DES MOINES, Iowa (AP) — Iowa leads the nation in the amount of human and animal waste it produces, and the management of animal manure continues to be a pressing environmental concern, according to a University of Iowa research engineer.

Chris Jones’ study found that the state — with 3.2 million people and a total livestock population of 110 million — produces as much manure as a human population of 168 million, the Des Moines Register reported.

“Just to ensure clarity, in Iowa, we are generating as much fecal waste in every square mile as 2,979 people,” Jones wrote in a blog on the university’s website.

“For reference, Iowa City is the second most densely populated city in Iowa and has 2,775 people per square mile. So imagine an Iowa-sized Iowa City,” he continued in the blog discussion explaining his study.

Pigs are responsible for driving Iowa to the top of the rankings, Jones found.

Jones flagged his concerns about the impact of livestock waste on water quality.

“Our statewide nitrate load has increased over the last 20 years or so, and part of that is due to an increase in the number of livestock animals that we have, especially hogs,” Jones said.

In a 2018 study , the scientist found that water in areas with more livestock contains higher levels of nitrate. The study found that nitrogen pollution flowing out of Iowa to the Gulf of Mexico grew by nearly double over almost two decades, despite hundreds of millions of dollars spent to stop it from entering the state’s waterways.

Jones acknowledged that livestock production helps rural economies and that manure is a good fertilizer that promotes healthy soils. But manure can be difficult to manage with extreme weather, he explained. Applying manure to snow-covered fields means that the fertilizer is more likely to wash into nearby streams.

“Managing the waste from these animals is possibly our state’s most challenging environmental problem,” Jones wrote in his first blog calculating Iowa’s fecal equivalent population.

The nation’s top manure-producing states after Iowa are Delaware, Wisconsin, Nebraska and Pennsylvania, based on waste created per square mile, the study shows.