2.5. Species Sensitivity Distribution

The species sensitivity distribution (SSD) selects the Burr type Ⅲ distribution of cumulative exposure concentration curves fitting the Burr type Ⅲ distribution of skewness and kurtosis; in covering the larger range, it can make it more flexible fitting the parameters [38,39,40]. The Burr type Ⅲ distribution fits the parameters of the function equation for:

HC5 is the cumulative concentration on the species sensitivity distribution curve corresponding to the damage to 5% of species. The smaller the hazardous concentration for 5% (HC5), the more toxic the heavy metals represented [41,42]. The formula for calculating HC5 by using Burr type Ⅲ distribution is:

where q is the corresponding protection level. potentially affected fraction (PAF) is the proportion of hazardous species corresponding to the measured concentrations on the species sensitivity distribution curve, and the calculation formula is as follows:

where x is the measured concentration of pollutant. multi-substance potentially affected fraction (MSPAF) is the proportion of hazardous species caused by multiple pollutants, which can reflect the combined pollution caused by multiple pollutants in water. The calculation formula is: