A process for insitu treatment of at least one contaminant in a groundwater plume, the process comprises the steps of: installing a permeable reactive barrier in the flow path of the groundwater plume, wherein the installation of the permeable reactive barrier comprises the steps of: boring a first borehole with a portion longitudinally extending substantially parallel with an upper surface of the groundwater plume; filling the longitudinally extending portion of the first borehole with a reactive material comprising a solid reactant configured to react with the at least one contaminant in the groundwater plume to produce at least one product having less hazardous characteristics than the at least one contaminant; boring a second borehole with a portion proximate, substantially adjacent or overlapping the longitudinally extending portion of the first borehole; and filling the portion of the second borehole proximate, substantially adjacent or overlapping the longitudinally extending portion of the first borehole, with the reactive material.