In the Jan. 22, 2025 issue of Grainews, I wrote an article about selenium (Se) — a naturally occurring element that can be found in soil, our drinking water and food. I had a number of inquiries for more information on its direct effects on agriculture, the environment and irrigated crop production.
In the past five years, there have been increasing concerns about pollution risk from proposed coal mining in the Eastern Slopes in southern Alberta. This region of southern Alberta extends from the upper alpine slopes of the Rocky Mountains down to the foothills and onto the Prairies. This area forms the headwaters of the Oldman, Bow and Red Deer rivers, which merge to form the South Saskatchewan River.
In 1911, Canada’s department of the interior recognized the importance of the Eastern Slopes watershed that supplied water to the river systems that in turn provide water to the great plains to the east. Policies were developed to protect the Eastern Slopes and to keep lands from being privatized. It’s very important that the Eastern Slopes continue to be protected into the future. These lands have become very important as the major source of water to downstream users in Alberta, Saskatchewan and Manitoba.
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If coal mining proceeds in the Eastern Slopes of southern and central Alberta, downstream water users could be seriously impacted. Selenium and other elements potentially could contaminate water quality for humans, livestock and irrigated crop production and cause various other environmental and ecological concerns. In this article, I would like to focus more deeply on effects of Se on irrigation water quality, soil quality, and irrigated crop production and quality.
Selenium in drinking water and for aquatic life
Health Canada has an upper limit of 50 micrograms per litre (µg/L) of Se for drinking water, which is the limit used by the Alberta government. Alberta uses an upper limit of one to two µg/L for aquatic life. British Columbia uses an upper limit of 10 µg/L for Se for drinking water and of one µg/L for aquatic life.
Selenium in soils, forages and animals
First, it is important to note that Se is important for agriculture. It is an essential micronutrient for animals and humans. Selenium is not required by plants, but roots naturally take up very small amounts from soil. Plants are the main source of dietary Se for livestock.
Selenium can be found in soil in various forms, including selenide, selenite, selenate, elemental selenium and organic selenium. Selenium can also be found in different fractions in soil, including ion-exchangeable, oxide-bound, organic, sulphide-bound, and residual forms. The majority of Se in agricultural soils exists as selenate (SeO4−2) or selenite (SeO3−2). The toxicity level of Se in soil depends on the amount, chemical forms and bioavailability of the element. Therefore, Se toxicity levels in soil are variable and a function of a number of chemical soil factors. For more detailed information on Se in soils, read Imran et al (see “Reading for reference” at the end of this article for details).
It’s important to keep in mind that Se accumulation in soil can pose serious threats to crop production and agro-ecosystems due to bioaccumulation. Selenium toxicity in soil and then into food chains depends on Se forms in soil, rather than total Se contents. In 1992, the Geological Survey of Canada noted Se levels in Alberta soils ranged from 0.1 to 2.7 milligrams per kilogram (mg/kg), Saskatchewan soils ranged from 0.1 to 3.1 mg/kg and Manitoba soils ranged from 0.1 to 4.7 mg/kg.
Beef cattle have a Se requirement of 0.1 mg/kg (100 µg/kg) of dry matter. Generally, for domestic animals, the Se requirement is in the range of 0.05 to 0.2 mg/kg (50 to 200 µg/kg) in dry matter. A Se toxicity threat to livestock can occur if forages are continually fed at marginal levels of Se at 0.3 to 0.5 mg/kg. Forage above 0.5 mg/kg of Se can cause acute toxic conditions in livestock.
Health Canada suggests adult humans have a daily requirement of 45 micrograms per day (µg/day) of Se, which is 0.000045 grams/day. The tolerable upper intake level (UL) for Se for adult humans is 400 µg/day or 0.0004 grams/day from food, water and other sources — very small but critical amounts of Se.
Conversions for context
1 milligram/kilogram (mg/kg) = 1 part per million (p.p.m.)
1 microgram/litre (µg /L) = 1 part per billion (p.p.b.)
1 milligram/litre (mg/L) = 1 part per million (p.p.m.)
1 milligram/litre (mg/L) = 1,000 micrograms/litre (µg/L)
Irrigation water quality
The level for Se in irrigation water for Alberta is 0.02 milligrams/litre (mg/L), which is 20 micrograms/litre (μg/L). The safe level of selenium in irrigation water in British Columbia is set at 0.01 mg/L (10 μg/L). The World Health Organization set a guideline value for Se in irrigation water at 0.04 mg/L (40 μg/L).
An Alberta Agriculture report on irrigation water quality in southern Albera, by Little et al, found Se was detected in 69 per cent of collected irrigation water samples (419 of 607) across a number of irrigation districts. Detection frequencies within the irrigation districts ranged from 34.4 per cent in the United district to 95.3 per cent in the Western district. The greatest mean concentration was observed was 0.0021 milligrams per litre (mg/L, or 2.1 μg/L), while the greatest maximum concentration was observed at the Lethbridge Northern district at 0.01 mg/L (10 μg/L). Selenium exceeded guidelines for the protection of aquatic life in 12.8 per cent of samples. Irrigation guidelines, however, were not exceeded. The lowest compliance rate was found in Magrath district return flows (28.6 per cent).
The good news about this 15-year-old study was that Se level in irrigation water was acceptable in all 607 water samples from an irrigation quality standpoint in 2010. The concerning news was that Se was not acceptable for aquatic life in almost 13 per cent of samples. Se was of greatest concern in the Lethbridge Northern district, which receives all its water from the Oldman River — which is downstream from the proposed Grassy Mountain project.
How much selenium could be applied in irrigation water?
Here the important questions to ask are:
• How much Se would be applied in irrigation water?
• How would Se impact irrigated soils?
• How would Se impact crop production and quality?
These concerns would depend on the level of Se in the water. Keep in mind, the Alberta government considers 0.02 mg/L (20 μg/L) of Se to be the upper limit for irrigation water; the British Columbia environment ministry considers 0.01 mg/L (10 μg/L) of Se as the upper limit for irrigation water.
Most irrigation farmers apply 250 to 500 millimetres (mm) of irrigation water per season to their crops. Amounts of 250 to 300 mm are typically applied to grain and oilseed crops, while 400 to 500 mm are applied to alfalfa and special crops such as sugar beets, potatoes or corn. The amount applied each season varies depending on the amount and timing of growing-season precipitation.
If the Se level in irrigation water is 0.05 mg/L (50 µg/L) — the upper limit for drinking water — how much Se would be added to soil? To determine Se addition to soil, I will use 250 and 500 mm of gross irrigation water application, which works out to 2,500 and 5,000 cubic metres of water per hectare (m3/ha) respectively. I used a Se irrigation water level of 0.05 mg/L, or 0.00005 kg of Se per cubic metre (kg/m3) of water. When 250 mm of water is applied, then about 0.125 kg per hectare (kg/ha) of Se is applied in a growing season. When 500 mm of water is applied, 0.25 kg/ha of Se is applied in a growing season.
Amounts of 0.125 and 0.25 kg/ha of Se seem like very small amounts, but over 10 years would amount to an accumulation level of 1.25 and 2.5 kg/ha in soil and likely would result in unsafe levels of Se in harvested plant material. Again, keep in mind, Se levels in forage of 0.3 to 0.5 mg/kg are a concern for livestock.
Consequences of Se contamination in irrigated soils
If coal mining in the Eastern Slopes of the Rocky Mountains proceeds, Se contamination of downstream water is a likely result. When contaminated irrigation water is used, Se will accumulate in soil and will be taken up by crops. As Se accumulates in soil over years, more Se will be taken up by plants, making crops less suitable or unsuitable for human or livestock consumption.
High-value crops such as potatoes, beans and other pulse crops, canola and various grains are grown under irrigation for human consumption. High-yielding silage corn is widely grown for feed for the intensive livestock industry and alfalfa is also commonly grown for feed. Se-contaminated water applied to irrigated land over a period of a just a few years could be devastating to the irrigation industry. Production of crops for human consumption and feed for livestock production would be affected.
The irrigation industry contributes significantly to Alberta’s economy. It generates a large portion of the area’s gross domestic product (GDP), creates substantial employment and ensures diverse crop production. The irrigation industry likely contributes nearly $8 billion to the regional economy in southern Alberta. If irrigated land is well managed, irrigated crop production would be sustainable for hundreds of years. Coal is a non-renewable resource — and mining coal would be short-term, with long-term environmental problems.
Irrigation farmers and the public must ask: “Can we afford the risk of contaminating and damaging our irrigated lands? What would the effects be on food and feed quality produced on irrigated lands? What would be the environmental and economic impact of damaging irrigated lands with Se?”
To sum up
Let’s review: Health Canada has an upper limit of 50 µg/L of Se for drinking water, which is used by the Alberta government. Alberta uses an upper limit of 20 µg/L for Se in irrigation water and an upper limit of one to two µg/L for aquatic life. British Columbia uses an upper limit of 10 µg/L for Se for drinking water, an upper limit of 10 µg/L for Se for irrigation water and an upper limit of 1.0 µg/L for aquatic life.
Decisions need to be made as to whether water is best managed based on drinking water quality, irrigation water quality or aquatic life and river health. Ideally, from a water quality standpoint, the health of our rivers and lakes should be our highest priority. If river health and aquatic life of our rivers is well maintained and cared for, this will ensure the health of our environment.
In my opinion, using a Se upper limit of one to two µg/L would be a very wise target to maintain healthy rivers and lakes. This would ensure Se levels are very safe for human consumption, livestock, irrigation water quality and aquatic life.
CORRECTION, July 7, 2025: The print and previous online versions of this article incorrectly noted a selenium irrigation water level of 0.05 milligrams per litre equaling 0.005 kg per cubic metre. That number has been corrected to 0.0005 kg per cubic metre in the version above.
Reading for reference
British Columbia Ministry of Environment. 2014. Ambient Water Quality Guidelines for Selenium.
British Columbia Ministry of Environment and Climate Change Strategy. 2025. British Columbia Approved Water Quality Guidelines: Aquatic Life, Wildlife & Agriculture – Guideline Summary. Water Quality Guideline Series, WQG-20. Prov. B.C., Victoria B.C.
Government of Alberta. 2018. Environmental Quality Guidelines for Alberta Surface Waters. Water Policy Branch, Alberta Environment and Parks. Edmonton, Alberta.
Health Canada. 2014. Guidelines for Canadian Drinking Water Quality: Guideline Technical Document — Selenium. Water and Air Quality Bureau, Healthy Environments and Consumer Safety Branch, Health Canada, Ottawa, Ontario. (Catalogue No H144-13/4-2013E-PDF).
Health Canada. 2021. Selenium and its compounds – information sheet.
Little, J., Kalischuk, A., Gross, D., and Sheedy, C. 2010. Assessment of Water Quality in Alberta’s Irrigation Districts, Second Edition. Alberta Agriculture and Rural Development, Alberta, Canada. 181 pp.
Fordyce, Fiona. 2010. Chapter 16 Selenium Deficiency and Toxicity in the Environment. In: Essentials of Medical Geology, Editor: Olle Selinus, Publisher: Springer Dordrecht.
Muhammad Imran, Zhikun Chen, Ayaz Mehmood, Shah Rukh, Wang Weixie, Waleed Asghar and Farhan Iftikhar. 2023. Distribution of Selenium in Soils and Human Health.
