Download Can arsenic in soil and water contaminate home

Fact Sheet-09-09
Can arsenic in soil and water contaminate
home-grown vegetables?
Mark Walker Associate Professor, College of Agriculture, Biotechnology and Natural Resources
State Extension Specialist, University of Nevada Cooperative Extension
JoAnne Skelly
Associate Professor, University of Nevada Cooperative Extension
Extension Educator, Carson City/Storey County
Kent McAdoo
Central Northeast Area Extension Specialist, Natural Resources,
University of Nevada Cooperative Extension
This Fact Sheet summarizes the results of research about the possibility that home‐grown vegetables will be unsafe to eat when irrigated with water that contains high concentrations of arsenic. Although some research has shown that arsenic can accumulate in plants, the research indicates that even if arsenic concentrations in soils and water are high, arsenic in roots, leaves and fruits will likely be less than the concentrations that the U.S. Food and Drug Administration has determined to be safe for consumption. Research also indicates that plants will likely be damaged, with reduced yields, if concentrations of arsenic are high. When concentrations of arsenic are high enough in soils and water to potentially lead to accumulations, the plants are unlikely to produce edible roots, leaves and fruits. Background
Many rural households in Nevada rely on private wells for water supply. In parts of Nevada, groundwater contains high concentrations of metals, including arsenic. Those who garden with water from private wells often are concerned that arsenic from groundwater could accumulate in roots, leaves and fruits in concentrations that are harmful to health. Although treatment can remove naturally occurring metals from drinking water, some treatments that are effective for removing arsenic, such as reverse osmosis, usually produce very small volumes of water, much less than would be needed for gardening. In this case, untreated water, which could contain high levels of arsenic and other metals, would be used for gardening. The standard for arsenic in food
The U.S. Food and Drug Administration (FDA) sets standards for food to protect public health. For a complete list of standards related to This publication was made possible by a Regional Water Quality Coordination Grant awarded as part of the U.S. Department of Agriculture’s National Water Program. chemical and microbiological contaminants, visit the FDA’s web site (www.fda.gov). The FDA standard for arsenic in food is based on its use as a food additive (21 Code of Federal Regulations, Ch. 1, part 173: Secondary Direct Food Additives Permitted in Food for Human Consumption). One part per million (ppm) equals one part of a substance (by weight) mixed completely in one million parts of another substance. In water, one ppm is the same as one milligram (mg, about 0.00004 ounces) of a substance dissolved in one liter of water (1.06 quarts), which weighs one kilogram (about 2.2 pounds). A ppm is the same as a mg/liter (mg/L). In soil, 1 ppm is the same as having 1 ounce of gold in 31.25 tons of ore. The standard allows no more than 2 parts per million of arsenic to be present in the fresh weight of a product. It is important to note that the standard is focused on fresh weight, because results of many experiments with arsenic are often reported for the dry weight of the parts of the plant studied. In most cases the concentration of arsenic reported for the dry weight of a plant would be much higher than for the fresh weight, because of the amount of water in plant tissues. Almost all soils contain some arsenic, although in many soils the concentrations are below typical laboratory detection limits. Soils that have not been treated with an arsenic‐containing pesticide have concentrations that are usually below 10 ppm, with a range of 0.2 to 40 ppm. Soils are the connection
Most gardeners’ concerns about potential accumulation of arsenic in edible plant tissues are related to high concentrations of arsenic in water used for irrigation. However, most home gardeners are not producing vegetables using techniques such as hydroponics, which relies only on water with added nutrients to grow vegetables in specialized systems. For the most part, when water that contains arsenic is added to soils, the arsenic may be chemically bound to soil particles in a way that makes it unavailable to plants and unlikely to dissolve in water. Not all arsenic in soil is readily available for plant uptake. In general, the capacity of soil to absorb and retain arsenic is related to the amount of the soil that is made up of fine particles, especially clays. The more clay a soil has, the more likely that arsenic will be tightly bound and not released to plants in water passing from the soil into the plant. Plant tolerance to arsenic
Sources of arsenic
In Nevada, most arsenic in groundwater and soils is present naturally. Although arsenic was once used as a pesticide—for example for livestock dips—the predominant source in natural waters is sediments from volcanic rock, eroded over the course of many millennia and deposited in the basins that trend north to south in Nevada. Over long periods of time, these arsenic‐bearing sediments can add arsenic to groundwater. Under the right subsurface chemical conditions, concentrations of arsenic may be very high and substantially exceed the drinking water standard of 0.010 ppm. Plants vary considerably in their tolerance to arsenic in soils. In general, before arsenic accumulates in leaves and fruits in concentrations that exceed the FDA standards, plants are likely to die or have severely reduced yields (Walsh, et al. 1977). Tolerance is related to many chemical characteristics of the soil, including the fertility level (especially phosphorus content), acidity and the kinds and amounts of minerals and organic matter present in the soil. The sections that follow discuss the toxicity of arsenic to specific fruits (tomatoes), leafy vegetables (spinach) and edible roots (radishes). Arsenic toxicity to three commonly grown vegetables (from Woolson, 1973): This study with tomatoes, spinach and radishes involved mixing an arsenic compound with soils to create 10, 50, 100 and 500 ppm concentrations of arsenic in three soils (a loamy sand, a silty‐clay loam and a clay loam). The results are discussed briefly below. and moved into plant roots and other tissues. Even though concentrations of arsenic in soils may be high, the levels in plants are likely to be much lower because of the capacity of soil to absorb and retain arsenic. Arsenic toxicity to tomatoes: Tomatoes showed substantially reduced growth (40 percent less growth) in loamy sand at soil concentrations of 50 ppm than tomatoes not grown in soils with arsenic. In silty‐clay and clay loam, yield was reduced by 6 percent to 21 percent at concentrations of up to 100 ppm. When concentrations reached 500 ppm, tomatoes grew very poorly, regardless of the soil type. This indicates that at a concentration of 500 ppm, which is very high, a gardener would expect to see poor growth. The amount of arsenic that accumulates in plant tissues is determined by several factors, including the type of plant, the portion of the plant that is to be eaten and soil types. In general, if concentrations of arsenic in soil and water are high, plant growth and yield are likely to be reduced, especially for fruits and leaves. Root crops such as radishes, potatoes and onions may accumulate arsenic to a greater degree than fruits and leaves. Arsenic toxicity to spinach: Arsenic concentrations of 500 ppm in soil led to no spinach growth. At lower concentrations, the type of soil influenced the effect on plant growth. For example, at arsenic concentrations of 100 ppm, spinach yields were reduced by 98 percent in loamy sand soil. However, in silty‐clay and clay soils spinach yields were reduced by 30 percent to
35 percent, respectively, compared to sandy soils that contained no arsenic. Arsenic toxicity to radishes: In a sandy soil, radishes either did not grow or showed 90 percent to 96 percent reduction in growth at a soil concentration of 500 ppm. At concentrations of 100 ppm in loamy sand, silty‐clay and clay soils, radishes showed 67 percent, 17 percent and 7 percent reductions in growth, respectively. This means that when arsenic concentrations are high (between 100 and 500 ppm) in soils, radish growth will be inhibited. In all of the experiments discussed above, the effects of having arsenic were greater in sandy soils than in soils that had high proportions of clay. Clay minerals sequester arsenic, making it very difficult for arsenic to be dissolved in water Arsenic accumulation in plants
A study of roots, stems, leaves and fruits in soils with varying arsenic levels found that the amount of arsenic in root crops, such as potatoes and onions, corresponded with the amount of arsenic in the soils in which they were grown (Dahal, et al. 2008). However, the researchers concluded that although potatoes and other vegetables took up arsenic from surrounding soils and irrigation water, arsenic accumulation was unlikely to exceed the FDA standard. This is similar to results reported by Huang, et al (2006), who found that radishes and onions accumulated arsenic when grown in soils with approximately 1 to 25 ppm arsenic, though not in concentrations of concern. Finally, another study by Gaw et al. (2008) found that radishes and lettuce grown in soils that had formerly been treated with arsenical pesticides also accumulated arsenic, though not in concentrations that exceeded the FDA standard. Bioaccumulation factors (BAF) represent the ratio of the concentration of a chemical in a plant to the concentration in soils. When the BAF is greater than 1.0, a plant accumulates a chemical in concentrations that are greater than in soil. A study by Gaw et al (2008) estimated the BAF for lettuce and radishes grown in soils that contained arsenic in concentrations of 0.4 to 35.6 ppm. Some of the soils had been treated with arsenical pesticides in the past, for fruit production. They found the BAF for both vegetables to be small, much less than 1.0. In fact, for lettuce the BAF ranged from too small to estimate to 0.13 and for radishes, it ranged from too small to estimate to 0.17. These studies reported the results in terms of the dry weight of the plant, which is typically much less than the fresh weight. Because of this, the BAF for each vegetable would likely have been much lower if the results were expressed for fresh weight, which is how the FDA standard is written. Conclusions
The studies that examined the connection between reduced growth in vegetables and arsenic concentrations in soil and water used concentrations that were very high, much higher than would be expected in soil and water found in Nevada. In general, yields and plant growth were affected before concentrations of arsenic exceeded the FDA standard in the edible portions of the plant. Some studies showed accumulations in root crops, such as radishes, to be higher than 2 mg/kg. There are many other factors besides arsenic toxicity, such as nutrient deficiencies, high or low pH and high salt content, which can adversely affect plant growth. To eliminate these other factors, gardeners should have soil tested if plants are not growing well. If other factors are suitable for plant growth, the homeowner may want to have additional tests performed for trace metals such as arsenic. County offices of the University of Nevada Cooperative Extension system maintain lists of laboratories that provide soil‐ and water‐testing services. Such analyses can be helpful, but there are no guidelines for determining whether concentrations of arsenic in soil and water are likely to lead to plant damage. References: Dahal, B.M. et al. YEAR. Arsenic contamination of soils and agricultural plants through irrigation water in Nepal. Environmental Pollution V. 155(1): 157‐163. Gaw, S.K. et. al. 2008. Uptake of ΣDDT, arsenic, cadmium, copper and lead by lettuce and radish grown in contaminated horticultural soils. Journal of Agricultural and Food Chemistry. V. 56, pp. 6584‐6593. Huang, R.Q. et al. 2006. Soil arsenic availability and the transfer of soil arsenic to crops in suburban areas in Fujian Province, southeast China. Science of the Total Environment. V. 368 (2‐3), pp. 531‐541. Walsh, L.M., E. Malcolm, S. Keeney and D.R. Keeney. 1977. Occurrence and distribution of arsenic in soils and plants. Environmental Health Perspectives. V. 19 pp. 67‐71. Woolson, E.A. 1973. Arsenic phytotoxicity and uptake in six vegetable crops. Weed Science. V. 21(6), pp 524‐527. For further information, please contact:
Mark Walker, Associate Professor, Hydrologist
Natural Resources and Environmental Science,
University of Nevada, Reno
University of Nevada Cooperative Extension
and College of Agriculture, Biotechnology and
Natural Resources
Phone: (775) 784-1938
FAX: (775) 784-4789
Email: [email protected]