ASSESSMENT OF FATE AND TRANSPORT OF HEAVY METALS (CR, CD) IN HARD-ROCK AQUIFERS

Abstract

In this work, we focused on chromium (Cr) and cadmium (Cd) behavior in varied hard-rock aquifer lithologies, exploring their sorption capacities and transport behavior. Hard rocks, known for their low porosity and heterogeneity, significantly influence the movement of contaminants through fractures and surfaces. Cadmium, a toxic metal primarily from human activities like mining and industrial discharge, poses significant risks to ecosystems. Chromium, particularly its hexavalent form, Cr(VI), originating from industrial and agricultural sources, is also a concern due to its carcinogenicity. WHO guidelines recommend limits of 0.003 mg/L for Cd²⁺ and 0.05 mg/L for Cr(VI) in groundwater, often exceeded in many regions, indicating environmental hazards and health risks. Alteration of fresh bed rocks in hard-rock aquifer systems leads to the formation of weathered secondary rock and mineral surfaces, resulting in a multi-layered sub-surface environment. These surfaces have active sites for contaminant interaction and can also change flowing water quality ultimately impacting contaminant retention properties. The study involved characterizing hard rock materials and conducting batch-sorption and column transport experiments to understand contaminant-rock interactions in such settings. Results indicated varying sorption capacities across lithologies, with Basaltic aquifers demonstrating superior chromium retention and Limestone aquifers showing superior cadmium retention compared to granitic ones. Ionic competition minimally affected sorption, prompting the use of NaCl solutions in subsequent experiments. Obtained sorption kinetics and varying contaminant concentrations results were modelled using pre-established models like Freundlich, Langmuir and Kinetic models. Rainwater showed a higher adsorption effect compared to river water due to presence of competitive ions. Crystallinity, surface charges and mineral compositions were characterised for the rock and mineral samples using XRD, Zetasizer, and XRF respectively. Column transport experiments highlight differences in chromium and cadmium retention capacities among different aquifer systems, with basaltic and limestone aquifers exhibiting higher retention capacities compared to granitic aquifers. Leaching evaluation of constituent ions from rocks and mineral powders elucidates ion exchange mechanisms and retention capacities, with basaltic and limestone aquifers showing greater retention capacities attributed to higher leaching of constituent ions and greater ion exchange potential.