The ecosytem of a banana plantation is extremely wet and hot. The soil is very loose, helping the banana plants grow but also making it easy for pesticides to spread throughout the system. It often rains in these areas, flushing pesticides into the ground and water table. The banana industry's answer to this dissipation has been to apply pesticides frequently. Airplanes drop toxic chemicals regularly from the air.
Pesticides fall on the plants, but also on workers, the ground and irrigation canals and streams.
Workers apply pesticides to the ground around the plants. These chemicals seep into the ground with every rainfall. Pesticides also get into water that is used to wash bananas in the packing plants. That water then flows back into the irrigation canals. Plastic bags with insecticide cover all the banana bunches from their inception.