Carbon isotopic constraints on the contribution of plant material to the natural precursors of trihalomethanes

The δ13C values of individual trihalomethanes (THM) formed on reaction of chlorine with dissolved organic carbon (DOC) leached from maize (corn; Zea maize L.) and Scirpus acutus (an aquatic bulrush), and with DOC extracted from agricultural drainage waters were determined using purge and trap introduction into a gas chromatograph-combustion-isotope ratio monitoring mass spectrometer. We observed a 16.8‰ difference between the δ13C values of THM produced from the maize and Scirpus leachates, similar to the isotopic difference between the whole plant materials. Both maize and Scirpus formed THM 12‰ lower in 13C than whole plant material. We suggest that the low value of the THM relative to the whole plant material is evidence of distinct pools of THM-forming DOC, representing different biochemical types or chemical structures, and possessing different environmental reactivity. Humic extracts of waters draining an agricultural field containing Scirpus peat soils and planted with maize formed THM with isotopic values intermediate between those of maize and Scirpus leachates, indicating maize may contribute significantly to the THM-forming DOC. The difference between the δ13C values of the whole isolate and that of the THM it yielded was 3.9‰, however, suggesting diagenesis plays a role in determining the δ13C value of THM-forming DOC in the drainage waters, and precluding the direct use of isotopic mixing models to quantitatively attribute sources.