What is the primary source of carbon dioxide in groundwater?

The primary source of carbon dioxide in groundwater is biological oxidation of organic matter. In this process, microorganisms break down organic materials, such as plant debris and animal waste, which releases carbon dioxide as a byproduct. This naturally occurs in soils and sediments that are rich in organic matter, particularly in anaerobic conditions where the breakdown process occurs without oxygen.

As organic matter decomposes, the resultant carbon dioxide can then dissolve in the groundwater, contributing to its concentration. This is a significant pathway for introducing carbon dioxide into aquifers compared to other sources.

While photosynthesis of plants also involves carbon dioxide, it primarily removes carbon dioxide from the atmosphere, rather than contributing it into groundwater. Industrial emissions can indeed release carbon dioxide, but these are typically associated with surface air pollution rather than being a direct source for groundwater. Infiltration of atmospheric air can introduce gases, including carbon dioxide, into groundwater, but this process is not as significant as the biological oxidation process occurring within the soil and subsurface environments, where organic matter is actively decomposed.