Multiple Discharges of Treated Municipal Wastewater Have a Small Effect on the Quantities of Numerous Antibiotic Resistance Determinants in the Upper Mississippi River

This study evaluated multiple discharges of treated wastewater on the quantities of antibiotic resistance genes (ARGs) in the Upper Mississippi River. Surface water and treated wastewater samples were collected along the Mississippi River during three different periods of 4 days during the summer of 2012, and quantitative real-time PCR (qPCR) was used to enumerate several ARGs and related targets. Even though the wastewater effluents contained 75- to 831-fold higher levels of ARGs than the river water, the quantities of ARGs in the Mississippi River did not increase with downstream distance. Plasmids from the incompatibility group A/C were detected at low levels in the wastewater effluents but not in the river water; synthetic DNA containing an ampicillin resistance gene (<i>bla</i>) from cloning vectors was not detected in either the wastewater effluent or river samples. A simple 1D model suggested that the primary reason for the small impact of the wastewater discharges on ARG levels was the large flow rate of the Mississippi River compared to that of the wastewater discharges. Furthermore, this model generally overpredicted the ARG levels in the Mississippi River, suggesting that substantial loss mechanisms (e.g., decay or deposition) were occurring in the river.