Physicochemical characteristics of groundwater
The physicochemical parameters of groundwater samples were analyzed at 110 different sites, comprising 62 wells, 40 qanats, and 8 springs (Table. 2). While pH does not directly influence consumer health, it serves as a crucial indicator of water quality (WHO 2017). The pH levels of the water samples were measured on-site and fell within the range of 6.62 to 8.63, with an average of 7.9. Except for the Giv station, all stations exhibited a neutral pH level, adhering to the permissible limits set by the WHO and the United States Environment Protection Agency (USEPA) (6.5-8.5) (Brown et al., 1978). The DigRostam well station recorded the lowest pH at 6.62, while the Giv qanat station exhibited the highest pH at 8.63.
The electrical conductivity (EC) measurements across all areas ranged from 0.33 to 6.37 (mμ cm-2). The Gazdez region recorded the highest EC value of 6.73. The physicochemical parameters of groundwater depend on both natural (e.g. geology and minerals) and human-based factors (e.g. agriculture and mining). Chronic water shortages, with an annual average rainfall of less than 100 mm in this region have led to increasing changes in the hadrochemical parameters in the groundwater (Behzadi et al. 2019; Sharafi et al. 2023). However, most of the studied sites showed the values for the water quality within the permissible range of WHO and USEPA.
Ni concentrations and standard limit
The concentrations of Ni in 110 stations were assessed and compared to the standard values set by the Canadian Standard and the Environmental Protection Organization of Iran, which are 25 μg l-1 and 20 μg l-1, respectively (Ghavidel & Moatar 2008). In the qanat water samples, the concentration of Ni varied between 0.02-132.39 μg l-1, with an average concentration of 5.09 μg l-1 across all studied qanats (Table 3). Among all the stations, Halvan qanat had the highest concentration at 132.39 μg l-1, followed by Grymanj at 24.37 μg l-1. Mining activities have had a significant impact on the surface and groundwater conditions over the past century (Singh & Kamal, 2015). Water pollution caused by heavy metals in abandoned mines is a critical environmental issue, particularly in arid and semi-arid regions where water resources are limited (Bhuiyan et al., 2010). Heavy metal pollution in water is considered as one of the most serious environmental problems globally (Storelli 2008). In studied area, it is highly possible that metal pollution causes by the permeation from the mining sites to the rock soil and then groundwater. Ni in low concentrations has no adverse effects on the human health, but exceeding concentration of Ni could cause health risks in target consumers. USEPA suggested an acceptable concentration for Ni at 100 μg l-1, while we found the highest Ni concentrations at 132.39 μg l-1 in Halvan station, where mining activities have increased over the last three decades. Kazemi et al. (2022) stated that there is a relationship between the density of active mines and groundwater pollution, meaning that the concentrations of Ni in the groundwater of heavily mining sites are higher than the areas with fewer mines. Having a long horizontal water channel underground, water in qanat experiences many geochemical changes, including dissolution of metals and salts. Ni is more mobile in the acidic waters relative to the alkaline conditions. Therefore, one reason for the higher Ni concentrations in some of our sites might be due to the presence of acid-intensifying factors (e.g. soil and rock).
Non-carcinogenic risk assessment of Ni
The findings of the non-carcinogenic risk assessment for Ni indicate that only the Halvan station, with a hazard quotient (HQ) value of ≥ 1, poses a potential non-carcinogenic risk to the residents. Conversely, the remaining sites, with HQ values < 1, present a negligible risk. The distribution of non-carcinogenic risk across the studied sites plotted using the zoning map created using ArcGIS (v) (Fig. 2). Additionally, non-carcinogenic average daily dose (ADDnc) for each of the sampled groundwater sites was mapped using ArcGIS (v) (Fig. 3). Assessing the risk of groundwater contamination is an essential component of evaluating groundwater quality to the consumers. Albint (1968) introduced the concept of groundwater vulnerability, and focused on the sensitivity of groundwater to changes resulting from human activities and natural incidents, emphasizing the adaptability of the groundwater environment (Zhang et al. 2022). Elevated levels of toxic metals in water are usually associated with industrial, mining, and agricultural operations. Ni, the 23rd most abundant element in the earth's crust, is among these toxic metals (Ghavidel & Moatar 2008). Acute exposure to Ni can alter the gene expression of normal cells, increasing the risk of Ni-induced cancers. Ni also contributes to the development of malignant properties in cells (Zadhush 2012). It is used in steel and other alloys, water treatment, batteries, and acts as a catalyst (Ghavidel & Moatar 2008), therefore, Ni is ubiquitous in the environment and threats people’s health.
Our findings indicated that the concentration of Ni exceeded the standards in 5.5% of the sampled qanats, particularly in the Halvan and Grymanj stations. The HQ analysis revealed that 98.21% of the samples posed negligible risks (HQ < 1), while 1.78% of stations presented non-carcinogenic risks to consumers (HQ ≥ 1). The northwestern regions of South Khorasan exhibited the highest severity of non-carcinogenic risks associated with Ni contamination. Conversely, the western areas did not show any significant non-carcinogenic effects related to Ni. This difference can be attributed to the increasing mining and industrial activities in South Khorasan, leading to metal pollution in these areas. For instance, the Halvan region, located 80 km away from Tabas, is in proximity to various active mines in the Tabas region, which may explain the high Ni concentration in the Halvan station. Furthermore, the analysis of physicochemical parameters indicated that the areas with the highest pH generally exhibited the highest Ni concentrations. However, Pearson's statistical correlation calculation did not show a significant relationship between Ni and pH (Kazemi et al. 2022).
With the growing concern for drinking water quality, extensive studies worldwide have been conducted to address the decline in water quality. For example, in a study conducted by Rajaei et al. (2011) on the physicochemical analysis of irrigation and drinking water sources in the South Khorasan province. The analysis of water samples from various stations in South Khorasan revealed that the electrical conductivity (EC) varied from 0.33 to 6.37 mS/cm, with an average of 1.78 mS/cm. The station with the lowest EC was Gazakht (agricultural land), while the highest EC was recorded at Gazdez station. pH analysis indicated that the Giv station had the highest pH, while the Gezdez station had the lowest pH. Only two of the qanats exceeded the standard limit of 20 μg/l for drinking water, as declared by the World Health Organization (WHO). The concentrations of these qanats were measured at 132.39 μg/l and 24.37 μg/l, respectively (Valinejhad et al. 2016).
Furthermore, the health risk assessment based on the hazard quotient (HQ) revealed that 98.21% of the studied qanats had HQ values below 1, indicating a non-carcinogenic risk to consumers. Conversely, 1.78% of the qanats exhibited HQ values equal to or exceeding 1, suggesting a potential non-carcinogenic risk. Pearson tests conducted on the data showed a strong and significant relationship (at the 0.01 level) between the nickel concentration in each area and the HQ value of the corresponding point across all the studied stations in South Khorasan, including wells, qanats, and springs. In response to this crisis, the utilization of groundwater reservoirs has become a crucial strategy for accessing untapped water resources (Mostafaeipour 2010). While the exploitation of mineral resources has been essential for human development, it has also led to detrimental environmental impacts, often wrongly attributed to natural causes, posing a threat to both aquatic and terrestrial ecosystems (Khelfaoui et al. 2022). Mining operations, throughout the lifecycle of a mine and even after its closure, have a significant impact on water sources, including surface and groundwater. Processes such as extraction, flooding, dewatering, and the discharge of untreated water are all associated with water pollution (A. K. Singh et al. 2016). Given the prevalence of mining activities in this area, there is a possibility of groundwater contamination by toxic and carcinogenic metals, resulting from soil and underground rock pollution. Despite the risks, people in South Khorasan still rely on groundwater for drinking, industrial use, agriculture, and livestock due to its quality and mineral content. However, the abundance of mining activities increases the potential for groundwater sources to become contaminated by toxic and carcinogenic metals seeping through the soil and rocks of the aquifer.