Environmental Science & Technology Article Figure 2. Flow diagram of water discharge in the Beijing region (Haihe river system). Encircled numbers and letters refer to the sites labeled in Figure 1. The discharge amounts for the Chaobaixin River and the Qinglongwan River do not account for additional tributaries along their way. mm yr−1,10 which is more than twice the average annual precipitation. Huang et al.24 observed that rain events of up to 80 mm in rural areas (plain of the Haihe basin) did not produce surface runoﬀ to rivers and channels, and events with >80 mm occurred only once in ﬁve years.23 Another study found water runoﬀ during heavy summer precipitation in some areas of Beijing City.25 However, in agreement with our observations on water discharge and nutrient sources (see below), these reports concluded that rainwater runoﬀ from agricultural soils was generally marginal. Of the annual average of 18.5 m3 s−1 of river water discharged below Beijing (site 9, Figure 2), a surprisingly small fraction of <10% (1.6 m3 s−1) actually Tianjin at site 13, and eventually the sea. Our investigation revealed that the missing 16.9 m3 s−1 vanished from the river as a result of water withdrawal for irrigation. In fact, several dams along the river divert water to a network of irrigation channels from where countless farmers pump water to irrigate their crops. Similarly, the large amount of water channeled from downstream Beijing to the Chaobaixin and Qinglongwan rivers (13.4 m3 s−1 on average) is also used to irrigate agricultural land. This raises concerns, since the river’s water pollution status surpasses grade V of the national quality standard, which means that this water should not be used for any purpose.26 In addition, groundwater from a 60−100 m depth is excessively pumped to supplement the demand for irrigation, despite the pronounced decline in groundwater levels (currently some 20 m) observed over the past three decades.8,9,27 Spatial Patterns of Nutrient Concentrations. In contrast to the commonly observed increase in pollutants in rivers passing urban and/or agricultural areas, the concen- trations of TN and TP decreased signiﬁcantly from the Shahe reservoir to Tianjin (sites 1−12, Figures 3). This was due to the very high nutrient concentrations in the upstream Shahe reservoir of up to 3.2 mgP L−1 DIP and 33 mgN L−1 NH4+. These nutrients originate largely from the discharge of untreated domestic sewage and/or sludge from animal husbandry, mainly poultry.24 Downstream from the reservoir, between sites 1 and 4, countless small sewers release untreated sewage to the river, some of which discharge as little as 1 L s−1. Treated wastewater from 14 million inhabitants of Beijing (A−D in Figure 1) enters the river between sites 4 and 7. This water had lower nutrient levels, thereby diluting the river’s N and P concentrations downstream. Only the Gaobeidian WWTP, which is the biggest facility of Beijing to date, with a Figure 3. Spatial variations of N species (a and b) and P species (c) along the investigated 240 km of the Haihe river system in April and July 2009. P species in April 2009 (see Figure SI-2 of the SI) displayed the same trends as in July 2009. Note that the sampling sites are 5−25 km apart from each other. hydraulic load of around 1 million m3 d−1 (11.5 m3 s−1) of sewage discharged to the Tonghui River (downstream site 5), increased the concentration of TP again considerably at site 6 (Figure 3). Below sites 13−16, the TN concentrations increased again to 30−50 mgN L−1 as a result of the draining of Tianjin wastewater into this ﬁnal river section. This pattern was not observed for P, perhaps due to a better elimination of P by the Tianjin WWTPs, but most probably also as a result of adsorption onto suspended particles in this mixing zone of freshwater and seawater, which is reﬂected in PP being the dominant species (Figure 3). Individual N species, plotted in Figures 4 and SI-3 of the SI, illustrate that the Tonghui River drains wastewater with the highest NO3− and NO2− levels (site 6), indicating only partial nitriﬁcation in the upstream WWTPs. Concentrations of almost 4 mgN L−1 NO2−, in combination with about 1 mg L−1 NH3 5296 dx.doi.org/10.1021/es3004415 | Environ. Sci. Technol. 2012, 46, 5294−5301