How heating oil can affect groundwater

GROUNDWATER

Groundwater is water held below the ground within the pores and fractures of soil, sediment and rock. An oil spill does not need to remain visible at the surface to create a potential pathway towards it.

Whether groundwater is at risk depends on the volume and nature of the release, the geology beneath the site, the depth to groundwater, rainfall, groundwater movement, nearby wells and other potential pathways and receptors.

The presence of oil in soil does not automatically prove that groundwater has been affected. Equally, the absence of visible oil at the surface does not demonstrate that groundwater is safe.

Groundwater can occur at different levels

Groundwater should not be thought of as one single underground body of water.

Depending on the geology of a site, water may occur as:

• perched groundwater, where water accumulates locally above the main groundwater body because it is held up by a less-permeable layer;
• shallow groundwater within superficial deposits, such as sands, gravels or other unconsolidated materials; or
• groundwater within an aquifer, where geological formations are capable of storing and transmitting significant quantities of water.

More than one groundwater body may therefore exist beneath the same property at different depths.

This is important because the first groundwater encountered during an investigation is not necessarily the principal aquifer or regional water table.

Regulatory terminology such as controlled waters differs between jurisdictions, so the relevant legal classification should be confirmed for the location concerned.

THE PATHWAY

How oil moves towards groundwater

Once heating oil enters the ground, it begins moving through the pore spaces and pathways available within the soil and geology.

The ease with which it penetrates the ground depends partly on the structure of the material.

Coarse sands, gravels and permeable made ground generally allow easier migration than dense, fine-grained soils such as clay.

Entry into the soil

For oil to enter a pore space, it must overcome the resistance presented by the soil structure and, where present, water already occupying the pores.

This is sometimes described in terms of capillary or entry pressure.

Migration depends on the size, continuity and capillary properties of pore throats as well as their saturation. Coarse, connected pathways often permit faster movement, while fine pores can exert stronger capillary forces but may require greater pressure for non-aqueous-phase liquid to enter when they are water-filled.

A significant accumulation of oil above the ground can also create additional hydraulic head, increasing the driving force for penetration into available pathways.

However, oil does not necessarily travel vertically in a straight line.

Its movement may be slowed or redirected by:

• clay and other low-permeability layers;
• changes in soil type;
• water-filled pores;
• foundations and buried structures;
• service trenches;
• fractures and fissures; and
• other preferential pathways.

Oil may therefore travel laterally for some distance before reaching groundwater.

What happens when oil reaches groundwater?

Where downward migration continues far enough, oil may eventually reach the capillary fringe and the saturated zone.

Heating oil is generally less dense than water and does not readily mix with it as a bulk liquid. Mobile oil reaching groundwater will therefore usually accumulate within and around the capillary fringe and upper saturated zone. It may nevertheless move below the apparent water table where sufficient pressure displaces water from pore spaces, particularly in heterogeneous ground.

This type of mobile petroleum contamination is often described as a light non-aqueous phase liquid, or LNAPL.

The situation can become more complicated where groundwater levels fluctuate.

As the water table rises and falls, oil can become distributed through a vertical interval of soil known as a smear zone, leaving residual contamination above and around the changing groundwater level.

This means that the absence of a measurable layer of free oil at one point in time does not necessarily mean that petroleum contamination is absent from the surrounding ground.

Oil and water do not behave as a simple two-layer system

It is commonly said that oil and water do not mix.

For the bulk fuel, that is broadly true. However, heating oil is not a single chemical substance.

Kerosene and other petroleum fuels are complex mixtures containing many different hydrocarbon compounds, each with its own physical and chemical characteristics.

Some components have very low water solubility and tend to remain predominantly within the oil phase.

Others are more soluble and can dissolve into groundwater.

The dissolved contamination can therefore have a very different chemical composition from the original oil that was spilled.

This distinction is important because dissolved petroleum compounds can migrate with groundwater beyond the area containing visible or free-phase oil.

Certain aromatic hydrocarbons, including compounds such as benzene where present in the fuel, are more water-soluble than many of the heavier petroleum components.

Their presence and concentration will depend on the particular fuel product, its composition, weathering and the circumstances of the release.

For this reason, groundwater cannot be assessed simply by looking for floating oil.

Dissolved hydrocarbons may be present even where there is no visible sheen, free product or obvious petroleum contamination.

WHY LOCATION MATTERS

Wells, springs and abstractions

Private wells, boreholes, springs and other groundwater abstractions can be particularly sensitive receptors.

Straight-line distance from the spill is only one consideration.

The risk to a well can also depend on:

• groundwater flow direction;
• the depth and construction of the well;
• which geological unit it draws water from;
• the pumping rate;
• fractures or highly permeable pathways;
• seasonal groundwater levels; and
• the relationship between the source and the abstraction.

Pumping itself can alter groundwater flow locally and may draw contaminated groundwater towards an abstraction.

A well that appears to be some distance from the spill should therefore not automatically be assumed to be unaffected simply because of its position on a map.

DESKTOP STUDY

The investigation should begin before anyone arrives on site

A competent groundwater investigation normally begins with a desktop study.

Before attending the property, an appropriately experienced environmental professional should review the available information about the site’s wider hydrogeological setting.

This may include:

• bedrock and superficial geology;
• known or potential aquifers;
• groundwater vulnerability;
• nearby wells, boreholes and springs;
• rivers, streams and other surface waters;
• drainage;
• historic land use;
• topography;
• possible groundwater flow direction; and
• other sensitive receptors.

Groundwater vulnerability matters

Groundwater vulnerability provides an indication of how easily contamination released at the surface may reach underlying groundwater.

A site underlain by thick low-permeability material may present a very different risk from one situated over shallow gravel or fractured rock.

Understanding this before attending the site allows the investigator to structure the investigation intelligently rather than sampling without a clear purpose.

It can help determine:

• where intrusive investigation should be undertaken;
• what depths may need to be examined;
• whether groundwater monitoring is required;
• where monitoring wells should be positioned;
• which receptors require particular attention; and
• whether specialist hydrogeological advice is needed at an early stage.

A good site investigation begins with a conceptual understanding of the site, not with the first sample.

INVESTIGATION

Evidence should answer a clear question

Groundwater investigation should be designed to answer specific questions.

These may include:

• Has oil reached groundwater?
• Is free-phase product present?
• Are dissolved hydrocarbons present?
• In which geological layer is contamination occurring?
• What direction is groundwater moving?
• Is contamination moving away from or towards a receptor?
• Could a private well, spring or surface water body be affected?
• Is the contamination stable, increasing or decreasing over time?

An assessment may combine site history, geology, groundwater levels, observations, field measurements and laboratory sampling.

Monitoring points should be positioned and constructed for the particular question being investigated.

A single groundwater sample, taken without an understanding of the geology or groundwater flow, may provide very limited information.

Likewise, an isolated laboratory result should be interpreted within the wider site context and with appropriate quality controls and an understanding of uncertainty.

RESPONSE AND MONITORING

Decisions develop as evidence improves

Where groundwater may have been affected, the response may include:

• stopping and removing the source;
• recovering mobile oil where practicable;
• removing or treating contaminated soil;
• protecting wells or other water users;
• treating affected groundwater where necessary; and
• monitoring groundwater over time.

The appropriate response will depend on the source, pathways, receptors and the evidence obtained during the investigation.

Groundwater levels and contaminant concentrations can vary over time, so monitoring may be required to understand trends rather than relying on a single measurement.

Professional judgement

Groundwater cases can become technically complex very quickly.

The important distinction is between:

free product + dissolved contamination + residual contamination in soil and the smear zone.

These different forms of contamination behave differently and may require different methods of investigation, monitoring and remediation.

A visible layer of oil on groundwater represents only one part of the problem. Dissolved hydrocarbons may have migrated farther, while residual oil may remain held within soil above and around the water table.

Private wells and boreholes deserve particular attention because they create a direct potential pathway between groundwater and people.

Where a spill has occurred near a private water supply, within highly permeable ground, over vulnerable groundwater, in fractured rock, or where the direction of groundwater movement is uncertain, early hydrogeological input can be considerably more valuable than attempting to reconstruct the pathway later.

The purpose of groundwater investigation is therefore not merely to establish whether oil can be seen in a monitoring well.

It is to understand:

where the contamination is, what form it is in, how it is moving and what it could affect.

A spill is happening now?

Prioritise safety.

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

Read next

• How heating oil behaves in soil
• What happens when heating oil reaches drains or surface water?
• How an oil-spill site is assessed and remediated
• Understanding oil-spill sampling, laboratory results and monitoring