What Is Groundwater? Groundwater, Aquifers and Contamination Explained

Groundwater is water that occurs below the land surface within saturated soil, sediment and rock. It forms an important part of the natural water cycle and is used across NSW for town water supply, agriculture, industry and other purposes. Groundwater can also support rivers, wetlands, springs and other groundwater-dependent environments.

For contaminated land and development projects, groundwater can be particularly important because it may provide a pathway for contaminants to migrate away from their original source.

Where a potential groundwater pathway is identified, groundwater assessment and monitoring may be required to establish groundwater conditions, investigate contamination and determine whether further management or remediation is necessary.

What Is Groundwater?

Groundwater is water stored below the ground surface within the spaces between soil and sediment particles and within fractures or openings in rock.

Rainfall is one of the main sources of groundwater recharge. Some rainfall flows across the land surface into creeks and rivers, while some infiltrates into the ground. Water that continues moving downward may eventually reach a zone where the available pores and fractures are saturated.

The upper surface of this saturated zone is commonly referred to as the water table.

Groundwater does not usually occur as large underground lakes or rivers. Instead, it commonly occupies small interconnected spaces within geological materials and moves through them over time.

Groundwater systems can be connected to rivers, streams, lakes and wetlands, meaning groundwater and surface water should not always be considered independently.

What Is an Aquifer?

An aquifer is a geological formation capable of storing and transmitting groundwater.

Aquifers can occur within different geological materials, including:

• sand and gravel;

• alluvial sediments;

• coastal sands;

• sandstone and other sedimentary rocks;

• fractured rock; and

• limestone or other formations containing interconnected openings.

Geoscience Australia identifies a range of groundwater settings across Australia, including alluvial aquifers, coastal aquifers, fractured-rock systems and sedimentary basins. Geoscience Australia

The characteristics of an aquifer influence how much groundwater it can store, how readily groundwater can move through it and how contaminants may migrate if groundwater becomes affected.

How Does Groundwater Move?

Groundwater is not static.

Water generally moves from areas of higher hydraulic head toward areas of lower hydraulic head through permeable soil, sediment and rock.

Groundwater enters a system through recharge and leaves through discharge.

Recharge may occur through rainfall infiltration, interaction with rivers or other pathways.

Groundwater may discharge to:

• rivers and creeks;

• wetlands;

• springs;

• lakes;

• coastal environments;

• groundwater bores; or

• other parts of the groundwater system.

Groundwater levels can also change over time due to rainfall, drought, groundwater extraction, construction dewatering and other natural or human influences.

This is one reason a single groundwater level measurement does not necessarily describe groundwater conditions over the life of a project.

Why Is Groundwater Important in Contaminated Land Assessment?

Groundwater can provide an important pathway for contaminant migration.

For example, a release of fuel into soil may migrate downward through the unsaturated soil profile and eventually reach groundwater.

Once contamination reaches groundwater, dissolved contaminants can potentially migrate away from the original source area.

This may create risks to:

• groundwater users;

• neighbouring properties;

• creeks and wetlands;

• ecological receptors;

• surface water;

• future development; or

• other environmental values.

Groundwater therefore forms an important part of some contaminated site assessments and environmental site investigations.

The ASC NEPM provides the nationally consistent framework for assessment of site contamination in Australia and includes groundwater within the site-characterisation and risk-assessment process.

Whether groundwater requires investigation should be determined through the Conceptual Site Model, rather than assuming every contaminated site requires groundwater monitoring.

How Does Groundwater Become Contaminated?

Groundwater contamination can occur where chemicals or other contaminants enter the subsurface and migrate into groundwater.

Potential sources can include:

• leaking underground petroleum storage systems;

• service stations;

• fuel depots;

• workshops;

• industrial facilities;

• chemical storage and handling;

• landfills;

• waste disposal;

• uncontrolled fill;

• wastewater systems;

• agricultural activities;

• PFAS sources;

• spills and leaks; and

• historical contamination within soil.

The way contamination behaves depends on both the contaminant and the groundwater system.

Some contaminants dissolve readily in groundwater and may migrate significant distances.

Others bind strongly to soil or sediment and may move more slowly.

Petroleum contamination can also occur as separate-phase product where sufficient fuel or oil has entered the subsurface.

Factors such as geology, groundwater depth, groundwater flow direction, contaminant characteristics and preferential pathways can all influence the extent of groundwater contamination.

What Contaminants Can Affect Groundwater?

There is no single analytical suite that applies to every groundwater investigation.

The contaminants selected for analysis should reflect the history of the site and the potential sources identified through the Conceptual Site Model.

Common contaminant groups can include:

• petroleum hydrocarbons;

• BTEXN compounds;

• PAHs;

• metals;

• PFAS;

• chlorinated solvents;

• pesticides;

• nutrients; and

• other industrial or site-specific chemicals.

A former service station will generally present a different groundwater contamination scenario from a landfill, former manufacturing facility or agricultural property.

Groundwater assessment should therefore be designed around the contaminants that could reasonably be associated with the site.

How Is Groundwater Contamination Assessed?

A groundwater assessment generally begins with an understanding of the potential contamination source and the hydrogeological setting.

The assessment may consider:

• site history;

• geology;

• groundwater bore information;

• expected groundwater depth;

• previous soil and groundwater data;

• nearby groundwater users;

• surrounding surface water;

• contaminant characteristics; and

• the Conceptual Site Model.

Where intrusive investigation is required, groundwater monitoring wells may be installed at selected locations and depths.

A groundwater investigation can then include:

• monitoring well installation;

• groundwater level gauging;

• assessment of groundwater flow direction;

• well development;

• groundwater sampling;

• field water-quality measurements;

• laboratory analysis;

• contaminant plume assessment; and

• interpretation of temporal trends.

Monitoring wells should be located according to the question the investigation is intended to answer.

For example, wells may be positioned hydraulically upgradient and downgradient of a suspected source to help determine whether groundwater contamination is associated with the site and whether impacts are migrating.

What Is Groundwater Monitoring?

Groundwater monitoring involves collecting groundwater data over one or more monitoring events to understand groundwater levels, groundwater quality or contamination conditions.

Groundwater monitoring services can be used for:

• baseline environmental assessment;

• contaminated land investigation;

• plume assessment;

• industrial sites;

• former fuel facilities;

• landfill and leachate monitoring;

• remediation monitoring;

• post-remediation monitoring;

• dewatering projects;

• environmental management plans; and

• regulatory or development requirements.

A groundwater monitoring program may include groundwater-level measurements, sampling, field parameters and laboratory analysis.

Field parameters can include:

• pH;

• electrical conductivity;

• dissolved oxygen;

• oxidation-reduction potential;

• turbidity; and

• temperature.

Monitoring frequency should reflect the purpose of the program.

Some projects may require only one or several rounds of monitoring. Others may require quarterly, six-monthly or longer-term monitoring to understand seasonal variation, plume behaviour or remediation performance.

Confluence Environmental provides groundwater monitoring services and environmental monitoring services for contaminated land, development, remediation and operational projects across NSW.

When Is Groundwater Assessment Required During a Detailed Site Investigation?

Groundwater investigation is not automatically required for every Detailed Site Investigation (DSI).

The need for groundwater assessment should be determined from the site history, contaminant characteristics, geology, groundwater depth, surrounding environmental setting and Conceptual Site Model.

Groundwater investigation may be appropriate where:

• soil contamination extends to depth;

• historical underground fuel infrastructure is present;

• mobile contaminants have been identified;

• groundwater is relatively shallow;

• contamination could migrate beyond the site boundary;

• groundwater users are located nearby;

• groundwater may discharge to a sensitive surface-water environment; or

• previous investigation has identified potential groundwater impacts.

Where a credible groundwater pathway exists, a DSI may include monitoring well installation, groundwater sampling and laboratory analysis alongside soil, soil vapour, surface water or other environmental media.

The results are then used to refine the Conceptual Site Model and determine whether further investigation, management or remediation is required.

Does a Preliminary Site Investigation Include Groundwater Testing?

A Preliminary Site Investigation (PSI) is primarily intended to establish the site history, identify potentially contaminating activities, understand the environmental setting and develop a preliminary Conceptual Site Model.

Groundwater may still be considered during the PSI.

For example, the assessment may review:

• groundwater bore records;

• expected groundwater depth;

• regional groundwater conditions;

• groundwater users;

• surrounding water bodies; and

• whether groundwater could provide a migration pathway.

However, installation and sampling of groundwater monitoring wells is not an automatic component of every PSI.

Where the PSI identifies a credible groundwater contamination pathway, the findings can be used to develop the scope of a subsequent contaminated site investigation.

What Is the Relationship Between Groundwater and Surface Water?

Groundwater and surface water can be closely connected.

Groundwater can discharge into rivers, creeks, wetlands and coastal environments, while surface water may also recharge groundwater systems.

This connection can become important during contaminated land assessment.

For example, contaminated groundwater migrating from an industrial property may eventually discharge to a nearby creek.

In that situation, the environmental site investigation may need to consider both groundwater and surface water to properly assess the potential contamination pathway.

This is why groundwater and surface water investigations are often considered together where the Conceptual Site Model identifies a connection between the two environments.

What Happens if Groundwater Contamination Is Identified?

Identifying groundwater contamination does not automatically mean that active remediation is required.

The significance of the contamination needs to be assessed in the context of:

• the contamination source;

• contaminant concentrations;

• lateral and vertical extent;

• groundwater flow direction;

• migration potential;

• groundwater uses;

• receiving environments;

• exposure pathways; and

• relevant environmental values.

Further investigation may involve:

• additional monitoring wells;

• plume delineation;

• contaminant trend assessment;

• source-area investigation;

• risk assessment;

• ongoing groundwater monitoring; or

• remediation planning.

The ASC NEPM adopts a risk-based approach to contaminated site assessment, including groundwater contamination.

What Is Groundwater Remediation?

Groundwater remediation involves managing or reducing unacceptable risks associated with contaminated groundwater.

The appropriate remediation strategy depends on the contaminant, hydrogeological conditions, plume behaviour, environmental receptors and remediation objectives.

Depending on the site, groundwater remediation may involve:

• removal or treatment of the contamination source;

• monitored natural attenuation;

• groundwater extraction and treatment;

• in-situ treatment;

• hydraulic containment;

• management controls; or

• other specialist remediation technologies.

Groundwater remediation should generally follow an adequate investigation of the source, extent and significance of contamination.

Where remediation forms part of a contaminated-land project, the strategy may be documented through a Remediation Action Plan (RAP) and supported by groundwater monitoring during and following remediation.

Confluence Environmental can support the investigation, remediation planning, groundwater environmental management, monitoring and validation components of groundwater remediation projects.

Why Does Groundwater Monitoring Continue After Remediation?

Groundwater contamination can respond more slowly to remediation than contamination in soil.

Even where the original source has been removed or treated, groundwater impacts may remain for a period of time.

Post-remediation groundwater monitoring can therefore be used to determine whether:

• contaminant concentrations are decreasing;

• the plume is stable;

• the plume is contracting;

• contamination continues to migrate;

• remediation objectives are being achieved; and

• additional management is required.

For some projects, longer-term groundwater environmental management services may be required to monitor groundwater conditions over time.

Who Undertakes Groundwater Assessment and Monitoring?

Groundwater assessment associated with contaminated land should be undertaken by environmental professionals with appropriate experience in contaminated site investigation and groundwater monitoring.

An experienced environmental consultant should be able to integrate groundwater data with:

• site history;

• soil contamination;

• hydrogeology;

• surrounding receptors;

• Conceptual Site Model;

• proposed land use; and

• other environmental data.

Depending on the complexity of the project, groundwater investigations may also involve hydrogeologists, environmental scientists, drilling contractors, laboratories and remediation specialists.

The purpose is not simply to collect groundwater samples.

The value of the investigation lies in understanding what the groundwater data mean and what decision they support.

Frequently Asked Questions (FAQ):

Q: What is groundwater?
Groundwater is water stored below the land surface within saturated soil, sediment and rock. It can move through interconnected pores and fractures and may discharge to rivers, wetlands, springs or other environments.

Q: What is an aquifer?
An aquifer is a geological formation capable of storing and transmitting groundwater.

Q: Is groundwater the same as the water table?
No. Groundwater is the water within the saturated subsurface. The water table generally describes the upper surface of the saturated zone.

Q: Does every contaminated site require groundwater monitoring?
No. Groundwater should be investigated where the Conceptual Site Model identifies a credible groundwater source, pathway or receptor. It is not automatically required for every contaminated land investigation.

Q: How is groundwater sampled?
Groundwater is commonly sampled from monitoring wells. The process can include groundwater-level measurement, purging or low-flow sampling, field parameter measurement, laboratory analysis and quality assurance procedures.

Q: Can groundwater contamination move off site?
Yes. Contaminants dissolved in groundwater can migrate with groundwater flow where geological and hydrogeological conditions allow.

Q: Does contaminated groundwater always require remediation?
No. The appropriate response depends on the contaminant, extent, migration pathways, groundwater uses and receptors. Some sites require remediation, while others may require further investigation, management or ongoing monitoring.

Groundwater Assessment and Monitoring in NSW

Confluence Environmental provides groundwater assessment, groundwater monitoring, groundwater contamination investigation and surface water services across NSW for contaminated land, development, remediation, infrastructure and environmental management projects.

Our groundwater and environmental services include monitoring well installation, groundwater sampling, water-level monitoring, laboratory analysis, contamination assessment, plume investigation, environmental monitoring and remediation support.

Programs are developed around the site setting, Conceptual Site Model and project objectives so that monitoring is directed toward the environmental questions that actually need to be resolved.

Where groundwater forms part of a broader contaminated land investigation, learn more about our Detailed Site Investigation services.