Which of the following is NOT a water quality issue related to changes in source water quality?

Water hardness is primarily determined by the concentration of calcium and magnesium ions in the water and is largely a characteristic of the geological environment through which the water flows. While it can be affected by the source of the water in the long term, changes in source water quality typically relate more directly to factors that can fluctuate seasonally or due to pollution — such as temperature, fecal contamination from sewage or animal waste, and algal blooms resulting from nutrient runoff.

Changes in temperature, for example, can influence the solubility of gases like oxygen, alter chemical reactions, and affect the metabolism of aquatic organisms. Fecal contamination can lead to serious health risks and often indicates issues with the source water that can arise from various human activities. Algae growth is directly linked to nutrient levels in the water, often exacerbated by runoff that increases during rainfall or after agricultural activities.

Thus, while water hardness is an important factor in overall water quality, it does not fluctuate significantly due to immediate source water quality changes like the other options do.