Abstract: Apparatus is disclosed for separating an amount of a substance from groundwater, comprising an elongate chamber (18) having an inlet (22) which is arranged in use to admit groundwater into the chamber near a lower first end (24). There is also a gas sparger (26) located near the first end (24) which admits gas into the chamber for inducing groundwater to flow from the first end (24) of the chamber toward a second end upper end, and for producing a froth layer (32) which rises above an interface with the groundwater including a concentrated amount of the substance. A suction hood (38) can be moved downward from the top of the chamber (18) into a position to collapse the froth layer (32) and to cause it to be removed from the well body (14). The suction hood (38) (acting as a froth depth regulation device) controls the amount of groundwater in the froth layer (32), which influences the concentration of the contaminant substance achieved in the froth layer (32).