Water scarcity is usually described in volumes: how many liters per person, how much storage remains, how far the water table has fallen. Volume is measurable and comparable, which makes it the default frame.
For a large number of communities, the volume is adequate and the water is unusable. That is a different problem with different solutions.
Where contamination comes from
Agricultural runoff carries fertilizer and pesticide residue. Industrial discharge introduces heavy metals and chemicals. Inadequate sanitation infrastructure introduces pathogens. Some contamination is natural, including arsenic and fluoride in certain aquifers.
How quality and quantity compound
The two problems make each other worse. Lower river flows concentrate pollutants, since the same contaminant load is diluted by less water. Depleted coastal aquifers admit saltwater intrusion, which renders wells unusable while the water is still physically present. Drought therefore reduces both supply and quality at once.
Why measurement lags
Quantity is easier to monitor than quality. Stream gauges and satellite gravity measurements provide continuous volume data. Quality requires sampling, laboratory analysis, and testing for specific contaminants, which is slower and more expensive.
The consequence is that quality problems are often discovered late, after exposure. Any serious account of water security has to treat contamination as a first-order issue rather than a footnote to volume.