Leeks grow in sandy soil that’s mounded up around them, which is how most of the stalk stays white. Grit nestles easily between the many leaf sheaves, so thorough cleaning is always required. This takes only a few minutes.
Swish chopped or sliced leeks in a water bath. Leave the leeks floating in lukewarm water for 5 to 10 minutes so any grit settles to the bottom, and then lift them out carefully. Repeat if necessary.