*2.5. Sediment Contamination and Ecological Risk Assessment*

The geo-accumulation index (*Igeo*) provides a good explanation of the contamination or pollution status of sediment and thereby gives a better understanding of the possible threat of metal pollutants to natural ecosystem settings [31]. For that reason, *Igeo* was applied in this study as a quantitative indicator, putting into consideration the classifications that evaluate the level of pollution as described by Müller [32]. This index is expressed mathematically as:

$$I\_{\rm gcp} = \log\_2[\frac{\mathcal{C}\_n}{1.5 \times B\_n}] \tag{1}$$

where *Cn* and *Bn* are the metal concentration in the sediment and geochemical background value of the element (*n*).

The levels of metal contamination were classified into seven levels: uncontaminated (<0), uncontaminated to moderately contaminated (0–1), moderately contaminated (1–2), moderately to strongly contaminated (2–3), strongly contaminated (3–4), strongly to extremely contaminated (4–5) and extremely contaminated (>5) [32].

In aquatic ecosystems generally, sediment quality guidelines (SQGs) are utilized to assess the potential risk posed by pollutants to the natural ecosystem [33]. Sub variables of SQGs, which are threshold effect limit (TEL) and probable effect limit (PEL), were used for comparison with metal concentrations in sediment to conclude possible potential ecological risk. The concentrations of metals less than the threshold effect limit (TEL) denote a minimal effect, below which there is no expectation for adverse effects. However, metal concentrations at or greater than the probable effect limit (PEL) denote likely frequent occurrence of adverse biological effects (Table 1).

The ecological risk factor (ErF) was used to further assess the status of metal pollution in sediment and its possible toxicological effect with a comprehensive evaluation of ecological risk constituted by metal contaminants. The basis for the classification of ErF values for metal pollutants was as described by Hakanson [34]. For a particular metal, ErF was expressed as:

$$\text{ErF} = \text{Tr}^i \times \text{CF}^i \tag{2}$$

where *Tri* is the toxic response factor of a given metal (*i*) (Pb = 5, Zn = 1, Cu = 5 Cd = 30, Cr = 2, Ni = 5, Mn = 1) and *CF<sup>i</sup>* is the contamination factor of metal (*i*).

The classification of ErF is in five classes according to Hakanson [34]: low risk (ErF < 30), moderate risk (30 ≤ ErF < 60), considerable risk (60 ≤ ErF < 120), high risk (120 ≤ ErF < 240) and very high risk (ErF ≥ 240).
