Arsenic collects in top soil, contaminating rice, reducing yield.

Jul 12, 2010

Asaduzzaman Khan, M, M Rafiqul Islam, GM Panaullah, JM Duxbury, M Jahiruddin and RH Loeppert. 2010. Accumulation of arsenic in soil and rice under wetland condition in Bangladesh. Plant and Soil

Synopsis by Thea M. Edwards

Arsenic carried to rice fields by contaminated irrigation water tends to accumulate in top soil layers where the rice takes it up, contaminating the grain, lowering its nutritional value and exposing large populations who depend on it for food.

Arsenic in irrigation water or soil congregates in the top layers of soil where rice roots grow, allowing the plants to incorporate the toxic metal into the grain. The arsenic tends to stay in the top soil regardless of growing season, type of rice or other farming factors, according to a two-year study comparing water and soil sources of the metal.

While it is known that arsenic accumulates in soil and contaminates rice, this study corroborates others that document how far and under what conditions arsenic moves through the soil. A better understanding of arsenic's absorption and movements through soil and water could improve rice farming techniques in ways that would lessen the toxic metal's accumulation in the food over time.

People are exposed to arsenic primarily through diet and drinking water. Rice is one of the largest sources of arsenic exposure for people in Bangladesh and India. It represents about half the total intake of arsenic, according to the authors, who note that rising arsenic contamination in water and food supplies threatens food security, food safety and quality, and long-term agricultural sustainability of rice crops.

In the United States, drinking water levels of arsenic are regulated at 10 parts per billion (ppb) by the U.S. Environmental Protection Agency (EPA). However, data collected by the EPA from 1980-1998 shows that arsenic concentrations in public water systems can exceed 200 ppb in some states, including California and Alaska.

In Bangladesh, where the rice study was conducted, drinking water arsenic concentrations range from 1 to 450 ppb.

Other sources of arsenic include treated lumber, fertilizers and pesticides, and some medications. Until 2004, arsenic was widely used to preserve wood against pests. Playsets, decks and other home and community projects were built from the lumber, which exposed children and contaminated the underlying soils. Like lead, arsenic creates a legacy of contamination, even if additional arsenic applications are stopped.

Arsenic exposure can cause stomach pain, nausea, nerve damage and blindness. It is linked to several forms of cancer. Arsenic is also associated with poor cognitive development in children, and low birth weights related to poor fetal muscle development.

The World Health Organization recommends that daily arsenic intake be limited to 2.1 μg/kg body weight, equivalent to approximately 1 μg/pound. So a person who weighs 160 pounds should not exceed 160 μg of arsenic per day, or 13 grams, which is about one serving of the most contaminated rice from this study.

In the two-year study, researchers grew two varieties of rice in PVC tubes under five separate treatments: irrigation water spiked with arsenic at 1 or 2 parts per million (ppm), top soil with arsenic added at 10 or 20 ppm or control plots with no arsenic. For comparison, the irrigation water in this study had arsenic concentrations that were 2 to 10 times higher than some drinking water sources in the United States and Bangladesh. 

The two-year, two-crop farming cycle used in the study mimicked Bangledesh's rice farming rotations during the annual rainy and dry seasons, when crops are irrigated. After harvesting, the rice was dried and analyzed for nutrients. Soil samples were collected after the second year and analyzed for arsenic in 5-centimeter increments to a depth of 40 centimeters.

Arsenic – whether mixed with the soil or applied through irrigation water – concentrated in the top soil layers above 20 centimeters. Amounts measured were seven times higher than control levels. Between 51 and 57 percent of the arsenic was retained in this level, regardless of treatment type.

This is the depth that is plowed and where the rice roots grow. Indeed, arsenic levels measured in the roots varied with depth and treatment: more was measured at higher treatments and in the roots at the lower levels of 5-10 centimeters than the 0- to 5-centimeter depth. Less than 16 percent of the applied arsenic was detected in the 20- to 40-centimeter layers.

The applied arsenic also made its way into the rice grains, husks and straw. Arsenic was measured at concentrations between 0.2–12.5 micrograms per gram (μg/g).

Yield was reduced by up to 80 percent and nutritional value was lowered. Yield drops depended on the type of rice, the season in which it was grown and the arsenic treatment.