Carbon dating is not flawed
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.
I am not aware of any authentic research which supports this claim.
Also, it does not coincide with what creationist scientists would currently anticipate based upon our understanding of the impact of the Flood on radiocarbon.
It is not difficult to see how such a claim could arise, however.
Radiocarbon is not used to date the age of rocks or to determine the age of the earth.
The shells of live freshwater clams can, and often do, give anomalous radiocarbon results.
However, the reason for this is understood and the problem is restricted to only a few special cases, of which freshwater clams are the best-known example.
Long tree-ring chronologies are rare (there are only two that I am aware of which are of sufficient length to be of interest to radiocarbon) and difficult to construct.
They have been slowly built up by matching ring patterns between trees of different ages, both living and dead, from a given locality.