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An operator‐splitting approach is often used for the numerical solution of advection‐dispersion‐reaction problems. Operationally, this approach advances the solution over a single time step in two stages, one involving the solution of the nonreactive advection‐dispersion equation and the other the solution of the reaction equations. The first stage is usually solved with a finite difference, finite element, or related technique, while the second stage is normally solved with an ordinary differential equation integrator. The only generally published guidelines on numerical accuracy suggest that the discretization errors associated with each stage must be small in order to achieve high accuracy of the overall solution. However, in this note we demonstrate that there is an inherent mass balance error present in the operator‐splitting algorithm for problems involving continuous mass influx boundary conditions. The mass …