Persistence of heavy metals and human health risk assessment in the South Indian industrial area

Groundwater has become a major and important mineral worldwide due to its reliability and quality for various applications such as drinking, irrigation, and industrial applications (Wang et al. 2020). Continuous monitoring of these valuable resources is essential due to contamination from various sources, particularly heavy metals to industrial expansion and uncontrolled development activities associated with groundwater exploitation (Hussain Alfaifi et al. 2021). Trace elements are chemical elements and are known as heavy metals because of their higher density greater than 5 g/cc that is generally found in relatively small concentrations in soil and water, but may be potentially harmful to organisms if present in high concentrations due to toxicity, persistence, and bio-accumulation (Pourret & Bollinger 2018; Hussain Alfaifi et al. 2021). Heavy metals are detrimental to human health, and exposure to these metals has increased by modern industrialization (Herojeet et al. 2020; Balali-Mood et al. 2021). The vast development of industrial zones is vulnerable to soil pollution with metals, which significantly increases the risks to human health (Adimalla et al. 2020). Bio-accumulation of heavy metals leads to diverse toxic effects on human body tissues and organs (Balali-Mood et al. 2021). Many studies on heavy metal contamination in the soils in prominent industrial areas reported significant deterioration of soil and water quality (Harikrishnan et al. 2016). To evaluate pollution levels and their effects on the environment and human health, metal contaminations in surface dust from industrial and urban regions are being examined globally (Hu et al. 2013; Pathak et al. 2015; Yang et al. 2015; Ahmed et al. 2016; Krishna & Mohan 2016; Gabarron et al. 2017; Adimalla & Wang 2018; Khademi et al. 2019). According to recent investigations, heavy metal contamination in soils and groundwater has become a serious problem for the ecosystem and human health, specifically with the development of industrialization (Adimalla et al. 2019; Adimalla 2020; Wang et al. 2019). Several researchers have reported that infants and children are at high risk of soil contamination with these heavy metals (Jiang et al. 2017; Kusin et al. 2018; Yang et al. 2018). Different indexes are used widely to assess the level of contamination that includes the geo-accumulation index (I_geo), which assess the level of contamination by relating the present concentration of elements in the soil samples with those from pre-industrial times (Herojeet et al. 2020). This approach was developed for bottom sediments (Müller 1981), but it might also be used to assess soil and dust pollution (Li et al. 2015; Qing et al. 2015; Benhaddya et al. 2016; Mathur et al. 2016). For example, Khademi et al. (2019) employed this method to evaluate the heavy metal contamination of soil and dust in the industrial area located in Murcia city, Spain. Monged et al. (2020) used this classification to account for fluctuations of heavy metals in the agricultural soil of the north-eastern Nile Valley, Egypt due to industrial activity. Su et al. (2022) used I_geo in South China Industrial areas. Adimalla et al. (2019) has used it as an indicator for the assessment of the level of heavy metal contamination in the agricultural soil of northern Telangana and uninhibited industrial waste dumpsites in Ibadan, Nigeria. The presence of these heavy metals in soils and groundwater can affect the human body through two different pathways: ingestion and dermal contact (Gu et al. 2020). Metal-contaminated soil and dust can harm human health through skin contact and hand-to-mouth contact, notably through inadvertent uptake by children in playgrounds and city streets (Saeedi et al. 2009; Pan et al. 2018). Mercury (Hg), lead (Pb), chromium (Cr), cadmium (Cd), and arsenic (As) have been the most common heavy metals with potentially dangerous effects on human health. In particular, As and Pb are more harmful and have been linked to both carcinogenic and non-carcinogenic health consequences in humans (Sun & Chen 2016). Long-term exposure to Hg, Cd, As, Cr, Ni, Zn, and Pb can have negative impacts on human health, including nervous and endocrine system issues, kidney and liver damage, and various types of cancer such as lung, stomach, and skin damage (Duruibe et al. 2007; WHO 2007; Li et al. 2013; Gao & Wang 2018). The current study is focused on the highly industrialized area where industrial contamination is reported (Surinaidu et al. 2020). Many inhabitants were living downstream of this industrial area of the study. Hence, the present study is important to understand the associated human health hazards caused by heavy metal contamination in soil and water. In this study, we have evaluated the degree of soil pollution by estimating the geo-accumulation index (I_geo), contamination factor (CF), pollution load index (PLI), and potential health risks to the residents of all age groups (infants, children, teens, and adults) due to long-term exposure to heavy metal-contaminated groundwater.

To quantify the heavy metal contamination in soils, a total of 96 samples were collected from three different depths from 32 locations using a hand soil auger with an 8 cm diameter (Figure 1). The samples were transferred to the laboratory after being packed in polyethylene bags with labels for subsequent analysis. Groundwater samples were collected from 28 locations in the same areas where soil samples have been collected (Figure 1). Before collecting the water sample from the well, the well was pumped out for 5–10 min to remove storage water from the casing. All water sample bottles were thoroughly rinsed with double-distilled water after being cleaned with nitric acid and water (1:1). Then, all samples were analyzed at the VIMTA laboratory over the space of a week.

The soil samples were analyzed by X-ray fluorescence (XRF) and the water samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) following methods of the American Herbal Products Association (APHA 2009). For the estimation of trace (heavy) metals, solutions were prepared for ICP-MS analysis. 0.5 g of each sample was placed in a 14 mm diameter polyethylene tube, followed by 8 ml of HNO₃ 65%, 5 ml of HCl 37%, and 5 ml of H₃BO₃ 5%. All samples were shaken for 40 min in a rotary shaker. Following the extraction technique, the solution was filtered in a 100 ml volumetric flask using Whatman No. 42, ash-free filter paper (125 mm diameter). Purified water of 18.2 MΩ/cm was used to bring the volume up to 100 ml. The resultant solution was labeled and stored in a high-density polyethylene (HDPE) bottle. For the XRF spectrometry analysis, soil samples were dried at 60 °C for 2 days, then the dried sample was crushed using a mortar and pestle. The finely powdered sample was collected in 250 mesh size (US standard) using a swing grinding mill. Sample pallets were prepared using a backing of boric acid and pressed at 25 tons of pressure. All soil and water sample analyses were performed at Vimta Labs Ltd in Hyderabad. In the present study, Mn, B, Zn, Cr, Pb, Ni, Cd, As, and Hg were analyzed in both soil and water. Descriptive analysis of chemical parameters in soil and water samples (mean, median, minimum, maximum, standard deviation, and upper and lower quartiles) was performed in Microsoft Excel (Table 1).

Table 1

Categorization of soils based on pollution indices

Index and values .	Equation and classification .	References .
Geo-accumulation index (Igeo)		Muller (1969)
≤ 0	Practically no contamination	Men et al. (2018)
0 < < 1	No contamination to moderate contamination	Khademi et al. (2019)
1 < < 2	Moderate	Adimalla et al. (2019)
2 < < 3	Moderate to heavy	Monged et al. (2020)
3 < < 4	Heavy	Adimalla et al. (2020)
4 < < 5	Heavy to extreme	Su et al. (2022)
5 <	Extreme contamination
Contamination factor (CF)		Hakanson (1980)
< 1	Low contamination factor	Loska et al. (2004)
1 ≤	Moderate
3 ≤	Considerable	Aguilera et al. (2021)
6 ≤	Very high
Pollution load index (PLI)		Tomlinson et al. (1980)
PLI < 1	Unpolluted
1 < PLI < 2	Moderate	Monged et al. (2020)
2 < PLI < 10	Strong	Aguilera et al. (2021)
PLI > 10	Extreme

Index and values .	Equation and classification .	References .
Geo-accumulation index (Igeo)		Muller (1969)
≤ 0	Practically no contamination	Men et al. (2018)
0 < < 1	No contamination to moderate contamination	Khademi et al. (2019)
1 < < 2	Moderate	Adimalla et al. (2019)
2 < < 3	Moderate to heavy	Monged et al. (2020)
3 < < 4	Heavy	Adimalla et al. (2020)
4 < < 5	Heavy to extreme	Su et al. (2022)
5 <	Extreme contamination
Contamination factor (CF)		Hakanson (1980)
< 1	Low contamination factor	Loska et al. (2004)
1 ≤	Moderate
3 ≤	Considerable	Aguilera et al. (2021)
6 ≤	Very high
Pollution load index (PLI)		Tomlinson et al. (1980)
PLI < 1	Unpolluted
1 < PLI < 2	Moderate	Monged et al. (2020)
2 < PLI < 10	Strong	Aguilera et al. (2021)
PLI > 10	Extreme

is the measured concentration of the heavy metal (n); is the background concentration or reference value of the measured heavy metal ‘n’; is the single element index.

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Soil contamination can be evaluated by comparing contemporary metal concentrations to pre-industrial levels. Geo-accumulation index (I_geo), which is proposed by Muller (1969) for identifying and defining metal pollution in soil and sediments, is used in the study. In the I_geo equation, factor 1.5 is used because of possible variations in the background values of a particular metal in the environment, i.e., lithogenic influence, as well as possible limited anthropogenic effects (Egbueri et al. 2020; Monged et al. 2020). Using this analysis, the soil can be classified into seven groups based on I_geo (Table 1). The CF was determined using the Hakanson (1980) model, which is the ratio of the metal concentration in the sample collected to the background concentration. The background values for evaluated metals are (in mg/kg) taken from Taylor and McLennan (1995) and are 600 for Mn, 15 for B, 71 for Zn, 35 for Cr, 20 for Pb and Ni, and 0.1 for Cd. The PLI was derived using CF values (Tomlinson et al. 1980), to determine the level of metal contamination in the soil (Table 1).

The parameters used (USEPA 1989, 1992; Mondal et al. 2012; Mukherjee et al. 2019) in quantifying human health risk assessment are presented in Supplementary Table S2.

IngR is the groundwater ingestion rate in L/day; SA is the exposed skin surface area (cm²); is the ith target heavy metal's dermal permeability coefficient (cm/h) (USEPA 1992); F is the proportion of the skin's contact surface with groundwater (no units); EF (day/year), ET (h/day), and ED (years) are the exposure frequency, groundwater exposure time, and exposure duration, respectively; BW_A and ET_A are the average body weight of the exposed individual (kg) and average exposure time (day), respectively; CF is the volumetric conversion factor for groundwater (1 L/1,000 cm³).

RfD is the reference dose (mg/l/day), ‘i’ is the number of exposure pathways, and IRIS (https://www.epa.gov/iris) was used to acquire the RfDs for each heavy metal. HQ is the hazard quotient and HI is the hazard index which represents the sum of the HQ's for the two exposure pathways. If HI > 1, there is a high possibility to incur adverse non-carcinogenic health risks to humans, while HI < 1 has no obvious health risk to humans (USEPA 2002).

SF is the cancer slope factor for two-exposure pathways to ith target heavy metals, respectively, and SF values for oral exposure were taken from the USEPA regional screening tables (USEPA 2011). CR is the carcinogenic risk of two-exposure pathways and TCR is the total carcinogenic/cancer risk. According to USEPA (2002), if the value of (CR and TCR) < 1 × 10⁻⁶, the carcinogenic risk can be negligible or has no effect on the human body; if 1 × 10⁻⁴ < (CR and TCR) < 1 × 10⁻³, it indicates moderate risk; if the value is observed in the range of 1 × 10⁻³ < (CR and TCR) ≤ 0.1, it signifies a higher risk of cancer due to exposure to heavy metals.

In groundwater, the heavy metal concentrations are in the order of Mn > Zn > Ni > B > Pb > Cr > Cd. This indicates that the concentrations of Mn, Cd, and Pb are abundant in the groundwater of the study region and also exceeded the geo-chemical background values in mg/l, which were Mn – 0.1; Cd – 0.003; Pb – 0.01; Cr – 0.05; Ni – 0.02; B – 2.4; Zn – 3. At some locations (S24, S20, S16, S10, and S21), groundwater shows high concentrations of Mn, B, Pb, Ni, and Cd, which may be attributed to leaching metals from soil to water. The heavy metals can be accumulated in the soil and further continuous load of the contaminants with groundwater recharge can enrich the heavy metal concentrations in groundwater. Metals can be expected to be mobilized as groundwater flows along flow pathways and over the metal-contaminated upper soil layer, slowly contaminating groundwater over decades (Atyen et al. 1980; Gurunadha Rao et al. 2001). It is also observed that higher concentrations of heavy metals are noticed in groundwater where the groundwater is very shallow <3 m bgl that shows the faster leaching of heavy metals due to less soil thickness on the top of the water table. The shallow groundwater regions or groundwater mounds were created in the region due to the occurrence of crystalline rock at shallow depth, local point source recharge, and no groundwater pumping that tends to leach pollutants in groundwater in these regions (Surinaidu et al. 2020).

Human health risk assessment of groundwater trace elements (Mn, B, Zn, Cr, Pb, Ni, Cd) through dermal and ingestion was performed for infants, children, males, and females in the study region. The HI and total cancer risk (TCR) for non-carcinogenic and carcinogenic health risks were summarized in Table 5.

The computed mean value for HI of non-carcinogenic risk is less than 1 for all the age groups, signifying that the concentration of groundwater heavy metals does not have a harmful effect on the human body in terms of non-carcinogenic risks. However, the mean value of TCR in all age groups indicates lower to moderate risk since the value is ranging between 1 × 10⁻⁴ and 1 × 10⁻³. But infants and children are prone to higher risk at >20% locations. Hence, any use of groundwater for any domestic purpose without proper treatment should be avoided. Surinaidu et al. (2020) reported a reduction in pollution load after the implantation of remedial measures in the study area. However, a complete assessment of groundwater quality and soil quality may be assessed for further management and protection of soil and groundwater contamination.

The present study has been carried out to evaluate the impact of industrialization impact on soil and groundwater pollution and associated human health risks in the South Indian industrial watershed. The study results indicated that the mean concentration of heavy metals at all depth levels of soil is in the order of Cd > Ni > Cr > Pb > B > Mn > Zn and in groundwater Mn > Zn > Ni > B > Pb > Cr > Cd. Soils are not contaminated by Mn and Zn in all three depth levels of soils. The study results also indicated moderate contamination of soils by Cr, Pb, and Cd. However, less pollution load is observed at deeper depths (80 cm). The groundwater is contaminated at a few locations and it is only restricted to shallow groundwater table locations due to underlaid crystalline rock at shallow depth. The health risk assessment revealed the carcinogenic risk of heavy metals of moderate risk for all age groups and very high risk to infants and children at >20% of locations. The outcome of this study will help the local community and regulatory authority to understand heavy metal contamination and the threats it poses to human health, as well as affording scientific baseline data for managing groundwater in the area. In our first paper (Surinaidu et al. 2020), we have explained how to demarcate the pollution sources and relevant remedial measures for the same study area that can be applied elsewhere, and the present study can help to understand the impacts of such pollution persisting in groundwater.

The authors are thankful to Prof V. M. Tiwari, Director of CSIR-National Geophysical Research Institute, for his kind permission to publish this paper. The generous financial support of the Model Industrial Association and their help during the field investigation are highly appreciated, and the encouragement of the Telangana State Pollution Control Board is acknowledged.

M.J.N. and L.S. designed the study, and conceptualized and executed the project. K.A., F.B., and U.A. collected samples. K.A. and F.B. analyzed the data and prepared the first draft; L.S. did extensive editing.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.