this has been a hot topic in NY as of late. when the WTC was built the edge of Manhattan island a big hole was excavated. to reach bedrock (good old manhattan schist,) they had to go down 70 feet.. at that point, they also reach ground water, and the excavation risked being flooded by the Hudson (North) river.(less than a half mile away.)
so they build a giant retaining wall (called the bath tub) it was anchored to the bed rock, made of waterproof concrete and designed to keep the WTC site free of water.(it is real the opposite of a bathtub, instead of holding water in, it keeps it out--but hey, this is NY, and we are not going to quibble about words here!)
with the collapse of the towers, the south tower collapsed onto the bathtub wall. as the final excavation of the site nears, there have been some near disasters. the damaged bathtub is exposed, as rubble, which has been acting as an earthern dam is removed. the site has come close to being flooded several times.
a complex array of siesmigraphs track movement of the bathtub wall. there are also test going on to check the strenght of the wall-- and each crack is carefully monitored.
meanwhile NY is in the 17 month of a drought. there are water restrictions in place, but twice a day, the streets of lower manhattan are still being washed, as part of a program to control the very fine particles of dust from the WTC.
an excess of water (at WTC) is one problem, the lack of water (in reservois) is an other.
