Effects of daily precipitation on spatiotemporal variations in river water quality and trace metal bioavailability

Trace metal contamination from abandoned mines poses major risks to river ecosystems. However, their concentration and speciation in river environments exhibit complex, high-frequency spatiotemporal dynamics due to changes in surrounding physicochemical conditions, including water quality parameters and rainfall events. Here, we conducted a 27-day consecutive field survey of four rivers surrounding the Ashio Copper Mine, Japan, to evaluate daily variations in trace metal concentrations and their speciation in response to precipitation. A Bayesian state-space modeling approach was employed to statistically decompose time-series water quality data into trends, daily precipitation-driven fluctuations, and random variations from unmodeled factors. Dissolved metal concentrations exhibited significant day-to-day variations even within the limited spatial extent of the study area (approximately 15 km); for example, copper, zinc, and cadmium concentrations ranged from 0.305 to 210 µg/L, 0.253-155 µg/L, and 0.0027-0.925 µg/L, respectively. The trend patterns-decreasing, increasing, or no distinct trend-differed among sites and parameters. Additionally, dissolved trace metal concentrations, particularly those of copper and zinc, exhibited site-specific responses to daily precipitation. Concentrations of other elements (e.g., calcium) and pH generally decreased or became more acidic in response to rainfall across most sites, while dissolved organic carbon concentrations generally increased. Due to these site- and parameter-specific differences in trends and responses to precipitation, ratios of ionic to dissolved concentrations of trace metals, particularly cadmium, exhibited substantial spatiotemporal variations. This study concludes empirically that trace metal bioavailability varies markedly across time and space, driven by prevailing daily hydrological conditions.