why is the water in a quarry so blue?

Answer 1

Two reasons:
1) It has no organic matter. This is because it is artificial and because the lime from the limestone makes the water inhospitible to growth.
2) It's bottom is made of pale colored limestone.

Answer 2

Quarry water is often blue because it contains tiny particles of suspended clay, which scatter the blue spectrum of light, in a similar way to the scattering effect that makes the sky appear blue (search for question 'why is the sky blue? for a complete answer).