From the water main to the water glass: your drinking water system
When water leaves the treatment plant in your distribution system, it contains variable amounts of common ions such as Ca++, Mg++, Na+, Cl-, etc., but the water is normally free of heavy metals such as Cu, Cr, Fe, Ni, Pb, V, Zn and others. However, if you sample the water at your tap, it is not unusual to find these substances. For the most part they are produced by the corrosion of the metallic components in the distribution system, many of which are inside or close to your house:
Components of the System
The water main - usually cast iron or cement-lined ductile iron. The unlined pipes develop tubercles that can break off and shed iron-rich particles into the water. Sometimes these have toxic metals attached to them, which then reach the customer's tap.
The service branch - copper, galvanized iron, or lead. If lead, the service branch is capable of producing high lead levels at the downstream tap. Galvanized iron services are also associated with elevated lead.
Internal plumbing
(1) Lead-soldered copper. Solder tends to leach Pb in small amounts and cases are known of Pb poisoning from this source. Source of Cu, Ni, Zn.
(2)Brass components - valves, fittings, meters, and faucets. Most brass has appreciable Pb that can leach to yield Pb at the tap [see Pb content of brass used in plumbing], so even houses without Pb service lines can experience dangerous Pb levels. Source of Cu, Ni, Pb, Zn [see Brass additives].
(2) Galvanized iron. A common treatment for iron and steel is to apply a zinc coating, a process called galvanizing. Among the components commonly used for internal plumbing, this is the one most prone to corrosion.
(3) Plastic-
(4) Junctions between different metals. A house will have dozens of joins of copper to brass or lead to brass or iron to copper or brass. Joining two different metals can create a difference in electric potential and hence an electric current. This current accelerates corrosion of one of the metals.
Scale Minerals - Fe, Cu, and Pb oxides and carbonates, Cu sulfates, Pb phosphates and vanadates line the inner surfaces of the plumbing system. In some cases these coatings provide protection from corrosion; in others they themselves are sources of high levels of metals at the tap. [see tables of common minerals - Pb - Cu - Fe]. These scales may adsorb toxic elements such as Ba, Mn, Ra, and V and, if bits of scale become detached, transport these substances to tap water.
Organisms - despite the presence of disinfectants in drinking water, many bacteria live and even thrive within the protective cover of corrosion scales. Most distribution systems also have a biofilm of organic material coating the insides of the pipes. Finally external microorganisms, frequently diatoms, end up being incorporated in pipe scales.