An underground storage tank riser assembly comprises a riser member and at least one riser alignment spacer assembly. The riser member comprises an inferior riser end and an outer riser surface. The outer riser surface at the inferior riser end comprises a plurality of paired, optionally color-coded, curvilinear score indicia. The curvilinear score indicia enable a user to remove paired inferior end portions from the inferior riser end for forming a tank-engaging, saddle-shaped mouth at the inferior riser end. The saddle-shaped mouth comprises a vertical riser radius of curvature substantially equal in magnitude to a determinable tank radius of curvature. The saddle-shaped mouth is engageable with an outer tank surface in radial adjacency to a conduit extending therefrom. The riser alignment spacer assembly functions to center the riser member relative to the conduit.