Q.Pampas Grass
I have five Pampas Grass plants in my front yard. All are at the base of my front hill near the street. Four have beautiful white plumes, but the one at the base of the driveway has plumes but they are a “dirty brown” color rather than white. Water does run down my driveway toward the street. Could that plant be getting too much water to cause the discoloration? Or perhaps it is a male plant, and the plumes are not as white as female plants?? Thanks for your help.

Pampas grass plumes do turn brown when they dry out at the end of the season, or when the plant itself is having trouble absorbing water. Check whether the plumes and the leaves on the brown plant are brittle, as if they have been dried, and whether the leaves of that plant are also turning brown. If so, this could be due to underwatering or, as you suggest, overwatering, because excessive water could damage the roots and cause the plant to have trouble delivering water to its plumes and leaves.
Check this article:
https://www.gardeningknowhow.com/plant-problems/environmental/signs-of-plants-affected-by-too-much-water.htm