in B is toward the land. Because of the opposing directions of the 

 net flow velocities, a surface cd must exist where the net horizontal 

 transport of salt is equal to zero. Moreover, there must exist through 

 the surface cd a net volume transport of salt per tidal cycle from 

 B into A, equal to that entering the seaward boundary of B in a tidal 

 cycle. If, now, the volume of river water received from the land in 

 a tidal cycle is designated by R, and Qg represents the net volume 

 transport of sea water in B, the net volume transport of mixed water 

 in A will be represented by: 



Q* = R + Q. 



(1) 



The hatched zone in Figure 1 is assumed to contain water which has 

 very little motion, and hence this zone does not relate to the problem 

 at hand. Also, the water in the shaded zone of Figure 1 will be con- 

 sidered to be dead, because the sea water arriving at point g is 

 expected to pass from f rather than uphill from some point e. 



When an estuary is shallow, some river water will flow toward the 

 land in B, but when the depth is relatively great, the flow in B will 

 consist chiefly of sea water. 



It is now assumed that a contaminant will be dispersed uniformly 



C — 



FIGURE 2 



SCHEMATIC MOVEMENT OF A TYPICAL WATER PARTICLE 

 AS A RESULT OF TIDAL ACTION 



