Contamination patterns in river water from rural Beijing: A hydrochemical and multiple stable isotope study

Highlights: • Rural Beijing river water is hazardous for human health and the aquatic ecology. • Higher contribution of urban effluents to the downstream rivers in the rural south. • Urban wastewater is the prime anthropogenic pollution source. • High fluoride concentrations have a largely natural source. -- Abstract: The pollution of urban river has reached a critical level. In the present study, the hydrochemical composition of local surface water collected during two seasons from the rural area around urban Beijing, China, was examined. Concentrations of selected cations and anions reveal the temporal and spatial distribution of anthropogenic and natural pollution. Multiple stable isotopes (δ¹⁵N_nitrate, δ¹⁸O_nitrate, δ³⁴S_sulfate, δ¹⁸O_sulfate, Δ³³S_sulfate) analyses were applied for detecting of specific hydrochemical processes and identifying the major sources of contamination. Twenty-eight percent of the river water samples from the wet season and 34% from the dry season exhibit a minor water quality of class IV (poor) and below due to high pH values as well as high nitrate, sulfate and fluoride concentrations. This water is hazardous for human health and the aquatic ecology. The seemingly better river water quality in the wet season is caused by higher precipitation and, hence, dilution of the water constituents. Multiple isotopes identify urban wastewater as the prime pollution source. The higher fraction of deteriorated river water in the southern rural area (42%) compared to the north (24%) points to a higher contribution of urban effluents to the downstream rivers in the rural south. It can, thus, be concluded that less discharge of wastewater to the local surface and subsurface environments by better controls of public and private sewage disposal combined with the renewal of leaking sewer pipes would reduce the risk of anthropogenic contamination of river water.