To contribute to sustainability, products' manufacture, use, and disposal should not lead to harmful amounts of chemicals in the environment. To estimate what chemical concentration might be harmful, laboratory tests are usually conducted with different species of plants and animals. The results of these studies - called a hazard profile - suggest which species are most sensitive and what concentrations are likely to cause a problem.

For boron, tests of numerous microorganisms, plants and animals have suggested that rainbow trout is the most sensitive species of all those tested. Early reports suggested that trout growth or development might be affected by boron concentrations of about 0.1 mg-B/L. However, further study suggested that adverse effects could be consistently observed only at boron concentrations of one mg-B/L or more. When the early studies were repeated using river water (instead of laboratory water), no effects were observed even though one of the rivers had boron concentrations of 0.75 mg-B/L (the Firehole River in Yellowstone National Park, Wyoming, U.S.). In addition, work with trout in low-boron waters showed that boron is essential: trout embryos showed decreased growth when water concentrations were less than about 0.1 mg-B/L.¹

Conclusions from reviews of these and other laboratory studies were that acceptable levels of boron exposure to trout and other aquatic organisms are up to 1.3 mg-B/L.² Field studies of trout in rivers in the western US have shown that sustainable trout populations exist in river systems having up to 1.0 mg-B/L.³

With this background, it was interesting to discover that a self-sustaining rainbow trout population exists in the Rio de los Patos of northwest Argentina. This high plateau region (about 4,000 meters above sea level) is the location of a significant borate deposit. Boron concentrations in the river range from about two mg-B/L in the upper portion to 18 mg-B/L in the lower portion.

The prediction based on existing laboratory studies would be that this trout population should not be able to survive. According to standard approaches to estimate safe concentrations of chemicals, sensitive species like trout would not persist under such conditions. However, the rainbow trout population in the Rio de los Patos was apparently established by a single stocking event in 1973 and has sustained itself ever since. Additionally, a study conducted by the Universidad Nacional de Salta, Argentina in 1997 indicated adult rainbow trout live in the parts of the river with boron concentrations up to 15 mg-B/L.

To further investigate this site, Borax sought assistance from Dr. Joe Meyer of the University of Wyoming. Dr. Meyer had previously investigated trout populations in the Firehole River in Wyoming.⁴ A reconnaissance survey in December 2002 confirmed that adult rainbow trout were present at boron concentrations up to 18 mg-B/L in the upper and lower portions of the Rio de los Patos and in its only tributary, and that the trout appeared to be reproducing in areas with boron concentrations up to 3 mg/L . A more extensive study is being conducted by Ms. Gladys M. de Gonzo and her graduate students at the Universidad Nacional de Salta. She analyzed water chemistry values at several locations in the river system in late 2003 and collected trout to measure their size, body condition, and reproductive status. Numerous spawning redds (riverbed areas used by trout as egg hatcheries) were located in waters containing up to 3 mg-B/L, but were inactive during the observation period. This work is continuing in 2004 and is expected to be an interesting addition to the hazard profile of boron.

Goldstein, J.N., W.A. Hubert, D.F. Woodward, A.M. Farag, J.S. Meyer. 2001. "Naturalized salmonid populations occur in the presence of elevated trace element concentrations and temperatures in the Firehole River, Yellowstone National Park, Wyoming, USA." Environ. Toxicol. Chem. 20: 2342-2352.