without regard to their discharge rate or volume. However, for reasons 

 already noted, I cannot fully approve any waste-disposal regulations that 

 are entirely independent of local conditions and needs, the manner of dis- 

 charge of the wastes, and the ability of receiving waters to assimilate 

 them without impairment of any beneficial uses of the waters. I believe 

 that toxicity emission rates can best be limited so as always to permit 

 reasonable utilization of the assimilative capacity of the receiving 

 waters with no undue risk of injury to aquatic life by toxicants, the 

 calculation and control of these rates may prove especially useful in pro- 

 tecting valuable organisms in waters that receive toxic wastes in con- 

 siderable amounts from several sources. 



Procedures for equitable apportionment of the assimilative capacities 

 of waters for toxic pollutants among multiple sources of these wastes 

 have not yet been thoroughly developed, and will not be discussed in de- 

 tail here. Much attention recently has been given to this difficult 

 regulatory problem in the United States, especially in California. 

 Various proposals for its solution have been advanced and studied, but 

 none, as yet, have been shown to be entirely sound nor widely accepted. 

 When there are several important sources of toxic pollution of a water 

 body, evaluation and consideration of the persistence, as well as the 

 dilution, of the discharged toxicants in the receiving water may be 

 necessary. The acute toxicity of combined, fresh effluents from different 

 sources not separated by large distances can often be easily determined 

 experimentally. Perhaps the combined toxicity can also be estimated with 

 sufficient accuracy for regulatory purposes by measuring the toxicity of 

 each effluent, and assuming an additive interaction of the poisons in the 

 mixture. This assumption is implied by the proposed summation of toxicity 

 emission rates. But when effluents from several plants are discharged 

 into a body of water at points that are remote from each other, the non- 

 persistent toxicants from the different sources do not occur together in 

 the receiving water. Unless most of the toxicants involved are very per- 

 sistent, the safe discharge rates for the effluents can be much greater 

 than they would be if all the effluents were discharged together at one 

 point into a common mixing zone. The proper allowances to be made for 

 natural self-purification of waters are not easily determined, and very 

 little is known about the interaction of poisons at sublethal concentra- 

 tion levels. Measurements of rates of loss of toxicity under appropriate 

 conditions in the laboratory can be useful in estimating levels of resi- 

 dual toxicity of polluted waters in nature. Without some such correction 

 for natural self-purification, a value for the "toxicity concentration" 

 (in t.u.) at a given point in a stream, computed by dividing the sum of 

 the toxicity emission rates from all upstream sources by the stream flow 

 (in m3/min., for example), is not meaningful. But a correction for loss 

 of toxicity can introduce an error in the opposite direction when the re- 

 sults of the calculation are applied in regulating the waste discharges. 

 Because slowly acting, accumulative poisons tend to be also highly per- 

 sistent, the appropriate application factor for the acute toxicity of a 

 solution of nonpersistent and persistent poisons is likely to decrease as 

 the solution ages. 



