Thin, roughly horizontal low-permeability layers are a common form of large-scale
heterogeneity in geological porous formations. In this paper, the dynamics of a buoyancydriven plume in a two-dimensional layered porous medium is studied theoretically, with
the aid of high-resolution numerical simulations. The medium is uniform apart from
a thin, horizontal layer of a much lower permeability, located a dimensionless distance
L 1 below the dense plume source. If the dimensionless thickness 2εL and permeability
Π of the low-permeability layer are small, the effect of the layer is found to be well
parameterized by its impedance Ω = 2εL/Π. Five different regimes of flow are identified
and characterized. For Ω L
1/3
, the layer has no effect on the plume, but as Ω is
increased the plume widens and spreads over the layer as a gravity current. For still
larger Ω, the flow becomes destabilized by convective instabilities both below and above
the layer, until, for Ω L, the spread of the plume is dominated by convective mixing
and buoyancy is transported across the layer by diffusion alone. Analytical models for
the spread of the plume over the layer in the various different regimes are presented
