Water pollution is a serious concern for a long time, and this problem has now further elevated with the finding of the latest class of contaminants known as emerging contaminants (ECs) (Gavrilescu et al., 2015). The main kinds of ECs, found in wastewater, are pharmaceutical and personal care products (PPCPs), endocrine-disrupting compounds (EDCs), pesticides, antibiotic-resistant bacteria (ARB), and antibiotic-resistant gene (ARG) (ME Stuart et al., 2011; Chen et al., 2017, Lopez et al., 2015). These ECs are highly toxic, variable, persistent, and bioaccumulating in nature (B. M. Sharma et al., 2019). After contaminating almost all of our surface water resources, we have now started finding their fingerprints in the groundwater (Postigo & Barceló, 2015). Rapid urbanization, industrialization, and all other irresponsible anthropogenic activities are responsible for groundwater contamination (Naidu et al., 2016). Out of the total freshwater present on earth, the groundwater contains 68.70% of it as shown in Figure 1.1, and its central region is in surface aquifers and below bedrocks (Sorensen et al., 2015). This hidden water has an essential visible role in maintaining a healthy water chain, maintaining subsurface aquatic ecosystems, and maintaining the water level in rivers and other water bodies (Taylor et al., 2013; Pinasseau et al., 2019).

Groundwater is generally considered pure and free from any contamination, and its contamination is a severe issue affecting the natural ecosystem and humans. The continuously depleting surface water resources and increasing population will increase our dependency on groundwater in the near future (Díaz-Cruz & Barceló, 2008). Therefore, it is essential to emphasize groundwater contamination, and proper study needs to be done to know the actual groundwater condition (Banzhaf et al., 2017). Only a limited groundwater study has been done and with the specific pollution source in a limited geographical region. The data from this preliminary study have shown that ECs’ concentration is relatively lower in groundwater than surface water (Sui et al., 2015). With this much low concentration of the order below 1 ng/L, there is no acute health risk, but the continuous exposure of a mixture of ECs to humans can cause some chronic health effects (Lapworth et al., 2012). These ECs reach groundwater either from a point source or a diffused source (Tran et al., 2019).

The point source is responsible for the entry of some specific contaminants such as pesticides from agricultural land, antibiotics, and pharmaceuticals from hospitals. In contrast, the diffused sources spread over a wider area and contribute various ECs to groundwater. The ECs must bypass multiple natural attenuations and filtration processes before reaching groundwater. Only the most mobile, persistent, and degradation by-products can reach groundwater (Ramakrishnan et al., 2015). The environmental condition in groundwater is different from the surface water and is not favorable for ECs’ degradation, further increasing ECs’ persistence. In low degrading conditions, the ECs can reside in groundwater for multiple decades and act as a legacy source of pollution (Sardiña et al., 2019). The excessive use of chemicals, improper disposal techniques, and inefficient ECs degradation result in the continuous increase in the concentration of legacy pollutants in groundwater. These groundwater contaminants, then for an extended period by the regular natural process of water exchange, continuously pollute the freshwater bodies coming in contact (Sophocleous, 2002). The water exchange happening at aquifers is the primary transport mechanism of legacy ECs from groundwater to other freshwater bodies in contact. The shallow water table regions are at greater risk than the deep-water table regions due to the less natural barriers resulting in easy reach. With the geography, the climate condition also affects the groundwater pollution, and in the rainy season, the increased water helps increase the mobility of ECs (Charbeneau, 2006; Jyrkama & Sykes, 2007). Proper and reliable studies are not present, and therefore regular studies including all the aspects of seasonal variation, accidental spillage, pandemic outbreak, etc. are needed. In high groundwater use and high contamination load regions, regular monitoring must be ensured as they are at a higher risk of groundwater contamination. ECs’ transformational products and the microbial contaminants (ARB and ARG) should also be given special attention along with the chemical contaminants. Currently, the fecal bacterial test is the main microbial test, whose primary purpose is to know the microbial load in order to prevent the spread of any infectious disease (Pantha et al., 2021; Szekeres et al., 2018), while the risk by ARB and ARG is more as they can transfer antibiotic resistance in the environment (Bouki et al., 2013). Such a groundwater study will help determine actual contamination conditions, ECs’ behavior, transport mechanism, and ultimate fate. The results of this study will also be used to prioritize the most toxic ECs based on their occurrence, physicochemical properties, and toxicological effects.

All this information will be beneficial and is very much needed by the researchers and regulating bodies. Using these data, proper rules and regulations, standards, and thresholds will be set, helping preserve groundwater. In the absence of any such rules and regulations, the ECs continuously get added to the groundwater. This is the right time to protect groundwater quality and quantity, as once it gets contaminated, there is no rectification method available for its treatment. The present wastewater treatment system is ineffective against the ECs’ complete removal (Bolong et al., 2009). Thus, researchers are working on new and advanced wastewater treatment methods such as plasma techniques for ECs’ complete removal.

The continuous addition of ECs to groundwater and their long-term persistence will convert this hidden water source to a hidden pollution source. The ECs persist in the groundwater due to unfavorable groundwater conditions for their degradation, due to which the groundwater pollutant becomes a legacy pollutant source. Between the groundwater and surface water, natural exchange of water occurs, and during this process, the ECs also get exchanged. The ECs found in groundwater are of two types: recent and old legacy contaminants (Díaz-Cruz & Barceló, 2008). The primary mechanism of ECs’ degradation in groundwater is biodegradation, transformation, and sorption. A significant portion of recent ECs gets attenuated by these methods, and the remaining nondegraded or partially degraded compounds get added up to the class of legacy contaminants. The ECs’ concentration in groundwater is low, but with the continuous addition of contaminants, the ECs in groundwater continuously increase and deteriorate groundwater health. The continuously increasing groundwater pollution has reached the stage where ECs are found to be p...