  Transported chemical reactions in unsaturated porous media are relevant
across a range of environmental and industrial applications. Continuum scale
dispersive models are often based on equivalent parameters derived from analogy
with saturated conditions, and cannot appropriately account for processes such
as incomplete mixing. It is also unclear how the third dimension controls
mixing and reactions in unsaturated conditions. We obtain 3$D$ experimental
images of the phases distribution and of transported chemical reaction by
Magnetic Resonance Imaging (MRI) using an immiscible non-wetting liquid as a
second phase and a fast irreversible bimolecular reaction. Keeping the P\'eclet
number (Pe) constant, we study the impact of phases saturation on the dynamics
of mixing and the reaction front. By measuring the local concentration of the
reaction product, we quantify temporally resolved effective reaction rate
($R$). We describe the temporal evolution of $R$ using the lamellar theory of
mixing, which explains faster than Fickian ($t^{0.5}$) rate of product
formation by accounting for the deformation of mixing interface between the two
reacting fluids. For a given Pe, although stretching and folding of the
reactive front are enhanced as saturation decreases, enhancing the product
formation, this is larger as saturation increases, i.e., volume controlled.
After breakthrough, the extinction of the reaction takes longer as saturation
decreases because of the larger non-mixed volume behind the front. These
results are the basis for a general model to better predict reactive transport
in unsaturated porous media not achievable by the current continuum paradigm.
