Which is not a measure of water turbidity?

The choice that is not a recognized measure of water turbidity is the Siple turbidity unit. The other units mentioned are established methods for quantifying turbidity in water.

Nephelometric turbidity units (NTU) are widely used to express the turbidity of water, based on the scattering of light by suspended particles. This method is common in many laboratory and field settings for water quality assessment.

Formazin turbidity units (FTU) arise from the use of a formazin polymer, which is a standard reference material used to calibrate turbidity measurements. This unit represents turbidity in a way that is specific to the formazin standard.

Jackson turbidity units (JTU) are also an older method used for measuring turbidity by comparing the depth at which a standard black disc can no longer be seen through the water sample.

The Siple turbidity unit, however, is not widely recognized or utilized in the context of turbidity measurement, making it the correct answer. Understanding these units is crucial for environmental health specialists when assessing water quality and potential impacts on public health.