Run-off Mechanism of Domestic Wastewater into Lagoon-side Coast on Funafuti Atoll, Tuvalu

Abstract

 Our previous paper reports that the major pollution source in lagoon-side coast on Funafuti atoll is domestic wastewater originating from bottomless septic tanks. To elucidate the run-off mechanism of domestic wastewater, dynamic change in water level of a septic tank was examined. Periodic variation in water level was corresponded to tide changes with a 3-4 hours of time lags. In addition, the concentration of dissolved inorganic nitrogen and the number of Escherichia coli (E. coli) in the coast at low tide was significantly higher than that at high tide. These results indicates that seawater penetrated into septic tanks during flood tides and subsequently domestic wastewater in septic tanks rans-off into the coast during ebb tides. Moreover, the transition from neap tide to spring tide had up to 26,600MPN/100mL of E. coli during ebb tide, whereas that just after spring tide showed up to 1,080MPN/100mL. From the results, we conclude that the history of tide changes controls the run-off load of domestic wastewater derived from bottomless septic tanks into the lagoon-side coast.