Out of sight but not out of mind

Are environmental consultants using instrumentation that finds all hidden gases on a contaminated land site? Recent advances in technology have helped demonstrate that once-contaminated land is now safe to build on.

Methane found on a brownfield site is never going to be good news, but the days when it filled developers with horror are rapidly coming to and end, say two pioneering companies in Warwickshire.

Until now, this has largely been due to the lack of suitable gas flow monitoring equipment, which has either been incapable of determining flow with any degree of accuracy, or has proved downright awkward, cumbersome and time-consuming to use.

However, a collaboration between instrument designer and manufacturer, Gas Data, and geoenvironmental consultancy, Integrated Geotechnical and Environmental Services (IGES), has developed a new technique for gas flow measurement and the use of this data has made it possible to achieve great improvements in the extent and source of gas contamination. Together, these two companies have saved developers a significant amount of time and money by demonstrating that land once contaminated with methane, is in fact safe to build on.

Mike Bevan, principal engineer and marketing director at IGES, explains, “If we find any methane at all during a site investigation, we now automatically measure flow. Nowadays, there is less of a panic reaction to methane, but there are still too many developers, structural engineers and architects who initially obtain only basic gas results, which then often results in costly over-design of remediation measures.”

Gas contamination
Bevan added: “Since Gas Data developed its GF60, which is the only instrument on the market of its type, it is now possible to ascertain gas flow at a very early stage. We can now quickly and accurately answer important questions about methane we find such as: Is it safe? How much is there? Is it moving, and if so, where to?”

The joint study between Gas Data and IGES used the GF60 on a site in Dorset, where it immediately proved a very valuable tool to help locate the precise source and degree of gas contamination. In creating the first stand alone monitor for gas flow, Gas Data set out the following objectives to fill the serious gap in the availability of instruments to engineers working with the problems of gas within landfill or contaminated land. The GF60:

  • is at least an order of magnitude more sensitive than hot wire anemometers;
  • can be calibrated for any mix of landfill gas;
  • gives immediate results in the field so no cumbersome plotting of graphs is needed;
  • connects easily to standard borehole gas tap fittings;
  • is suitable for field operation.
Trials covered the use of the instrument on both landfill and contaminated land. It became obvious that flow measurement readings were a practical proposition, with the instruments providing all of the desired benefits. It also had the capability to average the readings in order to overcome the effects of wind motion, and was able to integrate the flow readings to give volume discharge measurements.

Acute sensitivity of the instrument allowed repeatable measurement of flow rates of less than 0.2 l/hour, with data logging capability providing a record of flow against time. Another added bonus was its ability to resolve flow in either direction so that motion of air into the soil could also be detected.

Most recently, IGES has successfully been using Gas Data’s GF 60 on a site much closer to its offices in Warwickshire. At a former scrap yard, also once used as a tip, a developer wanted to build 140 homes. During its very early investigations, in which it found methane, IGES dutifully referred to CIRIA’s (The Construction Industry Research & Information Association) Report 149, which described detailed measures on ‘Protecting Development From Methane’. Then, one of the most influential papers on the subject was published by SA Wilson and GB Card of Card Geotechnics, (entitled ‘Reliability and Risk in Gas Protection Design’). Instead of just recognising the basic facts about methane detection, the report was the first to show the true benefit of detecting and monitoring gas flow on contaminated land.

At the same time, Gas Data was developing the appropriate technology through the success of its LMS landfill gas analyser, which was the first intrinsically safe landfill gas analyser to offer built-in borehole flow and pressure measurement.

Chris Dakin, Gas Data’s founder and managing director says: “The Card report was a major breakthrough. Until then, guidelines were simplistic, yet demanded overly robust measures. Site investigators had to inform developers all too often that they couldn’t proceed with their building plans, (or that there would be a prolonged investigation) because the slightest detection of methane was a problem, causing the affected land to be significantly devalued. Now though, by analysing gas flow, companies such as IGES can prove to a local authority and to their customers that in many cases, the methane is negligible.”

Flow readings
Back at the former scrap yard in Warwickshire, the levels of landfill gas (methane and carbon dioxide) detected were of concern. However, in light of consistent flow readings from Gas Data’s G60, which indicated ‘no-flow’ to ‘very low flow’, together with the information contained in Card Geotechnics’ report, it was possible to demonstrate that the actual flow of landfill gas was not a problem. The local authority, who had initially blocked planning permission for the 140 homes, gave their go-ahead.

Mike Bevan from IGES continues: “Using flow rates of detected landfill gas has helped us attribute a risk factor to the proposed development. For example, methane emanating from a buried topsoil or natural peat has a very low flow rate, and is low risk. High flows of methane from an actively gassing landfill are obviously high risk. Before the advent of accurate and reliable measurement of flow, it was difficult to make this distinction. As these new houses are being built, we are carrying out further monitoring to prove our risk assessment model. We have two monitoring points for gas and gas flow and no methane has been found.

He added: “This is all a far cry from the recent past when as soon as methane was found, it meant trouble.”



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