Organic halides are compounds created by the reation of halogens (flourine, chlorine, bromine, and iodine) with organic chemicals. Chlorination of water supplies to control bacterial contamination can produce trihalomethanes (THMs). Organic halides can enter the groundwater from the application of pesticides and herbicedes, either from nearby agricultural activities or from the use of these materials around the home. Contamination of groundwater with organic halides can also result from industrial activities or dump sites.