an arrangement for driving out volatile impurities from a ground water with a well shaft extending to the region of the ground water to be purified , the arrangement comprises an element for producing a negative pressure in the well shaft and supplying a gas underneath a water level in the well shaft , an element for providing a plurality of capillary openings in an upper sieve wall portion of the well shaft at least in a purification region between a gas inlet and a water level in the well shaft to enhance a laminary flow of ground water with relatively high speed , the capillary openings forming a greater free throughflow surface than a remaining portion of the well shaft .