(CNN) — Voluntary blood testing will begin Thursday for Newark public school students, a day after officials announced that lead levels in the district’s water have been elevated since at least 2012.

The Newark Public School system’s early childhood programs will be the first to receive testing, Newark Public Schools Spokeswoman Dreena Whitford told CNN.

Overall, 17,000 students attend schools with elevated lead levels. But because testing is voluntary, it’s not clear yet how many will opt to undergo the blood test.

The move comes after testing revealed that nearly half the district’s schools had issues with elevated lead levels in water, according to the Newark Public School system.

Of 66 schools, 30 locations have tested positive for lead exposure over the federal legal limit of 15 parts per billion. Lead levels as high as 558 parts per billion were measured, but most results were below 100 parts per billion.

The New Jersey Department of Environmental Protection and Newark Public Schools claim they took quick action after learning the results of water testing. The school system shut off all drinking fountains last week at the affected schools and is providing alternate sources of water. The DEP also announced it has developed a new water testing regimen for all Newark Public Schools.

In a joint statement released last week, the DEP confirmed that lead had not been found in the Newark Water Department’s source water and said that lead in water often occurs when it leaches from “either lead pipes, household fixtures containing lead or lead solder.”

Lead-level data from 2012 to 2015 will also be released by the Newark Public School System on Thursday, according to a news release.

The data, compiled and reviewed by the New Jersey Department of Environmental Protection, will show that “out of about 2,067 water quality samples collected between 2012 and 2015, approximately 12% reflected lead levels above the federal action level of 15 parts per billion,” according to the release.

This data is similar to data compiled between December 2015 and February 2016, which shows about 10% of samples above the federal action level, according to the release.

Allegations of long-term neglect

The new blood testing comes in light of allegations by the Newark Teachers’ Union and the New Jersey chapter of the Sierra Club that the school system knew about lead concerns for years without taking action.

The Sierra Club released an internal memo sent to principals, head custodians and other administrators near the beginning of the 2014 school year with instructions to flush drinking water sources on a daily basis to “reduce the risk of possible lead contamination.”

“Instruct students and staff to run each fountain or drinking faucet for at least thirty (30) seconds before drinking,” the memo reads. It also instructs custodians to run and flush each fountain for two minutes before the start of school each day.

“This memo is a smoking gun,” said New Jersey Sierra Club director Jeff Tittel in a statement. “They would never have sent out this memo if they didn’t know that there was a problem with lead and that the problem had been going on for some time,” he also said.

But the school system says it began taking precautions in 2004 in conjunction with EPA regulations and that letters of that nature have been sent out each year since 2004. Whitford provided CNN with similar memos dated in 2007 and 2008. Whitfield also said in a statement that the school system installed filters as a part of their lead reduction program.

The Union and the Sierra Club allege that the maintenance of these filters is yet another misstep by the school system.

On Monday, Newark Teachers Union President John Abeigon released photographs he says show expired water filters at 10 Newark schools not included in the 30 that showed positive results for lead in the water. The images show filters with dates written on them in marker, ranging from 2009 to late 2015. Abeigon says these dates mean that the filters were expired.

In Wednesday’s news release, Newark Schools Superintendent Chris Cerf refuted the allegations about the filters, calling Abeigon’s claims “irresponsible.”

“The dates on the filter housing do not necessarily correspond to the dates of the filter replacement,” Cerf said in the statement.

Calls for resignation

The Newark Teachers Union has called for Cerf’s resignation alleging that Cerf chose to conceal the lead issue from the public.

“All the while, he has aided and abetted the growth of budget-sapping corporate charters in the district even at the expense of knowingly poisoning the students and staff whose safety during the school day he is responsible for,” Abeigon wrote in a statement.

The school system, though, defends its actions. Cerf asserts that he and his team are aggressively tackling the problem with document reviews and personnel interviews. And in Wednesday’s statement, Cerf shared his own concerns with the situation.

“As a parent, I too find the fact that the district has identified elevated levels of lead in water in each of these past years extremely concerning,” he said.

CNN’s Dominique Debucquoy-Dodley contributed to this story.