Fate and retention behavior of varying charged heavy metals in monolithic and multi-layered hard-rock subsurface...

Understanding contaminant fate and transport in hard rock aquifers under varying geochemical conditions is crucial for assessing groundwater contamination possibilities. Present study examines the retention and transport behavior of heavy metals in multi-layered subsurface aquifer systems. Three subsurface mimicking multi-layered columns were prepared representing basaltic (basalt-serpentine-bentonite), limestone (limestone-calcite-shale), and granitic (granite-feldspar-kaolinite) aquifer materials to evaluate their contaminant retention potential. Batch experiments revealed that serpentine (486.9 mg/kg for Cr and 3094 mg/kg for Cd) and calcite (3952 mg/kg for Cd) exhibited the highest retention capacities among the respective aquifer lithologies. Ionic competition studies suggested that retention of chromate ions was largely unaffected in the presence of Cd²⁺. Results from column transport experiments demonstrate distinct contaminant retention behaviors in monolithic and multi-layered rock systems. Cadmium exhibited higher retention than chromium, with values reaching 5781.7 mg/kg in serpentine and 5791.7 mg/kg in limestone triplets, compared to 260 mg/kg for chromium in limestone. Multi-layered systems showed altered water chemistry, significantly affecting retention. For instance, in the limestone triplet, chromium retention was 2.9 mg/kg compared to 87.2 mg/kg for averaged monolithic columns, while cadmium retention exceeded 5791.7 mg/kg compared to monolithic columns average of 917.7 mg/kg. Similarly, basaltic and granitic systems exhibited deviations between calculated and actual retention values. These results underline the critical roles of variations in mineral composition between multiple subsurface layers, geochemical conditions, and water chemistry in controlling contaminant mobility in hard-rock aquifer systems.