Environmental effects of a domestic heating-oil spill

ENVIRONMENTAL EFFECTS

The environmental significance of a heating-oil spill depends on more than the number of litres lost.

What matters is where the oil travels, what it can reach, how people or the environment may be exposed, and how long that exposure continues.

A modern contaminated-land assessment therefore uses a risk-based approach rather than assuming that every detectable trace of petroleum must automatically be excavated and removed.

The source-pathway-receptor model

A core principle of contaminated-land assessment is the source-pathway-receptor model.

For a significant risk to exist, three elements normally need to be present:

• a source of contamination;
• a receptor that could be harmed; and
• a pathway connecting the two.

For example, contaminated soil in a back garden may be the source.

Potential receptors could include:

• people living at the property;
• neighbouring properties;
• groundwater;
• surface water;
• plants and wildlife;
• the building itself; or
• other sensitive land uses.

The pathway is the route by which contamination reaches that receptor.

Human exposure pathways

Where people are the receptor, potential pathways may include:

• inhalation of petroleum vapours;
• direct skin contact with contaminated soil or materials; and
• accidental ingestion of soil, particularly where young children regularly play in an affected garden.

The presence of contamination alone therefore does not define the risk.

The assessment also considers whether exposure can realistically occur, at what concentration, how frequently and for how long.

Groundwater as a receptor

Where groundwater is the receptor, the pathway may involve downward migration through soil and geological layers.

The assessment considers whether contamination can realistically reach groundwater, how vulnerable that groundwater is and whether it could subsequently migrate towards another receptor such as a well, spring or surface-water body.

A source without a viable pathway may present a very different risk from the same contamination where a clear pathway to a sensitive receptor exists.

A risk-based approach

Contaminated-land assessment in Northern Ireland and elsewhere in the UK generally uses a risk-based approach, rather than treating the presence of petroleum alone as proof that excavation is required.

The important questions are:

What contamination is present?

Where is it?

How can it move?

What can it affect?

Does that exposure represent an unacceptable risk?

Heating oil is a refined petroleum product containing naturally occurring hydrocarbon compounds, but a domestic spill introduces those compounds into an environment and at concentrations where they would not normally be expected.

Where that contamination creates unacceptable risks to people, water, buildings, neighbouring land or ecological receptors, corrective action may be required.

However, there are also circumstances where relatively low levels of residual contamination are stable, inaccessible and do not present a significant pathway to a sensitive receptor.

Following proper investigation and risk assessment, monitoring or proportionate management may sometimes be more appropriate than intrusive excavation.

The objective is not simply to remove contamination.

The objective is to identify and manage unacceptable risk.

Effects on soil and plants

Significant petroleum contamination can affect both the physical and biological condition of soil.

Oil occupying pore spaces can reduce the ability of soil to transmit air and water.

In heavily contaminated ground, drainage may deteriorate and rainfall may begin to pond or leave the soil waterlogged.

Additional water can also influence the migration of an existing spill. Heavy rainfall, leaking water services or inappropriate washing-down may mobilise contamination and move it towards:

• buildings;
• drains;
• groundwater;
• neighbouring land; or
• other sensitive receptors.

Effects on soil ecology

Heating oil can also adversely affect the living component of soil.

High petroleum concentrations may damage or suppress:

• bacteria and other microorganisms;
• earthworms;
• insects and other invertebrates;
• plant roots; and
• the biological processes that contribute to healthy soil.

The severity of these effects depends on the product, concentration, duration of exposure and environmental conditions.

Visible vegetation stress can therefore provide useful evidence of contamination, but the absence of visible damage does not demonstrate that the soil beneath is unaffected.

Water and wildlife

Where oil reaches drains, groundwater, ditches, ponds, wetlands, streams or rivers, the significance of the incident can increase considerably.

Potential impacts depend on:

• the fuel involved;
• concentration;
• duration of exposure;
• flow conditions;
• habitat;
• season;
• the sensitivity of the receptor; and
• how quickly and appropriately the spill is controlled.

Potential receptors may include:

• aquatic plants;
• fish;
• aquatic invertebrates;
• birds;
• mammals;
• wetlands;
• private water supplies;
• livestock watering points; and
• downstream abstractions.

A relatively modest release can therefore become environmentally significant where it reaches a sensitive receptor.

People and buildings

In domestic cases, some of the most important risks concern people, buildings and neighbouring properties.

Vapour migration into homes

Where heating oil has accumulated beneath or against a building, more volatile components of the fuel may evaporate into soil gas.

These vapours can enter living areas through pathways such as:

• cracks in floors;
• service penetrations;
• gaps around walls and foundations;
• voids beneath suspended floors; and
• gaps around skirting boards and other building interfaces.

Persistent petroleum odours may therefore occur indoors even where no oil is visibly present inside the property.

For many domestic heating-oil cases, inhalation of petroleum vapours can be an important human-exposure pathway.

Direct skin contact and accidental ingestion may also need to be considered depending on the circumstances.

The significance of vapour exposure depends on the compounds present, their concentration, ventilation, exposure duration and the people occupying the property.

Odour alone should not therefore be treated as a complete measure of health risk.

Effects on building materials

A common misconception is that heating oil simply causes cured concrete to “rot”.

In existing buildings, the more usual concern is that porous materials such as:

• concrete;
• blocks;
• screeds;
• render; and
• plaster

can absorb petroleum compounds and retain persistent odours.

These materials may then act as a continuing source of vapours into occupied spaces.

The situation may differ where fresh or uncured cementitious materials come into contact with petroleum contamination, as contamination during curing can affect normal material performance.

Other building products – including membranes, insulation, sealants, coatings and plastics – may respond differently to hydrocarbons and may require separate assessment.

Neighbouring property and third-party impacts

Heating oil does not recognise property boundaries.

Contamination may migrate into:

• neighbouring gardens;
• adjoining foundations;
• shared drainage;
• groundwater beneath neighbouring land;
• commercial premises;
• agricultural land; or
• surface-water systems.

Once another property is affected, the spill may create additional environmental, financial, insurance and legal consequences.

A person who becomes aware of a spill should therefore take reasonable steps to stop the source and limit further migration.

Allowing a known spill to continue spreading onto neighbouring land may create significant civil-liability and insurance issues.

The precise legal position depends on the facts and jurisdiction and should be considered separately where required.

Success needs a defined outcome

Remediation should begin with a clear understanding of what condition the site needs to achieve for its intended use.

Different land uses create different exposure scenarios.

For example, the assumptions used when assessing:

• a residential garden where home-grown produce may be eaten;
• residential land without produce cultivation;
• public open space; and
• commercial or industrial land

are not the same.

A domestic garden used by adults and children and capable of producing fruit or vegetables generally represents a more sensitive human-exposure scenario than commercial or industrial land.

Environmental assessment criteria should reflect those differences.

Suitable-for-use assessment criteria

In UK contaminated-land practice, one commonly used set of human-health screening criteria is the LQM/CIEH Suitable 4 Use Levels, or S4ULs.

These provide assessment values for different contaminants and generic land-use scenarios.

They can help environmental consultants determine whether measured concentrations warrant further assessment.

However, they should not automatically be treated as clean-up targets.

A result above a generic screening value does not necessarily mean that all affected soil must be excavated.

It indicates that the potential risk requires further consideration.

Equally, achieving a particular numerical soil concentration does not by itself prove that every relevant environmental risk has been resolved.

Separate assessment may still be required for:

• groundwater;
• surface water;
• ecological receptors;
• vapour migration;
• neighbouring property; and
• impacts on buildings.

The final remediation objectives should therefore arise from the site-specific source-pathway-receptor assessment.

What successful remediation should demonstrate

A remediation project should have clearly defined objectives.

Completion should demonstrate that identified unacceptable risks have been:

• removed;
• reduced; or
• otherwise controlled to an appropriate level for the intended use of the site.

Evidence may include:

• excavation records;
• validation sampling;
• groundwater monitoring;
• vapour assessment;
• building-material assessment; or
• other site-specific evidence.

The objective should be to protect people and the environment without causing unnecessary secondary damage through excessive excavation, waste generation, transport or disruption.

Professional judgement

The presence of heating oil does not automatically tell us how serious a spill is.

The same concentration of contamination can have very different significance depending on whether it is:

• isolated beneath inaccessible ground;
• releasing vapours into a house;
• affecting a garden used by children;
• migrating towards groundwater;
• entering drainage; or
• crossing onto neighbouring property.

The most important question is therefore not simply:

“Is oil present?”

It is:

“Is there a significant source-pathway-receptor linkage, and does it create an unacceptable risk?”

That distinction is the basis of proportionate, evidence-led and sustainable remediation.

A spill is happening now?

Prioritise safety.

Stop the source only if it is safe to do so, prevent further spread where practicable, and use the emergency guidance rather than relying solely on this explanatory article.

Read next

• How domestic heating-oil spills spread
• How heating oil behaves in soil
• How heating oil can affect groundwater
• How an oil-spill site is assessed and remediated