Neutralisation of Acid Rock Drainage by Youngest Toba Tuff Leachate Revealed by Hydrogeochemistry

The Youngest Toba Tuff (YTT) supervolcanic eruption occurred 75000 years ago, and resulted in distinctive ash fall deposition in different locations encompassing marine, estuarine, lacustrine, and fluvial sedimentary basins. Of the different sedimentary basins, the YTT crypto-tephra horizon preserved in the South Kerala Sedimentary Basin (SKSB) of the western coast of India is hosted by a paleo-estuarine carbonaceous clay layer. Along the eastern margin of SKSB, confined aquifers hosting highly acidic groundwater is associated with this YTT ash and associated organic matter (OM)-rich carbonaceous clay layer, creating worse acid rock drainage (ARD), which eventually gets neutralised during summer, signalled by the crystallisation of halotrichite. Hydrogeological investigation gave insights on some of the unique geochemical processes, which facilitated the neutralisation of ARD. The main aquifers in the area include laterite and clayey-sand, which is separated by this impervious layer hosting YTT ash. Wells tapping the clayey-sand aquifer, beneath this layer, is affected by the ARD condition due to the interaction with pyrite, manifested as low pH of groundwater (3.7). Simultaneously, leaching from YTT ash, which constitutes 11.91% of Al₂O₃, facilitates Al content to reach groundwater in high concentration (2879.97 ppb). During dry season, when the surface of YTT-hosting OM-rich carbonaceous clay layer is exposed, the leached Al interacts with the acid derived from the YTT-hosting OM-rich carbonaceous clay layer and results in the precipitation of halotrichite. The two processes, one resulting in ARD condition and the other as formation of halotrichite, occur in succession. Thus, the crystallisation of halotrichite signals the neutralisation of water as well as heralding the potability of water.