Statement 1: In Enrichment assessment analysis, Geoaccumulation Index (Igeo) value for Cd is negative with the range of (-4 to -2). 

Answer: This statement (1) above indicates that your igeo data indicate no contamination or enrichment above background value with reference to the Bn.

Statement 2. The same Cd value is in Enrichment Factor (EF) and Pollution Load Index (PLI) are showing (17.9 to 118.5 and 2 respectively). When, the accumulation is nill... How the Enrichment value and Pollution load indexes were showing Positive.

Answer: Based  on  the second statement above I think the issue relating to your data has to do with your reference values. Pollution load index (PLI)  gives the estimation of metal contaminants status and Enrichment factor (EF) gives the relative abundance of a contaminant over background values. In your statement you did not state if you normalized your data with a conservative element usually Fe or what background value you applied in the case of the calculation of EF. However,  where the Igeo is negative (no contamination ) and PLI and EF are positive (enrichment over background value or contamination) could arise due to the reference values you applied in each case for Bn, M/Fe background and  CF as stated below:

Geoaccumulation index according to Muller (1981)

                                      Igeo   =    [log2Cn/1.5 X Bn ],                        

        Where,

                        Cn =    the concentration (mean) of heavy metals as calculated   

                                       (mg/kg)

                        Bn =     geochemical background value of shale

                                        (mg/kg) or soil in the area (reference)

                        1.5 =    correction factor due to lithological variations  

Enrichment factor (EF), Birch (2003):

                    Enrichment factor (EF) =    (M/Fe of Sample) / (M/Fe Background)    

    Where,

         M/Fe of sample - ratio of metal and Fe concentration in the samples

         M/Fe background - ratio of metal and Fe concentration in the subsurface         

         Water of the study area (reference)

Pollution load index:

PLI = (CF1 X CF2X CF3…….. CFN) 1/N

note: contamination Factor (CF) =  Chs/Cbs                                 

    Where;

               Chs =     Heavy metal concentration in sediment samples (mg/kg)

               Cbs=     Background concentration of heavy metal in sediment in the study 

                              area(mg/kg)

                N =   is the number of metal studied

         Therefore, in  your case  the interpretation of your data should be discussed or presented  based on each index of contamination  you  have used. Based on the fact that PLI and EF of the index you employed  signify contamination then the overall conclusions can be based on that that there exist contamination in the study area and appropriate measures should be taken.

Dear Mustapha Atta,

Thank you very much for your response and answers for my questions. I will check once again. Thank you so much for your precious time. 

I do agree with Mr. Mustapha Atta. Thanks for his details response. Moreover,  in case of geoaccumulation, if we can use the original background value of a particular area (if available), then we can omit  correction factor. I think this is one reason behind your result. Furthermore, I would like mention that PLI is a cumulative indication about pollution of an area, and little bit different then Igeo and EF.

Pollution load index and Geoaccumulation index (Igeo)

The Tomlison’s pollution load index, PLI, is calculated according to the equation:

         PLI = [CF1 x CF2 x ... x CFn]1/n

where n = the number of metals (n=5); CFn = the contamination factor of metal n given by:              

                  CF= Cmetal/ Bn

The PLI represents the number of times by which the metal content in the sediment or soil exceeds the natural background concentration, and gives a summative indication of the overall level of heavy metal toxicity in a particular sample. Where the natural background concentration is not available, the geochemical background value of metal n in reference average shale can be used (Turekian and Wedepohl, 1961). The Igeo is a common technique employed for assessing the magnitude of heavy metal contamination in environmental samples. The Igeo values for the n number of metals can be computed using the equation developed by Müller (1979):

                                     Igeo = log2(Cn / 1.5 x Bn)

where Cn is the nth heavy metal concentration in the sample, Bn is the geochemical background value of metal n in reference average shale, and 1.5 is a factor employed to address the possible deviations in background concentration of metal, which may be associated with lithogenic and anthropogenic effects. According to Müller’s geoaccumulation indices, the magnitude of contamination with respect to individual element concentration is categorized as:

PLI Grade        Degree of contamination

0                            Perfection

1                          Polluted

Igeo Index            Igeo class             Degree of contamination

≤ 0                            0                          Uncontaminated

0 <  – ≤ 1                  1                          Uncontaminated to moderately contaminated

1 < – ≤ 2                   2                          Moderately contaminated

2 < – ≤ 3                   3                          Moderately to highly contaminated

3 < – ≤ 4                   4                          Highly contaminated

4 < – ≤ 5                   5                          Highly to extremely contaminated

>5                             6                          Extremely contaminated

Enrichment Factor

Enrichment factor (EF) is also regarded as an effective tool for the evaluation of the magnitude of contaminants in an environmental matrix. EF is very useful especially when considering the assessment of elemental anthropogenic influences in the environment. It is calculated using the following formula:

         EF = [Cmetal/CAl]sample / [Cmetal/CAl]Bkg

where (Cmetal/CAl)sample is the ratio of concentration of heavy metal (Cmetal) to that of aluminum (CAl) in the sample, and (Cmetal/CAl)Bkg is the same reference ratio in the background sample. An EF value of about 1 indicates that the heavy metal may have originated from natural weathering processes. EF >1.5 suggests possible anthropogenic influences.  Generally, the following EF categorizations are applicable: 1.5 – 3 (minor modification), 3 – 5 (moderate modification), 5 – 10 (severe modification), and >10 (very severe modification). 

From your results, the Igeo values ranged between -4 and -2, indicating an Igeo index that was less than zero. It means that your environmental matrix (sediment, soil, etc.) was not contaminated by Cd at all investigated sites. Assertively, your samples indicated an Igeo class 0 degree of contamination by Cd. On the other hand, obtaining a PLI of 2.71 showed that environmental matrix is contaminated by Cd. This is of course in contrast to the case when the geoaccumulation index was applied. 

In addition, your result indicated an EF that ranged between 17.9 and 118.5. The EF gradation according to your result is considered an evidence of very severe contamination of the studied matrix by Cd. Evidently, the EF is indicative of human mediated source(s) of Cd. 

It is important for one to understand that there are a variety of quantitative tools, indices and guidelines proposed and employed in evaluating elemental enrichment and contamination degree in environmental matrices. These approaches have their peculiarities, advantages and limitations. Some tools do give contrasting categorizations and by implication conflicting degrees of contamination. I will advice you apply a suite of other tools such as modified degree of contamination, hazard quotients, modified hazard quotients, etc. to further ascertain the contamination status of Cd in your studied matrix.

Dear H. M. Zakir and Nsikak Benson 

Thank you very much for your response and precious time.

Thank you