How do you account for radioactive isotopes either entering or leaving rock when dating it?

David P asked:


There are over forty methods in use for dating rock samples. Each involves a radioactive isotope, (the parent), and a product, (the daughter) that could easily wash in or out throwing the ratio way off.
Water is constantly seeping through the rocks all the time. That is how water gets under the ground into wells. You cannot find rocks that this does not happen of has not happend to.