Which of the following conditions is most likely to create excessive lead in water?

The presence of excessive lead in water is most greatly influenced by the corrosion of household plumbing, particularly in older homes equipped with lead or lead-soldered pipes. When water sits in these pipes for extended periods, it can lead to the dissolution of lead into the water supply. Factors like water acidity, temperature, and flow patterns can accelerate this corrosion, leading to higher lead concentrations.

Pipes made of stainless steel generally do not corrode in the same manner as lead pipes and are not a source of lead contamination. Filtering through sand is primarily a method for removing larger particles and sediment and does not introduce lead. Use of soft water can actually increase the potential for corrosion if the water is aggressive or not properly managed; however, it is the direct corrosion of plumbing systems that poses the greatest risk for lead contamination. Thus, the corrosion of household plumbing is the most critical factor leading to excessive lead in water.