Environmental Science & Technology Article Figure 6. Flow diagrams of (a) daily loads of total nitrogen and (b) daily loads of total phosphorus in the Beijing region. All values are based on independent measurements. Loads of the Chaobaixin river and the Qinglongwan river do not account for additional tributaries and discharges along their way. Wastewater of Tianjin enters the river downstream of site 13, where tidal conditions prevent the measure- ment of riverine water discharge and thus nutrient ﬂuxes to Bohai Bay. However, a rough estimation of nutrient loads discharged at Tianjin into the sea is provided in the text. the same per capita production of N and P as determined for Beijing, the 5 million inhabitants of Tianjin release 19.2 tN day−1 and 1.6 tP day−1 into the Bohai Bay, thereby contributing signiﬁcantly to the eutrophication of this part of the East China Sea. Future Trends. Over the past few decades, concentrations of N and P species have increased sharply in the Haihe watershed (Figure 7). Despite the huge eﬀorts spent, such as the construction of more than 4000 km of sewers and 9 new WWTPs to treat about 90% of the wastewaters of Beijing, these measures have had no apparent eﬀect, as concentrations continue to increase exponentially. When compared to N and P loads measured in other river systems of the world during their nutrient peak years in 1965− 1970, such as in the Mississippi,40 Seine and Scheldt,41 and Rhine rivers,42 the regional loads in the Haihe River system of 1330 kgN km−2 y−1 and 110 kgP km−2 y−1 today are quite similar. However, due to the semiarid climate and the much smaller water discharge, as well as the continuing drought, the concentrations of N and P species in the Haihe river system are 2−3 times higher than the maximum concentrations observed in the 1970s in Europe and the U.S.43−45 From the 1950s to the 1990s, N and P ﬂuxes in European rivers increased due to the combined eﬀect of three causes: (i) intensiﬁcation of agriculture involving the massive use of synthetic N and P fertilizers; (ii) introduction of polyphosphate-containing detergents that resulted in a three-to-4-fold increase in domestic P-release, and (iii) treatment of urban wastewater primarily aimed at the mineralization of organic matter, Figure 7. Temporal trends of average NH4+ concentrations in the Haihe watershed. The concentration range determined in our 12- month study period (2009−2010, box plot) demonstrate that nitrogen levels are still steeply increasing despite the new Beijing WWTPs that have been in operation since 2008. The green shading represents overall uncertainties involved in the past measurements (data from refs 10 and 37−39). neglecting the elimination of N and P. The Beijing−Tianjin region currently displays all of these same features, and thus one could expect that peak levels of N and P may now be reached. If eﬃcient mitigation measures were to be introduced, such as banning of P-detergents and inclusion of state-of-the-art wastewater treatment, then signiﬁcant improvements in P to near-natural levels can be expected to be established over the following 30 years, according to the historical study of Billen and Garnier.41 Regarding N, the drastic increase was stopped in Europe and the U.S. in the 1980s, but 20−30 years later N ﬂuxes are still far above pristine levels due to countless nonpoint sources that are diﬃcult to control.40,41 However, in the Beijing−Tianjin region, where the climate is dry and the N-sources are largely restricted to wastewater, installation of suﬃcient wastewater treatment facilities with state-of-the-art denitriﬁcation eﬃciency has a high potential to drastically cut down nutrient concentrations and mitigate eutrophication. Banning the use of P-detergents in China would certainly be an additional prerequisite to achieving this goal. In fact, with little nutrient input derived from agriculture and with wastewater eﬄuents that are low in nutrient concentrations, the Haihe river system could recover from eutrophication even faster than has been observed in many regions in Europe and the U.S., where nutrient runoﬀ from agriculture remains substantial. Similar situations to that seen in China are likely present in sub-Saharan Africa and in further semiarid regions of the world. A key to success would be to build a well-designed monitoring network along the sewers and rivers that would enable the long-term documentation of changes in crucial water quality parameters and water quantity. The present study provides valuable grounding for the placement of monitoring sites. Besides adequate installations to collect water discharge- proportional samples and conserve them on-site, a dedicated analytical laboratory with well-trained personnel needs to be established. Finally, it has to be kept in mind that high N and P pollution from sewage is often accompanied by other harmful contaminants such as heavy metals, biocides, pharmaceuticals, and persistent organic pollutants.46−49 As a consequence of 5299 dx.doi.org/10.1021/es3004415 | Environ. Sci. Technol. 2012, 46, 5294−5301