An Evaluation of Indicator Bacteria Transport in Stormwater Runoff and Removal in Stormwater Control Measures.

摘要

Microbial quality in surface waters is a concern across the United States, Europe, Australia, and elsewhere due to human reliance on surface waters for food, recreation, and other life sustaining activities. Although pathogens are of utmost concern, indicator bacteria are typically used for regulatory purposes to indicate the presence of fecal matter, and thus the possible existence of pathogens. Total Maximum Daily Loads are established for surface waters impacted by excessive indicator bacteria. Analyses are required to categorize sources of indicator bacteria, and a plan is developed to restore water quality in the impacted water by way of various management/control practices. Stormwater runoff has been shown to have high indicator bacteria concentrations, contributing to microbial degradation in surface waters.;Although numerous studies have been performed to establish patterns of indicator bacteria transport and export in estuarine and riverine systems, relatively little research has been performed for urban stormwater (prior to runoff entering surface water). Chapter 2 provides an analysis of variables which may influence indicator bacteria export from an urban watershed. Event Mean Concentrations (EMCs) of E. coli and fecal coliform exhibited significant seasonal variation (p 0.05). Based on multiple linear regression analyses, EMCs were also influenced by antecedent meteorological conditions, with temperature and moisture being important in explaining variability among sampling events. Further analysis in Chapter 3 provided a traditional first flush assessment of data collected from the urban watershed. Although total suspended solids (TSS) exhibited a first flush in the watershed, no first flush effect was noted for E. coli and enterococci, and the first flush effect for fecal coliform was relatively weak. Seasonal variations in first flush strength were observed, likely due to differences in pollutant sources between seasons. These studies emphasized the importance of seasonality and antecedent conditions in indicator bacteria transport and export from urban watersheds. Further, the lack of a substantial first flush effect suggests stormwater control measures ("SCMs," also known as Best Management practices or "BMPs") cannot sequester proportionally more indicator bacteria as a result of greater mass delivery during the beginning of storm events.;Stormwater runoff is typically managed by implementation of SCMs. Although SCMs have been shown to sequester numerous pollutants, relatively little is known regarding their ability to sequester indicator bacteria. The effectiveness of SCMs in Charlotte, NC, and Wilmington, NC, was examined in Chapters 4 and 5, respectively. Differences in performance were noted between the two locations, potentially due to differences in particle association of indicator bacteria between the relatively clayey soils in Charlotte, NC, and the sandy soils in Wilmington, NC. High water tables in Wilmington, NC, likely also influenced results, particularly for wet ponds. Although some SCMs showed statistically significant reductions of indicator bacteria (p 0.05), some SCMs appeared to export indicator bacteria. Further, effluent indicator bacteria concentrations were observed to vary seasonally. Well performing filtration/infiltration-based SCMs were examined in both locations; however, a paired watershed study in Wilmington, NC, showed differing performance between two bioretention cells. These differences were explored in Chapter 6, and soil media depth was identified as the most likely difference between the two cells leading to differences in indicator bacteria sequestration. These data suggest SCMs do possess treatment mechanisms which are effective at sequestering indicator bacteria; however, an environment may be present in some SCMs which allows indicator bacteria to persist and/or regrow. Infiltration-based SCMs offer some advantage, as mass removal of indicator bacteria can be realized through infiltration of runoff into subsoils. However, the impact of these practices on groundwater microbial quality should be investigated. For bioretention cells, a minimum soil media depth appears to exist, below which poor sequestration of indicator bacteria may occur due to high soil water flux and low contact time. Finally, seasonal variations in effluent indicator bacteria concentrations from SCMs should be considered in TMDLs.