TRURO — As the weather warms up and summer crowds flock to the beaches to cool off, something unwelcome has been detected in the water — and it’s not sharks.

The Cape Cod National Seashore closed Head of the Meadow Beach in Truro and Race Point Beach in Provincetown on June 22 after high bacteria levels were found in water samples. A week before that, Wellfleet closed Gull and Duck ponds for the same reason. In both instances, the bacteria levels quickly returned to normal, and the beaches were reopened the next day.

According to Geoff Sanders, chief of natural resource management for the Seashore, the park’s first water-quality tests of the season on June 20 yielded high levels of enterococci bacteria at two of the Seashore’s six beaches, prompting their closures. High levels of E. coli were found in the Wellfleet ponds, interim Health and Conservation Agent Meredith Ballinger said.

While high levels of bacteria are common along bayside beaches because of road runoff, they are much less frequent in ponds and at ocean beaches, according to sample data collected by Barnstable County.

The levels of enterococci and E. coli bacteria, which are not pathogenetic themselves, are tested because they indicate the presence of other dangerous bacteria that live in human and animal waste, which if ingested can lead to serious illness.

Sanders said that exceedances of bacteria at the ocean beaches were most likely caused by wild animals. “Waste matter from marine animals can lead to an exceedance,” Sanders said, but high bacterial levels are “ephemeral and hard to attribute to any one thing.”

According to Ballinger, high levels of bacteria in ponds are often caused by runoff from recent rainfall. Sanders said that storm runoff is also a possible contributing factor at the ocean beaches, “but with the volume of water you have in the ocean, dilution is much higher,” he said.

That is why it is much rarer to see exceedances at oceanside beaches, he said: “The ocean flushes so well, so it’s really not a common occurrence.” Last summer, the Seashore closed Herring Cove for a day after tests found high bacteria levels, Sanders said. Before that, “the park hadn’t exceeded in quite a while.”

Sanders added that “last year’s exceedance did come later in the summer. I was surprised we got one this early in the year.”

He said that while time of year doesn’t directly correlate to high bacterial levels, a greater influx of people, pets, and wildlife can lead to a higher probability of exceedances later in the season. Sanders maintained that there is not “any evidence that we will find exceedances more often later in the season.”

Nonetheless, “the surprising aspect is that we saw the exceedance at two beaches, and exceedances have been so rare recently,” Sanders said.

The Seashore tests its six beaches weekly during the summer season as does Wellfleet at its ponds, Ballinger said. According to Truro Assistant Health Agent Courtney Warren, Truro tests Head of the Meadow Beach once a month. Its last test on June 6 found normal bacteria levels.

Sanders said that the duration of a beach closure has more to do with the time it takes to receive test results than a continual presence of bacteria. It typically takes one to two days for a lab to run a sample, and the Seashore immediately retests if the first sample yields high bacteria levels. Both this year and last year, the retest found normal levels of bacteria, Sanders said.

Ballinger said that Gull and Duck ponds as well as most public beaches won’t be closed unless two consecutive samples exceed the bacteria limit. A third test the next day confirmed that E. coli levels had subsided, Ballinger said.

According to Barnstable County’s beach monitoring program, a Mass. Dept. of Public Health (DPH) analysis of beach samples from the past 13 years found that retest results come back within allowable limits a great majority of the time, suggesting that high bacteria levels are often caused by a brief contamination event.

The DPH’s 2022 annual beach report found that, of the 592 beaches sampled last summer, 33 percent had at least one bacterial exceedance. Last year’s exceedance rate of 5.3 percent for all samples was slightly higher than the historic average of 5.1 but was a sharp decline from 2021’s exceedance rate of 8 percent.