The chapter discusses the geochemistry of natural waters, focusing on the role of carbon dioxide ($\mathrm{CO}_{2}$) and bicarbonate ($\mathrm{HCO}_{3}^{-}$) in controlling water chemistry. It explains how these variables influence carbonate concentration and pH, particularly in the context of photosynthesis, respiration, and aerobic and anaerobic decay. The text provides equations to illustrate these relationships, such as the simplified equation for anaerobic decay involving sulfate reduction. An example is given to demonstrate how the pH and calcium concentration of pure water in equilibrium with calcite vary with changes in $\mathrm{CO}_{2}$ pressure. The chapter also includes simplified pe-pH diagrams for systems involving uranium and dissolved species, showing stability fields and solubility boundaries.The chapter discusses the geochemistry of natural waters, focusing on the role of carbon dioxide ($\mathrm{CO}_{2}$) and bicarbonate ($\mathrm{HCO}_{3}^{-}$) in controlling water chemistry. It explains how these variables influence carbonate concentration and pH, particularly in the context of photosynthesis, respiration, and aerobic and anaerobic decay. The text provides equations to illustrate these relationships, such as the simplified equation for anaerobic decay involving sulfate reduction. An example is given to demonstrate how the pH and calcium concentration of pure water in equilibrium with calcite vary with changes in $\mathrm{CO}_{2}$ pressure. The chapter also includes simplified pe-pH diagrams for systems involving uranium and dissolved species, showing stability fields and solubility boundaries.