Trihalomethanes formed from natural organic matter isolates: Using isotopic and compositional data to help understand sources
Over 20 million people drink water from the Sacramento-San Joaquin Delta despite problematic levels of natural organic matter (NOM) and bromide in Delta water, which can form trihalomethanes (THMs) during the treatment process. It is widely believed that NOM released from Delta peat islands is a substantial contributor to the pool of THM precursors present in Delta waters. Dissolved NOM was isolated from samples collected at five channel sites within the Sacramento-San Joaquin Rivers and Delta, California, USA, and from a peat island agricultural drain. To help understand the sources of THM precursors, samples were analyzed to determine their chemical and isotopic composition, their propensity to form THMs, and the isotopic composition of the THMs. The chemical composition of the isolates was quite variable, as indicated by significant differences in carbon-13 nuclear magnetic resonance spectra and carbon-to-nitrogen concentration ratios. The lowest propensity to form THMs per unit of dissolved organic carbon was observed in the peat island agricultural drain isolate, even though it possessed the highest fraction of aromatic material and the highest specific ultraviolet absorbance. Changes in the chemical and isotopic composition of the isolates and the isotopic composition of the THMs suggest that the source of the THMs precursors was different between samples and between isolates. The pattern of variability in compositional and isotopic data for these samples was not consistent with simple mixing of river- and peat-derived organic material.