不同雨强对太湖河网区河道入湖营养盐负荷影响

为揭示太湖河网区不同雨强下入湖河道面源污染规律,以太湖入湖负荷最大的河道大浦河为例,通过一周年逐日高频监测水体各形态氮、磷及溶解性有机碳等营养盐情况,结合河道流量及降雨量的自动监测资料,分析了大雨、中雨、小雨及无雨等4种降雨强度下太湖河网区典型河道的流量和营养盐负荷特征.结果表明,作为太湖河网区的典型入湖河道,大浦河时常发生往复流现象,观测的365 d内,有60 d日均流量为负值,占16%;河道流量对雨强的响应较为迟缓,仅强降雨事件( 25 mm·d~(-1))下,降雨当日河道流量才显著增加;中雨期平均流量仅比无雨期高了29%,在统计上不显著.河道水体营养盐浓度在不同降雨强度下差异不显著,大雨、中雨、小雨及无雨事件下河道总氮浓度分别为(3. 00±0. 58)、(3. 34±0. 93)、(3. 55±1. 05)和(3. 37±1. 14) mg·L~(-1),小雨事件当天水体总氮浓度均值最高,而4种类型降雨事件下河道总磷含量分别为(0. 228±0. 068)、(0. 258±0. 121)、(0. 219±0. 083)和(0. 225±0. 121) mg·L~(-1),差异性也不显著,就平均值而言,中雨时河道总磷浓度最高.夏季典型降雨过程分析表明,不同雨强发生之后河道溶解性有机碳和各形态氮的浓度变化不大,但大雨之后次日河道各形态磷浓度有明显增高,持续时间为2 d,中雨后次日河道总磷和颗粒态磷有明显增高,持续时间仅为1 d,小雨后磷浓度基本无变化.大雨、中雨、小雨和无雨时总氮日负荷分别为7. 64、3. 19、3. 21和2. 62 t·d~(-1),总磷日负荷分别为0. 59、0. 26、0. 22和0. 20 t·d~(-1),受入湖流量影响,大雨期营养盐日负荷显著高于中雨及以下强度的降雨;然而,由于一年内大雨出现频次较少,大雨期氮和磷总入湖负荷占年负荷的比重不大,大雨期总氮和总磷分别入湖61. 11 t和4. 72 t,占观测周年的5. 6%和5. 8%,这与山区河道降雨负荷间的关系有着显著区别.本高频观测表明,太湖流域平原河网区河道面源污染汇集过程复杂,入湖负荷受降雨强度的影响相对较小,入湖水量是营养盐负荷的重要影响因素.本研究结果对太湖流域平原河网区湖泊的面源污染的估算及控制对策的制定具有参考价值.

Influence of Rainfall Intensity on the Nutrient Loading from an Inflowing River in the Plain River Network of the Taihu Catchment

To reveal the law of external pollution in the plain river network of the Taihu catchment, we investigated nitrogen, phosphorus, and dissolved organic carbon daily, as well as automatic recording data for flow rate and rainfall in Dapu River from 1^st March, 2017 to 28^th February, 2018. Dapu River, a typical inflowing river of Lake Taihu, usually has a reciprocating flow. A reciprocating flow is a common condition in the plain river network located in the Yangtze Delta. The response flow rate in the river was relatively slow to different intensities of rainfall. Flow rate significantly increased only when the rainfall intensity reached heavy rain (>25 mm·d^-1). The concentrations were statistically non-significant under different rainfall intensities, and the concentrations of total nitrogen averaged 3.00±0.58, 3.34±0.93, 3.55±1.05, and 3.37±1.14 mg·L^-1 under heavy rain (>25 mm·d^-1), medium rain (10 mm·d^-1-25 mm·d^-1), light rain (<10 mm·d^-1), and no rain. The concentrations of total phosphorus averaged 0.228±0.068, 0.258±0.121, 0.219±0.083, and 0.225±0.121 mg·L^-1, respectively. The concentration of dissolved organic carbon and nitrogen in the Dapu River changed little during the 3 typical rainfall processes, but the concentration of phosphorus increased significantly after heavy rain when the duration was 2 days. The concentration of total phosphorus and particulate phosphorus increased significantly after medium rain when the duration was 1 day. The concentration of phosphorus did not show an obvious change after light rain. The concentration of total nitrogen was the highest under light rain, and the concentration of total phosphorus was the highest under medium rain. The daily loadings of total nitrogen from the Dapu River to Lake Taihu were 7.64, 3.19, 3.21, and 2.62 t·d^-1 under conditions of heavy rain, medium rain, light rain, and no rain, respectively. The daily loadings of total phosphorus were 0.59, 0.26, 0.22, and 0.20 t·d^-1 under conditions of heavy rain, medium rain, light rain, and no rain, respectively. Although the daily loading of nutrients under heavy rain conditions was the highest, the contribution of annual nutrient loading was small during heavy rain days. The reason was that the percentage of heavy rain during the entire year was small. The total nitrogen loading in heavy rain days was 61.11 tons in the observatory year, which was only 5.6% of annual loading. The total phosphorus loading in heavy rain days was 4.72 tons, which was only 5.8% of the annual loading. These low percentages of nutrient loading under heavy rainfall are quite different from situations in mountain catchments. This research showed that the transport processes of nutrients in the plain river network was complicated and its influence on nutrient concentrations and loadings in connected rivers was slow and indirect. The amount of inflow water was an important source of external nutrient loading in the plain river network. These conclusions have reference value for estimating the external loading and deciding non-point control policy of lakes in plain river network of the Taihu catchment.