(CNN) - Residents and officials in Newark, New Jersey, are trying to determine exactly when lead began leaching into the water supply.
It's a vital question, since lead contamination in water can damage a child's health. Even low levels of lead have been linked to serious, irreversible damage to developing brains. For this reason, experts say that there is no safe level of lead in drinking water.
The local government in New Jersey's largest city has begun to address the unsafe level of lead in water supplied to some residents, but still struggles to answer when -- and how -- this began.
Generally, city officials say the elevated levels of lead in Newark's drinking water are "due to the releasing of lead from plumbing and lead service lines on private property between the street and approximately 15,000 homes." Also, the Pequannock water treatment plant's corrosion control system "became less effective at reducing the corrosion of lead pipes and resulted in rising lead levels in some homes with lead service lines throughout Newark," the city said.
The long-term fix involves replacing service lines, but until that multi-year project can be completed, Newark is implementing short-term solutions.
Here's what we know:
2010-16 school years
The Newark Board of Education tested water samples in city schools beginning in the 2010-11 school year. At that time, 12.2% of all samples tested above the 15 parts per billion (ppb) federal actionable lead level as defined by the US Environmental Protection Agency. (The EPA's "lead and copper rule" states that action must be taken if lead concentrations exceed 15 ppb in more than 10% of sampled taps.)
The board of education said it turned off or remediated all fountains with elevated lead levels, including removing galvanized water lines and installing new "lead free" lines and fixtures. The city also offered free blood lead level tests to families who desired one and of nearly 500 students tested; just one student was found to be above the action level of 10 micrograms per deciliter, the board of education reported (citing The New York Times).
During the 2011-12 school year, water testing showed that 11.7% of the samples rose above the 15 ppb actionable lead level, according to the Newark Board of Education.
For the 2012-2013 school year, just 6.84% city school water samples tested above the actionable lead level. But in 2013-2014, Newark schools once again slipped into the unsafe zone when 13.31% of all samples showed too much lead.
A record 15.27% of samples tested above the 15 ppb lead action level the following school year, but lead levels dipped during the 2015-16 school year to just under 10% of samples testing above 15.5 ppb.
June 30, 2017
Citywide testing that had begun in January showed that more than 10% of homes across Newark had nearly twice the 15 ppb of lead in drinking water -- the amount permitted by federal law.
September 30, 2017
The Natural Resources Defense Council, a non-profit environmental advocacy group, sends a letter to officials in Newark stating that the city has failed to respond comprehensively to contamination of its drinking water.
December 31, 2017
Newark is once again in violation of the federal action level for lead in drinking water for a second consecutive water monitoring period (beginning in July 2017), according to water quality reports.
June 26, 2018
The Natural Resources Defense Council and Newark Education Workers caucus, a group of educators who teach in the city's public schools, sue state and city officials for ongoing violations of the federal Safe Drinking Water Act. Erik D. Olson, a drinking water expert at the council, says Newark is not using enough of the chemical treatment that creates a protective layer inside old lead pipes to prevent leaching of lead into the water supply.
June 30, 2018
Newark exceeds federal limits on lead in drinking water for the third consecutive monitoring period beginning in January 2018. Quality reports have found drinking water in parts of Newark to be unsafe for 18 months.
October 12, 2018
City officials announce a plan to distribute more than 40,000 National Sanitation Foundation certified water filters to Newark residents. Roll-out begins later in the month.
December 31, 2018
Based on testing begun in July, Newark violates the federal action level for the fourth time, according to the Natural Resources Defense Council complaint. Throughout two years, many Newark residents have been supplied with water contaminated by illegal amounts of lead.
January 14, 2019
An open letter to President Trump authored by Newark Mayor Ras Baraka asks for federal assistance to repair the water infrastructure in New Jersey and around the country.
February 22, 2019
The Natural Resources Defense Council asks the court to mandate bottled water to certain Newark residents, including all families with children under 6 years old, pregnant women and homes with elevated lead levels or lead service lines.
Newark begins construction on the first phase of its multi-year Lead Service Line Replacement Program with the goal of removing all the lead service lines in the water system. The intention of the program is to remove 15,000 lead services lines and replace them with copper pipes. By the end of March, more than 38,000 water filters and over 31,000 replacement cartridges have been distributed to residents.
June 30, 2019
Newark violates the federal action level for lead in drinking water for the fifth consecutive monitoring period, running from January through July, according to the Natural Resources Defense Council.
August 9, 2019
The City of Newark conducts follow-up testing and finds two out of the three water filters tested may not be removing lead at the level expected, according to information provided by Newark to CNN.
August 12, 2019
The city of Newark and the state of New Jersey begin providing bottled water to residents in the Pequannock service area who have lead service lines.
Correction: A previous version of this story incorrectly stated the name of the Natural Resources Defense Council.