The boil order decade

It happened again. 

Residents in the Lower 9th Ward have been asked to boil water if they intend to use it after a pump was taken offline at a Sewerage and Water Board plant.

People in that area are asked not to drink, make ice or brush their teeth with tap water until further notice.

Water pressure in the area dropped below 20 psi, which automatically triggers such an advisory. The pressure dropped after a pump at the Carrollton Water Plant was taken offline about 8 a.m., and another pump was brought online.

Other area of the city were not affected. 

This thing where they lose pressure at Carrollton but it only affects the Lower Nine is relatively new. I think this is the second one this year. What causes that?