  Random Walk (RW) is a common numerical tool for modeling the
Advection-Diffusion equation. In this work, we develop a second order scheme
for incorporating a heterogeneous reaction (i.e., a Robin boundary condition)
in the RW model. In addition, we apply the approach in two test cases. We
compare the second order scheme with the first order one as well as with
analytical and other numerical solution. We show that the new scheme can reduce
the computational error significantly, relative to the first order scheme. This
reduction comes at no additional computational cost.
