Monitoring & metering


common practice to deploy field-use screening devices to speed up the process of contamination assessment, and to reduce the number and cost of samples requiring laboratory analysis. It is usually necessary for field testing to be supported by confirmatory sampling and laboratory analysis. Field-use screening tools help to identify contaminated areas quickly and cost- effectively. For example, portable XRF analysers enable rapid measurement of several metals, and PID instruments can detect hundreds of volatile organic compounds (VOCs). The ION Science Tiger XT, for example, has an internal gas table for over 700 gases, with response factors for each – for further information, download the ION Science Response Factor datasheets from IONScience.com.


HYDROCARBON CONTAMINATION Hydrocarbon contamination is frequently encountered in site investigations at brownfield sites, where elevated concentrations of oil and petroleum hydrocarbons in soil and groundwater systems present a risk to human health and the environment. Hydrocarbon contamination is therefore one of the most common forms of pollution, which explains why portable photoionization detectors (PIDs) are the most commonly employed tool in the screening and characterisation of potentially contaminated sites.


The more significant groups of hydrocarbons that are either indicative of oil-based pollution or represent a significant human health or environmental hazard are BTEX (benzene, toluene, ethylbenzene and xylenes) and PAHs (polycyclic aromatic hydrocarbons). Portable PIDs can be used to test the air surrounding a potentially contaminated sample, or the sample of water or soil may be contained within a tube, jar or sample bag so that the VOCs are allowed to volatilise and accumulate. The PID can then be used to test the headspace inside the sample container. This is a useful test for water samples that may have been contaminated by leaking underground storage tanks for example. VOCs such as methyl tertiary-butyl ether (MTBE) are highly soluble in water, and are therefore extremely mobile and capable of contaminating large volumes of water such as aquifers. Odour may be detected from water contaminated by hydrocarbons, but subjective assessments such as these should be verified by PID measurements. Early identification is therefore vital because remediation of underground water resources can be problematic and costly.


WHY USE PORTABLE PIDS? Portable PIDs are commonly deployed at every stage of a contaminated land project. This includes preliminary risk assessments involving site visits to define project objectives, and to identify sources, pathways, receptors and pollutant linkages. Portable PIDs are also used


Instrumentation Monthly April 2023


during subsequent site investigations to undertake risk assessments using site specific data, and to refine the conceptual model. Once onsite work is underway, the operators of machinery, such as diggers and remediation equipment, need to know whether a sample is contaminated or not. A precise analytical measurement is frequently not required for ongoing operational purposes. All that is necessary is a fast indication of whether significant levels of VOCs are present, so that the operator can make an informed decision, such as where to excavate. Similarly, a fast indication of contamination is necessary when onsite remediation is undertaken – for samples both before and after treatment.


It is important to emphasise that portable PIDs do not replace laboratory analysis; they complement such work and help to mitigate the cost, delays and labour involved with the analysis of collected samples. It is also important to note that the collection and transport of samples for VOC analysis in a laboratory involves stringent procedures to ensure that VOC levels do not dissipate before analysis. Nevertheless, analysis in a suitably accredited laboratory is necessary for a prescribed number of samples to demonstrate that a site is complying with its licence. Portable PIDs are also employed to test the headspace of soil probes that are sometimes deployed for site characterisation and to check the ongoing effectiveness of remediation. A variety of alternative methods, test kits and instruments exist for the onsite measurement of VOCs, but most of these are either costly, cumbersome, not portable, time- consuming, or too specific in their target species. Flame ionization detectors (FIDs) are also responsive to a wide variety of VOCs, including alkanes, but this includes methane (PIDs do not respond to methane) which is often present in soil for reasons unrelated to contamination. Also, FIDs require an ignition source such as hydrogen, which makes them less well suited to field work.


THE MAIN ADVANTAGES OF PORTABLE PIDS ARE:


Fast response


Sensitive to hundreds of VOCs including BTEX and PAHs


Highly sensitive. With a broad range Relatively low cost


Simple to use by untrained staff Battery powered and portable


Different lamps available to target specific groups of compounds


Underlining the importance of PIDs in the measurement of hydrocarbons, CONCAWE, an environmental research group comprising many of the world’s leading oil companies, published a report in 2021 entitled ‘Overview of Field-Based Analytical Techniques, Devices and Kits to Determine Petroleum Hydrocarbons in Soil’. The report concluded: “It is important to acknowledge that there is currently not a single field analytical technology that allows to determine and quantify the entire range of petroleum hydrocarbons in soil and therefore a combination of analytical technologies (i.e., combining PID with vis-NIR or GC-MS with vis-NIR) has the potential to offer a more robust approach in quantifying petroleum hydrocarbons in soil by providing greater prediction accuracy.”


PIDs are also employed during emergency response situations that arise when land contamination occurs from a leak or spill of hydrocarbons. For example, the USEPA has an established set of emergency response procedures to investigate a site, evaluate the threat, and determine the best course of action. For example, the choice of PPE is


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