se: ATSDR has reviewed a number of documents relating to various aspectsof the aluminum and nickel smelting processes including the referenced DOE report on the"Metals Recovery Program" . While that document does discuss the types of documentsthat were reviewed, neither that document or the Phase II Report  contain specificreferences or bibliographies. Consequently, ATSDR cannot determine if we have reviewedthose same documents. However, ATSDR did review, and the Health Consultation doesreference, the report produced by that investigative team (as well as the report of anotherteam investigating related issues) [1,2]. ATSDR has not reviewed the "1972 study ofradionuclides in scrap." Although, it is mentioned, as stated by the commentor, in the PhaseII Report, it is not specifically referenced so ATSDR has no way of finding or tracking thatdocument.
As stated in the Health Consultation, smelting activities in C-746A did apparentlycommence in the 1950's. However, the summary statement describing the years of smeltingoperation are the only reference to smelting in the 1950's (other documents suggest thatearlier smelting activities mainly involved aluminum). Based on available reports, the yearsof significant nickel smelting were restricted to 1977 to 1986 (inclusive). While limitedquantities of nickel may have been smelted during other years, the exposure assessment inthis consultation focuses on those years of maximum nickel smelting (1977-86) and airborneemissions. As the airborne nickel concentrations estimated using conservative (healthprotective) assumptions during years of peak smelter operation are below levels of healthconcern, emissions during any earlier years of lower emission concentrations are also below levels of public health concern.
ATSDR Response: In determining the contaminants of concern for the Public CommentRelease of the Public Health Assessment, ATSDR did not evaluate specific sources of nickelor lead. That evaluation was based on the measured concentration of the contaminants invarious media, such as soil, water, and sediment. The rationale for conducting this healthconsultation (this appendix) is that we did not include metal particulates released fromsmelting operations in assessing airborne contaminant distributions. The form of nickelevaluated in the Public Comment Release of the Public Health Assessment for the groundwater, surface water, soil and sediment, and biota pathways is total nickel. For the airpathway in this consultation (this appendix), the chemical form is, as referenced, nickeloxide(s); however, the nickel health comparison values used are applicable to elemental ormetallic nickel, and both soluble and insoluble forms.
With regard to the chemical form of nickel compounds, only the nickel carbonyl form has asignificantly higher toxicity than other forms (and only via inhalation). As nickel carbonyl isnot thermodynamically stable in soil, air, or water, it will be converted to other morethermodynamically stable forms, such as nickel hydrates in water and nickel ferrite in soil. Nickel also tends to strongly adsorb onto larger particles in both soil sediment whichlimits its subsequent mobility . Although in some cases, it may be important to assess thepotential distribution and concentrations of the most toxic chemical forms, the limitedchemical stability of nickel carbonyl precludes significant off-site transport and exposure.For the determination of the off-site contaminants of concern at PGDP, if the totalenvironmental nickel concentrations are below levels of health concern, it is not necessary todelineate the specific sources of the nickel.
With regard to the total nickel distribution in the environment surrounding PGDP, there area number of potential sources including naturally occurring background levels in all media(nickel is the 24th most common element in the earth's crust) . In addition to the PGDPsmelter and steam plant and the TVA facility, minor sources include forest fires, volcanoes,fuel oil combustion, and other manufacturing processes. Any measurement of nickel in an