EVALUATION OF NATURAL ATTENUATION MECHANISMS ASSOCIATED WITH - - PowerPoint PPT Presentation

EVALUATION OF NATURAL ATTENUATION MECHANISMS ASSOCIATED WITH PETROLEUM MECHANISMS ASSOCIATED WITH PETROLEUM- HYDROCARBON RELEASE SITES AS MEASURED IN SOIL-VAPOUR Philip Schulz & Casey O’Farrell Philip Schulz & Casey O Farrell

The Australian Context Science The Australian Context – Science

• Federal Government legislated a move away from pure research.

Research should be a business and thereby needs to able to make a return on the investment.

• Industry are reticent to invest in anything which cannot yield an

immediate return...how much is your soul worth? immediate return...how much is your soul worth?

• Regulatory agencies adopt wait and see approaches. Let the rest of

world work it out and then co-opt it into guidance and/or policy world work it out and then co-opt it into guidance and/or policy.

The Australian Context Soil vapour The Australian Context – Soil-vapour

• No federal, state, territory or industry-based regulatory guidance or

policies.

• Variances in the scope and quality of work amongst practitioners.

Ignorance amongst many statutorily endorsed environmental

• Ignorance amongst many statutorily endorsed environmental

auditors and reviewers.

• Minimalistic approaches have currency, the marketplace is

amazingly cost sensitive.

Indicators of Natural Attenuation in Soil vapour Indicators of Natural Attenuation in Soil-vapour

• Primary lines of evidence (i.e. reduction in contaminant mass and

extent) – slow move towards multiple sampling rounds...number not comparable to groundwater sampling.

• Secondary lines of evidence (i.e. geochemical indicators of

biological activity) – much slower move in expansion in the scope of biological activity) much slower move in expansion in the scope of works.

– Petroleum hydrocarbons: oxygen, carbon dioxide, methane. – Chlorinated compounds: ethane and ethylene. p y

Benefits of Collecting Evidence of Natural Attenuation in Benefits of Collecting Evidence of Natural Attenuation in Soil-vapour

• Helps in framing the jump from where we observe volatile organic

compounds to potentially explaining why the concentrations are elevated.

• Potential problem and solution – in Australia, methane is back in
• fashion. At petroleum-release sites, methane is considered to be a
• fashion. At petroleum release sites, methane is considered to be a

by-product, not a contaminant of potential concern (COPC).

• Assists in formulating monitored natural attenuation (MNA) as a
• Assists in formulating monitored natural attenuation (MNA) as a

remedial approach (secondary lines of evidence in groundwater can be unreliable).

Explaining How and Why Explaining How and Why

1,600,000 1,800,000 2,000,000 800,000 1,000,000 1,200,000 1,400,000 VOCs [g/m3] 200,000 400,000 600,000 800,000 V 10-foot 5-foot Sub-slab

Explaining How and Why Explaining How and Why

Sub-slab ~5-feet ~10-feet

Methane is Back in Fashion Methane is Back in Fashion.

• “Alright let’s talk about gas.

Imagine if your home was filling up with a dangerous gas that you couldn't see or smell. Scary? Well that's what happened to some families in Melbourne and it was so bad they had to move out of their homes”

ABC 774 Melbourne (11-Sep-2008)

Methane is Back in Fashion Methane is Back in Fashion.

• “Thirty families have already

fled the terror of living with the constant threat of a gas explosion tearing their Brookland Greens, Cranbourne, homes apart.”

Cranbourne Leader (17-Sep-2008)

Methane is Back in Fashion Methane is Back in Fashion.

• “This sort of thing has occurred
• verseas, and the landfills

have exploded, but we are still trying to find one of this scope and scale, to the extent of affecting up to 400 homes.”

Cranbourne Leader (01-Oct-2008)

Methane is Back in Fashion Methane is Back in Fashion

Dataset for this Investigation Dataset for this Investigation

• Dataset comprises general gas analyses from 24 petroleum-release

sites around Australia (over 200 discrete sample points). Issue of methane source confounder.

• All sample points were permanent constructions to permit additional

rounds of sampling. rounds of sampling.

• Every sample point met quality assurance protocols including helium

tracer shroud testing tracer shroud testing.

– Note, helium forms part of the general gas analysis profile.

Potential Risk Factors for Elevated Methane Potential Risk Factors for Elevated Methane

• Statistically significant relationships.

– Vadose zone soil type. – Surface covering. Surface covering. – Depth to groundwater. – Presence of petroleum hydrocarbon at sample location...

• Poor statistical correlations.

– Petroleum hydrocarbon analyte concentration. Aquifer seepage velocity – Aquifer seepage velocity...

Vadose Zone Soil Type Vadose Zone Soil Type

• Relationship: the higher the soil air filled porosity, the more likely

that methane would be elevated...seems to be counterintuitive when

• xygen diffusion is considered as a counter balance.
• Hypothesis: elevated soil-vapour flux in highly permeable soils

reduces potential for methanotrophic bacteria to degrade methane in reduces potential for methanotrophic bacteria to degrade methane in the near-surface environment.

Surface Covering Surface Covering

• Relationship: if a surface covering is present, the more likely that

elevated methane would be measured.

• Hypothesis: a surface barrier allows for the concentration of

methane and acts as an impediment for oxygen recharge into the sub-surface. sub surface.

• Issue: how do we predict this at undeveloped sites?

Depth to Groundwater Depth to Groundwater

• Relationship: the shallower the groundwater occurrence, the more

likely methane concentrations would be elevated.

• Hypothesis: distance most likely serves to ‘buffer’ the system.

Note: this relationship did not correlate well with the petroleum

• Note: this relationship did not correlate well with the petroleum

hydrocarbon impact locations and thereby may potentially be statistically anomalous – at least for methane.

Presence of Petroleum Hydrocarbons Duh! Presence of Petroleum Hydrocarbons...Duh!

• Relationship: elevated methane correlated with the occurrence of

petroleum hydrocarbon vapour at the sample point.

• Hypothesis: hope I don’t need to explain.

Implication: that the lateral migration of methane from petroleum

• Implication: that the lateral migration of methane from petroleum

hydrocarbon sources may not potentially be as critical as other from sources, landfills for example.

The Poor Correlations The Poor Correlations

• Natural systems are not all that simple...source concentrations,

including the presence of LNAPL is a very poor predictor of methanogenesis.

• Aquifer seepage velocities were predominately literature-based. As

such, the relationship may potentially be present but the dataset is such, the relationship may potentially be present but the dataset is

• biased. The legacy of a competitive marketplace...

Takeaway Messages Takeaway Messages

• Make your time on-site count!
• Oxygen and carbon dioxide are not the whole of the story for
• Oxygen and carbon dioxide are not the whole of the story for

petroleum hydrocarbons. Characterising natural attenuation in soil vapour provides benefits

• Characterising natural attenuation in soil-vapour provides benefits

to...

– Data interpretation. Informed remedial options – Informed remedial options. – Indicators of explosibility.

Last Slide I promise! Last Slide, I promise!

Questions?