A tense question!

Has been occuring / has become

These are natural tenses to use in your example.

why? could you explain?

"has been occuring" (present continuous tense)
"How long has this been going on??"
"We have been fixing up the house."

It may be hard to think of these as present tense when you're speaking of something which has been going on for centuries, but the important thing is that it's still going on today.

"has become" (present perfect tense)
"The neighbors have become a n

In isolation, "has been occurring" has a stronger sense of continuous or repeated action than "has occurred". "has occurred" could mean that pollution occurred on at least one, and presumably several, sporadic occasions, whereas "has been occurring" suggests it has been going on all the time, or much of the time.

However, in this sentence the words "for centuries", and the

Sorry about the spelling, MrWordy. I used the double "rr," then looked at Windy's and changed it.

Thanks.