Thousands of bottles of water await distribution. (Dale G. Young/The Detroit News/AP)

The Flint, Mich., water system is in much better shape six months after the city switched its water source and began adding chemicals to control corrosion of aging pipes, but the threat of lead contamination remains, researchers said Tuesday.

“There are very positive trends. . . . The system is definitely on its path to recovery,” said Marc Edwards, the Virginia Tech professor whose work helped bring Flint’s water crisis to light. But because recent samples showed that potentially dangerous levels of lead persist, “at present, no one should be drinking unfiltered water in Flint.”

On the whole, Edwards said, a lower percentage of homes tested last month showed high lead levels, and even in those where levels still exceed federal standards, the amount of lead appears to be falling. The corrosion control efforts also appear to be reducing levels of iron in the water.

But the researchers found that too many homes still had lead levels above the federal “action level” of 15 parts per billion. In addition, they said, lead levels throughout Flint’s water system are “highly variable.”

“Thus, virtually all homes in Flint must be considered at risk, at the present time, for elevated lead in water,” researchers wrote in a summary of their findings, “unless the homeowner is certain that there is no lead plumbing . . . in the home.”

At the same time, the fact that many Flint residents are using very little tap water is hindering the recovery of the city’s system, Edwards said. “If we want to help this system recover, we have to get water moving through these pipes,” he noted.

Many residents are avoiding their tap water and relying primarily on bottled water — much of it provided at no charge — both to lower their utility bills and to avoid possible contact with lead and other contaminants. But that means too little flow of water to carry the chlorine and other chemicals added by the city to reduce the leaching of lead from pipes and to combat bacteria.

The latest water samples in Flint were collected in March by Virginia Tech students alongside local residents, with funding from the Environmental Protection Agency.

The testing involved 174 of the 269 homes where Edwards’s team collected water samples last summer.

Flint’s lead problems began to surface after the city temporarily switched to the Flint River for its water supply in April 2014 as part of a bid to save money. The state failed to ensure that anti-corrosion chemicals were added to the water, which was contaminated when lead leached into it from aging underground pipes.

As a result, nearly 9,000 children younger than 6 have potentially been exposed to the toxin. Lead can cause permanent learning disabilities, behavior problems and, at higher levels, a number of diseases. Public health officials say there is no safe level of lead in the body.

Under current lead-in-water regulations, at least 90 percent of homes tested by a water utility must remain below the federal 15 ppb “action level.” The 90 percentile of samples taken last month in Flint was 23 ppb, Edwards said.

Flint in crisis: Tainted water, little hope

The release of the summary report comes days after state and federal officials, along with the Virginia Tech researchers, agreed that although the quality of Flint’s water is improving, more work must be done before all residents will be able to drink it safely.

“It will take months and months to recover, and no one is willing to put a time limit,” Robert Kaplan, acting regional EPA administrator, said at a Flint Water Interagency Coordinating Committee meeting Friday, according to local news reports.

Michigan Gov. Rick Snyder (R), whose administration has been heavily criticized for not acting sooner on the problems in Flint, sounded cautiously optimistic in a statement late last week.

“Whenever we see a positive trend in Flint’s water quality, that’s good news,” Snyder said. “But we still have much work to do to get people the quality of water they need and deserve.”

In the meantime, residents have been encouraged to flush their kitchen water faucets to remove any particles of lead that might be lingering in their home’s plumbing and to help move anti-corrosion chemicals through the plumbing system.