Frequent Questions about the Delisting Risk Assessment Software (DRAS)
On this page:
- DRAS Doesn't Work on My Computer
- Export to Table Doesn't Work
- Surface Impoundment Module Requires Corrections
- My Detection Levels for Non-Detected Entries Exceed the Delisting Limits
- How Do I Assess Toxicity Characteristic Leaching Procedure (TCLP) or Leachability in a Liquid Waste Sample?
- DRAS Doesn't Calculate Maximum Allowable Concentrations for Non-Detect Entries. How Do I Determine Them?
- Should I Use the Maximum, Median, or Mean Concentration to Enter into DRAS?
- DRAS Lists Fluoride as a Carcinogen
- What Changes in DRAS if the Carcinogenicity Designation is Changed?
- Aggregate Hazard Index and Cancer Risk Output Table Correction
Some users have reported that DRAS does not work on computers running versions of Windows Vista. The software was written and tested on computers running Windows XP. We are unable to address this issue at this time and recommend that users that have trouble running DRAS in a Windows Vista environment access computers with Windows XP in order to run DRAS.
The Export to Table function in DRAS version 3 does not save to the Table Export (*.html) format. The Export to Table function does save to the MS Excel Tab Delimited Text File (*.txt) format for all tables except the Aggregate Risk and Hazard Quotient Results Table (see explanation below). EPA recommends saving to the MS Excel Tab Delimited Text File (*.txt) format when using the Export to Table function.
It has come to our attention that the surface impoundment module in DRAS version 3.0 has a number of problems. Users reported that changes to the Flow rate of Liquid parameter under Step 2 - Waste Management Unit and Risk/HQ Information had no impact on model results. EPA confirmed this was the case and discovered that this parameter had become disabled in the code. In order to properly process the groundwater pathway for surface impoundments, DRAS needs to be able to apply scaling factors to the base dilution attenuation factors in the database. The surface impoundment calculation for scaling factor requires the "lifetime surface impoundment waste volume" (see RCRA Delisting Technical Support Document, Chapter 2: Estimation of Chemical Releases and Media Concentrations, page 2-15). The "lifetime surface impoundment waste volume" is the annual liquid waste flow rate to the impoundment multiplied by the surface impoundment lifetime (default is 1 year for one-time delisting projects and 50 years for multi-year or on-going delistings).
Since DRAS is not correctly using this parameter, users must either replace the DRAS 3.0 executable file with the September 2010 version (recommended for users with many user-defined customer chemicals of concern in their database) or install the complete updated version of DRAS posted in September, 2010. Existing projects must be exported to file in a separate location on your hard drive and later imported into the revised version of DRAS 3.0 after installation.
For Step 2 - Waste Management Unit and Risk/HQ Information, you must enter the annual liquid waste flow rate (in either cubic meters, cubic yards, or cubic feet) into the Waste Volume parameter box. Be sure to select the correct units for the volume you entered (m3, yd3, or ft3) from the drop down list. The parameter Flow rate of Liquid continues to be disabled.
While reviewing the groundwater issues with the surface impoundment module, we also discovered issues with the volatile emission calculations for surface impoundments. Since the volatile emission calculation requires both the annual liquid waste flow rate and a surface impoundment capacity, the single waste management unit parameter entry (since Flow Rate of Liquid continues to be disabled) will not be sufficient to complete the calculation. Furthermore, the methodology presented in the RCRA Delisting Technical Support Document, Chapter 2: Estimation of Chemical Releases and Media Concentrations does not appear to result in conservative estimates of average volatile emission rates. As a result, EPA recommends preparing an independent calculation for volatile emissions from surface impoundments. The methodologies presented in Chapter 5.0 Surface Impoundments and Open Tanks are the same basic methodologies upon which DRAS was based but provide more conservative results. In particular, we believe the emissions can be estimated with either equation 5-5 solving for KACL (the volatile emission rate for quiescent well-mixed impoundments) or equation 5-12 solving for E (the volatile emission rate for surface impoundments with plug-flow characteristics). The approach for plug-flow results in higher values, but only by a few percent. We believe either will suffice for a delisting evaluation.
You will need several parameters from the DRAS database for each waste constituent including the Henry's Law Constant, diffusivity in air and water, and the constituent concentration in the liquid waste. You will also need the area of your surface impoundment. If you have the volume of your impoundment, but not the area, you can calculate it from equation 2-12 from Chapter 2 of the RCRA Delisting Technical Support Document. Regardless of what the DTSD says about this equation, use the actual capacity of your surface impoundment for V (volume) to get the area of your impoundment. We also recommend using wind speed at 10 meters above the liquid surface (U10) of 5.73 meters per second for the calculation. For converting the Henry's Law constant to the dimensionless version, you can use a gas constant value of 8.21x10-5 atm-m3/g-mol K and a standard temperature of 298 K.
The emission rate will need to be run through a dispersion model to estimate possible downwind concentrations. The methodology described in equation 2-47 from section 18.104.22.168 (Calculation of Downwind Waste Constituent Concentration in Air at the POE Surface Impoundment) of Chapter 2 of the RCRA Delisting Technical Support Document can be used to estimate downwind concentrations. Risk from these downwind concentrations and maximum allowable delisting concentrations can be determined using the methods presented in Chapter 4 (Risk and Hazard Assessment) of the RCRA Delisting Technical Support Document.
EPA insists on an Agency-approved Quality Assurance Project Plan (QAPP) in which we check to see that laboratory reporting concentrations are at or below the maximum allowable concentrations from DRAS 3.0 using region-specific target risk levels and waste-specific info such as the annual volume of waste generated. If this process results in analytical reporting limits that are not sensitive enough, in consultation with our analytical chemist, we look to following resolutions:
Select a commonly available analytical method with greater sensitivity. For example, lower detection limits for PNAs are typical of HPLC Method 8310, compared to Method 8270. If that doesn't work:
Ask the question "Do you expect the constituent to be present in the waste?"
- If so, look for more sophisticated analytical approaches such as GC/MS with Selected
Ion Monitoring (SIMs) etc.
- If not, consider accepting the higher detection limit. You may want to increase the amount of documentation for justifying that the constituent is not likely to be present.
In the conduct of the TCLP test, free liquids in a sample are removed and reserved and only the solids are subject to the leaching procedure. After leaching, the reserved free-liquids are added back to the leachate before analysis. This means that a 100% liquid sample would be analyzed "as is" (as there are no solids to leach) essentially the same as a "totals" analysis.
DRAS Doesn't Calculate Maximum Allowable Concentrations for Non-Detect Entries. How Do I Determine Them?
Please refer to the Limiting Pathways table. The maximum allowable concentrations for both total composition (mg/kg) and leachate (mg/L TCLP) appear regardless of detection status.
The selection of the most appropriate data for input to DRAS is best determined in consultation with the regulatory authority for your delisting decision. For delistings decided by the federal delisting program, EPA typically uses the maximum value of all samples for each individual waste constituent. We have no objection to using statistical parameters; however, we rarely have enough data points to allow for a robust statistical analysis. The more robust the statistical analysis, the more likely we may consider values toward the mean, median, or an upper bound. Poor datasets, with too few observations or nonconforming distributions, will require consideration of upper-bound or maximum values for DRAS. The overall quality of the data must also be considered when selecting data for input. We currently recommend that a prospective petitioner work with us in the case of federal delistings, or their delisting regulatory authority, to develop an appropriate quality assurance project plan including sampling and analytical methodologies and strategies prior to sample collection.
Fluoride should not be listed as a carcinogen in DRAS. The user should change this selection when evaluating fluoride in order to correctly identify this property in model output. In Step 4 - Selecting Chemicals of Concern and Entering Concentrations, the user can scroll across the page and change the entry to noncarcinogen by means of a drop down selection. Even with fluoride mistakenly identified as a carcinogen, DRAS does not actually calculate any carcinogenic risk for fluoride because there are no toxicity reference values (cancer slope factors, etc) for fluoride.
This option in the database is a holdover from previous versions of DRAS and no longer affects the calculations. All exposure calculations are based on toxicity reference data entered. DRAS will choose limiting pathways based on the most conservative result, regardless of the carcinogenicity designation. Carcinogenicity does effect the selection of Dilution Attenuation Factors (DAFs); however, DRAS automatically uses the correct DAF in the calculations. DRAS version 3.0 is pre-loaded with two DAFs for each constituent. One DAF is based on a distribution of peak receptor-well concentrations and is used for non-carcinogenic exposure calculations since negative health effects can occur when exposure concentrations are exceeded for even short time periods. The other DAF is based on a distribution of receptor-well concentrations associated with an upper confidence limit and is used for groundwater exposure to carcinogens since a long-term, chronic exposure is more important than a peak concentration. The two DAFs are often similar and sometimes identical. Nevertheless, both DAFs are pre-loaded into DRAS and the result that predicts the greatest risk or hazard, depending on the selection of target risk and hazard, will be identified by DRAS as the limiting concentration.
The final summation of aggregate Hazard Index and Cancer Risk displayed within the Aggregate Risk and Hazard Quotient Results output table is incorrect. The final aggregate calculation in DRAS version 3.0 does not correctly handle the risk from constituents that were identified as not detected for either total or TCLP concentrations in Steps 4 - Selecting Chemicals of Concern and Enter Concentrations. The waste-constituent specific groundwater and surface pathway risks and hazard quotients displayed within the Aggregate Risk and Hazard Quotient Results Table are correct. Only the summing algorithm at the bottom of the table is not reporting the correct value. Further complicating the problem is that the Export to Table option under the File menu is not working for the Aggregate Risk and Hazard Quotient Results Table. This makes it difficult to export the results to a spreadsheet and enter the correct calculation.
In order to correct the aggregate calculation, the DRAS user must transfer the risk results from the Aggregate Risk and Hazard Quotient Results Table and perform the correct calculation. The correct calculation sums the hazard quotients (and, in a separate sum, the cancer risks) for detected constituents only. A second calculation adds a fraction (0.5) of the risk of constituents that were not detected to the sum of the detected risks. The user must perform this calculation separately from DRAS. Small projects (with few constituents) can be recalculated by hand or by manually entering the results into a spreadsheet.
For large projects, the user may export the Groundwater Pathway Hazard Quotient - , Groundwater Pathway Risk -, Surface Pathway Hazard Quotient -,and Surface Pathway Risk-Tables. The Export to Table function works with these tables and the user can take the columns summing the risk or hazard quotients (the last column in each table) and combine them to calculate the aggregates.
To obtain aggregate results using the Export to Table function with the Groundwater Pathway Hazard Quotient - , Groundwater Pathway Risk -, Surface Pathway Hazard Quotient -,and Surface Pathway Risk-Tables, the following procedure can be used:
Maximize the Groundwater Pathway Hazard Quotient Table after running DRAS and select Export to Table from the File menu. A file-dialog box appears allowing the user to choose where to save the document. EPA recommends changing the file type to MS Excel Tab Delimited Text File (*.txt). This file type seems to be accurately converted from text to the columnar data needed to do the calculations. Save the text file. Export the Groundwater Pathway Risk -, Surface Pathway Hazard Quotient -,and Surface Pathway Risk-Tables in the same manner remembering to give each a distinct filename.
Open each exported text file in a spreadsheet. The remaining directions recommended here correspond to using Microsoft Office Excel 2003TM. The spreadsheet will automatically offer text conversion options and recommends selecting delimited text. Keep this option and choose Next. The spreadsheet then recommends selecting Tab as the delimiting type. Keep this option and choose Next. The spreadsheet then gives the user options for formatting columns. Keep the General option selected and choose Finish. The spreadsheet will now open with all the numeric and text data properly separated into columns.
At this point, it may be clearer to widen some of the columns or reformat the column headings (with the wrap-text option for example) to make them clearer to read. EPA recommends uniformly sorting all columns and rows by the chemical name so that the columns from the individual tables (which otherwise sometimes appear in different orders) will be comparable.
Copy the last column from each of the tables exported and converted from step one into a new spreadsheet document or worksheet along with a column of chemical names. Arrange them as they would be in the DRAS Aggregate Risk and Hazard Quotient Results Table. See Example.
Add columns summing the hazard index results (groundwater and surface) and the cancer results (groundwater and surface).
Identify all non-detect results (i.e. with a background color) so you can easily identify them later.
To calculate the Aggregate Hazard Index or Cancer Risk for detected chemicals only, sum the results from each column. Be sure to subtract the results from non-detects. For example, you can sum all the values from a column with no non-detected values starting at cell B13 and ending at cell B23 by typing in "=@sum(B13..B23)". If cells C16, C17, C19, and C20 were non detect for a column starting at cell C13 and ending at cell C23, you can change the formula to "=@sum(C13..C23)-(@sum(C16,C17,C19,C20))" or "=@sum(C13..C23)-(C16+C17+C19+C23)".
To calculate the Aggregate Hazard Index or Cancer Risk including non-detects, sum the detected results in each column and add one half of the sum of non-detect results. For a column starting at cell G13 and ending at cell G23 with non detects at cells G16, G17, G19, and G20, and the detects-only sum already calculated at cell G27, you can use the following formula: =@sum(G27+(0.5*(@sum(G16,G17,G19,G20)))
Finally, add the groundwater pathway Aggregate Hazard Index to the surface pathway Aggregate Hazard Index to get the total Aggregate Hazard Index. Add the groundwater pathway Aggregate Cancer Risk to the surface pathway Aggregate Cancer Risk to get the total Aggregate Cancer Risk.