Salient features of major ion chemistry
Table 2 summarizes results of the various physical and chemical parameters including statistical measures such as minimum, maximum, average and standard deviation analysed groundwater samples from the study area. TDS values ranges from 530 to 10200 mg/l, with an average value of 3016 mg/l. The High concentration of TDS in the groundwater sample is due to leaching of salts from soil and also by anthropogenic activities. The EC value is measured in micro-semens per centimetre and is a measure of salt content of water in the form of ions. The EC values range from 350–19,100 μs/cm with an average value 4887 μs/cm. The spatial distribution of EC in groundwater of the study area is shown in Fig. 3. The Not Permissible (NP) limit of EC (3125 mg/l) was observed towards N–NE, N–NW and central directions. Highly soluble rocks contribute more ions and impart high conductivity. To determine the suitability of groundwater of any purposes, it is essential to classify the groundwater depending upon their hydrochemical properties based on their EC values (Selvam et al. 2014b, c). The negative logarithm of hydrogen ion concentration (pH) ranges from 7.1 to 10.2 with an average value 7.7. The pH value as low as 7.1 was recorded in Meelavitan and the highest was found in Jothi nagar near Sankarapari with a value of 10.2. This shows that the groundwater of the study area is dominantly alkaline in nature. The slight alkalinity may be due to the presence of bicarbonate ions, which are produced by the free combination of CO2 with water to form carbonic acid, which affects the pH of the water (Azeez et al. 2000). Amongst the cations, the concentrations of Na, Ca, Mg and K ions range from 27 to 1400, 11 to 570, 15 to 442 and 5 to 400 mg/l, with a mean of 409.00, 139.60, 118.00 and 63.00 mg/l during PRM seasons. The order of abundance of chemical concentration is Na+ > K+ > Mg2+ > Ca2+, respectively, during the PRM seasons. For the anions, the concentrations of HCO3, Cl, SO4, NO3, F and PO4 range from 0 to 756, 36 to 5885, 19 to 1272, 0 to 14, 0.16 to 4.8 and 0 to 0.1 mg/l with a mean of 293.25, 899.41, 354.72, 5.65, 0.76 and 0.1 mg/l, respectively. The order of abundance of chemical concentration is Cl− > SO4
2− > F > NO3 > PO4 during the period (Fig. 4).
Heavy metal distribution
For the protection of human health, guidelines for the presence of heavy metals in water have been set by different International Organisations such as United States Environmental Protection Agency, World Health Organization (WHO) and the European Union Commission (Marcovecchio et al. 2007). Thus, heavy metals have permissible limits in water as specified by these organizations. The summary of the heavy metal results of laboratory analyses conducted on the samples are in Table 1.
The concentration of lead in groundwater varies from 0.000 to 0.018 mg/l with an average concentration of 0.004 mg/l (Table 3), which is beyond the desirable limit of 0.01 mg/l as recommended by WHO (2004). The lead concentration in groundwater of the study area is within the maximum allowable limit in all the sample locations. The main sources of lead contamination are industrial discharges from smelters, battery manufacturing units, run off from contaminated land areas, atmospheric fall out and sewage effluents.
Arsenic concentration in the groundwater varies from 0.000 to 0.083 mg/l with an average concentration of 0.015 mg/l. The maximum allowable limit of arsenic ion concentration in groundwater is 0.01 mg/l as per WHO 2004 classification. According to WHO standards, 42 % of the samples have exceeded the permissible limits and 58 % of the samples are within the permissible limit (Table 3). Not Permissible limit of arsenic was observed towards North West, North East and central portion (Fig. 5). A higher concentration of Arsenic in the study area is due to industrial waste leaching or percolating through the subsurface. The study area STERLITE is one of the copper industries located near Thoothukudi town which produces copper from copper concentrates. Arsenic trioxide is obtained as a byproduct from dusts and residues that are produced during the treatment of other metal ores such as gold and copper (Puthiyasekar et al. 2010). The high arsenic concentration is due to the anthropogenic activities like poultry waste, brick making and agricultural practices (Selvam et al. 2014a).
Chromium concentration in the groundwater varies from 0.001 to 0.080 mg/l with an average concentration of 0.013 mg/l. As per WHO 2004 standard only one sample exceeds the permissible limit, which may be due to industrial activity. The most common man-made sources of chromium in groundwater are burning of fossil fuels, mining effluent, effluent from metallurgical, chemical and other industrial operations (Leung and Jiao 2006). The risk to human health is through ingestion only—drinking, cooking and teeth brushing. Well water with chromium levels greater than 0.05 mg/l may safely be used for bathing, hand washing and dishwashing (Selvam et al. 2015).
The concentration of iron in the groundwater varies from 0.001 to 0.357 mg/l, with an average of 0.123 mg/l. The maximum allowable limit of iron ion concentration in groundwater is 0.3 mg/l as per WHO 2004 classification. It is found that 85 % of the samples are within the desirable limit and 15 % of the samples have crossed the permissible limit in the area, according to the WHO standard 2004 (Fig. 6). The NP limit of 0.3 mg/l was observed in small patches of central portion due to anthropogenic activity. Iron can be found in meat, whole meal products, potatoes and vegetables. The human body absorbs iron in animal products faster than iron in plant products. Iron is an essential part of haemoglobin, the red colouring agent of the blood that transports oxygen through our bodies. Higher Fe concentrations in the aquifers might have been the results of interaction from oxidized Fe minerals and organic matters and subsequent dissolution of Fe2CO3 at a comparatively lower pH (Mondal et al. 2010). This type of water was clear when drawn from the well, but shortly changes into cloudy and then turns brown due to precipitation of Fe (OH)3. Another reason for high Fe concentration may be due to the removal of dissolved oxygen by organic matter, leading to reduced conditions. Under reducing conditions, the solubility of Fe-bearing minerals (siderite, marcasite, etc.) increase in water, leading to the enrichment of dissolved iron in groundwater (Applin and Zhao 1989; White et al. 1991).
The concentration of Aluminium in groundwater ranges from 0.011 to 0.837 mg/l with an average concentration of 0.084 mg/l. The maximum allowable limit of aluminium ion concentration in groundwater is 0.2 mg/l as per WHO 2004 classification. 88 % of samples fall within the maximum allowable limit, while 12 % of samples exceed the permissible limits (Fig. 7). This figure clearly shows that a not permissible limit of Aluminium (0.2 mg/l) was observed in the central portion and small patches of western portion of study area. The source of Aluminium in the study area groundwater samples may be through the weathering of bedrock and soil or it may be related sources like industries, which introduced Aluminium into groundwater. Aluminium occurs naturally in some rocks and drainage from mines. Aluminium formed during mineral weathering of feldspars, such as orthoclase, anorthite, albite, micas and bauxite, subsequently ends up in clay minerals.
The concentration of selenium in the groundwater varies from 0.000 to 0.149 mg/l with an average concentration of 0.086 mg/l. The maximum allowable limit of selenium ion concentration in groundwater is 0.01 mg/l as per WHO 2004 classification. According to WHO standards, 82 % of the samples exceed the permissible limits and only 18 % (Table 3) of the samples are within the permissible limit (Fig. 8). NP limit of selenium was observed in N–NE, N–NW, S–SE, S–SW direction and central portion. Selenium is a natural heavy metal associated with specific geological formations and in groundwater it occurs as a mixture of selenite and selenate. Diminutive amount of selenium was beneficial, but excess amount was toxic in groundwater. The potential health effects were on hair, finger nail loss and numbness in fingers or toes (Negrel et al. 2004).
The concentration of antimony in the groundwater varies from 0.000 to 0.007 mg/l with an average concentration of 0.001 mg/l. The maximum allowable limit of selenium ion concentration in groundwater is 0.005 mg/l as per WHO 2004 classification. Especially, people who work with antimony suffer the effects of exposure by breathing in antimony dusts. Human exposure to antimony may take place not only by breathing air, drinking water and by eating foods that contain it but also by skin contact with soil, water and other substances that contain it (Sang et al. 2008).
Cadmium concentration in the groundwater varies from 0.000 to 0.002 mg/l with an average concentration of 0.000 mg/l. The range of nickel concentration in groundwater varies from 0.000 to 0.011 mg/l, with an average concentration of 0.005 mg/l. The range of molybdenum concentration in groundwater varies from 0.000 to 0.008 mg/l, with an average concentration of 0.002 mg/l. The concentration of barium in the groundwater ranges from 0.000 to 0.057 mg/l with an average concentration of 0.012 mg/l. The concentration of rubidium in the groundwater varies from 0.000 to 0.850 mg/l, with an average concentration of 0.107 mg/l. In the study area, copper concentration varies from 0.002 to 0.236 mg/l with an average concentration of 0.031 mg/l. Zinc concentration in groundwater of the study area varies from 0.000 to 0.870 mg/l with an average concentration of 0.20 mg/l. Manganese concentration in the groundwater ranges from 0.000 to 0.424 mg/l with an average value of 0.040 mg/l. The concentrations of cadmium, nickel, molybdenum, barium, rubidium, copper and zinc in groundwater are within the maximum allowable limit as per WHO standard.
Cobalt concentration in the groundwater ranges from 0.000 to 0.027 mg/l with an average value of 0.002 mg/l, while Vanadium concentration in the groundwater varies from 0.002 to 0.052 mg/l with an average concentration of 0.010 mg/l. The range of silver concentration in groundwater varies from 0.000 to 0.002 mg/l, with an average concentration of 0.000 mg/l. The concentration of strontium in the groundwater varies from 0.000 to 2.000 mg/l, with an average concentration of 0.504 mg/l. BIS and WHO have not given any guideline value for Cobalt, Vanadium, Silver and Strontium concentration in the groundwater. The strontium concentration was higher in the study area indicating that the source could be anthropogenic through agricultural activities an input of strontium to some extent it depends on the content of fertilisers and carbonate additives and manure like cattle, poultry, etc. (Negrel et al. 2004). Strontium concentrations in soil may also be attributed to dumping waste and industrial wastes. Strontium in soil dissolves in water, so that it would be able to leach deeper into the ground and enter the groundwater.
In microbiological analysis, MPN test was conducted in 11 water samples in order to find out faecal coliforms (total coliform bacteria, faecal coliform bacteria, E. coli and faecal streptococci) through biochemical tests (Table 4; Fig. 9). Further, the samples were screened for Salmonella to study the reliability of faecal indicator bacteria as an index of human pathogenic bacteria.
The test indicated that the presence of Faecal coliform bacteria were detected at the maximum detectable limits and were detected in water samples near costal area (Coastal Beaches), which was found to be the more polluted sampling site. Samples of water from Mappillaiurani Beach, Iniko Nagar, Thirespuram present higher values indicating the contamination of ground water from fishing activity, making the water unfit for drinking purposes (Fig. 10). Total coliform bacteria were high in water samples of all coastal fishing centres and were at the maximum detectable level of over MPN 145 ml−1 and were detected at eastern part (Iniko Nagar) of the study. This may be due to the settlements nearby which lead to defecation along the beaches. Similar trend in total coliform bacteria was observed in Thirespuram, Mappillaiurani Beach, though the maximum level of MPN >140 ml−1 was recorded in the study period (Fig. 10). This might be due to the mixing of sewage from Thoothukudi town through the Buckle channel. E. coli and Faecal streptococci showed greater variation among samples during season and the highest values were recorded in water samples from Fishing old Harbour, Mappillaiurani Beach and Thirespuram. It may be due to the fishing activity which is one of the main reasons for contamination by E. coli and Faecal streptococci in the study area (Fig. 10). Although faecal streptococci gives only supplementary evidence of faecal pollution, they are still considered better indicators than coliforms because of their inability to grow and multiply in water or virgin soils (Vaidya et al. 2001). Faecal indicator bacteria such as total coliforms, faecal coliforms and E. coli that are excreted by humans and warm-blooded animals pass sewage treatment plants in large amounts, and survive, preserving their pathogenicity for a certain time. Salmonella might be isolated in samples of the Thoothukudi study area, even when the concentration of faecal coliform bacteria was low. On the contrary, Geldreich (1972) could establish a relationship and reported that when the faecal coliform was over MPN 20 ml−1, Salmonella was always present in water samples from canal communities along Texas coast (Goyal et al. 1977). However, in this study Salmonella was absent in many samples even when the faecal coliform bacteria were over MPN 145 ml−1.
A number of issues emerge which need to be incorporated into future strategies for groundwater management. Definitions of baseline are given here which can form the basis for distinguishing pristine from polluted waters as well as criteria for identifying modern waters with only traces of human impact from more polluted waters. In common, physico-chemical treatments offer various advantages such as their rapid process, ease of operation and control, and flexibility to change of temperature. Unlike in biological system, physico-chemical treatment can accommodate variable input loads and flow such as seasonal flows and complex discharge. Whenever it is required, chemical plants can be modified. In addition, the treatment system requires a lower space and installation cost. Their benefits, however, are outweighed by a number of drawbacks such as their high operational costs due to the chemicals used, high-energy consumption and handling costs for sludge disposal. However, with reduced chemical costs and a feasible sludge disposal, physico-chemical treatments have been found as one of the most suitable treatments for inorganic effluent (Kurniawan et al. 2006). In wastewater systems containing heavy metals with other organic pollutants, the presence of one species usually impedes the removal of the other. For instance, hydrometallurgy, a classical process to recover metals, is inhibited by the presence of organic compounds and a pre-treatment step, to remove or destroy organics, is generally required; pyrometallurgy which is able to decontaminate systems from organic pollutants and recover metals suffers from lack of controllability, demanding extremely high temperatures. The most promising methods to treat such complex systems are the photocatalytic ones which consume cheap photons from the UV-near visible region. These photo catalysts serve as electron relays, from the organic substrates to metal ions. Thus, they induce both degradation of organic pollutants and recovery of metals in one-pot systems, operable at traces of the target compounds (less than ppm). Regulations are in place to regulate the release of toxic metals in the environment to ensure safety and health of the workers as well as the public in general. The following remedial suggestion can reduce the risk of trace metals and biological contamination in the present study area:
Monitoring of water and soil in the vicinity of the toxic metal processing units needs to be carried out more rigorously for the specific metal.
Recycling/reprocessing of wastes containing toxic metals and biological contamination needs to be given greater emphasis not only from environmental and health considerations but also as a resource conservation measure.
Guidelines for proper management of tailings and slags containing toxic metals should be prepared taking into consideration techno-economic feasibility.
Tailings dumps and process wastes lying in locations close to the processing units need to be remediated on priority.
Health monitoring of workers engaged in the processing of toxic metals/compounds should be carried out regularly.