Article pubs.acs.org/est Sources and Pathways of Nutrients in the Semi-Arid Region of Beijing−Tianjin, China Benoît Pernet-Coudrier,† Weixiao Qi,‡ Huijuan Liu,‡ Beat Müller,§ and Michael Berg†,* †Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland ‡State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco- Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, People's Republic of China §Eawag, Swiss Federal Institute of Aquatic Science and Technology, 6047 Kastanienbaum, Switzerland *S Supporting Information ABSTRACT: Semiarid regions worldwide are particularly prone to eutrophication, which causes immense ecological and economic problems. One region that is in transition and requires systematic research for eﬀective intervention is the dry landscape of Beijing-Tianjin (P. R. China). We investigated the sources and spatiotemporal loads of nitrogen and phosphorus species over a one-year period in the Haihe catchment that drains the megacity of Beijing. Although wastewater treatment was improved in recent years, the rivers were heavily contaminated by 0.3−5.3 mgP L−1 and 3.0−49 mgN L−1, with toxic levels of nitrite (≥1 mgNO2−N L−1) and ammonia (≥0.6 mgNH3−N L−1). The average NH4+ (16.9 mgN L−1) increased by 160% compared to 1996-levels. Mass ﬂuxes and δ15N- signatures revealed that nutrients originated almost exclusively from sewage. Furthermore, the water balance demonstrated that >90% of the polluted river water was diverted for irrigation, thereby threatening food safety and groundwater quality. Per capita loads of 1.42 kgN/yr and 115 gP/yr were comparable to the peak discharges typical of Europe and the United States in 1970−1990, but concentrations were 2−3 times higher in the Beijing−Tianjin region. Our research identiﬁed sewage as the predominant nutrient source in this semiarid region, which suggests that state-of-the-art wastewater treatment would drastically mitigate eutrophication and even more rapidly than was previously observed in Europe. ■ INTRODUCTION Eutrophication of surface waters has become one of the most severe environmental problems in developing and transitioning countries. This issue is particularly pronounced in arid and semiarid regions of Asia and sub-Saharan Africa,1,2 where populations are constantly increasing. The consequences are oxygen depletion in rivers, ﬁsh kills, algal blooms, siltation of water bodies, and increases in drinking water treatment costs. In the western world, research on the mitigation of eutrophication conducted from the 1970 to the 1990s, and intervention strategies derived thereof, have triggered a marked improvement in the aquatic environment. A similar approach is now needed in the transitioning regions. Here, the situation diﬀers from the former eutrophication problems of the West, as booming cities are situated next to poor rural areas and cause diﬀerent patterns of nutrient contamination than what were experienced in Europe or North America. Northern China, which has several megacities in dry landscapes, is particularly suitable for initiating these types of eﬀorts, because serious eutrophication problems are more than evident, and funds for mitigation are more readily available than in other nations. Within only two decades, China has evolved into one of the largest economies, with a GDP growth rate of nearly 10%.3 However, awareness of environmental degradation and the corresponding legislation and investment lag behind the present speed of increasing productivity, which has con- sequences of rapid urbanization, high demand of resources including energy and water, and the subsequent pollution of air, water, and soil.3−6 This development is primarily felt in the centers along the coast and in the megacities of Northern China. The Beijing−Tianjin region has the highest growth rates in economy and population and therefore faces increasingly severe water scarcity. In 2007, the Beijing water availability dropped to 230 m3 y−1 inh−1, which is only 2.7% of the 8500 m3 y−1 inh−1 world average.7,8 Many rivers ran dry,9,10 partly due to 25 years of drought recorded since the 1970s, but also due to extensive water consumption of the growing population and industry, and to intensiﬁed farming that relies largely on irrigation.7−9,11 In addition, nutrient concentrations (N and P) in the Haihe river system have massively increased in the past 40 years,8,12 mostly because of draining of untreated sewage from the megacities8,13 and possibly the overuse of chemical fertilizers.14 Received: August 25, 2011 Revised: February 19, 2012 Accepted: February 22, 2012 Published: March 8, 2012 © 2012 American Chemical Society 5294 dx.doi.org/10.1021/es3004415 | Environ. Sci. Technol. 2012, 46, 5294−5301