In drinking-water supplies, iron(II) salts are unstable and are precipitated as insoluble iron(III)
hydroxide, which settles out as a rust-coloured silt. Anaerobic groundwaters may contain
iron(II) at concentrations of up to several milligrams per litre without discoloration or
turbidity in the water when directly pumped from a well, although turbidity and colour may
develop in piped systems at iron levels above 0.05–0.1 mg/litre. Staining of laundry and
plumbing may occur at concentrations above 0.3 mg/litre