Impacts of storm events on chlorophyll-a variations and controlling factors for algal bloom in a river receiving reclaimed water.

Harmful algal bloom is prevalent in the reclaimed-water-source (RWS) river caused by the excessive nutrient's inputs. Rainfall water may be the sole nutrient-diluted water source for the RWS river. However, the effects of storm events on the algal bloom in the RWS river are poorly understood. This study presents chlorophyll-a (Chl-a) variations before, during, and after the initial storm events (Pre-storm, In-storm, and Post-storm) at four representative sites with distinct hydraulic conditions in a dam-regulated RWS river system, Beijing. The response of Chl-a to the initial storm events mostly depends on the ecosystem status that caused by the river hydraulic properties. The upstream is more river-like and downstream is more lake-like. In the river-like system, elevated water temperature (WT, increased by 2 %) could support the dominating algae (diatom) growth (Chl-a increased by 130 %) from Pre-storm to In-storm period. In the lake-like system, the dominant algae (blue algae) declined (Chl-a sharply decreased by 96%-99 %) due to the lower WT (decreased by 3%-7%) and increased flow velocities from Pre-storm to In-storm period. During the Post-storm period, the dominant algae break out (Chl-a surged by 20%-319 %) in the lake-like system caused by the recovered WT (increased by 3%-6%) and flow velocity. The occurrence of algal bloom can be predicted by the Random Forest (RF) model based on water quality parameters such as total nitrogen (TN). The thresholds of algal bloom for the Pre-storm, In-storm, and Post-storm periods were identified as 30 µg/L, 10 µg/L, and 10 µg/L, respectively. The two driven factors were WT and nitrate (NO3-N) for the Pre-storm period and were WT and TN for the In- & Post-storm periods. A higher risk of algal bloom is highlighted during the initial storm events in the RWS river. We propose recommendations for improving water quality in the RWS river systems under the climatic change.

[Impact of Maximum Precipitation in 2017 on the Runoff Component of Reclaimed Water-Intaking River].

The hydrology of rivers recharged with reclaimed water is an important factor controlling its aquatic environment and biochemical processes, which change during the wet season. To understand the impacts of precipitation on hydrological conditions, water samples were collected from seven sites in three periods (before the wet season and during and after the maximum precipitation in July 2017, with 3.3 return periods) throughout a reclaimed water intake area of the Chaobai River in the Shunyi District, Beijing. The hydrogen-oxygen isotope characteristics and chloride content were measured. The results show that the hydrogen and oxygen isotopes of precipitation are mainly affected by the amount of the effect. The minor variation in the later period is due to changes in the sources of moisture. Within three days after precipitation, the slope runoff continues and the fraction of each section varies greatly. The reclaimed water reaches the downstream section through the preferred pathway. The water component ratio of the slope runoff increases from 2% to 85.6% in the direction of the flow, while the reclaimed water ratio decreases from 90% to 67%. The stream remains effluent from sections SY01 to SY05 that are recharged by the slope runoff, reclaimed water, and in-site river water, while the sections SY06 to SY07 are mainly recharged by the slope runoff and in-site river water within three days after the precipitation (the stream effluent is unremarkable).