Broadly speaking, "mineral water" is groundwater that has emerged from the ground and flowed over rock. Treatment of mineral water is restricted to removal of unstable elements such as iron and sulfur compounds. Treatment for such minerals may extend only to filtration or decanting with oxygenation. Free carbon dioxide may be removed only by physical methods, and the regulations for introduction (or reintroduction) of CO2 are strictly defined. Disinfection of natural mineral water is completely prohibited, including the addition of any element that is likely to change bacterial colony counts. If natural mineral water is effervescent, it must be labelled accordingly, depending on the origin of the carbon dioxide: naturally carbonated natural mineral water (no introduction of CO2); natural mineral water fortified with gas from the spring (reintroduction of CO2); carbonated natural mineral water (CO2 added following strict guidelines).