Sand in my basement window frames

Our house is a bi-level, and we have siding on the upper level of the house. The lower level, that is approximately half out of the ground, has a stucco finish. All of the basement windows are double paned sliders, and have a small metal lip at the top that (I believe) is supposed to help re-direct water away from the windows slightly. I have noticed recently that at the base of all my basement windows, where the glass meets the frame, and in the slider tracks, that it's full of sand. The volume of sand actually bulges the lower plastic (vinyl) of the window frame slightly. I can manage to clean it out painstakingly, with toothpicks, cloths, etc. But within a few months it's built up again. I think it is sand particulate from the stucco, running into the window frame on the outside.

My question is, what can I do about this? This cannot be good for the windows, and I don't want to deal with rotting walls or damaged windows. The house was built only 13 years ago, so is not that old.

Any help or suggestions would be greatly appreciated!

Thank you