The present invention removes contaminants from well water without creating the problem of large volumes of waste liquid disposal. It uses double-cone devices (102, 116, 125) as pressure amplifiers which due to their lack of moving parts avoid the risk of additional contamination of the well and provide a prolonged lifetime. For toxic contaminants, a periodic purging of the concentrated contaminants is required. In the case of (sea) salt contamination, there is no waste brine to be continuously handled at all and the system may be run without interruption over an extended period. In this case, the brine is dumped into the well simply by periodically stopping the plant. The brine pours out of the double-cone unit(s) (102, 302) and due to its higher density, sinks down in the well without disturbing the operation of the purification plant.