Table 4. Effect of P-Detergent Ban or P Removal on the N:P Ratio of 

 Wastewaters (after Mancini, et al.. in press) 



control phytoplankton growth. These factors include not only nutrient content but 

 also the rate of the circulation; the rate of grazing by herbivorous animals; tempera- 

 ture and the transparency of the water, which determines the depth of the euphotic 

 zone; and the total amount of photosynthesis that can take place in the water 

 column. 



CIRCULATION AND MIXING 



Circulation of the water in the environment in which the sewage is released will 

 determine the rate of dispersion and dilution of the effluent, and the advective 

 processes will transport the contaminated water away from the point of discharge. 

 The rates of these processes depend upon the geomorphology of the estuary, the 

 characteristics of the tidal regime, and the river flow. These are unique characteris- 

 tics of each estuary and must be separately evaluated for each. Circulation and 

 mixing within the estuary determine the fate of a pollutant, but these processes are 

 commonly inadequately evaluated or poorly understood in the planning of marine 

 disposal operations. 



A conceptual or mathematical model of a specific estuary is desirable in order to 

 understand clearly the dynamics of the circulation and to facilitate comparison with 

 other estuaries. The early conceptual models were based upon the assumption of the 

 steady-state distribution of properties (Tully, 1949; Ketchum. 1951a). The develop- 

 ment of computer technology has made it possible to model transient conditions in 

 the estuary. Some of the conceptual models are designed to take account of biologi- 

 cal processes, and several of these are discussed by O'Connor (in press). 



While each estuary is unique, certain generalizations apply to all estuaries in w hich 

 the water supplied by river flow is diluted by seawater. There is a gradient of salinity 

 from the virtually fresh water of the river to the salinity of coastal seawater, generally 

 30 to 32°/oo. The surface layers are fresher than the deeper waters within the estuary, 

 and these two layers are separated by a density discontinuity that generally depends 

 on the salinity distribution (the halocline). A density-driven circulation pattern is 

 established within the estuary so that the net transport during a complete tidal cycle 

 in the surface, fresher layer is seaward and the net transport in the deeper layer is 

 landward. Under steady-state conditions, the net effect of these two flows is to trans- 

 port seaward a volume of freshwater equal to the volume introduced by the river 

 during a complete tidal cycle. The net transport of salt through any complete cross- 

 section of the estuary is equal to zero. When the river flow is not constant, the steady- 

 state conditions will not be met. When the rate of river flow is declining, salinity will 

 increase at any location within the estuary and there will be an excess inflow of sea- 

 water. When the river flow is increasing, the estuary will become fresher and there 

 will be a net transport of both salt water and freshwater seaward. 



Any conservative soluble pollutant that is added to the estuary will be distributed, 

 diluted, and transported by the same processes that control the distribution of fresh- 

 water and salt water. If the pollutant is introduced at a mid-point in the estuary and 



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