Before revision:  The longest residence times have been observed for water molecules near the counterionsthat are localized in the minor groove of the double helix : about 30 ps in the case of Na^{+ K^{+} and Cs^{+} counterions. In the major groove and outside the double helix it is essentially lower. The counterions constrain water molecules too strong, and as the result the effect of negative hydration for K^{+} and Cs^{+} counterions was not observed in the simulations.
After revision:  The longest residence times have been observed for water molecules near the counterionsthat are localized in the minor groove of the double helix : about 30 ps in the case of Na^{+ K^{+} and Cs^{+} ). The estimated potentials of mean force for the hydration shells of the counterions show that the water molecules are constrained too strong, and as the result the effect of negative hydration for K^{+} and Cs^{+} counterions was not observed in the simulations.