Fish and other aquatic organisms that live in such water incorporate carbon from that water into their tissues.

Their tissues end up with less carbon-14 than they would if they were using only atmospheric carbon.

When you then radiocarbon date those fish, or the bones of people who ate large quantities of those fish, you get a date that appears older than it actually is.

The radiocarbon-dead carbon from the ancient geological sources has effectively made the sample look older than its true age.

This is the freshwater reservoir effect, and it has been documented in rivers and lakes around the world.