Yep. There is no portable filter or purifier that will take out chemicals or minerals.

Salt water, runoff from fertilized fields or cow pastures, water running through tailings of old mines... all suspect and all untreatable with typical backpacking chemical purification or filters, or even the almighty First Need purification system.
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"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

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