Methods and apparatus for mixing and delivery of process gases are provided herein. In some embodiments, a gas injection apparatus includes an elongate top plenum comprising a first gas inlet; an elongate bottom plenum disposed beneath and supporting the top plenum, the bottom plenum comprising a second gas inlet; a plurality of first conduits disposed through the bottom plenum and having first ends fluidly coupled to the top plenum and second ends disposed beneath the bottom plenum; and a plurality of second conduits having first ends fluidly coupled to the bottom plenum and second ends disposed beneath the bottom plenum; wherein a lower end of the bottom plenum is adapted to fluidly couple the gas injection apparatus to a mixing chamber such that the second ends of the plurality of first conduits and the second ends of the plurality of second conduits are in fluid communication with the mixing chamber.