Example calculation of a partition coefficient:  Partition coefficients are measured experimentally in a laboratory by measuring the concentrations of an element in phases that are held in chemical equilibrium.  Suppose that you do an experiment to measure the partitioning of a pollutant (we'll call it Death-X) between liquid water and clay.  You add 1 gram of Death-X to 10 grams of water and 5 grams of clay.  Concentration is defined as grams of Death-X for each gram of water or clay.  You stir the clay into the water, allowing the Death-X to reach equilibrium between clay and water.  You measure the amount (defined as grams Death-X) of Death-X in the water and find it to be 0.1 gram.
  Question 1:  Is most of the Death-X in the water or the clay?
  Question 2:  What is the partition coefficient for Death-X in Clay/Death-X in water?
  Question 3:  If you spilled the Death-X on a clay-rich layer of sediment above your neighbors well (oops), do you think it would be likely to reach the water table?

Question 1:  Since 0.1 grams Death-X is in the water, the rest, 0.9 grams, must be in the clay.  Thus, most of the Death-X is in the clay.

Question 2:  The concentration of Death-X in clay is 0.15 (0.9 grams Death-X in 5.9 grams total Death-X + clay).  The concentration Death-X in water is 0.0099 (0.1 grams Death-X in 10.1 grams total Death-X + clay).  The partition coefficient for Death-X in clay/water is thus 15.

Question 3:  Since the Death-X tends to stay with the clay, rain water seeping down toward the water table and the well intake will have the Death-X removed from it by the clay, and little will reach you neighbors well (hee, hee, maybe you won't have to tell them.....at least until everything dies at the surface of the ground).

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