Using these profiles and preliminary data about the concentration and frequency of these pollutants in sewage sludge, the EPA exempted from regulation those pollutants that presented no risk to human health or the environment at the highest observed concentration and deferred consideration of those for which it had insufficient data to make this risk determination.  See 58 Fed.Reg. 9264.  It initially proposed limits for 25 pollutants in sludge to be applied to land, see 54 Fed.Reg. 5761 (Table III-4), and concluded by regulating ten heavy metals in sludge applied to land in the final Round One sewage sludge regulations, see 58 Fed.Reg. 9392.  These portions of the regulations establish numeric limits on pollutants in sludge that is applied to agricultural land, forests, public contact sites, or reclamation sites.

II. THE LAND APPLICATION REGULATIONS

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In establishing the limits for the ten regulated heavy metals pollutants--arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc--the EPA generated two sets of data.

A. The Underlying Data

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The first data set on the ten pollutants describes their current concentration in sewage sludge.  The data is culled from the EPA's National Sewage Sludge Survey ("NSSS"), in which the EPA sent questionnaires to 479 POTWs--out of a national total of 11,407--and performed sampling and analysis at 208 of the 479.  See 58 Fed.Reg. 9269.2  Based on this sampling and analysis, the EPA identified the pollutant concentrations in current sludge output, and calculated 99th-percentile concentration numbers:  the pollutant concentration not exceeded by 99% of the sludge samples in the NSSS ("99th percentile caps").


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The second data set on the ten pollutants is risk-based.  Under its risk-based analysis, the EPA modelled 14 pathways by which pollutants in land-applied sludge could affect human health or the environment and then identified a hypothetical "highly exposed individual" ("HEI") for each pathway and calculated a pollutant limit that would