Groundwater contains a significant amount of carbon that can affect lakes, streams, and aquatic ecosystems. Researchers estimated the size and discharge of this carbon pool by studying groundwater data, depth distribution, and age. They found that a large amount of groundwater carbon is close to the surface, with an average turnover time of 10 years at 25 meters depth. Global groundwater discharge is 16,000 cubic kilometers per year, with dissolved organic carbon levels stabilizing at 2-4 mg/L at 100 meters depth. The total carbon content in groundwater is similar to marine sediments, and the carbon flux from land is higher than previously thought. Most of the carbon discharge occurs within the upper 100 meters of groundwater.