The U.S. Food and Drug Administration (FDA) provides guidance to state shellfish control authorities on establishing prohibitive closure zones in proximity to wastewater treatment plant (WWTP) discharges with the purpose of minimizing the exposure of molluscan shellfish to health hazards posed by bacterial and viral pathogens present in wastewater effluents. For more than 25 years, the FDA has recognized conditional area management as an option to minimize the size of a prohibitive closure zone, and to enlarge the size and productivity of shellfish growing areas. To use this option, the FDA has recommended achieving a 1,000:1 dilution of effluent within the perimeter of the prohibited closure zone. Using newly available analytical methods and hydrographic equipment, the FDA is undertaking studies to determine whether its 1,000:1 dilution recommendation is supported by the findings. From 2007 through 2009, the FDA conducted field investigations to assess the impacts of wastewater effluent from a large municipal WWTP that discharges into Alabama's Mobile Bay. The dilution of the effluent in the bay was ascertained by conducting a hydrographic dye study using rhodamine WT tracer dye. Submersible fluorometers fastened to oyster cages at sentinel stations were used to determine continuously the dilution of the dye-tagged effluent throughout a 4-day study period. In addition, dilution and dispersion of the dye-tagged effluent was tracked throughout Mobile Bay by fluorometric measurements made while conducting boat transects. The microbiological impacts of the wastewater on molluscan shellfish were assessed by testing oysters placed in cages at sentinel stations at various distances along the anticipated path of the effluent. Levels of fecal coliforms, Escherichia coli, male-specific coliphage, and norovirus genogroups I and II were determined. Norovirus genogroup II was detected in oysters that were located as far as 5.74 km from the discharge, an area in close proximity to the calculated 1,000:1 dilution line. Results also showed that the levels of indicator microorganisms and viral pathogens in the shellfish inversely correlated with increased dilutions of the wastewater effluent in Mobile Bay.