From where must all public water systems initially collect samples to monitor lead content?

The correct answer is that all public water systems must initially collect samples from high-risk residences to monitor lead content. This is crucial because high-risk residences typically include older homes and buildings that may have lead plumbing or fixtures. These locations are more likely to harbor lead contamination in the drinking water due to corroded pipes or the presence of lead solder used in plumbing systems, particularly those constructed before the 1986 federal banning of lead in plumbing materials.

Sampling from high-risk residences allows water system operators to identify potential issues proactively and ensure that corrective actions can be taken to protect public health. It helps in targeting the populations most at risk, such as children and pregnant women, who are particularly vulnerable to the harmful effects of lead exposure. Monitoring lead at these specific sites is a vital step in maintaining water quality and ensuring compliance with regulatory standards aimed at reducing lead exposure in drinking water.

The other options, while relevant to public drinking water systems, do not specifically focus on the highest risk areas for lead contamination. Public facilities, school buildings, and commercial properties could all contain lead but are not necessarily the starting points for monitoring lead content in the water supply as high-risk residences are.