Contamination is the presence of something undesirable or unsafe. If drinking water becomes contaminated with salt water, pollution or pesticides, then it is no longer safe to drink. Contamination can happen in a lot of different ways. If there is a leak from a treatment plant, factory or underground, then oil, sewage and dangerous chemicals can easily get into water supplies. Pesticides, fertilizers and herbicides from farms and lawns can also make it into the water supply if it is carried away by runoff.