The recreational waters near many large cities in the United States and Canada are severely impaired by pathogens that are present in the storm water runoff. In separated sewers the pathogen sources may be cross-flows between the sanitary and storm water systems. This paper presents the methodology that was used in developing a forecasting model for pathogen indicators for recreational sites in the receiving waters of multiple storm water outfalls. The objective of the model is to give a timelier indicator of beach water quality than conventional beach monitoring, which takes about 2 d for laboratory results. The model used for the study was based on the Princeton Ocean Model. The forecasting system consists of nested hydrodynamic models and a bacteria fate-transport submodel. Calibration and validation is based on 6 years of field studies, laboratory analyses, and experiments. The methodology is illustrated by a case study of the impact of storm water flows on the south shore of Lake Pontchartrain, Louisiana, which has been banned for swimming since 1985. The water quality data included: pathogen indicators (fecal coliform, Enterococci, and E. Coli), water chemistry parameters, turbidity, and nutrients.Résumé : L'eau des cours d'eau destinés aux loisirs près des grandes villes des États-Unis et du Canada est gravement détériorée par les pathogènes se trouvant dans le ruissellement des eaux pluviales. Dans des conduites d'égout séparées, les sources de pathogènes peuvent provenir d'une contamination croisée entre les systèmes d'égouts pluviaux et d'égouts sanitaires. Cet article présente la méthode utilisée pour développer un modèle prédictif des indicateurs pathogènes pour les sites récréatifs dans les eaux recevant plusieurs exutoires d'égouts pluviaux. L'objectif de ce modèle est de fournir un indicateur plus rapide de la qualité des eaux à la plage que celui offert par la surveillance conventionnelle des plages, laquelle nécessite environ deux jours pour avoir les résultats du laboratoire. Le modèle utilisé pour cette étude est basé sur le modèle océanique de Princeton. Le système de prédiction consiste en des modèles hydrodynamiques emboîtés et d'un sous-modèle de transport-destin des bactéries. L'étalonnage et la validation sont basés sur des études de terrain d'une durée de six ans, sur des analyses en laboratoire et des expériences. La méthode est illustrée par une étude de cas de l'impact du ruissellement des eaux pluviales sur la rive sud du lac Pontchartrain, en Louisiane, dans lequel la baignade est interdite depuis 1985. Les données sur la qualité de l'eau comprennent : les indicateurs pathogènes (coliformes fécaux, Enterococci et E. coli), les paramètres de chimie de l'eau, la turbidité et les nutriments.
Modern tools for management of recreational waters include field monitoring, laboratory analyses and computer modeling. A case study of a brackish receiving water body subjected to periodic discharges of contaminated stormwater runoff is presented to illustrate the field and laboratory support that is needed to develop a numerical model. Field sampling protocols and laboratory procedures are outlined and the results are presented in the form required for a numerical model. Results conclusively showed that pathogen counts are strongly correlated to storm-generated flows. It was also found that fine sediment is a significant transporting medium for pathogen indicators. A discussion of the implications of these studies on numerical modeling is presented. A simple dilution model is introduced as a screening level model for shoreline contamination due to attached plumes from relatively wide storm drainage canals.
An advisory discouraging swimming and other primary contact recreation in Lake Pontchartrain was issued in 1985 by the Louisiana Department of Health and Hospitals (LDHH). The advisory is still in effect today for the south shore area of the lake and names fecal coliform bacteria as the causative pollutant. The suspected source of the contamination in this area is urban stormwater runoff that is collected and pumped to the lake and may be contaminated by sanitary sewer cross-flows. A water quality shoreline study was initiated in the south shore area of the lake in New Orleans by the Department of Civil & Environmental Engineering at the University of New Orleans (UNO). The objective was to determine if the reduced bacteria levels are a result of decreased pollution or if this is a temporary phenomenon caused by a short-term climatic effect. Five monitoring stations were selected for study on the basis of proximity to drainage canals that discharge the stormwater runoff and current or previous use for primary contact recreation. Fecal coliform concentrations was found to be "wet" weather-dependent at all stations except one. There appears to be an active continuous bacteria source near this site since fecal coliform levels there cannot be directly linked to urban runoff. For the remaining areas a general rule of thumb for recreational use of these south shore water is that the user should assume that the water is unsuitable for primary contact recreation, especially in the near vicinity of urban drainage canals, for at least two to three days following a storm event. Precipitation analysis showed a reduction in mean total annual rainfall during the study period amounting to nearly one-third of the typical mean total annual rainfall for the area. Therefore, lower fecal coliform concentrations observed may be due to uncharacteristic drought conditions rather than decreased pollution.
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