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OK, very simple answer. The WHO recognise various water quality parameters, e.g. physical, chemical, microbial and radiological. However one aspect is often overlooked and it is the most important and that is "aesthetic". If the water looks, smells and tastes odd/ unpleasant then people won't drink it. So whilst knowing the cause is nice to know, it may be very expensive and difficult to find out - even if the testing equipment is available. Even if you do not know what the chemicals are, you still need a practical way forward.So accept the water does not meet the standards as it's yukky. If you wouldn't drink it then it's not good enough. The problem is people may prefer good tasting surface water (but polluted with bacteria) than horrible tasting groundwater (even if it's bacteriologically safe).
So... - carry out a sanitary survey/ water source survey to look for any obvious sources of contamination/ sudden changes in water quality. This might help identify the type of pollution. - chemical pollution is often a long-term problem (e.g. a bit of arsenic for a few weeks is less of a problem than drinking a little over 20 years). Microbial contamination is much more of a short-term concern. - chemical treatment is often hard and expensive, even if you know the problem - consider rapid RWH if rainfall is enough - even if just for drinking water. Simple tarpaulin systems work e.g. https://wedc-knowledge.lboro.ac.uk/resources/conference/34/Burt_M_-_196.pdf - consider temporary HHWT such as a filter (settlement/ chlorination systems don't necessarily reduce chemicals and may not work well anyway) - that may improve taste enough. - consider trucking/ bottled water for drinking only / for vulnerable groups.