Parameter Tree:
EARTH SCIENCE > TERRESTRIAL HYDROSPHERE > WATER QUALITY/WATER CHEMISTRY > GASES > DISSOLVED CARBON DIOXIDE

Definition:
Naturally present as a result of animal respiration, the decay of organic matter, and the decomposition of certain minerals. It is the major source of acidity in unpolluted water samples. Surface waters typically contain less than 10 ppm (mg/L) dissolved CO2, while ground waters, particularly if deep, may contain several hundred ppm (mg/L).

Reference:
Houghton, J. 1997. Global Warming: The Complete Briefing, 2nd. ed. Cambridge University Press, Cambridge, UK. Maunder, W.J. 1994. Dictionary of Global Climate Change, 2nd ed. Chapman and Hall, NY.