**4. Commonly used pollution indices**

Pollution indices are calculated estimates of the degree of soil and sediment contamination, usually associated with heavy metals. Pollution indices are also employed to (i) assess soil quality and health, (ii) predict ecosystem sustainability, and (iii) discriminate between natural processes and anthropogenic processes to explain heavy metal distributions in soil profiles [20, 21]. Many of the pollution indices rely upon a proper selection of a geochemical background (GeoBase) [20, 21]. The geochemical background, if properly selected, will permit an estimation of the intensity of the heavy metal or rare earth element pollution.

The geochemical background (GeoBase) for an element estimates the natural variations in concentration in the surficial environment [22] or is a "measure that is used to differentiate between the concentration of the natural compound and the concentrations with an anthropogenic influence in a given environmental sample" [23]. Kowalska et al. [21] assessed 18 pollution indices and estimated their strength and weaknesses. Diwa [24], Elvira [25], Lawrence et al. [26], Barbieri et al. [27], Ghrefat et al. [28], and Gargouri et al. [29] provided additional information on interpreting the environmental impact of pollution indices values. In the Czech Republic, Weissmannová et al. [30] chronicled the potential ecological risk and human health risk assessment in soils influenced by coal mining and metal processing.


#### **Table 3.**

*Frequently employed pollution indices.*

Frequently, employed pollution indices are briefly described in **Table 3** and more fully described below.

The geoaccumulation index (Igeo) is determined from the elemental concentration and the GeoBase. The GeoBase is the concentration value of the element selected from a geochemical reference background, and it is critical to select an appropriate geochemical background, with various research investigations using PAAS, NASC, UCC, or local baselines perceived as preindustrial or non-impacted. The Igeo value and the corresponding pollution level is (i) less than or equal to 0 is not impacted, (ii) 0 to 1 is at most moderately impacted, (iii) 1 to 2 is moderately impacted, (iv) 2 to 3 is moderate to highly impacted, (v) 3 to 4 is highly impacted, (vi) 4 to 5 is high to very highly impacted, and (vii) 6 is very highly impacted. The single pollution index evaluates the degree of heavy metal or rare earth element accumulation in soil or sediment relative to a reference GeoBase. The single pollution index estimates the total amount of an element's accumulation and does not indicate the bioavailability of the heavy metals. The single pollution index (PI) allows the comparison of sites or different soils over time. The enrichment factor (EF) estimates the heavy metal's anthropogenic impact and is determined as EF = [HMconc/LV] of sample/GeoBase/LV] of background, where LV (low variability reference) is the selected element concentration considered as not supplied or depleted. Typically, Fe, Al, Ca, Ti, Sc, and Mn have been used as the reference element. If the EF value ranges from 0.5 to 1.5, the likelihood of anthropogenic activity is low. The selection of the GeoBase that reduces the metal variability is critical for assessment. If the EF value and the enrichment level are: (i) less than 1, there is no likelihood of element enrichment (impact), (ii) 1 to 3 is minor enrichment, (iii) 3 to 5 is moderate enrichment, (iv) 5 to 10

#### *Evaluation of Rare Earth Element Mine Sites for Environmental Impact DOI: http://dx.doi.org/10.5772/intechopen.109161*

is moderately severe enrichment, (v) 10 to 25 is severe enrichment, (vi) 25 to 50 is very severe enrichment, and (vii) more than 50 is extremely severe enrichment.

The contamination factor (CF) estimates the preindustrial increase of heavy metals or rare earth elements in soil and sediment. CF is estimated as the mean of more than five samples for a particular heavy metal or rare earth element relative to the heavy metal concentration from preindustrial samples. The selection and evaluation of the quality of the preindustrial reference samples may be difficult. Appropriate ratings are: (i) less than 1 is low contamination (impact), (ii) 1 to 3 is moderate contamination, (iii) 3 to 6 is considerable contamination, and (iv) greater than 6 is considered high contamination [25]. The biogeochemical index (BGCI) is estimated as a ratio of the heavy metal or rare earth element concentration in the soil's O horizon to that element's concentration in the soil's A horizon. If the BGCI has values greater than 1, then there exists an increased heavy metal or rare earth element adsorption in the O horizon. The BGCI is well suited for forest soils; however, the biogeochemical index lacks consideration of the heavy metal biological availability.

The sum of contamination (PIsum) involves a suite of heavy metals or rare earth elements and is simply the sum of the individual PI values for the sampled heavy metals. The calculation of the PIsum must contain all of the relevant heavy metals. The selection of the geochemical database must be appropriate to the site assessment, typically employing local and preindustrial sampling. Similar to the BGCI, the Pisum does not consider heavy metal biological availability. The Nemerow pollution index (PInem) is defined as PInem = {[(1/n)∑PI)2 + PImax2 ]/n}0.5, where n is the number of heavy metals sampled, and PImax is the maximum PI value for all of the heavy metals. The usage of the appropriate geochemical database, baseline values, or threshold levels must be ascertained. The Nemerow pollution index directly reflects the soil or sediment environmental pollution and highlights the heavy metal having the greatest environmental presence or intensity of pollution.

The pollution load index (PLI) is calculated as {∏ PI}1/n and is simply the harmonic mean of the PI's for the analyzed heavy metals or rare earth elements. The pollution load index does not consider heavy metal biological availability and is based on the reliability of the PI values. The average single pollution index (PIaverage) is estimated as the average of the individual PI values. The average single pollution index does not consider heavy metal or rare earth element biological availability and is based on the reliability of the PI values. Appropriate ratings are: (i) less than 1.5 is very low contamination, (ii) 1.5 to 2 is low contamination, (iii) 2 to 4 is moderate contamination, (iv) 4 to 8 is high contamination, (v) 8 to 16 is considered very high contamination, and (vi) 16 or greater is considered extreme contamination [25]. The vector modulus of pollution index (PIvector) is estimated {(1/n)∑ PI2 } 0.5. The vector modulus of pollution index does not consider heavy metal or rare earth element biological availability and is based on the reliability of the PI values.

The multi-element contamination index (MEC) is estimated as MEC = {∑(HMconc of element i/tolerable level for element i)}/n. The multi-element contamination index does not require an assessment of the variation in natural processes. The degree of contamination (Cdeg) is estimated as Cdeg = ∑CF. The degree of contamination does not require an assessment of the variation in natural processes.

The potential ecological risk factor (Eir) is estimated as Eir from an estimate of the element's toxicity (Ti factor) and associated PI value, where the Ti has values of 1 for zinc (Zn), 2 for chromium (Cr), 5 for Nickel (Ni), Copper (Cu) and lead (Pb), 10 for arsenic (As), and 30 for cadmium (Cd). Ti values for the rare earth elements are not yet determined. The potential ecological hazard index (RI) is estimated as IR = ∑Eir.

If the Eir risk values are: (i) less than 40, which implies low ecological risk, (ii) 40 to 80 implies moderate ecological risk, (iii) 80 to 160 implies appreciable ecological risk, (iv) 160 to 320 implies high ecological risk, and (v) more than 320 implies series ecological risk. If the IR risk values are: (i) less than 150, which implies low ecological risk, (ii) 150 to 300 implies moderate ecological risk, (iii) 300 to 600 implies high ecological risk, and (iv) more than 600 implies series ecological risk [24]. The hazard quotient (HQ ) of a rare earth element is estimated as the rare earth element concentration relative to the rare earth element concentration, where no environmental effect was observed.
