Advice for discoloration of granite

Hello,

I was doing some online research on granite and came across a website with
questions and aswers regarding stone and your email address.  You seem very
knowledgeable about granite and I wondered if you might be able to assist me
with a project I am working on.

I have a client that just constructed a MULTI MULTI million dollar
government building.  They installed granite pavers and granite benches
outside the building.  The pavers are imported from China and are very
expensive.  Many of the pavers and other granite stones have developed a very unsightly discoloration on the outside surfaces.  The pattern of
discoloration is very random.   One very dark and discolored paver may be
adjacent to several others without discoloration.  Some outdoor pavers
discolored after installation.  Some pavers that have not been installed and are in storage have developed discoloration.  Some pavers stored in a
basement parking garage were exposed to some flooding during a sudden storm, but other pavers remained dry and are stilled discolored.  The pavers installed outdoors were sealed after installation (and before discoloration appeared), while unsealed pavers in storage also have discoloration.

The pavers are a very pale, sand colored granite without much marbling or
color variation.  The discoloration is appearing as dark gray or black spots
in large blotches (not uniform) on a paver.  There are also some pavers that have a dark brown discoloration that covers a larger, more uniform area.

We are going to be performing several tests to evaluate the nature of the
discoloration on the pavers so we can evaluate a method to clean the pavers.
Do you have any ideas or suggestions as to what may cause the staining?

Jennifer Bailey