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Semi-arid coastal aquifers face critical challenges characterized by lower rainfall, higher evaporation rates, and looming risk of over-exploitation. These conditions, coupled with climate change, are conducive to seawater intrusion and promote mechanisms associated with it. The understanding of metal behavior in such environments is limited, and hence, an attempt is made through this review to bridge the knowledge gap. A study on the behavior of trace metals within a specific context of semi-arid coastal aquifers was carried out, and 11 aquifers from 6 different countries were included. The review observed that trace metals within semi-arid coastal aquifers exhibit distinctive behaviors influenced by their surrounding environment. The prevalence of evaporation and continuous seawater intrusion played a pivotal role in shaping trace metal dynamics by curtailing groundwater flux. The findings suggest that the formation of stable Cl and organic ligands under increased alkaline conditions (pH > 8) has higher control over Zn, Pb, and Cd toxicity in a highly ionic reactive condition. In addition, dominant control of Fe/Mn-hydroxide association with Pb and high organic affinity of Zn played a pivotal role in controlling its bioavailability in aquifers such as WFB, Saudi Arabia NW-C and India. On the contrary, under prevailing acidic conditions (pH 8), and chloride complexes govern the distribution of trace metals further enhanced by prolonged water residence time. Nonetheless, specific conditions, such as a reducing and acidic environment, could potentially elevate the solubility of highly toxic Cr (VI) released from anthropogenic sources.
Gantayat et al. (Fri,) studied this question.