COMMON ARSENICAL PESTICIDE
Written by: Science News
Golfers can thank monosodium methanearsonate (MSMA) for flawless, weed-free fairways, but experts are questioning whether the arsenic-containing pesticide is safe for the environment and human health. New research reveals that, despite industry claims, MSMA applied to golf courses with certain types of soil degrades to toxic inorganic arsenic, which leaches into groundwater. A separate study has documented that MSMA can move through wildlife food chains. The news comes as Canada and the United States are re-evaluating registrations for MSMA.
According to the U.S. EPA, MSMA "can reasonably be anticipated to cause cancer in humans" and is converted in the environment to inorganic arsenic, a known human carcinogen. About 4 million pounds of MSMA is applied every year to golf courses and cotton fields in the United States to control weeds. The pesticide has been banned in India and Indonesia.
Concerns about the fate and transport of MSMA led to a collaborative study between Yong Cai and his colleagues at Florida International University and George Snyder, John Cisar, and their colleagues at the University of Florida. They dosed an experimental golf green at the University of Florida with MSMA and monitored the soil and the water percolating through the soil. "Presumably due to microbial activity in the soil, MSMA was transformed to As(V), As(III), monomethylarsonic acid, and dimethylarsinic acid, with As(V) being the major form," Snyder says. After 14 weeks, almost 20% of the arsenic in the MSMA percolated below the root zone of sandy soils in the form of inorganic arsenic, Cai says. The more clay there is in the soil, the more arsenic is retained, he notes. The study has been submitted to a peer-reviewed journal.
Some industry scientists have criticized Cai's study, saying that the apparatus used to collect leachate promotes the growth of bacteria, which boost the transformation of MSMA beyond what would be expected in the soil. However, Cai and Snyder's findings are corroborated by a Florida Department of Environmental Protection (FDEP) study that found contamination after examining 65 groundwater samples from 7 different golf courses where MSMA was legally applied. Shallow monitoring wells revealed that 86% of the samples contained more than 10 micrograms of total arsenic per liter, which is the new groundwater standard in Florida. In areas with high water tables, just a single application of MSMA is likely to render the underlying groundwater out of compliance with the new standard, the study says.
FDEP has recommended restricting the use of MSMA in vulnerable areas with sandy soils and high water tables. Florida's Department of Agriculture and Consumer Services (FDACS) has asked companies that sell MSMA in the state to conduct further field tests, says Dennis Howard, chief of the bureau of pesticides at FDACS. Representatives from these companies have maintained that MSMA is quickly adsorbed into soil and not easily released, he says.
Meanwhile, in Canada, the permit to inject pine trees with MSMA to curb outbreaks of mountain pine beetles has expired, and the manufacturer, United Agri Products Canada, Inc., has withdrawn MSMA from the Canadian market. Agencies are also evaluating work from the Canadian Wildlife Service, which shows that woodpeckers feeding on bark beetles in treated trees had 4-7 times the level of arsenic in their blood than species that don't feed on the beetles.
Although organic forms of arsenic were once thought to be less toxic than inorganic arsenic, new research on the metabolism and toxicity of both organic and inorganic arsenic has changed that view, says Mirek Styblo, biochemical toxicologist at the University of North Carolina, Chapel Hill. He says that human cells metabolize As(V) to As(III) and trivalent methylated forms-monomethylarsonous acid and dimethylarsinous acid. He and others have shown that all the trivalent forms, including methylated ones, are genotoxic and more toxic than the corresponding pentavalent forms.
"But we're lacking data on the chronic toxicity of methylated arsenicals, including the trivalent and pentavalent forms," Styblo says. "If [Snyder and Cai] have really shown that MSMA is demethylated, that is very important because it means that arsenic could end up in drinking water," he warns.
MSMA does not cause cancer in animal studies and is safe if used properly, says Barbara Beck, director of health sciences at Gradient Corp., a consulting firm. If it is ingested, 95% is converted in the gut to monomethylarsonic acid, which is rapidly excreted, and the remainder is metabolized to dimethylarsinic acid, she adds.
EPA is currently reviewing MSMA for re-registration, says Enesta Jones, a press officer with the agency. The re-evaluation will include consideration of new research on the carcinogenic potential of the organic arsenic herbicides and their degradation in the environment, she says. The re-registration eligibility decision is due July 2006.
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