Isn't the Water Supply unit considered a "Special Unit"? The Eng/Tkr was probably due on the 1st alarm, along with the Tkr. I imagine a Special alarm would be needed for the WS or possibly a "Tkr. Strike Force". The Officers on scene evidently felt things could be handled by utilizing units on scene.
Another question...Is/was the WS unit even in service?
Another question...Is/was the WS unit even in service?