Agriculture is the backbone source of the human food supply that aims provision of safe products with the least environmental impacts. Food obligation is attained by increasing the crop yield and is achieved by the use of certain agrochemicals (Rafique et al., 2016). Moreover, agriculture is principally based upon two natural resources i.e. soil and water, and dearth in case poses a potential threat to the production of growing food crops (Perveen et al., 2010). Like other developing countries in the world, Pakistan is also a developing agro-based country and is facing freshwater scarcity. To meet water requirements, farmers have a general practice to use sewage and industrial wastewater as irrigation sources which include different heavy metal species including Pb, Zn, Cd, Ni, Fe, Mn, Cr, etc. (Ali et al., 2014a; Ali et al., 2014b; Kanwal et al., 2020; Aslam et al., 2021; Rabiya et al., 2022). Different agrochemicals like synthetic fertilizers and pesticides (xenobiotics) also act as potential sources of heavy metals. Thus, the agricultural system of Pakistan is facing immense heavy metal pollution through such unwise agricultural practices. Such a kind of irrigation is radical for soils, plants, and consumers’ health as well. Different studies have addressed this issue in various cities of Pakistan i.e. in Lahore, Faisalabad, Attock (Rashid et al., 2018), Sargodha (Ahmad et al., 2019), in Karachi (Shehnaz et al., 2019; Aziz et al., 2021) and Sawabi (Ali et al., 2021). On the other, studies have revealed that only 2% of cities are availing wastewater treatment systems in Pakistan, whilst 80% of the cities use untreated discharge for irrigation purposes (Ullah et al., 2018; Shahid et al., 2020; Bashir et al., 2021). Estimates have revealed that >960 thousand million gallons of municipal (> 675) and industrial (>290) wastewater is generated annually (WB-SCEA, 2006), and is adding heavy metals into the intact environment (Sial et al., 2006; Riaz et al., 2020). Among other pollutants, heavy metals are the foremost contributing pollutants due to their potential in changing the physical and chemical characteristics of the ecosystem (Ali et al., 2018; Sobhanardakani, 2019).

Heavy metals are non-essential and non-biodegradable toxic substances with a density greater than 5 g/cm³ which is five times more than the density of water (Dai et al., 2006). They are very toxic even used in low concentrations, are deadly unsafe for human health, and produce oxidative stress after entry through the food chain either through natural or anthropogenic source. Moreover, heavy metals also cause liver cirrhosis, renal dysfunction, mental retardation, CNS breakdown, cardiovascular disorder, infertility, and degeneration of the basal ganglia of the brain and ultimately cause death (Sobhanardakani, 2017; Oyugi et al., 2021; Paithankar et al., 2021). For instance, As even at its minute exposure to humans acts as carcinogenic, destroys blood composition, causes anorexia, hair loss, kidney and liver damage, herpes, muscle spasms etc. (Tasleem Jan et al., 2015; Sobhanardakani, 2018; Sobhanardakani, 2018b). Cd is highly toxic element to the human body and specifically targets the vital organs including kidneys and liver, and reduces enzymatic activity related to kidney function (Wuana and Okieimen, 2011; Özcan and Al-Juhaimi, 2012). Chromium is another chemical element and is graded as human carcinogen (Sobhanardakani and Kianpour, 2016). Lead is considered to be the most absorbed environmental pollutant into the human body daily by 80% - 90% via consumption of different food items, and affects mental retardation in young children (Krejpcio et al., 2005; Liu et al., 2010). Nickel is an essential element for human body, but its above permissible concentration affects human body at large i.e. Ni associates with the hematotoxicity, genotoxicity, immune-toxicity and carcinogenicity (Akar et al., 2019; Sobhanardakani, 2019).

Focusing on the consequences of the issue, the present study was designed to evaluate the status of heavy metal species in the wastewater of the city Manka sewage canal and sewage wastewater of industrial sites. Likewise, different samples of irrigated soil, and vegetables and crop plants were also collected from different irrigated sites of the study area for metal analysis. Moreover, in present study potential health risk coupled with dietary use of these toxic metal species has been assessed by manipulating the Health Risk Index (HRI).

During the survey and sample collection, it has been observed that the farmers (respondents) of Dera Ghazi Khan District are using municipal and industrial-mixed sewage wastewater as a source of irrigation for a long. The farmers were also assessed about their knowledge of wastewater and HMs hazard through a questionnaire and were found unaware or have a bit of knowledge about the hazard to growing plants, irrigated soil, and consumers. During the present study, the heavy metal availability has also been confirmed in the collected samples of wastewater, soils, and even in the growing plants from different irrigated sites (SF1-SF17). The subsequent analyses have pretentiously confirmed the presence of different heavy metals in these collected samples, and have predicted the potential health hazards to the subsequent consumers of the area. Moreover, a mean value of pH of the wastewater has been observed within the safe limit except in a few sites following an increasing trend of TDS but within the safe limit. As concerns EC of the wastewater, the mean value was found to be exceeding the safe limit, and individually most of the sites (SF-1, 3, 5, 15, 16, 17) have an exceptional value of EC. From Table 1 , it is observed that in the wastewater upper range of trace elements like Cr, Cd, and Mn have exceeded the permissible limits, whilst Al, Cu, Fe, Pb, Zn, and Ni were observed within the safe limit. However, arsenic (As) was observed at the margin in some sites (SF-9, 13). These metals need careful management and monitoring from exceeding in the future. Hence, exceeding values of pH, TDS, EC and different heavy metals in the wastewater have turned it to be unfit for irrigation purposes in the study area. The findings of our study are supported by Kumar et al. (2018) that sewage and industrial wastewater were found to be unfit for irrigation purposes due to exceeding values of pH, TDS, and EC that changed the physic-chemical properties of irrigated soils and plants (spinach, fenugreek, and coriander). The wastewater was also found with a high amount of heavy metals, and the same was detected in the grown vegetables in an order of Cd > Zn > Fe > Mn > Pb > Cr > Cu. Likewise, the findings of Ishaq et al. (2020) showed the above permissible limits of pH, TDS, and EC in sewage wastewater of Peshawar city along with the varying concentration of heavy metals i.e. Cd, Pb, Cu, Ni, Zn, and Fe. These heavy metal traces were also observed in tomato plants irrigated with such metal-enriched sewage wastewater. According to a study by Hassan et al. (2022), the textile wastewater was also found with differentiated physiological parameters (EC, TDS) and heavy metals (Cd, Ni, Cr, Cu, Zn, Fe, Pb) that were exceeding the recommended limits. A high pH of wastewater increases the pH and alkalinity of irrigated soil which potentially tends to the availability of heavy metals in the soil and contaminate the food chain and vice versa. Such an affected food chain poses health risks to subsequent consumers (Ali et al., 2015a; Mwamba et al., 2020; Waheed et al., 2020).

Results about metals in the soils ( Table 1B ) irrigated with the wastewater have revealed the metal accumulation which was below the permissible limit; however, Cr and Pb were the most frequent (100%) among the metals. Likewise, the mean accumulation of heavy metals for all 17 stands has also revealed an elevated concentration of chromium (11.32 mg/kg). Foregoing deep insight, by differentiating the irrigated sites, ten sites SF-5, SF 7-10, and SF 12-17 have been found with the elevated metal contents (Cr, Cu, Ni, Pb, and Zn) under the cultivation of the cotton crop, might be due to excessive use of fertilizers and pesticides along with the wastewater application as reported earlier (Rafique et al., 2016; Gill et al., 2016; Gill et al., 2017). Briefly, available data from our analyses have revealed that sewage and industrial wastewater is unfit for irrigation purpose and has potential hazards due to the accumulation of metals. Earlier studies by Rafique et al. (2016) and Latif et al. (2018) also supported the findings of the present investigation and confirmed the availability of different heavy metals in cultivated soil of various areas of the Dera Ghazi Khan district. These areas were irrigated with sewage and industrial wastewater for a long time. Such a kind of irrigation was the potential source of heavy metal accumulation in agro-soils and grown vegetables. Heavy metals (mg/kg) were detected in the irrigated soils and vegetables were in the order of Cr [(17.89) > Ni (9.24) > Zn (2.31) > Pb (0.84) > Cu (0.29) > Cd (0.186); and Mn (137.3) > Cr (6.62) > Fe (968)]. Chromium, Mn, Fe, Ni, and Pb were exceeding the FAO/WHO limits (Aderinola et al, 2012; Rafique et al., 2016; Latif et al., 2018; Guo et al., 2021).

In the present study, besides heavy metal accumulation in the agro-soil system through wastewater, the collected samples of vegetables and crop plants have been revealed to be metal-enriched. It has shown an effective metal transfer from contaminated soil to the grown plants with the exceeding limits of Cr, Ni, Mn, and Pb. However, As, Cd, Cu, and Zn traces have been found within the safe limit ( Table 2 ). Interpretation of HRI has revealed the possible potential risk of heavy metal contaminated plants in the order of spinach > wheat > brinjal > tomato > red corn > apple gourd > white corn > cabbage > luffa. The health risk assessment of different metals has been found as Cu > Zn > Cr > Ni > Mn > Fe > Cd > Pb > As ( Table 5 ). These toxic metals put serious health risks to subsequent consumers interacting through the food chain (Ali et al., 2015b; Ali et al., 2015c; Dziwornua et al., 2018; Zuo et al., 2019; Tian et al., 2021) cause damage to DNA and proteins, and become carcinogenic in humans (Edelstein and Ben-Hur, 2018; Witkowska et al., 2021). Exposure to heavy metals may also result in skin-lung disorder, epigastric pain, hemorrhage, etc. (Das and Roychoudhury, 2014). Cattle milk is used as a nutritive source of food for all age groups including infants. But it possesses serious health risks due to heavy metals in the milk transferred from cattle after consumption of metal-contaminated fodder crops previously irrigated with sewage water, synthetic fertilizers, fungicides, pesticides, and industrial wastewater (Shar et al., 2021). In agreement with these earlier studies, our findings regarding the detection of different heavy metals in different vegetables and crop plants irrigated with metal-enriched wastewater have predicted potential health risks to the consumers of the study area through calculation of HRI ( Table 5 ).

Results of PCA have shown a distribution pattern of heavy metals in the agricultural system of the study area, and a pronounced variation along axis 1 following a strong correlation among the metals along axis 2. Figure 2 shows a strong variance in Cr and Zn in the wastewater collected from different sites (axis 1). The reason may be the exceptional concentration of these metals in sewage wastewater possibly receiving the metal residues from the textile mill (SF1, SF5) and pesticide plant wastewater (SF 13, SF 9). Heavy metals in soil samples irrigated with the wastewater have also shown a correlation of metals along axis 2, while Fe, Ni, and Cr are the exception on axis 1. We have observed the maximum concentration of these three metals in almost all the collected soil samples as compared to the concentration found in wastewater samples ( Figure 3 and Tables 1A, B ). This kind of metal accumulation in the soil samples may be due to the excessive use of pesticides or synthetic chemical fertilizers besides using municipal or industrial mixed sewage wastewater as a possible source of heavy metals (Rafique et al., 2016; Ulhassan et al., 2019). Likewise, subsequent plant samples have also shown a strong variance regarding Cu, Zn, and Fe metals ( Figure 4 ). This may be due to the specific mineral composition of collected plants (as in the case of spinach having more Fe-content), or excessive application of pesticides and synthetic fertilizers specifically absorbed by some growing plants.

Environmental pollution brought by addition of heavy metals in the agricultural system has been found as major health risk for humans and the environment. Due to their nature of toxicity and non-biodegradable property, these heavy metals remain present in the environment. Removal of such toxic entities is being made by imposing bioremediation technique - an effective tool to clean the environment. Microorganisms (aerobes, anaerobes) are used to treat contaminated soils for effective elimination, degradation and immobilization of heavy metals (Bala et al., 2022). Moreover, the source water pollution can be focused on remediation of wastewater and organic pollutants using some membrane or anaerobic microbial bioreactors (Ahmad et al., 2020; Naz et al., 2021).

The present study has revealed that wastewater samples collected from different sites of the study area have varying physico-chemical properties and are loaded with heavy metal trace elements. The concentration of As, Cd, and Cr was exceeding the acceptable limit, while Cu, Pb, and Mn showed to be marginal. Unremitting irrigation with wastewater has caused an accumulation of toxic metals in the subsequent agricultural land sites. However, soil samples showed metal accumulation below the permissible limit, and however, Cr and Pb were the most frequent (100%) among the metals. Likewise, the mean accumulation of heavy metals for all 17 stands has also revealed an elevated concentration of chromium (11.32 mg/kg) followed by Cu, Ni, Pb, and Zn. Samples of vegetables and crop plants were also revealed to be metal-enriched with the exceeding contents of Cr, Ni, Mn, and Pb. Based on DMI, HRI revealed the possible potential risk of vegetables and crop plants contaminated with heavy metals in the order of spinach > wheat > brinjal > tomato > red corn > apple gourd > white corn > cabbage > luffa. Likewise, health risk assessment of different metals was observed in the order of Cu > Zn > Cr > Ni > Mn > Fe > Cd > Pb > As. The present study also suggests that prolonged application of wastewater as an irrigation source may cause severe health risks to subsequent consumers. Further, it is also suggested that an imperative notice and awareness to the people is needed for appropriate regulation and monitoring the municipal and industrial effluents as wastewater. Different bioremediation techniques and water treatment plants could be a good tool to revive the contaminated agricultural system of the study area. Moreover, in the future, besides metal pollution the organic pollution and role of microorganisms will be considered. However, it needs attention of the scientific community.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

MIA, SZ and HA created the research plan, performed the experiments of MIA Ph. D work, analyzed the data and write down the manuscript. HA, IA, MS, BA worked as technical advisors and helped in the experiments. KN, RI, D-QD, SB, IA, UAH helped in revising, editing and funding acquisition for the manuscript. All authors contributed to the article and approved the submitted version.