Infiltration drainage where the ground is contaminated
Clean water entering contaminated soil becomes a flushing mechanism towards groundwater. That is why the drainage hierarchy and groundwater protection pull in opposite directions on brownfield sites.
Infiltration drainage on contaminated ground is the point where the drainage hierarchy and groundwater protection collide. The SuDS hierarchy pushes you towards infiltration as the first option. The Environment Agency's position is that infiltration systems should not be constructed in ground affected by contamination, because clean water entering contaminated soil becomes a flushing mechanism that drives soluble contaminants towards the water table. On a brownfield site those two positions cannot both be satisfied without doing something about the contamination.
The sequence that avoids a late objection is to settle the ground conditions before the drainage strategy is fixed. Most of the schemes that get stuck did it the other way round.
Why the two objectives conflict
Infiltration works by introducing water into the ground at a point and letting it percolate downwards. Where the soil contains residual contamination, that water does several things at once.
It dissolves soluble contaminants and carries them with it. It creates a hydraulic head that can mobilise non-aqueous phase liquids sitting on or in the soil. It changes pH and redox conditions in the unsaturated zone, which can make metals that were previously immobile soluble. And it concentrates all of this at a single point rather than spreading it over the site as rainfall would.
The receptor is groundwater, and groundwater is a controlled water. Causing or knowingly permitting a discharge that pollutes it engages the Environmental Permitting (England and Wales) Regulations 2016, so this is a regulatory question rather than a matter of good practice.
Where the Agency will object
The Environment Agency publishes its groundwater protection position and applies it consistently in consultation responses on planning applications.
Objections arise in a recognisable set of circumstances. Infiltration proposed through or into ground affected by contamination, without remediation. Infiltration within Source Protection Zone 1, where the travel time to a public water supply abstraction is shortest. Deep bore or shaft soakaways that bypass the unsaturated zone and deliver water close to or directly into the aquifer. Systems with insufficient unsaturated thickness beneath the base of the infiltration surface, where a minimum separation from the highest groundwater level is expected, commonly in the order of a metre, with the figure to be confirmed for the site. And infiltration into made ground of unknown provenance, which on former industrial sites is most made ground.
The objection is usually framed as a holding objection pending further information, which is recoverable. An objection on a scheme where the drainage design cannot change without redesigning the layout is much harder to recover from, which is the argument for front-loading the ground investigation.
Establishing what is actually there
A conceptual site model is the document that drives this, and it has to be built on real data rather than on a historical map review alone.
A Phase 1 desk study identifies the potential sources, pathways and receptors from historical mapping, previous uses, geology and hydrogeology. A Phase 2 intrusive investigation tests the hypotheses with boreholes, trial pits, soil analysis, groundwater monitoring wells and, where volatiles are suspected, soil gas and vapour sampling. Groundwater levels need monitoring over time rather than a single reading, because the separation distance to the water table is a design input and the water table moves seasonally.
Land contamination work is one of the areas where a suitably qualified and experienced professional is legally expected. Reports should follow BS 10175 and the government's land contamination risk management guidance, and a contaminated land assessment drafted with AI assistance is a starting point for that specialist rather than a substitute for their investigation and judgement. The same applies to the ground investigation itself, which belongs with the author of the Geotechnical Assessment.
Infiltration testing should be carried out at the depth and location of the proposed system, not at a convenient spot. A test pit in clean cover material tells you nothing about the horizon the soakaway will actually sit in.
Remediation that unlocks infiltration
Where infiltration is commercially important, usually because there is no outfall and no sewer capacity, remediation can make it acceptable.
The route is a remediation strategy agreed in advance with the Agency and the local planning authority, source removal or treatment to criteria set for the specific receptor, and a verification report demonstrating that the criteria were met. Validation sampling at the base and sides of the excavation, with the results assessed against the agreed criteria rather than against generic screening values, is what the Agency will look for.
Two practical points. Remediation to a standard adequate for human health is not necessarily adequate for groundwater protection, and the controlling criterion for infiltration is the water environment. And partial remediation beneath the footprint of the infiltration system, leaving contamination elsewhere on the site under cover, is sometimes acceptable, because the pathway rather than the source is what the infiltration creates. That is a site-specific argument to put to the Agency rather than an assumption to build into a design.
Treatment stages, and what they do not fix
A treatment train upstream of an infiltration feature deals with the pollutants in the runoff itself: silt, hydrocarbons from parking areas, metals from roofs. The SuDS manual sets out the stages and the number of treatment stages appropriate to the pollution hazard of each surface.
That is necessary and it is not the answer to this problem. Treatment protects groundwater from what the new development generates. It does nothing about contamination already in the ground beneath the system, because the mechanism there is the water moving downwards rather than the quality of the water arriving.
Confusing the two is a common defect in drainage statements, and reviewers spot it quickly. Say plainly which risk each measure addresses.
Alternatives when infiltration is ruled out
Most brownfield schemes end up with a lined system, and the design consequences follow.
- Lined attenuation storage with a controlled discharge to a sewer or watercourse, which requires the undertaker's or the riparian owner's agreement and a flow rate the receiving system will accept.
- An impermeable membrane beneath permeable paving, converting it from an infiltration feature into a storage and conveyance one, which changes the volume calculation.
- Discharge to a surface water sewer, which needs capacity confirmation and usually a restricted rate.
- Pumped discharge where levels do not allow gravity, which is a whole-life maintenance liability and a last resort.
- Partial infiltration in the cleaner part of the site, with the contaminated area drained by a lined system, which is often the most economic answer.
Where the design moves to a lined solution, the storage volume generally increases because none of the runoff is lost to the ground. Rework the calculation rather than assuming the previous volume will do. Excavation for lined storage on a contaminated site also brings its own consents, including those covered in our note on construction dewatering.
The evidence that resolves the objection
A submission that answers the Agency's concerns usually contains the following, and the Drainage Strategy should cross-refer to each rather than repeating it.
- A conceptual site model identifying sources, pathways and receptors, with the aquifer designation and any Source Protection Zone stated.
- Phase 1 and Phase 2 reports with soil, groundwater and, where relevant, gas and vapour data.
- Monitored groundwater levels establishing the highest credible level and the available unsaturated thickness.
- Infiltration test results at the proposed depth and location.
- An infiltration risk assessment addressing mobilisation specifically, rather than a general contamination risk assessment.
- A remediation strategy and verification plan where infiltration is to proceed after remediation.
- The drainage design with the treatment stages, the exceedance route and the maintenance regime.
The honest difficulty is programme. Groundwater monitoring takes months to produce a defensible range, and a scheme that needs the answer in six weeks will usually have to design for the lined solution and accept the cost. That is a reason to start the ground investigation at acquisition rather than at the point the drainage engineer asks for an infiltration rate.
This is general guidance rather than legal advice. Check the position with the Environment Agency, your local planning authority and a suitably qualified contaminated land specialist before relying on it.
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Frequently asked questions
Why does infiltration make contamination worse?
Infiltrating water dissolves soluble contaminants and carries them downwards, creates a hydraulic head that can mobilise non-aqueous phase liquids, and alters pH and redox conditions so that previously immobile metals become soluble. It also concentrates all of that at one point rather than spreading it as rainfall would, which is what creates the pathway to groundwater.
Where will the Environment Agency object?
Typically where infiltration is proposed into or through ground affected by contamination without remediation, within Source Protection Zone 1, through deep bore or shaft soakaways that bypass the unsaturated zone, where there is insufficient separation from the highest groundwater level, or into made ground of unknown provenance. Objections are often framed as holding objections pending further information.
How much unsaturated zone is needed beneath a soakaway?
A minimum separation from the highest groundwater level is expected, commonly in the order of a metre, though the figure should be confirmed for the site with the Agency. Establishing the highest credible level needs monitored groundwater data over time rather than a single reading, because the water table moves seasonally.
Can remediation make infiltration acceptable?
Often yes, through a remediation strategy agreed in advance with the Agency and the council, source removal or treatment to criteria set for the water environment rather than human health, and a verification report with validation sampling at the base and sides of the excavation assessed against the agreed criteria rather than generic screening values.
Does a treatment train solve the problem?
No. Treatment stages deal with pollutants in the runoff itself, such as silt, hydrocarbons and metals, and protect groundwater from what the development generates. They do nothing about contamination already in the ground beneath the system, because the mechanism there is water moving downwards rather than the quality of the water arriving.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.