**4. Discussion**

order: Cd (70.74%) > As (7.12%) > Cu (5.71%) > Zn (5.01%) > Ni (4.39%) > Cr

Max 750.00 20.03 10.38 3.60 6.89 1.98 1.47 780.38 Mean 334.65 11.23 4.50 2.51 3.46 1.21 0.79 358.35 Reference lake ("unpolluted") 30 10 5 5 5 2 1 58

*<sup>f</sup>* value, Cd and As have a very high and moderate contamination factor, respec-

The resulting *Cd* values of each sample site ranged from 6.75 to 29.83. According to the category of *Cd* (**Table 6**), only sample L1 has the low degree of contamination. Ten sampling sites are classified as moderate and 7 sampling sites as having high contamination factors, sample L19 is classified into very high contamination factor. **Figure 3** clearly shows that Cd has the highest contamination factor. That means the Liaohe River is dominated by the pollution of one element—Cadium.

**Elements (***St<sup>i</sup>* **value) Cd As Cu Pb Ni Cr Zn**

**30 10 5 5 5 2 1**

*<sup>r</sup>) of different elements detected in sediments [11].*

tively. Whereas, Cu, Zn, Ni, Cr, and Pb have low contamination factors.

*<sup>r</sup> and ERI*

*<sup>r</sup>* is fixed by the *T<sup>i</sup>*

This means that the results of the given water body are compared with a reference

*<sup>r</sup>) of different elements detected in sediments.*

In the Liaohe River case study, the most toxic element is Cd and the *T<sup>i</sup>*

If the classification thresholds of *Cd* are modified, the *Ei*

*<sup>r</sup>* value of all the elements.

30. Therefore, the classification threshold of *Ei*

*<sup>f</sup>* value of Pb and Cr is less than 1.0. For the average

*<sup>r</sup>* and ERI should also

*ERI* <sup>¼</sup> <sup>P</sup>**<sup>7</sup>** *i*¼**1** *Ei r*

*<sup>r</sup>* of Cd is

*<sup>r</sup>* of all elements

*<sup>r</sup>* value of the most toxic element.

*<sup>f</sup>* = 1). Similarly, the first level of ERI is fixed by

*<sup>r</sup>* is 30. The sum of *T<sup>i</sup>*

(3.84%) > Pb (3.18%). Every *C <sup>i</sup>*

*The potential ecological risk factor (Ei*

*Water Quality - Science, Assessments and Policy*

*3.4.2 The potential ecological risk Ei*

be modified. The first level of *E<sup>i</sup>*

the sum of *T<sup>i</sup>*

**Figure 4.**

**82**

*The potential ecological risk factor (E<sup>i</sup>*

lake which has no contamination (*C <sup>i</sup>*

*C i*

**Table 7.**

The Liaohe River is used as a case study to illustrate this approach. The investigation of seven heavy metals (Cd, As, Cu, Ni, Pb, Cr, and Zn) in the sediments suggest that the Liaohe River is dominated by the pollution of Cd which contributes around 94% potential ecological risk. The *Ei <sup>r</sup>* means of the remaining sites are ranked as: Cd (93.39%) > As (3.13%) > Cu (1.26%) > Ni (0.97%) > Pb (0.70%) > Cr (0.34%) > Zn (0.22%). All elements except cadmium have low potential ecological risk. According to the ERI results, due to the serious pollution of cadmium, all the sampling sites have the considerable or very high potential ecological risk. Thus, it is important to control the pollution of cadmium. This study assesses the risk of Liaohe River by the modified risk classification criterion. Therefore, the results are different from [11], the risks assessed by this study are more serious. It is worth discussing how to use the risk classification criterion. This study suggests using the modified risk classification criteria.

Because of the "toxic-response" factor, compared with other approaches, the potential ecological risk index can distinguish the differences among substances and aquatic systems. Therefore, this approach has outstanding advantages to assess the risk of water system as a widely used approach which can provide a better overall ecological risk to the aquatic system. However, two main problems are neglected in the application of this method. (1) *T<sup>i</sup> <sup>r</sup>* is replaced by *Sti* . More attention should be given to the BPI value. Different aquatic systems have different sensitivities to toxic substances. According to Eq. (3) and **Table 4**, the effect of BPI value on the results depends on the degree of contamination of the aquatic system. If the pollution of the study aquatic system is serious, the BPI value will have large effect on the index calculation. Ecological risks can be evaluated more accurately by measuring the BPI value of the study aquatic system. (2) According to Håkanson's research [7, 23], the classification thresholds should be modified for different assessments. In this chapter, a reasonable suggestion for modification is suggested as well as applied. For *Cd*, the threshold for the "low risk" is modified by the number of substances. For *Ei <sup>r</sup>*, the threshold for the "low risk" is modified by the *T<sup>i</sup> <sup>r</sup>* value of the most toxic element. For ERI, the threshold for the "low risk" is modified by the sum of *T<sup>i</sup> <sup>r</sup>* of all elements. There are still other problems deserve researchers concerns in the application of this approach, for example, the determination of accumulation areas in the aquatic system and calculation of *St<sup>i</sup>* value. This study provides detail information for the potential ecological risk index and discusses several problems of the approach. And it is helpful for researchers to assess the ecological risk of aquatic system by this approach.

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