Properly abandoning unused wells is important to eliminate the physical hazard of an open well and to preserve the water yield and hydrostatic head of the aquifer. Proper well abandonment is also necessary to prevent cross-contamination from the vertical migration of poor quality water within the well casing and annulus. This is especially important for monitoring wells and other wells at hazardous waste sites where the ground water is known to be contaminated and the potential for a corrosive subsurface environment exists.