Ground gas protection in a contaminated land assessment

Ground gas protection is the part of a contaminated land assessment that most often holds up a discharge of conditions, and it fails for a predictable reason: the monitoring does not support the conclusion. A ground gas assessment needs concentrations and flow rates, measured over enough visits and under the right atmospheric conditions, before any characteristic situation can be assigned or any protection designed. Concentrations alone tell you almost nothing. If your consultant's report gives you methane and carbon dioxide percentages with no flow data, the assessment is not finished.

Why ground gas is assessed on its own

Soil and groundwater contamination is assessed by comparing measured concentrations against assessment criteria for the intended land use. Ground gas does not work that way.

The hazard is a mixture of what gas is present, how fast it can move, and what the building does with it. A modest methane concentration in a well-connected granular horizon beneath a sealed slab is a worse proposition than a higher concentration in a low-permeability clay under a suspended floor with a ventilated void.

That is why the assessment has its own method, its own monitoring regime and its own design standard, and why it sits in a separate chapter of the report rather than inside the human health risk assessment.

Sources worth expecting are made ground and former landfill, infilled quarries and ponds, organic alluvium and peat, and mine gas in coalfield areas. Radon is a different regime again, assessed against published mapping and dealt with through its own protective measures.

Monitoring rounds and the weather they need

Gas emission is driven by atmospheric pressure. When pressure falls, gas in the ground expands and migrates towards the surface, which is exactly when it enters buildings.

So a monitoring programme has to include visits during periods of low and falling barometric pressure, and it has to record the pressure and the trend at each visit. A set of readings taken on six sunny mornings in a stable high tells you nothing about the worst case.

The number of visits follows the risk. Published guidance points to a minimum programme spread over several months for a low risk site, with substantially more rounds where a landfill or mine gas source is present. Agree the programme with the local authority's contaminated land officer before you start rather than after the results arrive, because a regime rejected at the discharge stage has to be repeated from scratch and the repeat sits on the critical path.

Installation matters as much as frequency. Response zones should target the gassing horizon rather than spanning several strata, and the record should show the installation detail alongside the readings.

From concentration and flow to a screening value

Each visit should record, for every monitoring well: methane, carbon dioxide and oxygen concentrations, borehole flow rate, atmospheric pressure and its trend, and water level.

The gas screening value is derived from the flow rate and the concentration together, expressed as a gas flow in litres per hour. That single figure, taken at the worst credible combination rather than the average, is what drives the classification.

Two errors recur. Using peak concentration with average flow, which understates the hazard, and averaging across all wells rather than assessing the worst well in the relevant part of the site. Report the derivation well by well and show the working.

Characteristic situations and what protection follows

The classification places the site in one of a series of characteristic situations, running from very low risk through to very high, with boundaries defined by gas screening value. Read those boundaries from the current published classification rather than from a figure carried over from an old report, because the standards in this area have been revised.

The protection design then follows a points-based approach. Each element contributes a score. The structural barrier, meaning the floor slab type and its detailing. The gas resistant membrane. And ventilation, usually a passive ventilated subfloor void or a granular blanket vented to atmosphere. The total score required depends on the characteristic situation and on the building type, with housing in unrestricted private ownership requiring more than a commercial building under managed control.

The building type point is worth dwelling on. A private house can be extended, have its void filled, or have a conservatory built over a vent by an owner who has never heard of the gas assessment. That is why the standard asks for more protection in that case.

Designing the protection rather than specifying it

A protection system is a designed element rather than a product selection. Someone has to decide the slab detail, the membrane specification, the venting arrangement and every junction where they meet.

The junctions are where systems fail. Service entries, pile heads, column bases, the perimeter detail at the wall, and the interface with a lift pit or a below-ground drainage chamber. A membrane specification with no detail drawings at those points is an aspiration.

Make sure the design is recorded in the remediation or verification strategy submitted to discharge the condition, with drawings. Contractors build from drawings, and a strategy that describes the system in prose will be built from someone's memory of the last job.

Verification and the report that discharges the condition

Verification is a separate exercise carried out during installation by a suitably qualified person independent of the installer.

It covers inspection of the substrate before the membrane goes down, checking laps and seals, integrity testing where the design calls for it, inspection of every penetration, photographic recording with location references, and confirmation that the ventilation is open and will stay open.

The verification report is what actually discharges the condition. It should include the as-built drawings, the photographic record indexed to locations, the test results, the qualifications of the verifier, and a statement of any deviations from the design and how they were resolved.

Build the verification into the construction programme and the contract. A verifier appointed the week before practical completion cannot inspect a membrane that is already under a slab, and the only remedy at that point is opening up.

Where these assessments usually fall short

Seven failures account for most rejected submissions.

1. Concentrations recorded without flow rates, so no screening value can be derived.
2. Monitoring carried out entirely during stable or rising pressure.
3. Too few rounds, or rounds compressed into a fortnight.
4. Response zones spanning several strata, mixing gas from different horizons.
5. No conceptual site model linking source, pathway and receptor, so the protection is not justified against anything.
6. A protection system named but not designed, with no junction details.
7. Verification arranged after installation, or carried out by the installer.

Land contamination assessment, including the gas work, must be carried out by a suitably qualified and experienced person working to the relevant British Standard for the investigation of potentially contaminated sites. This is a legal and regulatory expectation rather than a preference, and the local authority will look at the author's credentials. A contaminated land assessment drafted with AI assistance is a starting point for that professional, never a substitute for them, and the same applies where a Geotechnical Assessment runs alongside it on the same boreholes.

An illustrative example. A 40 unit scheme on a partially infilled clay pit is monitored across eight visits over four months, two of them during falling pressure below 1000 millibars. The worst well produces a screening value that places the site in a moderate category, and the design uses a reinforced concrete slab, a gas resistant membrane and a passive ventilated void. Verification during installation finds three unsealed service penetrations, which are remedied before the slab is poured. The condition is discharged first time. Figures here are illustrative.

To see which ground-related documents a site with an industrial or infill history is likely to need, browse the report types by category.

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.