An error in the previously published equation (A.6) affects other equations in the paper, leading to a correction of equation (83) for J(u) and a replacement of figure 3, as presented in the associated PDF file.Qualitatively, however, the same J(u) behavior persists and therefore all the implications regarding the impurity measurements in Alcator C-Mod and the bootstrap current, previously discussed in section 7, persist as well.The authors thank Matt Landreman for bringing this error in the paper to their attention.