[Spatial changes and sources of nitrate in Beijing urban ecosystem surface water]

The spatial variation in nitrate-nitrogen (NO3- -N) concentrations in surface water of ten sampling sites in the Beijing urban ecosystem from Kunminghu Lake to Tonghui River were assessed using monitoring data from 2009 to 2010. Nitrogen sources were examined using a hydro-chemical method. The results showed that the average nitrate-N concentrations of surface water in the Beijing urban ecosystem ranged from 0.7-7.6 mg x L(-1), with concentrations at all sites affected by human activities to a varying degree. The nitrate-N concentrations in the Dongbianmen and Tonghui River located in the southeastern of Beijing ranged from 7.0-7.6 mg x L(-1) and were significantly higher than those in the upper reaches (P < 0.01). For all sampling sites, the chloride concentrations fell between 14.8-86.0 mg x L(-1). The chloride concentrations at the furthest downstream sites, in the Dongbianmen and Tonghui River, ranged from 81.5-85.0 mg x L(-1) and were 2.3-5.8 times higher than those in the upper reaches. This indicates that the surface water in Dongbianmen and Tonghui River is clearly affected by human activities and that there are outfalls or pollutant sources near these two sampling sites. Further, the Cl-/Na+, SO4(2-)/Ca2+ ratios showed that the majority of nitrate-N came from a single source. The information given by the nitrate-N concentrations combined with the NO3- -N/Cl- ratio implied that sewage effluent, including industrial waste water, leakage from solid waste disposal and domestic wastewater mainly controlled nitrate distribution in the Beijing urban surface water. The results from this study suggest that surface water management should focus on downstream sites located in the southeastern region of Beijing such as the Dongbianmen and Tonghui River in the future.