澎溪河回水区水体叶绿素a含量与流速相关关系研究

STUDY ON CORRELATIVITY OF CHLOROPHYLL-A CONTENT AND FLOW RATE IN THE REGION OF PENGXI BACKWATER

以三峡库区支流澎溪河回水区为特征区域,研究三峡水库回水区水动力条件对水体富营养化的影响。根据三峡水库的调蓄过程,在175m高水位、小流量、低污染负荷和145m低水位、大流量、高污染负荷两种设计条件下模拟澎溪河水体断面平均高锰酸盐指数(CODMn)、总氮(TN)、总磷(TP)浓度和叶绿素a(Chl-a)含量。研究表明:澎溪河回水段从上游到下游的CODMn、TN、TP浓度有增加的趋势,在接近河口处有趋同于长江水污染物浓度的趋势。145m低水位的大流量、高污染负荷设计条件下Chl-a含量较高,在弯道下游和河道水面开阔的高阳、双江大桥断面的流速减缓区段,出现Chl-a含量峰值。拟合澎溪河水体流速及Chl-a含量相关关系,在枯丰两种设计条件下建立水体流速(V)与Chl-a含量(S)的相关关系式,分别为S=0.186 0·V-0.353 6(相关系数r2=0.838 0)和S=10.844 9·V-0.293 9(相关系数r2=0.825 4)。模拟结果表明:加大水体流速有利于抑制Chl-a含量。通过改变水动力条件控制水体富营养化具有可行性。

The backwater of the the Pengxi River was a typical region to study the effect of water hydrology on the chlorophyll-a （Chla） content.The computed area was 53.5 km of the Pengxi River reach range from the river estuary to Qukou section in Yunyang city and the Yangtze River reach of 6.8 km in Shuangjiang city.Currently there are three methods to study the effect of water hydrology on the Chla content：laboratory research,field monitoring study and ecological dynamics model simulation.In this paper,the verified ecological dynamics model was used to simulate CODMn,TN,TP concentrations and Chla content.The average permanganate index （CODMn）,total nitrogen （TN）,total phosphorus （TP） concentrations and Chla content along the Pengxi River were on two design conditions：1） 175 m high water level with low flow and low pollutant load; 2） 145 m low level with high flow and high pollutant load.The low pollutant loads of CODMn,TN,TP were 10.28 t/d,2.06 t/d and 0.39 t/d,respectively,while the high pollutant loads of CODMn,TN,TP were 77.95 t/d,30.96 t/d and 5.78 t/d respectively.The computed results showed that the ranges of CODMn,TN,TP concentrations and Chla content in the backwater of the Pengxi River were 1.37-1.60 mg/L,0.55-0.80 mg/L,0.035-0.050 mg/L and 0.32-0.96 μg/L on the design condition 1,and 5.67-6.80 mg/L,3.02-4.08 mg/L,0.131-0.240 mg/L and 10.60-32.13 μg/L on the design condition 2,respectively.CODMn,TN,TP concentrations from upstream to downstream tended to increase,and the water quality at the estuary trended toward the Yangtze water quality.In the period of 145 m low water level with large flow and high pollutant load,the Chla content was high.In the curve or wide surface sections such as Gaoyang or Shuangjiang Bridge,the flow rate was slow and Chla content was high.The correlation of flow rate （V） and Chla content （S） was fitted under the two design conditions.The equations were S=0.186 0 ＆#183; V-0.3536 （related coefficient r2 =0.838 0） and S=10.844 9 ＆#183; V-0.293 9 （related coefficient r2 =0.838 0） respectively.The simulated results showed that the increase of flow rate tended to keep down the Chla content.Under the conditions of low flow with 175 m high water level,the pollutant load was low,average water temperature was 8.0℃,total solar radiation was 517 J/（cm2 ＆#183;d）,and average sunshine hours were 2.0 h/d.The Chla content increased quickly when the flow rate was lower than 0.01 m/s,and the Chla content tended to be stable when the flow rate was higher than 0.10 m/s.Under the conditions of high flow with 145 m low water level,the pollutant load was high,average water temperature was 20.0℃,total solar radiation was 1 464 J/（cm2＆#183; d）,and average sunshine hours were 5.8 h/d.Chla content increased quickly when the flow rate was lower than 0.05 m/s,and Chla content tended to be stable when the flow rate was higher than 0.40 m/s.This study suggested that 0.05 m/s was the critical flow rate of Chla content change in the Pengxi backwater under the appropriate environmental conditions.The simulated results showed that it was feasible to control Chla content through regulating hydrodynamic conditions.