On what story arc is the Swamp Thing based on?

Actually few things have been altered for the show, screenrant has a list of them.. On the story arc front, I couldn't find any credible source to put here. I'll updated this if I can find one.

1. Alec's Backstory

Dr. Alec Holland's backstory is vastly different between the DC Universe series and the comics. In the comics, Dr. Holland was a brilliant scientist working on finding a solution to end world hunger. Good intentions, right?

Meanwhile, the DC Universe Alec is...somewhat a disgrace in the scientific community. He still has good intentions, of course. But he did falsify records in order to make his results look better (we'd like to believe it was in order to get people to pay attention...but...). Now Alec is working in the bayou while trying to study it.

2. The Transformation Into Swamp Thing

The reasons behind Alec's transformations change slightly between the two versions. Though both seem to have been involved in an attack and explosion before becoming exposed. So some themes were kept the same.

In the comics, Dr. Holland and his wife were working on creating a bio-restorative formula to end world hunger. They were attacked when they refused to sell what they had created, and unfortunately, the bio-restorative formula spilled on him, and you know the rest.

In the DC Universe series, Dr. Alec Holland had believed he and Abby Arcane were on the verge of a breakthrough in researching a new disease. Or more accurately, a plant infection. He went back out into the bayou – by himself – to remove what he believed to be contaminants before anybody else got sick. During this trip, he was attacked and would have died had his transformation not begun.

3. Abby Arcane

In the comics, Abby Arcane's name is a little on the nose. In the DC Universe version, her name makes a little less sense. You see, comic Abby Arcane is the niece of Anton Arcane. A magical man who had been grooming Abby to step into the family business since she was a child. Abby resisted the family plans, and instead became a doctor. Her life did eventually bring her back into the path of Swamp Thing, but not as an enemy.

4. Matt Cable

Matt Cable is one of many characters that were altered to better fit the DC Universe series. In the comics, Matt is an Interpol agent. He gets involved in the series early on, when he gets assigned to protect Dr. Alec Holland and his wife while they work on their research. Matt witnesses the death of the Hollands – or so he believes – and blames the deaths on Swamp Thing. Thus starts a whole new series of events for the characters.

Meanwhile, the DC Universe version has changed Matt's job and perspective. In this series, Matt is the local sheriff – quite a change in job. And instead of working to protect Holland and family, he's more focused on Abby. He's an old friend of Abby's and gets pulled into the whole mess while trying to protect her.

5. Status as an Earth Elemental

Swamp Thing is an Earth Elemental, but many fans are already well aware of that. This means that Swamp Thing is something different and larger than any mere mortal. He's the guardian of the Green, and thus really has a sacred duty to protect it and everything associated with it.

That is less true for the DC Universe series. Though perhaps that will change in time. After all, this information wasn't revealed right away in the comics either. For right now though, Swamp Thing seems to be more of a transformed human/monster than anything else.

6. Liz Tremayne

Like many other characters on this list, Liz Tremayne has been altered between the two versions. In the comics, Liz was a very talented investigative reporter. She had a talent for letting her investigative nose lead her into messes and dangers. And she wasn't afraid to take on corrupt businessmen like Sunderland.

Meanwhile, the DC Universe of Liz has her working double duty. She's a reporter still, but for a very small local paper. The rest of her time is spent tending bars.

From CBR's list

Abby Arcane

Reed's Abby Arcane doesn't physically resemble the character from the comic books, who's known for her distinctive white hair with a black stripe. The two share troubled pasts, although in the comics it's because of Abby's family, which includes her uncle Anton, an early archenemy of the Swamp Thing. While Abby's comic book counterpart possesses telepathy, empathy and telekinesis, the TV version doesn't display such abilities -- yet, in any case.

"That's a secret!" Reed teased in an interview with CBR. "Yeah, we haven't delved into that completely, and there might be some hints of that moving towards the end of the season, but I don't want to give too much away."

Alec Holland

In the comics, Alec was a scientist who worked with his wife, Linda, in the Louisiana swamps to create a "bio-restorative formula" intended to solve the world's food-shortage woes. When his lab is bombed by a criminal organization, Alec runs from the facility, covered in the formula, and crawls back out of the water as the Swamp Thing, later revealed as the latest in a long line of Elementals, devoted to protecting plant life.

Matthew Cable

In comics, Cable is an agent with Defense Department Intelligence, Matthew Cable set out to find who was responsible for the deaths of his friend Alec and Linda Holland following the bombing of their Louisiana lab. He mistakenly believed the Swamp Thing was the killer, and tracked the creature to the Balkans, where he met, fell in love with, and eventually married, young Abby Arcane. Their relationship deteriorated, and a drunken Matthew was mortally wounded in a car accident, only for his body to be used by Abby's evil uncle Anton Arcane to permit him to escape Hell. Following Anton's defeat, Matthew dies.

Matt Cable is played by Henderson Wade on Swamp Thing, where he's an old friend of Abby's who has served for the past four years as a deputy with the fictional Montrivelle Parish Sheriff's Department.