The Seasonality of Phosphorus Concentrations in a Suburban River Catchment by the Assessment of the Dissolved and Particulate Phosphorus Species

Phosphorus (P) contamination in surface water is a major global concern affecting most suburban river catchments worldwide due to the discharge of industrial effluents and urban sewage, a consequence of rapid urbanization and industrialization. The objective of this study was to investigate the concentrations and seasonal patterns of various phosphorus (P) species in water from the Yamato River. Total phosphorus (TP), total particulate phosphorus (TPP), and particulate inorganic phosphorus (PIP) were determined using the molybdenum blue method; particulate organic phosphorus (POP) and total dissolved phosphorus (TDP) were calculated by difference; and particulate phosphorus was fractionated using the sedimentary phosphorus sequential extraction (SEDEX) method. Average TP concentration was 0.31 mg/L (0.14 mg/L–0.77 mg/L), TPP was 0.10 mg/L (0.03 mg/L–0.47 mg/L), PIP was 0.07 mg/L (0.02 mg/L–0.46 mg/L), particulate organic phosphorus (POP) was 0.02 mg/L (0.00 mg/L–0.24 mg/L), and total dissolved phosphorus (TDP) was 0.22 mg/L (0.00 mg/L–0.71 mg/L). There was a decrease in TP levels with increased water levels in summer, possibly due to dilution effects, although other factors may be influencing TP levels. TDP consistently dominated over TPP but was more pronounced in low-flow conditions in winter compared to summer. PIP exceeded POP on most dates, although they both increased with the increased water levels witnessed in the summer. Fe-P was the dominant P species, increasing alongside Ca-P in the summer (irrigation period of paddy fields). The study indicated that the phosphorus pollutants in the river water were inorganic in nature and may have originated from the leaching of inorganic fertilizers from nearby paddy fields, urban surface runoff, wastewater discharge, and construction debris carried as erosion products.