Natural Filtration Dam Possible?
Answer:
The river needs to be looked at as water shed management. Placing a structure (dam) across it is of little use when talking about pollutants. Perhaps there is some use at a structure on a tributary to the stream/river. Most likely there needs to be a total water shed analysis done and point sources (factories) and non point sources (neighbor hoods, cities, fields) be identified that are contributing to the overall pollution. Once the individual pollution sources are identified, then they can be prioritized and individual solutions for the given pollution can be determined. There are many different solutions, but most solutions can only take care of specific forms of pollution.
Pollution in water can be many different things, if it is silt and sediment, then a structure is frequently a good sollution. Some chemicals such as nitrogen and phosphorus (from fertilizers on lawn and fields and even animal waste like dogs) can be effectively handled with a well placed wetlands. Other chemicals require very expensive remediation methods that may be very expensive. The most effective method is to stop the pollution at the source and give the river time to heal itself.
Problem is that the upstream side of the river gets very polluted as the concentration of chemical rises. A local river has a problem with PCB contamination caused by the chemicals settling out behind a low head dam.
One could filter out particulates (silt) but the filter would tend to clog. To remove dissolved chemicals is much harder, reverse osmosis can do it, but that would be more complex than pilling up gravel and sand, not really a "natural" process. Wetlands can remove some pollutants by sedimentation and various biological actions, but require a lot of space and suitable terrain.
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