Quantifying the Relative Contributions of Environmental Sources to the Microbial Community in an Urban Stream under Dry and Wet Weather Conditions

Abstract

Investigating sources of microbial contamination in urban streams, especially when there are no contributions from combined sewer overflows or sewage effluent discharges, can be challenging. The objectives of this study were to identify the sources of microbes in an urban stream and quantify their relative contributions to the microbial community in the stream under dry and wet weather conditions. A microbial source tracking method relying on the 16S rRNA gene was used to investigate the microbial communities in water samples of an urban stream (i.e., from 11 dry and 6 wet weather events), as well as in streambed sediment, soils, street sweepings, sanitary sewage, an upstream lake, and feces of animals and birds collected between 2013 and 2015. The results showed that the Escherichia coli levels in the stream were significantly higher in wet weather flow than in dry weather flow. The upstream lake contributed approximately 93% of the microbes in dry weather flows. Water discharged from storm drain outfalls was the biggest source of microbes in wet weather flows, with a median contribution of approximately 90% in the rising limb and peak flow and about 75% in the declining limb of storms. Furthermore, about 70 to 75% of the microbes in the storm drain outfall water came from materials washed off from the street surfaces in the watershed. Fecal samples did not appear to contribute substantially to the microbes in environmental samples. The results highlight the significance of street surfaces in contributing microbial loads to urban streams under wet weather conditions.IMPORTANCE Identifying the sources of microbial contamination is important for developing best management practices to protect the water quality of urban streams for recreational uses. This study collected a large number of water samples from an urban stream under both dry and wet weather conditions and provided quantitative information on the relative contributions of various environmental compartments to the overall microbial contamination in the stream under the two weather conditions. The watershed in this study represents urban watersheds where no dominant fecal sources are consistently present. The findings highlight the importance of reducing the direct contribution of microbes from street surfaces in the watershed to urban streams under wet weather conditions. The methods and findings from this study are expected to be useful to stormwater managers and regulatory agencies.

Keywords: dry weather condition; microbial source tracking; urban stream; wet weather condition.

FIG 1

Boxplot showing E. coli concentrations in Antelope Creek water under dry weather conditions and in the three stages under wet weather conditions. The bottom and top of each box represent the 25th and 75th percentiles, respectively. The bold line inside each box represents the median concentration, while the dot represents the average concentration. Whiskers on either side of each box represent data within 1.5 times of the interquartile range (25th percentile to 75th percentile).

FIG 2

Mantel test results showing significant correlation (P < 0.05) between weighted UniFrac distance for overall microbial community and Bray-Curtis dissimilarity coefficient for water quality parameters and environmental factors. The sizes of the boxes and the fill color both represent the r value. Boxes are shown only for significant correlations (P < 0.05).

FIG 3

Conceptual model for SourceTracker analysis showing major sources of microbes to Antelope Creek for dry weather conditions (A) and wet weather conditions (B).

FIG 4

Five runs of SourceTracker analysis showing the relative contributions of potential environmental sources to the overall microbial community in dry weather water.

FIG 5

Five runs of SourceTracker analysis showing the relative contributions of potential environmental sources to the overall microbial community in the three stages of wet weather flow.

FIG 6

Five runs of SourceTracker analysis showing the relative contributions of potential environmental sources to the overall microbial community in storm drain outfall water.

FIG 7

Map of the study area (in green) marked with in-stream sampling sites, storm drain outfalls, and a USGS gaging station. Direction of flow in Antelope Creek is from Holmes Lake toward AC1. OpenStreetMap was used to create this figure (https://www.openstreetmap.org/).