Nutrients in runoff degrade water quality. The development of schemes to mitigate such degradation requires a characterization of the underlying dynamic processes of nutrient loss. The drinking-water source protection area in the Lake Hongfeng watershed of Guiyang City, the capital of Guizhou Province, China, has been delimited for effective conservation. However, no systematic observations have provided data on nutrient losses from these areas that could support optimal management. We selected one typical watershed in the area. Automatic gauges were installed to record the water levels and calculate runoff rates during 2010 and 2011. A total of 1523 runoff samples were collected at an interval of 3 h during a day; total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) were tested. The results indicated that surface runoff rates were primarily less than 15 L/s but rapidly increased 1-30 times 15 L/s when it rained. TN, TP, and COD concentrations primarily fluctuated between 0.06 and 18.79 mg/L, between 0.01 and 1.57 mg/L, and between 0.01 and 160 mg/L, respectively. TN and COD concentrations in 98.98% and 52.04% of the runoff samples, respectively, exceeded the upper limit required by the Environmental Quality Standards for Surface Water (EQSSW) in China. Conversely, 94.29% of the runoff samples had lower concentrations than the upper limit of TP concentration. Surface runoff has been seriously polluted by nitrogen and organic pollutants. The occurrence frequency of different runoff rates and TP and COD concentrations showed different distributions, but TN concentrations had a normal distribution. There was a significant relationship between runoff rates and TP concentration and TN, TP, or COD loss. TN, TP, and COD loss primarily occurred on vegetable lands, rice fields, and residential sites. Effectively controlling nitrogen fertilizer that is applied on vegetable lands and paddy fields and managing wastewater and solid waste are urgent. The results reported here will also provide references for many other regions facing similar problems.
Keywords: Nitrogen; Nonpoint source pollution; Phosphorus; Runoff; Water quality.