Some environmental groups have expressed concerns about the operation of Indian Point, including radiation pollution and endangerment of wildlife, but whether Indian Point has ever posed a significant danger to wildlife or the public remains controversial. Though anti-nuclear group Riverkeeper notes "Radioactive leakage from the plant containing several radioactive isotopes, such as strontium-90, cesium-137, cobalt-60, nickel-63 and tritium, a rarely-occurring isotope of hydrogen, has flowed into groundwater that eventually enters the Hudson River in the past, there is no evidence radiation from the plant has ever posed a significant hazard to local residents or wildlife. In the last year, nine tritium leaks have occurred, however, even at their highest levels the leaks have never exceeded one-tenth of 1% of U.S. Nuclear Regulatory Commission limits.