Don’t eat bass, sunfish or catfish caught in the 51-mile stretch of Tonto Creek that starts one mile south of Bear Flat and extends all the way to Roosevelt Lake, the Arizona Department of Environmental Quality warned residents.
The stretch of creek in question runs through the Hellsgate Wilderness Area and does not include the upper creek with its many picnic and camping areas and pools stocked each week from the Arizona Game and Fish Hatchery at the head of the creek.
Tests show that if a person eats too many of the fish in the lower, wilderness stretch of the creek, they could consume enough mercury to cause a variety of woes, including nerve damage, heart attacks and perhaps cancer. Children and pregnant women are especially vulnerable, warned ADEQ.
Experts have not pinpointed the source of the mercury contamination, but it likely comes from naturally occurring cinnabar mineral veins in the volcanic rocks through which the creek cuts. Such veins of red, flaky, often-beautiful minerals have high concentrations of mercury, for which they have been mined since ancient times.
Mercury contamination also stems from left-over mine tailings. Miners often use mercury to extract gold and silver from other ores, which can leave toxic tailings that leach into streams.
Bacteria in the creek alter the mercury and concentrate it in the sediments. Insects munch on the bacteria and are consumed by small fish that are then consumed by large fish. The mercury concentrations increase at each step up the food chain, which means the big, top predator fish like bass end up with the highest concentrations of mercury. Curiously, trout caught in the creek will likely not have worrisome levels of mercury.
The fish in a host of Arizona lakes can have high levels of mercury, including Roosevelt Lake. Other lakes subject to health advisories include Alamo, Arivaca, Coors, Pleasant, Lyman, Parker Canyon, Pena Blanca, Soldier, Long and both upper and lower Lake Mary.
The advisory stressed that the creek remains safe for recreational fishing and camping.
ADEQ recommended that in that wilderness section people should not eat smallmouth or largemouth bass, green sunfish or black bullhead catfish.
Common carp have lower levels, but should not be eaten by children under 6. Children between the ages of 6 and 16 should limit consumption of carp to no more than one 8-ounce serving per month — with a limit of two servings a month for those over 16.
The advisory for possible mercury contamination in Roosevelt Lake fish dates back to 2009. That recommendation says children under 6 should not eat largemouth bass or channel catfish from the lake and women of childbearing age should not eat more than one fish meal per month of largemouth bass or bluegill or two fish per month of pike.
Men are advised to limit themselves to three largemouth bass a month, five bluegill or six pike.
“Consuming fish contaminated with mercury is the most common method of human exposure to mercury,” said ADEQ Director Henry Darwin. “While we continue to work to reduce mercury pollution in Arizona’s waterbodies, these fish consumption advisories are an important part of our effort to protect public health.”
Tonto Creek runs through many relatively young layers of volcanic rock, veined with deposits of cinnabar. The granular, sometimes scarlet mineral has a crystal structure similar to quartz. It forms in hot springs in the fractures of layers of volcanic rocks and has been mined for its mercury content for at least 10,000 years. The Romans noted that sending slaves and convicts to work in the Spanish cinnabar mines amounted to a death sentence and the Mayans used cinnabar to decorate royal tombs.
Bacteria readily transform mercury into methylmercury by binding the metal to carbon molecules, which makes it much easier for the body to absorb.
Mercury has a bewildering array of health effects at high enough concentrations, most of them related to how the metal affects the nervous system — especially among children both born and unborn. Studies suggest that even small exposures can reduce a child’s IQ.