Robert F. Kennedy Jr. plunged into a creek deemed a "hazard" to human health as part of his Mother's Day celebrations on Sunday, May 11. The Department of Health and Human Services secretary, 71, posted photos to X of himself and his family at Rock Creek Park in Washington, D.C. In one of the images, Kennedy is fully submerged with his head underwater. In another, he holds one of his grandchildren as the child's feet dip into the creek. The National Park Service banned swimming and wading in the creek due to "high bacteria levels" in the water. "Mother’s Day hike in Dumbarton Oaks Park with Amaryllis, Bobby, Kick, and Jackson, and a swim with my grandchildren, Bobcat and Cassius in Rock Creek," Kennedy captioned his post on Sunday. The NPS warns that possible infectious pathogens in Rock Creek — a tributary of the Potomac River — could pose a risk to human and pet health. The agency urges visitors, "Please protect yourself and your pooches by staying on trails and out of the creek." Many of D.C.'s waterways are not fit for recreational swimming due in part to contamination from the city's sewer system, which is more than 200 years old, according to The Hill. The District of Columbia Water and Sewer Authority recently found a 200-foot-long fracture in the ceiling of a sewer tunnel next to Rock Creek. Kennedy, who challenged the idea that germs cause disease in his 2021 book The Real Anthony Fauci, has a long history of dubious health claims. He recently incorrectly stated that the MMR vaccine includes "aborted fetus debris," and said bird flu should be allowed to spread on farms, suggesting the infected birds should be isolated and left to breed, not culled. In late March, Dr. Peter Marks, a top vaccine official at the U.S. Food and Drug Administration (FDA), resigned, and listed Kennedy's work at HHS as one of the factors that drove him out. “It has become clear that truth and transparency are not desired by the Secretary, but rather he wishes subservient confirmation of his misinformation and lies,” he wrote at the time. The HHS secretary most recently came under fire for his remarks about people with autism, alleging that they are unable to live normal, full lives and are "suffering." His words sparked immediate backlash, including from Holly Robinson Peete, whose son was diagnosed with autism, and Big Brother contestant Kyland Young, who was diagnosed at age 30. Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories. Kennedy's cousins even penned an open letter blasting his "slurs and statements" about people diagnosed with autism. "We have been reminded again and again that people with autism can and will continue to be wonderful children, brothers and sisters, and members of families," read the joint letter from Best Buddies Founder & CEO Anthony K. Shriver and his brother, Special Olympics International Chairman Timothy Shriver. HHS did not immediately respond to PEOPLE's request for comment.