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1. ATSDR > Public Health Assessments & Consultations

PUBLIC HEALTH ASSESSMENT

MONTICELLO MILL TAILINGS (DOE) AND
MONTICELLO RADIOACTIVELY CONTAMINATED PROPERTIES
(aka MONTICELLO VICINITY PROPERTIES)

ENVIRONMENTAL CONTAMINATION AND OTHER HAZARDS

Contaminants discussed in later sections of this public health assessment are evaluated to determine whether exposure to them has public health significance. ATSDR staff members select and discuss contaminants based on several factors: sample design, field and laboratory data quality, and comparison of chemical concentrations to levels that could cause cancer or other health effects. We also consider community health concerns.

Evaluating the sample design involved reviewing Department of Energy Remediation Program regulations and the approach to locating contamination. ATSDR scientists consider spatial distribution of sampling locations, sampling frequency, concentration changes over time, medium-to-medium differences, and correlation between the selected list of analytical parameters and suspected environmental contaminants when determining the contaminants to which 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 to verify 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 other agencies have developed the comparison values to provide guidelines for estimating contaminant concentrations in media at which adverse health effects are not expected to occur. A standard daily ingestion rate and body weight are assumed in deriving these values. These values, in many cases, have been derived from animal studies. Health effects are related not only to the exposure dose but also to the route of entry into the body and the amount of chemical absorbed by the body. For those reasons, comparison values used in public health assessments are contaminant concentrations in specific media and for specific exposure routes. Several comparison values may be available for a specific contaminant. ATSDR scientists use the most conservative assumptions (that is, we assume exposure to the maximum concentration) in order to protect the most sensitive segment of the population. The Public Health Implications section of this document contains a discussion of the potential for adverse health effects from exposure to 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, below which adverse health effects are not expected to occur as a result of likely exposures. Comparison values are used to determine which contaminants require additional evaluation concerning possible exposure scenarios and adverse health effects. These levels are derived using conservative assumptions about exposures. Because of their conservative nature, and because they are not derived using site-specific information, comparison values should never be used as clean-up levels. Their use should be limited to the initial screening of site contamination information.

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 as BDL if the concentration detected is below the minimum concentration verifiable (can be duplicated through multiple analyses) by the analytical technique specified for that chemical. Analytical techniques have both a lower (minimum concentration detectable) and an upper (maximum concentration detectable) limit.
CREG	cancer risk evaluation guide (ATSDR). Derived by ATSDR from the EPA cancer slope factor. It represents a concentration in water, soil, or air at or below which excess 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's minimal risk level (MRL). It is the concentration in water, soil, or air at or below which daily human exposure is unlikely to result in adverse noncancerous effects.
RMEG	reference dose (or concentration) media evaluation guide (EPA). Derived by ATSDR from the EPA oral reference dose. It is the concentration in water or soil at or below which daily human exposure is unlikely to result in adverse noncancerous effects.
MCL	maximum contaminant level. Enforceable drinking water regulation established by EPA that is protective of human health to the "extent feasible" over a lifetime. MCLs represent contaminant concentrations that EPA scientists deem protective of public health over a lifetime (70 years) at an exposure rate of 2 liters of water per day. MCLs are also regulatory concentrations. MCLs take into account technological and economic feasibility.
mg/kg	milligrams per kilogram (parts per million). The unit applied to express contaminant concentrations in soil.
N/A	not applicable
NA	not available
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 contaminant concentrations in air or water.
µg/L	micrograms per liter of water (parts per billion). The unit applied to express contaminant concentrations in water.
RAC	reasonably anticipated to be a carcinogen (National Toxicology Program designation).
Class A Carcinogen	human carcinogen (EPA designation).
Class B1 Carcinogen	probable human carcinogen (EPA), based on limited human studies and sufficient animal studies.
Class B2 Carcinogen	probable human carcinogen (EPA), based on inadequate human studies and sufficient animal studies.

EMEGs and CREGs are the first choice for comparison values. In addition, any contaminants will be contaminants of concern if they have no CREG, but have been designated as carcinogens or potential carcinogens by 1) the National Toxicology Program in the Department of Health and Human Services, 2) the EPA, or 3) the International Agency for Research on Cancer. If a contaminant is not a carcinogen, and has no EMEG, then the following values (in order of preference) will be chosen for the comparison value if available: the RMEG, the lifetime health advisory (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, noncancerous adverse 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 water health goal recommended by EPA and set at a level at or below which no known or anticipated adverse human health effects are expected), the MCL, or the action level (derived by EPA for use in evaluating drinking water, the concentration in water at or below which daily human exposure is unlikely to result in adverse noncancerous effects).

We conducted a search of EPA's Toxic Chemical Release Inventory (TRI) for the San Juan County 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 each environmental medium (air, water, and land) by manufacturing facilities that employ more than 10 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 releases were listed as originating from communities in San Juan County.

A. Surface Soil Contamination

A.1 On-Site Surface Soil

Surface soils (0-6 inches) have been contaminated in various ways, including storage of ore in open stockpiles, emissions from the roaster stack (heat process used to convert vanadium minerals to a soluble form), overflow of tailings ponds, and the erosion of tailings piles by wind and water. Results of a radiometric survey show that most of the mill site surface soil contains concentrations of radium-226 exceeding EPA Standard 40 Code of Federal Regulations (CFR) 192.12 for cleanup of land and buildings contaminated with residual radioactive materials from inactive uranium processing sites. 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 at selected peripheral properties. The result of the analysis would provide data to support whether or not there is a correlation between radioactive and nonradioactive materials. As of yet, there are no convincing analyses or evaluations to support this unproven assumption.

Analytical results of soil samples, together with results of in-situ spectrometer measurements, 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 (0-15 cm, or approximately 6 inches) is 20 pCi/g over the mill site. Contamination of cover material 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 order of 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 during the Remedial Investigation in 1990 and again as part of the Monticello Remedial Action Program in 1991. Nonradioactive composite samples were taken from borings and pits when radium-226 measurements were above 15 pCi/g. Borings were drilled to various 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:



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)

Maximum concentrations of each nonradioactive element found in the tailings piles were evaluated for potential health implications. Seven contaminants of concern were identified: arsenic, beryllium, chromium, copper, lead, nickel, and vanadium. Table 4 contains the concentration found and comparison values for each. The Pathways Analysis and Public Health Implications sections of this public health assessment contain further discussions of each contaminant of concern.

A.2 Off-Site Surface Soil

North and east off-site areas contaminated with radium-226 above EPA standard 40 CFR 192.12 (5 pCi/g above background in the top 15 centimeters (cm) of soil or 15 pCi/g above background in any 15 cm layer below the top 15-cm) are predominantly farming lands but include some residences. Windblown surface soil contamination is found as far as 0.5 mile north and 0.25 mile east of the mill site. Radium-226 concentrations above EPA standards ranged from 6 pCi/g to 494 pCi/g and averaged 27 pCi/g (1).

The major source of nonradioactive contaminants is confined to the tailings piles on site. A much smaller area with lower concentration of contamination occurs in stream sediments 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 on the Montezuma Creek flood plain. Samples were taken in pasture soil south and east of the mill site in the flood plain, then were analyzed for tailings-related contaminants. Maximum concentrations of each contaminant were evaluated for potential health implications for any children who might play in the area. Additional sampling of sediments for nonradioactive contaminants were collected during the Operable Unit (OU) III study. The purpose of the OU III study was to collect sufficient information and data to characterize the nature and extent of environmental contamination in OU III, identify the sources of contamination, assess changes in contamination patterns over time once on-site sources (tailings piles) have been removed, and to calculate the levels of risk to human health and the environment from the contaminants associated with OU III. The OU III soil and sediment area, which is located entirely on private land, begins approximately 0.5 miles east of the eastern mill site boundary and extends downstream approximately 14,100 feet. The area is currently used for cattle grazing and recreational purposes; no residences are located within the OU III soil and sediment study area. Soil and sediment characterization began in 1994 and continued through September 1996. The primary source of soil and sediment contamination in the OU III soil and sediment study area is the mill site. Montezuma Creek, which flows through the tailings piles on the mill site, has been the primary transport mechanism for soils and sediments (38). Table 5 contains a list of contaminants of concern chosen for further consideration based upon sampling data collected during environmental monitoring program activities and the most recent OU III study. The Pathways Analyses section of this public health assessment addresses each contaminant of concern.

B. Surface Water Contamination

B.1 On-Site Surface Water Contamination

Surface water monitoring of Montezuma Creek has included collection of samples from upgradient (upstream), on site, and downgradient (downstream with respect to the mill site) locations. The creek, which flows through the mill site property, has frequently contained contaminants at levels exceeding comparison values as far as 3 miles downgradient of the property. Appendix F, Figure 11, depicts on-site surface water sampling locations, and Table 6 lists on-site surface water contaminants. Some contamination in the creek resulted from discharge of the contaminated alluvial aquifer beneath the mill site, although the primary source of contamination appeared to be past surface runoff from the tailings piles. Current controls in place collect and treat surface water before it is discharged. Alluvial groundwater is still providing base flow contaminants to Montezuma Creek. Montezuma Creek is used for both irrigation and livestock watering downgradient of the mill site.

To facilitate comparison of upgradient, on-site, and downgradient concentrations, the on-site and the off-site surface water contamination discussions of this public health assessment are combined.

B.2 Off-Site Surface Water Contamination

The two upgradient surface water sources used by the city of Monticello public water system are monitored in accordance with the Safe Drinking Water Act and Utah state requirements; those standards have not been exceeded for site-related contaminants. ATSDR representatives have used concentrations of chemicals detected during the monitoring program to depict naturally occurring concentrations for comparison with site-related data.

Montezuma Creek, the main surface water body in the project area, flows from west to east 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 bodies on the mill site include ponds, seeps, and drainages. Surface water sampling at the mill site has had four primary goals: 1) compare upstream water quality conditions of Montezuma Creek with conditions on site and downstream from the mill site, 2) characterize the type and extent of contamination in surface water sources, 3) verify compliance 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 supply about 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 and downgradient 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 sampling locations. Two sampling locations (W-3 and W-5) have been the historic sources of upgradient water quality samples from Montezuma Creek. In November 1992, locations SW92-01, SW92-02, and SW92-10 replaced W-3 and W-5 as upgradient sampling locations (40).

Before November 1992, on-site sampling was limited to three locations: the drainage between the Carbonate and Vanadium Tailings Piles (W-2), the seep-fed pond adjacent to the Carbonate Tailings Pile (designated Carbonate Seep), and the low spot between the Carbonate and Vanadium Tailings Piles (designated North Drainage). In November 1992, on-site sampling was expanded to include two locations on Montezuma Creek, SW92-04 and SW92-05 (41).

Figures 12a and 12b, in Appendix F depict downgradient surface water sampling locations. In past years, downgradient water quality within Montezuma Creek was monitored at three locations: the W-4 site, approximately 325 feet downstream of the east boundary of the property; the Sorenson site, approximately 1.25 miles downstream of the mill site, and the Montezuma Canyon site, approximately 6 miles downstream of the mill site. In November 1992, four additional locations were sampled downstream of the property (SW92-06, SW92-07, SW92-08, and SW92-09) (41).

From 1987 through April 1992, surface water samples were analyzed for the following constituents: gross alpha, radium-226, radium-228, uranium-234, uranium-238, thorium-230, arsenic, molybdenum, nitrate, selenium, and vanadium. Tests in the field measured alkalinity, pH, and specific conductance. In November 1992, the list of surface 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. During this same period, biological oxygen demand, chemical oxygen demand, and levels of fecal 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/May 1993, July 1993, October 1993, May 1994, October 1994, April 1995, October 1995, February 1996, April 1996, and June 1996) furnished surface water contamination data for comparison with concentrations detected upgradient, on site, and downgradient from 1984 to 1992. Those concentrations exceeding comparison values are selected as contaminants of concern. Table 6 presents those surface water contaminants detected in concentrations exceeding comparison values during the most recent and historical sampling rounds. Listing the contaminants of concern maximum concentrations for the most recent surface water sampling rounds and the historical maximum concentrations for those contaminants of concern portrays the site's actual impact on downgradient water quality over time. In the case of nitrate contamination, although the site did contribute to contamination of downgradient surface water, screening values were exceeded upgradient. The Pathways Analyses section of this public health assessment contains further discussions of the contaminants.

Concentrations of arsenic, molybdenum, selenium, vanadium, gross alpha, radium-226, uranium-234, and uranium-238 increase within Montezuma Creek as the creek flows across the mill site and downgradient. Seeps from the shallow aquifer are visible along the creek downstream of the eastern mill site boundary, and creek discharge increases throughout this section for approximately 1.25 miles. Historical assessments of water quality 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 values at 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 exceeded comparison values regularly. Selenium concentrations are also consistently below comparison values 6 miles downstream of the mill site at the Montezuma Canyon location. 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 the W-4, Sorenson, and Montezuma Canyon sampling locations. The trend continued through the 1995 sampling rounds (43).

Higher concentrations of mill tailings-related contaminants have been detected in the ponds and seeps on the mill site than in Montezuma Creek because the ponds and seeps are surface expressions of the groundwater (see On-Site Groundwater Contamination). Analyte concentrations in the seeps and ponds are similar to those in alluvial aquifer groundwater 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 in surface water because of historical and recent detection upgradient, on site, and downgradient at concentrations exceeding comparison values. Common agricultural activities around the mill site, such as the use of fertilizers, are known to cause nitrate contamination of surface water. Nitrate detected in upgradient surface water samples is not 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 least in part, site-related, resulting from former process operations at the mill. During the last 4 years of the mill site's active operations, ammonium nitrate and other miscellaneous oxidizers were added to a process for extracting and concentrating uranium from a liquid solution. A maximum of 2 tons per day of ammonium nitrate was used in the process, with the residual waste effluent from the process discharged to the Acid and East Tailings Piles (30). Nitrate was therefore selected as a contaminant of concern because both historical and recent concentrations in upgradient, on site, and downgradient surface water are elevated. The Public Health Implications (A. Toxicological Evaluation) section of this public health assessment contains further discussion of potential health effects of nitrate ingestion.

A wastewater treatment plant, designed to treat contaminated surface water runoff from the mill site and groundwater encountered during excavation to the tailings piles, began operation in May 1995. The wastewater treatment plant is designed to remove heavy metals and radionuclides from ground and surface wastewaters at an average flow rate of 60 gallons per minute. During operations, through August 21, 1995, influent and effluent samples were obtained and, with the exception of mercury and silver concentrations, effluent limits set by the Utah Department of Environmental Quality, Division of Water Quality, were not exceeded. A position paper was submitted to the Department of Water Quality in March 1996 to propose higher effluent limits for mercury and silver; approval was subsequently granted (44).

C. Groundwater Contamination

C.1 On-Site Groundwater Contamination

Two aquifers underlie the Monticello Mill Tailings Site (MMTS) and the surrounding area. Unconsolidated materials (such as loose sands and gravel) deposited by Montezuma Creek constitute an alluvial aquifer along the valley bottom. An underlying sandstone aquifer, the Burro Canyon Formation, is separated from the alluvial aquifer by the Mancos Shale Formation and by fine-grained units of the Dakota Sandstone Formation, both of which act as aquitards in the mill site area. Aquitards are low permeability geologic formations or groups of formations that impede groundwater flow from one aquifer to another (1).

The alluvial aquifer is approximately 16 feet thick near Montezuma Creek in the vicinity of the Carbonate Tailings Pile and thins gradually upgradient and downgradient from this location and toward the valley sides. Montezuma Creek is hydraulically connected (joined) with the alluvial aquifer on the upstream side of the East Tailings Pile. However, because of a realignment of the stream channel, the alluvial aquifer and Montezuma Creek are separated in the vicinity of the East Tailings Pile. 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 sediment gravel on the valley sides. Like the local surface waters, water levels within the aquifer fluctuate seasonally. The alluvial aquifer discharges into Montezuma Creek. Transmissivity values for the alluvial aquifer beneath the East Tailings Pile were determined from a pump test and ranged from 3.3 x 10^-4 to 5.4 x 10^-4 square meters per second. As the alluvial groundwater moves to the east and southeast across the mill site, it is degraded by contaminants leached from the mill tailings. Groundwater from the alluvial aquifer is not used in the vicinity of the mill site as a water source for human 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 from the alluvial aquifer by an aquitard consisting of the Mancos Shale Formation and fine-grained units of the Dakota Sandstone Formation. Those geological units limit downward migration from the alluvial aquifer. The EPA and Utah Department of Environmental Quality (UDEQ) have challenged DOE's interpretation of the hydrologic conditions. Site-specific information indicates that there is a joint/fracture system possibly related to the uplift of the Abajo mountains. The Mancos Shale on the mill site is weathered and varies in thickness from 0 to less than 40 feet. Studies of the hydrologic heads of paired wells in the Mancos Shale, Dakota Sandstone, and Burro Canyon Formation in the vicinity of the mill site indicate that the movement is downward. Although EPA and UDEQ remain confident that contamination has not reached the Burro Canyon Formation, there is conflicting data as to whether radiological contamination from the mill site is present in the Dakota Sandstone.

The Burro Canyon Aquifer is recharged through the tilted, exposed area of the formation located along the margin of the Abajo Dome west of the mill site. Discharge from the aquifer occurs across the Great Sage Plain, along erosional margins, and in areas where canyons dissect the formation. Numerous stock ponds and marshy areas are created as a result of spring-fed discharge from the aquifer (1). Residences in the Monticello area not connected to the municipal water supply use the deep Burro Canyon 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 area come from sampling of selected monitoring wells that were installed beginning in 1982. Although some other wells were installed before 1982, the validity of samples collected from those wells is questionable because of poor well completion records. Data cited in this public health assessment are from those wells considered to yield reliable samples on the basis of satisfactory well completion records and relatively consistent well performance over several years. ATSDR staff members reviewed the DOE-validated groundwater sampling data from the March 1984 through the June 1996 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 8 downgradient wells (5 alluvial, and 3 Burro Canyon). The upgradient wells characterize groundwater quality before contact with mill site contamination, the on-site wells characterize the extent of groundwater contamination on site, and the downgradient wells characterize the impact of the mill site contamination on groundwater before the water leaves the mill site. Appendix F, Figure 13, depicts groundwater monitoring well locations upgradient and on site; Appendix F, Figure 14, depicts groundwater monitoring well locations downgradient (46, 47). Groundwater samples were analyzed for organic and inorganic chemicals, and radioactive parameters. Organic analytes included EPA's target compound list (TCL): volatile organic 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); total dissolved 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/December 1992, March 1993, April/May 1993, July 1993, December 1993, May 1994, October 1994, April 1995, October 1995, February 1996, April 1996, and July 1996) provide data for comparing groundwater contamination that exceeds comparison values in the alluvial and Burro Canyon Aquifers, with concentrations detected upgradient, on site, and downgradient from 1984 to 1992. Table 7 presents groundwater contaminants detected in concentrations exceeding comparison values during the most recent and historical sampling rounds. The Exposure Pathways section of this public health assessment contains discussions of those contaminants.

Contaminant concentrations detected in samples from upgradient alluvial and Burro Canyon wells did not exceed comparison values during the most recent sampling rounds, with the exception of elevated nitrate concentrations detected in the alluvial aquifer. Nitrate detected in upgradient alluvial aquifer samples is not a site-related contaminant but is rather the result of agricultural activities; however, nitrate detected in 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 the mill site's active operations, ammonium nitrate and other miscellaneous oxidizers were added to a process for extracting and concentrating uranium from a liquid solution. A maximum of 2 tons per day of ammonium nitrate was used in the process, with the residual 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 and recent elevated concentrations in upgradient, on-site, and downgradient alluvial aquifer groundwater. The Public Health Implications (A. Toxicological Evaluation) section of this public health assessment contains further discussion of potential health effects of nitrate ingestion. Historical upgradient sampling (1984 to 1992) detected selenium and gross alpha contamination in excess of comparison values.

Groundwater sampled from the alluvial aquifer on site is contaminated by elements leached from the tailings piles. In general, the highest contaminant concentrations are found in the vicinity of the Vanadium and East Tailings Piles. Historically, levels of arsenic, molybdenum, selenium, vanadium, gross alpha, radium-226, radium-228, uranium-234, and uranium-238 have, at times, exceeded comparison values. During the 1993 sampling rounds, levels of those compounds continued to exceed comparison values in one or more on-site groundwater samples.

A sample collected during the November/December 1992 sampling round, from one on-site Burro Canyon well (84-77) had uranium (43.43 pCi/L) and gross alpha (46.67 pCi/L excluding uranium and radon) activities above the comparison value of 15 pCi/L. Subsequent sampling rounds, up to July 1996, did not detect concentrations above the comparison values. This well will continue to be sampled to determine whether 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.

Downgradient alluvial aquifer monitoring wells on private property east of the mill site have provided evidence of contaminant 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 concentrations exceeding comparison values. Limited historical sampling data did not indicate downgradient vanadium contamination in excess of comparison values; however, the more comprehensive recent sampling has detected vanadium at concentrations exceeding comparison values. Comparison values have not been exceeded in groundwater samples collected from downgradient off-site Burro Canyon Aquifer wells (84-74, 83-70, and 92-10) during either historical sampling or the recent sampling rounds (40).

Sampling for TCL (EPA's target compound list)--volatile organic compounds, semivolatile organic compounds, pesticides/PCBs, and herbicides--in the alluvial and Burro Canyon Aquifers has been conducted both historically and during the recent sampling rounds (see Appendix B). With the exception of a few semivolatile and volatile organic compounds detected and confirmed as common laboratory contaminants and introduced during the sampling and analysis process (acetone, bis[2-ethylhexyl] phthalate, chloroform, and methylene chloride), all concentrations of TCL volatile organic compounds, TCL semivolatile organic compounds, TCL pesticides/PCBs, and TCL herbicides have been below comparison values (40).

Semivolatile and volatile organic compounds that were not TCL analytes but were detected in groundwater samples were reported as tentatively identified compounds (TICs). A TIC is a chemical that is detected during analysis, but cannot be confirmed because the laboratory instrument utilized was not calibrated for that specific chemical. The result is an estimated concentration. Because of the low estimated concentrations detected (<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. This project included DOE's abandonment of three wells that were used for water production during operation of the uranium mill, and four bedrock core holes that were installed for investigative purposes in 1982. In 1996 numerous wells on the mill site were abandoned. Abandonment was necessary because of the age, unknown construction information, and lack of use of the wells. Abandonment also eliminated a potential 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 sources by people living outside the city of Monticello. However, those wells are screened in the lower Burro Canyon Aquifer, which has not shown evidence of site-related contaminant concentrations in excess of comparison values. A definitive well survey followed by initiation of private well monitoring should be considered if site-related contaminants begin to appear in downgradient Burro Canyon Aquifer samples.

D. Air Contamination

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 contained in the buried uranium mill tailings, and 2) airborne radioactive and nonradioactive particles associated with the tailings (1).

D.1 Radon in Air

Extensive measurements of radon concentrations were done at 19 sampling locations from November 2, 1983, to November 19, 1984. Duplicate samplers were placed 1 meter (3.3 feet) above ground level at each location. The 19 sample stations were divided among 3 different regions; 4 on site near the center of each tailings pile, 7 at the 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 an average of the data points. This background value was added to the allowable increase of 0.5 pCi/L to yield an administrative limit of 0.91 pCi/L. (The limit of 0.5 pCi/L comes from the 40 CFR 192 regulation for Inactive Uranium Processing Sites). Table 8 shows the maximum amount of radon-222 found at each sampling location from November 2, 1983, through November 19, 1984.

All but five locations exceeded the administrative limit of 0.91 pCi/L. The 19 measurement locations used during this time were reduced to 8 thereafter. These 8 locations are shown in Appendix F, Figure 16 (31). In response to increased remediation activities, seven off-site locations were added during the third quarter of 1993. Annual surveys of these 15 stations show elevated radon concentrations at three points (2 on site and 1 off site about 0.5 kilometer east of the mill site boundary). The three points that exceed the administrative limit of 0.91 pCi/L range from 1.0 to 3.3 pCi/L (see Table 9).

Two new radon monitors were also installed adjacent to the mill site in 1992 to monitor the effect of increased construction activity at the mill site on ambient radon concentrations (see Appendix F, Figure 16). Average Monthly Real-Time Radon Monitoring Results are shown in Table 10. Station 1 exceeded the EPA standard during most of 1992, but concentrations at Station 2 were consistently below the EPA standard. During 1993 both stations were consistently below the EPA standard.

Throughout the period of active operations, tailings from the mill site were used in the city of Monticello as fill for open lands; as backfill around water, sewer, and electrical lines; as sub-base for driveways, sidewalks, and concrete slabs; as backfill against basement foundations; and as sand mix in concrete, plaster, and mortar. The total tonnage of tailings removed from the mill site is estimated at approximately 135,000 tons (3). A potential health hazard exists from the radon-222 gas generated by the radioactive decay of radium-226 in those construction materials. The primary potential for exposure to radon-222 gas exists in confined spaces, without adequate ventilation, such as buildings where the gas can accumulate over time. Routine monitoring of buildings in Monticello has detected concentrations of radon in excess of comparison values. Therefore, the potential for exposure to radon-222 gas is further evaluated in the 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.

D.2 Nonradioactive Particulates

Air particulate measurements were begun at the mill site in August 1983. Sampling stations were located to the north and east in the path of prevailing wind patterns, with one background station placed west of the mill site. Sample station locations are pictured in Appendix F, Figure 17 (1). EPA has not accepted DOE's sampling locations for background air particulate measurements. An audit will be conducted to determine the appropriate locations for air monitoring. The samplers were placed 9 feet above ground level and operated for 24 hours every sixth day. Samples were not collected 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 locations where they were detected are shown in Table 11. Detected concentrations were not significantly higher than ambient background concentrations; therefore, these analytes are not site-related contaminants (1).

D.3 Radioactive Particulates

Radium-226, thorium-230, and uranium-238 particulates were sampled from 1984 through 1986 at locations near the mill site. Appendix F, Figure 17, shows the sampling 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 of both thorium-230 (0.0011 pCi/m³) and uranium-238 (0.0011 µg/m³) (1). These concentrations are not at levels of public health concern.

D.4 Past Air Emissions

Earlier air emissions during plant operation consisted of end products, process chemicals, and reaction products. The end products were uranium oxide (U₃O₈) and vanadium pentoxide (V₂O₅) released during a salt roast process used to recover vanadium. Both are relatively nonreactive; however, vanadium pentoxide is an oxidation catalyst (45). The primary process chemicals added during different stages included sulfuric acid (a corrosive acid), sodium chlorate (a strong oxidizer), sodium carbonate (a base), and ammonium nitrate (a strong oxidizer with corrosive thermal decomposition fumes). The reaction products formed during the chemical reactions would 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 have encountered a large number of chemical valence states during the reactions. Emissions of these chemicals yielded about 1,200 kilogram per day of dust (46, 47). Increased corrosion of metal objects (fences, screen doors, and chrome automobile bumpers) presented evidence of these releases to the environment. Present atmospheric particulate concentrations are far below the EPA's National Primary Air Quality Standards defined in the Clean Air Act 1977, as amended. Uranium is typically measured three to four orders of magnitude below its respective Derived Concentration Guide (DCG), the concentration that would cause a member of the public to receive a dose 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 of the standard of 1 µg/m³. Consequently, lead measurements were discontinued in 1991 and, according to the data reports, will be "restarted at the time of tailings removal" (48).

E. Food Chain Contamination

E.1 On-Site Food Chain Contamination

Contamination in the soil and water represent a potential for contamination of game animals on the mill site. The security fence does not prevent large game animals from entering the mill site. Small game animals, such as rabbits, can also enter the mill site for grazing. Cattle are not presently pastured on the mill site, although cattle are pastured 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 game animals, domestic cattle, and any food crops grown in the Montezuma Creek area. Several ranchers run cattle on the Montezuma Creek floodplain and canyon downgradient from the mill site.

EPA and UDEQ staff were equally concerned about food chain contamination. In the fall of 1996, EPA and UDEQ conducted a study of the body burden of contaminants in tissues and organs of deer and cattle that consumed water and vegetation from the Montezuma Creek floodplain. EPA sampled cattle that were fenced in the middle and lower canyon. The deer that were harvested and sampled were the resident herd in the Montezuma Creek floodplain and canyon east of the mill site. Cattle and deer from a background reference area were also sampled. The meat, liver kidney, and ribs are being analyzed for radionuclides and nonradionuclide contaminants. Although the analyses have not yet been completed, preliminary results indicate little or no contaminant uptake in cattle or deer above the uptake in the reference area animals.

F. Quality Assurance and Quality Control

This public health assessment incorporates environmental sampling data provided by DOE and MACTEC Environmental Restoration Services (formerly RUST Geotech, Inc., then formerly Chem-Nuclear Geotech), the primary DOE contractor at the mill site. ATSDR staff members assumed that adequate quality assurance and quality control (QA/QC) measures, as outlined in the August 1992 Sampling and Analysis Plan for 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.

G. Physical Hazards

Physical hazards observed at the mill site were heavy equipment operation, vehicular traffic, and load handling. However, only DOE employees and contractors who have received prior safety training are permitted to work on site. General public access is restricted. Staff members from the mill site's Occupational Health and Safety (OH&S) Office are present during the workday, conducting safety inspections and monitoring personnel 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 of trespassing. ATSDR staff members did not observe any physical hazards that would threaten the general public's health.

ATSDR scientists will continue to review any future environmental contamination and other hazards resources that become available. Should additional information become available that alters the findings of this public health assessment or addresses issues described herein, this public health assessment will be modified as needed.

PATHWAYS ANALYSES

There are five main pathways into the human body for radioactivity and tailings-related substances from uranium mill sites:

1. inhaling radon and radon daughters,
   
2. inhaling and ingesting radioactive and chemical particles,
   
3. ingesting contaminated foods produced in the area contaminated by radionuclides and nonradionuclide chemicals,
   
4. drinking water contaminated by radionuclides and chemicals, and
   
5. encountering external gamma ray exposure (1).

ATSDR scientists reviewed substantial information regarding exposure to and uptake of radionuclides in the environment and the impact(s) on the public's health as we prepared this document. 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, ATSDR representatives evaluate the environmental and human components leading to human exposure. An exposure pathway consists of five elements: 1) a source of contamination, such as tailings piles or waste pits; 2) an environmental medium in which the contaminants might be present or from which they might migrate, such as groundwater or soil; 3) points of human exposure, such as 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 the contaminant source to a receptor population. Should a completed exposure pathway exist in the past, 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.

A. Pathways Model

Scientific studies identify the waste streams as they form and move through a plant such as the mill. The plant's waste streams can be solid, liquid, gas, or any combination of the three. Each stream will take some course through the environment and might eventually reach humans. This study traces those streams through the environment and shows ways they expose the human community. 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 both radioactive and nonradioactive materials. To use it, start with the top block, marked Operating Uranium Mill. Then trace along the arrows from block to block, noting the title of each block in order until the path ends. As an example, you could find out how radioactive material or chemicals from the mill site get into hamburgers. One way is the air-soil-pasture grass-grazing animal-meat pathway. This pathway existed when the mill operated. Uranium oxide left through the roaster stack and followed the gaseous waste pathway into the air. From there it took several paths, and in one it settled out onto the soil. The pasture grass absorbed it through the root system. Then grazing cattle ate the grass. The cattle were slaughtered for meat, and humans 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. Each individual's lifestyle, work and home locations, and eating habits constitute a unique pattern that results 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 Monticello community today. They are the ones that lead to inhaling radon gas and receiving direct radiation from radioactive material deposited on soil. Others would include direct radiation from working with construction materials and eating food crops that contain radioactive materials either inside or on the surfaces. The same food washing practices that are important from a hygiene standpoint will probably be effective in removing radioactive material from the vegetables' surfaces as well. Most pathways have low potential with little chance of producing measurable exposure. Human radiation exposure from both inhaled radon gas and the tailings themselves is expected to be negligible once the tailings piles are removed and capped and all other contaminated areas are remediated.

Special pathways can be added for unique circumstances, such as a child playing on the tailings piles. The solid waste-tailings piles-playing on tailings-direct radiation pathway would perhaps give the largest dose equivalent. Incidental ingestion of dirt might also be an important source of exposure.

B. Completed Exposure Pathways

As Table 12 shows, we identified two completed surface soil pathways and one completed air pathway.

B.1 On-Site Surface Soil Pathway

Past, current, and future completed exposure pathways are possible because of surface soil contamination. All soil contamination originated from the tailings piles. There has been no nonradioactive surface soil sampling on site before 1995. The assumption is that, because most of the surface soil has radium-226 levels above 15 pCi/g, nonradioactive contamination is also present. Workers employed in sampling and remediation activities at the mill site have the potential for occupational exposure to the previously discussed contaminants of concern through inhalation, ingestion, and dermal absorption. They could be exposed to chemicals at the mill site while handling of waste materials through soil disturbance. Adhering to proper work practices and procedures as defined by state or federal regulatory or permitting authorities can eliminate these possible exposures.

Workers may also be exposed by inhaling radon-222. Radon-222 comes from the radioactive decay of radium-226 in the tailings. Radon is a noble gas and therefore does not enter into chemical reactions that would fix or immobilize it; it subsequently migrates from the tailings into the atmosphere. On-site radon-222 measurements show levels above normal background. Radiation doses from inhaled or ingested radionuclides are adjusted using a series of modifying factors that account for the different 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 health hazard. The Thermoluminescent Dosimetry (TLD) Quarterly Measurement Program began in April 1991, and the data were reviewed for three quarters in 1991; no data are available for the period since 1992. Results from these stations, most of which are near the mill site boundary, indicate rather significant annual doses delivered to the near-boundary vicinity. Measured exposure rates, at three locations, are approximately 200 to 300 millirem per year (mrem/yr) above the nominal 100 mrem/yr background found at 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 are permitted to work on site. The general public's access is restricted. Staff members from the mill site's Occupational Health and Safety (OH&S) Office are on site during the workday, conducting safety inspections and monitoring personnel exposure. To limit radiation exposure, and comply with the site safety plan, OH&S staff members conduct routine radioactive surveillance that includes radiation surveys, surface contamination surveys, and air monitoring; they also establish controls for access to posted hazardous areas. Employees working on the mill site are required to participate in an occupational health program involving medical surveillance and exposure monitoring.

There is a chain-link fence around the mill site, and lockable gates control access. Site visits did not produce any evidence of trespassing. The restricted access to the mill site limits the potential hazard to workers involved in site characterization and remediation activities.

B.2 Off-Site Surface Soil Pathway

Past, current, and future completed exposure pathways are possible because of surface soil contamination. The major source of contamination is the tailings piles on the mill site. However, throughout the operating period, mill tailings from the Monticello Mill Tailings Site were used as fill for open lands; backfill around water, sewer, and electrical 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 of uranium mill tailings removed from the mill site for construction purposes, although never documented, is believed to be approximately 135,000 tons. The retrieval of contaminated tailings from the mill site was restricted by August 1975 (3).

Moreover, additional soils were windblown from the mill site to adjacent properties in Monticello and to stream sediments east of the mill site. The area east of the mill site is used for cattle pasture and crop (for cattle, not human consumption) production. The off-site elements deposited in pasture soils might enter the food chain when they are ingested with food crops and animal products. Contaminants have been and continue to 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 release significant amounts of contamination into the Montezuma Creek and floodplain. Contaminants have either leached from the tailings piles or been windblown into other environmental media. Caps on each tailings pile have controlled this movement to a degree; however, contaminants have been detected in soils and sediments north and east of the mill site.

Soil contamination off site might generate possible pathways of exposure for several populations. Residents whose properties were contaminated by windblown erosion or whose structures were constructed with tailings might be exposed through several routes. Ingestion of food potentially contaminated through uptake and accumulation of nonradioactive and radioactive substances by plants and animals is one route. Other routes include inhalation of contaminated dust particles and radon-222, dermal contact with contaminated soil, or direct exposure to gamma radiation. Hunters, ranchers, and farmers are potentially exposed to contaminants through ingestion of contaminated food, dermal absorption, inhalation of contaminated particulates, or direct exposure to gamma radiation.

The Public Health Implications (A. Toxicological Evaluation) section of this public health assessment contains further discussion of potential adverse health effects resulting from ingestion, inhalation, and dermal absorption of contaminants from off-site surface soils.

B.3 Air Pathway

Off-site past, current, and future completed pathways are possible because of radium contamination at the mill site. Various levels of radon-222 gas have been detected during routine monitoring of off-site structures. The radioactive decay of radium-226 in the soil generates radon-222 gas. Radon is a noble gas and therefore does not enter into chemical reactions that would fix or immobilize it; it subsequently migrates from the contaminated soil into the atmosphere. Inhalation of radon and its alpha-emitting decay products in confined spaces may increase human cancer risk. While this report has summarized outdoor concentrations of radon-222, we cannot make a complete evaluation of exposure to residents or workers until we analyze data from indoor measurements.

The Public Health Implications (A. Toxicological Evaluation) section of this public health assessment contains further discussion of potential adverse health effects resulting from inhalation of radon-222 gas.

C. Potential Exposure Pathways

Table 13 contains information on the groundwater, surface water, and food chain potential exposure pathways.

Table 13. Potential Exposure Pathways
Path Name	Compounds	Exposure Pathway Elements					Time
		Source	Media	Point of Exposure	Route of Exposure	Exposed Population
Groundwater Upper Aquifer	Arsenic Molybdenum Nitrate Selenium Vanadium Gross Alpha Radium-226,-228 Uranium-234,-238	Tailings Piles	Groundwater	Off-Site Private Wells Used for Drinking Water	Ingestion	Private Well Users Tapping the Upper Aquifer (unknown number) (Most likely an improbable occurrence and will remain incomplete, although there is a potential for use in the future.)	Future
Surface Water	Arsenic Molybdenum Nitrate Selenium Vanadium Gross Alpha Radium-226,-228 Uranium-234,-238	Tailings Piles	Surface Water	Off-Site Montezuma Creek	Ingestion	Farmers Ranchers Hunters	Past Current Future
Food Chain	Arsenic Molybdenum Nitrate Selenium Vanadium Gross Alpha Radium-226,-228 Uranium-234,-238	Tailings Piles	Surface Water Uptake Into the Food Chain	Contaminated Meat/Plants	Ingestion	Consumers	Past Current Future

C.1 Groundwater Potential Pathway

Contaminants from the tailings piles (arsenic, molybdenum, nitrate, selenium, vanadium, gross alpha, radium-226, radium-228, uranium-234, and uranium-238) have leached into the shallow alluvial aquifer. Those contaminants have been detected in the shallow aquifer at concentrations exceeding comparison values. However, direct human contact with groundwater from the shallow aquifer, resulting in a completed exposure pathway, appears unlikely for two reasons. First, the shallow aquifer is not presently used as a source of potable water and is unlikely to be used in the future as a public water supply because of the unreliable well yield and limited saturated thickness. Residents in the area downgradient of the mill site currently obtain their water 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 Creek alluvial gravels, is limited. The aquifer downgradient of the mill site is estimated to be no more than 500 feet wide, and the contamination plume extends no more than a mile downgradient before it discharges into the creek. The plume has, therefore, reached its maximum dimensions. To prevent use of the contaminated alluvial aquifer as a source of potable water, institutional controls (establishing local ordinances that prevent the installation 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 used as a drinking water source. The Mancos Shale and shale units on the Dakota Sandstone, which separate the Burro Canyon Formation from the alluvial aquifer, act as aquitards to limit downward migration from the alluvial aquifer.

Water sampling data for private residential drinking water wells in areas surrounding the Monticello Mill Tailings Site are not available; furthermore, it is possible that additional undocumented private wells border the mill site, although we do not know that specifically. Potential for future exposure exists if any residents should use water in the future from contaminated portions of the shallow alluvial aquifer.

The Public Health Implications (A. Toxicological Evaluation) section of this public health assessment contains further discussion of potential adverse health effects resulting from ingestion of contaminated groundwater from the shallow alluvial aquifer.

C.2 Surface Water Potential Pathway

Tailings-related contaminants enter Montezuma Creek where the contaminated alluvial aquifer discharges into the creek about a mile downstream of the mill site and by direct surface runoff from the tailings pile soil covers. Contaminants detected in surface water at concentrations exceeding comparison values include arsenic, molybdenum, nitrate, selenium, vanadium, gross alpha, radium-226, radium-228, uranium-234, and uranium-238. The major source of contamination is presently confined to the tailings piles on the Monticello Mill Tailings Site. A potential worst-case migration scenario would require that the pile cover be stripped away by a major flood and subsequently contaminate Montezuma Creek with contaminated mill site drainage. Ultimately, the tailings would be deposited with downstream sediments.

Historically, the highest off-site concentration of site-associated elements in the surface water occurs downstream, east of the tailings site, where the alluvial aquifer recharges Montezuma Creek. Further downstream, contaminant concentrations are diluted to levels below comparison values at the confluence of Montezuma Creek with the San Juan 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 from Montezuma Creek occasionally. Interviews with local residents indicate that the resulting exposures would be incidental and short term. The Public Health Implications (A. Toxicological Evaluation) section of this public health assessment contains further discussion of potential adverse health effects resulting from those potential exposures.

C.3 Food Chain Potential Pathway

The potentially exposed population includes farmers and ranchers living near the Montezuma Creek floodplain, one adjacent to the mill site and others within a few miles east of the mill site. The rancher raising livestock adjacent to the mill site uses Montezuma Creek as a source of water for his livestock. Another rancher raises cattle in a pasture along the creek and uses creek water to irrigate alfalfa, on which the cattle graze. We do not know whether additional farmers downstream use Montezuma Creek water. Because tailings-related contaminants have entered the creek through discharge of the shallow alluvial aquifer beneath the tailings site and in direct surface runoff from the tailings pile soil covers, this water might be a potential cause of elevated soil concentrations in the grazing area. By ingesting contaminated creek water, alfalfa, and soil, the cattle can potentially accumulate tailings-related contaminants in their flesh, and then humans consuming the beef could potentially be exposed. Humans could potentially experience exposure by eating vegetables that accumulate contaminants if they were to be grown in the area in the future. In summary, contaminants detected in surface 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 public health assessment contains further discussion of potential adverse health effects resulting from ingestion of contaminated beef and vegetables.

ATSDR scientists will continue to review any future exposure pathways resources that become available. Should additional information become available that alters the findings of this public health assessment or addresses issues described herein, this public health assessment will be modified as needed.

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