material. These substances take up dissolved oxygen from the water as they 

 decompose, often causing the oxygen concentration of the water to fall to 

 a level that is adverse to aquatic life. BOD is a measure of the oxygen 

 depletion potential of organic matter introduced into a water body (in 

 pounds of oxygen consumed). Occasionally, animals are killed by a sudden 

 oxygen drop, but the usual effect is to impair their health, or, if they 

 are mobile, to drive them away. An example is the blockage of fish spawn- 

 ing migrations in the Delaware River, where greatly reduced oxygen near 

 Philadelphia (from combined sewage and industrial effluent) appears to have 

 now eliminated striped bass spawning. 



The level of dissolved oxygen is one direct index of the health of the 

 system. High levels are generally indicative of a healthy system that will 

 support a diverse biota and multiple uses. The lower the concentration of 

 dissolved oxygen, the lower the potential carrying capacity of the system 

 becomes [84]. 



Dissolved Solids : This water quality parameter is the "filterable" 

 component of water chemicals and including the carbonates, sulfates, 

 chlorides, phosphates, and nitrates. The quantity and quality of 

 dissolved solids are major factors determining the variety and abundance 

 of plant and animal life in the aquatic system because they include the 

 primary nutrients. 



4.14.3 Assimilation Capacity 



The effect of pollutants on the estuarine environment depends on how 

 strong they are, where they go, and how rapidly they are assimilated or 

 flushed out of the environment. All of these conditions depend on water 

 movement and circulation patterns, which are in turn governed by the 

 relationship of tide and river flow to estuarine basin shape and size [84]. 



Active circulation and a good rate of flushing are usually considered 

 beneficial because they provide rapid transport of nutrients and cleansing 

 of the natural system, as well as performing other vital functions. To 

 an extent, good flushing protects ecosystems stressed by development because 

 it hastens the assimilation, dispersal, and dilution of pollution. 



A high proportion of U.S. coastal water basins have poor flushing char- 

 acteristics. For example, in the Texas gulf, inflows into the major estuaries 

 west of the Neches River are often small, leading to relatively slow flush- 

 ing of the estuaries. The upper Houston ship channel has an average flush- 

 ing time of 38 days and a flushing time as great as 80 days over 10 percent 

 of the time. The median flushing period for Galveston Bay is 175 days. 

 This combination of limited tidal mixing and limited freshwater inflow is 

 believed to create "a condition which is particularly susceptible to the 

 buildup of pollutants and, consequently, to a significant impact of these 

 pollutants in the water quality" [130]. 



163 



