Groundwater Characterisation in Urbanising Hard‐Rock Aquifers: Insights From Rock–Water Interactions in a Sub‐Humid Region of Central Kerala, India

ABSTRACT This study examines the hydrogeochemical characteristics and groundwater interactions across diverse aquifer systems in Palakkad, a water‐stressed and rapidly urbanising district in Kerala, southern India. Despite its significant dependence on groundwater, urbanisation has a profound impact on hydrological processes, particularly on groundwater quality and availability. This influence is mediated by local geology, where hard‐rock aquifers present hydrogeological complexities despite their storage capacity and contamination resistance. Understanding the interplay between urban development and groundwater quality through region‐specific studies is crucial, given the hydrochemical variations across different rock types. Using multiple analytical and geochemical modelling approaches, groundwater in Palakkad is classified as freshwater, predominantly of the calcium‐bicarbonate type, with most chemical parameters meeting WHO drinking water standards. Geochemical analysis reveals a cation dominance of Ca 2+ > Na + > Mg 2+ > K + and an anion dominance of HCO 3 − > Cl − > SO 4 2− > NO 3 − > F − . Groundwater chemistry is primarily controlled by geogenic processes, with rock–water interactions and reverse ion exchange being the dominant mechanisms. Key processes include the dissolution of silicate minerals, particularly ferromagnesian minerals and plagioclase feldspar, alongside contributions from secondary mineral precipitation (calcium carbonate, clay minerals), gypsum dissolution and cation exchange in soils. These findings highlight that urbanisation's impact on hard‐rock aquifers involves complex interactions among geology, land‐use changes and hydrochemistry, extending beyond merely reduced recharge or direct contamination of aquifers. This underscores the critical importance of incorporating local geological and hydrogeochemical knowledge to develop effective and sustainable urban water management strategies, with significant implications for groundwater resource management in rapidly urbanising regions worldwide.