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We investigate the deep water abundance of Neptune using a simple 2-component(core + envelope) toy model. The free parameters of the model are the totalmass of heavy elements in the planet (Z), the mass fraction of Z in theenvelope (f_env), and the D/H ratio of the accreted building blocks (D/H_build). We systematically search the allowed parameter space on a grid and constrainit using Neptune's bulk carbon abundance, D/H ratio, and interior structuremodels. Assuming solar C/O ratio and cometary D/H for the accreted buildingblocks forming the planet, we can fit all of median ~ 7%), and the rest theconstraints if less than ~ 15% of Z is in the envelope (f_env is locked in asolid core. This model predicts a maximum bulk oxygen abundance in Neptune of65 \times solar value. If we assume a C/O of 0.17, corresponding toclathrate-hydrates building blocks, we predict a maximum oxygen abundance of200 times solar value with a median value of ~ 140. Thus, both cases lead to anoxygen abundance significantly lower than the preferred value of Cavali\'e etal. (2017) (~ 540 times solar), inferred from model dependent deep COobservations. Such high water abundances are excluded by our simple but robustmodel. We attribute this discrepancy to our imperfect understanding of eitherthe interior structure of Neptune or the chemistry of the primordial protosolarnebula.

What is Neptune's D/H ratio really telling us about its water abundance ?