The meaning behind that simile is that ‘they’ are getting tons of gold like they were corn grains which if you’ve seen them, there’s a lot in just one corn. So basically those people were getting a lot of gold.
They are not synonyms of each other, but both are used as synonyms of the word "unbiased." Each word has a very broad meaning, and the meanings only intersect when describing an unbiased opinion or view.
Hospitals take strict precautions to assure that nothing adulterates the blood supply.
** adulterate : render something poorer in quality by adding another substance, typically an inferior one.