The electric field gradients at the central deuterons in one hundred clusters consisting of heavy water molecules, previously calculated in a supermolecular approach including many-body effects, is calculated assuming pair-additivity. Excellent pair-additivity of the electric field gradient is found for the water clusters. This result is confirmed for one hundred clusters extracted from a water-DMSO mixture. The use of pair-additivity results in substantial computer time savings in the quantum chemical calculation of the electric field gradients for nuclei in liquid systems using the so-called cluster approach, and hence their quadrupole coupling constants. It also permits the simulation of quadrupolar relaxation times from electric field gradient hypersurfaces obtained within the pair-additive approximation.