Oil-spill terms explained in plain English

OIL-SPILL GLOSSARY

Oil-spill investigations can quickly become full of technical language.

Terms such as LNAPL, PID, TPH, conceptual site model and validation may appear in reports from consultants, laboratories, insurers or regulators without much explanation.

This glossary explains the terminology most commonly encountered during a domestic heating-oil spill in straightforward language.

Absorbent

A material used to collect liquid oil from a surface.

Purpose-made oil absorbent pads, booms or granules are commonly used during spill response. Once contaminated, the material must be collected and disposed of appropriately.

Aquifer

A geological formation that can store and transmit groundwater.

Aquifers may consist of materials such as sand, gravel or permeable or fractured rock.

If oil reaches an aquifer, groundwater contamination may migrate beyond the original spill area.

Aquitard

A layer of soil or rock with relatively low permeability that restricts the movement of groundwater or contamination.

Clay can sometimes act as an aquitard.

Oil reaching a low-permeability layer may move laterally along its surface rather than continuing directly downwards.

BTEX

A group of petroleum-related aromatic hydrocarbons:

Benzene, Toluene, Ethylbenzene and Xylenes.

These compounds may be included in laboratory testing because some are relatively volatile or water-soluble and can be important when assessing vapour, groundwater and human-health risks.

Brownfield land

Land that has previously been developed or used, often for industrial, commercial or infrastructure purposes.

Brownfield sites can contain old foundations, rubble, fill, buried services and historic contamination.

These features can make the migration of a new oil spill more difficult to understand.

Capillary fringe

The zone immediately above the groundwater table where water is drawn upwards into soil pores by capillary forces.

Oil migrating downwards through soil may encounter the capillary fringe before reaching the fully saturated groundwater zone.

Chain of custody

A written record showing who collected, handled, transported and received an environmental sample.

It helps demonstrate that the sample arriving at the laboratory is the same sample collected on site and that its history is traceable.

Combined sewer

A drainage system that carries both foul wastewater and storm water within the same sewer network.

Combined systems are different from separate drainage arrangements where foul sewage and rainwater travel through different pipes.

Conceptual site model

A working model used by environmental professionals to explain:

• where contamination is;
• how it can move;
• who or what could be affected; and
• which pollutant linkages require investigation.

The conceptual site model is refined as new evidence becomes available.

It is one of the foundations of a good contaminated-land investigation.

Contaminant plume

The area through which contamination has spread from its source.

An oil plume may extend laterally and vertically and can be highly irregular because soil, geology, drains, foundations and other preferential pathways influence its movement.

Contaminated land

In everyday language, land containing unwanted contamination.

In legislation, “contaminated land” can have a more specific legal definition, which differs between jurisdictions.

Finding petroleum hydrocarbons in soil does not automatically mean that land meets the statutory definition of contaminated land.

Controlled waters

A regulatory term used in parts of the UK for waters protected by environmental legislation.

Its precise legal meaning differs between jurisdictions, so Spill-Response will use jurisdiction-specific guidance where the legal classification matters.

Dissolved contamination

Petroleum compounds that have dissolved into groundwater or surface water.

Oil and water do not completely mix as bulk liquids, but some individual chemicals within petroleum products are water-soluble.

Dissolved contamination can therefore migrate with groundwater even where no visible layer of oil is present.

DRO

Diesel-range organics.

A laboratory analytical grouping used to measure hydrocarbons falling broadly within a range associated with diesel and similar heavier petroleum products.

The exact analytical method should always be checked when interpreting results.

Environmental consultant

A professional who investigates contamination and advises on environmental risk, remediation and verification.

Their work may include:

• desktop studies;
• site investigation;
• sampling;
• conceptual site modelling;
• risk assessment;
• remediation design;
• monitoring; and
• validation reporting.

Field screening

Tests or observations carried out on site to help guide an investigation.

Examples include PID readings, observations of staining, soil type, odour and groundwater conditions.

Field screening provides rapid information but normally does not replace accredited laboratory analysis.

Foul sewer

A sewer designed principally to carry wastewater from:

• toilets;
• bathrooms;
• kitchens;
• washing machines; and
• other domestic or commercial wastewater sources.

It is different from a surface-water drain intended mainly to carry rainwater.

Free product

Liquid petroleum present in the ground in a form that remains sufficiently mobile to move or be recovered.

Where free petroleum product reaches groundwater, it may accumulate near the groundwater surface.

See also LNAPL.

Generic assessment criteria

Scientifically derived concentrations used as screening tools when assessing contaminated soil.

They help determine whether a measured concentration may require further investigation or risk assessment.

They should not automatically be treated as universal clean-up limits.

Groundwater

Water located beneath the ground within pores, sediments, soil and fractures in rock.

Groundwater may occur at several levels beneath a site and can include shallow, perched and deeper groundwater systems.

Groundwater vulnerability

An indication of how susceptible groundwater may be to contamination originating at or near the ground surface.

Factors include:

• soil and geology;
• permeability;
• groundwater depth;
• superficial deposits; and
• pathways through rock or made ground.

Vulnerability information is often reviewed during the desktop assessment before an investigator attends the site.

GRO

Gasoline-range organics.

A laboratory grouping used for lighter petroleum hydrocarbons broadly associated with petrol or similarly volatile fuels.

Heating oil

A general term for fuel used in heating systems.

The exact product may differ according to location and installation.

For domestic properties in the UK and Ireland, heating oil commonly means kerosene, but terminology should not be relied upon without confirming the actual product supplied.

Hydrocarbon

A chemical compound made principally from hydrogen and carbon.

Petroleum fuels such as kerosene contain complex mixtures of many different hydrocarbons.

Different hydrocarbons have different:

• volatility;
• solubility;
• toxicity;
• mobility; and
• persistence.

Kerosene

A relatively light petroleum fuel widely used in domestic oil-heating systems.

In Great Britain, domestic heating kerosene is commonly supplied as BS 2869 Class C2 kerosene.

Trade terminology may also include references such as 28-second oil.

LNAPL

Light Non-Aqueous Phase Liquid.

A liquid such as heating oil that does not readily mix with water and is generally less dense than water.

Where mobile heating oil reaches groundwater, it normally tends to remain around the top of the saturated zone rather than simply sinking through the water column.

Loss adjuster

A claims professional normally appointed by an insurance company.

They may investigate the circumstances of a claim, review reports and costs, coordinate specialists and advise the insurer regarding settlement.

A loss adjuster should not be confused with a loss assessor.

Loss assessor

A claims professional appointed by the policyholder rather than the insurer.

They may help quantify and present an insurance claim and communicate or negotiate with the insurer.

Loss assessors normally charge the policyholder for their services.

Made ground

Ground that has previously been excavated, filled, reworked or altered by human activity.

It can contain soil, rubble, hardcore, ash, building materials or other fill.

Because it is often highly variable, made ground can create unexpected pathways for oil migration.

Monitoring well

A purpose-built borehole or installation used to observe groundwater.

It can allow environmental professionals to measure:

• groundwater level;
• groundwater movement;
• free product;
• dissolved contamination; and
• changes over time.

PAHs

Polycyclic aromatic hydrocarbons.

A group of organic compounds associated with petroleum products and combustion.

Certain PAHs may be included within laboratory analytical suites because of their persistence and environmental or human-health significance.

Perched groundwater

A localised body of groundwater situated above the main regional groundwater table.

It commonly occurs where water is held above a less-permeable layer such as clay.

Encountering shallow groundwater during an investigation therefore does not necessarily mean that the principal aquifer has been reached.

PID

Photoionisation detector.

A portable field instrument used to detect certain volatile organic compounds in air or soil gas.

Environmental professionals can use PID readings to help locate areas affected by petroleum vapours and guide further investigation.

A PID is a screening instrument, not a substitute for laboratory analysis.

Pollutant linkage

A connection between:

source -> pathway -> receptor

For contamination to create a significant risk, there normally needs to be a mechanism connecting the source with something that can be harmed.

Breaking or adequately controlling an unacceptable pollutant linkage is a fundamental objective of remediation.

Preferential pathway

A route through which contamination can move more easily than through the surrounding ground.

Examples include:

• gravel;
• hardcore;
• service trenches;
• drains;
• ducts;
• fractured rock;
• old foundations;
• pipe bedding; and
• cavities.

Preferential pathways can allow oil to travel significant distances from its original source.

Receptor

A person, property or environmental feature that could potentially be harmed by contamination.

Examples include:

• occupants of a home;
• neighbouring properties;
• groundwater;
• rivers and streams;
• private wells;
• plants;
• wildlife; and
• building materials.

Remedial target

A defined objective established for a particular remediation project.

It may relate to:

• contaminant concentrations;
• vapour levels;
• groundwater quality;
• removal of free product; or
• interruption of a particular pollutant linkage.

A remedial target is site-specific and should not automatically be confused with a generic screening criterion.

Remediation

Work undertaken to remove, reduce, treat, contain or otherwise manage unacceptable contamination risks.

Remediation does not always mean excavation.

Depending on the site, it can involve physical removal, treatment, recovery, containment, pathway interruption, monitoring or combinations of several approaches.

Residual contamination

Contamination remaining within soil, groundwater or building materials after the more mobile contamination has moved or been removed.

Residual petroleum can become trapped within soil pores or absorbed onto soil and organic material.

The presence of residual contamination does not automatically mean that unacceptable risk remains; it must be assessed in context.

Screening criteria

Benchmark concentrations used to help determine whether contamination may require further assessment.

Exceeding a screening value does not automatically mean that remediation is required.

It indicates that the potential risk needs to be considered further.

Septic drainage field

The area of ground that receives partially treated liquid effluent from a septic tank or similar private wastewater system.

The soil then provides further treatment as the effluent infiltrates through the ground.

This should not be confused with a surface-water soakaway.

Smear zone

The vertical zone around a fluctuating groundwater table through which petroleum may become distributed as groundwater rises and falls.

Residual petroleum contamination can remain within this zone even when no measurable layer of free oil is present in a monitoring well.

Soakaway

An underground structure designed to receive rainwater and allow it to infiltrate gradually into the surrounding ground.

If heating oil enters a soakaway, it can introduce contamination directly into permeable ground beneath the surface.

Source

The contamination itself or the location from which it originates.

Examples include:

• leaking oil in soil;
• an oil-contaminated foundation;
• residual petroleum beneath a floor; or
• free product on groundwater.

Source-pathway-receptor model

A fundamental approach used in environmental risk assessment.

It asks three questions:

Source – Where is the contamination?

Pathway – How could it move or cause exposure?

Receptor – Who or what could it affect?

A significant risk generally requires a credible connection between all three.

Surface-water drain

A drainage system intended principally to carry rainfall from roofs, roads, patios, yards and other surfaces.

Some surface-water drains discharge directly or indirectly to streams, rivers, ditches or other receiving waters.

Oil entering these systems can therefore move off site very quickly.

TPH

Total Petroleum Hydrocarbons.

A broad laboratory term used to describe petroleum-related hydrocarbons measured within a sample.

More detailed analysis may divide TPH into different carbon ranges or aromatic and aliphatic fractions to provide information that is more useful for risk assessment.

Validation

The process of collecting evidence after remediation to demonstrate whether the agreed remedial objectives have been achieved.

Examples include taking samples from the bottom and sides of an excavation and submitting them for laboratory analysis.

Verification

The broader process of demonstrating that remediation has been completed in accordance with its objectives.

A verification or validation report normally documents:

• what contamination was identified;
• what remediation was undertaken;
• what evidence was collected afterwards; and
• whether the remedial objectives were achieved.

The terms validation and verification are sometimes used interchangeably in practice.

VOC

Volatile Organic Compound.

An organic chemical that can readily enter the vapour phase.

Heating oil contains volatile components that may evaporate from contaminated soil or materials and contribute to petroleum odours or vapour exposure.

Vapour intrusion

The movement of contaminant vapours from affected soil, groundwater or building materials into an occupied building.

Potential entry routes can include:

• floor cracks;
• service penetrations;
• wall/floor junctions;
• voids beneath suspended floors; and
• other gaps in the building structure.

Vapour extraction

A remediation or mitigation technique that uses controlled suction to remove contaminated vapour from soil or spaces beneath a building.

It may be used to reduce vapour concentrations or manage migration towards occupied areas.

Weathering

The physical, chemical and biological changes that occur to petroleum after it has been released.

These can include:

• evaporation;
• dissolution;
• biodegradation;
• movement through soil; and
• preferential retention of some hydrocarbon fractions.

Weathered heating oil may therefore look, smell and analyse differently from fresh fuel.

20-second, 28-second and 35-second oil

Historic and trade terms associated with the viscosity of different fuel oils.

Terminology varies by jurisdiction and supplier.

In Ireland, 20-second or 28-second oil may be used informally to describe kerosene-type domestic heating fuel, while 35-second oil has historically referred to a heavier gas-oil-type product.

Where product identity matters, the actual fuel supplied should be confirmed from delivery or supplier records rather than relying solely on informal terminology.

Why the terminology matters

The purpose of learning these terms is not to turn a homeowner into an environmental consultant.

It is to make it easier to understand:

• what a consultant is investigating;
• what a laboratory report means;
• why a particular remediation approach has been recommended;
• what an insurer or loss adjuster is asking for; and
• what evidence should exist before a project is considered complete.

If a technical term in a report is unclear, ask the person who prepared it to explain what it means for your property and why it matters to the decision being made.

Read next

• How domestic heating-oil spills spread
• How heating oil behaves in soil
• How an oil-spill site is assessed and remediated
• Understanding oil-spill sampling, laboratory results and monitoring