Arsenic is a toxic compound and it is most toxic in its inorganic form. Long‑term exposure to arsenic from drinking‑water and food can cause cancer and skin lesions. It has also been associated with developmental effects, cardiovascular disease, neurotoxicity and diabetes.
Arsenic in Soil and Plants
Arsenic naturally occurs in the soil and elsewhere in the environment and it is the 20th most common element on Earth. Industrial activity (such as mining) can increase natural levels. Arsenic is also found in CCA treated timber, termite control products and some herbicides designed for use on lawns. Extensive research has shown that arsenic is 'safe' or tolerable if ingested at rates below two µg/kg of body weight per day (World Health Organisation limit), or three µg/kg of body weight per day (Food Standards Australia limit). The NSW Department of Primary Industries suggests that agricultural soil should have less than 20 mg of arsenic per kg of soil. Different guidelines may be available in your area but this is a good general guide. If your soil has an arsenic concentration of more than 10 mg/kg, it would be a good idea to order a plant tissue test and seek expert advice on whether the amount of arsenic in your home grown food constitutes a health risk.
If you are not growing food in your garden or the food you grow makes up less than 10% of the fruit and vegetables you eat, Australian guidelines recommend a soil lead value of less that 100 mg/kg. Environmental protection agencies in California, Canada, Norway and the United States publish guidelines for arsenic in soil as well: 0.07 mg/kg, 12 mg/kg, 12 mg/kg and 0.61 mg/kg respectively.
Dealing with Excess Arsenic in the Soil
There are a number of ways to reduce the risk of ingesting arsenic from the soil (via ingesting plants and plant matter grown in that soil).
- Garden in raised beds. Raised beds containing imported (non contaminated) soil will not absorb arsenic from the ground soil below ‑ provided CCA treated timber is not used for the structure of the bed. To be on the safe side, edible plants grown in raised beds should not be able to sink a large proportion of their roots into the soil below the garden bed. Raised bed heights should thus be sufficient to cover the majority of root growth. Where there is a concern, plant tissue testing will demonstrate whether plants are absorbing arsenic in excess of accepted levels.
- Choose crops carefully. Studies have shown that certain types of crops are more likely to absorb arsenic than others. The CSIRO notes that the above ground fruits of plants grown in contaminated soil (such as tomatoes, cucumbers and grapes) do not contain arsenic whereas root vegetables (carrots and beetroot) do. The arsenic accumulates mostly within the skin of these vegetables so can be removed prior to consumption through peeling. The arsenic is also known to be in a safer, organic form.
- Bioremediate the soil. Certain plants accumulate large amounts of toxins without any ill effects. These plants can be removed when they reach maturity and the soil toxins with them. Over time, repetition of this cycle reduces the amount of the toxin in the soil, provided soil isn't being continuously contaminated (from uphill water run off or herbicide use for instance). Pityrogramma calomelanos var. austroamericana is an arsenic super accumulator that could be used for this purpose.
- [Australian] National Environment Protection (Assessment of Site Contamination) Measure 1999 Schedule B1 (F2013C00288, measures as amended, taking into account amendments up to National Environment Protection (Assessment of Site Contamination) Amendment Measure 2013 (No. 1)) http://www.comlaw.gov.au/Details/F2013C00288/Html/Volume_2