PCSiWaPro® is a software tool developed by the TU Dresden which simulates water flow and contaminant transport in the unsaturated soil zone. Due to the increasing demand in computational efficiency for large numerical meshes, simulation runs needed to be accelerated. This acceleration was carried out by manual parallelization of parts of the software's simulation kernel. After identifying the most time-consuming sections of the code several numerical equation solver libraries were implemented and tested. Different concepts of software parallelization as well as different types of mathematical solving strategies were tested for their suitability for PCSiWaPro®. With the SAMG software, a multigrid solver developed by the Fraunhofer Institute for Algorithms and Scientific Computing, the largest reduction of simulation time could be achieved