Article provided by Judith Nathanail (LQM), Geraint Williams (ALS), Mike Smith, Paul Nathanail (GHD)
“It’s not there!”
“You haven’t looked hard enough”.
Once the preliminary risk assessment is done, and volatile organic compounds (VOC) are contaminants of concern the sampling and analytical strategies need to reflect the ease with which VOCs can be lost from a sample resulting in a false negative analytical result.
In case you are wondering, a VOC is “any organic compound having an initial boiling point less than or equal to 250 °C (482 °F) measured at a standard atmospheric pressure of 101.3 kPa.”
Volatiles are lost rapidly if soil samples are left exposed. Losses of 25 – 50% have been recorded within 30 seconds of exposure. Sampling method has an even bigger effect – with losses of up to 99.9% recorded from bulk sampling. Where in situ VOC concentrations exceed an assessment criterion, such losses can result in false negatives, leaving behind unremediated soils or prematurely ending remediation.
BS 10175 recommends that samples intended for the determination of VOCs should be taken in a way that minimizes the loss of volatiles. The primary purpose of BS 10176 is to specify procedures that can be followed in the field to minimize loss of volatile organic compounds (VOCs) during sampling. These procedures need to be strictly adhered to in order to provide reliable and repeatable results.
The procedures described in BS 10176 are similar to those described in long standing guidance and standards across the world. The immersion methods require considerable time, resources, safe work practices, competent oversight and quality control.
The procedures involve taking a small sample of known weight and volume is taken using a coring device followed by either sealing the intact core or immersing the sample in a liquid to prevent losses through volatilisation in a subsequently sealed vial. BS 10176 describes procedures based on immersion in methanol, in sodium hydrogen sulfate (sodium bisulfate) (only for low VOC concentrations) or in de-ionized water. The core is then used directly in the laboratory without sub-sampling.
BS 10176 requires duplicate samples are taken from the same soil stratum and as close as possible to the location of the first sample to provide the laboratory with an additional sample in case re-analysis is required.
Soils up to coarse sand can be sampled. It is unlikely that samples representative of the in-situ VOC concentration can be obtained or tested from coarser soil fractions. For coarse gravel, cobbles, etc. alternative methods such as PID headspace screening or soil vapour sampling need to be adopted.
Standards like BS 10176 are drafted by working groups appointed by BSI’s Soil Quality Committee EH/4. EH/4 is responsible for developing British Standards in the fields of soil quality, soil pollution and contaminated soil. The committee contributes to European (CEN) and International (ISO) Standards. The EH/4 committee comprises representatives of relevant industry and academic bodies, learned and professional organisations and/or individual experts. Committee members volunteer their time and expertise to the development of standards. You can find out more at: https://standardsdevelopment.bsigroup.com/committees/50001294
The authors presented an introduction to BS 10176:2020 in a webinar in April – a link to the recording is at: https://attendee.gotowebinar.com/recording/5875920295717747975