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Contaminated land: dealing with hydrocarbon contamination Conceptual models for petroleum hydrocarbon sites.

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Presentation on theme: "Contaminated land: dealing with hydrocarbon contamination Conceptual models for petroleum hydrocarbon sites."— Presentation transcript:

1 Contaminated land: dealing with hydrocarbon contamination Conceptual models for petroleum hydrocarbon sites

2 Environmental Simulations International Contents of presentation n Key contaminants –Compounds of particular interest –The challenge of TPH analysis n Hydrocarbon-contaminated sites –Important source considerations –NAPL sources –Important pathways –Common receptors n Developing and refining the conceptual model n Contaminant interactions

3 Environmental Simulations International Key components n Will depend on the case but most common hydrocarbon risk drivers are: –BTEX –PAH’s n Weathering may change priorities with time n When relevant, consider MTBE (etc.), lead, other metals n You will need to consider component interactions –Discussed later

4 Environmental Simulations International Key components – more on BTEX n Main pathways are often migration as dissolved plume and/or as vapour –Relatively soluble in water n Benzene ~1790 mg/l –Relatively volatile n Toxicology –Benzene is a carcinogen –TEX relatively toxic but not carcinogenic

5 Environmental Simulations International Key components – more on PAH’s n Main pathways are often ingestion, dust inhalation, direct contact –Low to very low solubility in water n e.g, naphthalene ~30 mg/l; benzo[a]pyrene ~0.003 mg/l –Adsorb onto solids –Relatively low volatility (although naphthalene can be significant) n Variable toxicity –Some are carcinogen’s (e.g., benzo[a]pyrene) –Effects can be additive

6 Environmental Simulations International Key components n Don’t forget NAPL migration

7 Environmental Simulations International A brief digression – TPH analysis n TPH = Total petroleum hydrocarbons n A quick and easy screening assay –Everyone uses them –Can help identify the type of oil and the extent of weathering n Don’t always believe the lab.’s interpretation n But what do the concentrations tell us?

8 Environmental Simulations International TPH analysis n Various methods –Gravimetric n Not recommended! –IR –GC –GC/MS

9 Environmental Simulations International What’s wrong with TPH measurement? n Nothing, provided the data are used correctly –Do you know which standards were used? –Different methods give very different answers –No direct relationship to hazards posed by the components n Work in progress to provide a standard TPH analysis methodology that can link directly into standard risk assessment techniques –Environment Agency/Institute of Petroleum –US methodology

10 Environmental Simulations International The US approach 1. Define appropriate hydrocarbon fractions for assessment 2. Define key properties of each fraction 3. Determine toxicity based on data derived for the fraction or key components within it 4. Integrate fate/toxicity data and determine risk

11 Environmental Simulations International How does the process work? n GC/FID screen to derive TPH fractions n Screening concentrations per fraction for main exposure routes n More detailed GC/FID or GC/MS analysis of fractions n Procedure for deriving site-specific values

12 Environmental Simulations International Conceptual models Hardstanding River intercepts plume Deep borehole

13 Environmental Simulations International Why do we need conceptual models? n A ‘pollutant linkage‘ is the relationship between –a contaminant (source) –a pathway –a receptor n The presence of all three elements is required for land to be defined as ‘contaminated’ n So, we need a conceptual understanding of our site!

14 Environmental Simulations International A conceptual model … n Allows targeting of subsequent investigation –Use the model iteratively –And is a requirement for SI (BS10175:2001) n Provides the context for quantitative risk assessment n Demonstrates an appreciation of the problem –…beyond the purely factual data gathering n Aids communication n Is a good QA tool

15 Environmental Simulations International Some considerations for hydrocarbon sources - 1 n What? –What was released? n Source material n Additives (MTBE, lead?) n Co-contaminants? –e.g., degreasing solvents –When? n Weathering? –How? n Distribution and weathering?

16 Environmental Simulations International Some considerations for hydrocarbon sources - 2 n How much? –Estimate of source mass –NAPL likely? –Residual? n Where? –Weathering? –Spatial variability –Fracture versus matrix flow n And so on …

17 Environmental Simulations International NAPL sources n Flowing or immobile? –If flowing, where to? n Smearing? n How much? n How reliable are the thickness measurements? n Do it’s properties match theory?

18 Environmental Simulations International Chlorinated solvents Oil and petroleum products Ethanol Antifreeze Metals (e.g., lead) Some possible source behaviours

19 Environmental Simulations International NAPL sources

20 Environmental Simulations International NAPL smearing – spreading the source? Raising water table may not displace free product lens which remain in initial position. Idealized spreading on water table. (actually, capillary fringe) Soil heterogeneities result in “fingering” Water table moving below initial level results in further descent of free flowing NAPL.

21 Environmental Simulations International Some pathway considerations - 1 n Geological setting n Hydrology/hydraulics –Recharge processes and unsaturated zone flow –Groundwater flow n Essentially horizontal or vertical? –Discharge points n Natural or anthropogenic? –Fast pathways (  early arrival of contaminants) –Slow pathways (  persistence of contaminants) –Barriers to flow

22 Environmental Simulations International Some pathway considerations - 2 n Groundwater pathways –Advection and Dispersion –Dispersion –Sorption/retardation –Biodegradation n NAPL flow n Gradient of water table n Penetration of preferential flow paths

23 Environmental Simulations International Some pathway considerations - 3 n Vapour pathways –Advection and Dispersion –Sorption/retardation –Biodegradation n Solid phase contact –Dust transport –Direct contact n Inter-phase partitioning n Plant uptake?

24 Environmental Simulations International Some possible important receptors n Who/what? –Humans –Controlled waters n Aquifers n Surface waters –Water supply n Plastic pipelines –Utilities structures n Cabling n Sewers –Ecosystems –Buildings Fit for purpose ?

25 Environmental Simulations International Component interactions n Hydrocarbon components will behave very differently from each other –MTBE can behave very differently in groundwater – high mobility, often slow biodegradation n Components can affect the behaviour of others –Biodegradation –Sorption –Solubility –Vapour composition…

26 Environmental Simulations International Component interactions An example - benzene Parameter Pure benzene Typical petrol Solubility in groundwater (mg/l) Equilibrium vapour concentration (mg/l)4006.7

27 Environmental Simulations International Conclusions n We need conceptual models n Hydrocarbons have some specific properties that are important to consider –And many that are common to all contaminants n Consider the effect of the component mixtures when testing the model n Ask the questions


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