A review of boron effects in the environment

Boron (B) is a naturally occurring element that is found in the form of borates in the oceans, sedimentary rocks, coal, shale, and in some soils. Borates are released naturally into the atmosphere and aquatic environment from oceans, geothermal steams, and weathering of clay-rich sedimentary rocks. B is also released to a lesser extent from anthropogenic sources. B concentrations in air range from <0.5 to 80 ng/m3 with an average of 20 ng/m3, and in soils from 10 to 300 mg/kg with an average of 30 mg/kg. Concentrations of B in surface freshwaters are typically <0.1-0.5 mg/L; much higher concentrations are measured in a few areas, depending on the geochemical nature of the drainage catchment. B accumulates in both aquatic and terrestrial plants, but it does not appear to be biomagnified through the food chain. No observed effect concentrations (NOECs) for aquatic invertebrates tend to be in the range of 6-10 mg B/L with lower values of 1-2 mg/L for community studies. No effect concentrations for fish in natural waters are around 1 mg/L, although lower values have been recorded in reconstituted water. Comparing no effect concentrations with the general ambient environmental levels indicates that the risk to aquatic ecosystems from B is low. In a few B-rich areas, natural levels will be higher; however, there is some indication that organisms may be adapted to the local conditions. B is an essential micronutrient for higher plants with interspecies differences in the levels required for optimum growth. In general, there is a small concentration range between deficiency and toxicity; however, toxicity owing to excess B is much less common in the environment than B deficiency. Levels of B in aquatic plants growing in areas receiving B-rich runoff from irrigated fields are higher than dietary concentrations, which cause effects on the growth of young birds in the laboratory; however, the bioavailability in the field of such plant-accumulated B is uncertain.