Why can a satellite redness index flag rusting Arctic rivers but still fail to tell which streams are chemically hazardous?

Answer: It only reads surface color, so it cannot reveal the water's actual chemistry

It only reads surface color, so it cannot reveal the water's actual chemistry ✓ — Correct. A redness index can spot the optical fingerprint of iron-rich discoloration across remote watersheds, which is useful for finding candidates. But it is still a surface color signal. Two orange streams can differ in pH, sulfate, and dissolved metals, so chemistry has to be sampled before calling one a toxic hazard.

It measures the concentration of every metal dissolved in the water — Measuring all metals is exactly what the satellite does not do. Reflected light can show visible staining, not the concentration of zinc, nickel, aluminum, or cadmium in the water. The useful workflow is scout from above, then test on the ground.

It images the buried source of the contamination directly — Routine redness mapping does not image the buried source. It observes the river surface after groundwater chemistry has already reached open water. The hidden source layer has to be inferred from geology, permafrost, hydrology, and samples, which is why satellite maps are powerful but incomplete.