PUBLIC HEALTH ASSESSMENT
MONTICELLO MILL TAILINGS (DOE)AND
MONTICELLO RADIOACTIVELY CONTAMINATED PROPERTIES
(aka MONTICELLO VICINITY PROPERTIES)
Contaminants discussed in later sections of this public health assessment are evaluated todetermine whether exposure to them has public health significance. ATSDR staff membersselect and discuss contaminants based on several factors: sample design, field and laboratorydata quality, and comparison of chemical concentrations to levels that could cause cancer orother health effects. We also consider community health concerns.
Evaluating the sample design involved reviewing Department of Energy Remediation Programregulations and the approach to locating contamination. ATSDR scientists consider spatialdistribution of sampling locations, sampling frequency, concentration changes over time,medium-to-medium differences, and correlation between the selected list of analyticalparameters and suspected environmental contaminants when determining the contaminants towhich humans could be exposed.
Review of sampling field quality control procedures included interpreting data on background(or regional) concentrations of chemicals and checking the adequacy and number of replicate,spiked, and blank samples to verify detection of contaminants. We reviewed procedures used toverify instrument reliability to assess laboratory quality control.
Contaminant concentrations detected on and off site are compared with comparison values,contaminant concentrations in specific media that are considered protective of public health(values that are believed to be without adverse health effects upon exposure). ATSDR and otheragencies have developed the comparison values to provide guidelines for estimating contaminantconcentrations in media at which adverse health effects are not expected to occur. A standarddaily ingestion rate and body weight are assumed in deriving these values. These values, inmany cases, have been derived from animal studies. Health effects are related not only to theexposure dose but also to the route of entry into the body and the amount of chemical absorbedby the body. For those reasons, comparison values used in public health assessments arecontaminant concentrations in specific media and for specific exposure routes. Severalcomparison values may be available for a specific contaminant. ATSDR scientists use the mostconservative assumptions (that is, we assume exposure to the maximum concentration) in orderto protect the most sensitive segment of the population. The Public Health Implications sectionof this document contains a discussion of the potential for adverse health effects from exposureto contaminants.
The following paragraph is to provide additional clarification concerning comparison values. Comparison values are concentrations in environmental media, such as air, soil, or water, belowwhich adverse health effects are not expected to occur as a result of likely exposures. Comparison values are used to determine which contaminants require additional evaluationconcerning possible exposure scenarios and adverse health effects. These levels are derivedusing conservative assumptions about exposures. Because of their conservative nature, andbecause they are not derived using site-specific information, comparison values should never beused as clean-up levels. Their use should be limited to the initial screening of site contaminationinformation.
The following abbreviations are used in Tables 4-12:
|ATSDR||Agency for Toxic Substances and Disease Registry.|
|EPA||Environmental Protection Agency.|
|NTP||National Toxicology Program.|
|BDL||below detection limit. A chemical detected during chemical analysis is reported asBDL if the concentration detected is below the minimum concentration verifiable (canbe duplicated through multiple analyses) by the analytical technique specified for thatchemical. Analytical techniques have both a lower (minimum concentrationdetectable) and an upper (maximum concentration detectable) limit.|
|CREG||cancer risk evaluation guide (ATSDR). Derived by ATSDR from the EPA cancerslope factor. It represents a concentration in water, soil, or air at or below whichexcess cancer risk is not likely to exceed one case of cancer in a million (10E-6)persons exposed over a lifetime.|
|EMEG||environmental media evaluation guide (ATSDR). Derived by ATSDR from ATSDR'sminimal risk level (MRL). It is the concentration in water, soil, or air at or belowwhich daily human exposure is unlikely to result in adverse noncancerous effects.|
|RMEG||reference dose (or concentration) media evaluation guide (EPA). Derived by ATSDRfrom the EPA oral reference dose. It is the concentration in water or soil at or belowwhich daily human exposure is unlikely to result in adverse noncancerous effects.|
|MCL||maximum contaminant level. Enforceable drinking water regulation established byEPA that is protective of human health to the "extent feasible" over a lifetime. MCLsrepresent contaminant concentrations that EPA scientists deem protective of publichealth over a lifetime (70 years) at an exposure rate of 2 liters of water per day. MCLsare also regulatory concentrations. MCLs take into account technological andeconomic feasibility.|
|mg/kg||milligrams per kilogram (parts per million). The unit applied to express contaminant concentrations in soil.|
|ND||no data were collected.|
|pCi/g||picocuries per gram of soil. The unit applied to express radioactive contaminant concentrations in soil.|
|pCi/L||picocuries per liter of air or water. The unit applied to express radioactive contaminantconcentrations in air or water.|
|µg/L||micrograms per liter of water (parts per billion). The unit applied to expresscontaminant concentrations in water.|
|RAC||reasonably anticipated to be a carcinogen (National Toxicology Program designation).|
|human carcinogen (EPA designation).|
|probable human carcinogen (EPA), based on limited humanstudies and sufficient animal studies.|
|probable human carcinogen (EPA), based on inadequate humanstudies and sufficient animal studies.|
EMEGs and CREGs are the first choice for comparison values. In addition, any contaminantswill be contaminants of concern if they have no CREG, but have been designated as carcinogensor potential carcinogens by 1) the National Toxicology Program in the Department of Health andHuman Services, 2) the EPA, or 3) the International Agency for Research on Cancer. If acontaminant is not a carcinogen, and has no EMEG, then the following values (in order ofpreference) will be chosen for the comparison value if available: the RMEG, the lifetime healthadvisory (derived by EPA, a drinking water concentration at or below which adverse,noncancerous adverse health effects would not be expected) or child longer-term health advisory(derived by EPA, a drinking water concentration at or below which adverse, noncancerousadverse health effects would not be expected in children after exposure up to 7 years in duration)(whichever is lowest), the maximum contaminant level goal (non-enforceable drinking waterhealth goal recommended by EPA and set at a level at or below which no known or anticipatedadverse human health effects are expected), the MCL, or the action level (derived by EPA foruse in evaluating drinking water, the concentration in water at or below which daily humanexposure is unlikely to result in adverse noncancerous effects).
We conducted a search of EPA's Toxic Chemical Release Inventory (TRI) for the San JuanCounty area to determine the extent of reported environmental contamination releases. The TRI,established through the Superfund Amendments and Reauthorization Act of 1986 (SARA),requires the reporting of estimated annual releases of chemicals into the environment since 1987(35). The database includes the annual quantity of toxic chemicals discharged into eachenvironmental medium (air, water, and land) by manufacturing facilities that employ more than10 people and are in Standard Classification Codes 20 through 39 (as in effect since July 1,1985) (36). The Monticello Mill Tailings Site has not been in operation since 1960; therefore,no chemical releases were recorded on this database for the mill site. No local chemical releaseswere listed as originating from communities in San Juan County.
A.1 On-Site Surface Soil
- Antimony (Sb)
- Arsenic (As)
- Beryllium (Be)
- Cadmium (Cd)
- Chromium (Cr)
- Copper (Cu)
- Lead (Pb)
- Mercury (Hg)
- Molybdenum (Mo)
- Nickel (Ni)
- Selenium (Se)
- Silver (Ag)
- Thallium (Tl)
- Vanadium (V)
- Zinc (Zn) (1)
Surface soils (0-6 inches) have been contaminated in various ways, including storage ofore in open stockpiles, emissions from the roaster stack (heat process used to convertvanadium minerals to a soluble form), overflow of tailings ponds, and the erosion oftailings piles by wind and water. Results of a radiometric survey show that most of themill site surface soil contains concentrations of radium-226 exceeding EPA Standard40 Code of Federal Regulations (CFR) 192.12 for cleanup of land and buildingscontaminated with residual radioactive materials from inactive uranium processingsites. EPA Standard 40 CFR 192.12 identifies areas as contaminated if their radium-226 concentrations in soils exceed 5 pCi/g above background in the top 15 centimeters(approximately 6 inches) of soil or 15 pCi/g above background in any 15-centimeter(cm) layer below the top 15 cm (1).
DOE representatives are conducting a background analysis and taking samples atselected peripheral properties. The result of the analysis would provide data to supportwhether or not there is a correlation between radioactive and nonradioactive materials. As of yet, there are no convincing analyses or evaluations to support this unprovenassumption.
Analytical results of soil samples, together with results of in-situ spectrometermeasurements, indicate an average natural background radium-226 concentration of 1.0+/- 0.4 pCi/g. The average concentration of radium-226 in the surface soil layer (0-15cm, or approximately 6 inches) is 20 pCi/g over the mill site. Contamination of covermaterial has been attributed to redistribution of tailings by burrowing animals (4).
The tailings generated by the mill site operations are in four piles referred to, in orderof their construction, as the Carbonate Pile, Vanadium Pile, Acid Pile, and East Pile(see Appendix F, Figure 10). Field investigations of the piles were conducted duringthe Remedial Investigation in 1990 and again as part of the Monticello RemedialAction Program in 1991. Nonradioactive composite samples were taken from boringsand pits when radium-226 measurements were above 15 pCi/g. Borings were drilled tovarious depths to 50 feet or more; while the pits ranged from 9.5 to 21.5 feet deep. The samples were analyzed for the following nonradioactive contaminants:
Maximum concentrations of each nonradioactive element found in the tailings pileswere evaluated for potential health implications. Seven contaminants of concern wereidentified: arsenic, beryllium, chromium, copper, lead, nickel, and vanadium. Table 4contains the concentration found and comparison values for each. The PathwaysAnalysis and Public Health Implications sections of this public health assessmentcontain further discussions of each contaminant of concern.
|N/A = not applicable|
NA = not available
EPA-A = Environmental Protection Agency-human carcinogen
EPA-B2 = Environmental Protection Agency-probable human carcinogen
NTP-RAC = National Toxicology Program-reasonably anticipated to be acarcinogen
CREG = cancer risk evaluation guide
EMEG = environmental media evaluation guide
ppm = parts per million
* Value believed to be without adverse health effects upon exposure.
A.2 Off-Site Surface Soil
- North and east off-site areas contaminated with radium-226 above EPA standard 40CFR 192.12 (5 pCi/g above background in the top 15 centimeters (cm) of soil or 15pCi/g above background in any 15 cm layer below the top 15-cm) are predominantlyfarming lands but include some residences. Windblown surface soil contamination isfound as far as 0.5 mile north and 0.25 mile east of the mill site. Radium-226concentrations above EPA standards ranged from 6 pCi/g to 494 pCi/g and averaged 27pCi/g (1).
The major source of nonradioactive contaminants is confined to the tailings piles onsite. A much smaller area with lower concentration of contamination occurs in streamsediments east of the mill site in an area used to pasture cattle and produce some crops. During milling operations, tailings, mixed with stream sediments, were deposited onthe Montezuma Creek flood plain. Samples were taken in pasture soil south and east ofthe mill site in the flood plain, then were analyzed for tailings-related contaminants. Maximum concentrations of each contaminant were evaluated for potential healthimplications for any children who might play in the area. Additional sampling ofsediments for nonradioactive contaminants were collected during the Operable Unit(OU) III study. The purpose of the OU III study was to collect sufficient informationand data to characterize the nature and extent of environmental contamination in OUIII, identify the sources of contamination, assess changes in contamination patternsover time once on-site sources (tailings piles) have been removed, and to calculate thelevels of risk to human health and the environment from the contaminants associatedwith OU III. The OU III soil and sediment area, which is located entirely on privateland, begins approximately 0.5 miles east of the eastern mill site boundary and extendsdownstream approximately 14,100 feet. The area is currently used for cattle grazingand recreational purposes; no residences are located within the OU III soil andsediment study area. Soil and sediment characterization began in 1994 and continuedthrough September 1996. The primary source of soil and sediment contamination inthe OU III soil and sediment study area is the mill site. Montezuma Creek, whichflows through the tailings piles on the mill site, has been the primary transportmechanism for soils and sediments (38). Table 5 contains a list of contaminants ofconcern chosen for further consideration based upon sampling data collected duringenvironmental monitoring program activities and the most recent OU III study. ThePathways Analyses section of this public health assessment addresses each contaminantof concern.
|ppm = parts per million|
N/A = not applicable
NA = not available
EPA-A = Environmental Protection Agency-Human Carcinogen
EPA-B1 = Environmental Protection Agency-Probable Human Carcinogen
EPA-B2 = Environmental Protection Agency-Probable Human Carcinogen
NTP-RAC = National Toxicology Program
EMEG = environmental media evaluation guide
CREG = cancer risk evaluation guide
RMEG = reference dose media evaluation guide
* Value believed to be without adverse health effects upon exposure.
B.1 On-Site Surface Water Contamination
Surface water monitoring of Montezuma Creek has included collection of samplesfrom upgradient (upstream), on site, and downgradient (downstream with respect to themill site) locations. The creek, which flows through the mill site property, hasfrequently contained contaminants at levels exceeding comparison values as far as 3miles downgradient of the property. Appendix F, Figure 11, depicts on-site surfacewater sampling locations, and Table 6 lists on-site surface water contaminants. Somecontamination in the creek resulted from discharge of the contaminated alluvial aquiferbeneath the mill site, although the primary source of contamination appeared to be pastsurface runoff from the tailings piles. Current controls in place collect and treat surfacewater before it is discharged. Alluvial groundwater is still providing base flowcontaminants to Montezuma Creek. Montezuma Creek is used for both irrigation andlivestock watering downgradient of the mill site.
To facilitate comparison of upgradient, on-site, and downgradient concentrations, theon-site and the off-site surface water contamination discussions of this public healthassessment are combined.
B.2 Off-Site Surface Water Contamination
The two upgradient surface water sources used by the city of Monticello public watersystem are monitored in accordance with the Safe Drinking Water Act and Utah staterequirements; those standards have not been exceeded for site-related contaminants. ATSDR representatives have used concentrations of chemicals detected during themonitoring program to depict naturally occurring concentrations for comparison withsite-related data.
Montezuma Creek, the main surface water body in the project area, flows from west toeast through the middle of the mill site property. Although flow is generally perennial,the creek can be quite low or dry during the late summer. Other surface water bodieson the mill site include ponds, seeps, and drainages. Surface water sampling at the millsite has had four primary goals: 1) compare upstream water quality conditions ofMontezuma Creek with conditions on site and downstream from the mill site, 2)characterize the type and extent of contamination in surface water sources, 3) verifycompliance with state surface water quality standards, and 4) detect changes in water quality resulting from remedial actions (39).
Montezuma Creek is one source for the city of Monticello municipal water supplyabout 1 mile upgradient of the mill site. Utah state regulations (Title 26, Chapter 11,Utah Code Annotated) place the segment of Montezuma Creek that flows through anddowngradient from the mill site into four use classifications: 1) Domestic Source lC, 2)Recreation and Aesthetics 2B, 3) Agriculture, and 4) Aquatic Wildlife 3B. Downgradient surface water is used primarily for livestock watering and agricultural irrigation (1).
Appendix F, Figure 11, depicts upgradient and on-site surface water samplinglocations. Two sampling locations (W-3 and W-5) have been the historic sources ofupgradient water quality samples from Montezuma Creek. In November 1992,locations SW92-01, SW92-02, and SW92-10 replaced W-3 and W-5 as upgradientsampling locations (40).
Before November 1992, on-site sampling was limited to three locations: the drainagebetween the Carbonate and Vanadium Tailings Piles (W-2), the seep-fed pond adjacentto the Carbonate Tailings Pile (designated Carbonate Seep), and the low spot betweenthe Carbonate and Vanadium Tailings Piles (designated North Drainage). InNovember 1992, on-site sampling was expanded to include two locations onMontezuma Creek, SW92-04 and SW92-05 (41).
Figures 12a and 12b, in Appendix F depict downgradient surface water samplinglocations. In past years, downgradient water quality within Montezuma Creek wasmonitored at three locations: the W-4 site, approximately 325 feet downstream of theeast boundary of the property; the Sorenson site, approximately 1.25 miles downstreamof the mill site, and the Montezuma Canyon site, approximately 6 miles downstream ofthe mill site. In November 1992, four additional locations were sampled downstreamof the property (SW92-06, SW92-07, SW92-08, and SW92-09) (41).
From 1987 through April 1992, surface water samples were analyzed for the followingconstituents: gross alpha, radium-226, radium-228, uranium-234, uranium-238,thorium-230, arsenic, molybdenum, nitrate, selenium, and vanadium. Tests in the fieldmeasured alkalinity, pH, and specific conductance. In November 1992, the list ofsurface water analytes was expanded to include aluminum, ammonia, antimony, boron,barium, beryllium, gross beta, calcium, cadmium, chlorine, cyanide, cobalt, chromium,copper, fluorine, iron, herbicides, lead, mercury, potassium, magnesium, manganese,nickel, nitrite, pesticides/polychlorinated biphenyls (PCBs), polonium-210, radon-222,semivolatile organic compounds, silver, sodium, sulfate, total dissolved solids,thorium-232, thallium, total uranium, volatile organic compounds, and zinc. Duringthis same period, biological oxygen demand, chemical oxygen demand, and levels offecal coliform, total coliform, total suspended solids, and total organic carbon were determined from samples collected at locations SW92-01, SW92-02, and SW92-10 (41).
The most recent sampling rounds (November/December 1992, March 1993, April/May1993, July 1993, October 1993, May 1994, October 1994, April 1995, October 1995,February 1996, April 1996, and June 1996) furnished surface water contamination datafor comparison with concentrations detected upgradient, on site, and downgradientfrom 1984 to 1992. Those concentrations exceeding comparison values are selected as contaminants of concern. Table 6 presents those surface water contaminants detectedin concentrations exceeding comparison values during the most recent and historicalsampling rounds. Listing the contaminants of concern maximum concentrations for themost recent surface water sampling rounds and the historical maximum concentrationsfor those contaminants of concern portrays the site's actual impact on downgradientwater quality over time. In the case of nitrate contamination, although the site didcontribute to contamination of downgradient surface water, screening values wereexceeded upgradient. The Pathways Analyses section of this public health assessmentcontains further discussions of the contaminants.
|Constituent||Maximum Concentration1||Historical Maximum Concentration2||Comparison Value3||Source|
|454 µg/L||15.1 µg/L||3.9 µg/L||3,500 µg/L||27 µg/L||0.02µg/L |
|Molybdenum||2.6 µg/L||321 µg/L||90.9 µg/L||178 µg/L||3,420 µg/L||340 µg/L||50 µg/L |
|Nitrate||24,600 µg/L||18,500 µg/L||7,190 µg/L||3,000 µg/L||64,100 µg/L||10,007 µg/L||10,000 µg/L||MCL|
|Selenium||9.7 µg/L||41.4 µg/L||17.5 µg/L||6 µg/L||3,110 µg/L||42 µg/L||30 µg/L |
|Vanadium||5.9 µg/L||7,830 µg/L||280 µg/L||142 µg/L||63,600 µg/L||750 µg/L||30 µg/L||EMEG|
|Gross Alpha *||BDL|
|1,209 pCi/L||350 pCi/L||17 pCi/L||3,300 pCi/L||547 pCi/L||15 pCi/L||MCL|
|Radium-226 *||0.5 pCi/L||7.4 pCi/L||2.2 pCi/L||0.5 pCi/L||23.8 pCi/L||13 pCi/L||15 pCi/L||MCL|
and -238 *
|5.5 pCi/L||1,638.5 pCi/L||350.7 pCi/L||9.7 pCi/L||2,336 pCi/L||399.8 pCi/L||15 pCi/L||MCL|
| NOTE: Table 6 includes the following abbreviations and footnotes:|
BDL = below detection limit (analytical lower detection limit is in parentheses)
1 Maximum concentration detected during the most recent surface water sampling rounds: November/December 1992, March 1993, April/May 1993, July 1993, October 1993, May 1994, October 1994, April 1995, October 1995, February 1996, April 1996, and June 1996.
2 Maximum concentration detected for all surface water sampling rounds 1984 through 1992, excluding the November/December 1992 sampling round.
3 Value believed to be without adverse health effects upon exposure.
Concentrations of arsenic, molybdenum, selenium, vanadium, gross alpha, radium-226,uranium-234, and uranium-238 increase within Montezuma Creek as the creek flowsacross the mill site and downgradient. Seeps from the shallow aquifer are visible alongthe creek downstream of the eastern mill site boundary, and creek discharge increasesthroughout this section for approximately 1.25 miles. Historical assessments of waterquality data indicate that the highest downgradient concentrations of mill tailings-related constituents occur at either the W-4 site or at the Sorenson site. Both sampling sites are downgradient of the mill site (41).
Since 1985, selenium concentrations have consistently been below comparison valuesat the upgradient (W-5) location. Samples from the W-4 and Sorenson locations,which are 0.06 and 1.2 miles downstream of the mill site, respectively, have exceededcomparison values regularly. Selenium concentrations are also consistently belowcomparison values 6 miles downstream of the mill site at the Montezuma Canyonlocation. Selenium concentrations exceeded the comparison values at on-site locations during the recent sampling.
Since 1987, gross alpha levels have been below detection limits at the upgradient(W-5) location. Gross alpha concentrations have exceeded comparison values at theW-4, Sorenson, and Montezuma Canyon sampling locations. The trend continuedthrough the 1995 sampling rounds (43).
Higher concentrations of mill tailings-related contaminants have been detected in theponds and seeps on the mill site than in Montezuma Creek because the ponds and seepsare surface expressions of the groundwater (see On-Site Groundwater Contamination). Analyte concentrations in the seeps and ponds are similar to those in alluvial aquifergroundwater samples collected from wells near the Vanadium and East Tailings Piles. Levels of gross alpha (140 pCi/L), arsenic (245 µg/L), and selenium (26 µg/L)exceeded comparison values in at least one of the ponds or seeps.
Nitrate, although not totally a site-related contaminant, is a contaminant of concern insurface water because of historical and recent detection upgradient, on site, anddowngradient at concentrations exceeding comparison values. Common agriculturalactivities around the mill site, such as the use of fertilizers, are known to cause nitratecontamination of surface water. Nitrate detected in upgradient surface water samples isnot a site-related contaminant but is rather the result of those agricultural activities;however, nitrate detected in on site and downgradient surface water samples is, at leastin part, site-related, resulting from former process operations at the mill. During thelast 4 years of the mill site's active operations, ammonium nitrate and othermiscellaneous oxidizers were added to a process for extracting and concentratinguranium from a liquid solution. A maximum of 2 tons per day of ammonium nitratewas used in the process, with the residual waste effluent from the process discharged tothe Acid and East Tailings Piles (30). Nitrate was therefore selected as a contaminantof concern because both historical and recent concentrations in upgradient, on site, anddowngradient surface water are elevated. The Public Health Implications (A.Toxicological Evaluation) section of this public health assessment contains furtherdiscussion of potential health effects of nitrate ingestion.
A wastewater treatment plant, designed to treat contaminated surface water runoff fromthe mill site and groundwater encountered during excavation to the tailings piles, beganoperation in May 1995. The wastewater treatment plant is designed to remove heavymetals and radionuclides from ground and surface wastewaters at an average flow rateof 60 gallons per minute. During operations, through August 21, 1995, influent andeffluent samples were obtained and, with the exception of mercury and silverconcentrations, effluent limits set by the Utah Department of Environmental Quality,Division of Water Quality, were not exceeded. A position paper was submitted to theDepartment of Water Quality in March 1996 to propose higher effluent limits formercury and silver; approval was subsequently granted (44).
C.1 On-Site Groundwater Contamination
Two aquifers underlie the Monticello Mill Tailings Site (MMTS) and the surroundingarea. Unconsolidated materials (such as loose sands and gravel) deposited byMontezuma Creek constitute an alluvial aquifer along the valley bottom. Anunderlying sandstone aquifer, the Burro Canyon Formation, is separated from thealluvial aquifer by the Mancos Shale Formation and by fine-grained units of the DakotaSandstone Formation, both of which act as aquitards in the mill site area. Aquitards arelow permeability geologic formations or groups of formations that impede groundwaterflow from one aquifer to another (1).
The alluvial aquifer is approximately 16 feet thick near Montezuma Creek in thevicinity of the Carbonate Tailings Pile and thins gradually upgradient anddowngradient from this location and toward the valley sides. Montezuma Creek ishydraulically connected (joined) with the alluvial aquifer on the upstream side of theEast Tailings Pile. However, because of a realignment of the stream channel, thealluvial aquifer and Montezuma Creek are separated in the vicinity of the East TailingsPile. The creek and the aquifer are reunited downstream of the East Tailings Pile (31).
The alluvial aquifer is recharged from infiltration of precipitation (rainfall and snow),surface water, and water that has percolated the Mancos Formation and the sedimentgravel on the valley sides. Like the local surface waters, water levels within the aquiferfluctuate seasonally. The alluvial aquifer discharges into Montezuma Creek. Transmissivity values for the alluvial aquifer beneath the East Tailings Pile weredetermined from a pump test and ranged from 3.3 x 10-4 to 5.4 x 10-4 square meters persecond. As the alluvial groundwater moves to the east and southeast across the millsite, it is degraded by contaminants leached from the mill tailings. Groundwater fromthe alluvial aquifer is not used in the vicinity of the mill site as a water source forhuman consumption, but it is used to irrigate crops and provide water for livestock (1).
The Burro Canyon Formation is a confined aquifer under the mill site, separated fromthe alluvial aquifer by an aquitard consisting of the Mancos Shale Formation andfine-grained units of the Dakota Sandstone Formation. Those geological units limitdownward migration from the alluvial aquifer. The EPA and Utah Department ofEnvironmental Quality (UDEQ) have challenged DOE's interpretation of thehydrologic conditions. Site-specific information indicates that there is a joint/fracturesystem possibly related to the uplift of the Abajo mountains. The Mancos Shale on themill site is weathered and varies in thickness from 0 to less than 40 feet. Studies of thehydrologic heads of paired wells in the Mancos Shale, Dakota Sandstone, and BurroCanyon Formation in the vicinity of the mill site indicate that the movement isdownward. Although EPA and UDEQ remain confident that contamination has notreached the Burro Canyon Formation, there is conflicting data as to whetherradiological contamination from the mill site is present in the Dakota Sandstone.
The Burro Canyon Aquifer is recharged through the tilted, exposed area of theformation located along the margin of the Abajo Dome west of the mill site. Dischargefrom the aquifer occurs across the Great Sage Plain, along erosional margins, and inareas where canyons dissect the formation. Numerous stock ponds and marshy areasare created as a result of spring-fed discharge from the aquifer (1). Residences in theMonticello area not connected to the municipal water supply use the deep BurroCanyon Aquifer as a source of potable water (water used for drinking, cooking,showering, etc.).
Water quality data used to characterize groundwater chemistry in the mill site areacome from sampling of selected monitoring wells that were installed beginning in1982. Although some other wells were installed before 1982, the validity of samplescollected from those wells is questionable because of poor well completion records. Data cited in this public health assessment are from those wells considered to yieldreliable samples on the basis of satisfactory well completion records and relativelyconsistent well performance over several years. ATSDR staff members reviewed theDOE-validated groundwater sampling data from the March 1984 through the June1996 sampling round (38, 42).
DOE's current groundwater monitoring strategy is to sample 6 upgradient wells (3 alluvial, 3 Burro Canyon), 10 on-site wells (7 alluvial, 3 Burro Canyon), and 8downgradient wells (5 alluvial, and 3 Burro Canyon). The upgradient wellscharacterize groundwater quality before contact with mill site contamination, the on-site wells characterize the extent of groundwater contamination on site, and thedowngradient wells characterize the impact of the mill site contamination ongroundwater before the water leaves the mill site. Appendix F, Figure 13, depictsgroundwater monitoring well locations upgradient and on site; Appendix F, Figure 14,depicts groundwater monitoring well locations downgradient (46, 47). Groundwatersamples were analyzed for organic and inorganic chemicals, and radioactiveparameters. Organic analytes included EPA's target compound list (TCL): volatileorganic compounds, semivolatile organic compounds, herbicides, and pesticides/PCBs(see Appendix B). Inorganic analytes included major anions (chloride, cyanide,fluoride, nitrate, nitrite, and sulfate); major cations (ammonium, calcium, magnesium,potassium, and sodium); metals (aluminum, antimony, arsenic, barium, beryllium,boron, cadmium, chromium, copper, iron, lead, manganese, mercury, molybdenum,nickel, selenium, silver, strontium, thallium, uranium, vanadium, and zinc); totaldissolved solids; gross alpha; gross beta; and radionuclides (polonium-210,radium-226/228, thorium-230/232, uranium-234/238, and radon-222) (41).
Detected concentrations from the most recent sampling rounds (November/December1992, March 1993, April/May 1993, July 1993, December 1993, May 1994, October1994, April 1995, October 1995, February 1996, April 1996, and July 1996) providedata for comparing groundwater contamination that exceeds comparison values in thealluvial and Burro Canyon Aquifers, with concentrations detected upgradient, on site,and downgradient from 1984 to 1992. Table 7 presents groundwater contaminantsdetected in concentrations exceeding comparison values during the most recent andhistorical sampling rounds. The Exposure Pathways section of this public healthassessment contains discussions of those contaminants.
Contaminant concentrations detected in samples from upgradient alluvial and BurroCanyon wells did not exceed comparison values during the most recent samplingrounds, with the exception of elevated nitrate concentrations detected in the alluvialaquifer. Nitrate detected in upgradient alluvial aquifer samples is not a site-relatedcontaminant but is rather the result of agricultural activities; however, nitrate detectedin on-site and downgradient samples from the same aquifer is, at least in part, site-related because of former process operations at the mill. During the last 4 years of themill site's active operations, ammonium nitrate and other miscellaneous oxidizers wereadded to a process for extracting and concentrating uranium from a liquid solution. Amaximum of 2 tons per day of ammonium nitrate was used in the process, with theresidual waste effluent from the process discharged to the Acid and East Tailings Piles(30). Nitrate was therefore a contaminant of concern because of both historical andrecent elevated concentrations in upgradient, on-site, and downgradient alluvial aquifergroundwater. The Public Health Implications (A. Toxicological Evaluation) section ofthis public health assessment contains further discussion of potential health effects ofnitrate ingestion. Historical upgradient sampling (1984 to 1992) detected selenium andgross alpha contamination in excess of comparison values.
Groundwater sampled from the alluvial aquifer on site is contaminated by elementsleached from the tailings piles. In general, the highest contaminant concentrations arefound in the vicinity of the Vanadium and East Tailings Piles. Historically, levels ofarsenic, molybdenum, selenium, vanadium, gross alpha, radium-226, radium-228,uranium-234, and uranium-238 have, at times, exceeded comparison values. Duringthe 1993 sampling rounds, levels of those compounds continued to exceed comparisonvalues in one or more on-site groundwater samples.
A sample collected during the November/December 1992 sampling round, from oneon-site Burro Canyon well (84-77) had uranium (43.43 pCi/L) and gross alpha (46.67pCi/L excluding uranium and radon) activities above the comparison value of 15pCi/L. Subsequent sampling rounds, up to July 1996, did not detect concentrationsabove the comparison values. This well will continue to be sampled to determinewhether the uranium and gross alpha activities measured in the July 1993 sample were anomalous or represented contamination in the aquifer. Other detected contaminant concentrations from on-site Burro Canyon wells were below comparison values.
|Constituent||Maximum Concentration1||Historical Maximum Concentration2||Comparison Value3||Source|
|Arsenic *||12 µg/L||166 µg/L||131 µg/L||10 µg/L||190 µg/L||54 µg/L||0.02 µg/L |
|812 µg/L||190 µg/L||60 µg/L||1,440 µg/L||213 µg/L||50 µg/L |
|Nitrate||20,900 µg/L||198,000 µg/L||28,600 µg/L||19,600 µg/L||67,766 µg/L||33,308 µg/L||10,000 µg/L||MCL|
|57 µg/L||51 µg/L||13 µg/L||160 µg/L||42 µg/L||30 µg/L |
|2,920 µg/L||2,890 µg/L||NA||3,630 µg/L||90 µg/L||30 µg/L||EMEG|
|Gross Alpha *||BDL|
|5,060 pCi/L||1,400 pCi/L||15 pCi/L||7,280 pCi/L||547 pCi/L||15 pCi/L||MCL|
|Radium-226 and -228 *||0.6 pCi/L||12 pCi/L||0.1 pCi/L||0.2 pCi/L||44 pCi/L||13 pCi/L||15 pCi/L||MCL|
and -238 *
|6 pCi/L||4,440 pCi/L||2,870 pCi/L||13 pCi/L||8,525 pCi/L||533 pCi/L||15 pCi/L||MCL|
|NOTE: Table 7 includes the following abbreviations and footnotes: |
BDL = below detection limit (analytical lower detection limit is in parentheses)
Downgradient alluvial aquifer monitoring wells on private property east of the mill site have provided evidence ofcontaminant migration. Previous and current groundwater sampling has detected levels of arsenic, molybdenum, selenium,gross alpha, radium-226, radium-228, uranium-234, and uranium-238 in concentrationsexceeding comparison values. Limited historical sampling data did not indicatedowngradient vanadium contamination in excess of comparison values; however, themore comprehensive recent sampling has detected vanadium at concentrationsexceeding comparison values. Comparison values have not been exceeded ingroundwater samples collected from downgradient off-site Burro Canyon Aquifer wells(84-74, 83-70, and 92-10) during either historical sampling or the recent samplingrounds (40).
Sampling for TCL (EPA's target compound list)--volatile organic compounds,semivolatile organic compounds, pesticides/PCBs, and herbicides--in the alluvial andBurro Canyon Aquifers has been conducted both historically and during the recentsampling rounds (see Appendix B). With the exception of a few semivolatile andvolatile organic compounds detected and confirmed as common laboratorycontaminants and introduced during the sampling and analysis process (acetone, bis[2-ethylhexyl] phthalate, chloroform, and methylene chloride), all concentrations of TCLvolatile organic compounds, TCL semivolatile organic compounds, TCLpesticides/PCBs, and TCL herbicides have been below comparison values (40).
Semivolatile and volatile organic compounds that were not TCL analytes but weredetected in groundwater samples were reported as tentatively identified compounds(TICs). A TIC is a chemical that is detected during analysis, but cannot be confirmedbecause the laboratory instrument utilized was not calibrated for that specific chemical. The result is an estimated concentration. Because of the low estimated concentrationsdetected (<58 µg/L), those chemicals are not considered potential contaminants of concern in groundwater (40).
A well abandonment project at the mill site was completed in September 1992. Thisproject included DOE's abandonment of three wells that were used for waterproduction during operation of the uranium mill, and four bedrock core holes that wereinstalled for investigative purposes in 1982. In 1996 numerous wells on the mill sitewere abandoned. Abandonment was necessary because of the age, unknownconstruction information, and lack of use of the wells. Abandonment also eliminated apotential conduit for contaminant migration from the alluvial aquifer into the Burro Canyon Aquifer (41).
C.2 Off-Site Groundwater Contamination
There has been no off-site monitoring of private wells used as domestic water sourcesby people living outside the city of Monticello. However, those wells are screened inthe lower Burro Canyon Aquifer, which has not shown evidence of site-relatedcontaminant concentrations in excess of comparison values. A definitive well surveyfollowed by initiation of private well monitoring should be considered if site-relatedcontaminants begin to appear in downgradient Burro Canyon Aquifer samples.
Air investigations have centered around two potential types of contaminants: 1) radon-222, a radioactive gas produced by the natural decay of radium-226, which is containedin the buried uranium mill tailings, and 2) airborne radioactive and nonradioactiveparticles associated with the tailings (1).
Extensive measurements of radon concentrations were done at 19 sampling locationsfrom November 2, 1983, to November 19, 1984. Duplicate samplers were placed 1meter (3.3 feet) above ground level at each location. The 19 sample stations weredivided among 3 different regions; 4 on site near the center of each tailings pile, 7 atthe edge of the mill site boundary, and 8 at off-site locations. These locations are shown in Appendix F, Figure 15.
The measured value for background was determined to be 0.41 pCi/L based on anaverage of the data points. This background value was added to the allowable increaseof 0.5 pCi/L to yield an administrative limit of 0.91 pCi/L. (The limit of 0.5 pCi/Lcomes from the 40 CFR 192 regulation for Inactive Uranium Processing Sites). Table8 shows the maximum amount of radon-222 found at each sampling location fromNovember 2, 1983, through November 19, 1984.
|Sampling Location||Maximum Concentration (pCi/L)|
|Edge of Site|
All but five locations exceeded the administrative limit of 0.91 pCi/L. The 19measurement locations used during this time were reduced to 8 thereafter. These 8locations are shown in Appendix F, Figure 16 (31). In response to increasedremediation activities, seven off-site locations were added during the third quarter of1993. Annual surveys of these 15 stations show elevated radon concentrations at threepoints (2 on site and 1 off site about 0.5 kilometer east of the mill site boundary). Thethree points that exceed the administrative limit of 0.91 pCi/L range from 1.0 to 3.3 pCi/L (see Table 9).
|Note: DOE is using 0.91 pCi/L as the maximum allowable based on a limit of0.50 pCi/L measured above a background of 0.41 pCi/L (40 CFR 192,Inactive Uranium Processing Sites).|
Two new radon monitors were also installed adjacent to the mill site in 1992 to monitorthe effect of increased construction activity at the mill site on ambient radonconcentrations (see Appendix F, Figure 16). Average Monthly Real-Time RadonMonitoring Results are shown in Table 10. Station 1 exceeded the EPA standardduring most of 1992, but concentrations at Station 2 were consistently below the EPA standard. During 1993 both stations were consistently below the EPA standard.
(40 CFR 192)
|NOTE: Table 10 includes the following abbreviations:|
pCi/L = picocuries per liter
Throughout the period of active operations, tailings from the mill site were used in thecity of Monticello as fill for open lands; as backfill around water, sewer, and electricallines; as sub-base for driveways, sidewalks, and concrete slabs; as backfill againstbasement foundations; and as sand mix in concrete, plaster, and mortar. The totaltonnage of tailings removed from the mill site is estimated at approximately 135,000tons (3). A potential health hazard exists from the radon-222 gas generated by theradioactive decay of radium-226 in those construction materials. The primary potentialfor exposure to radon-222 gas exists in confined spaces, without adequate ventilation,such as buildings where the gas can accumulate over time. Routine monitoring ofbuildings in Monticello has detected concentrations of radon in excess of comparisonvalues. Therefore, the potential for exposure to radon-222 gas is further evaluated inthe Pathways Analyses section, and the health effects resulting from exposure to radon-222 gas are presented in the Public Health Implications (A. Toxicological Evaluation)section of this public health assessment.
Air particulate measurements were begun at the mill site in August 1983. Samplingstations were located to the north and east in the path of prevailing wind patterns, withone background station placed west of the mill site. Sample station locations arepictured in Appendix F, Figure 17 (1). EPA has not accepted DOE's samplinglocations for background air particulate measurements. An audit will be conducted todetermine the appropriate locations for air monitoring. The samplers were placed 9feet above ground level and operated for 24 hours every sixth day. Samples were notcollected in winter months due to weather and snow cover on the tailings piles. Nonradioactive analytes that were detected included barium, copper, iron, lead,manganese, potassium, and vanadium. Maximum concentrations and the locationswhere they were detected are shown in Table 11. Detected concentrations were notsignificantly higher than ambient background concentrations; therefore, these analytes are not site-related contaminants (1).
|Barium||0.0135||5 North||0.52||EPA HEAST|
|Copper||0.0766||5 North||140||EPA HEAST|
|Vanadium||0.1305||4 East||26.0||EPA HEAST|
NOTE: Table 11 includes the following abbreviations:
µg/m3 = micrograms per cubic meter (air)
* Value believed to be without adverse health effects upon exposure.
Radium-226, thorium-230, and uranium-238 particulates were sampled from 1984through 1986 at locations near the mill site. Appendix F, Figure 17, shows thesampling locations. Sampling station 5 North had the highest concentration of radium-226 (0.0022 pCi/m3) and the 4 East sampling station had the highest concentration ofboth thorium-230 (0.0011 pCi/m3) and uranium-238 (0.0011 µg/m3) (1). Theseconcentrations are not at levels of public health concern.
Earlier air emissions during plant operation consisted of end products, processchemicals, and reaction products. The end products were uranium oxide (U3O8) andvanadium pentoxide (V2O5) released during a salt roast process used to recovervanadium. Both are relatively nonreactive; however, vanadium pentoxide is anoxidation catalyst (45). The primary process chemicals added during different stagesincluded sulfuric acid (a corrosive acid), sodium chlorate (a strong oxidizer), sodiumcarbonate (a base), and ammonium nitrate (a strong oxidizer with corrosive thermaldecomposition fumes). The reaction products formed during the chemical reactionswould have included a wide variety of compounds. This is because the ore contained a range of uranium and vanadium compounds, and the process chemicals would haveencountered a large number of chemical valence states during the reactions. Emissionsof these chemicals yielded about 1,200 kilogram per day of dust (46, 47). Increasedcorrosion of metal objects (fences, screen doors, and chrome automobile bumpers)presented evidence of these releases to the environment. Present atmosphericparticulate concentrations are far below the EPA's National Primary Air QualityStandards defined in the Clean Air Act 1977, as amended. Uranium is typicallymeasured three to four orders of magnitude below its respective Derived ConcentrationGuide (DCG), the concentration that would cause a member of the public to receive adose of 100 millirem per year from inhalation of a specific radionuclide. Lead (Pb),the contaminant closest to its DCG, showed concentrations typically less than 1/10 ofthe standard of 1 µg/m3. Consequently, lead measurements were discontinued in 1991and, according to the data reports, will be "restarted at the time of tailings removal" (48).
E.1 On-Site Food Chain Contamination
Contamination in the soil and water represent a potential for contamination of gameanimals on the mill site. The security fence does not prevent large game animals fromentering the mill site. Small game animals, such as rabbits, can also enter the mill sitefor grazing. Cattle are not presently pastured on the mill site, although cattle arepastured on lands immediately adjacent to the mill site.
E.2 Off-Site Food Chain Contamination
Contamination in the soil and water represent a potential for contamination of gameanimals, domestic cattle, and any food crops grown in the Montezuma Creek area. Several ranchers run cattle on the Montezuma Creek floodplain and canyondowngradient from the mill site.
EPA and UDEQ staff were equally concerned about food chain contamination. In thefall of 1996, EPA and UDEQ conducted a study of the body burden of contaminants intissues and organs of deer and cattle that consumed water and vegetation from theMontezuma Creek floodplain. EPA sampled cattle that were fenced in the middle andlower canyon. The deer that were harvested and sampled were the resident herd in theMontezuma Creek floodplain and canyon east of the mill site. Cattle and deer from abackground reference area were also sampled. The meat, liver kidney, and ribs arebeing analyzed for radionuclides and nonradionuclide contaminants. Although theanalyses have not yet been completed, preliminary results indicate little or nocontaminant uptake in cattle or deer above the uptake in the reference area animals.
This public health assessment incorporates environmental sampling data provided byDOE and MACTEC Environmental Restoration Services (formerly RUST Geotech,Inc., then formerly Chem-Nuclear Geotech), the primary DOE contractor at the millsite. ATSDR staff members assumed that adequate quality assurance and qualitycontrol (QA/QC) measures, as outlined in the August 1992 Sampling and Analysis Planfor Environmental Monitoring, were followed with regard to chain-of-custody,laboratory procedures, and data validation/reporting. The QA/QC measures applied to the media sampling data in the documents provided to ATSDR scientists appear to be consistent with standard protocols for environmental sampling and analysis.
Physical hazards observed at the mill site were heavy equipment operation, vehiculartraffic, and load handling. However, only DOE employees and contractors who havereceived prior safety training are permitted to work on site. General public access isrestricted. Staff members from the mill site's Occupational Health and Safety (OH&S)Office are present during the workday, conducting safety inspections and monitoringpersonnel exposure. There has been a fence around the mill site since August 1975,and lockable gates control access. Site visits did not produce any evidence oftrespassing. ATSDR staff members did not observe any physical hazards that wouldthreaten the general public's health.
ATSDR scientists will continue to review any future environmental contamination and otherhazards resources that become available. Should additional information become available thatalters the findings of this public health assessment or addresses issues described herein, thispublic health assessment will be modified as needed.
There are five main pathways into the human body for radioactivity and tailings-related substances from uranium mill sites:
- inhaling radon and radon daughters,
- inhaling and ingesting radioactive and chemical particles,
- ingesting contaminated foods produced in the area contaminated by radionuclides and nonradionuclide chemicals,
- drinking water contaminated by radionuclides and chemicals, and
- encountering external gamma ray exposure (1).
ATSDR scientists reviewed substantial information regarding exposure to and uptake ofradionuclides in the environment and the impact(s) on the public's health as we prepared thisdocument. We used a pathway model to look at the movement through entry points into the human body.
To determine whether people are exposed to contaminants migrating from a site, ATSDRrepresentatives evaluate the environmental and human components leading to human exposure. An exposure pathway consists of five elements: 1) a source of contamination, such as tailingspiles or waste pits; 2) an environmental medium in which the contaminants might be present orfrom which they might migrate, such as groundwater or soil; 3) points of human exposure, suchas drinking water wells or work areas; 4) routes of exposure, such as inhalation, ingestion, or dermal absorption; and 5) a potentially exposed population.
A completed exposure pathway occurs when the five elements of an exposure pathway link thecontaminant source to a receptor population. Should a completed exposure pathway exist in thepast, present, or future, the population is considered exposed.
A potential exposure pathway exists when one or more of the five elements are missing. Potential pathways indicate that exposure to a contaminant could have occurred in the past,could be occurring now, or could occur in the future.
Scientific studies identify the waste streams as they form and move through a plant such as themill. The plant's waste streams can be solid, liquid, gas, or any combination of the three. Eachstream will take some course through the environment and might eventually reach humans. Thisstudy traces those streams through the environment and shows ways they expose the humancommunity. Placing the streams on a chart known as a pathways model makes them easy to understand.
Figure 18, Appendix F, is a pathways model for a typical uranium mill. It applies to bothradioactive and nonradioactive materials. To use it, start with the top block, marked OperatingUranium Mill. Then trace along the arrows from block to block, noting the title of each block inorder until the path ends. As an example, you could find out how radioactive material orchemicals from the mill site get into hamburgers. One way is the air-soil-pasture grass-grazinganimal-meat pathway. This pathway existed when the mill operated. Uranium oxide leftthrough the roaster stack and followed the gaseous waste pathway into the air. From there ittook several paths, and in one it settled out onto the soil. The pasture grass absorbed it throughthe root system. Then grazing cattle ate the grass. The cattle were slaughtered for meat, andhumans ate hamburgers and steaks. Figure 18, Appendix F, shows several pathways by which the uranium oxide exhausted into air reached humans via the meat they ate.
Some pathways are more important than others for exposing people to radiation. Eachindividual's lifestyle, work and home locations, and eating habits constitute a unique pattern thatresults in various ways an individual might be exposed to radiation. Figure 19, Appendix F,shows the pathways that are perhaps most significant to the average person in the Monticellocommunity today. They are the ones that lead to inhaling radon gas and receiving directradiation from radioactive material deposited on soil. Others would include direct radiationfrom working with construction materials and eating food crops that contain radioactivematerials either inside or on the surfaces. The same food washing practices that are importantfrom a hygiene standpoint will probably be effective in removing radioactive material from thevegetables' surfaces as well. Most pathways have low potential with little chance of producingmeasurable exposure. Human radiation exposure from both inhaled radon gas and the tailingsthemselves is expected to be negligible once the tailings piles are removed and capped and allother contaminated areas are remediated.
Special pathways can be added for unique circumstances, such as a child playing on the tailingspiles. The solid waste-tailings piles-playing on tailings-direct radiation pathway would perhapsgive the largest dose equivalent. Incidental ingestion of dirt might also be an important sourceof exposure.
As Table 12 shows, we identified two completed surface soil pathways and one completed air pathway.
Past, current, and future completed exposure pathways are possible because of surfacesoil contamination. All soil contamination originated from the tailings piles. There hasbeen no nonradioactive surface soil sampling on site before 1995. The assumption isthat, because most of the surface soil has radium-226 levels above 15 pCi/g,nonradioactive contamination is also present. Workers employed in sampling andremediation activities at the mill site have the potential for occupational exposure to thepreviously discussed contaminants of concern through inhalation, ingestion, and dermalabsorption. They could be exposed to chemicals at the mill site while handling ofwaste materials through soil disturbance. Adhering to proper work practices andprocedures as defined by state or federal regulatory or permitting authorities caneliminate these possible exposures.
Workers may also be exposed by inhaling radon-222. Radon-222 comes from theradioactive decay of radium-226 in the tailings. Radon is a noble gas and thereforedoes not enter into chemical reactions that would fix or immobilize it; it subsequentlymigrates from the tailings into the atmosphere. On-site radon-222 measurements showlevels above normal background. Radiation doses from inhaled or ingestedradionuclides are adjusted using a series of modifying factors that account for thedifferent decay types and energies. By this method, internal and external doses can be summed.
Exposure to external gamma radiation from the tailings also poses a potential healthhazard. The Thermoluminescent Dosimetry (TLD) Quarterly Measurement Programbegan in April 1991, and the data were reviewed for three quarters in 1991; no data areavailable for the period since 1992. Results from these stations, most of which are nearthe mill site boundary, indicate rather significant annual doses delivered to the near-boundary vicinity. Measured exposure rates, at three locations, are approximately 200to 300 millirem per year (mrem/yr) above the nominal 100 mrem/yr background foundat the mill site. The three points with highest exposure are sites 5, 6, and 12 (Figure 20, Appendix F) (31).
Only DOE employees and contractors who have received prior safety training arepermitted to work on site. The general public's access is restricted. Staff membersfrom the mill site's Occupational Health and Safety (OH&S) Office are on site duringthe workday, conducting safety inspections and monitoring personnel exposure. Tolimit radiation exposure, and comply with the site safety plan, OH&S staff membersconduct routine radioactive surveillance that includes radiation surveys, surfacecontamination surveys, and air monitoring; they also establish controls for access toposted hazardous areas. Employees working on the mill site are required to participatein an occupational health program involving medical surveillance and exposuremonitoring.
There is a chain-link fence around the mill site, and lockable gates control access. Sitevisits did not produce any evidence of trespassing. The restricted access to the mill sitelimits the potential hazard to workers involved in site characterization and remediationactivities.
Past, current, and future completed exposure pathways are possible because of surfacesoil contamination. The major source of contamination is the tailings piles on the millsite. However, throughout the operating period, mill tailings from the Monticello MillTailings Site were used as fill for open lands; backfill around water, sewer, andelectrical lines; sub-base for driveways, sidewalks, and concrete slabs; and in backfill,plaster, and mortar for construction in the city of Monticello. The total amount ofuranium mill tailings removed from the mill site for construction purposes, althoughnever documented, is believed to be approximately 135,000 tons. The retrieval ofcontaminated tailings from the mill site was restricted by August 1975 (3).
Moreover, additional soils were windblown from the mill site to adjacent properties inMonticello and to stream sediments east of the mill site. The area east of the mill siteis used for cattle pasture and crop (for cattle, not human consumption) production. Theoff-site elements deposited in pasture soils might enter the food chain when they areingested with food crops and animal products. Contaminants have been and continueto be released from the tailings piles through natural events, such as rain and wind. Rain has washed contamination into Montezuma Creek. A major flood could releasesignificant amounts of contamination into the Montezuma Creek and floodplain. Contaminants have either leached from the tailings piles or been windblown into otherenvironmental media. Caps on each tailings pile have controlled this movement to adegree; however, contaminants have been detected in soils and sediments north andeast of the mill site.
Soil contamination off site might generate possible pathways of exposure for severalpopulations. Residents whose properties were contaminated by windblown erosion orwhose structures were constructed with tailings might be exposed through severalroutes. Ingestion of food potentially contaminated through uptake and accumulation of nonradioactive and radioactive substances by plants and animals is one route. Otherroutes include inhalation of contaminated dust particles and radon-222, dermal contactwith contaminated soil, or direct exposure to gamma radiation. Hunters, ranchers, andfarmers are potentially exposed to contaminants through ingestion of contaminatedfood, dermal absorption, inhalation of contaminated particulates, or direct exposure togamma radiation.
The Public Health Implications (A. Toxicological Evaluation) section of this publichealth assessment contains further discussion of potential adverse health effectsresulting from ingestion, inhalation, and dermal absorption of contaminants from off-site surface soils.
Off-site past, current, and future completed pathways are possible because of radiumcontamination at the mill site. Various levels of radon-222 gas have been detectedduring routine monitoring of off-site structures. The radioactive decay of radium-226in the soil generates radon-222 gas. Radon is a noble gas and therefore does not enterinto chemical reactions that would fix or immobilize it; it subsequently migrates fromthe contaminated soil into the atmosphere. Inhalation of radon and its alpha-emittingdecay products in confined spaces may increase human cancer risk. While this reporthas summarized outdoor concentrations of radon-222, we cannot make a completeevaluation of exposure to residents or workers until we analyze data from indoormeasurements.
The Public Health Implications (A. Toxicological Evaluation) section of this publichealth assessment contains further discussion of potential adverse health effectsresulting from inhalation of radon-222 gas.
Table 13 contains information on the groundwater, surface water, and food chain potentialexposure pathways.
Contaminants from the tailings piles (arsenic, molybdenum, nitrate, selenium,vanadium, gross alpha, radium-226, radium-228, uranium-234, and uranium-238) haveleached into the shallow alluvial aquifer. Those contaminants have been detected in the shallow aquifer at concentrations exceeding comparison values. However, directhuman contact with groundwater from the shallow aquifer, resulting in a completedexposure pathway, appears unlikely for two reasons. First, the shallow aquifer is notpresently used as a source of potable water and is unlikely to be used in the future as apublic water supply because of the unreliable well yield and limited saturatedthickness. Residents in the area downgradient of the mill site currently obtain theirwater from the Monticello public water supply, which uses uncontaminated,topographically upgradient surface water sources. Second, the extent of the aquifer,which is physically confined to the narrow boundaries of the Montezuma Creekalluvial gravels, is limited. The aquifer downgradient of the mill site is estimated to beno more than 500 feet wide, and the contamination plume extends no more than a miledowngradient before it discharges into the creek. The plume has, therefore, reached itsmaximum dimensions. To prevent use of the contaminated alluvial aquifer as a sourceof potable water, institutional controls (establishing local ordinances that prevent theinstallation of wells screened in the contaminated alluvial aquifer) are effective in ensuring that the aquifer is not used during the time required for restoration.
The shallow alluvial aquifer overlies the deeper Burro Canyon Aquifer, which is usedas a drinking water source. The Mancos Shale and shale units on the DakotaSandstone, which separate the Burro Canyon Formation from the alluvial aquifer, actas aquitards to limit downward migration from the alluvial aquifer.
Water sampling data for private residential drinking water wells in areas surroundingthe Monticello Mill Tailings Site are not available; furthermore, it is possible thatadditional undocumented private wells border the mill site, although we do not knowthat specifically. Potential for future exposure exists if any residents should use waterin the future from contaminated portions of the shallow alluvial aquifer.
The Public Health Implications (A. Toxicological Evaluation) section of this publichealth assessment contains further discussion of potential adverse health effectsresulting from ingestion of contaminated groundwater from the shallow alluvialaquifer.
Tailings-related contaminants enter Montezuma Creek where the contaminated alluvialaquifer discharges into the creek about a mile downstream of the mill site and by directsurface runoff from the tailings pile soil covers. Contaminants detected in surfacewater at concentrations exceeding comparison values include arsenic, molybdenum,nitrate, selenium, vanadium, gross alpha, radium-226, radium-228, uranium-234, anduranium-238. The major source of contamination is presently confined to the tailingspiles on the Monticello Mill Tailings Site. A potential worst-case migration scenariowould require that the pile cover be stripped away by a major flood and subsequentlycontaminate Montezuma Creek with contaminated mill site drainage. Ultimately, thetailings would be deposited with downstream sediments.
Historically, the highest off-site concentration of site-associated elements in the surfacewater occurs downstream, east of the tailings site, where the alluvial aquifer rechargesMontezuma Creek. Further downstream, contaminant concentrations are diluted tolevels below comparison values at the confluence of Montezuma Creek with the SanJuan River.
Montezuma Creek is not used for fishing or swimming or as a source of potable water;however, the potential exists for farmers, ranchers, and hunters to drink fromMontezuma Creek occasionally. Interviews with local residents indicate that theresulting exposures would be incidental and short term. The Public HealthImplications (A. Toxicological Evaluation) section of this public health assessmentcontains further discussion of potential adverse health effects resulting from those potential exposures.
The potentially exposed population includes farmers and ranchers living near theMontezuma Creek floodplain, one adjacent to the mill site and others within a fewmiles east of the mill site. The rancher raising livestock adjacent to the mill site usesMontezuma Creek as a source of water for his livestock. Another rancher raises cattlein a pasture along the creek and uses creek water to irrigate alfalfa, on which the cattlegraze. We do not know whether additional farmers downstream use Montezuma Creekwater. Because tailings-related contaminants have entered the creek through dischargeof the shallow alluvial aquifer beneath the tailings site and in direct surface runoff fromthe tailings pile soil covers, this water might be a potential cause of elevated soilconcentrations in the grazing area. By ingesting contaminated creek water, alfalfa, andsoil, the cattle can potentially accumulate tailings-related contaminants in their flesh,and then humans consuming the beef could potentially be exposed. Humans couldpotentially experience exposure by eating vegetables that accumulate contaminants ifthey were to be grown in the area in the future. In summary, contaminants detected insurface water, soils, and sediments can enter the food chain and ultimately result in exposure to humans who eat the contaminated meat and vegetables.
The Public Health Implications (A. Toxicological Evaluation) section of this publichealth assessment contains further discussion of potential adverse health effectsresulting from ingestion of contaminated beef and vegetables.
ATSDR scientists will continue to review any future exposure pathways resources that becomeavailable. Should additional information become available that alters the findings of this publichealth assessment or addresses issues described herein, this public health assessment will be modified as needed.