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Name: Chiaming
Status: other
Grade: other
Location: MA
Country: N/A
Date: April 2006

Question:
We know that ocean absorbs CO2. What about fresh water lakes and rivers? Does the absorption rate depend on the substance in the water? Obviously the absorption rate must depend on contact area and water temperature. Roughly what is the absorption rate per unit area per unit temperature?



Replies:
It is common to talk about absorption "rates" but "rates" are very difficult to measure and depend upon many uncontrollable factors. You have pointed out two -- contact area, temperature -- but there are many others too, such as speed of current, temperature differences between the air and water, sampling depth, wind speed, concentration gradient between the air and water... The list goes on and on. What scientists generally use are not "rates" but "concentrations" at equilibrium. This turns a horrifically uncontrolled irreproducible problem into just an extremely complex problem. But that is a step in the right direction. In practice measurements are usually equilibrium quantities.

Carbon dioxide is more soluble in fresh water than in salt water. The solubility of nitrogen, oxygen, and CO2 in fresh water at 10 C. is 14.53, 8.06, and 0.39 ml gas/ liter water, respectively. The trend is influenced strongly by the amounts of each gas in the atmosphere: 78%, 21%, and about 0.033%, respectively. But that is only the beginning of the story. The solubility of CO2 is especially sensitive to pH of the water. This in turn depends upon the composition of the lake or stream bed. If it contains carbonate minerals (and many do) this will be in equilibrium with the dissolved CO2 and will absorb or release CO2 depending upon the pH. The presence of cations in the water that form insoluble carbonates (mainly Mg(+2) and Ca (+2)) removes CO2 from the water. Rainfall, both in the short time span and in the long time span, alters the concentration of CO2 in one direction or the other. The attached website: http://www.marietta.edu/~mcshaffd/aquatic/sextant/chemistry.htm and many others you can find on the Internet address the chemistry of CO2 in fresh water, salt water, and the atmosphere.

To reemphasize: "Rates of solution" are virtually useless as a scientific tool in the study of the distribution of carbon dioxide in the biosphere because it only applies to the specific place, time and conditions that the concentration is measured. It would only be useful for some very localized study or problem. Because of the number of rogue variables "rates" will likely vary "all over the map" unless there is some "driving force" -- runoff from mine tailings might be an example -- that overwhelm all the other variables.

Vince Calder



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