Yes. Groundwater contamination can continue moving beyond a factory boundary long after the original fuel or chemical leak has stopped. Pollutants already dissolved in groundwater may travel through permeable ground, while contamination remaining in soil or rock can continue releasing substances into the water.
For an industrial landlord, repairing a leaking tank or removing visibly contaminated soil does not necessarily establish that the groundwater is clean. The important question is whether contamination remains mobile, where it could travel and whether it threatens groundwater resources or other receptors.
Why a stopped leak can leave a moving plume
When fuel or chemicals enter the ground, some material may be retained within soil pores or fractures. Depending on its properties, some can dissolve into groundwater and form a contaminant plume.
A plume is a zone of affected groundwater that may extend away from the original release. It does not necessarily remain directly beneath the building, tank or pipe where the incident occurred.
Three processes can continue after source control:
- Residual release: Pollutants retained in soil or rock can continue dissolving into passing groundwater.
- Groundwater transport: Dissolved substances can move with groundwater through connected pores and fractures.
- Plume redistribution: Dispersion, dilution, sorption and degradation can alter concentrations and distribution without immediately eliminating the affected area.
A particularly important distinction is between removing an ongoing source and addressing contamination already present in the aquifer. The Environment Agency’s groundwater compliance point guidance treats existing groundwater contamination as a separate consideration within historical land-contamination risk assessments in England.
What determines how far contamination travels?
Groundwater movement depends on local geology and hydraulic conditions, not simply the distance from the original spill.
The hydraulic gradient describes the change in groundwater head across a distance and helps establish the likely direction of flow. Permeability determines how readily water can move through the ground. A permeable sand or gravel layer may provide a more effective transport pathway than a relatively impermeable clay layer.
Fractured rock and made ground can complicate the picture. Contaminants may follow preferential pathways rather than spreading evenly in every direction.
Other important considerations include:
- Whether groundwater levels change seasonally or following nearby pumping.
- How readily individual chemicals dissolve, attach to soil particles or degrade.
- Whether contamination occurs at different depths.
- Whether the plume is approaching a river, spring, abstraction or neighbouring groundwater resource.
A factory boundary has no inherent ability to stop groundwater flow. Equally, contamination detected beneath a site does not automatically prove that neighbouring land has been affected. That requires evidence of the groundwater pathway and contaminant distribution.
Use monitoring to establish whether the plume is stable
Monitoring boreholes positioned at appropriate locations and depths can provide evidence about groundwater levels, flow direction and contaminant concentrations.
For a historic release, a single sampling visit offers only a snapshot. Repeated observations can help distinguish a declining plume from contamination that is persistent, fluctuating or expanding.
Useful evidence includes background groundwater quality, concentrations near the former source, results farther down-gradient and measurements near identified receptors.
Where the existing information is incomplete, a targeted Phase 2 Site Investigation may help establish the extent of contamination and refine the understanding of groundwater pathways.
When is remediation needed rather than continued monitoring?
The decision should reflect the pollutant, pathway and receptor risks, supported by appropriate remedial targets. Monitoring may provide sufficient evidence for a stable or declining plume where relevant risks can be demonstrated to be acceptable.
However, continued movement towards a sensitive receptor, unacceptable concentrations or evidence of an ongoing contaminant release may justify active intervention.
Options can include further source treatment, groundwater recovery, in-situ treatment or measures that interrupt contaminant migration. The appropriate approach depends on the site’s hydrogeology and the behaviour of the substances involved.
A Groundwater Remediation strategy should therefore distinguish work on the original source from treatment or management of the existing plume.
For the landlord, the most useful outcome is documented evidence showing whether the plume is controlled, whether relevant receptors remain protected and what further monitoring or treatment is necessary. A repaired leak alone cannot answer those questions.
