Sources and Pathways of Nutrients in the Semi-Arid Region of Beijing–Tianjin, China

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.