The major environmental issue in eastern Canada is acid precipitation. Acid precipitation refers to the deposition of acidic substances from the atmosphere onto the Earth`s surface. It is caused by emissions of sulfur dioxide and nitrogen oxides from human activities such as burning fossil fuels. These pollutants are released into the atmosphere and can travel long distances before being deposited in rainfall, snow, fog, or dust.

Acid precipitation has harmful effects on the environment. It can harm forests, lakes, and aquatic ecosystems. When the acidic substances are deposited on the ground, they can leach into water bodies and soil, making them more acidic. This can have negative impacts on the health and reproduction of plants and animals.

Groundwater depletion, option 2, refers to the excessive extraction of groundwater, which is a separate environmental issue not specific to eastern Canada. Land degradation, option 3, is the deterioration of the quality and productivity of land due to human activities such as deforestation and overgrazing. Desertification, option 4, is the process of fertile land becoming a desert, which is not a major issue in eastern Canada.

Therefore, the correct answer is option 1: Acid precipitation.