Emergence of robust carbon quantum dots as nano-tracer for groundwater studies

Chemical tracers, both artificial and natural, are the established tools for providing information on groundwater flow characteristics such as groundwater flow velocity and flow direction. Chemicals namely chloride, bromide, iodide, dye tracers (Rhodamine-WT) and radioactive elements (Tritium) are artificial tracers that have been used as injected tracers for hydrogeological studies. However, the use of conventional tracers are often limited due to high diffusion rates and difficulties in estimating very low water flow velocities of the order 0.00001 to 0.001 m/s. Herein, we report the first ever large scale synthesis and testing of carbon quantum dots (CQDs) as hydrogeological tracers. CQDs, as they are now popularly termed, are a new class of environmentally friendly and highly fluorescent nanomaterial mainly used for bio-imaging and catalytic applications. In this manuscript, we have used fluorescence emission and surface functionalization of the CQDs as parameters to study flow of water in simulated laboratory columns and in field. The robustness of the CQDs towards environmental factors has been tested. Comparative tracer results suggested that CQDs may be envisaged as nano-tracers for future hydrogeological investigations.