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The tables in this section list the contaminants of concern. These contaminants are evaluated insubsequent sections of the public health assessment to determine if exposure has public healthsignificance. The following criteria are used for selecting the contaminants of concern:

1.    concentrations of contaminants on and off the site,

2.    field data quality, laboratory data quality, and sample design,

3.    comparison of on-site and off-site concentrations with background concentrations, if available,

4.    comparison of on-site and off-site concentrations with public health assessment comparison values for noncarcinogenic endpoints and carcinogenic endpoints, and

5.    community health concerns.

In the following sections, On-Site Contamination and Off-Site Contamination, the listing of acontaminant does not mean that it will cause adverse health effects. Instead, the list indicateswhich contaminants will be evaluated further in the public health assessment.

The data tables include the following acronyms:

    Environmental Media Evaluation Guide
    Cancer Risk Evaluation Guide
    Reference Dose Media Evaluation Guide
    Maximum Contaminant Level
    Federal Maximum Contaminant Level Goal
    milligrams per kilogram
    parts per billion (equal to micrograms per liter)
    parts per million (equal to milligrams per kilogram)

Comparison values for the public health assessment are contaminant concentrations in specificmedia that are used to select contaminants for further evaluation. EMEGs are media-specificvalues developed by ATSDR to serve as an aid in selecting environmental contaminants thatrequire further evaluation for potential health impacts. EMEGs are based on noncarcinogenichealth endpoints and do not consider carcinogenic effects. CREGs are media-specific valuesderived from EPA's Cancer Slope Factors to serve as an aid in selecting contaminants for follow-up that are potential carcinogens. Cancer Slope Factors provide an indication of the relativecarcinogenic potency of a particular chemical. RMEGs are derived from EPA's Reference Doses(RfDs). RfDs are an estimate of a daily exposure to a particular compound that is unlikely tocause adverse, noncarcinogenic health effects.

Maximum Contaminant Levels (MCLs) are contaminant concentrations set for public drinkingwater supplies that EPA deems protective of public health (considering the availability andeconomics of water treatment technology) over a lifetime (70 years) at an exposure rate of 2 literswater per day. EPA's MCLG is a drinking water health goal. It represents a contaminantconcentration, calculated with an adequate margin of safety, where no known or anticipatedadverse health effects should occur in people exposed to the contaminant. While MCLs areregulatory concentrations, EMEGs, CREGs, MCLGs, and RMEGs are not.


Previous investigations have shown the site to have 9 to 14 feet of unconsolidated soil and fillabove dolomite bedrock of the Shell Rock Formation. The fill consists of clays and silts as well asother materials. The upper portions of the bedrock are relatively fractured and weathered above azone of interbedded shales. Below the shale zone, the bedrock is more competent and intact. Groundwater in the fill and upper bedrock layers are hydraulically connected with Willow Creekat the east and west ends of the retaining wall. Groundwater in the bedrock below the shale zonegenerally flows toward the north-northwest and does not appear to be intimately connected to theupper groundwater or to Willow Creek. The direction of groundwater flow at the site isnortheast in the shallow aquifer and north-northwest in the deep aquifer.

Data from previous investigations and subsequent clean-up activities reveal two principal sourceareas of contaminants on-site: the northwest corner of the site (where the former gasmanufacturing facilities were located), and near the center of the site in the vicinity of the formerlarge gas holder and oil storage tank. These areas were found to contain relatively highconcentrations of contaminants (PAHs, volatile organics, phenolics, and cyanide). Other isolatedpockets of contaminated material may exist. The waste pile is also a potential source ofcontinuing contamination. However, based on the physical characteristics of the material in thepile, and the fact that it is completely covered by an impermeable tarp and surrounded by a claydike, it is unlikely that contaminants will migrate from the pile.

Samples of environmental media were collected and analyzed during site investigations. Sampleswere collected from the waste material, surface and subsurface soils, surface water, andgroundwater. The samples were analyzed for metals, cyanide, pesticides, PCBs, volatile organiccompounds (VOCs), base neutral acid extractable organics (BNAs), and PAHs.

A.    On-Site Contamination

Analytical results from the RI and previous sampling have consistently indicated on-sitecontamination of soil (surface, subsurface, and waste pile) and groundwater (Figures 2-A and 2-B).

Surface and Subsurface Soil

Soil samples were collected from soil borings and monitoring well locations on and around thesite (Figures 2-A and 2-B). Surface and subsurface soils on-site have been found to containPAHs, BTEX, arsenic, and lead. Analytical results from previous investigations of total PAHsand total carcinogenic PAHs in soils, and their locations, are presented in Figures 3-A and 3-B. These concentrations were present prior to the initial clean-up at the site and do not reflectcurrent site conditions. Selected contaminant concentration ranges in on-site soils are listed inTable I which are relatively lower than previous findings. The zero to two- foot depth at whichsamples were collected does not correspond with ATSDR's definition of surface soil, which iszero to three inches deep. BTEX were detected in soils at relatively low concentrations. PAHconcentrations were below the guidelines prescribed in the first EPA Consent Order. That actionrequired the removal of all oily sludges containing total PAH concentrations above 100 ppm.


ON-SITE SOIL CONTAMINANTS (mg/Kg)(RI, November 1993)
ContaminantDepthComparison Value
0 - 2 Feet4 - 6 Feet
10 - 12 Feet
Benz(a)anthracene1.04 - 1.610.2 - 5.060.78 - 6.32NA
Benzo(a)pyrene1.47 - 1.870.2 - 5.570.656 - 6.730.1 CREG
Chrysene1.19 - 2.560.2 - 6.350.987 - 7.69NA
Carcinogenic PAHs (total)5.37 - 9.280.9 - 24.563.73 - 32.96NA
Total PAHs11.14 - 26.452.11 - 63.4810.07 - 85.0NA
Arsenic5.3 - 8.37.670.10.4 CREG
Lead40.843.2J - 177 1,740.0 JNA

NA      = none available
CREG = ATSDR cancer risk evaluation guide
J         = estimated concentration
mg/Kg = miligrams per kilogram

Waste Pile

Detected compounds are those generally associated with manufactured gas plant coal-tarproducts, namely PAHs as listed above. Air monitoring stations around the periphery of the siteduring the waste pile trenching showed no elevated concentration of airborne contaminants, (i.e.,benz(a)anthracene and naphthalene). The waste pile contains soil and waste excavated fromknown contaminated areas. It is completely covered with an impermeable tarp and surrounded bya clay dike.


Contaminants detected in shallow aquifer monitoring wells (MWs) include PAHs, VOCs, andsome metals. The shallow aquifer includes the upper unconfined aquifer down to the shale zone. The concentrations for total PAHs detected ranged from 0.27 µg/L at MW-6 to approximately12,293 µg/L at MW-23 (Figure 2-B). The intermediate zone was monitored by wells screenedbelow the shale zone, but above the first transmissive zone. Total boring depths of monitoringwells ranged from 10.5 feet for shallow wells (MW-11,16) to 101.5 feet for the deepest well(MW-25). MW-25 is located on-site near MW-8. MW25 is not shown in Figure 2-B. Notable increases in PAH concentrations were observed in several wells on-site when compared to off-site wells. The deep aquifer on-site did not contain any significant levels of contaminants. Groundwater analytical results are presented in Table II.


ContaminantsRange of ConcentrationComparison Value (µg/L)
Benzene1J - 1301 CREG/5 MCL
Benz(a)anthracene0.027 - 4970.1 P
Benzo(a)pyrene0.053 - 233.005 CREG/0.2 P
Chrysene2.48 - 3030.2 P
Carcinogenic PAHs(total)0.027 - 1,200.5NA
Total PAHs0.266 - 12,293NA
Lead20.2 - 69.015 A

NA      = none available
CREG = ATSDR cancer risk evaluation guide
J          = estimated concentration
MCL   = EPA maximum contaminant level
P        = Proposed MCL
A        = EPA action level for drinking water
µg/L    = micrograms per liter

B.    Off-Site Contamination

Contaminants at locations which lacked a viable migration pathway were determined in the RI tobe indicative of local background levels or due to other off-site sources not related to the site(2, 4). We concur with this assumption.


Soil samples were collected from soil borings and monitoring well locations around the site. Soilsamples were taken west of South Delaware Avenue and north of Willow Creek, as well as otherlocations around the site (Figures 2-A and 2-B). The results of the soil sample analyses revealedthat PAH compounds were present at all sampling locations, with total PAHs ranging from .0042mg/Kg to .085 mg/Kg. The results also revealed that PAH contamination exists in relativelyhigher concentrations at off-site locations which are upgradient and topographically higher thanthe site. This suggests that these contaminants are indicative of background concentrations, orthe result of unidentified off-site source areas. Soil samples collected from the off-site RIAsampling locations around the site did contain evidence of PAH contamination. However, undercurrent site conditions, no viable mechanism exists for transport of contaminants from the site. Current on-site activities are minimal in scope. There are no air releases occurring on-site. Consequently, these relatively low levels of contaminants were considered background levels. The extent of site-derived soil contamination appears to be limited to the central and northwestportions of the site, extending under South Delaware Avenue toward MW-13, as identified in theRI.


Groundwater samples were collected from each of the monitoring wells around the site (Figure 2-B). The extent of the shallow groundwater contamination is defined by the wells north of WillowCreek. The presence of benzene and naphthalene at MW-17 indicate that the plume may havereached this location. These contaminants did not affect Willow Creek at significant levels. Amaximum concentration of benzene at 12,000 µg/L was detected in MW-17 near the automotivebody shop. However, as discussed in the RI report, the source of contaminants downstream ofthe dam is questionable due to the storm outlet on the south side of Willow Creek. With respectto the metal analyses, lead was detected in MW-13 and MW-14 at concentrations of 1,060 µg/Land 2,590 µg/L, respectively. The remaining wells contained either lower levels of contaminationor no detectable levels. The source of the lead is unclear; however, the extent is adequatelydefined by the existing suite of wells. MW-13 is located northwest of South Delaware Avenueand MW-14 is located east of the same street adjacent to the site. The MW-13 area is currentlyused as a parking lot.

Based on the off-site monitoring well data, contaminant migration in the groundwater appears tobe limited in scope. There is no evidence of contaminant migration into the deeper aquifer off-site. No domestic wells are currently being impacted by contaminants from the site as previouslydetermined in the Mason City Well Inventory because of their distance from the site.

Willow Creek Sediment, Surface Water, and Biota

Relatively low levels of PAHs have been detected in the Willow Creek sediment. The presence ofPAHs in the sediment is possibly the result of discharge from the shallow aquifer into the creek. During the most recent sampling event, the highest total PAH concentration in sediments adjacentto the upstream portion of the site was 25 mg/Kg. Lower concentrations of total PAHs weredetected adjacent to the site, 6.2 mg/Kg, compared to 120 mg/Kg downstream of the site. Willow Creek surface water contained benz(a)anthracene at 1.1 µg/L. Adjacent to the site, thesurface water of Willow Creek is contaminated with background levels of benz(a)anthracene. Thegeneral water quality of Willow Creek appears to be good. Although recreational fishing is notapparent in proximity to the site, fish from Willow Creek were assessed for site relatedcontaminants. Concentration of PAHs in fish were estimated by multiplying the concentration ofPAHs in sediment by a tissue concentration factor derived from the octanol/water partitioncoefficients for each of the PAHs. This method is considered very conservative for the followingreasons: it estimates the total concentration in the whole fish and not just the edible portions; fishare assumed to be in constant contact with the sediment; the equation is known to vary based onmetabolism, and fish are very effective in metabolizing PAHs. Based on this assessment, fish fromWillow Creek are not considered impacted by site related contaminants(1, 2).

C.    Toxic Chemical Release Inventory Information

Emissions from surrounding facilities may be contributing to an additional environmental burdento the nearby population and this site. Therefore, IDPH staff searched the Toxic ChemicalRelease Inventory (TRI) data for the years 1987 through 1992, and early 1993. The TRI containsinformation on estimated annual releases (emission rates) of toxic chemicals to the environment(via air, water, soil, or underground injection), whether these releases are routine releases, spillsand other accidental releases, or occasional releases from normal facility operation. Toxicchemical release information is submitted to EPA by certain industries as mandated under theEmergency Planning and Community Right-to Know Act of 1986. The TRI did not identify anynearby hazardous substance releases.

D.    Quality Assurance and Quality Control (QA/QC)

In preparing this public health assessment IDPH staff assumes that information provided in thereferenced documents was prepared from adequate QA/QC directed chain of custody procedures,laboratory procedures, and data reporting. The RI contained laboratory data validation reportswith appropriate quality assurance and quality control protocols as required.

The validity of the analyses and the conclusions section of this public health assessment aredetermined by the availability and reliability of referenced information.

E.    Physical and Other Hazards

During the site visit, no physical or other hazards were apparent.


A. Completed Exposure Pathways

No completed exposure pathways have been identified at this site.

B. Potential Exposure Pathways

Potential exposure pathways are summarized in the following table.

Table III

Exposure Pathways Summary
SiteSurface/subsurface soilOn-SiteIngestion,
Skin Contact
Trespassers, Site workersPast/ Potential
Present/ Potential
Future/ Potential
SiteGroundwaterTapIngestion, Inhalation, Skin ContactPrivate well owners Future/ Potential
Ambient airAirOn-Site and Immediate surrounding areasInhalation Site workers, Area residentsPast/ Potential
Future/ Potential

Source Areas and Surrounding Soils

Former workers at the facility may have been exposed to contaminants prior to and during thetrenching and excavation of the source area. These people could have been exposed throughdirect skin contact, inhalation of volatiles and entrained dust particles, and incidental ingestion ofdusts from waste materials. No adverse health effects were reported from these possibleexposures.

Piling and covering of the source area materials and contaminated soils has eliminated somepossibility of future exposure such as those that may have occurred with previous excavationactivities. However, with the presence of the waste pile, future removal activities could result inworker exposure through direct skin contact, inhalation of volatiles, and /or incidental ingestion ofcontaminated materials if workers are not adequately protected. Additionally, people near the sitecould be exposed to contaminants if the volatiles and entrained dust particulates are allowed toescape into the ambient air. This means that people living near the site could be exposed primarilythrough inhalation of airborne contaminants.


If precautions are not taken, people in proximity to the site and remedial workers could beexposed to airborne contaminants through inhalation during excavation and removal of the wastepile from the site.


People using off-site domestic water wells that are downgradient of the site could be exposed tocontaminants coming from the site if groundwater contamination at the site migrates. Peopleusing the well water could be exposed through ingestion, inhalation, and direct skin contact. No wells are known to currently exist that are contaminated at this time nor do any exist downgradient that are expected to become contaminated from site releases in the future.


A.    Toxicological Evaluation

In this subsection, the plausible health effects that could occur in people exposed to sitecontaminants are presented. A person must come into contact with the contaminants in order forthe chemicals to cause illness. Based on a thorough review of available data, no completedexposure pathways to contaminants leading to adverse health effects have been identified at thissite. As a result, and under current site conditions, no exposure and adverse health effects areexpected to occur if recommendations are followed.

Some possible exposure pathways, however, have been identified and are presented in Table III. Those exposure pathways involve possible past exposure to contaminated soil and air, future useof contaminated groundwater, direct contact with the waste pile, and possible emissions ofcontaminants to the ambient air during waste pile removal activities on-site. Possible past workerexposure to on-site soil and air contamination would be difficult to evaluate because an estimateddose cannot be calculated. No one that may have been exposed has reported any adverse healtheffects.

If the maximum concentration of contaminants detected in the shallow aquifer migrate to privatewells used for domestic purposes downgradient of the site, the levels of total PAHs, benzene, andlead would be a public health concern. The health concern from exposure to groundwatercontaminants is likely to be focused on potential chronic effects as a result of cumulative, long-term, low-dose exposure conditions. Long-term exposure could result in adverse health effects,including cancer.

No new well development is being planned for areas near the site. Moreover, drilling of newwells will require prior approval from the Iowa Department of Natural Resources. State lawforbids the drilling of new wells within city limits in Iowa. If wells are developed in these areas inthe future, they should be monitored for contamination prior to use and periodically testedthereafter to avoid possible exposure.

B.    Health Outcome Data Evaluation

Review of data from the Iowa Cancer Registry for the period 1973 through 1992 on all cancertissue sites indicates no significant increases in any of the cancer types reviewed for residents ofMason City, age groups 0-4 years through 85+ years, when compared to Cerro Gordo Countyand the State. This review was conducted prior to the start of this public health assessment toassess the potential public health impact of site contaminants.

Under current site conditions, no adverse health outcomes would be expected. No completedexposure pathways leading to adverse health effects have been identified. Area residents have notexpressed any specific health concerns related to the site. For those reasons, no other healthoutcome data will be evaluated at this time.

C.    Community Health Concerns Evaluation

IDPH is not aware of any specific health concerns that may be related to contamination at the site. Should new information become available suggesting that exposure to contaminants has occurredat levels of concern, that information will be evaluated and the community will be notified of theresults.

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