The hydrochemistry of landfill leachate and groundwater is affected by not only waste degradation processes, but also by external factors such as the geography of the landfilling site. Knowledge on the fate of landfill leachate in tropical countries will be beneficial for monitoring and regulatory purposes. We studied the Keputih landfill close to the sea at Surabaya, Indonesia: (1) to assess leachate and groundwater hydrochemistry with respect to contamination and seawater intrusion, (2) to investigate the seasonal effects on hydrochemical composition; and (3) to determine redox conditions in order to evaluate the potential for natural attenuation through microbe-mediated electron-accepting processes. We document an influence from sea water intrusion on groundwater hydrochemistry on top of the influences from the landfill itself. Leachate had a high electrical conductivity and high COD, and contained high concentrations of NH4+, HCO3-, SO42-, Fe2+ and Cl-. Concentrations were significantly influenced by season, except for COD and SO42-. The groundwater at locations surrounding the landfill was also contaminated by leachate and concentrations of groundwater contaminants were higher than national regulatory standards in Indonesia for drinking water. The abundance of SO42- in groundwater indicates a large potential for anaerobic biodegradation of organic compounds. Based on the relative concentrations of Cl- and SO42- an influence of the sea water on groundwater hydrochemistry was obvious. Landfilling in developing countries often occurs in coastal areas, therefore we emphasize the need to study microbial community structure and functioning in relation to degradation of landfill leachate in tropical coastal areas impacted by seawater infiltration.