1. Question: Who is required to use the Cumulative Risk Calculator?
Answer: Parties seeking to achieve compliance in the Iowa Land Recycling Program (LRP) are required to use the cumulative risk calculator, unless they are appealing to a site-specific standard per subrule 567 IAC 137.6(10) or (11) or institutional and/or technological controls will prevent any exposure to contaminants. Subrules 567 IAC 137.6(10) & (11) prescribe requirements for site-specific standards that are not specifically provided by rule.
2. Question: Do acceptable cumulative risk levels mean I do not have a problem at a site?
Answer: Not necessarily. In the LRP other compliance criteria must be achieved to attain a no further action determination. Outside of the LRP, other factors (e.g., land use, location of nearby drinking-water wells) will be assessed to determine if a significant problem might exist and, if it might, additional site characterization will likely be necessary to make a final determination.
3. Question: Can the cumulative risk calculator be used for purposes outside of the Iowa Land Recycling Program?
Answer: Yes, the cumulative risk calculator may be used as a screening tool in determining whether environmental sample results suggest a potentially significant problem. However, compliance with cumulative risk criteria outside of the LRP does not necessarily in itself indicate that no significant problem exists.
4. Question: When am I required to report identification of contamination?
Answer: Reporting of contamination is required when it constitutes a hazardous condition per chapter 567 IAC 131. In the case of contaminant identification from an environmental assessment, the DNR recommends reporting of contamination that is greater than statewide standards for soil and groundwater
5. Question: Do I need to calculate cumulative risk for a site where no contamination remains above a statewide standard?
Answer: In the LRP the answer is "yes". It is possible that cumulative risk criteria could not be met despite no exceedance of a statewide standard. (The site would not be eligible for the LRP initially without contamination above a statewide standard.) For a site for which an environmental assessment is being conducted that is not enrolled in the LRP, calculation of cumulative risk is not necessary since reporting of contamination is not recommended unless a statewide standard is exceeded.
6. Question: How are naturally occurring levels of arsenic in soils addressed in statewide standards and cumulative risk calculations?
Answer: The statewide standard for arsenic in soil is a risk-based value of 1.9 mg/kg. However, the naturally occurring background level of arsenic in soil is typically larger than the risk-based value. A site-specific background standard for arsenic in soil can be established in the LRP as the mean plus 3 standard deviations of a minimum of 10 background soil samples.

Based on a statewide sampling of soils conducted in 2004, a universal background level for arsenic of 17 mg/kg has been input into the cumulative risk calculator. This universal background level represents the mean plus 3 standard deviations of 532 shallow (i.e., <1 ft. deep) soil samples collected throughout the state. As a result, the DNR has made it a policy not to require any response to arsenic in soils less than 19 mg/kg — the universal background level plus the risk-based statewide soil standard. (Based on similar logic, an actionable level of 3.1 mg/kg is used for beryllium.)

Note: A child's exposure to soil containing 19 mg/kg of arsenic at the soil exposure rates used for statewide standards would result in only about 40% of the arsenic ingestion that would result from a child drinking 1 liter/day of water with arsenic at the drinking water standard. If arsenic is the only contaminant of concern and soil is the only medium of exposure, up to 39 mg/kg of arsenic yields an acceptable cumulative risk in a residential setting (13 mg/kg for beryllium). The level of arsenic exposure from soil at 39 mg/kg is still less than a child's exposure from water with arsenic at the MCL. Beryllium exposure via soil compared to water at the MCL is even less than it is for arsenic.

Universal background levels are applicable statewide, unlike site-specific background standards that apply only to the site for which they were derived. Universal background levels have only been established for several inorganic chemicals in soils, including arsenic and beryllium, and are built into the risk calculator. The cumulative risk calculator subtracts the larger of the concentration equal to the site-specific background standard or the universal background level from the exposure point concentration for determining cumulative risk.
7. Question: How are statewide standards determined?
Answer: Statewide standards for groundwater in a protected water source are equivalent to drinking water standards. Statewide standards for groundwater in a nonprotected groundwater source are similar to drinking water standards with fewer built-in factors of safety, but are still considered to be safe in the unlikely event a nonprotected groundwater source is used for a drinking-water supply.

Statewide standards for soil are calculated based on the toxicity of the contaminant and an assumed amount of exposure to soil by a site resident. A child resident is assumed to ingest 200 mg of soil 350 days out of the year for 6 years. The same person as an adult resident is assumed to ingest 100 mg of soil, 350 days out of the year for an additional 24 years. (Note: a standard aspirin weighs 325 mg.) In addition, exposure by dermal contact is assumed from soil covering the head, hands, forearms, lower legs, and feet of a child resident and head, hands, forearms, and lower legs of an adult resident both for 350 days a year. (Note: these are recommended exposure values from the USEPA.)
8. Question: Can the cumulative risk calculator results be appealed?
Answer: Yes, especially results that were based in significant part on factors (eg., toxicity values) that were selected by guidance in lieu being prescribed by rule. (The "Background" information under "Help" describes where guidance was used in lieu of rules.) It is also possible to propose site-specific exposure scenarios, which, if approved by the DNR, would necessitate adjustments to the risk calculator results.
9. Question: What soil depth applies to the site resident exposure scenario?
Answer: The focus of the site resident soil exposure is shallow soil, 0 – 6 inches deep, in which incidental exposure is likely. However, the statewide standards for soil, the exposure assumptions of which form the basis of the site resident scenario, are applicable to soil to a depth of about 10 feet. (The 10-feet depth may be adjusted based on site-specific conditions.) The logic behind the 10-ft. depth is that normal excavation, including basements, in a residential area does not exceed a depth of about 10 feet. It is common practice for the soil excavated for basements to be spread on the surface where direct contact is likely. The significance of contaminants in soil at depths greater than 6 inches depends on several site-specific factors like: current and potential future land use, source of contaminants (e.g., known or unknown, surface or buried), how well-defined contamination is (horizontally and vertically), etc.

Typically compliance with cumulative risk criteria in the LRP using the site resident scenario is sought in situations where unrestricted use (i.e., no institutional control) of the site is desired. In some cases residential use of a site may be approved in the LRP by attaining compliance criteria for residential use of shallow soil and placing institutional controls (e.g., limiting excavation) on contamination in deeper soils that does not pass residential exposure criteria. A site-specific deep residential soil standard, equal to ten times the statewide soil standard, also exists for soils generally greater than 10 feet deep (paragraph 567 IAC 137.6(6) "a"). An institution control is required with this standard.
10. Question: Does compliance with statewide standards in a medium (e.g., soil) ensure that the medium will not be a source of contamination affecting another medium (e.g., groundwater)?
Answer: No, statewide standards are intended to be protective only for direct exposure to the medium for which the standard has been established. In the LRP potential for cross-media transfer of contaminants is addressed by direct sampling of the potential receiving medium. For example, sampling of groundwater may be required based on identification of mobile contaminants in soil, even if statewide standards for soil are not exceeded. See Question #12 regarding potential cross-media migration of contaminants from soil or groundwater to vapors.
11. Question: Can statewide standards and the cumulative risk calculator be used for underground storage tank (UST) sites?
Answer: Statewide standards and the cumulative risk calculator should not be used for leaking UST sites. UST sites are under the jurisdication of the DNR UST program. The LRP and UST programs have the same groundwater standards for the actual groundwater receptor in the UST program and protected groundwater in the LRP. However, all other forms of standards/compliance criteria have different bases in each program. In particular, the UST program specifies standards based on cross-media transfer of contaminants (e.g., soil leaching to groundwater), while the LRP specifies only standards/compliance criteria for the medium in which exposure may occur.
12. Question: Under what circumstances should exposure to contaminants in air be assessed?
Answer: Air exposure is often difficult to evaluate. The risk-based levels for contaminants in air tend to be extremely small. Many contaminants don't have readily available information to enable assessment risk via inhalation. Inhalation of contaminants is typically an issue only in confined spaces (e.g., basements), which may not exist now, but might in the future. Unlike water and soil, direct measurement of contaminant concentrations in air at the point of exposure is often not possible. Even when a confined space exists, it is often not practical to obtain representative air samples. Therefore, the DNR recommends a screening assessment of potential air exposure before a substantial effort is made to address the air route of exposure.

Factors that will be evaluated in deciding whether to look closer at potential air contamination include:
  • The volatility of the contaminant.
  • The relative toxicity of the contaminant.
  • The magnitude (i.e., the concentration and extent) of soil and/or groundwater contamination that is a source of air contaminants.
  • The permeability of soils.
  • Potential for biodegradation.
  • The existence of subsurface structures, now and potentially in the future.
  • The existence of subsurface conduits, now and potentially in the future.
  • The number and characteristics of people potentially exposed.
In the LRP the DNR will make the final determination on the need to assess potential air contamination. The following guidelines are recommended for preliminary screening of potential air concerns. Subsequent screening is likely to involve limited soil-gas sampling.

Parameter (1) Value Magnitude of Contamination (2) Possible Vapor Concern? (3)
Vapor Pressure (mm of Hg) <1 mm Hg Any No
  1-100 mm Hg Small No
    Medium Maybe
    Large Yes
  >100 mm Hg Small Maybe
    Medium Yes
    Large Yes
Henry's Law Constant (unitless) < 10-5 Any No
  10-5- 10-3 Small No
    Medium Maybe
    Large Maybe
  10-3- 10-1 Small Maybe
    Medium Maybe
    Large Yes
  > 10-1 Small Maybe
    Medium Yes
    Large Yes

1. Chemical-specific values of vapor pressures and Henry's Law Constant can be found under "Chemical-Specific Information" on the toolbar.

2. The classification of magnitude of contamination is based on the area of soil or groundwater contamination relative to the size of actual or potential subsurface structures at the site and the contaminant concentration relative to its toxicity. The applicable statewide standard can be used as a gauge of toxicity of a chemical. For example, an isolated soil sample would be considered small unless the contaminant concentration was extremely high relative to the statewide standard for soil.

3. The DNR will assess situations classified as "Maybe" in greater detail before making the final determination on whether vapors need to be assessed for sites enrolled in the LRP.