Instream Bacteria Influences from Bird Habitation of Bridges

Abstract

The representativeness of ambient water samples collected from bridge crossings has occasionally been challenged because critics contend birds nesting on bridges elevate fecal indicator bacteria concentrations over samples collected from river reaches not spanned by bridges. This study was designed to evaluate the influence, if any, of bridge‐dwelling bird colonies on instream bacteria concentrations. Three bridges in central Texas were sampled under dry‐weather conditions for instream Escherichia coli. Two bridges were inhabited by migratory cliff swallows and one was devoid of birds. Numerous samples were collected from locations upstream, at the upstream bridgeface, and downstream of each bridge to determine whether significant increases in E. coli occurred in a downstream direction when birds were present. E. coli values increased significantly at bridgeface and downstream locations compared to upstream locations throughout the nesting season. During peak bird activity in May, bacteria geometric mean concentrations at bridgeface and downstream locations jumped from background levels <50 to >190 colony forming units (CFU)/100 mL, well above the state geometric mean criterion of 126 CFU/100 mL for primary contact recreation use. Results confirmed that under dry‐weather conditions bird colonies can have a significant impact on bacteria concentrations in the vicinity of the bridges they inhabit and therefore, to avoid this impact, monitoring should occur upstream of bridges.

Uncertainty in monitoring E. coli concentrations in streams and stormwater runoff

“…Another potentially large source of sample collection uncertainty is sampling location relative to bridges with nesting birds. Sampling immediately below bridges with high bird populations can contribute > 1000% systematic uncertainty according to Pendergrass et al (2015) and Wolfe (unpublished); however, this uncertainty source was not included in the present analysis because it would not occur in most sampling locations.…”

“…Several issues related to E. coli sampling are not well understood and warrant further study to support improved data collection methodology; these include:  Sampling immediately downstream of bridges with nesting birds. Additional study is needed to improve understanding of the magnitude, frequency of occurrence, and spatial differences and supplement information provided by Pendergrass et al (2015), Karthikeyan (unpublished), and Wolfe (unpublished); however, the potential for significant increases in E. coli concentrations immediately downstream of bridges with nesting birds should be carefully weighed when determining sampling locations.…”