The analysis of a water sample yielded a result of 4.0 mg/L CO2. What is the most likely source of this water sample?

• A. Shallow well
• B. Deep well
• C. Brackish lagoon
• D. Surface water

Answer: (D)

Dissolved CO2 in water mainly comes from gas exchange with the atmosphere and from biological/chemical processes in the water. Surface waters are in direct contact with air and experience ongoing respiration and decomposition by aquatic organisms, along with photosynthesis and organic matter breakdown near the surface. These processes typically yield measurable CO2 levels in the few milligrams per liter range, like 4 mg/L. Groundwater, whether from a shallow or deep well, tends to be more buffered by minerals in the rock and soils, so CO2 is often converted to bicarbonate and carbonate during transit, reducing the free CO2 you measure. Brackish lagoons can vary, but the steady moderate level at about 4 mg/L aligns most with surface water. So the water source is most consistent with surface water.