Long-term watershed management is an effective strategy to reduce organic matter export and disinfection by-product precursors in source water

Abstract

Watershed management practices such as prescribed fire, harvesting and understory mastication can alter the chemical composition and thickness of forest detritus, thus affecting the quantity and quality of riverine dissolved organic matter (DOM). Long-term effects of watershed management on DOM composition were examined through parallel field and extraction-based laboratory studies. The laboratory study was conducted using detritus samples collected from a pair of managed and unmanaged watersheds in South Carolina, USA. Results showed that dissolved organic carbon (DOC), total dissolved nitrogen (TDN) and ammonium (NH4+-N) concentrations were higher in water extracts from the unmanaged watershed than from the managed watershed (P<0.01). Pyrolysis gas chromatography–mass spectrometry analysis showed that water extracts from the unmanaged watershed contained more aromatic compounds than extracts from the managed watershed. For the field study, monthly water samples were collected for 1 year (2015) from the paired watersheds. DOC and TDN concentrations, as well as DOM aromaticity, were significantly higher in the unmanaged watershed than in the managed watershed for most of the year (P<0.05) and were linked to detrital thickness, precipitation and flow patterns. The formation potential of two regulated disinfection by-products was lower in the unmanaged watershed for most of 2015 (P<0.05). From this study, it appears that long-term watershed management practices may alter detrital mass and chemistry in ways that improve water quality.

The influence of wildfire on water quality and watershed processes: new insights and remaining challenges

“…Better understanding and tools are needed to help land managers compare the small, but potentially chronic, water quality changes from repeated prescribed burning and fuel reduction activities with the dramatic effects of severe wildfire. Here, Majidzadeh et al (2019) report that repeated prescribed burning contributed to reduced forest floor mass and decreased DOC concentration relative to an adjacent unmanaged catchment. It also influenced the abundance of aromatic C compounds in forest floor leachate and stream water and reduced the potential to form DBPs during drinking water treatment.…”

“…Additional information to help monetise potential benefits of mechanical and prescribed fire treatments aimed at reducing risks to aquatic ecosystems, other water quality attributes, watershed conditions and water treatment operations might alter the economic balance of pre-fire management. As one example presented here, Majidzadeh et al (2019) report that periodic prescribed burning in coastal plain forests reduced DOC leaching and potentially hazardous disinfection by-products (DPBs) precursor formation, a previously unaccounted benefit of management. Prescribed fire is commonly promoted as a strategy for avoiding the negative watershed and water quality consequences of severe wildfire (Gannon et al 2019).…”

“…Analytical advances continually uncover new contaminants of concern and help increase understanding of their post-fire responses and downstream consequences. Two papers from forests of the south-eastern USA apply novel laboratory (Majidzadeh et al 2019) and field approaches (Olivares et al 2019) to study chemical attributes of DOC and their post-fire responses that have implications for drinking water treatment. Similar to wildfires, the release of pyrogenic C and DOC following prescribed fires are synchronised with the initial post-fire rainstorms.…”