What causes the increase of soluble lead in surface water?

The correct answer is related to the chemical properties of lead and its solubility in water. An increase in the solubility of lead in surface water is significantly influenced by changes in water hardness. Water hardness primarily comes from the presence of calcium and magnesium ions, which can interact with lead to form insoluble compounds.

When the concentration of hardness ions decreases, there is less competition for lead ions, leading to an increased concentration of soluble lead in the water. This phenomenon occurs because soft water—characterized by low concentrations of these hardness ions—can dissolve more lead due to the absence of precipitating agents that would otherwise bind with lead and render it insoluble.

In contrast, an increase in water acidity generally leads to an increase in lead solubility, as acidic conditions can cause lead to dissolve more readily. However, this answer does not accurately pertain to the effect of hardness. An increase in temperature can affect the kinetics of dissolution but does not have the same direct impact as changes in hardness. Lastly, a decrease in organic matter may not significantly influence lead solubility because organic matter sometimes plays a role in complexing metals, which can either increase or decrease their solubility depending on the specific context.

Therefore, the relationship