Story posted April 05, 2005
When low-level veterinary antibiotics began showing up in groundwater and streams in the late '90s, federal agencies scrambled to determine their source and whether they posed a health concern to humans.
They identified livestock farms as an important source of these compounds, as antibiotics routinely are added to feed and water to prevent disease and enhance growth. Between 60 and 90 percent of the veterinary antibiotic dose is excreted and, presumably, released into soils, where it then can leach into surface waters and groundwater.
Although the concentration of antibiotics measured was well below levels that are toxic to test organisms or cause major environmental damage, the findings contribute to concerns that widespread exposure to antibiotics could eventually cause humans to develop antibiotic resistance.
Dharni Vasudevan, Bowdoin associate professor of chemistry/environmental studies, and her collaborator, University of Connecticut Assistant Professor Allison MacKay, are among the scientists studying the issue. Their work focuses on the early stages of contamination, when manure first hits the soil. Employing several kinds of chemistry - organic, inorganic, analytical, and environmental - Vasudevan is studying the interactions that occur between the soil surface and the released antibiotic to try to determine why these compounds reach surface waters.
"The idea is to see if we can get an understanding of what soil properties and what chemical characteristics cause these antibiotics to hang onto the soil. If we know that, we can gain insight into antibiotic leaching and transport through the soil. Also, we can anticipate where they will go in the environment, depending on the soil types they encounter."
Ultimately, Vasudevan hopes to develop a mathematical model that can predict how antibiotics affect and move through different soils, and the role of soil properties in aiding or reducing their entry into waterways.
For the first part of her research, Vasudevan and her research team collected 30 soil samples from the Eastern U.S. She selected sampling locations to ensure that soil composition included a wide range of organic matter, sand, and clay minerals, and varying properties of acidity and exchange capacity.
When feasible, soils were taken from regions where there was a high level of animal husbandry. To get a "clean" sample, the soils were taken from non-farm areas that were not contaminated with antibiotics.
After thoroughly characterizing each of the soils, Vasudevan measured the extent of their retention of oxytetracycline and ciprofloxacin - two of the most commonly used antibiotics. She found that soil texture, exchange capacity (ability to hold charge), and iron oxide content were key factors in determining the amount of retention of these antibiotics.
It's an important area of analysis because retention of older generation pollutants, such as DDT and PCBs, are proven to be predictable by the amount of organic matter in the soil.
"In a sense," says Vasudevan, "these antibiotic compounds don't fit our traditional model for determining retention in soils. The molecular structures of the antibiotics are different and their retention also is defined by other soil features, namely the metal oxide and clay silicate content."
Although Vasudevan's research has not yielded generalizations based on the orders, or types, of soils, her early findings are a starting point for understanding how antibiotics bind with the metal oxides and other clay minerals contained in those soils.
For the second phase of her research, Vasudevan and Bowdoin Research Associate Para Trivedi have begun adding antibiotics to purified samples of metal oxides and clay minerals to study the mechanism of retention using infrared spectroscopy.
"How does the molecule hang on? Which groups on molecules interact with the surface?" posits Vasudevan. "It's a complicated system. We're just adding pieces to the big puzzle."
Vasudevan began this research during her tenure at Duke University, before joining Bowdoin in 2004. In addition to Bowdoin support, her work is funded by the National Science Foundation and U.S. Department of Agriculture.
She says she is delighted with the College's commitment to her research and for the state-of-the-art instrumentation in the Chemistry Department.
"We are amazingly well set up, which makes the research easier," she says. "Also, the student researchers who have been working with me are truly inspiring. I couldn't get anything done without them. In lab-based research, it's a true team."
Vasudevan recently published a paper based on her early findings in the journal Environmental Toxicology and Chemistry, April 2005.