In what type of water is copper not effective?

Copper is a widely used algaecide and herbicide in aquatic pest control, primarily because of its effectiveness against various types of algae and aquatic weeds. However, its efficacy can be significantly influenced by the pH and alkalinity of the water.

In alkaline water, which has a higher pH (above 7.0), copper can form insoluble compounds with carbonate and bicarbonate ions. This process reduces the availability of free copper ions, which are responsible for the toxic effects on aquatic organisms. Therefore, in such conditions, copper is not very effective in controlling algae and aquatic plants because it is less bioavailable and cannot exert its full herbicidal or algaecidal effects.

Conversely, soft water generally has lower pH levels and lower concentrations of calcium and magnesium, which allows copper to remain more soluble and effective. Similarly, fresh water may present conditions where copper can still function effectively depending on other factors like nutrient availability and water chemistry. Brackish water, a mix of fresh and salt water, can also allow for some effectiveness, though this depends on the salinity levels.

Thus, the specific characteristics of alkaline water make it the environment where copper is least effective, making it the correct answer.