Investigating the Effects of Variable Water Chemistry on Bacterial Transport during Stormwater Infiltration

Abstract

Pathogenic microorganisms and heavy metals have frequently been detected in urban stormwater runoff. Pathogens transport to the groundwater table with the infiltrating water and cause groundwater contamination. A variety of physical, chemical and biological factors have been studied for their effects on bacterial transport. However, the effect of heavy metals has largely been ignored, despite the elevated concentrations common in stormwater runoff. This work examines changes in bacterial and soil surfaces using scanning electron microscopy and energy dispersive X-ray spectroscopy after exposure to synthetic stormwater amended with heavy metals. Sets of batch bacterial sorption experiments were conducted under different conditions by varying heavy metal concentrations in synthetic stormwater and soil exposure history. The results indicate that the presence of heavy metals increases bacterial attachment to soil surfaces.Modeling bacterial transport during stormwater infiltration is challenging due to the variability and complexity of the physical, chemical and biological interactions in the soil-water-bacteria system. This work quantified changes in bacterial attachment under variable solution chemistry using a newly combined rate equation, which varies temporally and spatially with changes in solution chemistry. The relative importance of physiochemical variation on the estimation of bacterial attachment was quantitatively described using two-phase Monte Carlo analysis. A semi-reactive microbial transport model was further developed in HP1 (HYDRUS1D-PHREEQC) with the incorporation of the newly combined rate equation. The model matched observed bacterial breakthrough curves in laboratory column experiments well. This method represents one step towards a more realistic model of bacterial transport in complex microbial-water-soil systems.The developed model was further applied to the investigation of bacterial removal in field bioretention systems. The influent and effluent water samples from bioretention systems in New York City were sampled and analyzed over the summer of 2012 for fecal indicator Escherichia coli. Reduction of the effluent bacterial concentrations was observed and the removal efficiency was up to 66%. The antecedent dry period was found to affect bacterial removal. Shorter antecedent dry period results in higher soil moisture which is favorable for bacteria in soil to persist. The semi-reactive microbial transport model was applied and the modeled bacterial removal efficiency agrees well with observed values with a slight overestimation. This is primarily due to the presence of preferential flow paths in the field bioretention systems, which are not considered in the model.%%%%Ph.D., Environmental Engineering – Drexel University, 2013