Ground gas monitoring regimes in a Phase 2 report

A ground gas monitoring regime is the part of a Phase 2 report most likely to be sent back, and the reason is almost always the same: too few visits, none of them during falling atmospheric pressure, and flow rates recorded as zero because nobody had the right instrument. Soil and groundwater chemistry is a sampling exercise you can complete in a week. Gas is a time series, and the programme has to be planned months before the ground floor slab is designed.

Land contamination assessment at this stage requires a suitably qualified and experienced person. A drafted report structure is a starting point for that specialist, never a substitute for them.

Why gas is a separate exercise from soil chemistry

Soil contaminant concentrations are a property of the material. Take a representative sample, analyse it, compare it with an assessment criterion. The answer does not change next Tuesday.

Ground gas generation and migration depend on the source, the ground, the groundwater level and the weather. Methane and carbon dioxide from degrading organic material, from made ground, from peat, from alluvium, or from mine workings in a coalfield referral area, all behave differently and all respond to barometric pressure and rainfall. A single visit tells you almost nothing.

That is why a Phase 1 desk study that identifies a credible gas source should trigger the monitoring programme immediately, in parallel with the intrusive investigation, rather than after the chemistry comes back. Every week of delay is a week added to the planning programme, because the local planning authority will condition a verified protection scheme before occupation and the design cannot be fixed until the characteristic situation is known.

Well design, before any visit happens

Bad wells produce bad data, and no amount of monitoring fixes them.

The response zone needs to sit within the horizon you are investigating, with a bentonite seal above it so that the headspace is not simply sampling the atmosphere down the annulus. A well screened across both the made ground and an underlying granular layer gives you a mixed reading you cannot attribute.

Practical points that separate a usable installation from a decorative one:

• A gas tap at the surface, in a lockable cover, with the headspace volume recorded so purge requirements are known.
• Separate wells at separate depths where more than one source horizon is credible, rather than one long screen.
• Gas wells kept distinct from groundwater monitoring wells wherever the programme allows, because pumping and dipping disturb the gas regime.
• A survey of each well position and depth, recorded on a plan, so later readings can be tied to the stratigraphy.
• Records of groundwater level at every gas visit, because a rising water table displaces gas and produces the highest flows.

How many visits, and over what period

There is no single prescribed number, and anyone who quotes one without reference to the risk has misread the guidance.

The approach in CIRIA C665 is proportionate. A low risk site with a thin layer of made ground and no organic content may be adequately characterised by a handful of visits spread over a few months. A former landfill, or a site with deep organic fill, needs a far longer programme with many more visits, and regulators frequently ask for a full year to capture seasonal variation.

What makes a programme defensible is not the count. It is whether the visits captured the range of conditions the site will experience, and whether the report says so with the data to prove it. Six well timed visits can be more persuasive than twenty taken on fine mornings in June.

Agree the programme with the council's contaminated land officer before it starts. A regime signed off in advance rarely gets challenged afterwards.

Atmospheric pressure and the visits that matter

Gas emissions rise as barometric pressure falls, because the pressure differential between the ground and the atmosphere drives flow. Monitor only in settled high pressure and you will record nothing and conclude nothing.

So the programme needs visits during low pressure, below roughly 1000 mbar, and during periods when pressure is actively falling rather than merely low. A falling trend over the preceding hours matters more than the absolute value at the moment of reading.

That has consequences for how the work is commissioned. A fixed schedule of monthly visits will miss the conditions that matter. What works is a baseline schedule with a weather triggered element, where the monitoring team is instructed to attend within a set window when forecast pressure drops below a threshold. It costs more in mobilisations and it is the difference between a report that stands and one that is returned.

Record at every visit: barometric pressure at the time of reading, the trend over the preceding 24 hours, air temperature, rainfall in the preceding period and groundwater level. Those five columns are what a reviewer reads first.

Flow rate and concentration together

The gas screening value approach combines borehole flow rate and gas concentration, because either alone is misleading.

A well reading 60 per cent methane with no measurable flow presents a very different risk from one reading 5 per cent methane with a steady positive flow. The screening value multiplies the two, giving a flow of hazardous gas in litres per hour, and that figure is what feeds into the characteristic situation.

Flow is where most datasets fail. Measuring low flows through a monitoring well needs an instrument with appropriate resolution, and the common reporting of "0.0 l/h" across a whole programme often means the instrument could not resolve what was there. Record the instrument type and its resolution, record both the initial and the steady state reading, and record whether flow was positive or negative.

Gas concentrations should cover methane, carbon dioxide and oxygen as a minimum. Oxygen depletion without raised methane or carbon dioxide is itself a finding, pointing to oxidation or to a confined space risk. Where volatile organic compounds are credible from the desk study, they need their own assessment and they do not belong in the gas screening value calculation.

Deriving the characteristic situation

The characteristic situation places the site in a band, and the band drives the protection measures.

The derivation uses the gas screening value, calculated from the maximum flow and maximum concentration observed, alongside the nature of the source and the type of development proposed. BS 8485 provides the framework for translating that band into protection for new buildings, using a points based approach across the structural barrier, the gas resistant membrane and any ventilation layer, so that different combinations can achieve the same level of protection.

Two things to get right in the report. State the data the derivation rests on, including the single highest reading and whether it was an outlier or part of a pattern, because reviewers will test whether a worst case has been smoothed away. And state the development type assumed, since the protection required for housing with gardens differs from that for a commercial shed on a slab.

A land contamination assessment that reaches a characteristic situation without showing the screening value calculation is asking the reader to take it on trust, and contaminated land officers do not.

Protection measures and their verification

Protection is a system rather than a product, and the planning condition will almost always require verification before occupation.

The structural barrier, meaning the slab type and its reinforcement and joint detailing, does part of the work. The membrane does part. A ventilated void or a passive subfloor ventilation layer does part. Change one and the others have to compensate.

Verification is where projects lose money late. The condition typically requires an independent verification report covering membrane integrity testing, inspection of every service penetration and joint, a photographic record and sign off before the floor is covered. Penetrations are the recurring failure, because they are formed by trades who were not briefed and inspected after the screed has gone down.

Plan the verification into the construction programme, name the verifier in the remediation strategy submitted for approval, and hold the slab inspection as a formal gate. The geotechnical assessment and the gas work should be coordinated, because the foundation solution and the gas protection solution interact. The wider list of report types shows how the contamination documents sit alongside the rest of a major application pack.

For specialist technical reports, qualified professional input may be required. 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.

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