Myth: Gas monitoring wells do not need designing

Gas monitoring wells should be designed on completion of the hole, based on the strata and groundwater encountered. Response zones should be designed so that they are above the groundwater table and isolated into a single stratum that is either a pathway or source of gas that could pose a hazard.

Designing a site investigation – ensure that there is supervision and flexibility to allow design of individual response zones on completion of each borehole. Make sure that the response zones are suitable for the risk assessment method you are adopting.

Not all gas monitoring data can be used to determine the worst case gas screening values (GSVs) – flow rate and gas concentration has to be from the same stratum as defined in BS8485.

Myth: Every ground investigation on brownfield land has to include gas monitoring

Not all brownfield sites require gas monitoring. The decision should be based on a robust preliminary risk assessment and Conceptual Site Model. Whilst on some sites it’s absolutely required – unnecessary monitoring can waste time, resources, and carbon.

Gas monitoring in natural superficial deposits often leads to elevated CO2, which doesn’t pose a risk of hazardous emissions but is often misinterpreted.  If wells are installed in Alluvium, the likelihood is that they become flooded and any data is not representative – i.e. useless!

If the only viable source is urban Made Ground, are you better off doing organic carbon testing on the soil? (see CL:AIRE RB17)

Or installing a groundwater monitoring well and collecting gas monitoring data from it, just because it’s present? Perhaps gas monitoring for a road or rail schemes, where there is no receptor? We see this kind of thing all the time, and it’s frustrating.

Follow this golden rule: Gas monitoring should only ever be in the gas source (in the unsaturated zone) or in a permeable (unsaturated) pathway between the source and the receptor. If you are doing anything else, STOP. (More on monitoring well response zones coming up on the MythBusters!)

What do you think? How often do you design out gas monitoring from your site investigation?