It was apparent that the onty pollution character of consequence 

 was the oxygen demand of the waste sulphite liquor. If pollution was 

 to be avoided it was necessary that the mill sewage should be displaced 

 from the mill-site to the open sea before the dissolved oxygen content 

 of the inlet waters was lowered b}^ 50 per cent. 



An intensive oceanographic investigation showed that the fresh water 

 from land drainage moved isostaticaUy seaward. Due to tidal energy 

 it mixed with the under-lying sea-water en roitte to form a brackish upper 

 zone. The thickness, properties, and the rate of seaward displacement of 

 water through this zone were quantitatively related to the discharge of 

 the principal river in the inlet. There is a converse movement of ocean 

 water in the zone immediately below this, which provides the sea- water 

 to be mixed into the upper zone. Any fresh water transferred down- 

 wards is held isostaticaUy adjacent to the boundary and preferentially 

 returned to the upper zone in the mixing process. Consequently all 

 fresh water leaves the inlet by way of the upper zone, and none 

 accumulates in the lower. 



Below the level of the threshold there is a deep zone, in which move- 

 ment is generated by frictional transfer of velocity from the overlying 

 middle zone. 



A hydraulic model of the head of the inlet was built, in which the tide^ 

 the fresh and sea-water, and the winds were represented. From this it 

 was learned that the outflow of the river was a jet function, whose stream 

 diverged until it occupied the whole width of the inlet. Seaward of this 

 limit of divergence there was no transverse salinity gradient, and the 

 system was effectivel}- two-dimensional. The tidal and isostatic flow 

 characteristics m the head of the inlet, and the course of the effluent from 

 various sewer outlets were studied in this model, and confirmed by 

 oceanographic observations and float experiments in nature. 



From these studies it was evident that if the pulp-mill sewage were 

 diluted with'aU the wash water from the mill so that it entered the upper 

 zone of the inlet at approximately the density of fresh water it would 

 remain in the upper zone and be continually displaced seaward. This 

 approach to the problem permitted the fresh water in the inlet to be 

 regarded as a tracing solution, indicating the behaviour of the proposed 

 effluent. It was concluded that no sewage would appear in the middle 

 or deep zones, which would therefore be free of poUution. If sewage were 

 dimiped in these zones it would accumulate and eventually cause 

 pollution. 



The degree of pollution (P) was defined as the ratio of oxygen demand 

 of the pulp-mill sewage (BOD) to the dissolved oxygen supply (DO). 

 The tolerable rate of discharge of sewage into the upper zone was 

 arbitrarily defined as the amount whose oxygen demand would not 

 exceed 50 per cent, of the oxygen supplv. The pollution was considered 

 tolerable when 



_BOI> = 

 0-5 (DO) < 



This provided a quantitative definition of pollution which could be 

 evaluated. 



The normal oxygen content of the incoming fresh and sea-water was 

 observed, and thfe oxygen supply in the upper zone was evaluated from 

 the observed proportions of each, and their rate of displacement through 

 the inlet. The oxygen demand of the pulp-mill sewage was estifnated 



181 



