= Solution
\b[Contamination requires a lead-bearing source and conditions that dissolve it or detach lead-containing particles. Soft, poorly buffered <water> standing in contact with <lead> is a classical risk, but neither hardness nor clarity guarantees safety.]
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.<On the Action of Water upon Lead by Robert Christison>[1844, pp.265–276: pure waters, natural waters and the effects of dissolved salts]{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.<Pipe and Fittings Corrosion by the Drinking Water Inspectorate>[Lead solubility, pH, alkalinity and carbonate deposits]{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.<Lead in Drinking Water by the Drinking Water Inspectorate>[Factors affecting lead concentration; lead pipes and fittings; standing water and supply condition]{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.
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