Climatic effect on water quality evaluation

An advisory discouraging swimming and other primary contact recreation in L ake Pontchartrain was issued in 1985 by the Louisiana Department of Health and Hospitals (LDHH). The advisory is still in effect today for the south s hore area of the lake and names fecal coliform bacteria as the causative po llutant. The suspected source of the contamination in this area is urban st ormwater runoff that is collected and pumped to the lake and may be contami nated by sanitary sewer crossflows. A water quality shoreline study was ini tiated 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 resu lt of decreased pollution or if this is a temporary phenomenon caused by a short-term climatic effect. Five monitoring stations were selected for stud y on the basis of proximity to drainage canals that discharge the stormwate r runoff and current or previous use for primary contact recreation. Fecal coliform concentrations was found to be " wet " weather-dependent at all st ations 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 recre ational use of these south shore water is that the user should assume that the water is unsuitable for primary contact recreation, especially in the n ear vicinity of urban drainage canals, for at least two to three days follo wing a storm event. Precipitation analysis showed a reduction in mean total annual rainfall during the study period amounting to nearly one-third of t he typical mean total annual rainfall for the area. Therefore, lower fecal coliform. concentrations observed may be due to uncharacteristic drought co nditions rather than decreased pollution.