Catchment topography and land cover modulate DOM photoreactivity in reservoirs

Dissolved organic matter (DOM) in reservoir surface water is an important mediator for the photochemical fate of organic pollutants and disinfection by-products in drinking-water treatment. However, the roles of catchment and reservoir conditions in influencing the photoreactivity of DOM in reservoirs remain poorly understood. This study investigated 50 reservoirs in temperate and cold climates across Japan to identify the key catchment/reservoir factors contributing to DOM concentration, optical properties, and photoreactivity (i.e., quantum yield). The samples showed wide variability in the quantum yield coefficients for photoproduction of excited triplet states of DOM (31.3–168 M−1) and quantum yield for photoproduction of singlet oxygen (1.46–6.21 ×10−2). Steep slopes and dominance of grassland/paddy fields in catchments and high latitudes of the reservoir emerged as important conditions across reservoirs. These conditions correspond to DOM with relatively low dissolved organic carbon concentrations and aromaticity, but enhanced photoreactivity which can accelerate pollutant transformation and have the potential to generate toxic products. In contrast, catchments with gentle slopes and extensive forest coverage were correlated with relatively high concentrations and aromaticity of DOM, resulting in diminished photochemical activity. In summary, DOM photoreactivity is modulated by the dominant land cover, topography, and solar irradiance. Our findings demonstrate that variations in catchment and reservoir conditions modulate the optical and photochemical traits of DOM on reservoir surfaces. These findings guide water management, prioritizing source control in reservoirs draining steep, grassland/paddy catchments, and optimizing treatment in gently sloped, forested catchments.