Spring supplies are therefore a source of groundwater, which in most instances has fewer micro-organisms (e.g. coliform bacteria and protozoa such as Giardia and Cryptosporidium) and chemical contaminants than a supply from surface water. The point at which the groundwater reaches the surface is prone to contamination, so must be protected using a structure called a spring box. This is often surrounded by a fence to keep animals out, with other common features being a ditch on the uphill side, an overflow pipe and a well fitting lid.