Abstract

This paper discusses an integrated approach of pollution indices techniques to assess the intensity of heavy metal pollution in irrigation and drinking water systems discharged from coal mining in Bangladesh. Mn, Fe, Cd and Pb levels in most of the water samples exceed the Bangladesh and international standards. The heavy metal pollution index (HPI), heavy metal evaluation index (HEI) and degree of contamination (C_d) schemes indicate that the mine drainage/irrigation waters and the adjoining groundwaters are highly contaminated. The groundwater system in the vicinity of the coal mine site is also heavily polluted by anthropogenic sources. The pollution status of irrigation and drinking water systems in the study area are of great environmental and health concerns.

1. Introduction

Pollution of water and soil due to discharged water is a major environmental concern worldwide. Barapukuria coal mine is the only one coal mine in Bangladesh. The problems of water quality are more severe in areas where the mining and mineral processes’ industries are present. In mining processes, several classes of wastes are produced which may turn into ultimately the sources of water quality and environmental degradation. The discharge mine water through drainage system is generally used for irrigational purpose in the adjoining irrigation area. Coal mine water contains high concentrations of metals and metalloids that can create problem for surface water, ground water and top soils of mine water discharged area ¹. Metals from coal easily dissolve and mobilize into water and residue deposits ². The polluted water with metals make bad impact on aquatic life and the surroundings vegetation area of coal mine ³.

Today heavy metals pollution of the groundwater is one of the serious environmental problems. Some of the heavy metals considered as micronutrients can cause adverse effects to human health when their contents exceed the permissible limit in drinking water ^{4, 5}. Thus, heavy metals assessment in groundwater used for drinking purpose is very significance from the human health viewpoint.

Heavy metals as an environmental pollutant, occurrence in waters from natural (such as chemical weathering of minerals and soil leaching) or anthropogenic sources (such as industrial and domestic effluents, urban storm, water runoff, landfill leachate, mining activities, atmospheric sources etc.). For evaluation of water quality pollution several methods such as the contamination index (C_d), the heavy metal pollution index (HPI) and the heavy metal evaluation index (HEI) were developed. These indices help assessing the present level of pollution in water resources and combine all the water pollution parameters into some easy approach ^{6, 7, 8}.

2. Study Area

The study area is covered the adjoining agricultural land and settlement area of Barapukuria Coal Mine Company Limited (BCMCL) at the northern part of Bangladesh (shown in Figure 1). The area lies between latitudes 25031' N to 25035' N and longitude 88057' E to 88059' respectively.

3. Methodology

Thirteen water samples, consisting of six from mine drainage and nearby wetlands prefixed drainage water (DW), six groundwater samples from irrigation pumps and hand-dug wells prefixed groundwater (GW) and 1 sample from coal mine subsidence areas (MSW-1) prefixed surface water (SW), during the time of January, 2016. Samples were collected in plastic bottles which were pre-conditioned with 5% nitric acid and rinsed with distilled water. The DW samples were collected from the outlet of mine. Total Dissolved Solids (TDS) and Electrical Conductivity (EC) were measured with portable meter equipped with membrane electrode (Model: HANNA HI 2300) while pH and Dissolved Oxygen (DO) were measured with bench type pH meter (Model: Jenway 3510) and DO meter (Model: HANNA HI 2400) respectively. The collected water samples were preserved in a refrigerator at 4°C for further elemental analysis. Heavy metal analysis was performed by Atomic Absorption Spectrophotometer (AAS) (Model: Varian AA240). All the Chemical analysis of water samples were performed in the Laboratory of the Institute of Mining, Mineralogy and Metallurgy (IMMM), Bangladesh Council of Scientific and Industrial Research (BCSIR), Joypurhat.

Usually, pollution indices are estimated for a specific use of the water under consideration. The indices used in this study, namely heavy metal pollution index (HPI), heavy metal evaluation index (HEI) and degree of contamination (C_d) are determined for the purpose of evaluating water pollution both drinking and agricultural use, where the formulas deal with the similar characteristics of heavy metals. The HPI and HEI methods provide an overall quality of the water with regard to heavy metals. On the other hand, in the C_d method, the quality of water is evaluated by computation of the extent of contamination.

The HPI method was developed by assigning a rating or weightage (W_i) for each chosen parameter and selecting the pollution parameter on which the index was to be based. The rating is an arbitrary value between zero and one and its selection reflects the relative importance of individual quality considerations. In this study, the concentration limit (i.e., the highest permissible value for drinking water, S_i) is taken from the both international (WHO and FAO) and regional (Indian and Bangladesh) standards ^{10, 11, 12, 13}. The uppermost permissive value for drinking water (S_i) refers to the maximum allowable concentration in drinking water in absence of any alternate water source. The HPI, assigning a rating or weightage (W_i) for each selected parameter, is determined using the expression below ^{14, 15}:

Where Q_i and W_i are the sub-index and unit weight of the ith parameter, respectively, and n is the number of parameters considered.

The sub-index (Q_i) is calculated by

Where V_i, and S_i are the monitored heavy metal and standard values of the ith parameter, respectively. While Prasad and Bose ⁷ considered unit weightage (W_i) as a value inversely proportional to the maximum admissible concentration (MAC) of the corresponding parameter as proposed by Siegel ¹⁶.

HEI, like the HPI, gives an overall quality of the water with respect to heavy metals ¹⁷, and is computed as:

Where, H_c and H_mac are the monitored value and maximum admissible concentration (MAC) of the ith parameter, respectively.

The contamination index (C_d) summarizes the combined effects of several quality parameters considered harmful to household water ¹⁸ and is calculated as follows:

Where,

C_fi, C_Ai and C_Ni represent contamination factor, analytical value and upper permissible concentration of the ith component, respectively and N denotes the ‘normative value’. Here, C_Ni is taken as MAC.

4. Results and Discussion

General characteristics of groundwater physicochemical parameters for the study area are summarized in Table 1. All the samples (n = 13) showed the pH values ranged from 6.5 and 7.5 with a mean value of 7.3 in drainage water and 6.7 in ground water, indicating acidic to slight alkaline in nature.

The EC was found higher in drainage water (335-339µs/cm) than ground water (149-246µs/cm) samples. Considering the EC values, this water is suitable for irrigation. A similar finding was found in a study of Sultana et al ¹⁹. Total dissolved values of DW at all points cross the BMAC limit (100ppm). In case of ground water (GW-1, GW-4 and GW-6) three points was over BMAC limit.

Turbidity values of DW (116-45.3 FTU) were so high compare to BMAC limit value. According to maximum permissible limit of DO is 6mg/l. In this sense, DO values in both DW and GW were suitable for irrigation. One sample of mine subsidence area (MSW-1) showed similarity (DO, Turbidity) with ground water but have some exception in EC and TDS value.

The mean concentration of heavy metals is followed the descending order: Fe>Pb >Co>Mn>Cd>As. However, the mean value of Fe, Mn, Pb, Cd are higher than the water quality standards set by Bangladesh Standard (1997), Indian standard (2012) and international organization WHO (2011); FAO (1972). It is found that most of the groundwater samples showed the high concentrations of Fe, Pb , Mn, Co and Cd values in the study area.

The results of pollution evaluation indices are shown in Table 3. The heavy metal pollution indexes was computed using the Indian Standard (2012), Bangladesh Standard, (1997) and international organization FAO (1972) and WHO (2011) standards and were represented by HPI^a and HPI^b, HPI^c and HPI^d respectively. The range and average values of HPI^a for the drainage water and groundwater samples were 987.24–1641.01 and 1262.52, and 1202.85-1876.20 and 1497.47 respectively. The range and average values of HPI^b for the drainage water and groundwater samples were 954.74–1618.95 and 1223.89, and 1164.22-1804.31 and 1461.07 respectively. The range and average values of HPI^c for the drainage water and groundwater samples were 1044.12–1731.92 and 1314.12, and 1448.32-2050.17 and 1589.00 respectively.

In this study, the existing Contamination levels for HPI, HEI and C_d have also been categorized according to Bhuiyan et al ²⁰ at Table 4. The HPI^a, HPI^c, HPI^d and C_d are consistent in showing that the drainage water, mine subsidence water and ground water samples fall in the categories of high contamination (Table 4).

The degree of contamination (C_d) ²¹ was used as a reference of estimating the extent of metal pollution. The range and mean values of C_d of the drainage water and groundwater samples were respectively 51.75–110.99 and 81.27, and 75.89–121.80 and 101.86, with about 50% of the samples falling below the mean values in both cases. C_d may be classified into three categories ^{17, 18} as follows: low (C_d < 1), medium (C_d = 1–3) and high (C_d > 3). All the drainage water, groundwater and surface water samples have high C_d values (all analyzed samples exceed 3) suggesting that they are highly polluted.

Following the approach of Edet and Offiong ¹⁷ the values of HEI in the present study have been classified in terms of pollution levels as low, medium and high. Different HEI criteria values have been developed for surface water and groundwater, guided by their respective mean values, and the different levels of contamination are demarcated by a multiple of the mean values. HEI criteria for the surface and ground water samples using the scheme Bhuiyan et al. ²⁰, surface water (drainage water and mine subsidence water) samples show low contamination, whereas groundwater samples GW-4 and GW-5 shows low and GW-1, 2, 3 and 6 are moderately contaminated.

Geostatistical modeling for spatial distribution of the groundwater parameters has done by inverse distance weighting interpolation method; using Arc GIS (10.2 versions). The sample locations are shown in Figure 2.

The spatial map of HPI scores demonstrated a complex distribution pattern (Figure 3). The discrepancies were found along the mine drainage line of the study area. The spatial map of HPI values indicated that high values were observed in both side of the mine drainage water line. Poor quality water could be happened due to leaching of ions, over exploitation of groundwater, direct discharge of effluents from mine drainage and agricultural impact.

The C_d and HEI exhibit more or less similar distribution patterns with an increasing trend in the southern direction, which suggested the existence of similar point sources. However, it has been suggested that anthropogenic sources are likely to be attributed the high scores of the HEI and C_d in the study area.

5 Conclusions

The study shows that the drainage water and ground water of surrounding areas of Barapukuria Coal Mine area exhibits high concentration of heavy metals like Cd, Pb, Mn and Fe. The contamination index C_d place water quality in high contamination level and heavy metal pollution index HPI on the other hand consider the level of contamination critical. The concentrations of the physicochemical parameters and heavy metals in most DW and GW samples exceed the Bangladesh and international standards. The possible source of these harmful elements (e.g., Mn, Co, Pb, Cd) released from mine effluent have a high potential of contaminating the important water reservoirs within and outside the mining area in the irrigation zone. Due to heavy metal contamination some diseases like Itai-Itai, Arsenicosis, and skin diseases can be occurred in that area. In fact, the contamination of the water systems by heavy metals poses serious threat to human health and ecological habitat in the study area, which therefore require attention.

Acknowledgements

The authors are indebted to the authorities of the Institute of Mining, Mineralogy and metallurgy (IMMM), Bangladesh Council of Science and Industrial Research (BCSIR), Joypurhat.

References