Radiocarbon dating is wrong
Since no reliable historically dated artifacts exist which are older than 5,000 years, it has not been possible to determine the relationship of radiocarbon years to calendar years for objects which yield dates of tens of thousands of radiocarbon years.
Radiocarbon is not used to date the age of rocks or to determine the age of the earth.
If this water is in contact with significant quantities of limestone, it will contain many carbon atoms from dissolved limestone.
Since limestone contains very little, if any, radiocarbon, clam shells will contain less radiocarbon than would have been the case if they had gotten their carbon atoms from the air.
Some may have mistaken this to mean that the sample had been dated to 20,000 radiocarbon years.
The second characteristic of the measurement of radiocarbon is that it is easy to contaminate a sample which contains very little radiocarbon with enough radiocarbon from the research environment to give it an apparent radiocarbon age which is much less than its actual radiocarbon age.