Boron
Boron is one of the micronutrients essential for plant growth. It may be present naturally in water, or find its way into a watercourse through industrial waste effluents. Boron in excess of 2.0 mg/L in irrigation water can be detrimental to plant growth, some plants may even be adversely affected by concentrations lower than 1.0 mg/L (ppm). The United States Department of Agriculture (USDA) reports the following classification:
| Boron (ppm) | Effect on crops |
| < 0.5 good | except for very sensitive crops |
| 0.5 to 2.0 some risks | many crops must be excluded |
| > 2.0 dangerous | may only be used for very tolerant crops |
The average abundance of boron in soils is 18 to 63 ppm, in streams it is 10 μg/L, and in groundwaters it is 0.01 to 10 mg/L. The most common form of boron in natural waters is H3BO3. The most important mineral is borax, which is used in the preparation of heat-resistant glasses, detergents, porcelain enamels, fertilizers, and fiberglass.
Drinking water rarely contains more than 1 mg B/L and generally less than 0.1 mg/L, concentrations considered safe for human consumption. The ingestion of large amounts of boron can affect the central nervous system, with protracted ingestion resulting in a syndrome known as borism.
