Characteristics of diffuse pollution of nitrogen and phosphorous from a small town in the hilly area of the central Sichuan Basin,China

Hydrological and hydro-chemical monitoring of nitrogen (N) and phosphorus (P) in a small urbanized catchment was conducted in the hilly area of the central Sichuan Basin, China, from 2010 through 2011. The diffuse N and P loadings in different forms of total nitrogen (TN) and phosphorus (TP), dissolved nitrogen (DN) and phosphorus (DP), as well as particulate nitrogen (PN) and phosphorus (PP) were calculated based on runoff discharges and chemical analyses. The results revealed that the diffuse pollution concentrations of TN, DN, PN, TP, DP and PP exhibited large variations during rainfall events, with peak concentrations occurring during the initial period. For all of the measured parameters, the event mean concentrations (EMCs) were observed to clearly vary among rainfall events. The EMCs of TN, DN, PN, TP, DP and PP (for all of the observed rainfall events) were 10.04, 6.62, 3.42, 1.30, 0.47 and 0.83 mg/L, respectively. The losses of diffuse N and P exhibited clear seasonal patterns and mainly occurred during the period from July through September, when the losses totaled 99.3 and 9.6 kg/ha for TN and TP, respectively, accounting for 75% and 74% of the total annual loadings. The mean annual loadings of TN and TP were 124.6 and 12.9 kg/ha, respectively. The results indicate that residential areas in the hilly area of the central Sichuan Basin are subject to high diffuse N and P loadings, posing a serious risk to the receiving water quality. Ecological buffering belts are recommended to incorporate into the urbanized catchment to reduce diffuse pollution.