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.
After revision:  The longest residence times have been observed for water molecules contacting with the counterions, localized in the minor groove of the double helix : about 30 ps in the case of Na^{+ K^{+} and Cs^{+} counterions.