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Heavy metals are one of the most poisonous and serious groups of pollutants due to their high toxicity, abundance, and ease of accumulation from human and other various species. The behavior of heavy metals in the environment depends on their inherent chemical properties (1).
Pollution of the natural environment by heavy metals is a worldwide problem because these metals are permanent and most of them have toxic effects on living organisms when they exceed a certain concentration (2).
Water is one of the most important factors for every living organism on this planet. The 3% of global fresh water is large enough to meet the requirements of man for millions of years (1). The pollution of water resources by heavy metals released from various sources as a consequence of industrialization and urbanization have been an increasing worldwide concern for the last few decades (1,3,4). In this regard, groundwater contamination can occur by infiltration recharge from surface water, direct migration and inter-aquifer exchange (5). Therefore various pollution indices have been develop and apply for assessing water quality pollution for human consumption with respect to metal contamination (1,6). The objective of water quality indices is to turn complex water quality data into information that is understandable and used by the public. A single number cannot tell the whole story of water quality parameters that are not included in the index. However, a water quality indices such as Heavy metal pollution index (HPI) and Heavy metal evaluation index (HEI) based on some very important parameters can provide a single indicator of water quality (7). In general, water quality indices incorporate data from multiple water quality parameters into a mathematical equation that rates the health of a lake and river with number (8).
Iran is located within the dry and semi dry regions and groundwater is an important water source for drinking water, agricultural and industrial uses in many regions of this country. So that the estimation results indicate that almost 90% of the required water in Iran is secured through the use of groundwater resources (9,10). Due to groundwater is one of the major sources of drinking water in the study area, so it was important to assess the ground water quality with respect to heavy metal contamination.
Aims of the study:
The present study was carried out to assessment and monitoring of heavy metals (As, Zn, Pb, Cd and Cu) pollution in groundwater resources of Asadabad Plain using water quality pollution indices (HPI and HEI).
 

Materials & Methods

Study area
Asadabad Plain with aquifer area about 962 km² and 1650 m above the sea level is located in southwest of Hamedan township in the west part of Iran (11).
Sampling and sample analysis
Groundwater samples were collected from 30 different locations including open and tube wells in 3 replicates to evaluate the heavy metal contamination during spring and summer seasons in 2012. Figure 1 shows the sampling stations in the study area. The sampling locations were selected on the basis of different land use pattern, including agricultural and residential areas. The samples were taken in pre-cleaned, acid-soaked 200 ml polyethylene bottles to avoid unpredictable changes in characteristic as per standard procedures (12,13). The collected samples were filtered (Whatman no. 42), preserved with 6N of HNO₃ (suprapur Merck, Germany) and keep at 4 °C for further analysis (12,14). Concentrations of heavy metals (As, Zn, Pb, Cd and Cu) in samples were determined using inductively coupled plasma- optical emission spectrometry (ICP-OES) (Varian, 710-ES, Australia).

Figure 1) Map of sampling stations
Evaluation methods
Two documented methods evaluated in this study are the heavy metal pollution index (HPI) proposed by Prasad and Bose (2001) (15), and the heavy metal evaluation index (HEI) proposed by Edet and Offiong (2002) (14).
Heavy metal pollution index (HPI)
The HPI represent the total quality of water with respect to heavy metals. The HPI is based on weighted arithmetic quality mean method and developed in two steps. First by establishing a rating scale for each selected parameter giving weightage and second by selecting the pollution parameter on which the index is to be based. The rating system is an arbitrarily value between zero to one and its selection depends upon the importance of individual quality considerations in a comparative way or it can be assessed by making values inversely proportional to the recommended standard for the corresponding parameter (14,16-18). In computing the HPI, Prasad and Bose (2001) (15) considered unit weightage (W_i) as a value inversely proportional to the recommended standard (S_i) of the corresponding parameter as proposed by Reddy (1995) (19).
The HPI model (Mohan et al., 1996) (18) is given by

where Q_i is the sub-index of the i_th parameter. W_i is the unit weightage of the i_th parameter and n is the number of parameters considered. The sub-index (Q_i) of the parameter is calculated by

Where M_i is the monitored value of heavy metal of i_th parameter, I_i is the ideal value of the i_th parameter and S_i is the standard value of the i_th parameter. The sign (−) indicates numerical difference of the two values, ignoring the algebraic sign. Low heavy metal pollution (HPI<100), heavy metal pollution on the threshold risk (HPI=100) and high heavy metal pollution (critical pollution index) (HPI>100. If the samples have heavy metal pollution index values greater than 100, water is not potable (14-16,18).
In computing the HPI for the present study, As, Zn, Pb and Cu were used. The weightage (W_i) was taken as the inverse of MAC, S_i the WHO standard for drinking water and I_i the guide value for the chosen element (Table 4).
Heavy metal evaluation index (HEI)
Heavy metal evaluation index is a way of estimating the water quality with focus on heavy metals in water samples (20). The water quality index classify into three categories which include: low heavy metals (HEI <400), moderate to heavy metals (400 <HEI < 800) and high heavy metals (HEI> 800). The index is calculated from the following equation (21):

Where H_c is the monitored value of the i_th parameter and H_mac the maximum admissible concentration of the i_th parameter (14,20).
Statistical analysis
To test the correlation matrix between elements, correlation between computed indices average and correlation between mean concentration of elements and computed indices, Pearson Correlation Coefficient was performed using the SPSS 18.0 (SPSS Inc., Chicago, IL, USA) statistical package.
 

Results

In the present study levels of As, Zn, Pb, Cd and Cu in groundwater samples of Asadabad Plain during spring and summer in 2012 were determined. Tables 1 and 2, shows the mean concentrations of five elements in the water samples during spring and summer respectively. Also the correlation matrix between elements for spring and summer seasons are presented in Table 3.
The results indicate that mean concentrations of As, Zn, Pb, Cd and Cu in groundwater samples collected from Asadabad Plain in spring season were 52.53±13.62, 15.51±23.45, 10.10±2.80, 4.48±1.80 and 8.63±10.87 μg l⁻¹, respectively and in summer season were 57.60±16.90, 14.99±17.66, 9.28±2.46, 4.57±1.73 and 10.45±10.30 μg l⁻¹, respectively.
   The results of Pearson Correlation Coefficient at 1% level of significance (P<0.01), show significant correlation between the following pairs Zn/Pb, Zn/Cu and Cd/Cu in water samples for spring and summer seasons. These indicate that these metals are the main contributory parameters.
The computed HPI and HEI values for each location, correlation between index values and concentration of metal and correlation between different indices values for spring and summer seasons are presented in Tables 5 to 7 respectively.
 

Table 1) Concentration of metals (µg l^-1) in groundwater samples collected from Asadabad Plain in spring season

Station	As	Zn	Pb	Cd	Cu
1	42.12	54.90	10.08	4.24	23.10
2	36.68	93.37	8.38	4.09	36.20
3	72.92	37.68	10.54	4.10	7.21
4	52.18	6.80	9.16	3.90	4.11
5	52.69	3.64	10.42	4.03	4.56
6	62.99	4.04	8.54	4.10	9.58
7	59.21	5.99	10.40	4.16	8.62
8	47.58	3.58	5.28	4.30	1.88
9	64.39	8.72	10.61	4.12	5.22
10	36.31	2.86	9.83	4.21	10.42
11	38.49	6.82	7.07	4.08	6.67
12	52.24	6.46	10.54	4.03	0.72
13	30.49	7.25	8.62	4.64	3.24
14	45.73	6.84	11.72	4.05	6.38
15	57.43	3.86	0.91	4.27	3.24
16	38.77	5.64	11.44	4.13	4.10
17	52.18	89.30	14.74	13.94	52.84
18	54.37	4.12	13.16	3.10	11.32
19	69.70	4.78	12.61	4.44	3.31
20	68.13	9.96	10.10	4.17	6.40
21	48.10	5.50	10.85	4.30	4.53
22	66.92	5.28	5.25	4.32	8.99
23	68.97	26.43	11.10	4.31	1.88
24	66.60	6.20	14.04	4.14	0.79
25	20.68	7.09	10.90	4.20	2.61
26	50.33	8.53	9.99	4.13	7.16
27	54.78	5.44	10.57	4.45	8.15
28	53.53	4.90	10.11	4.14	7.25
29	75.76	19.50	13.53	4.06	2.16
30	35.78	9.70	12.65	4.34	6.29
Min	36.31	2.86	0.91	3.10	0.72
Max	75.76	93.37	14.74	13.94	36.20
Mean	52.53	15.51	10.10	4.48	8.63
S.D.	13.62	23.45	2.80	1.80	10.87

 
 
Table 2) Concentration of metals (µg l^-1) in groundwater samples collected from Asadabad Plain in summer season

Station	As	Zn	Pb	Cd	Cu
1	53.45	41.32	10.01	4.32	10.11
2	38.48	81.23	11.47	4.46	11.46
3	77.22	19.98	7.96	4.02	7.36
4	49.62	9.57	9.40	4.32	9.42
5	68.66	9.06	12.23	4.21	9.75
6	65.03	4.74	6.23	4.09	7.24
7	49.04	7.70	13.98	4.48	8.95
8	69.67	7.47	11.03	4.36	9.18
9	75.70	7.83	11.32	4.07	6.35
10	44.48	8.73	10.49	4.34	9.37
11	22.98	9.56	8.56	4.50	5.90
12	52.18	9.26	8.13	4.21	4.58
13	41.11	10.20	4.81	4.26	12.66
14	69.28	16.96	6.66	4.47	0.26
15	57.43	9.66	8.17	4.19	8.14
16	88.91	9.75	15.17	4.46	8.05
17	75.53	69.27	6.67	13.67	59.77
18	39.12	8.93	11.01	3.81	2.16
19	29.80	9.79	8.51	3.99	8.98
20	83.02	9.57	10.00	4.45	11.79
21	58.51	9.69	9.16	4.25	10.93
22	57.02	8.76	8.12	4.01	10.77
23	45.70	8.27	12.94	4.36	7.70
24	61.17	9.50	10.72	4.17	5.36
25	44.98	8.15	7.99	4.47	6.74
26	78.76	8.28	9.53	4.51	10.25
27	81.54	8.80	6.90	4.34	7.15
28	34.83	7.07	5.22	3.90	27.18
29	56.40	11.68	7.62	4.18	8.73
30	58.46	9.06	8.55	4.39	7.13
Min	22.98	4.74	4.81	3.81	0.26
Max	83.02	81.23	15.17	13.67	59.77
Mean	57.60	14.99	9.28	4.57	10.45
S.D.	16.90	17.66	2.46	1.73	10.30

Table 3) Correlation matrix between elements

	As	Zn	Pb	Cd	Cu
	Spring
As		-0.073	0.105	-0.019	-0.162
Zn			0.184	0.589**	0.881**
Pb				0.276	0.179
Cd					0.748**
	Summer
As		-0.019	0.150	0.219	0.100
Zn			-0.008	0.595**	0.551**
Pb				-0.168	-0.271
Cd					0.890**

**. Correlation is significant at the 0.01 level (2-tailed).