Docket Report

EPA Region 8, Solid and Hazardous Waste

ConocoPhillips Billings Refinery, Billings, Montana Delisting Petition

RE: Risk Evaluation for Management in a Subtitle D Landfill

The Environmental Protection Agency (EPA) and the ConocoPhillips Billings Refinery used the 

Delisting Risk Assessment Software (DRAS) version 3.0.34 to evaluate the petitioned waste. 

Both EPA and ConocoPhillips entered site-specific parameters into DRAS including maximum 

annual waste volume and maximum observed total and leachate concentrations of hazardous 

constituents in the waste. EPA used Region 8 specific target cancer and hazard levels (1x10-6

excess aggregate cancer risk, hazard index of 0.1) to evaluate the waste and ConocoPhillips ran 

the DRAS program at the model default hazard levels of 1x10-5excess aggregate cancer risk, and 

hazard index of 1.0. EPA independently ran the DRAS at the same hazard levels as 

ConocoPhillips to confirm their results. Identical risk assumptions and model inputs produced 

identical outputs for both the ConocoPhillips and EPA DRAS model runs; the ConocoPhillips 

program output is attached.

In addition to the lower target risk levels EPA Region 8 set the DRAS nominal total limits to 

1,000 mg/kg rather than the DRAS model default of 10,000 mg/kg (1%). Because this is a 

potential multiyear delisting Region 8 felt the lower target risk limits were appropriate. Use of a 

nominal upper total concentration of 1,000 mg/kg also defines a maximum nominal TCLP limit, 

since the latter can be no more than 1/20th the total value to conform to the TCLP analytical

procedure using a 20:1 dilution of a solid sample. This results in several compounds having 

TCLP concentration limits of 50 mg/L. Constituents that were analyzed in the ConocoPhillips 

samples but are not a preloaded in the DRAS model were evaluated at the nominal limit of 1000 

mg/kg, resulting in a TCLP limit of 50 mg/L..

Some geological considerations of prevalence as well as known potential risks were taken into 

account. The nominal upper limit for barium and zinc was set at 10,000 mg/kg due to low 

overall toxicity and higher natural abundance. For beryllium, cadmium and mercury with high 

toxicities and low prevalence, low overall total limits (25-50 mg/kg) were chosen. Arsenic is the 

only constituent that did not meet the target risk criteria for TCLP. For arsenic a risk level of 10-

5

; HQ=1.0, was selected as an appropriate limit and one that is very constraining (roughly twice 

the current maximum). This limit will maintain levels close to the present. Two additional 

constituents, naphthalene and 1,4-dioxane, have DRAS limits set at 10-5; HQ=1.0, to allow for 

future variability in analytical detection limits. 

If the allowable risk levels exceed the RCRA definitions of characteristic hazardous levels 

(flammability, corrosivity, reactivity and especially toxicity) the latter levels prevail. Hazardous 

concentration limits for toxicity (40 CFR 261.24) are based on TCLP analysis. Concentration 

limits for nine constituents are based on the TCLP limits for RCRA hazardous toxicity 

characteristics found in 40 CFR 261.24.

The aggregate cancer risk and hazard index for the petitioned waste meet the criteria set by 

Region 8. The risk analysis estimated the total aggregate cancer risk from constituents detected 
in the waste to be 6.32 x 10-6

. The risk analysis estimated individual constituent cancer risks 

well below EPA Region 8’s target of 1 x 10-6. With the exception of arsenic, individual cancer 

risks ranged from 10-8to 10-13. Arsenic total cancer risk is estimated to be 5.24 x 10-6. 

Furthermore, the analysis estimated total aggregate hazard index (a sum of hazard quotients) 

from constituents detected in the waste to be 0.0376, well below EPA Region 8’s target of 0.1. 

Note that the final aggregate calculation does not agree with the final table Aggregate Risk and 

Hazard Quotient Results from DRAS version 3.0. The Agency noted a problem with aspects of 

the DRAS version 3.0 aggregate calculation related to the handling of non-detects. The 

calculation was redone on a spreadsheet and is attached.