Leeks tend to trap a lot of earth, sand, and grit in between their layers. It's important to wash leeks twice before using them. For dishes where the leek will be used unchopped (such as Braised Leeks), trim the roots leaving the root core still attached to the leek. Fan through the leek layers with your thumb and middle finger while running the leek under cold, clean water. Repeat several times.
Chopped leeks should be submerged in a large amount of clean water, rubbed together with your hands, and then skimmed off. Rinse the bowl of any accumulated grit, and wash the leeks this way again. Make sure to agitate the water vigorously with your hands to dislodge stubborn sand.