Dissolved organic matter (DOM) from forested watershed is an important source of precursors for carcinogenic disinfection byproducts (DBPs) in drinking water. Severe wildfire such as the Rim Fire (lasting for 3 months and covering 104131 ha) near Yosemite National Park can cause intensive forest mortality and produce high loads of burned materials, which may alter the source water quality and DBP budget and speciation for millions of customers in San Francisco. Here, water extracts of surface soil cores (0-5 cm) with black ash (SB-moderate temperatures) or white ash (SW-hot temperatures) from five forests in the Tuolumne River watershed were characterized and evaluated for their potential contribution to surface water quality. Compared to the SB extract, the SW extract showed consistently higher pH, specific conductivity, and aromaticity of DOM (all P<0.05), comparable or higher NH4+-N and NOx--N (P<0.05 for most sites), and consistently lower phosphorus (P<0.05). The specific DBP formation potential (DBP-FP) of extracts from soil/ash reached 25.6�7.2 mg/mg-C for trihalomethanes, 2.36�0.79 mg/mg-C for haloacetonitriles, 4.69�0.98 mg/mg-C for chloral hydrate, and 0.613�0.396 mg/mg-C for haloketones. The specific trihalomethane formation potential did not differ between SB and SW extracts, with the exception of the shrub-dominated site (chaparral), which had almost three-fold greater DBP-FP in SB than SW samples. Although SW extracts have comparable or even lower total dissolved nitrogen and higher C/N ratios than SB extracts, its specific haloacetonitrile formation potential was consistently higher than SB samples, probably due to the high NH4+-N in SW extracts. Simulating the effects of multiple rainfall events, successive water extractions on the soil/ash samples showed a continuous decrease of specific conductivity, extractable carbon, nutrient concentrations and optical density, but increasing aromaticity and specific DBP-FP. Our results clearly indicated that the fire severity and forest types affected DBP precursors and the resulting DBP budget and speciation.

Impacts of the rim fire on disinfection byproduct precursors in forested watersheds

248th National Meeting (2014)

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