A significant increase in chlorine demand from 0.9 mg/L to 3.0 mg/L during well chlorination indicates what?

A significant increase in chlorine demand during well chlorination from 0.9 mg/L to 3.0 mg/L suggests the presence of contaminants or pollutants in the water. When the chlorine demand rises this dramatically, it indicates that there are substances in the water that are consuming the chlorine, thus necessitating a higher dosage to achieve effective disinfection.

Pollutants can include organic materials, bacteria, or other chemical substances that react with chlorine. These contaminants can significantly affect the water quality, requiring more chlorine not only to disinfect but also to neutralize these unwanted compounds.

While water hardness can affect chlorine efficiency, it is not directly correlated with such a steep rise in chlorine demand. Similarly, chlorine shock refers to an initial high chlorine dose to eradicate bacteria or algae and is not indicated by the increased demand observed here. An algal bloom could contribute to chlorine consumption, but the high demand suggests a broader issue of pollution rather than specifically identifying algae as the cause. Therefore, the increase in chlorine demand primarily points to some form of pollution affecting the water's quality.