Chlorine
What is it? Chlorine, on its own, is a greenish-yellow, highly reactive and diatomic gas that is almost never found free in nature by itself. When found in freshwater, chlorine is most often found bonded to other elements and ions.
Why is it important? We use chlorine to disinfect public water supplies from pathogens, such as bacteria and viruses. Chlorine acts as an important part of the cleaning process to ensure a healthy drinking water supply in cities. Chlorine is used in water treatment plants, our water distribution systems, in supply reservoirs, including cisterns. It follows that we sometimes find leftover traces of chlorine from water treatment in our freshwater systems including lakes, rivers, and streams.
What does it mean? Remember — all the water that comes out of your tap, as well as down your drain, comes from a water source and eventually finds its way back. You likely will only find small traces of chlorine in most freshwater systems. If you read your test results and note levels of chlorine in your water, you may be near a water treatment plant, or close to someone dumping pool water. Runoff from common domestic water uses, such as watering gardens, car washing, swimming pool draining, and driveway or road flushing, can all carry chlorine some distance, and end up in our lakes, rivers, and streams.