Aluminum sulfate can remove phosphates.
Mycoremediation would definitely be worth looking into, considering that fungi are able to break down/make inert a wide range of pollutants, including pesticides and hydrocarbons (PAHs, POPs, PFAs). Certain species e.g. wine cap/garden giant Stropharia annulus rugosa can lower coliform bacteria from animal manure too. This can be implemented in relatively low tech ways but would likely require collaboration with local mushroom growers in terms of sourcing substrate and spawn (e.g. spent mushroom substrate/grow bags). Intercepting the water and converting it to a slower drip system would be good if possible, but alternatively hessian sacks containing spawn/substrate can be placed as 'bunker spawn'. I recommend reading the works of Paul Stamets.
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