Abstract: A portable water injection plant for use either in waterflooding operations in secondary recovery of oil from an underground formation penetrated by oil recovery wells and water injection wells, or in water disposal operations, includes a basic portable plant module comprising a portable foundation having a water pump mounted thereon and having electrical wiring and fluid conduits preassembled on the foundation, the fluid conduits including an intake header coupled to the water inlet of the pump and a discharge header coupled to the water outlet of the pump. Coupling means are provided for readily detachably coupling the intake header to one or more associated water sources and the discharge header to one or more water injection wells. Auxiliary portable plant modules may be readily detachably coupled to the basic plant module and to additional water sources to increase the pumping capacity of the water injection plant.