Evidence of the bright-green fluorescein water dye that has been added to an Oxfordshire chalk stream to confirm the source of a pollutant that is contaminating the stream. Environment and water resource agencies use fluorescein dyes to trace ground water flows to assist in the location or presence of pollutants. Fluorescein is a fluorescent water dye considered ecologically safe and often chosen for tracer studies because of its low toxicity, water solubility, ease of detection, ready availability and low cost. Dye tracking may be either qualitative, that is, showing the presence of particular flow and quantitative, when the amount of the traced dye can be measured by special instruments.