A pit or depression in which water or other liquid is collected.

Sometimes the creation of a sump is desirable. For instance, if you installed an observation well alongside an underground tank surrounded by an impermeable liner as a means of checking for a release of gasoline, you would want to create a liquid-tight sump directly below the lower end of the observation-well pipe. Released gasoline would collect in the sump, making it easier to observe.

Some sumps, on the other hand, are undesirable. If there is a sag in the vapor recovery piping between an underground storage tank and the dispenser island, product may collect in this low point–the sump–and block the flow of vapors back into the storage tank after the pump is turned off. This can create operational problems in the vapor recovery piping.

See also Tank top sump.