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Abstract
Studies were carried out in two small urban watersheds with separated sewer system located in Lincoln, Nebraska to quantify the relative contribution of nitrate, microbes, and antibiotic resistance genes (ARGs) from potential sources. The role of atmospheric nitrate in two watersheds was investigated by comparing concentrations of stormwater nitrate with regional wet deposition and solving an isotope mixing model based on the isotopic ratios of nitrogen and oxygen in nitrate. Atmospheric deposition was found to be a major source of the nitrate in stormwater during smaller storm events (<32 mm) when most of the runoff is likely to be generated from impervious cover. The relative contribution of microbes and ARGs from potential environmental sources were quantified using Bayesian inference based on abundance data. The abundance data for microbial composition were obtained by analyzing the 16S rRNA gene (V4 region) sequences for samples of Antelope Creek water and potential sources. The composition of ARGs and microbes for samples of potential environmental sources and stream water were also obtained from high throughput metagenomic sequencing data. Results showed that an upstream lake was the biggest source of microbes in dry weather flows. Runoff discharged from storm drain outfalls was the biggest source of ARGs and microbes in wet weather flows. Street sweepings were estimated to account for the largest portion of both microbes and ARGs in storm drain outfall water, making it a major known source of contamination to the creek during wet weather events.
The watersheds where the studies were carried out represent urban Midwestern watersheds with separated sewer systems where contamination from agricultural runoff is not present. The findings revealed that the stormwater runoff is responsible for washing off nitrate, microbial loads, and ARGs from impervious surfaces into urban streams. Identifying the sources of contamination is important for developing best management practices to protect the water quality of urban streams for recreational uses. Results from these studies underscore the importance of reducing direct connections between impervious surfaces such as roads or parking lots and urban streams in order to reduce contamination of receiving water bodies in urban watersheds.
