It's very common for leeks to hold onto sand and grit from the soil they're grown in. To wash them thoroughly, first thinly slice them then soak them in a large bowl of cold water. Give them a gentle stir or shake to remove any soil or grit between the layers then allow them to sit for a few minutes, during that time impurities will fall to the bottom of the bowl. Use your hands or a slotted spoon to scoop the leeks out of the bowl and into a colander, give them a final rinse and they're good to go.