Water quality modeling of bacterial contamination in surface water from bridge nesting birds

To understand the effect of direct droppings from the bridge nesting birds on bacterial contamination of surface water, a water quality model using QULA2Kw is developed for the bacterial impaired section of Guadalupe River near Kerrville, Texas. A huge number of migratory birds reside under the bridge at STH 16 over the River and the direct dropping of those birds to the river water is considered as the primary source of bacteria for the impaired section.

Concentration of three indicator bacteria, such as fecal coliforms, E. coli, and Enterococci are monitored at upstream, below the bridge, and downstream of the River for one and a half year. The concentration of all indicator bacteria under the bridge was significantly higher than the background concentration (measured at upstream) and slightly higher than that measured at downstream. The concentration decreases along the downstream of the River due to dilution, decay, and settlement.

The water quality model is developed to estimate the reach of the impaired section due to the direct dropping of the birds from that particular bridge. The geometry of the River was measured at different sections and the flow of the River and water temperature was collected from the nearby USGS monitoring station. The model was calibrated with the measured bacterial concentration at different locations along the bridge. A decay constant estimated from the model was found to be 6.5/day for E. coli, and 5.5/day for fecal coliforms and 3.5/day for Enterococci.

A parametric study is also conducted to investigate the effect of the input parameters, such as decay rate, flow at the River, water temperature, and settling velocity. The reach of the impaired section depends significantly on decay rate of the bacteria and the flow of the River and slightly on water temperature and settling velocity. Since the decay constant does not vary significantly in that ambient environment at this location, the length of impaired section primarily depends on flow.