And to add to the above post, drinking water is only helpful on the day of the U/A. Drinking a bunch of water days or weeks before a U/A is of no help. It must be understood that THC is not water-soluble, unlike other types of drugs. Therefore you cannot rid the body of THC by drinking copius amounts of water. Does not work that way (more on that in a moment).

THC is fat-soluble. THC is stored in the fat cells and remain there until the fat cells are burned for energy. When fat cells burn, the THC is metabolized into THCA and is released back into the bloodstream. From there, the kidneys filter the THCA from the bloodstream and is deposited into the bladder for the eventual exit (this is the main reason why THC carries a longer detection period than other types of drugs). The rate in which the THCA is filtered from the bloodstream by the kidneys is unaffected by copius water consumption.

As already stated, drinking water is only helpful on the day of the U/A. Thats because the water consumption serves to dilute the THCA concentration that is present in the bladder. It is not ridding the bladder of THCA, but rather it "thins out" the THCA concentration present. Its like adding paint-thinner to paint - the paint is not eliminated, instead its concentration is reduced. The idea is to drink enough water to dilute the THCA concentration to where the urine sample registers below the cutoff thresholds of the drug screen. This dilution phenomena, however, is only temporary - once the fluids ingested had all been urinated out, the bladder is replenished with fresh THCA-laden urine and thus, the donor would resume testing positive for THC.