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Nitrogen and Phosphate Retention in a Drainage Ditch Dominated by Domestic Sewage in a Suburban Area of Hefei City

QIAN Jing;LI Ruzhong;TANG Wenkun;DONG Yuhong;YANG Jiwei;School of Resources and Environmental Engineering,Hefei University of Technology;  
Nitrogen and phosphorus retention for a typical drainage ditch in Guanzhenhe Branch,a first-order stream dominated by domestic sewage in the rural-urban continuum of Hefei,was characterized.Field tracer experiments were carried out for five replications by addition of a conservative tracer(Na Cl) and nutrients(NH4 Cl and KH2PO4) along the study reach.Based on the experimental data,hydrological parameters(e.g.,dispersion coefficient D,stream channel cross-sectional area A,storage zone cross-sectional area As,and stream storage zone exchange coefficient α) and the first-order nutrient decay coefficients(e.g.,decay coefficient in main channel λ and decay coefficient in storage zone λs),were estimated with the transient storage OTIS(one-dimensional transport with inflow and storage)model.Nutrient spiraling metrics(i.e.,uptake length Sw,mass transfer coefficient Vf,and nutrient uptake rate U) were subsequently calculated based on the nutrient spiraling theory.Regression and correlation analysis were further conducted with SPSS statistical software.The results showed that:1) The average value of As/A was 0.35,and was significantly negatively correlated with discharge(Q).2) The order of magnitude of α was 10-3,while the mean value of α was 2.49 × 10-3s-1.Sw-NH4 and Sw-PO4(Swof NH4-N and PO4-P)reached their maximum values of 934,020 and 47,518 m,respectively,indicating that Guanzhenhe Branch was not capable of retaining nutrients efficiently.Meanwhile,the negative values of Sw-NH4 and Sw-PO4 suggested that the study reach even acted as a nutrient source.3) Correlation analysis results showed that Vf-NH4 was negatively correlated with water depth and Q,respectively.In addition,both Vf-NH4 and Uf-PO4 increased with stream width.4) Nutrient spiraling metrics were not statistically related to ambient concentrations of NH4-N and PO4-P in regression analysis.In general,high nutrient concentrations restricted the effect of nutrient retention in Guanzhenhe Branch.Excessive nutrient sources coming from domestic sewage should be reduced to improve the stream retention capacity.
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