Brick is but one example of how lead contamination can elude rules and authorities, potentially for years.

“We need an aggressive program to get rid of lead service lines, starting with an inventory so we know where they are,” said Lynn Thorp, the national campaigns director for Clean Water Action, an advocacy group. “Water systems need to up their game and take this problem more seriously.”

Both scientists and advocates say the rules governing contamination from lead pipes are ridden with loopholes. For example, the E.P.A.’s lead rule requires water systems to test in only a small number of homes with lead pipes — 50 to 100 for large systems — and intervals between testing can stretch to three years.

Water systems use various protocols for tap water tests, and rules allow ordinary homeowners to conduct them unsupervised, raising questions about their consistency. Officials must disclose contamination and take remedial action only if tests show more than 10 percent of sampled homes exceed the standard. Advocates say that lets utilities declare their water safe even if contamination is uncovered.

”Over the last decade we’ve learned that the testing routines did not detect true risk from lead, that there are forms of lead that we’re not testing for and that testing was too infrequent,” said Dr. Griffiths, the former chairman of the E.P.A.’s Drinking Water Committee. “It’s hard to see how the status quo in lead testing for water is adequately serving the public.”

In December, the Drinking Water Committee endorsed recommendations by an advisory group to strengthen the lead rule in several critical areas. The group said water systems should bolster their anti-corrosion efforts and test more often to ensure that they are working. It called for the E.P.A. to set a standard for lead in drinking water based on its effect on people’s health, likely below the current level, and to require water systems to tell homeowners and public-health officials whenever it is exceeded.