Authorities in North Carolina have launched an investigation into widespread underreporting of dangerous toxins in dozens of feces-filled cesspools on giant hog farms that dot the eastern part of the state.

Testing of 55 waste lagoons at 35 hog-raising operations by regulators showed large discrepancies in levels of key pollutants compared with what was self-reported to the state by farmers. Excessive nutrients such as phosphorus and nitrogen, which can poison the water supply, were, in many cases, much higher than that reported by the farms.

The presence of hazardous heavy metals such as zinc and copper in the waste lagoons was even greater – zinc levels were as much as 101,108% higher in the regulators’ testing compared with what was reported to them.

Sign up to receive the top US stories every morning

Inspectors from North Carolina department of environmental quality (DEQ) took the samples from farms on 13 April and compared them with what was self-reported on that day. The farms are supposed to adhere to permits that restrict harmful pollutants befouling the air and water of nearby residents.

A million tons of feces and an unbearable stench: life near industrial pig farms Read more

There are about 2,000 industrialized hog farms in North Carolina, making the state the second largest pork producer in the US. The pigs are kept in huge barns where their waste falls through slatted floors and is kept in huge cesspools before being sprayed on to nearby fields. The pig farms of North Carolina produce around 10bn gallons of feces a year, which is more than the volume of waste flushed down toilets by the human population of Germany.

“This manipulation of data is an insult to the community members suffering from the industry’s continued use of the lagoon and spray field system,” said Devon Hall, co-founder of local anti-hog farm group Rural Empowerment Association for Community Help. “We demand real enforcement. The response to this slap in the face should be more than a slap on the wrist.”

The pork industry, dominated by the Chinese-owned business Smithfield, insists that contracted farmers abide by strict permits that ensure nearby communities are not affected by the huge pits of animal waste that sit beside them.

But environmental groups and a band of North Carolina residents claim the industry has tainted the air and water for decades with little intervention from supine state regulators. More than two dozen North Carolinian communities are in the process of suing Smithfield, complaining of pig waste sprayed on to their homes, polluted drinking water wells and unbearable odors.

In letters to the 35 hog farm operators, state authorities apologized “for the short notice” given ahead of inspections. A spokeswoman for the DEQ confirmed an investigation was under way but would not provide any further details.