Dilution of wastewater discharges from moving cruise ships

The U.S. Environmental Protection Agency (EPA) conducted a survey using EPA´s Ocean Survey Vessel Peter W. Anderson to track and quantify the dilution and dispersal of wastewater discharges behind four large cruise ships. Wastewater holding tanks were spiked with rhodamine dye before discharge. During the discharge of the wastewater, drogued buoys were released at two minutes intervals to permit visual tracking of the plume. A tow fish with a fluorometer was towed through the plume to measure the dimensions of the plume and dye concentration. Dilution of the discharge was calculated based on the concentration of dye in the plume and the initial concentration of the dye in the wastewater holding tanks. Results indicate that discharges behind cruise ships moving at between 9 and 17 knots are rapidly diluted by a factor of 260,000:1 to 580,000:1. These results are larger than dilution factors estimated by previous modeling efforts. This suggests that previous studies underestimate the impact of turbulence caused by the propellers and displacement of the ship´s hull.