The white lines on footwell carpets after winter are salt carried in on shoes. Vacuuming alone does not lift them.
Mix warm water with a little white vinegar, mist the stain, agitate with a soft brush and blot with a clean microfibre towel. Repeat until the towel comes up clean.
Leave the windows open a crack to dry, or ask us for an extraction clean, which rinses and dries the carpet in one pass.