How did DDT get into lakes and oceans?

When DDT is applied to soil, water or other materials it does not decompose or biodegrade rapidly. As consequence it is slowly washed out with the water flows into groundwater or surface flows into streams. These flows eventually reach lakes and oceans with the DDT content still intact.

The biggest problem was the over application of DDT which exceeded the absorptive capacity of the environment leaving a surplus to become mobile in water flows.