The present invention relates to a method for decreasing the contents of metals, metalloids, nitrate and nitrite, respectively, in raw water flowing through a basin (1), whereby water containing either oxygen, oxygen-releasing substances, metabolic reactions of naturally occurring micro-organisms or containing denitrification organisms and/or substrates therefore is intermittently introduced to the basin (1) through a number of satellite wells (2) arranged around at least one extraction well (3) for purified water. Water containing oxygen and/or oxygen-releasing substances or metabolic reactions of naturally occurring micro-organisms or denitrification organisms and/or substrates therefore is homogeneously distributed vertically in said basin (1), whereby the water to be purified is fed into the basin by horizontally arranged feeding pipes (5) and that the extraction of purified water from said at least extraction well (3) is such that a substantially negative pressure is created around said at least one extraction well (3). The invention relates also to an apparatus for carrying out the method and to a satellite well to be used in the apparatus.