Sustainable drainage on a site with contaminated ground

Infiltration drainage and contaminated ground are incompatible, and on a brownfield site that single fact can overturn a drainage strategy written on the assumption that soakaways would work. Driving surface water into the ground where contaminants are present risks mobilising them towards groundwater, and the Environment Agency's published position on groundwater protection is clear that infiltration through contaminated ground is unacceptable. In a Source Protection Zone the restriction is tighter still.

The answer is almost always a lined system with a positive outfall. The expensive part is discovering that in week eight of a determination rather than before the layout was drawn.

Why infiltration and contamination conflict

The national discharge hierarchy puts infiltration first, then a watercourse, then a sewer. Approved Document H and the Defra non-statutory technical standards for sustainable drainage both push designers up that hierarchy, and local policy usually repeats it. So an engineer's starting assumption is a soakaway or permeable paving.

On a site with a history of industrial use, that assumption carries a specific risk. Infiltration increases the volume of water moving downwards through soils that may contain hydrocarbons, metals, solvents or made ground of unknown composition, and that water becomes the transport mechanism. Where there was a stable, dry contaminant mass, there is now a plume.

The regulators treat this as a pollution risk rather than a drainage preference. That distinction matters in correspondence: a departure from the hierarchy justified by contamination is not a weak case, it is the correct answer.

Mobilising contaminants through the unsaturated zone

The mechanism is worth stating properly, because officers and consultees respond to a reasoned explanation rather than an assertion that infiltration is unsuitable.

Contaminants in made ground sit in the unsaturated zone above the water table. Introducing concentrated recharge at a point, which is what a soakaway does, does three things. It dissolves soluble contaminants and carries them downwards. It can mobilise fine particles with contaminants adsorbed to them. And it can raise the water table locally, bringing saturated conditions into contact with material that was previously dry.

Ground gas adds a second pathway. On sites with buried organic material or former landfill, changing the moisture regime can alter gas generation and migration, which is a building design issue as much as a drainage one.

Where a site has any of that history, the drainage strategy and the contamination assessment have to be written by people who have read each other's work.

Source protection zones and groundwater

Source protection zones define the catchments of public water supply abstractions, with the innermost zone representing the shortest travel time to the source. They are mapped and freely available, and checking them takes minutes.

Within the innermost zone the regulatory position is restrictive, and infiltration of anything other than clean roof water is generally unacceptable. Across wider zones and on principal aquifers the position is a matter of risk assessment rather than prohibition, and the assessment has to address the contaminant source, the pathway and the receptor explicitly.

Three things to establish and record early:

• Whether the site falls within a source protection zone, and which one.
• The aquifer designation of the underlying strata, which determines the regulator's level of concern.
• Depth to groundwater, including any perched water in made ground, measured rather than estimated.

That last point is where strategies come unstuck. A measured groundwater level from a single borehole in August is not a groundwater regime, and the unsaturated thickness available for infiltration is one of the inputs an infiltration design depends on.

Lined systems as the answer

Once infiltration is ruled out, the design becomes conventional attenuation with a controlled discharge. The components are familiar and the detailing is where the contamination constraint shows up.

Lined attenuation basins and swales, with an impermeable membrane protected by a geotextile and a positive outfall. Geocellular tanks within a sealed wrap, which also protects the structure from aggressive ground conditions. Permeable paving laid as a tanked system over a liner, which still delivers water quality treatment and storage while discharging to a pipe rather than to ground. Where ground gas is present, a gas-resistant membrane specification becomes part of the same detail.

Two practical consequences follow. Attenuation volume increases, because none of the water is being lost to ground, and that volume has to come from somewhere on the layout. And the outfall becomes critical, because the whole system now depends on it, so the invert level and the receiving capacity have to be confirmed rather than assumed.

Water quality treatment still has to be demonstrated. Removing infiltration removes one treatment mechanism, so the treatment train needs an alternative: filter strips above a liner, proprietary treatment devices, or sufficient staged storage with settlement.

Evidence the regulators need

Three regulators have an interest and they want different things. The lead local flood authority wants the surface water design, the discharge rate and the exceedance route. The Environment Agency wants the groundwater risk assessment. The council's contaminated land officer wants the conceptual site model and the remediation strategy.

What satisfies all three is one coherent set of documents in which the drainage strategy cites the contamination work and the contamination work addresses the drainage proposal. Specifically:

1. A conceptual site model identifying sources, pathways and receptors, including the proposed drainage as a potential pathway.
2. A statement of why infiltration has been discounted, referencing the ground conditions and the groundwater position.
3. The proposed lined system, with the liner specification and the discharge arrangement.
4. Confirmation of the receiving system's capacity and the agreed discharge rate.
5. The water quality treatment train, with the mechanisms identified.

A Contaminated Land Assessment is one of the report types where the local planning authority expects a report prepared in accordance with BS 10175 by suitably qualified and experienced persons, so an AI-assisted draft is a structured starting point for that professional rather than a substitute for them. The same applies to the ground investigation feeding a Geotechnical Assessment.

Sequencing the investigations

The usual mistake is running infiltration testing and contamination investigation as separate commissions at separate times, which produces soakaway test results for a site where soakaways were never going to be acceptable.

A sequence that works:

1. Phase 1 desk study, establishing the site history and a preliminary conceptual site model.
2. A decision point: does the history indicate contamination or a sensitive groundwater setting.
3. Where it does, intrusive investigation designed to answer the contamination and the geotechnical questions together, with groundwater monitoring over a period rather than a single visit.
4. Infiltration testing only where the contamination and groundwater position leaves infiltration viable.
5. Drainage strategy written against the actual ground conditions.

Running step 4 before step 3 wastes the testing cost. Running step 5 before step 3 wastes the design fee.

An illustrative example

A 1.4 hectare former printing works is proposed for 40 dwellings. The desk study identifies solvent use and underground tanks, and the site falls within a wider source protection zone over a principal aquifer. Monitoring over three months shows groundwater 4m below ground level with perched water in made ground. Infiltration is discounted in writing, and the strategy uses tanked permeable paving to the parking courts, a lined basin on the public open space and a vortex control to the surface water sewer. The contamination report and the drainage strategy cross-reference each other. This example is illustrative and the conclusions would differ on another site.

Where the two disciplines must be commissioned together

The honest position is that these two workstreams are usually procured separately, at different times, by different people, and that is the root of the problem rather than any technical difficulty.

Commission them with a shared brief. Ask the contamination consultant to comment specifically on the acceptability of infiltration and on any constraint the drainage design places on remediation. Ask the drainage engineer to design against the conceptual site model rather than against a generic assumption. Put both in the same Drainage Strategy submission so the officer reads one position rather than two.

Groundwater also matters for flood risk in its own right, and evidencing groundwater flooding in a flood risk assessment covers the monitoring that supports that case. The same borehole data serves both.

This is general guidance, not legal advice. Contaminated land and groundwater risk assessment are specialist disciplines with legal consequences, so take advice from suitably qualified professionals and confirm the regulatory position with the Environment Agency and your local planning authority.

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