The modification of the bulk resistivity of semi-insulating GaAs caused by incorporation of copper has been studied. Wafers cut from an ingot annealed, intentionally Cu contaminated LEC grown crystal were investigated optically and electrically. A characteristic variation pattern of the Cu related photoluminescence intensity, gradually decreasing towards the center, is observed. The resistivity exhibits a quite similar radial dependence. The results convincingly show that Cu is diffusing across macroscopic distances into GaAs and is responsible for the radial resistivity variation. The topographic correlation allows to quantify the Cu acceptor content.