A properly sealed well cap protects against all types of contamination. It is the first line of protection against nonpoint source pollution, which constitutes the majority of groundwater contamination. Nonpoint source pollution includes runoff of pesticides and herbicides, soil erosion, airborne dust and debris, and elements from the street.
Well caps also keep out insects, such as earwigs, which prefer a dark, damp environment to nest, wasps and bees that frequently build nests in wells, spiders (spiders love the moist well environment), and small animals such as rodents and snakes. All of these can cause major problems in a well. Bacteria levels can rise from their droppings, and sometimes the bugs, snakes and rodents themselves can get trapped in the wells, die, and decompose in the well water.