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Human health risk assessment of heavy metal and pathogenic contamination in surface water of the Punnakayal estuary, South India

Variation in levels of toxic heavy metals in river system during the COVID-19 pandemic lockdown might potentially assist in development of a public health risk mitigation system associated with the water consumption. The water quality of Punnakayal estuary in the Thamirabarani River system from the south India, a vital source of water for drinking and domestic purposes, industrial usage, and irrigation was assessed here. A comparitive assessment of physico-chemical variables (pH, EC, TDS, DO, BOD, turbidity and NO3), microbiological parameters (total coliform bacteria, fecal coliform bacteria, fecal streptococci and escherichia coli) and toxic metals (As, Cr, Fe, Cu, Zn, Cd, and Pb) suggested a decrease of 20% in the contaminant ratio during the lockdown period in comparison to the pre-lockdown period. The Health risk assessment models (HQ, HI, and TCR) highlighted carcinogenic and non-carcinogenic hazards for both children and adults through the ingestion and dermal adsorption exposures. The HI values for both As and Cr exceeded the acceptable limit (>1) during the lockdown period, but the potential risk for children and adults remained low in compaisio with the pre-lockdown period. Our results suggested that the Thamirabarani River system remained hostile to human health even during the lockdown period, and it requires regular monitoring through a volunteer water quality committee with private and government participations.

• The study measured hazardous heavy metal in water during the COVID-19 lockdown. • Order of heavy metal enrichments (Zn > Fe > Cu > As > Cr > Pb > Cd) in both phases. • TCR values also demonstrated "higher risk of cancer" in children and adults. • The study suggested that the Thamirabarani River system remained hostile to human health. Variation in levels of toxic heavy metals in river system during the COVID-19 pandemic lockdown might potentially assist in development of a public health risk mitigation system associated with the water consumption. The water quality of Punnakayal estuary in the Thamirabarani River system from the south India, a vital source of water for drinking and domestic purposes, industrial usage, and irrigation was assessed here. A comparitive assessment of physico-chemical variables (pH, EC, TDS, DO, BOD, turbidity and NO 3 ), microbiological parameters (total coliform bacteria, fecal coliform bacteria, fecal streptococci and escherichia coli) and toxic metals (As, Cr, Fe, Cu, Zn, Cd, and Pb) suggested a decrease of 20% in the contaminant ratio during the lockdown period in comparison to the pre-lockdown period. The Health risk assessment models (HQ, HI, and TCR) highlighted carcinogenic and non-carcinogenic hazards for both children and adults through the ingestion and dermal adsorption exposures. The HI values for both As and Cr exceeded the acceptable limit (>1) during the lockdown period, but the potential risk for children and adults remained low in compaisio with the pre-lockdown period. Our results suggested that the Thamirabarani River system remained hostile to human health even during the lockdown period, and it requires regular monitoring through a volunteer water quality committee with private and government participations.

Introduction
The outbreak of corona virus or COVID-19 has led to a complete lockdown in 213 countries (World Health Organization, 2020;Bherwani et al., 2020;Gautam and Trivedi, 2020;Sivakumar, 2020). The first case of COVID-19 was reported in China (Travaglio et al., 2021;Kachroo, 2020;Selvam et al., 2020a;Sinha, 2020) and subsequently, it caused an epidemic of acute respiratory syndrome (SARS-CoV-2) all over the globe (Gautam and Hens, 2020). Till now, India has a total number of cases of 1,32,05,926 with a mortality rate of 1.28% i.e., 1,68,436 deaths. The Tamil Nadu state in south India has 33,659 active cases with a mortality rate of 1.40% i.e., 12,863 deaths (Source: https: //www.mygov.in/ covid-19/). This epidemic has also caused irreversible changes to the socio-economic and environmental conditions as the prolonged closure of various industrial edifices had detrimental effects on the economic prosperity. However, the progressive effects on different ecosystems and environment were applauded. For example, Selvam et al. (2020a, India), Lian et al. (2020, China), Nakada and Urban (2020, Brazil), Ropkins and Tate (2021, UK) and Stratoulias and Nuthammachot (2020, Thailand) documented better air quality. Similarly, the upgrades in surface water qualities was noticed by Selvam et al. (2020b, India, Qiu et al. (2020, and Kassem and Jaafar (2020, Lebanon). Dutta et al. (2020) and Patel et al. (2020) documented water quality improvements in the Ganges and Yamuna rivers as a result of the lockdown in India.
The water pollution has been evaluated through estimation of heavy metals (Selvam et al., 2020a). It has two vital sources, i.e. (i) natural (ii) anthropogenic. The natural resources include metals released from rock weathering and their eventual discharge to the water bodies (Ravindran and Selvam, 2014). The anthropological sources include the emission of heavy metals by emancipation of industrial effluents, fossil fuels/industrial burning, and discharge of sewage into the surface water bodies (Selvam and Sivasubramanian, 2012;Singaraja et al., 2015;Panneerselvam et al., 2021).
Recent studies have reported that human health risk assessment using Heavy Metal Pollution Index (HMPI), and Heavy Metal Toxicity Load (HMTL) approaches, carcinogenic and non-carcinogenic health risk approach and human health risk index analysis. The chronic exposure to low concentrations of metals like Pb, Cd, Cr might cause brain and kidney damage, and other chronic kidney disease (CKD). Heavy metals like Cd, Pb, Cu, Ni, U, As, and Fe, are the main nephrotoxic heavy metals that can cause tubular damage and glomerulopathies (Bineshpour et al., 2020;Karaouzas et al., 2020;Mukherjee et al., 2020;Proshad et al., 2020;Tokatli and Ustaoglu, 2020).
Thamirabarani River system is one of the vital water sources in the state of Tamil Nadu (south India) for drinking, irrigation and middle scale industrial usages (Source: https://en.wikipedia.org/wiki/Thamira barani_River). Past researches in this region have reported heavy metal contamination in this river and related estuaries through the industrial (tannery) effluent, municipal sewage effluent, and urban development Singaraja et al., 2015;Muthukumaravel et al., 2021). Therefore, the general assumption was that the reduced activities of above sources during the lockdown would have improved the surface water quality of the Thamirabarani channel. However, there has been no study to assess the effects of COVID-19 lockdown. Thus, the purpose of this study was to (i) estimate hydro-chemical variances between the lockdown and pre-lockdown periods, (ii) assess the pollution vulnerability of industrial and anthropological demeanor on surface water quality, (iii) measure possible health hazards to children and adults from domestic, irrigational and livestock usages of surface water during the lockdown and their differences with the pre-lockdown period, (iv) identify the pollution sources and estimate the reduction percentage of toxic heavy metals, and (v) provide the community with suggestions and solutions to protect the water system through eco-friendly environmental activities. Fig. 1. Location map of the study area along with sampling points.

Study area
The Thamirabarani River basin is located in the coastal district of Thoothukudi, Southern Tamil Nadu, India. This district was detached from the adjacent Tirunelveli district in 1986 on the basis of primary augmentation of industrial efficiencies and relevant coastal economic evolutions (Selvam et al., 2013). Geographically, it extends between N 8.5838889-8.6194444 latitude and E 77.9225-78.1297222 longitude and includes 223.32 sq.km of the total delta basin (Fig. 1). The Thamirabarani River originates from the Pothigai hills of the Western Ghats and flows through the districts of Thoothukudi and Tirunelveli before joining the Bay of Bengal. It provides water for irrigation and electricity production (Arisekar et al., 2020). The primary crops include paddy, banana, ground nut, brinjal, ragi, sorghum, coconut, pulses, ginger, tea and rubber in upper reaches of the river course (Arisekar et al., 2018).
The delta region is underlined by the Archaean gneisses followed by granites and charnockites . Alluvium (Quaternary), Dune Terri sands and Tertiary sediments outcrop in the delta region and the marine and fluvial-marine deposits are present along the coast (Narayanaswamy and Lakshmi, 1967;Magesh et al., 2016;Satheeskumar et al., 2020). The primary land use encompasses agricultural lands, barren lands and salt pans other than emigrations and aquifers (Satheeskumar et al., 2020). Both the groundwater and surface water are vulnerable to anthropogenic activities related to fishing and tourism , both of which generated annual revenue of 8.9 crores in 2017-2018 (source: http://www.townpanchayat.in/ti ruchendur). All chemical plants are pertinent to salt industry, petrochemicals and plastics industry . Small scale industries belong to paper, soft drink manufacturing, textile, PVC pipe manufacturing, and soap manufacturing.

Sampling and analysis
A total of 20 water samples were collected uniformly from the Punnakayal estuary of Thamirabarani River basin, and subsequently located in Fig. 1 using GPS (HANNA 2130). These samples were collected in the pre lockdown period (28-29 January 2020), and during the COVID-19 lockdown (6-7 May 2020). The water samples were collected from 10 cm below the water surface and stored in 2 L polyethylene containers. As per the guidelines of American Public Health Association (APHA), we used ultrapure HNO 3 for on-site acidification (pH < 2) to avoid microbial activity and adsorption/precipitation on the bottle walls (APHA, 1995). Different physical parameters such as pH, total dissolved solids (TDS), electrical conductivity (EC), turbidity, and dissolved oxygen (DO) were measured using deluxe water and soil analysis kit (model no: 191).
In laboratory, the UV-visible spectrophotometer (Systronic), quantified NO 3 and the AAS (Atomic Absorption Spectroscopy Perkin Elmer, Elan Drce) measured the absorptions of metals (Cr, Cu, Fe, Cd, Pb and Zn) and metalloids (As) by using The NIST (National Institute of Standards and Technology) standard (1640a) for the QA/QC (Quality Assurance and Quality Control) resolution."Cetripur" was used for the multi-element (Merck) calibration. The MPN (Most Probable Number) method (ISO, 2000) and estimation of total coliform, fecal coliform, Escherichia coli, and fecal streptococci determined the microbial quality. The MPN method also estimated the number of coliforms of lactose enzymes that produce gas per 100 ml of water sample.

Heavy metal pollution index (HMPI)
HMPI comprehensively assessed the influence of each dissolved heavy metal on overall surface water quality (Mohan et al., 1996;Vetrimurugan et al., 2016;Jorfi et al., 2017;Wagh et al., 2018;Rezaei et al., 2019;Karaouzas et al., 2020). It was computed using the formula: Where n and W i refer to number of heavy metals and unit weight of the ith heavy metal, respectively. In step 1, the sub-index (Q i ) of ith heavy metal was computed using equation (2): Where M con (μg/L) refers the computed value of ith heavy metal. S i is the standard permissible of ith metal for drinking purpose (World Health Organization, 2017) for the heavy metals (μg/L) and I i refers to the ideal limits of ith heavy metal. In step 2, the unit weight (W i ) of each metal was assessed using the equation (3): Where k refers to proportionality constant and it is considered as 1 for all the metals (Wanda et al., 2012;Qu et al., 2018). Finally, HMPI was computed using the Eq. (1) and it classified the heavy metal pollution in surface water bodies into three categories such as low contamination (HMPI<15), medium contamination (HMPI: 15-30) and high contamination (HMPI>30, Edet and Offiong, 2002;Wanda et al., 2012;Qu et al., 2018;Zakir et al., 2020).

Human health risk index analysis
Consumption of drinking water contaminated with toxic metals increases the risk of non-carcinogenic and carcinogenic diseases in humans (Bineshpour et al., 2020: Qu et al., 2018. We utilized the methods specified by U.S. Environmental Production Agency (USEPA) to appraise the non-carcinogenic (As, Cr, Fe, Cu, Zn, Cd, and Pb) and carcinogenic risks from the dissolved metals (As, Cr, Cd, and Pb) by  (2004) initiated a health risk technique for measuring the non-cancerous human health risks from the heavy metal elements in groundwater and surface water through ingestion, inhalation and exposure to skin. Primarily, the risk was caused by direct water intake and absorption or skin contact (Saha et al., 2017;Qu et al., 2018;Mukherjee et al., 2020;Saleem et al., 2019;USEPA, 2020). It computes the pollutant dose consumed in human using chronic daily intake (CDI), which reflects the dose of pollutants in kilogram per day captivated through the digestion pathway (CDI ingestion )and dermal absorption (CDI dermal ) using Eqs. (4) and (5), respectively (USEPA, 2011;Zhang et al., 2017;Jehan et al., 2020;Tokatli and Ustaoglu, 2020).
where, Con water refers to trace metal concentration in surface water (μg/ L). Table 1 reveals the non-carcinogenic health impact parameters and their input assumptions used for estimating the exposure to heavy metals through intake and skin absorption. In second step, we calculated the Hazard quotient (HQ) from CDI (CDI ingestion and CDI dermal ) and RfD (RfD ingestion and RfD dermal ) using Eq. (6) (Saha and Paul, 2019;Imran et al., 2019); At the final step, the total potential non-carcinogenic risks were appraised by estimating the hazard index (HI) by using Eq. (7) (Rupakheti et al., 2017;Jehan et al., 2020;Karthikeyan et al., 2021); The toxic metals with HI and HQ of >1 can have adverse effects and with <1 have no adverse effects on human health (USEPA, 1989;Vetrimurugan et al., 2016Vetrimurugan et al., , 2017Yang et al., 2017;Mohammadi et al., 2019).

Carcinogenic health risk approach
We considered the heavy metals as carcinogenic to humans (As) and likely to be carcinogenic to humans (Cd) in order to assess the carcinogenic and non-carcinogenic risks according to the IARC report (IARC, 2013). Both Pb and Cr were included in the non-carcinogenic risk assessment. Even though, Cu, Fe and Zn are not classified in the IARC report, they were involved in the non-carcinogenic risk assessment (e.g. Chan et al., 1998).

Table 2
Statistical measures and evaluation of physico-chemical, heavy metals and biological parameters during COVID-19 before and after lockdown period against WHO, USEPA and BIS standards.

Heavy metal toxicity load approach
HMPI or HI (HMPI>100 or HI > 1) indicate the suitability of water as it reveals the accurate ratio of surplus metal (Saha and Paul, 2019;Proshad et al., 2020). It concedes many ideas for predicting and mitigating the pollution of water bodies. This estimation technique predicts the concentration of excess metal in water and the amount that must be removed to make it harmless for human use. HMTL (Heavy metal toxic load) estimates toxic heavy metal in a water source that seduces human health, and it was computed using the following equation (8) (Saha and Paul, 2019): where c, n and HIS denote the concentration of heavy metal, number of heavy metals and risk severity score, respectively. HIS scores for As (1676), Cr (1149), Cu (805), Zn (913), Cd (1318), and Pb (1531) were considered from ATSDR (2018). They are based on the frequency of hazardous material occurrence on National Priority List (NPL), prepared by ATSDR. The permissible limit of HMTL was below 5888.527 mg/l (Saha and Paul, 2019;Proshad et al., 2020). The HIS score was multiplied by acceptable limit of specific concentration, which is considered the permissible limit for toxicity load and the permissible toxicity load is given in Table 9. HTML Result identifies that the surplus percentage of removal of heavy metals from surface aquifers beyond the permissible toxicity load is essential for human health.

Results and discussion
We assessed water quality of the Punnakayal estuary during the lockdown and pre-lockdown periods using the physic-chemical variables, dissolved metals and microbiological parameters.

Physicochemical parameters
Mean pH of the pre-lockdown (7.78), and lockdown (7.63) periods did not show significant difference (Table 2). Higher pH of TSW -11, 12, 20 during the lockdown period was due to warmer water temperatures. Both the mean EC and TDS of the lockdown period were 22.04% and 29.96% lower than the pre-lockdown. TDS of 75% of surface water samples in the pre-lockdown and 50% of samples of lockdown interval exceeded the permissible limit of WHO (2017; 1000 mg/l) (Fig. 2).
These changes are due to absence of agricultural activities during the first phase of lockdown as well as the dilution effect from rainfall. It also revealed the reduction in water consumption for the industrial purposes. Compared to the Central Pollution Control Board (CPCB) (1979) and BIS (1982) water quality standards, about 45% of both the pre-lockdown (1.26-9.26 mg/l) and lockdown (1.11-6.89 mg/l) samples had DO in class C, revealing unsuitability for drinking. The mean BOD in both periods (5.4 mg/l in pre-lockdown and 4.8 mg/l in lockdown) showed no significance difference and it grouped only few samples in class A, representing the suitability for drinking without conventional treatment but after disinfection (Source: Central Pollution Control Board (CPCB), 1979 andBIS, 1982). Turbidity limits before and during the lockdown are 1.56-9.65 (NTU) and 1.21-6.62 (NTU), respectively. Most samples from both periods exceeded the acceptable limit of BIS (2012). Higher turbidity loads also revealed natural causes such as erosion of more silt   and mud and anthropogenic causes such as agriculture, sand mining, construction, and algae from domestic wastes. NO 3 − is a prime contaminant in agro-terrains (Adimalla et al., 2018;Zhang et al., 2018;Chandrasekar er al., 2021). It was quantified as 35-72 mg/l during the pre-lockdown and 12-56 mg/l during the lockdown. Comparison with World Health Organization (2017) suggests 50% of pre-lockdown and only 15% of lockdown samples were under high risk. We detected 27% discrepancy between the two periods and plenty of samples of the pre-lockdown period showed high health risk (e.g.Adimalla and Qian, 2019, Fig. 3).

Metal concentrations
Concentrations of chromium (Cr), copper (Cu) and zinc (Zn) showed <20% difference between the lockdown and pre-lockdown periods (Table 2 and Fig. 4). However, the concentrations of arsenic (As), iron (Fe), lead (Pb) and cadmium (Cd) decreased >20% during the lockdown in comparison to the pre-lockdown period.
Arsenic: It is one of the most dangerous toxic components, and can lead to immune disorders, reproductive dysfunction and skin cancer (Kabata-Pendias and Szteke, 2015;Kacmaz, 2020;Tokatli and Ustaoglu, 2020). Concentration of 0.012-0.099 mg/l in pre-lockdown and 0.001-0.098 mg/l during the lockdown suggested that all of them exceeded the permissible limit (World Health Organization, 2017) for drinking (0.01 mg/l). Despite the COVID-19 lockdown, the anthropological activities related to poultry waste, fertilizer plants, brick making plants, pot design making plants and agricultural practices continued in this region (Selvam et al., 2014a(Selvam et al., , 2017 Chromium: The maximum Cr concentration was 0.08 mg/l in prelockdown samples and reduced to 0.07 mg/l in the lockdown samples. About 45% of the pre-lockdown samples exceeded the permissible limit (0.05 mg/l) and only 15% of the lockdown samples exceeded the permissible limit (Fig. 5). In both the phases, all samples near the coast exceeded the allowable limit of 0.05 mg/l (World Health Organization, 2017). Reduction in chromium concentration during the lockdown indicated decline in the activities related to the chemical industries (e.g. DCW Industry) (Selvam et al., 2017). Similarly, there was less utilization of petroleum product in heavy vehicle workshops of this region (e.g. Hua et al., 2016;Sakthivel et al., 2016).
Iron: For local aquifers the main contributors of iron (Fe) are industrial effluent, acid-mine drainage, and sewage. In this research the iron (Fe) concentration is varied from 0.123 to 0.458 mg/l pre lockdown and 0.102-0.321 mg/l during the lockdown (Fig. 6). This result shows that 55% of pre-lockdown samples have an acceptable limit of 0.3 mg/l (World Health Organization, 2017), while 5% of samples exceed the allowable limit during the lockdown period. The maximum occurred value of Fe on COVID-19 phases is very close to permissible limit (0.3 mg/l) and shows that this may be due to the shutting of metallurgical industries during the COVID-19 lockdown period, from which wastes are discharged into water bodies and from landfills (Milivojević et al., 2016).
Copper: This trace element is an important nutrient for the human body (Muhammad et al., 2014;Samantara et al., 2017). We detected almost no change between the two COVID-19 stages with mean values of 0.218 mg/l during pre-lockdown and 0.201 mg/l during the lockdown. None of them exceeded the drinking limits (2 mg/l) of the World Health Organization (2017) standard.
Zinc: Absence of Zn affects the metabolism and the immune system, resulting in infections in humans, anemia and birth defects in pregnant women and delayed sexual maturity in men (ATSDR, 2005; Samantara  Karunanidhi et al., 2021). Concentration of Zn in pre-lockdown samples (0.568-0.999 mg/l) and lockdown period (0.523-0.999 mg/l) remained similar. All of them remained suitable for drinking (<5 mg/l). It showed the absence of potential sources of Zn such as industries related to rubber, paint, bronze, die-casting metals, brass and other alloys in this region.
Cadmium: It ranged from 0 to 0.002 mg/l in pre-lockdown samples and 0-0.001 mg/l in the lockdown samples. Both remained within the permissible limits (World Health Organization, 2017; <0.003 mg/l). The two periods (pre-lockdown and lockdown) showed a significant difference of up to 42%. Cadmium can flow from phosphate fertilizers into the soil and surface water, and it is also sourced from cadmium-based batteries and cadmium coated materials (ATSDR, 2008;Tokatli and Ustaoglu, 2020). Since there is no homeostatic mechanism to control, the exposure to very low levels of Cd can cause adverse overall effects on humans (Carter and Fernando, 1979).
Lead: It varied from 0.006 to 0.019 mg/l with an average of 0.012 mg/l in the pre-lockdown and between 0.004 and 0.009 mg/l with an average of 0.007 mg/l during the lockdown. About 65% of the surface water samples of the pre-lockdown interval exceeded the recommended limit (0.01 mg/l), but all the lockdown samples were within the allowable range for drinking (Fig. 7). Industrial discharges from foundries, battery production amenities, contaminated land runoffs and sewage are the main bases of Pb in the Thoothukudi coastal region. (Selvam et al., 2017). Deficiency or minimal disposal of effluents from these industries and manufacturing unit and reduction in petroleum related transportation during the lockdown interval might have led to lower Pb concentration.

Microbiological parameters
The expressive statistics of microbiological concentration are presented in Table 2. Total coliform bacteria population varied between 6.2 and 189 MPN ml/l and 1.2-95 MPN ml/l in the lockdown and prelockdown samples, respectively. The maximum population of fecal coliform bacteria was 42.92 MPN ml/l in the lockdown and 97.09 MPN ml/l in the pre-lockdown period. Escherichia coli bacterial population ranged from 0 to 62 MPN ml/l in the lockdown samples and 0 to 77 MPN ml/l in the pre lockdown samples. The reduction in bacteria during the lockdown period was due to closure of fishing companies and tourism contamination (Fig. 8). However, all fecal streptococci population was low (Est <10) both in the pre-lockdown and lockdown samples. Escherichia coli Total coliforms, and faecal coliforms are contributed to the water system by humans and other warm-blooded animals. They survive the sewage treatment plants in large numbers and protect their pathogens for a longer period (Selvam et al., 2017). Selvam et al. (2020a) have observed that the closure of industries and other commercial activities in the study area provide favorable condition for the growth of large bacterial population.

Pollution Indices
Based on the method used by Edet and Offiong (2002), we evaluated the HMPI (heavy metal pollution index) for the pre-lockdown and lockdown phases for As, Cr, Fe, Cu, Zn, Cd, and Pb (Table 3)   The HMPI values of the central part were transferred from the high pollution to low pollution class and there was no change in the rest as they were continued to receive sewage and municipal effluents as well as pollutant from agricultural activities (i.e. Phosphate fertilizers) (Fig. 9).

Health risk assessment
The hazard indices (HI) of non-carcinogenic risk and carcinogenic risk were based on hazard quotients (HQ) of the ingestion and dermal adsorption pathways. They showed the total potential human health risks on children and adults from various heavy metals.

Non-carcinogenic health risk
The non-carcinogenic risk for children and adults was evaluated for the toxic As, Cr, Fe, Cu, Zn, Cd and Pb (Tables 4 and 5). In the prelockdown period for children, the HQ ingestion ranged from 1.280-10.560, 0.107-0.853, 0.013-0.049, 0.098-0.239, 0.061-0.107, 0-0.128 and 0.137-0.434 for As, Cr, Fe, Cu, Zn, Cd and Pb, respectively. Similarly, the dermal pathway HQ ranged from 0.032-0.266, 0.440-3.520, 0.001-0.003, 0.003-0.008, 0.003-0.005, 0.000-1.320           Table 7 The total carcinogenic risk (TCR) among children and Adults in the Thamirabarani River water before and during COVID-19 lockdown.   In the lockdown period, the HQ ingestion pathway of As also exceeded the limit (>1) in 100% of the samples and HQ dermal results of Cr remained above the hazard quotient limit in 60% of the surface water samples.
The pre-lockdown and lockdown HI indices (HI; sum of all HQ values by intake or skin absorption pathway) for children and adults were higher for the water intake pathway compared to the skin contact pathway (Table 6). Two-term HI values for ingestion and skin absorption exposure to As and Cr were higher than the acceptable range of noncarcinogenic metals. However, the pollution impact or pollution rate of lockdown period was relatively lower than the pre-lockdown. For example, the average HI intake for children was 6.616 for As and 2.485 for Cr during the pre-lockdown. They decreased to 5.149 (As) and 1.988 (Cr) during the lockdown. The closure of several industries in this region, limited use of petrochemicals in agriculture and reduction of other anthropological contributions such as discharge of domestic wastewater, municipal waste, and chemical waste from industries during the lockdown might have led to less heavy metal contribution and recued health risks.

Carcinogenic health risk
The carcinogenic risk (CR) was computed for As, Cr, Cd and Pb and Table 7 presents the total carcinogenic risk (TCR; sum of CR from ingestion and dermal contact exposure) for children and adults. In the pre-lockdown samples and for children, the TCR varied between 0.0005819-0.0048010, 0.0001633-0.0013064, 0-0.0003944 and 0.0000017-0.0000052 for As, Cr, Cd and Pb, respectively. Similarly for adults, the TCR values in the pre-lockdown samples varied between 0.0005171-0.0042658, 0.0001462-0.0011693, 0-0.0003526 and 0.00000155-0.0000047 for As, Cr, Cd and Pb, respectively. In carcinogenic elements the following elements are As, Cr and Cd (>95% samples) beyond the permissible carcinogenic limit. Only Pb (100%) remained within the acceptable or tolerable carcinogenic limit (0.000001-0.0001) for children and adults.
In the lockdown samples, the TCR for children varied between 0.0000485-0.0047525, 0.0016033-0.0112231, 0-0.0001972 and 0.0000011-0.0000025 for As, Cr, Cd and Pb, respectively. The TCR values for adults varied between 0.0000431-0.0042227, 0.0001444-0.0010108, 0-0.0001752 and 0.0000009-0.0000022 for As, Cr, Cd and Pb, respectively. Again all the toxic metals (As, Cr and Cd), except for Pb exceeded the carcinogenic range (0.000001-0.0001) causing risk to children and adults. In this study did not show any significant improvements in surface water pollution but compare to prelockdown period, during lockdown period the pollution level is much reduced.

Heavy metal toxicity load
We computed HMTL to evaluate the concentration of pollutants that might cause non-carcinogenic health risk and furnished the data about the percentage of metals that needs removal from the specific samples (Table 8). The ranges of As, Cr, Cu, Zn, Cd, and Pb that are the most threatening to human health were selected from the ATSDR material priority list to calculate HTML (ATSDR, 2017). It varied between 773.70 and 1382.30 mg/l with an average of 1101.78 mg/l for the pre-lockdown samples and between 665.63 and 1311.45 mg/l with a mean of 990.04 mg/l for the lockdown samples. DCW industrial waste leaks and increased usages of agro-based petrochemicals in the study area may lead to increased HTML results. It is necessary to remove 90%, 38%, 47% of As, Cr, and Pb in the pre-lockdown samples. Similarly, almost similar amount of As (90%) and slightly less Cr (29%) must be removed from the lockdown samples to make it suitable for human health. However, Pb remained below the permissible toxicity load in all the lockdown samples. In both the pre-lockdown and lockdown periods, the concentrations of Cu, Zn and Cd were suitable for human activities (Table 9).

Remediation for human welfare
Our results suggested that the surface water of Punnakayal estuary in the Thamirabarani River system poses severe non-carcinogenic and carcinogenic hazards to human health. The pollution sources such as industrial effluents, domestic wastewater, sand mining and agroventures can be minimized by proper management of the surrounding petrochemical, and beverages manufacture units as well as improving the wastewater drainage system. The lockdown period bestowed a good opportunity to understand the paramount importance of nature in our daily lives. Furthermore, it also provided an insight to realize that the conservation and sustainability of natural water systems can be inhibited by effectively managing the pollution sources. According to HTML results, As, Cr and Pb required greater attention in the pre-lockdown samples, whereas only As and Cr were peril to human in the lockdown samples. Therefore, the implementation of pertinent strategy technique of water quality management might help to minimize the pollution of water bodies. In environmental studies various researchers were proposed various innovative solutions for arsenic and chromium remediation, especially Marinho et al. (2019) was discussed various aqueous solutions and the analytical methods used for their detection and quantification of arsenic and chromium elements. This study advocates to form a special panel to routinely monitor the surface water quality and mitigate the risk from exposure to potential heavy metals, especially from As and Cr in the Thamirabarani River ecosystem.

Conclusions
The present study evaluated the influence of COVID-19 pandemic lockdown on surface water quality of the Punnakayal estuary in the Thamirabarani River system of south India by estimating reduced absorptions of As, Cr, Cu, Cd, Fe, Pb and Zn. Toxic heavy element contamination risk assessment codes, health risk assessment methods and some pollution load approach described the water quality prior to lockdown and the lockdown periods. We did not observe any changes in the order of heavy metal enrichments (Zn > Fe > Cu > As > Cr > Pb > Cd) in both phases as the industrial ejects, domestic sewage and agricultural applications continued during the lockdown period. However, the quantity or impurity ratio was reduced compared to the prelockdown period. In the pre-lockdown surface water, the concentrations of Cu, Zn and Cd remained within permissible limits of World Health Organization (2017) in all samples and hazardous As, Cr, Fe and Pb exceeded the permissible limits in 100%, 40%, 45% and 65% samples, respectively. During the lockdown period, As remained similar with 100% samples exceeding the permissible limit, but relatively less samples had Cr (15%) and Fe (5%) above the permissible limits. HQ non-carcinogenic risk on children and adults from the ingestion and skin absorption of hazardous Fe, Cu, Zn, Cd and Pb were within the reliable range for both the periods. HI results, however, indicated more delicate to non-cancer risks in children compared to adults from both As and Cr. TCR values also demonstrated "higher risk of cancer" in children and adults from As, Cr, and Cd, even during the lockdown and "no carcinogenic dilemma" from Pb.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence Table 9 Percentage of removal of heavy metal to reduce pollution load in the Thamirabarani River surface water with respect to pre-lockdown and lockdown period. the work reported in this paper.