中文摘要：

在建设项目地表水环境影响预测与评价中,排污口外受纳水域污染混合区范围计算是一项主要内容,允许浓度升高值是污染混合区范围计算的重要影响因素之一。通过分析发现,当背景浓度与水质标准限值比较接近时,受纳水体背景浓度的一个微小误差,就会引起允许浓度升高值和污染混合区几何特征参数误差传递的放大效应。提出了确定背景浓度与允许浓度升高值的新方法：针对背景浓度的随机特征,提出了根据河流断面实测资料计算水质频率曲线的方法和以75%或90%水质保证率确定背景浓度的原则,以及采样60%～90%之间部分频率分布的曲线拟合方程计算背景浓度的方法。最后,由水质标准值减去背景浓度得到允许浓度升高值;按照河流上、下游水环境容量的公平合理分配和留有余地的原则,提出了以水功能区水质标准值的5%直接作为允许浓度升高值,以此计算污染混合区范围。

英文摘要：

In the prediction and evaluation of surface water environmental impacts from construction projects,the calculation of the water pollutant mixing zone at the sewage outlet is a major objective for which the allowed increase of pollutant concentrations is an important factor. Analysis shows that when the background concentration is close to water quality standards,a tiny error in the background concentration of the receiving waterbody will cause an error amplification due to error propagation during the calculation of the allowed increase of concentrations and the geometrical parameters of the pollutant mixing zone. This study proposes new methods to determine the background concentration and the allowed concentration increase. According to the random nature of the background concentration,a method has been proposed that first calculates the water quality probability distribution curve based on the data measured across the river section,then obtains the background concentration either by a principle that ensures the water quality ratio to be 75% or 90%,or a curve fitting an equation for the range of 60% to90% probability. Lastly,the allowed increase in concentrations acquired from this method will be the subtraction of the background value from the water quality standard. In accordance with the principle that both the upstream and downstream water environmental capacity of the river require fair and reasonable assignments and also allowing for unforeseen circumstances,5% of the water quality standard value in the water functional area is directly taken as the allowed increase in the concentration,and then used to calculate the range of the pollutant mixing zone.