An inflatable downhole isolation packer seals a water well bore above a pump assembly to prevent contamination from surface water or superficial aquifers. The device features an elastomeric expansion bag containing a dual-chamber chemical system. A frangible capsule isolates an activating agent from a base reactant. Upon deployment to a target depth, a mechanical, hydraulic, or electrical trigger breaks the frangible capsule. The resulting chemical reaction generates an expanding foam or gas, inflating the bag to conform tightly against the rough internal diameter of the well bore, creating a