Take a look at the journey your water makes before it reaches you.
Groundwater is water held underground in the soil or in pores and crevices in the rock. Our drinking water is mostly abstracted from the chalk aquifer which lies to the south and east of Cambridge.
The aquifer is very porous, like a sponge, which means that it can hold large quantities of water in between the sand grains and in cracks in the rock. As the sandstone can hold large volumes of water, the levels do not fall very far in dry periods, which makes groundwater sources a very reliable supply of water.