Newark, with 285,000 people, is the largest city in New Jersey, but also one of the poorest in the country. It has long struggled with lead contamination, both in the water and from paint in homes.
No concerns have been raised about the source of the water — reservoirs in northern New Jersey. The lead has leached into the tap water from 15,000 antiquated service lines that connect water pipes to homes and businesses.
City and state officials have known for years that the infrastructure was a major risk, but they lacked the funding to replace the aging service lines.
So, the city turned to an approved chemical, sodium silicate, that prevents corrosion and the leaching of lead from pipes into water. For more than two decades, it worked as expected, and no tests showed elevated levels of lead.
Then in 2016, the chemical seemed to stop working.
Here is what appeared to have happened, according to interviews and public records: The year before, the city had tinkered with the water, increasing its acidity to tamp down on possible carcinogens.
But the increased acidity seemed to reduce the effectiveness of the sodium silicate.