A change to the date of the annual Mirror Lake jump didn’t prevent about 30,000 people from visiting the lake between 9 p.m. and 3 a.m. Tuesday night, according to OSU Police.

Muddy sidewalks, abandoned flip-flops, towels hanging from branches and the faint odor of the Columbus Police Department’s horses lingered around the lake Wednesday morning, but they weren’t the only remnants of the festivities.

For some students, the night will be imprinted on their police records as well as their memories. OSU Police received 12 calls for service Tuesday night and made six arrests. The calls were down from 25 last year, and police made one more arrest on Mirror Lake jump night 2009.

The Columbus Police Department was not available for comment, but a Columbus Dispatch video showed police restraining a man using an aerosol spray can as a flamethrower.

The OSU Medical Center’s emergency department saw 14 people between 10 p.m. Tuesday and 1 a.m. Wednesday. The emergency medical squad came to treat people at Mirror Lake 26 times for various injuries and transported nine people to the hospital.

“Typically, we usually run around 30 extra patients,” said Jason Walsh, an OSU medical center emergency room manager. “We actually have to staff up for it every year.”

Walsh said three extra nurses were on duty at the OSU emergency room.

He said medical workers see a lot of intoxicated patients on Mirror Lake jump night, but alcohol poisoning is uncommon. Most patients are treated for hypothermia or minor injuries.

Hospital workers typically warm the patients up, give them fluids and send them on their way, Walsh said.

Richard Morman, deputy chief of OSU Police, said that at the peak of the evening, as many as 15,000 people were in the area surrounding Mirror Lake.

“There seemed to be a lot more spectators than in years past,” Morman said. “It’s quite the phenomenon, though.”

Student Safety Services had to escort at least five of those people back to their residence halls, Morman said. Student Life team members assisted students as well.

Most students found the way back to their residence halls to warm up with free hot chocolate and other food.

Mack Hall on South Campus offered free hot dogs to students, and there were 1,200 donuts up for grabs in the lobby of Blackburn House on North Campus.

Residence hall workers took measures to keep out visitors who don’t live in their buildings.

“Since we’re the closest hall to the lake, we had a lot of folks from Morrison, Canfield and Siebert trying to come through because it was quicker,” said Dakota Butch, resident manager of Mack Hall and a fourth-year in English.

Butch said housekeeping workers in Mack locked some of the bathrooms to contain the mess, laid down plastic in the bathrooms, put cardboard in the elevator and left towels for office assistants to hand out.

“There were water stations outside most of the halls where people got hosed off if they were too muddy,” Butch said. “The biggest problem was a few people coming in who had had way too much alcohol.”

A group of graduate and undergraduate earth science students are researching the jump’s effect on Mirror Lake.

“A lot of people say they always hear about the supposed urine content,” said Justin Von Bargen, a fifth-year in geological sciences. “I just told people to keep their head above the water and they should be fine.”

Von Bargen leads a group that took water samples from three sites around Mirror Lake at noon Tuesday, several times throughout the jump and at 8 a.m. Wednesday. The group tested for a change in many of the water’s properties, including temperature, and nitrogen and ammonia levels.

An increase in nitrogen and ammonia levels in the lake could indicate two things: sediment on the bottom of the lake being stirred up or an increase in urine levels in the water, said Brandon McAdams, a first-year master’s student in hydrogeology. A combination of both could also lead to a rise in the levels.

The majority of the group’s research is still in progress. The results so far have shown a spike in nitrogen and ammonia at 12:30 a.m. on the east side, the shallow end where most people jump in the lake. The nitrogen and ammonia levels are too high to be from stirred-up bottom sediments alone and suggest additional input, such as drunk people peeing, Von Bargen said.

Results from 2009 show that the change in nitrogen concentration could indicate an addition of almost 140 liters of urine in the lake.

Urine wasn’t the only thing jumpers left in the water. Paul Walsh, Facilities and Operations Development zone leader for most of South Campus, including the Mirror Lake area, said groundskeepers have to rake flip-flops out of Mirror Lake.

“Groundskeepers pick up trash and items of clothing that were left,” Walsh said.

The groundskeepers came in at 3 a.m. Wednesday to clean up the mess students made, even though their shift doesn’t normally start until 7 a.m.

To prepare, Walsh said groundskeepers put up signs warning students about jumping into Mirror Lake, and they removed the lake’s fountain, which stays out through winter.

An area east of the lake will be resodded.

The groundskeepers’ clean-up efforts were not in vein, as far fewer students returned Thursday night.

Between about 10:30 and 11:30 p.m. on Thanksgiving, only two students did not heed warnings from OSU officials that they could be charged with criminal trespassing if they entered Mirror Lake on Thursday.

The students, strangers to each other, jumped into Mirror Lake at about 11:15 p.m. Thursday.

“I’m not willing to break tradition just because of a holiday,” said Chad Shields, a sixth-year in construction systems management.

As he was preparing to take the chilly plunge on the rainy Thanksgiving night, Alex Slivinski, a third-year in actuarial science and economics, arrived at the lake with two friends and decided to join the jump.

It appeared that no police were monitoring the lake, and the two students’ 30-second swim went uninterrupted.

Chief Paul Denton said in an e-mail that OSU Police received no calls or reports of people in the lake Thursday.

“Officers checked the lake and buildings in proximity randomly throughout the day,” Denton said in the e-mail. “No one was reported as being observed in the lake while doing so.”

Collin Binkley contributed to this story.