Environmental Science & Technology Article Figure 5. Temporal variability of water discharge, water isotope signatures (δ2H), and concentrations of N and P species at site 9. (a) and (d), monthly measurements during one year (July 2009−June 2010). (b) and (e), daily measurements during one week (July 20−28, 2009). Parts (d) and (f) are hourly measurements during 24 h (July 23−24, 2009). river system. The major source of both nutrients were wastewater eﬄuents (>90%), with the Tonghui River carrying the eﬄuents from the Gaobeidian WWTP contributing >50%. Raw sewage discharges upstream from the Shahe reservoir and in the Wenyu River were signiﬁcant point sources of the upper subreach, causing high concentrations. However, loads were small and contributed only about 3% to the overall budget. The nutrient ﬂow diagrams presented in Figure 6 consist of independent measurements and are well balanced, which emphasizes our conclusion that sources other than wastewater, such as industrial input, agricultural nonpoint sources, and atmospheric deposition, were negligible. Moreover, δ15N and δ18O isotopic signatures of NO3− (Figure SI-4 of the SI) conﬁrm that sewage was the dominant source of N in the river. Volatilization of NH3 from the rivers was negligible, with an estimated 0.01−0.3 t d−1 between sites 1−12 (river surface area 20 km2, average NH3−N 1 mg L−1, temperature 4−30 °C).31 A previous study32 of 16 large river basins in China, including the Haihe, reported that agricultural fertilizer was the dominant source of DIN, while sewage was the major source for DIP between 1970 and 2000. The study predicted that these sources would still dominate in 2030 and 2050, whatever the chosen scenarios. However, for the ﬁrst time in China, our study highlights the dominant contribution of sewage (treated or not) and the negligible contribution of agricultural activities not only for P but also for N loads in this river system. This important ﬁnding shows that (i) the share of wastewater borne nutrients could be generally underestimated in regions with similar water regimes as the Haihe River catchment, and (ii) there is a need to develop methodologies to reliably and comprehensively assess nutrient pathways and ﬂuxes in other regions of China, preferably complemented by a permanent monitoring network. On the basis of the nutrient loads of the rivers and the estimated 14 million inhabitants of the Beijing watershed, we assessed a production of 1420 gN y−1 inh−1 and 115 gP y−1 inh−1, or 3.9 gN day−1 inh−1 and 0.32 gP day−1 inh−1. This amount of N is close to the values reported for untreated wastewater around the globe (1650−1850 gN y−1 inh−133,34), whereas typical values for treated wastewater are considerably smaller (110−800 gN y−1 inh−133,35). The estimated P discharge rate, however, was within the global range published for treated eﬄuents (91−211 gP y−1 inh−1).35,36 This observation reﬂects the higher treatment eﬃciency for P, which is partly removed by ﬂocculation during treatment and by the settling of particles in the river. In contrast, sluggish nitriﬁcation/denitriﬁcation processes in the WWTPs hinder N removal. The city of Beijing is still rapidly growing, with an increasing number of households being connected to WWTPs. However, the treatment facilities, which were planned in 2002/ 2003 for the estimated number of inhabitants living in Beijing in 2008, are today often operating beyond their projected capacity as a result of the considerably larger population. Wastewater from Tianjin spills into the Yongdingxin River and other channels that discharge directly to the sea. Assuming 5298 dx.doi.org/10.1021/es3004415 | Environ. Sci. Technol. 2012, 46, 5294−5301