Fresh Water Absorption of CO2

name         Chiaming
status       other
grade        other
location     MA

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?
---------------------------------------------------------
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
====================================================================