I wish I could see a map of the water system that explains this. From the surface it looks so unlikely!

The water main break in Arlington has impacted parts of upper NW DC. Some areas areas are under a boil water alert. Here is an interactive map showing which areas are affected:

https://www.arcgis.com/home/webmap/viewer.html?webmap=2ab86ced18ab41a3b7c3d0517b5bff16&extent=-77.1629,38.8733,-76.9093,38.9963