Throughfall Dissolved Organic Matter as a Terrestrial Disinfection Byproduct Precursor

More than half of the drinking water supply in the United States originates from forest watersheds, where terrestrial dissolved organic matter (DOM) is known as an important disinfection byproduct (DBP) precursor. Throughfall-derived DOM, a significant portion of terrestrial DOM, has seldom been evaluated for its formation potential of DBPs. Here, we collected throughfall and leaf extracts of an evergreen (loblolly pine, Pinus taeda L.) and a deciduous tree species (turkey oak, Quercus cerris L.) in South Carolina to explore their seasonal DOM quantity, optical properties, and DBP formation potential. Elevated dissolved organic carbon (DOC) from rainwater (1.2 ± 0.4 mg/L) to pine (26.0 ± 19.7 mg/L) and oak throughfall (38.8 ± 37.8 mg/L) indicated canopy can be a significant DOM source. DOM aromaticity (indicated by specific ultraviolet absorbance at 254 nm) was higher in oak than pine throughfall and higher in throughfall than leaf extracts. The throughfall DOM characteristics were seasonally more stable for the evergreen pine than for the deciduous oak. The specific DBP formation potential of pine and oak throughfall both varied greatly across seasons, with values of 52.7 ± 17.3 and 58.6 ± 15.1 μg/g-DOC for trihalomethanes, 0.82 ± 0.35 and 0.64 ± 0.11 μg/g-DOC for haloacetonitriles, 0.59 ± 0.60 and 0.22 ± 0.05 μg/g-DOC for haloketones, and 4.51 ± 2.25 and 4.20 ± 2.76 μg/g-DOC for chloral hydrate, respectively. We estimated the contribution of canopies on runoff DOC yield, and results suggested that the highest contribution would occur in the fall season. Results suggest that throughfall DOM is an important and overlooked terrestrial DBP precursor, and its seasonal variation is forest-type-dependent.