What is primarily responsible for acidity in surface water?

The primary factor responsible for acidity in surface water is carbon dioxide. When carbon dioxide is dissolved in water, it reacts to form carbonic acid, which contributes to the overall acidity of the water. This process occurs naturally in the environment and is a critical part of the carbon cycle. The presence of carbon dioxide in the atmosphere and its ability to dissolve in water lead to the formation of acids, impacting the pH levels of surface waters.

While sulfide compounds, nitrogen oxides, and phosphates can influence water quality and may have ecological impacts, they do not play as direct a role in the acidity of surface water as carbon dioxide does. Sulfide compounds can lead to the formation of sulfuric acid in specific conditions, and nitrogen oxides can contribute to acid rain; however, the primary mechanism for acidity in the broader context of surface water remains the dissolved carbon dioxide. Phosphates are more closely related to nutrient loading and eutrophication rather than directly affecting acidity. Thus, the foundational role of carbon dioxide in forming carbonic acid makes it the main contributor to surface water acidity.