Modeling the impacts of point-source inputs on nitrogen retention in an
urban river under low-flow conditions
Abstract
Excessive dissolved inorganic nitrogen (DIN) added to the urban river
systems by point-source inputs, such as untreated wastewater and
wastewater treatment plant (WWTP) effluent, constitutes a water-quality
problem of growing concern in China. However, very little is known about
their impacts on DIN retention capacity and pathways in receiving
waters. In this study, a spatially-intensive water quality monitoring
campaign was conducted to support the application of the river water
quality model WASP7.5 to the PS-impacted Nanfei River, China. The DIN
retention capacities and pathway of a reference upstream Reach A, a
wastewater-impacted Reach B and an effluent-dominated Reach C were
quantified using the model results after a Bayesian approach for
parameter estimation and uncertainty analysis. The results showed that
the untreated wastewater discharge elevated the assimilatory uptake rate
but lowered its efficiency in Reach B; while the WWTP effluent discharge
elevated both denitrification rate and efficiency and made Reach C a
denitrification hotspot with increased nitrate concentration and hypoxic
environment. The effects of the point-source inputs on the DIN retention
pathways (assimilatory uptake vs. denitrification) were regulated by
their impacts on river metabolism. Despite different pathways, the total
DIN retention ratios of Reaches A, B and C under low-flow conditions
were 30.3% km-1, 14.3% km-1 and 6.5% km-1, respectively, which
indicated the instream DIN retention capacities were significantly
impaired by the point-source inputs. This result suggests that the DIN
discharged from point-source inputs to urban rivers will be transported
downstream with the potential to create long-term ecological
implications not only locally but also regionally.