At recent meetings of the San Martin Perchlorate Community
Advisory Group, and in local news articles, area farmers have
expressed concerns about the levels of perchlorate in produce grown
in the area
EDITOR:
At recent meetings of the San Martin Perchlorate Community Advisory Group, and in local news articles, area farmers have expressed concerns about the levels of perchlorate in produce grown in the area, using irrigation water that contains trace amounts of perchlorate. In my view there is no danger to any consumer of the local produce grown with water containing the trace amounts of perchlorate reported in local wells.
Perchlorate, ClO4-, is a negatively charged ion about the same size as the iodide ion. In sufficient amounts it can interfere with the thyroid uptake of iodide, and has been used medicinally for that purpose for 50 years.
The solid perchlorate salts used in rocket fuels and highway flares are very powerful oxidizing agents. These highly reactive salts are very soluble in water, but in dilute solution perchlorate is one of the least reactive and most inert of anions. It is this inert character that determines its path in produce, and also explains the great difficulty in removing perchlorate that has leached into water supplies.
How does perchlorate get concentrated by plants? If a gallon of water with 10 parts per billion (ppb) perchlorate is evaporated or boiled down to one half gallon, the perchlorate level would then be 20 ppb. Perchlorate is not volatile, and remains behind in whatever fluid is left.
In a growing plant, not all the water that enters the plant remains in the plant. Some of the water is evaporated (transpired) through pores (stomata) in the leaves or diffuses out through plant membranes. The perchlorate from all the water entering the plant is left behind on or in the plant, just as are all the other non-volatile dissolved salts in the irrigation water.
If five volumes of water are required to grow a plant retaining only one volume of water at harvest, then the perchlorate concentration in the plant would be five times that of the water taken up by the plant.
Leafy vegetables such as lettuce appear to “concentrate” perchlorate because a relatively large volume of water is transpired by the plant. If the transpiration is through stomata, the residue would be near the surface of the leaf, and should be fairly readily washed off. Watery fruit on a plant, such as tomatoes or watermelon, would not transpire large volumes of water like leaves do, and would probably have a perchlorate level close to that of the incoming water.
Another factor is the manner in which irrigation water gets to the plant. If water is applied in nearby furrows, where water is said to “sub-up” to the plant by capillary action, there will be evaporation of water from the soil. In hot, arid climates that process will concentrate the salts in the water before it gets to the plants. That salt build up will also remain in the soil for the next crop unless the soil is flushed with fresh water and drained to remove the salts.
This concentration of perchlorate by plants is primarily a physical evaporation process determined by the plant physiology. Perchlorate just stays with the liquid water or ends up in the residue if the water evaporates completely. If perchlorate reacted chemically with the plants, it would no longer be perchlorate and detectable as such. (At higher concentrations, perchlorate would make an excellent tracer for studying plant transpiration and water movement.)
What levels of perchlorate might be expected in plants watered with 10 ppb perchlorate? In watery fruit like tomatoes, values higher than 20 ppb would seem unlikely, and therefore not detectable with an analytical sensitivity limit at 50 ppb. In leafy parts of a plant where transpiration would evaporate a large amount of water, values of 50 to 100 ppb are possible, assuming the residual salts are not washed off the plant stomata before analysis. Measuring those concentrations is less of a problem than in getting agreement on what levels are “acceptable.”
Are these concentrations of perchlorate in produce a problem? The Environmental Working Group claim that “there is human health harm at 1 ppb” does not seem credible or supported by facts.
On the other hand, a very credible clinical study on human volunteers has shown that drinking water with perchlorate concentrations of 200 ppb has absolutely no effect on the normal adult thyroid function. (MA Greer, et al., Environmental Heath Perspectives, 110:927-937 (2002)) Like drinking water, eating produce with levels of perchlorate up to 200 ppb should be totally safe.
Richard M. Peekema,
retired chemist, San Jose
