The well's defenses against contamination are its surface sanitary seal, annular seal, and well casing. If the annular seal is too shallow, the casing ruptured, or the surface seal broken by holes or missing gaskets, contaminants can enter the well. If a well is located too close to a source of contaminants, these may enter through one of these routes or percolate down through the ground, and enter the well at depth.