Sulphate is well-known for its laxative effect; in fact, it is used as magnesium sulphate (Epsom salts) for just this reason. Sulphate in drinking water, especially in the presence of magnesium, can have the same effect. This is more pronounced in children and individuals not used to water containing higher concentrations of sulphate.
Sulphate can be present in water due to man-made and natural processes, but it is often added during the water treatment processing as aluminium sulphate. Sulphate increases the conductivity of water and so enhances corrosion processes which can then lead to a higher concentration of metals in the water. High levels of sulphate affect the taste of the water.