Export, biodegradation, and disinfection byproduct formation of dissolved and particulate organic carbon in a forested headwater stream during extreme rainfall events

Despite an increasing recognition of the impor- tance of extreme rainfall events for organic carbon export to inland waters, little attention has been paid to the export and reactivity of particulate organic carbon (POC) and dis- solved organic C (DOC) in mountainous headwater water- sheds under monsoon climates. To investigate environmental implications of storm-enhanced export of POC and DOC in mountainous headwater streams, we examined the relation- ships between storm magnitude and C export from a forested watershed in the Haean Basin, South Korea, during 13 storm events over 4 years and compared potentials of DOC and POC for biodegradation and disinfection byproduct (DBP) formation during an extreme rainfall event with a total rain- fall of 209 mm. Event mean concentrations and export of POC increased nonlinearly above thresholds of precipitation and discharge, far exceeding the relatively small increases of DOC. The export of POC during a few storm events with a total rainfall above 200 mm per event exceeded the annual organic C export during dry years. During the 209 mm storm event, concentrations of total trihalomethanes formed by POC-derived dissolved components changed synchronously with POC concentrations, exhibiting lower levels than those formed by DOC. During a 30-day incubation at 25 C, DOC exported during peak flow exhibited rapid biodegradation of labile components within 7 days. In contrast, the concentra- tions of DOC leached from POC gradually increased fol- lowing the initial decline. Gradual transformation of POC- derived dissolved materials resulted in greater increases in the intensity of fulvic- and humic-like fluorescent compo- nents compared to the DOC treatment. The results highlight the significance of extreme rainfall events as "hot moments" for POC export from mountainous watersheds and also sug- gest that storm pulses of POC can provide potential sources of reactive organic components that can rapidly biodegrade and form DBPs after being released into headwater streams.