Fairfax County and a good bit of Northern Virginia seems to have been chock full of ADAMS during that time period. If you look in Geo. Washington's Diary, you'll also see reference to another neighboring landowner, Abednego Adams, as well as Francis Adams. There is also a Daniel Jenifer Adams in his diary and a Robert Adam[s], amongst others.
Oh, and let's not forget George's VP, John Adams of Mass., who is mentioned frequently in his diary which just complicates the search results! ;-)
Abednego, Francis and Daniel Jenifer Adams are all from the same family that originate in Charles Co., MD. I do not know at this point if Robert is connected to them. The name William was often used in this line, too.
The Gabriel, Peter, Nathan group of ADAMS is reported to also be from Maryland but I believe they originate on Maryland's eastern shore from what I've read. They seem to be an entirely different group from Abednego, Francis and kin.
Both families migrate across the Potomac River into Fairfax County. There are other ADAMS families in Fairfax, too, just to make it a bit more interesting!
I believe that Gabriel or Peter Adams left a will in Fairfax Co. but don't quote me on it. You might want to try the Fairfax Co. VA genforum to see if anyone can help you there.