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U.S. Geological Survey (USGS) scientists studying a midwestern stream conclude that pharmaceuticals and other contaminants in treated wastewater effluent discharged to the stream are transported into adjacent shallow groundwater. Other mobile chemicals found in wastewater are expected to have similar fates.
The study was conducted at Fourmile Creek, a wastewater-dominated stream near Des Moines, Iowa, during two sampling periods, October and December 2012. Wastewater effluent contributed approximately 99 and 71 percent of the flow in Fourmile Creek during these sampling periods, respectively. Persistent dry conditions predominated in the watershed through the study period.
The scientists determined that conditions suitable for significant infiltration of effluent into shallow groundwater can be created during persistent dry conditions, and during times of the day when effluent discharges are greatest. A network of water-level measurement devices (piezometers) provided data showing stream water is driven into shallow groundwater as a result of higher stream levels caused by the addition of treated wastewater to the stream.
Both stream and shallow groundwater samples were collected during October and December 2012 and analyzed for 110 pharmaceuticals. Scientists chose to track the movement of pharmaceuticals between the stream and shallow groundwater because pharmaceuticals are bioactive, can be highly mobile, are good indicators of domestic wastewater, and wastewater is the only source of pharmaceuticals in the study reach.
Analysis of stream-water samples collected downstream of wastewater effluent discharge revealed that 43 percent and 55 percent of pharmaceuticals analyzed were detected in October and December, respectively. Relatively fewer pharmaceuticals were detected in shallow groundwater. However, 16 percent and 6 percent were detected during October and December, respectively, at a distance of 20 meters from the stream bank. The pharmaceuticals detected 20 meters from the stream bank included antivirals and antibiotics, muscle relaxants, and antidepressants and tranquilizers, as well as medications for treating cancer, diabetes, and hypertension; their concentrations as high as 87 nanograms per liter (ng/L).
This study has important implications for the application of bank filtration for indirect water reuse. Bank filtration is the engineered movement of water between surface water bodies and wells located a short distance away on the streambank. Bank filtration is routinely used to pretreat surface-water for drinking water supply (raw surface water moves from the stream to a shallow groundwater extraction well), or as a final polishing step for the release of treated wastewater (treated wastewater moves from infiltration wells or lagoons through the bank to the stream).
This study was supported by the USGS Toxic Substances Hydrology Program.
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