Environmental Science & Technology Figure 4. Spatial variations of NH3, NO2−, and PN along the investigated 240 km of the Haihe river system in July 2009. The decrease in NH3 and NH4+ (shown in Figure SI-3 of the SI), together with the increase of NO2− and NO3− (Figure SI-3 of the SI), point to partial nitriﬁcation. PN also increases due to assimilation by algae. Note that the sampling sites are 5−25 km apart from each other. measured in July 2009 at site 6, are highly toxic28 and may trigger the frequently observed ﬁsh kills. NH3 concentrations between 0.1 and 10 mgN L−1 (87% ≥0.6 mgN L−1) persisted throughout the year at all sites, with levels generally peaking at sites 1, 8, and 13, where pH values >9 were common (Figure 4). NO2− levels surpassed the acute toxicity level of 1 mgN L−1 in 46% of the samples. At an average oxygen saturation of only 30% (0−75%) between sites 1 and 9, nitriﬁcation in the rivers is sluggish. Nevertheless, algal growth incorporates a substantial amount of dissolved nitrogen, particularly during the summer months. This process was reﬂected in increasing PN concentrations along the course of the river in July 2010 (Figures 4 and SI-3 of the SI), when abundant algae were visually more than evident at the sampling sites. Sparse Temporal Variability of N and P. Whatever the site or the season, the quality of the river water was always worse than grade V (the lowest) of the Chinese national standard due to low oxygen (<2 mgO2 L−1), high total N (>2 cmognNcenLt−ra1)ti,oann(d>e0s.4pemcigaPllyLN−1H).4+ (>2 mgN L−1), and high P Diurnal Variation. The nutrient concentrations showed no diurnal pattern during the 24 h of observation (Figure 5f), although a heavy rain event occurred on the day of the investigation, with a magnitude of 90 mm in less than 2 h, which represents at least a decennial rain (Figure 5c). Neither the composition of major ions (see Table SI-2 of the SI) nor the water isotope signatures (Figure 5c) showed any evident inﬂuence of rain or runoﬀ input. Hence, our data suggest that dilution of river water by rain was negligible even for the exceptionally intense rain event experienced here, an Article observation also reported by Huang et al.24 The steep increase in water discharge (Figure 5c) is therefore more likely a result of upstream river-gate opening, performed as a precautionary measure to reduce the risk of ﬂooding. Weekly Variation. In analogy to the stagnant diurnal situation, variations in N and P concentrations were likewise marginal over the course of one week in July 2009 (Figure 5e), despite pronounced discharge ﬂuctuations ranging from 6 to 33 m3 s−1 (Figure 5b). However, the isotopic signature of water increased in the aftermath of the heavy rain event, reﬂecting a changing ratio of water sources in the catchment. Annual Variation. A seasonality of TN, NO3−, and NO2− concentrations was evident only over the course of one year, with higher levels in winter and lower levels in spring and summer (Figure 5d). However, DIP and TP remained fairly constant over the seasons. Figure 5 shows temporal variations from site 9 only, but the same annual trends were observed at the other investigated sites (1, 5, 12, and 13; see Table SI-2 of the SI). The seasonal pattern of NO3− and NO2− likely reﬂected a varying eﬃciency of biological N-removal in the WWTPs (and in the river), with better performance during the warm seasons (i.e., March-September). This conclusion was corroborated by our three sampling campaigns conducted in May 2009, December 2010, and March 2011 at the ﬁve biggest Beijing WWTPs (see Materials and Methods section). These results displayed a large variability (over time and among the WWTPs) of the N-removal eﬃciency, at 16 to 93% (average: 66 ± 28%). Variable performance in seasonal nutrient removal was also reported in a recent study that investigated nutrient concentrations in Beijing’s WWTPs over a 2 year span.29 The winter situation is characterized by temperatures between +10 and −10 °C, which lowers the biological activity of both the treatment plants and the biota in the river. Summers are very hot with temperatures up to 32 °C, which increases: (i) the eﬃciency of WWTP processes like nitriﬁcation/denitriﬁcation, (ii) volatilization of NH3 due to alkaline conditions (pH 8−9) caused by photosynthesis and lower Henry coeﬃcient, and (iii) biological uptake of N in the river. The phosphorus dynamics showed a slightly diﬀerent picture, with less variation and smaller amplitude over the year, most probably due to P exchange between water, particles, and sediment; i.e., settling of PP and partial release of DIP in these anoxic river waters. In summary, N and P concentrations did not correlate with water discharge, nor were seasonal patterns evident in any of the studied river sections (apart from N loads that were aﬀected by the seasonally varying eﬃciency of WWTPs and the natural processes in the river such as NO3− uptake in the summer months, Figure 5d). Thus, these observations provide further evidence that wastewater is the major source of nutrients in the river. The main constituents of the TN and TP loads were NH4+ and DIP, representing 70% and 67% of the total loads (average on all sites over the 12 months of sampling). Ammonium is known to originate mainly from urine and feces of domestic and animal husbandry wastewater, whereas a signiﬁcant fraction of P also stems from P-containing detergents, which are not yet banned in China and usually contain 1% P by weight.30 Loads of Nitrogen and Phosphorus. The nutrient budgets presented in Figure 6 are the ﬁrst of this kind established for the Beijing−Tianjin region. The ﬂow diagrams of nutrient loads reveal that Beijing and its suburbs discharged an annual average of 54.5 tN d−1 and 4.4 tP d−1 into the Haihe 5297 dx.doi.org/10.1021/es3004415 | Environ. Sci. Technol. 2012, 46, 5294−5301