A novel fluorescent tracer for real-time tracing of clay transport over soil surfaces

• A novel clay-sized tracer has been created by labelling montmorillonite with Rhodamine B.
• The tracer can be detected and tracked in real time and during rainfall events.
• The application of the tracer in soil erosion studies is demonstrated.

Clay is an important vector for the transport of pollutants in the environment, including nutrients, pesticides and metals; therefore, the fate of many chemicals in soil systems is closely linked to that of clay. Understanding the mechanisms responsible for clay transport has been hampered by the lack of a suitable tracer. Producing a tracer that accurately mimics clay transport is challenging, due to the small size of the particles and their unique physical properties. Here we describe the design and synthesis of a tracer using natural clay particles as a foundation, exploiting the natural ability of clay to sorb molecules to coat the clay with a thin layer of fluorophore. Application of the tracer has been demonstrated through the collection of real-time images of the tracer moving over the surface of a soil box during a rainfall event. These images allow, for the first time, clay to be tracked spatially and temporally without need to remove soil for analysis, thus resulting in minimal experimental artefacts. Custom written software has been used to extract high resolution data describing tracer movement and extent throughout the experiment.

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