The pH of fresh groundwater slightly decreases upon exposure to air because—

The pH of fresh groundwater decreases upon exposure to air primarily because carbon dioxide (CO2) from the atmosphere dissolves into the water. When CO2 dissolves in water, it reacts to form carbonic acid (H2CO3), which subsequently releases hydrogen ions (H+) [1]. This increase in hydrogen ion concentration directly leads to a reduction in the pH level, making the water more acidic [1]. While groundwater often contains high levels of dissolved CO2 that can escape (degas) upon aeration—which would actually increase the pH [3]—the specific scenario of pH 'decreasing' upon exposure to air refers to the absorption of atmospheric CO2 into water that is not already saturated with it. Rainfall, for instance, is naturally acidic (pH ~5.6) due to this exact process of CO2 dissolution from the air [2].

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 18: Ocean Acidification > How it reacts? > p. 264
2. [3] https://www.osti.gov/servlets/purl/1981791
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Anthropogenic Solution Weathering > p. 91