What indicators are commonly used to assess acid deposition impact on freshwater systems, and what rain pH threshold is acidic?

• A. Chloride concentration; rain pH below 7.0 is acidic.
• B. Soil moisture; rain pH below 6.5 is acidic.
• C. Lake pH and ANC; rain pH below 5.6 is acidic.
• D. Oxygen saturation in lakes; rain pH below 5.0 is acidic.

Answer: (C)

Acid deposition changes freshwater chemistry by lowering acidity and reducing the system’s buffering ability. The best indicators to track its impact are lake pH and the water’s acid-neutralizing capacity (ANC). Lake pH shows how acidic the water is right now, while ANC reflects how much acid the system can neutralize before the pH drops further. A rain pH threshold of about 5.6 is used to define acidic rain because natural, unpolluted rain is around pH 5.6 due to dissolved CO2 forming carbonic acid; values below 5.6 indicate acidity from pollution. Other options don’t directly measure the effect of acid deposition on freshwater: chloride concentration relates to salinity, soil moisture to water content in soil, and oxygen saturation is influenced by many factors and isn’t a direct indicator of acidification.