Why did contamination continue and spread after the initial contamination of the Hudson River was stopped? Will give brainliest if you give a good answer!

The Hudson River was contaminated when General Electric dumped a huge amount of polychlorinated biphenyls (PCBs) into the Hudson River between 1947 and 1977.

General Electric proposed a plan of sediment dredging to clean up the polychlorinated biphenyls (PCBs) in the river but the PCBs are not safely buried in the sediment and dredging was not enough to clean up the PCB. This PCB has already entered the food web and the underground water of the Hudson River was also already spread in different places, so dredging was not enough to clean up the PCB.