Toxic Metals.  Arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver can all cause serious health problems. Lime soda softening and reverse osmosis can both be used to reduce the concentrations of toxic metals. Precipitation with alum is also effective for certain metals.  Lead can be a serious problem even if not found in raw water.  Corrosive waters may leach lead from pipes and fixtures, often exceeding the 0.015 mg/l maximum safe limit.