Contamination of surface water associated with birds nesting on highway bridges
Highway bridge structures provide habitat to many different species of birds. To investigate the significance of the bacterial loadings from direct dropping of bridge nesting migratory birds, fecal coliform, E. coli, and Enterococci concentrations were measured in the Gaudalupe River at Kerrville, TX. Samples were collected at three locations, such as, upstream, downstream and below the bridge for one and a half year which included two nesting seasons. The dry and wet weather samples were analyzed separately to obtain the combined effect of runoff and bird defecation on the pathogenic concentration of the river water. To obtain the diurnal variation of bacterial concentration, samples were collected every 2--4 hours in a day. The bacterial loadings under the bridge increased as soon as the birds started nesting on the bridge. The samples from downstream and below the bridge showed remarkably higher bacterial concentrations than that of the upstream in dry weather. In wet weather, E. coli, fecal Coliform and Enterococci concentrations went up to 1200, 1300, and 940 CFU/100 ml that are much higher than the bacterial water quality standards of 394, 400, and 89 CFU/100 ml respectively for contact recreational water designated by the State of Texas.
Statistical analysis showed the concentrations of E. coli and fecal coliform remained significantly higher below the bridge than the concentrations upstream of the bridge. Also, the wet weather and dry weather samples for all three bacteria concentrations were significantly different. The probability distribution plots constructed for below the bridge location showed that the concentrations of E. coli, fecal coliform and Enterococci are likely to remain within the TCEQ specified contact recreation limit in 72, 65 and 35 percent of the days respectively.