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

Abstract

Abstract.Despite an increasing recognition of the importance 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 dissolved organic C (DOC) in mountainous headwater watersheds under monsoon climates. To investigate environmental implications of storm-enhanced export of POC and DOC in mountainous headwater streams, we examined the relationships 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 rainfall 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 concentrations of DOC leached from POC gradually increased following the initial decline. Gradual transformation of POCderived dissolved materials resulted in greater increases in the intensity of fulvic-and humic-like fluorescent components 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 suggest 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.

Riverine carbon export in the arid-semiarid Wuding River catchment on the Chinese Loess Plateau

“…A conservative calculation using 400 the sampling results at Baijiachuan gauge indicates that 85% of the annual downstream carbon export occurred during the three extreme floods ( Figure S1 in Supplementary). Therefore, any sampling strategies missing episodic high-discharge events would create great uncertainties for annual-scale carbon export estimates (Lee et al, 2017;Jung et al, 2014). This is particularly true for arid-semiarid catchments, such as the Wuding River studied here, where episodic rainfall 405 events make an exceptionally large share of annual water and sediment export.…”

Multiple Time-Scale Monitoring to Address Dynamic Seasonality and Storm Pulses of Stream Water Quality in Mountainous Watersheds

“…For example, early warning signals can allow water treatment operators to add treatment modules or shut down the intake valves until "slugs" of suspended sediment pass by. These signals can also help predict potential increases in disinfection byproducts during rainfall events based on the well-established relationship between the amount of organic substances (particularly humic substances) and disinfection byproduct formation potentials [38,39,52].…”

“…In particular, high-frequency, in situ measurements of turbidity can provide high-resolution time series data that can be used to estimate the transport of suspended sediment and POC based on the empirical relationships established for several rainfall events of various duration and intensity. These data can also be used as early warning signals for suspended sediment and POC surges during intense rainfall events and to identify potential increases in disinfection byproduct formation by natural organic matter in drinking water source areas based on some empirical relationships between suspended sediment (or POC) and disinfection byproduct formation potentials that were established in a previous study conducted at the same watershed [39]. The positive relationships between the concentrations of suspended sediment and metals (Pb) reported from the same study site [20] might be used to predict the potential range of metal fluxes carried by suspended sediment.…”

“…There have been many reports on storm-induced water quality deterioration in large watersheds where storm responses might be more moderate compared to more rapid responses at smaller watersheds (e.g., [10,36,37]). Humic-like substances associated with aromatic and condensed structures have been shown to increase in forested watersheds during intense rainfall events, elevating the potential of disinfection byproduct formation upon chlorination in drinking water facilities [38,39]. These studies provide rare examples of climate-induced risks to water quality in headwater streams draining forested watersheds, yet more high-resolution monitoring data should be collected across a range of climatic conditions and watershed types to enhance our capacity to predict stream water quality changes during extreme rainfall events of varying duration and intensity.…”

Preface: Climate extremes and biogeochemical cycles in the terrestrial biosphere: impacts and feedbacks across scales

“…A modelling study disentangles the role of physiological responses versus ecosystem properties and boundary layer feedbacks for explaining unexpectedly higher sensible heat fluxes over forest compared to grassland during heatwaves (van Heerwaarden and Teuling, 2014). Focussing on the effects of extreme rainfall events, Jung et al (2014) analyse the export and reactivity of particulate and dissolved organic carbon in a headwater watershed under monsoon climate. While all these above papers report on direct concurrent effects of extreme events, some also point towards potential lagged effects (see Frank et al, 2015) of climate extremes on biogeochemical cycles, e.g.…”