361 
The costs per exceedance eliminated for categorical standards may appear 
to be high because the costs of all of the POTWs affected have been loaded onto 
the smaller number of POTWs where exceedances are actually eliminated. This 
is illustrated in Table 4-5A which allocates the total costs for each option 
into three categories: cases where there are no exceedances in the first place, 
cases where there are exceedances but they are not eliminated and cases where 
exceedances are eliminated. As shown in the table, the actual mean cost per 
exceedance eliminated is about 5 million dollars, and the median cost is 350,000 
dollars. Table 4-5B shows the effect that ambient levels of toxics can have on 
the elimination exceedances and on the incidence of cost across the three 
categories. For an ambient level of 50Z, the number of initial exceedences 
would increase to 57Z from 46Z and the number of exceedances eliminated would 
increase to about 80 from 6l. The importance of reducing POTW toxic discharges 
increases as the ambient levels come down to the aquatic water quality criteria. 
Tables 4-5 A and B raise three important questions: 1) to what extent 
should costs be borne in cases where there are no immediate water quality 
problems, 2) to what extent should costs be borne in cases where there are 
immediate water quality problems, but where the problems persist in spite of 
control measures and 3) is it worth the cost to eliminate the exceedances at 
all? These questions are discussed in the following section on benefits. 
4.6 LOCAL BENEFITS OF PRETREATMENT 
This report monetizes some benefits of the pretreatment program. Analysis 
was first limited to recreation benefits, which past studies have indicated 
,are most important. Because it was not clear how to relate degrees of recrea- 
tion activity to ambient toxics levels, the analysis was next limited to cases 
in which all exceedances of federal water quality criteria for toxics were elim 
inated by control options. The basic assumption is that eliminating exceedances 
makes valuable recreation uses feasible. Finally, analysis was limited to a 
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