In the past, we have talked about the sources of water that are available in city and urban environments. One thing that we have talked about using is ponds in parks and subdivisions. I have recently found some information that you need to consider if these are on your list of water sources.
Many of these ponds particularly in the southwest or other hot dry areas are ringed by plants that are not native to the area and take lots of care. Often large amounts of fertilizer and pesticides are used to keep these plants looking green. Runoff and leeching flush these toxins directly into the ponds. Golf courses are one of the worst offenders.
Another problem in many city pools and small lakes is storm water runoff. The runoff can contain all types of hazardous chemicals including gasoline and motor oil from the streets.
Now I am not saying that you have to totally discount these water sources. But become familiar with them, learn whether the ones in your neighborhood are a hazard. Depending on the type of hazard, you may be able to mitigate it by the use of the correct type of filtration.
Knowledge of your area many be the difference between life and death.