namely a net flux of freshwater through every complete cross-section equal to the 

 amount of freshwater added at all upstream locations of the boundary selected and a 

 net flux of salt water equal to zero, are the essential conditions for the mainte- 

 nance of a steady-state distribution of salt and volume within the estuary. 



When a pollutant is added to the estuary, it will be distributed by the same 

 processes that distribute the salt water and freshwater. If the pollutant is introduced 

 into the bottom layers, it will be carried in a net motion toward the head of the estu- 

 ary in the lower layer flow. Turbulent mixing, which derives most of its energy from 

 the ebb and flow of the tide, will disperse the pollutant horizontally in both a longitu- 

 dinal and lateral direction, and it will be vertically dispersed into the surface layers 

 where it is carried seaward again. Some of the pollutant in the surface layer is mixed 

 downward into the deeper layer and will be carried again toward the head of the estu- 

 ary. For a single discharge, there will be a peak concentration that will both gradu- 

 ally diminish and move seaward on each successive tidal cycle. If the pollutant is 

 "conservative," that is, not decomposed, changed by biological activity, or seques- 

 tered in the bottom deposits, the changes in its distribution can be derived from the 

 known salinity distribution of the estuary (O'Connor and Thomann, 1971). 



In terms of domestic pollution, the continuous discharge at a more or less steady 

 rate of the pollutant into the estuary is of great concern. With time, such a pollutant 

 will become uniformly mixed laterally and, in proportion to the flux in each layer of 

 a stratified estuary, will be transported landward in the deeper water and seaward in 

 the surface layers. Upstream from the point of introduction, the concentration of the 

 pollutant will be greater in the deep layers than in the surface layers, while seaward of 

 the point of introduction, the converse will be true. The pollutant is ultimately 

 flushed from the estuary in the seaward-directed flow of the surface layers. 



Based upon these fundamental considerations, a comparison among estuaries, 

 rivers, and lakes as places for the disposal of pollutants may be useful. In a river, the 

 volume available for the dilution of a pollutant is equal to the volume of river flow in 

 a unit period of time. In contrast, in an estuary the available diluting volume is aug- 

 mented by the participation of seawater in the circulation, and the volume increases 

 as the salinity in the sample increases. This occurs progressively as one moves from 

 the location of the maximum penetration of salt into the estuary toward the mouth of 

 the estuary. This has important implications in terms of the selection of a location for 

 an outfall in an estuary. A downstream placement of the outfall location will always 

 decrease the upstream concentration of the pollutant but will have no effect on the 

 downstream distribution ( Ketch um, 1955). Thus, if the water quality in the river or at 

 the head of the estuary is of principal importance, the outfall should be placed as far 

 downstream as is economically practicable to obtain the maximum improvement. 



When the pollutant is biologically degradable or otherwise changed with time, the 

 situation is somewhat more complicated because the residence time in various parts 

 of the estuary must be considered (Ketchum, 1955; O'Connor and Thomann, 1971; 

 O'Connor, in press). For such a time variable pollutant, the upstream concentration 

 will always be decreased by a downstream movement of the outfall, but the concen- 

 tration at the location of the outfall and downstream of this location may actually be 

 increased. Consequently, if the water quality of the beaches at the mouth of theestu- 

 ary are of prime consideration, an upstream location of the outfall might be prefer- 

 able, though an offshore location in the coastal water would generally be even better. 



The receiving capacity of a given part of the aquatic environment is related to the 

 concentration of the pollutant that is a function of the rate of dilution and the resi- 

 dence time within the estuary. Because of the augmented transport in estuaries, the 

 residence time tends to be short in comparison to that in rivers and in most lakes. In 

 estuaries, the residence time is found by dividing the volume of freshwater within any 

 given segment of the estuary by the rate of river flow. The freshwater fraction is cal- 

 culated as illustrated in Table 6. This, multiplied by the total volume within the seg- 

 ment, gives the volume of freshwater. In lakes and rivers, the entire volume of water 

 in a given part or segment is fresh, so that the total volume is divided by the rate of 



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