Permeable paving where the ground is contaminated

Permeable paving on contaminated land is usually still the right answer, but as a lined system rather than an infiltrating one. The drainage hierarchy pushes designers towards infiltration first, and on a site with made ground, a former industrial use or a position inside a groundwater source protection zone, infiltration is the one option the Environment Agency is most likely to resist. Putting clean rainwater through contaminated soil creates the pathway that was previously missing.

That is the whole of the conflict. The soil holds a source, the aquifer or watercourse is the receptor, and infiltration supplies the pathway between them.

Why infiltration can mobilise contamination

Risk assessment for land contamination works on the source, pathway and receptor model. A hydrocarbon plume sitting in made ground 1.5m below a yard surface may present an acceptable risk while the ground above it is sealed and the water table is well below it. Introduce a soakaway or an unlined permeable pavement sub-base and you have added a flushing mechanism that drives dissolved contaminants downwards and sideways.

Several contaminant groups behave badly under that treatment. Chlorinated solvents move as dense non-aqueous phase liquids and sink. Sulphates leach readily from colliery spoil and demolition rubble. Metals mobilise when the pH of the infiltrating water differs from the pH of the soil. Where ground gas is present, a drainage system that alters the moisture regime can also change the gas generation and migration pattern.

None of that is a reason to abandon surface water management at source. It is a reason to design the system so water is stored, treated and released without ever entering the ground.

Source protection zones and groundwater vulnerability

The Environment Agency maps source protection zones around public water supply abstractions, graded by the time groundwater takes to travel to the abstraction point. Zone 1 is the inner protection zone, Zone 2 the outer, and Zone 3 the total catchment. The Agency's groundwater protection position statements set out what it will and will not accept in each, and infiltration of anything other than clean roof water through made ground inside Zone 1 is the clearest objection you will meet.

Check three datasets before the drainage concept is fixed:

• Source protection zones and aquifer designations, which show whether the underlying strata are a principal or secondary aquifer.
• Historical mapping, which tells you where the yards, tanks, filled ground and railway land were.
• Borehole and groundwater records, which give you the standing water level and the unsaturated thickness available.

A principal aquifer with thin unsaturated cover and a history of industrial use is a combination where infiltration will not survive consultation, whatever the percolation test says.

Lined permeable systems as an alternative

A lined, or tanked, permeable pavement keeps almost everything the designer wanted. The surface still intercepts rainfall at source, the sub-base still stores it, silt and hydrocarbons are still intercepted in the upper layers, and the discharge is still controlled to an agreed rate. What changes is the base and sides, which take an impermeable geomembrane, and the addition of a positive outfall with a flow control.

Design points that matter when the system is lined:

1. Storage volume has to be calculated without any infiltration credit, which typically increases the sub-base depth or the plan area.
2. The liner needs protection geotextiles above and below, and a specification for welded joints rather than overlaps.
3. The outfall must have somewhere to go. A lined system is only as good as the discharge point, so the receiving sewer, watercourse connection or downstream attenuation needs to be confirmed before the layout is committed.
4. Upward migration of ground gas through the construction needs considering where gas protection measures are in place for the buildings.
5. Maintenance access and a silt management regime should be stated, because the treatment function sits in the upper layers.

Where the discharge runs to a watercourse, the outfall detail is its own exercise, and our note on outfall design, bank stability and scour protection covers the points the lead local flood authority raises.

Evidence needed from the ground investigation

The drainage case and the contamination case have to be written from the same ground investigation, and the commonest failure is that they are not. A drainage engineer who has only the infiltration test results, and a contamination specialist who has only the chemical suite, will produce documents that contradict each other.

What the investigation needs to deliver for the drainage design:

• Infiltration testing to a recognised method, with the test locations plotted on the proposed layout rather than on the site boundary.
• Groundwater levels, with seasonal variation where the programme allows, and the depth of the unsaturated zone beneath any proposed infiltration base.
• A chemical suite covering the contaminants of concern identified in the desk study, with leachate testing where infiltration is still being pursued.
• Made ground thickness and permeability, because a permeable made ground layer over clay spreads water laterally into foundations rather than downwards.

Reports on land contamination are expected to be prepared in accordance with BS 10175 by suitably qualified and experienced persons, and the Phase 1 and Phase 2 stages of that work are among the report types where a competent professional is legally necessary rather than merely advisable. An AI-assisted draft is a starting point for that specialist, not a substitute for them.

Consultee positions on infiltration

Three bodies take a view and they do not always agree.

The Agency will also expect a piling risk assessment where piles or ground improvement could puncture a low-permeability layer. That request surprises applicants who have already removed infiltration from the drainage design, and it is a separate pathway question.

Alternatives where infiltration is ruled out

Removing infiltration does not remove the obligation to manage surface water at source or to meet a discharge rate. The options that remain are a lined permeable pavement, lined swales and basins with a controlled outlet, blue and green roofs, rainwater harvesting with a managed overflow, geocellular storage with a liner, and discharge to sewer with the undertaker's agreement and pre-treatment.

Keep the treatment train honest. Lining a system removes the soil as a treatment medium, so the pollution mitigation has to come from the upper layers of the pavement, from a filter strip, from a proprietary interceptor, or from a combination. On a steeply sloping site the sequencing of those elements changes again, which we cover in managing runoff on a steeply sloping site.

Construction stage water is a separate consent question. Dewatering an excavation in contaminated ground needs its own permissions, set out in our guide to construction dewatering and the consents you actually need.

Conditions that manage the conflict

Expect the permission to carry a cluster of related conditions, and read them as a set rather than individually. The usual ones are a detailed surface water drainage scheme to be approved before development above slab level, an express prohibition on infiltration drainage unless otherwise agreed in writing, a remediation strategy and verification report requirement, an unexpected contamination clause, a piling risk assessment where relevant, and a maintenance and management regime for the life of the development.

Two of those are worth negotiating at draft stage. A condition that bars infiltration outright will block a later soakaway for a garden area where that would be acceptable, so ask for wording that allows infiltration with prior written approval. And a verification condition tied to "prior to first occupation" is easier to discharge than one tied to completion of the paving, which on a phased site can be years apart.

A Drainage Strategy that states plainly why infiltration has been discounted, with the datasets and test results behind it, saves a consultation round. Officers rarely argue with a clear negative finding. They argue with silence.

Enter your postcode on PlanScope to see which drainage and ground condition reports your site is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.