Past exam of the medicine course of the University of Oxford / 1864 / Second examination / Forensic medicine and hygiene / 9

Under what conditions is drinking water usually contaminated by Lead?

The source may be a pipe, cistern, service connection or fitting. Robert Christison's pre-examination experiments addressed the action of different waters on metallic lead: very pure waters, including distilled water or rainwater, could attack it, whereas salts in many natural waters modified the process. This provides a contemporary basis for treating the character of the supply, as well as the material of the cistern or pipe, as important.^[ref]

The dissolving tendency is called plumbosolvency. In modern chemical terms, low pH and low alkalinity can favour the release of lead. Dissolved oxygen supports corrosion of the metal; dissolved carbon dioxide affects both acidity and carbonate chemistry. Carbonate deposits can provide a partly protective surface layer, while conditions favouring their dissolution can increase release. It is therefore misleading to say simply that all carbon dioxide prevents contamination or that all waters containing it dissolve lead to the same degree. Water hardness and alkalinity are related features of many waters but are not interchangeable quantities.^[ref]

Long contact, especially stagnation overnight or through disuse, can increase contamination. Warmer conditions, changes in the supply chemistry, disturbance of an old pipe and the condition of accumulated deposits can also matter. Water can carry dissolved lead or particles containing lead; an old apparently coated pipe is not an assurance that release has ceased. Modern supplies may additionally encounter lead-bearing solder or brass, which should be recognized as a present comparison rather than assumed to be the construction of a particular 1864 installation.^[ref]

A clear appearance and acceptable taste cannot exclude lead. Investigate the actual plumbing materials and analyse appropriately collected samples, recording the period of standing and other conditions instead of inventing a concentration. The most direct way to remove the source is replacement of lead-bearing components; treatment that controls corrosion is chemistry-dependent and is not equivalent to a universal guarantee from a hard-water scale. These considerations answer when contamination is likely without imposing an unsupported period numerical threshold.