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

The delta(13)C values fi individual trihalomethanes (THM) formed on reactio n of chlorine with dissolved organic carbon (DOC) leached from maize (corn; Zea maize L.) and Scirpus acutus (an aquatic bulrush), and with DOC extrac ted from agricultural drainage waters were determined using purge and trap introduction into a gas chromatograph-combustion-isotope ratio monitoring m ass spectrometer. We observed a 16.8 parts per thousand difference between the delta(13)C 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 parts per thousand lower than C-13 than wh ole plant material. We suggest that the low value of the THM relative to th whole plant material is evidence of distinct pools of THM-forming DOC, rep resenting different biochemical types or chemical structures, and possessin g different environmental reactivity. Humic extracts of waters draining an agricultural field containing Scirpus peat soils and planted with maize amy contribute significantly to the THM-f orming DOC. The difference between the delta(13)C values of the whole isola te and that of the THM it yielded was 3.9 parts per thousand, however, sugg esting diagnesis plays a role in deermining the delta(13)C value of THM-for ming DOC in the drainage waters, and precluding the direct use fo siotopic mixing models to quantitatively attribute sources. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.