Organic contaminants in groundwater

A guest post from Stephen Buss, Assistant Scientific Editor (hydrogeology) of the Quarterly Journal of Engineering Geology and Hydrogeology, and Jonathan Smith, QJEGH Associate board member

This month, the Quarterly Journal of Engineering Geology and Hydrogeology (QJEGH) publishes a set of papers on ‘organic contaminants in groundwater’. There are five papers in the set, covering topics that span the range of interest in organic contaminants today.

While organic compounds are not the most common groundwater contaminant – agricultural nutrients, such as nitrate have that dubious ‘honour’ – the presence of organic compounds in groundwater is of interest because they can be particularly difficult to remediate and can lead to significant legal and commercial liabilities.

Some organic compounds are sparsely soluble and can travel significant distances underground in their original form (e.g. as fuel or organic solvents). Some organic liquids are denser than water and tend to sink to the base of the aquifer, where they can move under gravity down the slope on the top of an impermeable barrier. Kevin Leahy and colleagues discuss these flow processes in fractured rocks. They describe how, at two superficially similar sites, the movement of contamination can be significantly affected by quite subtle differences in geological structure and weathering.

Fate and transport of dissolved organic compounds is of enduring interest because there are so many contaminants, each with a unique set of characteristics. We publish two papers on the attenuation in groundwater of organic compounds that have very different potential for attenuation. FAME (fatty acid methyl esters) compounds are biofuel additives to diesel, and Alan Thomas and colleagues provide a review of literature that explores the fate and transport: while FAME compounds are quite mobile in groundwater they also appear to be quite degradable. On the other hand, compounds described as ‘heavy hydrocarbons’ (heaviness referring to the many C atoms in the molecule) are barely mobile in groundwater but generally persistent. David Brown and colleagues review the relevant literature on heavy hydrocarbons.

A conceptual model of heavy hydrocarbon fate and transport in the environment. Heavy hydrocarbons are typically released together with lighter hydrocarbon fractions that are found in most crude oils. This modifies the physical and chemical properties that significantly affect the transport on soils and in the subsurface. Releases of viscous refined bitumen or extremely heavy crude oils (APIo < 12) are unlikely to travel any significant distance in the soil or subsurface. From Brown et al., ‘Heavy hydrocarbon fate and transport in the environment’, QJEGH Vol 50, issue 3

Often organic contaminants contain volatile fractions which diffuse out of groundwater, into the air of the unsaturated zone and move upwards towards the atmosphere. When homes have been built above the contamination, or when the contamination has migrated under homes, there is a clear risk to the health of residents. Hannah White and colleagues describe how they approached the problem of characterising such a site, and develop a conceptual model of the processes that affect the rate of vapour ingress to the building. Matthew Lahvis reviews field evidence to develop a threshold vertical distance for total petroleum hydrocarbons (TPH) above a hydrocarbon spill, above which no petroleum vapour intrusion risk is indicated (his findings are also presented in a webinar on the CL:AIRE website ).

The editors and publishers are particularly keen to thank the site owners and operators, from which the datasets were obtained. It is difficult to release data pertaining to a contaminated site, so we applaud those who do, for the sake of improving the science and evidence base that allows environmental improvement across our industries.

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