Effect of Heavy Rainfall on Nitrogen and Phosphorus Concentrations in Rivers at River-net Plain

Abstract

To understand the quantitative effect of heavy rain on nitrogen and phosphorus pollution in river-net plain, daily observations of nutrient concentrations in two rivers, flowing into Lake Taihu, were conducted from 1st September, 2017 to 31st August, 2019. The daily rainfall was recorded by auto-recording meteorological stations located on the two rivers and the Taihu Laboratory for Lake Ecosystem Research. Intensive sampling in different sections of the two rivers during Super Typhoon Lekima was also conducted in August 2019. Using these datasets, the influence of heavy rainfall on various forms of nitrogen and phosphorus concentrations in the rivers, and its environmental effects, were analyzed. The results showed that 16 heavy rainfall events (19 d) were observed in two years, 50% of which occurred in the summer season. In addition, heavy rainfall accounted for as much as 41.33% of the total rainfall over the entire year. After the period of heavy rainfall, the concentrations of various forms of nitrogen and phosphorus increased, and the particulate P generally exhibited the fastest response, usually peaking on the day of heavy rainfall. In contrast, the peaks of N were delayed for 2-5 days with the occurrence of heavy rain. In general, the duration of the increase in the concentration of nutrients in the study river caused by heavy rainfall was short (usually 1-2 days), and sometimes was lower than the concentration before the rains. The Dapu River exhibited a slower response to heavy rains than the Yincun River because it has a larger and longer catchment area than the Dapu River. In addition, the effect of heavy rain on N and P concentrations was also strongly influenced by the land-use situation around the river basin. The increase of nitrogen in the reach, affected by agricultural non-point sources, was dominated by granular nitrogen, and the increase of nitrogen in the reach affected by urban non-point sources was dominated by dissolved nitrogen. The increase of phosphorus was dominated by granular phosphorus in the entire process. The conclusions of this study are as follows:In the plain river network area, the fluctuations of nitrogen and phosphorus concentrations in the river water body caused by heavy rainfall are small, and the response of various forms of nitrogen and phosphorus are significantly affected by the local environmental background. Therefore, the water quality generally exhibited limited variation. Due to the large proportion of water entering the lake during heavy rainfall events, a high level of the nutrient loading was also observed, and the occurrence of heavy rainfall was occasional. The short-term effect of heavy rainfall on the nitrogen and phosphorus loading entering the lake in the river channel in the plain river network area is therefore, also significant, and requires further investigation.