Wood preservation facilities, inorganic boron (borate): chapter I-3
3. Environmental Effects
Boron is ubiquitous in the environment; it is widespread in nature at relatively low concentrations (3) and is occurring naturally in over 80 minerals. The boron content of soils ranges from 0.002 to 0.1 mg/g dry weight (4); it is highly mobile in this medium and is easily leached. Factors influencing boron adsorption to soil include soil pH, texture, organic matter, cation exchange capacity, moisture and temperature (5). The concentration of boron in Canadian coastal waters is reported to range between 3.7 and 4.3 mg/L, and estuarine waters are generally rich in boron (6).
Boron is an essential trace element for the growth of terrestrial crop plants and some algae, fungi and bacteria, but can be toxic in excess. Toxicity to aquatic organisms, including vertebrates, invertebrates and plants, can vary depending on the organism’s life stage and environment. Early life-cycle stages are more sensitive to boron than later ones, and the use of reconstituted water shows higher toxicity in lower boron concentrations than natural waters. In mammals, excessive consumption can adversely affect growth, reproduction or survival (7).
Although boron is present naturally in Canada, industrial potential releases can be found in much greater concentrations that can potentially become harmful to the environment and human health.
The data indicate that a wide variety of properties must be considered in order to safely manage borate.
3.1 Aquatic Toxicity
The toxicity of boron compounds is generally expressed in terms of boron itself.The predominant form of boron in water is boric acid. Conversion may be necessary to reflect the element concentration.
Because boric acid is a weak acid with an acid dissociation constant (pKa) of 9.2, it exists primarily as the undissociated acid H3BO3 in aqueous solution at physiological pH, as do the borate salts (3). Therefore, the toxicity associated with these compounds is expected to be similar based on boron equivalents. Boron oxide will also produce similar effects because it is an anhydride that reacts exothermically with water in the body to form boric acid (8).
The guidelines and limitations noted in Table 3 are based on total concentrations, reflecting the recommendations of many scientific reviews.
Provincial guidelines are applicable and should be consulted. Provincial guidelines may differ from or be more specific than national guidelines. Provincial regulations may require additional measures that may enhance, but not reduce, protection.
Boron’s effects on aquatic plants are highly species-specific (9). Borate, like silicate, is an essential micronutrient for the growth of aquatic plants. Boron, under conditions of excess, alleviates nutrient deficiency in some phytoplankton and may cause temporal variations of phytoplankton composition in coastal waters (9). Phytoplankton can tolerate up to 10 mg inorganic B/L in the absence of stress from pH adversity and nutrient deficiency, although higher borate concentrations up to 100 mg/L are expected to cause species redistribution by favoring the growth of some species and suppressing that of others (10). Boron has been shown to accumulate in aquatic plants, which may be evidence for its importance in plant nutrition. Despite a tendency to accumulate in plants and algae, boron does not appear to biomagnify through the food chain (11).
Data are limited for aquatic invertebrates and boron. Juvenile Pacific oysters (Crassostrea gigas) accumulated boron in relation to availability, but showed no prolonged retention following cessation of exposure. At current industrial discharge levels of about 1.0 mg B/L, no hazard is clear to oysters and aquatic vertebrates (12).
The most sensitive aquatic vertebrates tested for which data are available were coho salmon (Oncorhynchus kisutch), with a median lethal concentration (LC50) value of 12 mg B/L in seawater (16-day exposure), and sockeye salmon (O. nerka), showing elevated tissue residues after exposure for 3 weeks in seawater containing 10 mg B/L.
Boron concentrations between 0.001 and 0.1 mg/L had little effect on survival of rainbow trout embryos after exposure for 28 days. These low levels may represent a reduction in reproductive potential of rainbow trout, and > 0.2 mg B/L may impair survival of other fish species, according to Birge and Black (13); however, additional data are needed to verify these speculations. Birge and Black reported that concentrations of 100-300 mg B/L killed all species of aquatic vertebrates tested; that embryonic mortality and teratogenesis were greater in hard water than in soft water, but that larval mortality of fish and amphibians was higher in soft water than in hard water; and that boron compounds were more toxic to embryos and larvae than to adults (14).