After injecting a coloured dye into a discharge, staff collect samples around the discharge point (which may be a diffuser section on the end of an ocean outfall, or a pipe discharging into a stream).
The dyes used for these studies are dyes that fluoresce, or glow, when exposed to light of a particular wavelength.
Very low concentrations of the dye can therefore be measured - down to a few parts per billion and the movement and concentration of even a few cupfuls of dye can be tracked for several kilometres in the sea.
This allows the rate of movement, directions of travel, and dilution of effluent plumes to be determined, so that for example, concentration of a contaminant on the water's edge of a beach or over a shellfish bed can be calculated.