Integrating Soil and Groundwater Remediation on Petroleum-Impacted Sites
Petroleum contamination can appear deceptively straightforward.
A leaking tank or fuel line is identified. Hydrocarbon-impacted soil is excavated. Validation samples are collected. The excavation is backfilled.
But if petroleum has reached groundwater, removing the contaminated soil may address only one part of the problem.
Fuel released into the subsurface can occur across several environmental media and in several different forms. Residual hydrocarbons may remain within soil pores. Light non-aqueous phase liquid, or LNAPL, may accumulate around the water table. Dissolved petroleum hydrocarbons may migrate with groundwater. Volatile constituents may generate soil vapour capable of migrating away from the original source.
These processes are connected.
Effective soil and groundwater remediation therefore requires an understanding of the entire source–pathway–receptor system rather than treating contaminated soil and contaminated groundwater as separate problems.
For petroleum-impacted sites, the central remediation question is often not simply:
How do we remove the contaminated soil?
It is:
What is sustaining the contamination, where has it migrated, and what combination of remediation measures is required to interrupt that system?
Petroleum Contamination Is a Source-and-Plume Problem
Petroleum contamination typically begins with a source.
This may include:
leaking underground petroleum storage systems;
former underground storage tanks;
fuel lines and pipework;
bowsers and dispensing areas;
above-ground storage tanks;
spills;
workshops and maintenance areas; or
historical fuel handling infrastructure.
The NSW EPA identifies leaking underground petroleum storage systems as a recognised source of soil and groundwater contamination, with UPSS potentially affecting soil, groundwater, surface water and indoor air through vapour migration.
Once a release occurs, petroleum can move through the subsurface.
Some hydrocarbons may remain sorbed to soil.
Some may volatilise.
Some may dissolve into infiltrating water or groundwater.
Where sufficient product is released, LNAPL can migrate through permeable soil and accumulate around the groundwater interface.
The resulting contamination therefore needs to be considered as a system comprising:
Primary source → residual soil source → LNAPL → dissolved groundwater plume → vapour and other exposure pathways
Not every petroleum site will contain every component.
But understanding which components are present is fundamental to selecting an appropriate remediation strategy.
Why Soil Remediation Alone May Not Resolve the Problem
Excavation is often highly effective for removing petroleum source material.
If a leaking tank and heavily contaminated surrounding soil can be physically removed, the contaminant mass remaining at the site may be substantially reduced.
This can be an important first step.
However, groundwater contamination can persist after the original source has been removed.
Imagine an underground fuel tank that leaked over many years.
By the time the tank is removed, hydrocarbons may already have migrated vertically through the soil profile and reached groundwater.
Removing the tank and surrounding impacted soil prevents the source from continuing to release fuel.
It does not automatically remove hydrocarbons that have already:
dissolved into groundwater;
migrated downgradient;
become trapped as residual LNAPL;
sorbed to deeper soil below the practical excavation depth; or
generated a vapour source.
The remediation strategy therefore needs to distinguish between source removal and plume management.
Both may be necessary, but they are not the same task.
Source Removal Is Often the Foundation of the Remediation Strategy
Where practicable, dealing with the source is generally a critical part of petroleum remediation.
Continuing to treat groundwater while leaving a significant ongoing source in place can create an inefficient remediation system.
Contaminants removed from groundwater may simply be replenished from petroleum remaining within soil or as residual product.
Source-zone remediation may include:
removal of leaking fuel infrastructure;
excavation of grossly impacted soil;
recovery of mobile petroleum product;
treatment of contaminated soil;
soil vapour extraction;
multiphase extraction; or
other in situ source-treatment methods.
The specific method depends on the site.
A shallow source on an open redevelopment site may be readily excavated.
The same contamination beneath an operating service station, building or major road may require an entirely different approach.
This is why soil remediation needs to be developed around both the contamination and the practical site constraints.
Groundwater Changes the Remediation Problem
Once groundwater is affected, hydrogeology becomes part of remediation design.
The consultant needs to understand more than simply whether petroleum hydrocarbons have been detected in a monitoring well.
Important questions include:
What is the groundwater depth?
Which direction does groundwater flow?
How does groundwater level vary over time?
What is the hydraulic conductivity of the aquifer?
Is LNAPL present?
What petroleum constituents are present?
Where is the dissolved plume located?
Is the plume stable, expanding or contracting?
Are concentrations changing with time?
Could contamination be migrating off site?
Are groundwater users, surface waters, buildings or other receptors potentially affected?
A groundwater concentration without this context provides limited information about the actual remediation problem.
This is why petroleum-contaminated sites can require both contaminated-land and hydrogeological interpretation.
LNAPL and Dissolved Contamination Require Different Thinking
A particularly important distinction is the difference between petroleum product and dissolved contamination.
Where LNAPL is present, it may act as a continuing secondary source to groundwater.
As groundwater moves through or interacts with the petroleum-affected source zone, soluble hydrocarbon components can continue to enter the dissolved phase.
This is one reason reducing the contaminant mass in the source zone can materially influence the longevity of the groundwater plume.
Douglas Partners has described this approach on petroleum-impacted sites by combining active remediation of the plume core and residual LNAPL with monitored natural attenuation for the outer dissolved plume. The objective was to reduce the continuing source load so that natural attenuation processes could address the remaining dissolved contamination more effectively.
The broader principle is important:
Different parts of the plume may warrant different remediation strategies.
The highest contaminant mass close to the source may justify active treatment.
Lower-concentration contamination further downgradient may require monitoring rather than the same intensive treatment.
The Most Aggressive Remedy Is Not Necessarily Required Everywhere
Petroleum groundwater plumes are rarely uniform.
There may be a relatively small source zone containing the majority of the contaminant mass and a much larger dissolved-phase plume at lower concentrations.
Applying the same remediation technology across the entire footprint may therefore be inefficient.
A more targeted strategy might involve:
Source zone: excavation, product recovery or active in situ treatment.
Plume core: active groundwater or multiphase remediation.
Outer plume: monitored natural attenuation, subject to demonstrating that the plume is stable or contracting and that receptors remain protected.
That is not a prescription for every petroleum site.
It illustrates why groundwater remediation should be based on the distribution and behaviour of contamination rather than simply the presence of a groundwater exceedance.
Monitored Natural Attenuation Is Not Simply “Leave It and Monitor”
Natural attenuation is sometimes misunderstood as doing nothing.
Properly applied, monitored natural attenuation is a remediation and management strategy that relies on naturally occurring physical, chemical and biological processes while monitoring is used to demonstrate that the required outcome is occurring.
For petroleum hydrocarbons, these processes can include biodegradation, dispersion, dilution, sorption and volatilisation.
But natural attenuation should not simply be assumed.
A defensible strategy needs evidence that the contaminant plume is behaving consistently with the conceptual model and that unacceptable risks are not being allowed to persist or increase.
Monitoring may need to demonstrate:
groundwater flow conditions;
plume stability or contraction;
changes in contaminant concentrations over time;
degradation of key petroleum constituents;
protection of downgradient receptors; and
progress toward the remediation objectives.
Natural attenuation can be particularly useful after the significant source mass has been reduced.
It is much less persuasive where a substantial ongoing source remains and the plume continues to expand.
Active Groundwater Remediation Has to Match the Site
Where active groundwater remediation is required, the most appropriate technology depends on the contaminant distribution, aquifer conditions and remediation objective.
Possible approaches may include:
groundwater extraction and treatment;
free-product recovery;
multiphase extraction;
soil vapour extraction;
air sparging;
enhanced bioremediation;
chemical oxidation;
permeable reactive treatment systems; or
combinations of different technologies.
The objective might be contaminant-mass reduction, source control, protection of a receptor, plume containment or acceleration of natural degradation.
Selecting a technology because it has worked on another petroleum site is not enough.
A pump-and-treat system, for example, may be effective for hydraulic containment or contaminant recovery in some aquifers but inefficient in a low-permeability formation.
In situ treatment relies on being able to adequately contact the target contamination.
Vapour extraction requires suitable contaminant volatility and subsurface air permeability.
The remediation technology needs to respond to the contaminant and the hydrogeological setting.
Douglas Partners similarly describes groundwater remediation as requiring site characterisation, monitoring, fate-and-transport assessment and selection from techniques including monitored natural attenuation, bioremediation, chemical oxidation, reactive barriers and extraction systems.
Soil Excavation Can Change Groundwater Conditions
Soil and groundwater remediation should also be coordinated because the act of excavating can influence groundwater.
A deep excavation may intersect the water table.
Dewatering may be required.
Groundwater entering the excavation may itself be contaminated.
Removing source material can also alter the ongoing contaminant mass entering groundwater.
For remediation planning, this creates several practical questions:
Will the excavation extend below groundwater?
Will contaminated water require pumping?
How will extracted groundwater be characterised and managed?
Could dewatering alter groundwater flow or contaminant migration?
Can the source zone be safely excavated?
Does excavation provide an opportunity for simultaneous LNAPL recovery?
What groundwater monitoring is required after source removal?
These issues should be addressed in the Remediation Action Plan rather than being discovered after an excavation fills with contaminated groundwater.
Vapour May Be the Link Between Soil, Groundwater and the Building Above
Petroleum contamination also requires consideration of vapour.
Volatile petroleum constituents present in soil, LNAPL or groundwater may partition into soil gas.
Where buildings are present or proposed, vapour migration can become an important exposure pathway.
This means a remediation strategy cannot always be developed by looking at soil and groundwater concentrations independently.
A groundwater plume beneath a proposed building may be relevant even where direct groundwater exposure is unlikely.
Similarly, residual petroleum contamination beneath a slab may require assessment because the building changes the receptor and pathway configuration.
The NSW EPA recognises vapour and indoor-air impacts as potential consequences of leaking UPSS, reinforcing the need to consider petroleum contamination as a multi-media problem.
Where vapour is a potentially complete pathway, it should be addressed within the Conceptual Site Model and investigation strategy.
Monitoring Wells Are Not Just Sampling Points
Groundwater monitoring wells can provide far more information than a single laboratory result.
A well-designed monitoring network can help establish:
groundwater elevations;
groundwater flow direction;
seasonal variability;
LNAPL occurrence and thickness;
contaminant concentrations;
plume geometry;
temporal trends;
downgradient conditions; and
remediation performance.
For petroleum remediation, the location and construction of wells therefore matter.
One well immediately beside a former tank may confirm contamination.
It may provide very little information about whether that contamination has migrated 20, 50 or 100 metres downgradient.
Likewise, a clean result from one monitoring well does not necessarily demonstrate that the plume has been delineated if the well is poorly positioned relative to groundwater flow.
The groundwater network should be designed around the Conceptual Site Model and the decisions the data need to support.
Petroleum Remediation Should Be Staged Around Decisions
Complex petroleum sites often benefit from a staged remediation process.
A typical decision sequence might involve:
1. Define the source
Identify tanks, fuel lines, dispensing areas and other likely release points.
2. Characterise soil impacts
Establish the nature and distribution of petroleum contamination within the source area.
3. Determine whether groundwater is affected
Install and monitor groundwater wells where the CSM identifies a credible groundwater pathway.
4. Assess LNAPL and dissolved-phase impacts
Determine whether mobile or residual product is present and characterise the dissolved plume.
5. Assess other relevant pathways
Consider vapour, surface water, off-site migration and sensitive receptors.
6. Remove or reduce the source
Implement practical source remediation where required.
7. Reassess plume behaviour
Determine how the groundwater system responds to source removal.
8. Apply targeted groundwater remediation
Use active treatment, monitored attenuation or a combination of measures based on the remaining risk.
9. Validate and monitor
Demonstrate that the remediation objectives have been achieved and that remaining contamination is stable and appropriately managed.
This approach allows each stage to respond to the evidence from the stage before it.
Source Removal Can Change What Happens Next
Post-source-removal monitoring can be particularly valuable.
A groundwater plume may respond differently once the continuing petroleum source has been removed.
Concentrations may decline.
The plume may stabilise or contract.
Residual product may remain.
Alternatively, monitoring may demonstrate that the contaminant mass is greater or more persistent than anticipated and additional active remediation is required.
This is why the completion of contaminated soil excavation does not necessarily mark completion of the overall remediation program.
Where soil and groundwater are linked, the performance of the groundwater system after source remediation can be an important line of evidence.
Validation of Groundwater Remediation Takes Time
Soil excavation can often be validated relatively quickly.
Groundwater is different.
Groundwater conditions vary over time.
A single round of acceptable groundwater results may not be enough to demonstrate that a petroleum plume has been successfully remediated or is stable.
A robust validation program may need to consider:
multiple monitoring events;
seasonal groundwater variation;
rebound after active remediation ceases;
contaminant trends;
LNAPL observations;
groundwater flow;
downgradient monitoring;
protection of receptors; and
whether the remediation objectives remain satisfied without active intervention.
The required monitoring period will depend on the site and remediation strategy.
This is one reason groundwater contamination can continue to influence development programs long after the obvious soil source has been removed.
Rebound Can Reveal Residual Source Mass
A further consideration following active remediation is rebound.
Groundwater concentrations can fall substantially while extraction or treatment systems are operating.
After the system is switched off, concentrations may increase again as contaminants redistribute from lower-permeability soil, residual product or other source material into groundwater.
A temporary reduction during active treatment therefore does not necessarily demonstrate that the source has been sufficiently controlled.
Post-treatment monitoring can help determine whether the improvement is sustained.
This becomes important when deciding whether active remediation can cease or whether another treatment phase is required.
Development Timing Can Influence the Strategy
Petroleum contamination is frequently encountered during redevelopment of former service stations, depots, workshops, industrial facilities and commercial properties.
The remediation strategy therefore needs to function within a development program.
Construction may create opportunities.
Tank removal may expose the source zone.
Bulk excavation may remove shallow contaminated soil.
Basement construction may allow deeper source material to be accessed.
At the same time, development can create constraints.
Buildings may restrict access.
Shoring may affect groundwater.
Dewatering may influence plume behaviour.
New structures may introduce vapour considerations.
The remediation program should therefore be developed alongside the project design rather than independently from it.
This is discussed further in our article Designing Soil Remediation Around the Proposed Development — Confluence Environmental
Common Failure: Treating the Groundwater Result Instead of the Source
One of the more important remediation principles on petroleum sites is understanding what is controlling the groundwater contamination.
Imagine an extraction system repeatedly removes hydrocarbons from groundwater, but petroleum-impacted soil and residual LNAPL remain within the source zone.
The groundwater system is being treated.
The mechanism continuing to contaminate it has not necessarily been removed.
This can extend the remediation timeframe substantially.
In contrast, reducing the source mass may allow the remaining dissolved plume to decline more effectively through a combination of active treatment and natural attenuation.
This is the difference between treating a concentration and treating a contamination system.
Common Failure: Assuming Tank Removal Equals Site Remediation
Removal of petroleum storage infrastructure is another point where the distinction matters.
The tank may have been removed successfully.
That does not demonstrate that historical leakage has been remediated.
In NSW, UPSS are specifically regulated because leakage can cause soil and groundwater contamination, and EPA guidance addresses monitoring, leak response and decommissioning requirements.
Where historical UPSS have been removed without adequate contamination assessment or validation, a subsequent Detailed Site Investigation may be required to establish whether residual soil or groundwater impacts remain.
The relevant question is not simply:
Was the tank removed?
It is:
Was the contamination associated with the former system adequately assessed and managed?
The Remediation Endpoint Needs to Be Defined Upfront
An effective petroleum remediation program needs a clear endpoint.
The objective is rarely simply:
Remove all hydrocarbons.
More useful remediation objectives may include:
removal of an ongoing petroleum source;
recovery of mobile LNAPL to the extent practicable;
reduction of soil contamination to an appropriate remediation criterion;
protection of groundwater receptors;
containment of a plume;
elimination of unacceptable vapour risk;
demonstration that the dissolved plume is stable or contracting;
achievement of defined groundwater criteria; or
establishment of a condition that can be safely managed over the long term.
These objectives should be established through the RAP and linked to validation requirements.
Without a clear endpoint, active groundwater remediation can continue for long periods without an objective basis for determining when the work is complete.
An Integrated Remediation Strategy
The most effective remediation strategy for a petroleum-impacted site may therefore use several methods rather than a single technology.
For example:
Source area: removal of former fuel infrastructure and excavation of heavily impacted soil.
Residual source: recovery or treatment of LNAPL and petroleum mass remaining around the groundwater interface.
Groundwater plume: active treatment within the higher-concentration plume core.
Downgradient plume: monitoring and natural attenuation where supported by the evidence.
Vapour pathway: assessment and mitigation where required.
Validation: soil validation combined with ongoing groundwater monitoring to demonstrate that the integrated remediation objectives have been achieved.
The exact combination will vary from site to site.
What remains consistent is the need to understand how each part of the contamination system interacts with the others.
Soil and Groundwater Remediation in NSW
Confluence Environmental provides soil and groundwater remediation and validation services for petroleum-impacted sites across NSW.
Our contaminated land team can support projects through investigation, remediation planning, implementation and validation, including:
petroleum source assessment;
soil and groundwater investigation;
groundwater monitoring;
UPSS and former fuel infrastructure assessment;
contaminant delineation;
Conceptual Site Model development;
Remediation Action Plans;
contaminated soil remediation;
groundwater remediation planning;
remediation supervision;
waste classification;
validation sampling; and
Site Validation Reports.
For petroleum-impacted sites, our approach is to understand the relationship between the source, soil, groundwater, vapour and potential receptors before selecting the remediation strategy.
This allows remediation effort to be directed toward the parts of the contamination system that are actually controlling risk and project outcomes, rather than treating soil and groundwater as unrelated problems.