  The continuum-scale electrokinetic porous-media flow and excess charge
redistribution equations are uncoupled using eigenvalue decomposition. The
uncoupling results in a pair of independent diffusion equations for
"intermediate" potentials subject to modified material properties and boundary
conditions. The fluid pressure and electrostatic potential are then found by
recombining the solutions to the two intermediate uncoupled problems in a
matrix-vector multiply. Expressions for the material properties or source terms
in the intermediate uncoupled problem may require extended precision or careful
re-writing to avoid numerical cancellation, but the solutions themselves can be
computed in typical double precision. The approach works with analytical or
gridded numerical solutions and is illustrated through two examples. The
solution for flow to a pumping well is manipulated to predict streaming
potential and electroosmosis, and a periodic one-dimensional analytical
solution is derived and used to predict electroosmosis and streaming potential
in a laboratory flow cell subjected to low frequency alternating current and
pressure excitation. The examples illustrate the utility of the eigenvalue
decoupling approach, repurposing existing analytical solutions and leveraging
simpler-to-derive solutions or numerical models for coupled physics.
