They can be, in NZ there are is a huge amount of land that has been converted to dairy through massive irrigation schemes which has caused massive problems for the rivers that flow naturally through these places… I imagine there are other places in the world used for dairy that wouldn’t be suitable if not for irrigation?
The half life of C14 is about 4500 years. Due to the age of coal, generally millions on years it tends to contain crazy small amounts of C14, just like petrol.