The correct answer is option 1: carbon dioxide from air is dissolved in the water.
When groundwater is exposed to air, it can absorb carbon dioxide (CO2) from the surrounding atmosphere. This is because both air and water contain CO2, but the concentration of CO2 is usually higher in the air than in the water. As a result, CO2 molecules from the air dissolve into the water, leading to an increase in the concentration of carbonic acid (H2CO3). Carbonic acid is formed when CO2 reacts with water, creating a weak acid that lowers the pH of the water.
Option 2 is incorrect because oxygen (O2) from the air does not have a significant effect on the pH of the groundwater. Dissolved oxygen levels can vary in groundwater, but it does not directly affect the pH.
Option 3 is incorrect because it describes the opposite process - the dissolution of CO2 from the groundwater into the air, which would actually increase the pH rather than decrease it.
Option 4 is also incorrect because the escape of dissolved oxygen into the air does not affect the pH of the groundwater.
Therefore, option 1 correctly explains that the pH of fresh groundwater slightly decreases upon exposure to air due to the dissolution of