Groundwater is a major source of fresh drinking water in both rural and urban regions in the developing world. Groundwater quality is deteriorating due to different anthropogenic activities. Current urbanization, industrialization and mining developments release various pollutants to surface and groundwater bodies. Among these, metals are a major pollutant in water sources (Kumar et al. 2003). Metal pollution in water bodies may be from natural (geological) or possibly anthropogenic (from fertilizers, pesticides, untreated sewage and solid wastes) processes (Singh 2005). Groundwater with excess metals is an environmental concern because they are non-biodegradable. Metals tend to accumulate in plants and animals causing chronic adverse effects on human health (Shrivastava and Mishra 2011). Metal contamination of groundwater and its toxicity was studied by various authors (Ramesh et al. 1995; Karatas and Dursun 2006; Obri 2007; Venugopal et al. 2009; Thomas et al. 2011; Ghosh et al. 2012; Hussain and Sheriff 2013). The Heavy Metal Pollution Index (HPI) is an effective method to evaluate the metal pollution of surface and groundwater (Prasad and Sangita 2008; Kumar et al. 2012; Sheykhi and Moore 2012).

Valiathura Sewage Farm is situated in the coastal area of Thiruvananthapuram district and is comprised of 43.706 hectares. The Sewage Farm is maintained by the Dairy Development Department, Kerala state, where fodder grass cultivation is carried out. Sewage water from Thiruvananthapuram city is collected in the sewage pumping station at Kuriathy and then pumped to Valiathura Sewage Farm before being disposed for irrigation. ‘Parvathy Puthanar’, a man made canal is flowing adjacent to the Sewage Farm. After irrigation, the excess sewage water directly drains to the nearby Parvathy Puthanar offering the potential for pollution (Unnikrishnan 2004; Chithra and Jaya 2005). Since the study area is densely populated, people usually dispose their solid wastes to adjacent water sources and open yards.

The main groundwater abstraction structures for domestic and agricultural purposes are dug wells, bore wells and tube wells. In the crystalline and lateritic terrain the groundwater is developed through dug wells and bore wells, while along the coastal alluvium the groundwater is developed through dug wells, filter point wells and tube wells. Since the degree of geological weathering is low, the shallow bore wells vary in depth from 6 to 13 m. The most potential aquifer in the area has an average yield of 10–60 m3 day−1 (CGWB 2008).

The people living around the Sewage Farm depend on various groundwater sources for their domestic needs. A literature review shows no previous studies conducted on the metal content of groundwater in Valiathura. Therefore the present study is an attempt to examine the metal pollution of potable groundwater sources in the vicinity of Valiathura Sewage Farm in Thiruvananthapuram district.

Materials and Methods

The study area is located between longitude 76°54′51″E–76°57′33″ E and latitude 8°26′26″N–8°29′29″N. The location of the study area is provided in Fig. 1. For this study, 42 groundwater bodies were selected which include dug wells (n = 29) and bore wells (n = 13), at an interval of 0.5–2 km in the northern, southern, eastern and western direction of the Sewage Farm during April 2010. Dug wells in the study area were shallow with a total depth ranging from 1.5 to 5.1 m, and the diameter ranged from 0.7 to 1.15 m. The depth of bore wells ranged between 7 and 25 m. Water samples were collected in pre-cleaned polyethylene bottles (1 L) and labeled appropriately. Water samples were carried to the laboratory within an hour and subjected to nitric acid digestion for metal analysis (APHA 1995). To 500 mL water sample, we added 5 mL concentrated nitric acid and digested the sample in a digestion chamber. Afterwards, the sample was made up to 50 mL volume and filtered using 0.45 µm membrane filter paper. Filtered water samples were analyzed for Cu, Zn, Cd, Pb and Fe using an Atomic Absorption Spectrophotometer (GBC 932 AA model, Dandenong, Victoria, Australia). The instrument is equipped with single element hollow cathode lamp and air-acetylene burner. The AAS was calibrated with different concentrations of standard solutions of the metals. Working standards were prepared from the standard stock solutions (Merck, Darmstadt, Germany) of Zn (HCO90983, 1,000 ± 2 mg L−1), Cd (HCO86902, 1,000 ± 2 mg L−1), Fe (HCO81154, 1,000 ± 2 mg L−1), Pb (HCO95295, 1,000 ± 2 mg L−1) and Cu (HCO96351, 1,001 ± 2 mg L−1). Mean values of three replicates were taken for each metal determination. Limits of detection for Zn, Cd, Fe, Pb and Cu were 0.008, 0.009, 0.05, 0.06 and 0.025 µg mL−1, respectively. Correlation analysis was performed using Statistical Package for Social Sciences (SPSS, Version 17). The HPI was calculated using the equation given by Mohan et al. (1996)

$${\text{HPI}} = \frac{{\sum\nolimits_{i = 1}^{n} {W_{i} Q_{i} } }}{{\sum\nolimits_{i = 1}^{n} {W_{i} } }}$$
(1)

where Qi is the sub index of the ith parameter, Wi is the unit weight of the ith parameter and n is the number of parameters considered. The sub index (Qi) of the parameter is calculated by

$${\text{Q}}_{i} = \sum\limits_{i = 1}^{n} {\frac{{\{ M_{i} ( - )I_{i} \} }}{{(S_{i} - I_{i} )}}} \times 100$$
(2)

where Mi is the monitored value of metal of the ith parameter, Ii is the ideal value of the ith parameter, Si is the standard value of the ith parameter. The sign (−) indicates the numerical difference of the two values, ignoring the algebraic sign.

Fig. 1
figure 1

Location map of study area showing Sewage Farm and sampling wells

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HPI was developed by assigning unit weightage (Wi) for each metal. The rating is based on the value between 0 and 1 which reflect the importance of the individual water quality considerations and defined as inversely proportional to the recommended standards for each metal (Mohan et al. 1996; Kumar et al. 2012). For HPI calculation, Indian drinking water standards (BIS 1991) were selected.

Results and Discussion

Results of the metal concentrations in dug wells and bore wells are presented in Table 1. Samples were compared with regional (BIS 1991) and International (WHO 2011) drinking water standards. The Fe content in about 27.5 % dug wells and 38.4 % bore wells were above the desirable limit (0.3 mg L−1) prescribed by BIS (1991). Among the dug well and bore well samples, high Fe concentration was recorded at BW7, which was adjacent to the Sewage Farm and Parvathy Puthanar canal. This may be due to either the leaching of wastewater from the Sewage Farm, houses and Parvathy Puthanar canal or from the geology associated with the groundwater in the study area. Hussain and Sheriff (2013) conducted a study on groundwater samples around the Cooum River at Chennai and reported the concentration of Fe in bore well water samples ranged from 0.55 to 2.74 mg L−1. They concluded the high Fe content in water was due to the discharge of domestic waste water. Effects of exposure to high Fe concentrations include abdominal discomfort, lethargy and fatigue. Excess Fe deposition leads to shrinkage of the liver, followed by fibrosis and cirrhosis. Ingestion accounts for most of the toxic effect of Fe, since Fe is rapidly absorbed in the gastrointestinal tract (Thomas et al. 2011).

Table 1 Metal concentration (mg L−1) in groundwater sources around the Valiathura Sewage Farm
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According to WHO (2011), the permissible limit of Pb in drinking water is 0.01 mg L−1. Current results show that most of the bore well samples and dug well samples (except DW23, DW24, DW25) exceeded the permissible limit of drinking water quality standards. This may be due to the leaching of wastewater from the Sewage Farm and solid waste materials like batteries and pipes dumped near the well in the residential area. Hem (1985) showed sources of Pb contamination in groundwater are from gasoline, pipes, pigments and batteries. Pb has serious cumulative effects and can accumulate in bones, causing nausea, nervous and reproductive disorders and kidney damage in humans (Ramakrishnan 1998; Anonymous 2002). It is harmful to developing brains of fetuses and young children and also pregnant woman (WHO 2011). In the present study, the mean concentration of Cu in dug wells (0.10 mg L−1) and bore wells (0.12 mg L−1) exceeded the desirable limit (0.05 mg L−1) prescribed by BIS (1991). Elevated levels of Cu in drinking water results from corrosion of pipes and fittings (Sonon et al. 2006). Excess Cu in the human body is toxic and causes hypertension, produces pathological changes in brain tissues (Rajappa et al. 2010), and gastrointestinal disorders (Shrivastava 2009). All dug well and bore well samples in the study area were within the guideline value for drinking water quality standards prescribed by WHO (2011) for Zn and Cd.

Pearson’s correlation analysis was applied to study the interrelationship among the metals in groundwater samples in the study area (Table 2). A high positive correlation between Cu and Pb (p < 0.01) implied the corrosion of pipes in the study area as a likely source of Cu and Pb in groundwater. Fe had a strong, positive correlation with Cu and Pb (p < 0.01). The relationship among metals in groundwater samples implied sources of these metals were due to anthropogenic activities such as wastewater from the Sewage Farm, Parvathy Puthanar canal and leaching of solid wastes dumped in the residential area. The sandy aquifer in the study area may facilitate the mobility of metals to the groundwater. The sandy aquifer increases the possibility of interaction between groundwater and polluted surface water (Kumar et al. 2012).

Table 2 Correlation matrix of metals in groundwater around the Valiathura Sewage Farm
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Heavy Metal Pollution Index (HPI) gives the composite influence of metals on the total quality of water (Sheykhi and Moore 2012). The result of the HPI calculations of groundwater and HPI of various sampling wells were presented in Tables 3 and 4. HPI can be classified into three categories: low (<19); medium (19–38); and high (>38) (Kumar et al. 2012). Based on current study HPI values, about 21 samples fall in the low category, 20 in the medium category and only one in the high category (41.79). The maximum HPI value of 41.79 was observed at DW29 which is adjacent to the polluted Parvathy Puthanar canal and Sewage Farm. The lowest HPI value (9.47) was recorded at DW24 (Table 4). This area is 1.9 km from the Sewage Farm and unaffected by anthropogenic activities. The percentage deviation for each sampling wells were calculated from the mean HPI value. The deviation shows the quality of groundwater in the study area. Negative deviation indicates better quality and positive deviation indicates poor quality of groundwater. The study revealed that about 41.4 % dug wells and 38.5 % bore wells show positive deviation which indicates that the quality of water is deteriorated.

Table 3 HPI calculations for groundwater compared with BIS permissible limits (1991)
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Table 4 Heavy metal Pollution Index (HPI) of sampling wells around the Valiathura Sewage Farm
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The study concluded that the majority of groundwater samples in the study area are contaminated with metals like Fe, Pb and Cu, and this may be due to the leachates entering into the water bodies from the Sewage Farm, Parvathy Puthanar canal and solid wastes dumped in the residential areas. The consumption of this water by people without prior treatment may cause different health problems. The study recommends that the groundwater sources in the study area need regular water quality monitoring, and should further monitoring result in continued exceedances, appropriate action should be taken.