Research conducted by associate professor Allison MacKay (Civil & Environmental Engineering) is aimed at helping scientists better understand how antibiotics and other organic compounds enter the nation’s waterways, disperse and change over time.
In 2004, the U.S. Geological Survey (USGS) published disturbing findings from a study of water and fish in tributaries of the Potomac River, among them a “high incidence” of male smallmouth bass endowed with oocytes, or eggs, in their testes. Analyses of water samples taken in the study revealed measurable levels of antibiotics, animal feed additives, arsenic, pesticides and other so-called “endocrine disruptors” – pharmaceutical or natural compounds that alter the ordinary functioning of hormones in living things.
The results added yet another dimension to growing worldwide concern about the prevalence of organic wastewater contaminants (OWCs) in water resources. The USGS conducted the first national study of organic contaminants between 1991-2000. Using new analytical methods, the team measured concentrations of 95 OWCs in water samples from a network of 139 streams deemed susceptible to contamination, in 30 states. The reconnaissance team found OWCs in 80% of the streams sampled.
Similar results have been found in Europe and Canada. The scope of the problem is particularly disturbing, since the sources of such contaminants are so widespread: industrial effluent pipes, municipal sewage treatment plants, animal feedlots, residential and senior living developments among them. Some of the more nettlesome contaminants are antibiotics.
With the emergence of antibiotic-resistant strains of bacteria, the prevalence of antibiotics in aquatic systems – including municipal reservoirs – is cause for concern. A major source of antibiotics in the environment is animal feedlots. In 2004, the U.S. agricultural industry used 21.7 million pounds of antibiotics, which are commonly added to animal feed to prevent disease and promote growth. Scientists believe that a very large percentage – more than 60 percent – of ingested antibiotics are excreted by livestock and eventually enter the nation’s waterways. The American Medical Association and American Academy of Pediatrics are among the more than 300 health, consumer, environmental, sustainable agriculture, and other organizations that have called for an end to the routine use of medically important antibiotics as feed additives.
Dr. MacKay, along with Dr. Dharni Vasudevan, associate professor of Chemistry at Bowdoin College, is interested in better understanding what happens to these agricultural antibiotics after they leave the animal. Their research, supported by the U.S. Department of Agriculture and the National Science Foundation, seeks to unveil the so-called “fate” of such contaminants in soil and water: how – and how far – they travel; how they are changed over time; how they are degraded, etc. Dr. MacKay explained that sunlight, temperature, flow rate, bacteria and other microorganisms, soil types and mineral composition – all may affect how these antibiotics are degraded.
“Antibiotics are designed to be biologically active even at low levels, so their impacts and environmental interactions can be much subtler and complex than many contaminants,” said Dr. MacKay.
According to Dr. MacKay, the most widely used agricultural antibiotic is tetracycline, which is added to the feeds of cattle, swine and even farmed fish. After leaving the animal, ingested antibiotics typically begin their journey in surrounding soil before being washed into bodies of water or seeping into groundwater. Dr. MacKay commented that antibiotics tend to remain active longer in soil than in water, for a variety of reasons. “In water, for example, if these compounds remain close to the surface, they may be broken down by sunlight. Antibiotics may also be degraded by bacteria more quickly in water than in soil.” It is here, in the soil, that Drs. MacKay and Vasudevan have focused their research in an effort to determine how different soil compositions may affect the movement and active life of tetracycline.
To date, most of Dr. MacKay’s research has been conducted in the lab, where different soil types have been tested and characterized chemically. The team’s studies have focused on soils containing high levels of iron oxide or clay. Dr. MacKay has found that soil containing greater amounts of clay – in contrast with porous, sandy soils – tends to bind the antibiotics, thus hampering their movement through the soil and into groundwater, rivers and streams.
Once Dr. MacKay and her team finish characterizing the soils and complete tests regarding the movement of tetracycline through these soils, they will develop a mathematical model that replicates the movement and fate of the antibiotics as they move through soils of different composition. They ultimately hope to expand the scope of the model to accurately reflect the movement of not only tetracycline but a wider array of antibiotics. In seeking to develop a macro- or generic model, they are looking at the interactions of tetracycline and soil at a molecular level.