Electron transport in Si low-angle bicrystals is analyzed by a novel Monte Carlo molecular dynamics simulation scheme. The effect of discrete charges at the grain boundaries is studied and compared to results from one-dimensional treatments. The average grain boundary charge density strongly influences transport, and a field-dependent threshold effect is predicted. Details of the internal charge arrangement are shown to be quite important at low fields and/or high grain boundary charge densities. Substantial increases in current conduction are predicted at lower temperatures over the thermionic emission model. Finally, analyses of interacting grain boundaries indicate site-correlation effects and a strong dependence of conductivity on the separation distance.