13 



initially 



3k. M 



Qi = \^ (19) 



a 



For a sustained point source in an infinite medium , in the absence of a 



mean current, Sutton (p. 135) gives the formula 



We can apply this for moderate r (a«:r< D), provided tliat an image source 

 of eqvial strength is taken adjacent to the actual source at the bottom . In this 

 case the concentration at moderate distances is given by 



o k. M. 



or in dimensionless form 



_:L = _6 _L i_ (22) 



C^i u K r 



3 

 where C . = M./4a as before. The coefficient k. represents an effective 



diffusion coefficient throu^ the pearmeable walls , and may be taken as the molecular 



diffusivity times the porosity of the walls. If we consider the order of magnitude 



of kj as 10 cm /sec and K as 1cm /sec then 



-3_ ^ 2x10'^ -^ (23) 



C^i r 



This relation is plotted as curve (3) in Fig. 4 , and as indicated corresponds to the 

 condition of infinite depth of water . 



If D is taken finite then the source can be considered as a line source dis- 

 tributed over a vertical column of depth D when considering tlae effect at very 



