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I believe in terms of cost, and simplicity - one can't beat the 'presence-absence' (P/A) test vials. They can be purchased, or if one purchases the broth ingredients, they can be made by local technicians or teams. They are imprecise (as has been pointed out by others), but can be left overnight at the household level and inspected the next day.

You didn't mention what type of water harvesting - and that could influence your decision. Are these rooftop systems at household level? In which case,P/A tests are probably your best bet. If this is a community system, a higher-accuracy test might be warranted, such as what Lisa suggested, or whatever is available locally. Local availability and supply chain are very important considerations.