Guidelines for Canadian drinking water quality boron: Rationale for maximum acceptable concentration

Boron can be present in drinking water supplies through weathering of boron-containing rocks and soils, seawater spray and volcanoes as well as from anthropogenic sources such as wastewater from municipal sewage treatment and industrial processes, pesticide application and other human activities. In most Canadian drinking water supplies, boron is below 0.1 mg/L. Higher concentrations of boron (1 mg/L to 8 mg/L) can be found in certain areas of Canada, particularly in groundwater supplies in areas with naturally occurring boron. The main source for Canadians' exposure to boron is through food, although consumer products can also contribute significantly to exposure. Exposure through drinking water can contribute up to 16% of total dietary exposure.

Reproduction and development are considered to be the most sensitive health endpoints for boron toxicity. Studies in humans have found possible associations between boron and effects to reproduction and development. However, these have several design limitations and cannot be used for risk assessment. Studies in animals have consistently observed adverse effects to the male reproductive system following ingestion of high levels of boron, which supports effects seen in human studies. Testicular effects observed in dogs were the most sensitive endpoint and are used to establish the HBV of 0.1 mg/L.

The guideline development process considers the ability to measure a contaminant in drinking water. There are several analytical methods available for the analysis of total boron in drinking water at concentrations below the HBV.

The guideline development process also considers whether the maximum acceptable concentration (MAC) is practically achievable using current drinking water treatment technologies. For municipal-scale treatment plants, the 2 most effective and commonly used treatment technologies for boron removal are RO and IX using a boron selective resin. Data from treatment plants indicate that a treated water concentration at the HBV of 0.1 mg/L is not achievable for most systems. While IX with a boron selective resin may be theoretically capable of removing boron from drinking water to below 0.1 mg/L, the operating conditions that would be required would be extremely challenging and are not considered to be practical. In general, when source concentrations are 8 mg/L or less, treated water concentrations of less than 5 mg/L are achievable using RO and IX (with BSR) that have design and operating conditions for boron removal.

A MAC of 5 mg/L is established for boron in drinking water, based on the following considerations:

• The HBV for boron in drinking water is 0.1 mg/L, derived using a conservative approach intended to protect against potential reproductive effects. Some studies indicate that boron may be beneficial to human health, and background exposures, coming primarily from food, are estimated to be similar to the TDI used to derive this HBV.
• A treated water concentration at the HBV of 0.1 mg/L is not achievable for most drinking water systems.
• A treated water concentration of 5 mg/L or less is achievable by municipal-scale treatment plants using RO or IX technologies with common design and operating conditions.
• The MAC provides flexibility for small groundwater systems to install a single RO system which is the simplest treatment technology for boron removal.
• Currently, boron is not included in NSF/ANSI standards for residential treatment units. However, RO and distillation treatment units are expected to be capable of removing boron to 5 mg/L or less in treated water.
• The MAC is risk managed to take into consideration the treatment challenges of lowering the MAC (especially for private wells and small systems).

In considering treatment achievability and the health risks associated with exposure from boron in drinking water, Health Canada, in collaboration with the Federal-Provincial-Territorial Committee on Drinking Water (CDW) have established a MAC of 5 mg/L. This MAC generally aligns with established limits in other jurisdictions. As this value exceeds the HBV, efforts should be made to reduce boron in drinking water to levels lower than 5 mg/L where possible. Lower concentrations can be achieved by some drinking water treatment systems depending on the source water quality, the type of treatment technology in place and the operational conditions of the treatment plant.

As part of its ongoing guideline review process, Health Canada will continue to monitor new research in this area and recommend any change to the guideline that is deemed necessary.