July 4 fireworks, barbecues and swimming on the South Shore. Which one might be canceled?

"Jaws" isn't the only reason to avoid swimming on July 4 at beaches and other swimming areas throughout Massachusetts.

The state Department of Conservation and Recreation posts twice-daily water quality test results for beaches and ponds under its purview at 9:30 a.m. and 12:30 p.m. on the Mass.gov website. As of Wednesday, July 3, 30 such areas were closed to swimming, with most due to high bacteria levels and an occasional cyanobacteria bloom popping up here and there.

Health officials advise residents to avoid closed swimming areas an additional 24 hours

State health officials said the technology currently available takes 24 hours to return results, meaning by the time the posting goes up, the information is not current. Residents are encouraged to wait for 24 hours after a closure posting before checking back to see if a swimming area has reopened, meaning that Wednesday's results should be applied for safety's sake to July 4, as well.

Why all the bacterial ickiness?

Heavy rain can lead to polluted runoff by carrying untreated and contaminated water into swimming areas. That runoff can expose swimmers to harmful microorganisms called pathogens, according to the EPA.

Enough science, can I swim July 4 or not?

Overall, yes. Locally, only Houghton's Pond in Milton and Smith Beach in Braintree were on the state's July 3 closure list. Going by the 24-hour rule, those areas should be considered off-limits for splishing and splashing on Independence Day.

Houghton's Pond is particularly susceptible to the spread of pathogens, especially during the last two years of periods of heavy rain. The pond has been closed to swimming for most of the summer.

Wollaston Beach, which has faced spot closures since last month, has been given the all-clear, and Nantasket Beach has yet to make it on the naughty list this season.

City-managed beaches and ponds in Quincy open for swimming

According to the last weekly water-quality update for Quincy on June 27, all swimming areas under the town's purview are safe to enter. The city conducts weekly water sampling and posts its results on Thursday or Friday. The latest testing results are available by calling 617-376-1288.

State of the water:How is the water quality at Quincy and Hull beaches? Do you worry about the bacteria?

State advises against eating fish caught at Houghton's Pond

The state also issued an advisory in May ahead of the summer season saying people should not eat any fish caught in Houghton's Pond due to their high PFAS levels. The pond is among dozens of fishing areas carrying similar warnings.

The chemicals under the PFAS umbrella were first used in the 1940s and are now in hundreds of products including stain- and water-resistant fabrics and carpeting, cleaning products, paints and firefighting foams, according to the FDA's website, which also said certain PFAS are authorized by the FDA for limited use in cookware, food packaging and food processing equipment.

Do you worry about the bacteria?How is the water quality at Quincy and Hull beaches in general?

The FDA went on to say that the chemicals persist in nature and pollute the environment.

"While the science surrounding potential health effects of bioaccumulation is developing, exposure to some types of PFAS have been associated with serious health effects," according to the FDA.

David R. Smith covers breaking, trending and restaurant news for The Patriot Ledger. He can be reached at Drsmith@gannett.com.