Studies on the environmental distribution of radionuclides, namely 226 Ra and 228 Ra are important in the context of external and ingestion hazards evaluation. The Cochin backwater, located in the south-west coast of India is a potential sink for anthropogenic inputs of natural radionuclides. The estuary in its surroundings is located downstream of industrial complexes. The present study aims at measuring the activity concentrations of 226 Ra and 228 Ra in the Cochin backwater and systematically evaluates the zonal and seasonal variations in the distribution of these radionuclides and in estimating the resultant ingestion dose to critical population.