Introduction: Subrule 567 Iowa Administration Code 137.10(7) specifies cumulative risk criteria that must be complied with to acquire a no further action certificate in the Iowa Land Recycling Program (LRP). Cumulative risk is the summation of cancer and noncancer risks, determined separately, based on exposure to multiple contaminants from the same medium and exposure of the same individual to contaminants in multiple media. This cumulative risk calculator may be used to demonstrate compliance with the cumulative risk criteria of the LRP.

The cumulative risk criteria are: cumulative cancer risk shall not exceed 1 in 10,000 and a noncancer health risk to the same target organ shall not exceed a cumulative hazard quotient (a.k.a. hazard index) of one. (A target organ is a major organ or system of the human body, such as the liver and respiratory system.)

This risk calculator determines cumulative cancer and noncancer risks based on three potential exposure scenarios involving three categories of individuals who may be exposed to contaminants in groundwater, soil and/or air. These off-the-shelf exposure scenarios may be used in the appropriate land-use setting as described below for each scenario.

Other site-specific exposure scenarios may be used if approved by the department; however, the risk calculator cannot be used to calculate risks directly for other scenarios. Simple adjustments to output from the risk calculator may be possible in such situations and the department will advise an LRP participant of such use with the approval of a non-standard (i.e., not specifically provided in rule), site-specific exposure scenario.

Exposure Scenarios

The three classes of individuals in the risk calculator are site residents, site workers, and construction workers, described as follows:

  • The site resident is an individual who is assumed to live at the site for 30 years starting at birth. This scenario may include exposure to groundwater from an on-site drinking-water well. The site resident is assumed to be exposed to soil via both ingestion and dermal contact. Exposure to contaminants in air may also be included in this scenario when warranted. The site resident scenario must be used when compliance with background or statewide standards is sought with no associated land-use restriction. It must also be used when appealing to the site-specific standard involving cumulative risk for residential exposure to soils per subrule 567 IAC 137.6(7).
  • The site worker is an adult who is assumed to work at the site for 25 years. The site-worker scenario may include drinking water from an on-site well. This scenario includes exposure to soil less than 2 feet deep by both ingestion and dermal contact. Exposure to contaminants in air may also be included in this scenario when warranted.The site worker scenario, in combination with the construction worker scenario, may be used to comply with site-specific standards in any nonresidential setting. (See the definition of "nonresidential land-use" in the "Glossary" under "Help") In particular, the site worker scenario must be used when appealing to site-specific soil standards, based on soils less than 2 feet deep and nonresidential land use, per paragraph 567 IAC 137.6(6)"b". The construction worker scenario must be used to address soil deeper than 2 ft. in a nonresidential setting.
  • The construction worker is an adult who is assumed to work in construction involving excavation at the site for 200 days during 1 year. The construction worker is assumed not to drink contaminated groundwater from the site, but is exposed to soil greater than 2 feet deep via both ingestion and dermal contact. Exposure to contaminants in air may also be included in this scenario when warranted. As previously described for the site worker, the construction worker scenario may be used in combination with the site worker scenario to comply with site-specific standards in any nonresidential setting. The construction worker scenario must be used when appealing to site-specific soil standards, based on soils greater than 2 feet deep and nonresidential land use, per paragraph 567 IAC 137.6(6)"b".

Exposure Assumptions: Table No. 1 summarizes the exposure assumptions used in the risk calculator for each of the 3 categories of exposed individual and 3 types of environmental media.

Table No. 1
Exposure Assumptions
Site Resident 567 IAC 137.5
Formula I
Table I (Protected) with the following exceptions:
AT = 6 yrs. (noncancer)
EFc = 365 days/yr. (cancer & noncancer)
EFa = 0 days/yr. (noncancer)
EDa = 24 years (cancer)
(See Note #1)
567 IAC 137.5
Formula I
Table II
567 IAC 137.6
Formula II
AF = 2.333 (cancer)

Formula III
AF = 1 (noncancer)
Site Worker 567 IAC 137.5
Formula I
AT = 25 yr. (noncancer)
ERa=1 l/day
EFa = 225 days/yr
(See Note #2)
567 IAC 137.5
Formula I
Table I
567 IAC 137.6
Table III(< 2ft.)
567 IAC 137.6
Formula II
AF = 13.627 (cancer)

Formula III
AF = 4.867 (noncancer)
Construction Worker Not Included 567 IAC 137.5
Formula I
Table I
567 IAC 137.6
Table III(> 2ft.)
567 IAC 137.6
Formula II
AF = 383.25 (cancer)

Formula III
AF = 5.475 (noncancer)


  1. The groundwater exposure-duration assumptions used for the site-resident groundwater scenario have been established to be consistent with the site-resident soil scenario. The site-resident soil scenario is consistent with the statewide standards for soils. As such, site-resident groundwater exposures assume 6 years of childhood exposure only for assessing noncancer risks and 24 years of adult exposure in addition to the 6 years of childhood exposure for assessing cancer risks. (Statewide standards for groundwater are based on 70 years of exposure by an adult only, which is inconsistent with the 30-year child plus adult residence assumed for statewide soil standards.)
  2. The water exposure scenario for the site worker is not specified in the LRP rules. Therefore the exposure assumptions used in this scenario constitute guidance only.
  3. Values for the adjustment factors (AFs) are not specified by rule and therefore constitute guidance only. AFs have been determined based on relative hours of exposure compared to a baseline of 365 days/yr., 24 hr/day for 70 years for a cancer outcome and 365 days/yr., 24 hr/day for 1 year for a noncancer outcome as follows.
    Individual Outcome Days/yr Years Hrs/day AF
    Site Resident cancer 365 30 24 2.333
    noncancer 365 1 24 1
    Site Worker cancer 225 25 8 13.627
    noncancer 225 1 8 4.867
    Construction Worker cancer 200 1 8 383.25
    noncancer 200 1 8 5.475

Toxicity Information

Table No. 2 lists the sources of toxicity information used for determining standards and calculating cumulative risks in order of priority.

Table No. 2
Maximum Contaminant Level (MCL) MCL, DWS & HA Yes
Lifetime Health Advisory Level (HAL) DWS & HA Yes
Cancer Group 1 - IRIS Yes
2 - DWS&HA No
3 - HEAST No
4 - Default "D" Yes (2)
(Chronic) Oral Reference Dose (RfDo) 1 - IRIS Yes
2 - DWS&HA No
3 - HEAST No
4 - MRL No (3)
5 - RAIS No
(Chronic) Dermal Reference Dose (RfDd) 1 - RAGS "E" No (4)
Oral Slope Factor(SFo) 1 - IRIS Yes
2 - DWS&HA No (5)
3 - HEAST No
4 - RAIS No
Dermal Slope Factor(SFd) 1 - RAGS "E" No (4)
(Chronic) Inhalation Reference Concentration (RfC) 1 - IRIS Yes
2 - HEAST No
3 - MRL No (3)
4 - RAIS No
Inhalation Unit Risk (UR ) 1 - IRIS Yes
2 - HEAST No
3 - RAIS No
Dermal Absorption Factor (Abs) 1 - RAGS "E" No (6)
2 - RAIS No (7)
Target Organ IRIS, HEAST, MCL & MRL Yes (IRIS) No (others) (8)
SOURCE REFERENCES for Sources Listed in Table No. 2
DWS & HA 2006 Edition of the Drinking Water Standards and Health Advisories, EPA 822-R06-013, Office of Water U.S. Environmental Protection Agency, Washington, DC, Summer 2006
Internet Link:
IRIS Integrated Risk Information System, U.S. Environmental Protection Agency, Office of Research and Development, National Center of Environmental Assessment
Internet Link:
HEAST Health Effects Assessment Summary Tables FY 1997 Update, EPA-540-R-97-036,PB97-921199, U.S. Environmental Protection Agency, July 1997
MCL Drinking Water Contaminants & MCLs
Internet Link:
MRL Minimal Risk Levels, Agency for Toxic Substances and Disease Registry, December 2006
Internet Link:
RAGS"E" Risk Assessment Guidance for Superfund, Volume I: Human Health Evaluation Manual (Part E, Supplemental Guidance for Dermal Risk Assessment) Final, EPA/540/R/99/005, Environmental Protection Agency, Washington DC, July 2004
Internet Link:
RAIS Risk Assessment Information System, U.S. Department of Energy (DOE), Office of Environmental Management, Oak Ridge Operations Office, February 2006
Internet Link: ???


  1. Table No. 3 lists those chemicals with associated toxicity values of reference dose (RfD), oral cancer slope factor (SF), inhalation reference concentration (RfC), and inhalation cancer unit risk (UR) that are not from IRIS. The associated toxicity values may be challenged since IRIS is the only specific source for such information given by rule.
  2. A default Cancer Group "D" is assigned a chemical that does not have a cancer group classification provided by one of the three sources listed, per paragraph 567 IAC 137.5(3)"b". As a matter of policy, a different classification may be made based on the recommendation of the Iowa Department of Public Health. This has been only been done for methyl tert-butyl ether (mtbe), which has been classified as a Cancer Group "C" and tetrachloroethylene, which has been classified as a Cancer Group "B".
  3. The MRL for chronic duration is used for the RfDo or RfC. If a chronic MRL does not exist, the MRL for the intermediate duration divided by 10 is used for the RfDo or RfC. The factor of 10 is a typical adjustment used to account for a change in duration from intermediate to chronic.
  4. The dermal reference dose (RfDd) and dermal cancer slope factor (SFd) have been derived from the oral reference dose (RfDo) and oral slope factor (SFo), respectively, based on GI absorption values (ABSGI) in accordance with recommendations in Section 4.2 and Exhibit 4-1 of RAGS "E". ABSGI is the portion of the administered oral dose (for which the RfDo or SFo was determined) that was absorbed by the study animal. The dermal absorption factor (Abs) accounts for the dermal dose that is absorbed. Therefore, if the ABSGI is less than 100% of the actual absorbed oral dose that caused the adverse effect is less than he administered dose for which the oral RfD oral SF is based. The adjustment to the dermal RfD and dermal SF is made by multiplying the RfDo by the ABSGI or dividing the SFo by the ABSGI. RAGS "E" recommends that no adjustment be made if the ABSGI is greater the 50%. Exhibit 4-1 in RAGS "E" provides ABSGI factors for a variety of organic and inorganic compounds. No adjustments are recommended for organic compounds, i.e., it is assumed that RfDd = RfDo and SFd = SFo for all organic compounds. Adjustments have been made to RfDo or SFo to derive RfDd and/or SFd in accordance with values of ABSGI from Exhibit 4-1 in RAGS "E" for inorganic compounds as follows:
    Compound ABSGI
    Antimony 0.15
    Barium 0.07
    Beryllium 0.007
    Cadmium (soil) 0.025
    Chromium(III) 0.013
    Chromium (VI) 0.025
    Manganese 0.04
    Nickel 0.04
    Silver 0.04
    Vanadium 0.026
  5. An oral cancer slope factor (SFo) is back-calculated from the 10-4 Cancer Risk from the DWS&HA based on Formula I in paragraph 567 IAC 137.5(3)"a" as follows: SFo = 0.0035 ÷ (10-4 cancer concentration in mg/l from DWS&HA). Units = (mg/kg/day)-1
  6. Dermal absorption factors (Absd) have been obtained from Exhibit 3-4 of RAGS "E". Exhibit 3-4 also recommends Absd = 0.1 for semi-volatile organic compounds that are not otherwise specified. Organic compounds have generally been classified as semi-volatile based on low (i.e., < 0.001) Henry's Law (unitless) coefficients and/or low (<0.1) vapor pressure of the compound.
  7. For volatile organic compounds (VOCs) and inorganic compounds, Absd values from the RAIS source have been used, which are generally Absd = 0.01 for VOCs and Absd = 0.001 for most inorganic compounds.
  8. All target organs for a chemical from all four sources have been included. The Department of Public Health (DPH) has provided oral target organs for some of the chemicals for which target organs were not provided by any of the four listed sources. Assignment of target organs sometimes involves judgment. Therefore, the DPH can be requested to conduct a review of the appropriateness of a target organ assignment.
Table No. 3
Chemical Rfd SF RfC UR
Acetone - - MRL -
Acrylonitrile HEAST - - -
Alachlor - HEAST - -
Aldicarb Sulfoxide DWS&HA - - -
Atrazine - HEAST - -
Barium - - RAIS -
Benzo [a]anthracene - RAIS - RAIS
Benzo [a]pyrene - - - RAIS
Benzo [b]fluoranthene - RAIS - RAIS
Benzo [k]fluoranthene - RAIS - RAIS
Beryllium - RAIS - -
Bis (2 chlororthyl)ether - - - MRL
Butylbenzene , n- DPH - - -
Boran & Borates - - HEAST -
Bromacil DWS&HA - - -
Bromochloromethane DWS&HA - - -
Butyl Benzyl Phthlate - RAIS - -
Carbazole - HEAST - -
Carbon tetrachloride - - MRL -
Chlorine - - RAIS -
Chlorobenzene - - HEAST -
Chlorodibromoethane - RAIS - -
Chloroform - RAIS MRL -
Chloromethane DWS&HA HEAST - HEAST
Chlorothalonil - HEAST - -
Chlorotoluene , p DWS&HA - - -
Chrysene - RAIS - RAIS
Chromium (total)(water) DWS&HA - - -
Cyanazine DWS&HA HEAST - -
DDD RAIS - - -
Diazinon DWS&HA - MRL -
Dibenz [a,h]anthracene - RAIS - RAIS
Dibromomethane HEAST - - -
Dichlorobenzene, 1,2- - - HEAST -
Dichlorobenzene, 1,3- DWS&HA - - -
Dichlorobenzene, 1,4- DWS&HA HEAST - -
Dichlorodifluoromethane - - HEAST -
Dichloroethane , 1,1- RAIS - HEAST -
Dichloroethane , 1,2- RAIS - MRL -
Dichloroethylene , 1,1- - RAIS - RAIS
Dichloroethylene , 1,2-cis- DWS&HA - - -
Dichloroethylene , 1,2-trans- - - MRL -
Dichloropropane , 1,2 MRL HEAST - -
Dimethrin DWS&HA - - -
Dimethyl methylphosphonate DWS&HA - - -
Dinitrotoluene , 2,6- DWS&HA - - -
Dioxane , 1,4- MRL - MRL -
Disulfoton - - MRL -
Epichlorohydrin DWS&HA - - -
Ethylene Thiourea - HEAST - -
Formaldehyde - - MRL -
Hexachlorocyclohexane , alpha MRL - - -
Hexachlorobutadiene DWS&HA - - -
Hexachlorocyclohexane , gamma - HEAST - -
Hexachloroethane - - MRL -
Hexane, N- HEAST - - -
Indeno [1,2,3-cd]pyrene - RAIS - RAIS
Lead See note - - -
Malathion - - MRL -
Mercury DWS&HA - - -
Methylphenol , 4 HEAST - - -
Methyl Isobutyl Ketone HEAST - - -
Methy tert-Butyl Ether MRL - - -
Methylene Chloride - - HEAST -
Nickel - - MRL -
Nitrobenzene - - HEAST -
Nitrophenol , p- DWS&HA - - -
Nitrosodiphenylamine , N- RAIS - - -
Octyl Phthalate, di-N- HEAST - - -
Phorate HEAST - - -
Simazine - HEAST - -
TCDD, 2,3,7,8- DWS&HA HEAST - -
Terbufos DWS&HA - - -
Tetrachoroethylene - RAIS MRL RAIS
Thallium DWS&HA - - -
Toxaphene DWS&HA - - -
Trichloro-1,2,2-trifluoroethane,1,1,2- - - HEAST -
Trichlorobenzene , 1,2,4- - - RAIS -
Trichloroethane , 1,1,1- DWS&HA - RAIS -
Trichloroethylene DWS&HA RAIS RAIS RAIS
Trichlorofluoromethane - - HEAST -
Trichlorophenol , 2,4,6- DWS&HA - - -
Trichloropropane , 1,2,3- - HEAST - -
Trimethylbenzene , 1,2,4- RAIS - RAIS -
Trimethylbenzene , 1,3,5 RAIS - RAIS -
Vanadium HEAST - - -
Note: A reference dose for lead has been back-calculated from the 400 mg/kg site-specific non-residential soil standard for purposes of cumulative risk calculation.

Procedure for Determining Toxicity Values Not Available from Standard References: When toxicity values are not available from one of the sources shown in Table 2, the Iowa Department of Public Health (DPH) will be consulted in accordance with paragraph 567 IAC 137.5(3)"c". If, based on their research, the DPH can confidently recommend an appropriate toxicity value, that value will be utilized until subsequently superseded by new information or until such time that the value is challenged and demonstrated to be inappropriate. If, after a reasonable effort in researching a chemical, the DPH is unable to recommend an appropriate toxicity value, the associated risk assessment will not be required. Such a determination will not preclude the department from imposing requirements associated with free product and gross contamination pursuant to subrule 567 IAC 137.9(6).

Background Contaminants : Subrule 567 IAC 137.10(7) states "Risks associated with background levels of contaminants shall not be included in the cumulative risk determination. Background levels of contaminants shall be determined in accordance with subrule 137.10(4) or, if approved by the department, by the use of generally available information on background levels of contaminants." The intent of this provision is to avoid penalizing LRP participants for contaminants that are not the result of a release at their site.

There are three forms of background contaminants — 1) naturally occurring, 2) man-made of widespread extent with no specific source known (e.g., roadside fallout from vehicle emissions), and 3) man-made from localized activities. The LRP gives background credit only to the first two forms listed above. The DNR will attempt to identify the party(ies) responsible for man-made contamination from localized activities and require them to address their share of the contamination. If a viable off-site responsible party cannot be identified, the DNR will meet with the LRP participant to assess alternatives. With the exception of lead in soils, site-specific background levels or universal background levels contaminants may be addressed in the cumulative risk calculator. The risk calculator automatically subtracts the larger of the universal background level, which is built into the risk calculator, or the site-specific background level that has been input by the user, if any, from the exposure point concentration to calculate risk.

Site-specific background levels can be entered on the contaminant input screen of the cumulative risk calculator. Paragraphs 567 IAC 137.10(4)"b" and "c" prescribe the processes for determining site-specific background standards for soil and groundwater, respectively. A site-specific background standard for soil can be established by first collecting 10 soil samples from a comparable area outside the influence of the site. A background standard for groundwater can be established in like fashion with 12 background samples. The background standard (site-specific background level) for soil or groundwater is then calculated as the mean plus three standard deviations of the concentrations of the background samples. Site-specific background levels of lead should not be input into the cumulative risk calculator.

Some universal background levels are built into the risk calculator. As described above, the DNR may establish background levels based on generally available information on background levels of contaminants. Universal background levels will only be established for contaminants that may reasonably be expected to exist at that concentration anywhere in the state due to naturally occurring conditions or widespread impacts from human activities not attributed to a single, readily identifiable source. Universal background levels have been established and are built into the cumulative risk calculator for the chemicals and environmental media shown in Table No. 4.

Table No. 4
Chemical Universal Background Concentration Environmental Medium Basis
Beryllium 2.6 mg/kg Soil 1
Manganese 1470 mg/kg Soil 1
Nickel 37 mg/kg Soil 1
Vanadium 133 mg/kg Soil 1
Basis 1: Background soil concentrations based on mean plus 3 standard deviations of 532 shallow (<1 ft. deep) soil samples from across the state that were collected by the Iowa Geological Survey in 2004 in cooperation with a project funded by the US Geological Survey. >

Other Nuances of the Risk Calculator

  1. If a contaminant in groundwater has a statewide standard that is based on a MCL or an HAL which has an associated cancer risk that is greater than 1 x 10-4 and/or non-cancer risk (i.e., hazard quotient) greater than 1, risk is computed assuming the statewide standard is at a concentration resulting in a cancer risk of 1 x 10-4 and/or a non-cancer risk (i.e., hazard quotient) of 1. For example, an arsenic concentration of 0.005 mg/l (half the MCL of 0.01 mg/l) has an associated cancer risk of 2.1 x 10-4, which exceeds the cumulative risk criterion. For purposes of cumulative risk determination the cancer risk associated with 0.005 mg/l of arsenic will be calculated as 0.005 ÷ 0.01 x (1 x 10-4) = 5 x 10-5, which does not cause a cumulative risk exceedance by itself. This provision is consistent with subrule 567 IAC 137.10(7).
  2. Cumulative cancer risk plus cumulative non-cancer risk are included in cumulative risk calculations for Cancer Group "A", "B", and "C" chemicals, if the necessary toxicity values are available. Cancer risk values shown as "NQ" indicate that toxicity values are not available to compute cancer risk. "NQ" values do not necessarily indicate 0 risk, although for the purposes of cumulative risk calculation they are treated as 0 risk.
  3. The risk calculator deals with lead differently than other chemicals. Lead is the only chemical in which specific standards have been prescribed by rule — 400 mg/kg for soils in a residential area and 1,100 mg/kg for soils less than 2 feet deep in a nonresidential area. The statewide standard for lead in groundwater in a protected groundwater source has been set as the action level of 0.015 mg/l for lead established by USEPA’s Office of Water, which has been assumed to be equivalent to an MCL. Lead is classified as a Cancer Group "B" chemical (probable human carcinogen) although no cancer slope factor is available for assessment of cancer risk from lead. The soil standards for lead are based on non-cancer health impacts; therefore, cumulative risk calculations involving lead are based only on non-cancer risks. The following paragraphs describe how lead is addressed for each exposure scenario.
    • Site Resident Exposure Scenario: An oral reference dose (RfD ) of 0.0051 mg/kg/day has been back-calculated for lead from the 400 mg/kg statewide soil standard. This RfD is used for oral exposure (i.e., groundwater and soil) in cumulative risk calculations for the site resident scenario.
    • Site Worker Exposure Scenario: The oral RfD that has been back-calculated from the 400 mg/kg statewide soil standard yields a risk that is inconsistent with the 1,100 mg/kg nonresidential soil standard, which is the basis of the site worker exposure scenario. Therefore, the cumulative risk calculator adjusts the risks from exposure to soil for the site worker scenario to be consistent with the 1,100 mg/kg non-residential site-specific standard for soil less than 2 feet deep. The back-calculated RfD of 0.0051 mg/kg/day is used in the site worker scenario for groundwater exposure.
    • Construction Worker Exposure Scenario: The cumulative risk calculator uses the 0.0051 mg/kg/day back-calculated reference dose for calculating risk from exposure to lead in soil greater than 2 feet deep in a non-residential area (i.e., the construction worker scenario). (An acceptable maximum concentration of nearly 2,000 mg/kg results, if lead in soil is the only concern.) Paragraph 567 IAC 137.6(6)"b" specifies of use of the most current exposure model for assessing risk associated with adult exposures to lead in soil by the USEPA for determining a site-specific standard for lead in soil greater than 2 feet deep in a nonresidential land use. The adult lead model may be used may be used in lieu of the cumulative risk calculator for assessing risk from lead in soil for the construction worker scenario. Current USEPA guidance for adult lead may be accessed from the following Internet link: ???
    • Background Lead: Background levels of lead are not addressed in the risk calculator and background levels of lead in soils should not be input in the risk calculator. If the background level of lead in soil causes noncompliance with cumulative risk criteria, the DNR should be contacted for a site-specific assessment of the situation.