Groundwater is the world's primary source of fresh water, accounting for roughly one-fifth of total freshwater supply (Jaseela et al. 2016; Jarray et al. 2017). It makes up the majority of the Earth's water circulatory system that occurs in permeable geologic formations known as aquifers (Kaur & Rosin, 2012). Groundwater is an essential and valuable natural resource for human life and economic development around the world (Baalousha, 2011; Kaur & Rosin, 2012). Currently, it plays a significant contribution in meeting demands on the water supply than surface water because of regional climate change and scarcity of surface water resources or their unsuitability for human consumption (Alwathaf & Mansouri, 2011; Jamil et al., 2011; Sener et al., 2009). Although groundwater is an essential freshwater resource, its management is complex, little is known about it (there is insufficient and reliable information), and restoring its quality is difficult. (Hyneman, 2014). Recently, rapid population growth, urbanisation, the use of agrochemicals in agriculture, and industrialization have resulted in the groundwater system being threatened and becoming a major environmental problem around the world. (Shirazi et al., 2013). Groundwater contamination is one of the world's most serious environmental problems and a major environmental issue in unconfined aquifers due to contamination from land use and other anthropogenic impacts. (Bouwer, 2003; Xu et al., 2019). Groundwater is both overexploited and contaminated as a result of recent dramatic population pressure and water stress caused by climate change and scarcity of surface water. (Mishra et al., 2014; Negasa et al., 2017; Pophare et al., 2014). As a result, groundwater pollution has been reported in various parts of the world as a result of anthropogenic factors that have drastically altered its chemical composition. (Bera et al., 2021; Gheisari, 2017). Various research findings (Shirazi et al., 2013; Jaseela et al., 2016; Tavassoli and Mohammadi, 2017) around the world informed that contaminants are currently contaminating groundwater. Unfortunately, its contamination is not easily remedied and/or is occasionally impossible to restore to its original quality. As a result, aquifer remediation becomes extremely difficult, and sometimes impossible, to restore to their original quality. (Michalopoulos & Dimitriou, 2018; Rao et al., 2018). As a result, protecting aquifers from contaminants is regarded as an important factor in groundwater resource management. Accordingly, assessing aquifer vulnerability is critical in order to provide solutions for preserving groundwater resources. (Abdelmadjid & Omar, 2013). In recent years, one of the subjects of intense research in the field of natural environment has been the evaluation of groundwater vulnerability to pollution, and various methods for predicting which areas are the most vulnerable have been developed. (Oroji, 2018, 2019).
The risk of groundwater contamination is caused by both the aquifer's vulnerability (natural characteristics) and contaminants above the soil surface (Bera et al., 2021; Kaur & Rosin, 2012). Its intrinsic vulnerability is determined by several factors, including the absorption process and contaminant travel time, the dynamics of contaminant flow in the saturated zone, and the pollutant's residual concentration as it spreads in the saturated zone. Unfortunately, anthropogenic factors have significantly deteriorated groundwater around the world due to unsustainable use and contamination release (Dorsey-spitz, 2015; Kaur & Rosin, 2012; Machiwal et al., 2018). Human and animal waste disposal, as well as harmful wastes; mining and oil operation byproducts; seepages from sanitary sewer lines and septic tanks; and farming have all caused significant contamination(Dorsey-spitz, 2015).
Different studies have been conducted all over the world (Balaji et al., 2021; Roha Jamil & Said, 2011; Mimi, Mahmoud, & Madi, 2012; Tavassoli & Mohammadi, 2017; Wu et al., 2018) and in Ethiopia (Asfaw & Mengistu, 2020; Michalopoulos & Dimitriou, 2018; Tilahun & Merkel, 2010), demonstrated that groundwater is vulnerable to pollution in various parts of the world, including Ethiopia, due to hydrogeological and anthropogenic factors. The researchers also claim that groundwater quality deterioration occurs in addition to potential vulnerability (Aragaw & Gnanachandrasamy, 2021; Baalousha Husam, 2020; Eriksson, 2013; Kahsay et al., 2019; Mahboob et al., 2017). As a result, groundwater management requires investigating its vulnerability and monitoring quality in order to make effective decisions about groundwater utilisation and future planning(Al-adamat, 2018; Boufekane et al., 2021; Moldovan et al., 2019). Groundwater vulnerability assessment and risk mapping are critical for resource management and monitoring, which can assist planners and decision-makers in managing land use and water resources(Baalousha, 2011; Baalousha Husam, 2020).
Modeling groundwater vulnerability to pollution is crucial for implementing development activities to protect groundwater quality (Jaseela et al. 2016; Asfaw and Mengistu 2020). To investigate the vulnerability of groundwater, different models such as statistical methods, process-based methods, and overlay and index methods were developed and applied. (Singh et al., 2015). Accordingly, groundwater vulnerability modelling is based on current hydrogeology and land use conditions(Oroji, 2018; Raipur, 2016). Different models, such as the GOD model, were used by different scholars to assess aquifer vulnerability(Ebrahimi, 2015; Musálem, 2015); SINTACS (Raipur, 2016); DRASTIC (Aller et al., 1987; Jang et al., 2017; Jaseela et al., 2016b;. Shirazi et al., 2014; Tilahun & Merkel, 2010); modified DRASTIC model (Al-awarded et al., 2014; Asfaw & Mengistu, 2020; Balaji et al., 2021; Boufekane et al., 2021; Mimi, Mahmoud, & Madi, 2012; Saidi et al., 2011; Sarkar & Chandra, 2021; Singh et al., 2015). As a result of integrating anthropogenic factors with hydrogeological parameters to examine groundwater vulnerability, the modified DRASTIC model is currently the most commonly and widely used model for groundwater assessment (Sarkar & Chandra, 2021). The drastic model includes seven hydrogeological parameters that influence the occurrence and movement of groundwater into the system. However, it does not take into account the anthropogenic factors that contribute significantly to groundwater vulnerability. As a result, it is critical to include anthropogenic (LULC) parameters in groundwater vulnerability assessments, such as the modified Drastic model(Shirazi et al., 2013; Singh et al., 2015). The most widely used methods are GIS and remote sensing-based overlay and index methods with modified DRASTIC model to evaluate aquifer vulnerability to pollution due to the use of hydrogeological parameters for evaluation and ease of application(Michalopoulos & Dimitriou, 2018; Musálem, 2015; Ouedraogo et al., 2020).
Rib watershed, located in the upper Blue Nile (Abay) sub-basin, is one of the areas most affected by anthropogenic activities such as irrigation, urban expansion, and land use/land cover change over time (Garede et al., 2014). These activities contribute significantly to pollutant contamination of groundwater. Groundwater is heavily used for domestic and irrigation purposes in the area (Andualem et al., 2020). Different researchers were conducted on the vulnerability of groundwater in different parts of Ethiopia such as (Asfaw & Mengistu, 2020), modeled aquifer vulnerability of the Megech watershed; (Tilahun & Merkel, 2010) assessment of groundwater vulnerability in Dire Dawa; and other researchers focused on surface water quality of Koga (Eriksson, 2013), groundwater potential, and water flow over the Tana basin including Rib watershed (Andualem et al., 2020), Land use/land cover dynamics of rib watershed (Garede et al., 2014) and groundwater quality of Tana-Gumara for drinking and irrigation purpose including Rib watershed (Andualem et al., 2020). Despite the fact that the watershed is affected by anthropogenic factors that may affect groundwater (irrigation dam project, rice farming with agrochemicals, urbanisation, and associated problems including waste products), none of them assessed the vulnerability of the aquifer (groundwater) to pollution. So, this study aimed to assess the suceptiability groundwater of to nitrate contamination using GIS-based modified DRASTIC Model in the Rib watershed, upper Abay Basin, Ethiopia.