitoring wells Answers in days rather than weeks or months

10 Key aim Identify whether site is low or high risk Manage the risk in an appropriate manner Avoid another Loscoe Provide a more consistent assessment method Avoid disputes between regulators and consultants

11 Outline of approach Are there any sources of gas? Are there any landfills nearby or below site? Is radon protection already required? Allows for air tight building construction that is required for Building Regulations Considers organic carbon content of Made Ground

12 Information Robust desk study is vital Site investigation to allow good visual assessment of source trial pits are best Forensic description not as complex as it sounds! TOC tests on soils

13 Flow chart

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15 Theory to practice The method has been used successfully on a number of sites Used where disputes over classification of site based on gas monitoring data Used other site investigation data to confirm classification of site Saved time and money by avoiding need to collect more gas data

16 Thanks to Hugh Mallett Richard Boyle Karen Thornton Simon Firth Heidi Hutchings Paul Nathanial For their time and effort reviewing the paper

17 Application of principles to a Scottish Landfill Site Used generation models together with simple migration models to assess risk of gas migration Minimises need for further gas monitoring Desk study and robust conceptual model are vital for gas risk assessment

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19 Desk Study

20 Conceptual model

21 Carbon dioxide only outside site

22 Use conceptual model to identify credible pathways for migration

23 Identify critical receptors and distance from source

24 Analyse gas generation and migration through the ground Assess dilution within buildings

25 Calculations to estimate rate of generation Rate of flow horizontally Rate of flow vertically Dilution in building Which is limiting factor? Estimate gas concentration in building

26 BOD/COD >0.4 BOD/COD <0.4 INCREASING DECREASING α t = gas formation at time t (years) α t = Σ A.C i.k i.e -k.t A = mass of waste (tonnes) C i = carbon content (kg/tonne) DOC k = rate constant (year -1 ) t = time elapsed (years) 26

27 Assessment Framework Copyright EPG Limited 2011

28 Multi level small diameter monitoring points 28

29 Dumble et al, 2006

30 A final thought

31 Thank you Your thoughts please