ee 58 Fed.Reg. 9283.  The antibacksliding rubric, however, cannot evade the requirement that the numeric limits on land application be risk-related--based on reasonably anticipated adverse effects.  The conclusion that current sludge composition is safe absent a showing that alternative sludge composition would not be safe does not justify a mandate to freeze current sludge quality.  In Natural Resources Defense Council v. EPA, 859 F.2d 156 (D.C.Cir.1988), we upheld an antibacksliding system that prevented a retreat by industrial facilities from standards regulating pollutant discharge that had been established in individual permits once the individual permitting process was replaced by national standards.  In that case, the EPA had been authorized in the first place to issue the standards from which backsliding was prohibited, and those standards were based on the same statutory factors as the subsequent national standards.  See 859 F.2d at 201.   In sum, when the statute mandates risk-based regulation, standards from which facilities may not retreat must also be risk-based.15


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We also note another significant distinction between the 99th percentile caps and the antibacksliding regime approved in Natural Resources Defense Council v. EPA.   The Natural Resources Defense Council system was site-specific and required only that each individual facility maintain its existing standards.  See 859 F.2d at 201.   The 99th percentile caps, by contrast, are not tailored to individual facilities.  There will, by statistical necessity, be a certain number of POTWs that currently produce sludge with chromium and selenium concentrations above the 99th percentile, and these facilities must do more than avoid sliding back in order to meet the clean sludge caps.  Without a finding of risk, the EPA is without a basis for imposing the antibacksliding mandate.


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C. The Application Rate and Duration Assumptions


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For all other pollutants except chromium and selenium, the risk-based concentration cap is more stringent than t