Isotopes Help Clear Up Concerns About Chloroform In Groundwater

When communities in Denmark pump groundwater for drinking water, officials monitor the levels of chemicals such as chloroform that might signal the presence of industrial contaminants. But not all chloroform travels with human-made pollution. Sometimes it comes from microbes living in the soil. Now researchers have demonstrated a method to distinguish between natural and human-made sources using carbon isotopes. In the new study, published in the journal Environmental Science & Technology, the researchers describe the new technique that has already helped regulators better determine whether groundwater is safe to drink. Chloroform is part of a family of halogenated compounds, including trichloroethylene and perchloroethylene, used as industrial solvents in manufacturing and dry cleaning. At high levels, the compounds can cause cancer in people. Regulators also worry about levels of chloroform in groundwater too low to cause health problems, because other industrial solvents, refrigerants, and additional contaminants may not be far behind. With that in mind, Danish regulators had set the chloroform limit for groundwater used for drinking water at 1 µg/L. But in the past decade, researchers have recognised that natural processes, such as the metabolism of soil-dwelling fungi, also create chloroform. If officials could spot naturally occurring chloroform, communities could continue pumping the groundwater as a drinking water source, even if its chloroform levels exceeded the regulatory limit by a few micrograms per litre, says study author Ole Stig Jacobsen of the Geological Survey of Denmark and Greenland. He and his colleagues had a hunch that soil fungi could be creating hotspots of chloroform in otherwise pristine groundwater in Denmark’s northern forests. They realised that natural and human-made chloroform are slightly different: Microbes use more of the lighter carbon isotope, 12C, to produce the compound whereas industrial processes make chloroform from methane, which contains more of the heavier 13C. To take advantage of this isotopic difference, the researchers partnered with scientists from the University of Neuchâtel, in Switzerland, to use a sensitive method that incorporates gas chromatography and isotope-ratio mass spectrometry to measure the abundance of carbon isotopes in dilute samples. They tracked the carbon isotopes in samples from wells in relatively pristine locations, such as forested sites, as well as in human-influenced areas, such as a city landfill. The team compared the ratios of 13C to 12C in the samples to those that they measured in commercial chloroform and in organic carbon from forest soils. The industrially made chloroform and the chloroform in water from urban sites had similar values, indicating to the team that the groundwater compounds may have come from an industrial source. But the forest groundwater contained chloroform with isotope ratios closer to those of the soil organic carbon, indicating, the team concludes, that the chloroform’s source was primarily the soil’s carbon, not human-made contamination. Urs Von Gunten of the Swiss Federal Institute of Aquatic Science and Technology calls the new method “sophisticated,” adding that carbon isotopes are the only means to distinguish between natural and anthropogenic chloroform. He also points out that the study’s findings have already had regulatory consequences: Upon learning of these results in 2007, before they were published, the Danish government raised the groundwater chloroform limit for drinking water to 10 µg/L in sites that officials determine have only natural sources of chloroform. Jacobsen says that although the method is expensive and time consuming, it could also find use at sites undergoing pollutant cleanup to determine when industrial chloroform contamination gives way to natural sources of the compound.

Chemical & Engineering News, 11 May 2012 ; ;