Abstract: A method of purifying underground water in a pollution zone comprises determining the rock components of the pollution zone and the flow velocity of the underground water; taking samples of the underground water from a plurality of wells without admitting air and determining their sulfate ion (SO.sub.4.sup.2.sbsp.-) concentration and pH value to reveal the acidity/alkalinity of the environment, a pH less then 4 or greater than 11 being changed to one in a range of values from about 4 to about 11. Then, sulfate-reducing bacteria are selected and a biomass of them is accumulated. The resulting biomass is introduced into the underground water through those wells whose water samples have a pH value lying within the range of values from about 4 to about 11. And then, the biomass is introduced into the underground water through the remaining wells. The biomass input depends on the sulfate ion concentration, the higher the sulfate ion (SO.sub.4.sup.2.sbsp.