A number of important local stories were leading the website of the Storm Lake Times, circulation 3,000, on Tuesday morning. Second-grader Alejandra Gonzales found a four-leaf clover in the field behind her school. A local woman had bought and renovated a building to house 25 elderly cats.

And in a modest announcement of just a sentence, another notable local happening: on Monday, Art Cullen, the paper’s owner and editor, was awarded a Pulitzer prize, the most prestigious award in global journalism, for his editorial writing. Fellow honorees for 2017 include the rather better read New York Times, Wall Street Journal and Miami Herald.

The twice-weekly newspaper, serving a town of a little over 10,000 people, may not previously have been widely read outside Buena Vista county, north-west Iowa, but Cullen’s editorials – “fuelled by tenacious reporting, impressive expertise and engaging writing that successfully challenged powerful corporate agricultural interests in Iowa” – certainly caught the attention of the Pulitzer committee. He beat writers from the Washington Post and Houston Chronicle to the award.

The paper will pocket $15,000 (£12,000) – no small sum for its nine staff, which include Cullen, his brother John (the paper’s publisher), son Tom (reporter), wife Dolores (photographer) and John’s wife Mary (author of the My Favourite Recipes column). The family dog Mabel (“the news hound”) is another newsroom regular.

The prize was not exactly expected. Cullen, 59, was watching a livestream video on the Pulitzer site, he told Poynter, “and they went through national reporting, local reporting, etc, and then got to editorial writing and said “Art Cullen”, and I started screaming to my brother, “Holy shit, we won!” ... I started screaming and he thought I had gone nuts.”

The office of the Times, which was founded in 1990 by John Cullen, does not have much in common with the flashy, glass-fronted newsrooms of many of those he beat to the prize, he said. “I sit with piles of newspapers around three-week-old page proofs, and people can come in and start yelling at me. The office is a wreck. It’s a whole different environment.”

So it proved on Tuesday when, newly famous award-winner or not, Cullen spent much of the day batting away interview requests from international media; Tuesday is press day in Storm Lake, and the paper wasn’t getting out without the hands-on attention of its editor.

Like any successful local paper, the Times knows what its readers want. “We strive to have a baby, a dog, a fire and a crash on every front page,” Cullen has said. But the Pulitzer was awarded for a very different kind of journalism, after the paper began looking into a lawsuit brought by Des Moines Water Works, the publicly owned water utility in the Iowa capital, against three counties, including Buena Vista, for releasing too much nitrogen from farming into the river from which it draws its drinking water.

The counties fought the suit, funded by undisclosed sources, and the Storm Lake Times wanted to find out if they were, as it suspected, “hundreds of secret sugar daddies from the seed/petrochemical industry”.

“All we are truly asking [is] how much is the bill and who is paying it?” Cullen wrote on 18 March, 2016. “You always follow the paymaster. If Farm Bureau [an agricultural lobby group] is signing the check, then you know who is really calling the shots on our behalf. We elected a board of supervisors and a county attorney to direct the policies and protect the taxpayers of this county. We did not elect the Farm Bureau or any other interest group to set our course.”

The water utility’s suit was ultimately unsuccessful, but Cullen said he felt vindicated that the paper, with the support of the Iowa Freedom of Information Council, had got the information released.

The newspaper is not afraid to take an unpopular position. Photograph: Storm Lake Times

Outspoken criticism of agribusiness is not necessarily a popular position in a state that is heavily dominated by agriculture. But in the 27 years of the paper’s existence, Cullen says he and John have been “obsessed” with changes to farming practice in the state, and the impact they have had on the local environment.

“Anyone with eyes and a nose knows in his gut that Iowa has the dirtiest surface water in America,” he wrote in one of his award-winning pieces. “It is choking the waterworks and the Gulf of Mexico. It is causing oxygen deprivation in north-west Iowa glacial lakes … Everyone knows it’s not the city sewer plant causing the problem. And most of us recognize that this is not just nature at work busily releasing nitrates into the water.”

It is not the only unpopular position the paper has taken. The fact that Buena Vista is a solidly Republican county (it voted by 59.2% for Donald Trump in November) did not stop Cullen describing him before the election as a “Frankenstein” [sic] created by the Republican party “through racism, sexism, nationalism and obstructionism”.

Winning the prize, meanwhile, would not signal an end to their coverage of the water pollution story. “They’ve changed the entire agricultural system since 1980, without any consideration [for] how it affects groundwater or surface water,” he told the Washington Post. “It’s a national story. It’s just that it happened in our front yard and we’re still reporting on it.”