  An understanding of the abundance and distribution of water vapor in the
innermost region of protoplanetary disks is key to understanding the origin of
habitable worlds and planetary systems. Past observations have shown H2O to be
abundant and a major carrier of elemental oxygen in disk surface layers that
lie within the inner few au of the disk. The combination of high abundance and
strong radiative transitions leads to emission lines that are optically thick
across the infrared spectral range. Its rarer isotopologue H2-18O traces deeper
into this layer and will trace the full content of the planet forming zone. In
this work, we explore the relative distribution of H2-16O and H2-18O within a
model that includes water self-shielding from the destructive effects of
ultraviolet radiation. In this Letter we show that there is an enhancement in
the relative H2-18O abundance high up in the warm molecular layer within 0.1-10
au due to self-shielding of CO, C18O, and H2O. Most transitions of H2-18O that
can be observed with JWST will partially emit from this layer, making it
essential to take into account how H2O self-shielding may effect the H2O to
H2-18O ratio. Additionally, this reservoir of H2-18O-enriched gas in
combination with the vertical "cold finger" effect might provide a natural
mechanism to account for oxygen isotopic anomalies found in meteoritic material
in the solar system
