A former US navy base in Maine has caused among the largest accidental spills of toxic PFAS “forever chemicals” ever recorded in the nation, and public health advocates suspect state officials are attempting to cover up its scale by reporting misleading and incomplete data.

Meanwhile, state and regional officials were slow to alert the public and are resisting calls to immediately test some private drinking water wells in the area despite its notoriously complex hydrology, which could potentially spread the contamination widely.

The spill was caused by a malfunctioning fire suppression system in a hangar at the Brunswick naval air station near Maine’s coast, which released about 51,000 gallons of PFAS-laden firefighting foam into nearby surface water, leading to astronomical levels of PFAS contamination.

The levels in the foam reached as much as 4.3bn parts per trillion (ppt) – the drinking water limit for some PFAS compounds is 4ppt.

The government’s communication has been “unconscionable” and the data reporting was “problematic”, said Sarah Woodbury, director of Defend Our Health, a Maine-based non-profit that works on PFAS issues.

“Causing confusion like that, however unintentional it was, just increases the distrust that people have when it comes to government dealing with catastrophes like this,” she said.

PFAS are a class of about 15,000 compounds most frequently used to make products water-, stain- and grease-resistant. They have been linked to cancer, birth defects, decreased immunity, high cholesterol, kidney disease and a range of other serious health problems. They are dubbed “forever chemicals” because they do not naturally break down in the environment.

The 19 August spill sent toxic firefighting foam into storm drains and floating through the air in a nearby residential and business area. It occurred at the Brunswick executive airport, which is part of the former naval base that was listed decades ago as a Superfund site, a federal designation for the nation’s most polluted areas.

The base, which is now under civilian control and being redeveloped, has long polluted the local environment with a range of toxins, and several other smaller PFAS spills have occurred.

PFAS has been a main ingredient in firefighting foam because it is effective at extinguishing jet fuel fires, and is a main source of PFAS water pollution nationwide. Water at and around more than 720 military sites has been found to be contaminated with PFAS, though not at levels seen near the spill.

In the week after the spill, the Maine department of environmental protection, which is leading the cleanup, issued “do not eat” advisories for fish and began testing local ponds and waterways.

A 26 August progress report listed a reading for PFOS, one of the most common and dangerous PFAS compounds, as 3,230 parts per million (ppm).

Typically, PFAS levels are reported in ppt, which would mean the PFOS levels were about 3.2bn ppt. The 3,230ppm figure appears smaller than the staggering 3.2bn ppt figure. Similarly, the state reported water levels at 700ppt as 0.0007ppm.

It is unusual for PFAS water levels to be reported in ppm, said Jared Hayes, a senior policy analyst with the Environmental Working Group non-profit, which tracks PFAS pollution. The lab reported the results in ng/l, which is the same as ppt, but the state still changed the unit of measurement to ppm, raising suspicion and frustration among residents and public health advocates.

In a statement to the Guardian, the state said it made the change for “ease of readability”.

Meanwhile, Maine only reported the number for PFOS, but it had also tested for 13 other PFAS compounds that tallied 1.1bn ppt. That included 64m ppt of PFOA, one of the most common and dangerous PFAS compounds.

The agency said it only disclosed the PFOS results because that showed the highest figure and was the primary chemical of concern.

The state said it would test a limited number of nearby wells, and would check more if needed. Woodbury said public health advocates were also urging the state to provide bottled water until the results are available in several weeks, and calling on it to do soil testing in the most affected areas.

The spill comes amid a military effort to switch to PFAS-free firefighting foam and dispose of the old, PFAS-laden product by October 2025, though it will probably not meet the deadline, Hayes said.

The foam was slated to be removed in October, and additional hangars still hold foam. It is unclear when that foam will be removed, and the situation highlights that “the [Department of Defense] needs to start acting swiftly to remove this from bases across the US”, Hayes said.

“The fact that this happens here means it can probably happen somewhere else, and with so much legacy foam out there it’s hard to say when or where it will happen next,” he added.