Antibiotics in the Cache la Poudre River
Livestock and human antibiotics documented in the Poudre.
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Human and veterinary pharmaceutical compounds in the environment have received increased attention in recent years. These medicines are used for therapeutic treatment of infectious diseases in humans and for treating and protecting the health of animals. In addition, veterinary antibiotics are used to promote growth and feed efficiency in a range of animals. For example, monensin is a common feed additive for beef cattle. The tetracycline class of compounds is the most widely used animal antibiotic in this country. Currently, two of the ten approved antibiotic growth promoters are tetracyclines, chlortetracycline and oxytetracycline. A fraction of the drugs is completely metabolized to inactive compounds, but a significant amount is excreted as active metabolites.
Researchers have shown that several classes of antibiotics (e.g., tetracyclines, sulfonamides, macrolides and ionophores) are present in hog waste lagoons at concentrations as high as 0.7 mg/L. A variety of residual antibiotics were found in sewage treatment plant (STP) effluents in Germany with concentrations as high as 5 μg/L. The U.S. Geological Survey measured concentrations of antibiotics in water samples from a network of 139 streams across 30 states during 1999 and 2000. This reconnaissance study indicated that these compounds were found in 80% of the streams sampled. Antibiotic concentrations as high as 1.9 μg/L were found and only 10 of 24 compounds measured were not detected in any of the streams. The frequency of detection of at least one antibiotic was 22%.
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| Figure 2. Occurrence of tetracyclines in the Poudre River |
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| Figure 3. Occurrence of sulfonamides in the Poudre River. |
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| Figure 4. Occurrence of ionophores in the Poudre River. |
The presence of antibiotics in the aquatic environment has created two concerns. The immediate concern is the potential toxicity of these compounds to aquatic organisms and humans through drinking water. In addition, there is growing concern that release of antibiotics to the environment contributes to the emergence of strains of disease-causing bacteria that are resistant to even high doses of these drugs. Indications of increased bacterial resistance in waste effluent from hospitals and pharmaceutical plants have been reported, raising potentially serious public health issues associated with the ultimate disposal of antibiotics.
The origin of antibiotic contamination in surface and ground waters is considered to be both point and non-point source discharges of municipal and agricultural wastewater. Since few studies have been conducted on the occurrence, fate and transport of antibiotics in the environment, there are several questions that need to be answered on a regional and even watershed level. The most important questions that need to be addressed for a particular watershed relate to the occurrence and source (urban or agriculture) of these compounds. After these issues have been addressed, and assuming that the goal is zero discharge of antibiotics to the environment, watershed stakeholders should identify approaches for minimizing release from both urban and agricultural sources. The objective of this article is to describe a study that is underway at CSU to answer these questions for the Cache la Poudre watershed in Northern Colorado.
A watershed-scale field study was conducted on the Cache la Poudre (Poudre) River (Figure 1). The Poudre River originates near the continental divide in Rocky Mountain National Park, flowing through steep mountainous terrain for approximately 43 miles before entering the Front Range city of Fort Collins. After traveling through Fort Collins, the river moves through approximately 45 miles of mostly agricultural landscape before joining the South Platte River in Greeley, CO. Due partly to the semi-arid nature of the Front Range of Colorado, there are no significant tributaries to the Poudre River and, therefore, the inputs to the river are predominantly point sources in the urban landscape of Fort Collins and non-point sources in the agricultural areas outside of the City. These factors coupled with the source being snowmelt with minimal anthropogenic influences make this an ideal watershed to study the occurrence and evolution of antibiotics through pristine, urban and agricultural landscapes.
Five tetracyclines including tetracycline (TC), oxytetracycline (OTC), chlortetracycline (CTC), doxycycline (DXC), and democlocycline (DMC), and six sulfonamides including sulfathiazole (STZ), sulfamerazine (SMR), sulfamethazine (SMT), sulfachloropyridazine (SCP), sulfamethoxazole (SMX), and sulfadimethoxane (SDM) were analyzed and quantified. In addition, the concentrations of three ionophore antibiotics (monensin, salinomycin, narasin) were determined at each of the five sites. The ionophore antibiotics are of interest since they are used exclusively in agricultural applications.
The results of the occurrence survey are shown in parts per billion (same as micrograms per liter) in Figures 2 through 4. At the five sites along the Poudre River that were monitored, the only site at which no antibiotics were detected was the pristine site in the mountains before the river had encountered urban or agricultural landscapes. By the time the river had exited Fort Collins (Site 3), 7 of the 13 compounds that were monitored were found in the samples. At Site 5 in Greeley, CO where the river converges with the South Platte River, all five of the tetracyclines monitored were present indicating both urban and agricultural influences. Although 3 out of 6 sulfonamides were detected in the river leaving Fort Collins, only 1 of 6 was found at Site 5. This result indicates that sulfonamides were not originating from agricultural sources and that significant natural attenuation mechanisms were active in the river between site 3 and 5. Of the ionophores, only narasin was found at sites 2-5. Since this antibiotic is only used therapeutically with poultry, it is not clear what the source is. Monensin, a growth promoter in beef cattle, and salinomycin, used for prevention of coccidiosis in broilers, were found in the agriculturally influenced sample sites (4 and 5).
A significant number and concentration of human and animal antibiotics have been measured in the Poudre River. The current phase of the study is identifying sources (wastewater treatment plants, animal waste lagoons, land application of wastewater, manure, and sludges) and future phases will identify strategies for minimizing the release of these compounds into the environment.
