a planar gravel surface, the intragravel flow 

 velocity will necessarily increase or decrease. 

 If an area of low gravel permeability occurs 

 between two areas of high permeability, inter- 

 change will occur as shown in figure 5. 



Looking again at the continuity equation (3), 

 a third cause of interchange is suggested: a 

 change in intragravel flow area or, in effect, 

 gravel bed depth. As gravel depth increases 

 in the direction of flow (assuming constant 

 slope and velocity) the total intragravel flow 

 must proportionately increase, and there will 

 be interchange into the gravel. 



A fourth possible source of interchange is 

 the roughness and irregularity of the stream- 

 bed. It is surmised that the composite effect 

 of surface irregularities and fluid inertia 

 causes a channeling of surface water into the 

 gravel bed. 



Since interchange may be either an upwell- 

 ing, a downdraft, or not present at all, it is a 

 controlling variable in the oxygen transport 

 process from air to gravel interior. 



Of final consideration is the actual intra- 

 gravel flow of water. By D'Arcy's law, intra- 

 gravel flow velocity depends upon stream 

 gradient and permeability. Since both stream 

 gradient and permeability may vary to restrict 

 or freely permit intragravel flow, they are 

 also controlling variables. 



Ground-water oxygen transport. — The 

 mechanisms of ground-water oxygen trans- 

 port are: 



1. Dissolution of atmospheric oxygen In 

 standing surface water (lakes, ponds) or rain. 



2. Diffusion of oxygen to lower levels of 

 standing water. 



3. Seepage of oxygenated water through 

 soil to the intragravel strata. 



This process is shown in figure 6, 



Ground-water oxygen transport is subject 

 to controlling variables in each step of the 

 series process. Diffusion of oxygen through 



Figure 5.- -The influence of varying gravel permeability on interchange. 



Seepage 



Figure 6.— Ground-water oxygen transport. 

 6 



