DECISION SUMMARY

SECTION 1

SITE NAME, LOCATION AND DESCRIPTION

The Richardson Flat Tailings (RFT) site (Site) is located 1.5 miles northeast of Park City, Utah, and is part of a 650 acre property owned by United Park City Mines (UPCM) Company (Figure 1). The Site is a tailings impoundment that covers 160 acres in the northwest corner of the UPCM property, a small portion of the much larger Upper Silver Creek Watershed (Figure 2). Silver Creek is the primary surface water source found in the area and is comprised of runoff from three significant drainages in the watershed, including Ontario Canyon, Empire Canyon and Deer Valley (Figure 3). Silver Creek is currently listed on Utah’s 303(d) list for zinc and cadmium and is targeted for total maximum daily load (TMDL) development. Historic mining activities in the canyons left behind six active Comprehensive Environmental Response, Compensation and Liability Information System (CERCLIS) sites, including Empire Canyon, Silver Creek Tailings, and Silver Maple Claims, each one impacting Silver Creek in some way. While zinc and cadmium are the primary heavy metals found in Silver Creek, lead and arsenic are the main contaminants in the sediments and soils of the watershed. Because of the volume of mining activity throughout the district and the dynamics of the watershed hydrogeology, it is difficult to target any one site as the main source of contamination affecting Silver Creek and the environmental media within the watershed. The overall remedial goal for the watershed is to clean up the surrounding sites, including the Site, thereby eliminating current and future hazards to human health and welfare and the surrounding environment.

The RFT site is a geometrically closed basin, bound by highway 248 to the north, a main embankment to the west, and diversion ditches to the south and the northeast (Figure 4). Silver Creek can be found on the northwest border of the Site, separated from the Site by a small stretch of wetlands and riparian vegetation. The impoundment was used as a mine tailings reservoir prior to 1950. The Site now houses approximately seven million tons of sand-sized carbonaceous particles and minerals containing zinc, silver, lead, and other metals. Use of the Site by UPCM ended in 1982. To date, the Site is not listed on the National Priorities List (NPL). The Site was considered for listing in both 1988 and 1992. UPCM, the primary potentially responsible party (PRP), has taken responsibility for funding the majority of the remedial action at the Site.

SECTION 2

SITE HISTORY AND ENFORCEMENT ACTIVITIES

2.1 HISTORICAL LAND USE

In 1953, UPCM was formed through the consolidation of Silver King Coalition Mines Company and Park Utah Consolidated Mines Company. At that time, the Site was already being used as an impoundment for mine tailings consisting primarily of sand-sized carbonaceous particles and minerals containing lead, zinc, silver and other metals. Additionally, tailings were transported to and placed in several distinct low elevation areas in the southeast portion of the Site just outside of the main impoundment.

In 1970, with renewed mining activity in the area, Park City Ventures (PCV), a joint venture partnership between Anaconda Copper Company and American Smelting Company (ASARCO), entered into a lease agreement with UPCM. This agreement allowed PCV to deposit additional mine tailings at the Site; however, the Site had to be partially reconstructed. Dames and Moore provided PCV with design, construction and operation specifications which were approved by the State of Utah. These specifications included installation of a large embankment along the western edge of the impoundment, and construction of containment dike structures along the southern and eastern boarders of the Site for additional tailings storage. PCV also created a diversion ditch system along the higher slopes north of the impoundment and outside of the containment dikes along the east and south perimeters of the impoundment to collect surface run off. As part of the approval process for the renewed use of the Site, the State of Utah required installation of groundwater monitoring wells near the base of the main embankment.

Over the course of PVC’s use of the Site, about 450,000 tons of tailings were deposited at the Site through a slurry pipeline that originated at their mill facility. Dames and Moore had recommended that the tailings be deposited around the perimeter of the Site, moving towards the center of the Site over time. However, PVC chose to deposit the tailings from the slurry pipeline in one constant area in the center of the impoundment, creating a steep, cone-like structure in the middle of the impoundment. After PVC discontinued their use of the Site in 1982, high winds caused tailings from the cone-shaped feature to become airborne, creating a potentially significant exposure pathway. These operations shaped the topography of the impoundment which still exists today.

From 1980 to 1982, Noranda Mining, Inc. leased the mining and milling operations and placed an additional 70,000 tons of tailings at the Site. Since then no further use of the Site has occurred, but UPCM began taking actions aimed at improving environmental conditions of the Site almost immediately after operations stopped. This work continued intermittently through the mid-1990s. These actions are described in the Site Characteristics Section of this Record of Decision (ROD).

2.2 INVESTIGATION HISTORY

EPA became aware of the Site in the mid-1980s. After initial site assessment work, EPA proposed the Site for listing on the NPL in 1988. After considering public comment, EPA did not pursue the Site for listing on the NPL. By 1992, the Hazard Ranking System (HRS) had been revised and EPA again proposed the Site for listing on the NPL. Ultimately, EPA decided not to pursue final listing on the NPL, and the Site remains proposed for the NPL at this time.

Subsequent to the second NPL proposal, the EPA Region 8 Superfund Emergency Response Branch conducted an investigation under the “Make Sites Safe” Initiative in 1993. This investigation concluded that conditions of the Site did not warrant emergency removal actions, but may present unacceptable risks to human health and the environment and should be addressed through long-term remedial action.

Throughout the 1990s, EPA and the Utah Department of Environmental Quality (UDEQ) were hoping UPCM would address the Site through the Utah Voluntary Cleanup Program. UPCM decided against this, but at the same time continued to voluntarily take steps to improve environmental conditions at the Site. Additionally, UPCM began collecting hydrogeologic data, which was used to better understand the groundwater flow and depth of tailings at the Site.

In 1999, EPA, UDEQ, UPCM, Park City Municipal Corporation, and other stakeholders formed the Upper Silver Creek Watershed Stakeholder’s Group (USCWSG). This community-based organization was formed to help EPA address Superfund-related environmental issues in the Park City area in a cooperative fashion, including issues related to the Site. The USCWSG has been very successful and several investigations and cleanups have occurred in Park City as a result. Early in USCWSG’s history, UPCM and EPA agreed to address the Site as an “NPL equivalent” site, using the same process for investigation and cleanup that is required for a NPL Site.

2.2 ENFORCEMENT HISTORY

EPA and UPCM signed an Administrative Order on Consent (AOC) on September 28, 2000 which called for UPCM to conduct a Remedial Investigation/ Focused Feasibility Study (RI/FFS) for the Site. EPA and UPCM have continuously worked well together since the inception of the USCWSG, and because of this, EPA was able to employ increasingly reduced oversight for the RI/FFS as it progressed. The RI/FFS conducted by UPCM provided the data and information used in this ROD.

EPA conducted two Potentially Responsible Party (PRP) Searches for the Site that identified several parties that may have some liability for cleanup of the Site. The Site owner, UPCM, has conducted the RI/FFS pursuant to an Administrative Order on Consent (AOC). EPA has been facilitating the allocation of costs of investigation and cleanup between the PRP’s and UPCM has indicated its willingness to enter into a Consent Decree (CD) with EPA for conduct of remedial design and remedial action.

SECTION 3

COMMUNITY PARTICIPATION

EPA recently published a Proposed Plan describing the preferred remedy at the Site. The Proposed Plan, released for public comment on September 4, 2004, was followed by a public meeting held on September 28, 2004. The public comment period on the proposed plan ran from September 5, 2004 to October 4, 2004. All comments received during this period are addressed in the Responsiveness Summary of this ROD

Throughout the 1980's and early 1990s, there was significant opposition to cleanup of the Site under CERCLA authority. Public participation consisted primarily of comments on the proposed listings and letters to EPA urging that neither site be listed on the NPL.

Since the formation of the USCWSG in 1999, community participation in Park City has increased and improved. The USCWSG meets regularly, in well-advertised open meetings. The participants receive updates on individual sites in the watershed and discuss issues in a cooperative format. The USCWSG has developed a web-site, funded by UPCM, which details actions related to the environmental investigations and cleanup. The EPA project manager discusses the Site periodically with the local radio talk show and the local newspaper reporter. An information repository, which includes the Administrative Record (AR) for the Site, was established at the Park City Library and Education Center. Numerous public meetings have occurred on both general issues and to fulfill requirements for particular sites in the watershed. Fact Sheets are produced annually with updates on progress. Throughout conduct of the RI/FFS at the Site, UPCM and EPA have provided information to the public through all of these routes.

SECTION 4

SCOPE AND ROLE OF RESPONSE ACTION

The Site is one of several historic mining sites in the Upper Silver Creek Watershed. At present, six of these sites are listed in the CERCLIS database, and several more are being considered for future Superfund action. The past and present impacts to surface water and sediment in Silver Creek result from the cumulative contributions of these sites over decades. Because of the high density of sites in a relatively small area, as well as the long history involved, it is often difficult to apportion specific problems to a particular site or time period. For example, sites upstream of Richardson Flat, such as Empire Canyon or Prospector Square, have impacted surface water and sediment conditions at and below Richardson Flat. However, it is difficult to determine exactly what contribution each made. For this reason, EPA has sought to investigate and remediate the Upper Silver Creek Watershed as a whole, rather than trying to investigate each site seperately. This ensures that remedies selected for the individual sites are complementary to each other and work toward the goal of cleaning up the entire watershed. This ROD addresses only the actions necessary to address actual and potential impacts specific to the Site, but it is part of a broader strategy to clean up the entire Silver Creek Watershed in a consistent, efficient manner.

The remedy selected by EPA and documented in this ROD includes remedial actions necessary to protect human health or welfare or the environment. The ROD is based primarily upon information set forth in the RI/FFS recently conducted by UPCM. An important purpose of the RI/FFS and associated risk assessment was to evaluate the efficacy of these voluntary actions and the risks posed by the Site in its current condition. For instance, there is a soil cover across the tailings impoundment that was put in place by UPCM in the 1990s. The RI/FFS evaluated the soil cover and showed it protects groundwater and other media at the site from becoming heavily contaminated. The risk assessment determined that under the current conditions, threats to human health are low. However, it is clear that in the absence of this soil cover, both human and ecological receptors would be exposed to high concentrations of heavy metals and contaminants would be free to migrate from the Site, thereby increasing the risk to human health and the environment. Thus, decisions on remedial actions must consider not only the risks posed by current conditions, but also the risks posed if current conditions changed. The selected remedy will enhance and ensure the integrity of the soil cover, reinforce the tailings embankment, and protect surface and ground waters from additional metals loading by containing the low level threat waste, thereby mitigating and abating the actual and potential risks to human health or welfare or the environment at the Site. Further, institutional controls will minimize potential, future, uncontrolled, human contact with contamination in any of the Site media.

SECTION 5

SUMMARY OF SITE CHARACTERISTICS

This section summarizes the information obtained through the investigations and feasibility studies. It includes a description of the Site conceptual model on which the investigations, risk assessments and response actions are based. The major characteristics of the Site and the nature and extent of contamination are summarized below. More detailed information is available in the Administrative Record for the Site.

5.1 SITE CONCEPTUAL MODEL

The illustrated site conceptual model depicted in Figure 5 is a representation of the location, and movement of contamination at the Site and any potential impacts that may occur to human health, the environment, or beneficial uses of resources. Presently, the tailings in the main impoundment (Area A) and the tailings south of the diversion ditch (Area B) are considered the primary waste sources. Impacted media at the Site include sediments in the south diversion ditch and the wetland area, and the surface waters. Surface water sources include the wetlands area, Silver Creek, the site pond, and intermittent flow in the diversion ditches and unnamed drainages. Seasonally, accumulated precipitation and snow melt can be found on the surface of the main impoundment. There is a clay layer underlying the tailings in Area A and Area B, so infiltration of groundwater into the underlying aquifer is limited. Additionally, heavy metal releases from the tailings are currently contained to a certain degree by a low permeability soil cap that was placed there by UPCM in the 1990's. Therefore, potential exposure to future Site users including high and low-intensity recreational visitors is limited. However, these possible exposure pathways include ingestion of soils/tailings and sediment, dermal exposure to surface water, and inhalation of particulates in air. The ecological exposure pathways and receptors are described in detail in Section 7.2, Ecological Risk.

5.2 OVERVIEW OF THE RICHARDSON FLAT TAILINGS SITE

The Site is located in a broad valley with undeveloped rangeland. The Site is about 6,570 feet above mean sea level and is characterized by a cool, dry, semi-arid climate (RMC, 2003). Meteorological stations located in Park City, Utah and Kamas, Utah estimate an annual precipitation of about 20 inches of water, an average low temperature of about 30°F, and an average high temperature of about 57°F (RMC, 2003).

5.2.1 Site Features

As described in the Site History, mine tailings have been deposited at the Site since 1950. For two decades, tailings were systematically deposited in the impoundment via a slurry line and eventually filled in all low lying areas (Area A). In 1970, PCV took over the use of the impoundment, which required several structural changes and improvements, including enlargement of the main embankment in the northwestern corner of the Site, construction of containment dikes along the southern and eastern borders of the impoundment, and construction of a diversion ditch system outside the impoundment along the east and south perimeters. On the south end of the impoundment, the diversion ditch was cut through an area of existing tailings, resulting in some tailings being located outside (south of) the present day boundaries of the impoundment (Area B). These additions, as well as the tailings south of the diversion ditch, make up the main surface features of the Site. The Study Area Boundary includes the tailings south of the diversion ditch and the main impoundment. The Site characteristics can be found in Figure 4.

Impoundment and Containment Dikes

The majority of the tailings at the Site are contained in the impoundment basin, with a large earth embankment in place along the western edge of the Site (Area A). The "main embankment" is vegetated and is approximately 40 feet wide at the top, 800 feet long, and has a maximum height of 25 feet. A series of man‑made dikes contain the tailings along the southern and eastern perimeter of the impoundment. The northern edge of the impoundment is naturally higher than the perimeter dikes.

Off‑Impoundment Tailings

Additional tailings materials are present outside and to the south of the current impoundment area (Area B). During historic operations of the tailings pond, tailings accumulated in three naturally low-lying areas adjacent to the impoundment. Starting in 1983, UPCM covered these off-impoundment tailings with a low-permeability, vegetated soil cover. However, recent surveys of off‑impoundment cover soils indicate that, at some locations, soil cover is thin or absent, leaving exposed surface tailings (RMC, 2001a). In addition to these off-impoundment tailings deposits, prevailing winds from the southeast carried tailings from the main impoundment and deposited them in the surrounding areas.

Diversion Ditches and Drainages

A diversion ditch system borders the north, south, and east sides of the impoundment to prevent surface water runoff from the surrounding land from entering the impoundment. Precipitation falling on the impoundment area creates a limited volume of seasonal surface water. The north diversion ditch collects snowmelt and storm water runoff from upslope, undisturbed areas north of the impoundment and carries it in an easterly direction towards the origin of the south diversion ditch. An unnamed ephemeral drainage to the southeast of the impoundment also enters the south diversion ditch at this point. Additional water from spring snowmelt and storm water runoff enters the south diversion ditch from other areas lying south of the impoundment at a point near the southeast corner of the diversion ditch structure.

Site Wetlands and Pond

Water in the south diversion ditch flows from east to west and ultimately empties into Silver Creek near the north border of the Site. Before its confluence with Silver Creek, water from the south diversion ditch enters a small one acre pond (RMC, 2003). Water exiting the pond flows in a discrete channel where it mixes with flow from Silver Creek in a wetlands area below the main embankment (RMC, 2003). Near the northwestern corner of the wetlands area, Silver Creek flows into the wetland beneath the rail trail bridge. Water flow exits the wetlands area back into Silver Creek via a concrete box culvert under State Highway 248 (RMC, 2003).

Silver Creek

Silver Creek flows approximately 500 feet from the main embankment along the west edge of the Site. The headwaters of Silver Creek are comprised of three signifigant drainages in the Upper Silver Creek Watershed; the Ontario Canyon, the Empire Canyon and Deer Valley. Flows from Ontario and Empire Canyons occur in the late spring to early summer months in response to snowmelt and rainfall, while Deer Valley flows appear to be perennial and originate from snowmelt and springs (RMC, 2000b). The largest contributor to water flow in Silver Creek near the Site is the Pace‑Homer (Dority Springs) Ditch, which derives most of its flow from ground water (USEPA, 2001). The outflow from the Pace‑Homer Ditch enters Silver Creek at several locations below the Prospector Square area. Significant riparian zones and wetlands exist near the Site in areas that consist of accumulated tailings piles.

5.2.2 Hydrogeology

Ground water of concern at the Site occurs in shallow aquifers below the original ground surface. These aquifers are primarily fed from local surface water recharge and are small and local in nature. They generally flow from southeast to northwest toward Silver Creek. Below these shallow aquifers, at varying depths, lies the bedrock aquifer of the Keetley Volcanics, which contains varying amounts of ground water depending upon local conditions. The hydraulic gradient in all aquifers is generally upward, but the connection between the bedrock aquifer and the shallow aquifers is weak.

The Site is located in a low gradient valley surrounded by small hills. The erosion and weathering of these hills, also part of the Keetley Volcanics, formed the original soil surface upon which the tailings were placed, as well as the soils used to cover the impoundment after its closure. These soils are rich in clay and exhibit a very low permeability, making them very important to the ground water and surface water hydrology of the Site. Beneath the tailings, the original ground surface acts as a confining unit for ground water movement, preventing water in the tailings from infiltrating downward into the shallow aquifers, as well as preventing water in the shallow aquifers from moving upward into the tailings. On the surface, the soils used to cover the tailings function as a nearly impermeable cap, effectively preventing infiltration of surface water into the tailings. The tailings are effectively encapsulated above and below by low permeability, clay rich soil. At present, the surface of the impoundment is convex and forms a closed basin, so precipitation that falls directly on the impoundment remains there until it evaporates or is used by plants. Spring snow melt and heavy rains cause a large, temporary area of ponded water on the east side of the impoundment. This ponded area remains for a significant duration after snow melt, with little recharge from precipitation, which shows the effectiveness of the cover soil in preventing significant infiltration into the tailings. The very small amount of water that does infiltrate into the tailings eventually seeps through the main embankment into a small wetland.

The diversion ditch is also critical to the Site’s hydrology. The diversion ditch serves as a barrier to both surface water and shallow ground water and captures water that flows toward the impoundment. The captured water is channeled around the impoundment, through a small retention pond, and into the small wetland at the foot of the main embankment. Here it mixes with water from Silver Creek and the small amount of water seeping through the embankment. All of this water is eventually used by plants in the wetland or flows north away from the Site as surface water or shallow ground water in the alluvium of Silver Creek.

5.3 SAMPLING STRATEGY

Sampling events for the RI took place in 2001 and 2002. The RI was designed to augment existing data that were collected in previous Site investigations and to collect additional data for the Ecological Risk Assessment. During these events each media was sampled as a separate entity. Samples were collected from the various site media, including surface water, ground water, Area A and B tailings, Area A and B soil cover, and lastly, sediments in the south diversion ditch and wetlands area.

Surface and Ground Water Sources

Surface water

Sample locations were chosen to provide sufficient data to characterize seasonal water quality and quantity in the South Diversion ditch and the two unnamed drainages flowing into the South Diversion Ditch, and Silver Creek. Data were also collected to determine the effects of the Site on Silver Creek and the metal concentrations in the surface water of the South Diversion Ditch. When sampling was not limited due to lack of flow, data was collected monthly at each location through one complete seasonal time period. All dissolved metal concentration data were screened against Utah Water Quality Standards. The most stringent of these standards are the Class 3A Aquatic Wildlife Chronic Criteria (AWCC). These standards are dependent on hardness and are adjusted appropriately for an average hardness measured at each sample location.

Ground water

Due to the amount of historic ground water data, additional data collection required the addition of two new monitoring wells which were installed adjacent to Silver Creek up and down gradient of the Site. These were established to determine any shallow alluvial groundwater impacts caused by the tailings. Samples were also taken from established wells close to the South Diversion ditch to determine the metals concentrations within the ground water associated with the Area B tailings, and to determine the hydraulic gradient

Tailings

Area A

Three test pits were created within Area A to sample the tailings. The test pits allowed for observation and documentation of the physical characteristics and spatial configuration of the interface. Additionally, at each location, five discrete samples were collected at one foot vertical increments to a depth of five feet below the soil cover. Acid/base potential data was used to assess the geochemical characteristics of the tailings materials.

Area B

Sampling in this area was completed first to determine the extent of the tailings outside of the main impoundment. The sample data were used in combination with areal photographs and historical information to determine the study area boundary. Backhoe test pits (63 total) and a series of hand tool excavations were completed in order to gather analytical and visual samples. Visual samples were used to establish the location of the tailings/clay layer interface. This sample data was also used to assess the thickness of the soil cover on top of the tailings in Area B. Analytical data was used to confirm the visual data. At seven sample locations one sample was taken from the tailings and one sample was taken from the clay layer below the tailings.

Soil cover

Area A

Soil samples (41 samples total, 0-2" each) were collected for analysis. The holes were dug down until tailings were collected from below the main impoundment soil cover to determine the depth of the soil cover and the chemistry of the surface soils. Samples were analyzed for lead and arsenic while 20% of the samples were analyzed for RCRA metals plus copper and zinc.

Area B

The same excavation and hand tool sampling techniques that were described in the Area B tailings section were used to determine soil cover thickness in this area. Additionally, this area was sampled to assess the extent and impact of windblown tailings. A series of samples were collected from three transects (28 total) and analyzed for lead and arsenic.

South Diversion Ditch Sediments

Six locations were chosen for sediment sample collection. Data were used to identify the source of zinc loading to the surface water found in the diversion ditch and to evaluate ecological risk.

Background Soils

Background surface soil samples (0-2") were collected from areas that have not been affected by tailings, found at least a mile away from the Site in all directions. All samples were analyzed for lead and arsenic, while 2 samples were analyzed for RCRA metals plus copper and zinc.

Study Area Boundary

Study area boundary samples were collected from two areas south of the tailings found outside the impoundment, and on the west and east perimeter of the main impoundment. These samples analyzed for lead and arsenic to aid in determining the study area boundary.

Ecological Sampling

Additional sampling was necessary to facilitate the completion of a thorough ecological risk assessment. Surface water and sediment sample data were collected from locations in the wetland area, site pond, and South Diversion Ditch. Vegetation samples and fish and macroinvertebrate samples were also taken. An analysis of these samples was necessary to complete the ecological risk assessment.

5.4 KNOWN AND SUSPECTED SOURCES OF CONTAMINATION

As previously described, the Silver Creek watershed is contaminated with heavy metals resulting from years of heavy mining activity in the Park City District. Surface water from the Site enters Silver Creek after passing through a wetland area in the northwest corner of the Site. There are three main sources of contamination at the Site: (1) the tailings contained within the tailings impoundment (Area A), (2) the tailings south of the diversion ditch (Area B) and (3) the tailings within the wetland area.

Metal contamination resulting from wind blown tailings distribution was investigated. Soil samples were taken along three transects (running west to east) that were oriented perpendicular to the prevailing wind direction. One transect was located north of the impoundment while the remaining two were located south of the impoundment. These samples were collected to determine the extent of wind blown tailings contamination and to aid in the study area boundary determination. The samples were analyzed for arsenic and lead and for eight RCRA metals, including zinc. Samples taken along transect two (south of the impoundment) had higher concentrations of lead than transects one and three. It is possible that these sample locations were not covered with top soil, while the other sample locations were. Sample locations with the highest concentrations of lead are included in the study area boundary.

5.5 TYPES OF CONTAMINATION AND AFFECTED MEDIA

The Site is contaminated with heavy metals, primarily zinc, lead and arsenic which are associated with the tailings found in the three locations described in Section 5.4. The media that are affected by these metals include the sediments and surface water of the south diversion ditch, the site wetland, and Silver Creek.

Surface water

Conclusions drawn from the sample data show that zinc exceeds the water quality criteria in some parts of the South Diversion Ditch, however, surface water zinc concentrations are below the criteria where the diversion ditch meets the wetland area. A Comparison of surface water data collected from Silver Creek to the AWCC shows that zinc exceeds the criteria at both sample locations. Peak concentrations of zinc appear during spring run-off conditions.

Ground water

Data gathered from the monitoring wells were used to determine the metals concentrations within the ground water associated with the Area B tailings, and to determine the hydraulic gradient. After data gathered from these two areas were compared to Primary and Secondary Drinking Water Standards (PDWS and SDWS) and Treatment Technology Requirement (TTR) they were also compared to each other to determine whether the Site tailings are contributing zinc or other metals to the Silver Creek alluvial aquifer. Results show that ground water within the Area B tailings had lower concentrations of metals than the Silver Creek alluvial aquifer. Dissolved zinc concentrations from the Area B tailings are approximately 500 times lower than the zinc concentrations measured in the up gradient Silver Creek alluvial aquifer. Lastly, there is no hydraulic connection between ground water stored in the Area A tailings and the underlying aquifers.

Tailings Metals Concentrations

Area A

The average lead concentration in the Area A tailings was 4,530 ppm, while the average arsenic value was 265 ppm.

Area B

The average lead and arsenic concentrations in the tailings above the clay layer were 10,434 ppm and 412 ppm respectively, while the average lead and arsenic concentrations in the clay layer below the tailings were 52 ppm and 9 ppm. Average lead and arsenic concentrations in the clay layer below the tailings in Area B are well below the background soil concentration.

Area A and B tailings data analysis

Based on the data presented above it appears that there are higher metals concentrations in the tailings in Area B as compared to Area A. However, metal concentrations in the clay layer below the tailings in Area B are lower than in background soil concentrations. Furthermore, the composition of the clay layer below Area B tailings is the same as the composition of the clay layer below the main impoundment. This leads to the conclusion that the clay layer below the tailings is serving as an adequate barrier to metals migration in Area B and A.

Soil Cover

Area A

Sample data indicate that the range of thickness of the soil cover is 0.5 to 4 feet. Analytical results show the average lead concentration to be 385 ppm, while the average arsenic concentration was 22 ppm. As there are no regulatory criteria for metals in soils, this data was used to analyze the risk of surficial soil exposure to recreational users and ecological receptors at the Site.

Area B

A series of samples were collected from three transects (28 total) and analyzed for lead and arsenic. Five of the samples were analyzed for eight RCRA metals plus zinc and copper. In conclusion, Transect 2 had a higher average concentration of lead and arsenic (1,446 ppm Pb, 75 ppm As) than transects 1 and 3, however, samples taken from this area may not have been covered by soil, causing the results to represent concentrations of lead and arsenic associated with the tailings that were already there, rather than concentrations associated with windblown tailings.

South Diversion Ditch Sediments

Analytical results show that the average concentrations for lead, arsenic and zinc are 2,578 ppm, 138 ppm and 7,878 ppm respectively. Concentrations are highest in the sample location found in the lower portion of the diversion ditch just east of the site pond.

Background Soils

The average lead concentration for the background soils is 43.3 ppm. The average arsenic concentration is 9 ppm. None of the background soil samples had elevated metals concentrations.

Study Area Boundary

Study area boundary samples were collected from two areas south of the tailings found outside the impoundment, and on the west and east perimeter of the main impoundment. These were analyzed for lead and arsenic to aid in determining the study area boundary. Analytical sample results were used to delineate the Study area Boundary. The boundary is drawn where background lead concentrations appear in the sample results.

Ecological Sampling

Additional sampling was necessary to facilitate the completion of a thorough ecological risk assessment. Surface water and sediment sample data was collected from locations in the wetland area, Site pond, and South Diversion Ditch. Vegetation samples and fish and macroinvertebrate samples were also taken. The resulting data was used to determine risk to ecological receptors in the Site area. A summary of the Ecological Risk Assessment including the findings from the ecological sampling is presented in section 7.2.

5.6 LOCATION OF CONTAMINATION AND POTENTIAL ROUTES OF MIGRATION

5.6.1 Surface water and Sediments

Sediments and surface water impacted by the tailings in Area A and B are found in the South Diversion Ditch and in the Wetland area. The contamination in these media is potentially affecting ecological receptors found in the area. Importantly, metal concentrations in the surface water of Silver Creek are higher than metals concentrations found in the surface water of the diversion ditch. Therefore, contaminated surface water found within the wetland is not adversely affecting Silver Creek.

South Diversion Ditch

Elevated concentrations of lead, arsenic, zinc and some cadmium were found in all water and sediment samples taken. The South Diversion Ditch is a dynamic environment, where elevated concentrations of metals, particularly zinc, fluctuate with seasonal runoff and correspond with peak groundwater elevation. Likely sources of elevated metals concentration found in surface water and sediments in the Diversion Ditch include the tailings located in the bottom if the ditch, the small pond area south of the Site, or from the tailings in Areas A or B.

Wetlands

Although concentrations of metals in the surface water and sediment of the wetland area are lower than those of the South Diversion Ditch, they are very likely to have impacts on the ecological environment at the Site. The average concentrations of lead, arsenic and zinc are just below those in the South Diversion Ditch. There is a mixing of surface waters that occurs in the wetland area; while water from Silver Creek enters the northern portion of the wetland, surface water also flows in from the Diversion Ditch in the southern portion of the wetland. Sample results indicate that water entering the wetland area from Silver Creek contains higher metals concentrations than the surface water of the South Diversion Ditch.

5.6.2 Ground water

• Ground water sampling results indicate that the Site ground water has much lower concentrations of metals than the ground water within the Silver Creek alluvial ground water. A large amount of this ground water is captured in the South Diversion Ditch. Based on this data, it does not appear that the Site ground water is impacting the Silver Creek alluvial aquifer.
• As a result of the native clay layer found beneath the Area A tailings there is no hydraulic connection between the ground water associated with these tailings and the shallow alluvial aquifers or the underlying Keetley Volcanic aquifers.
• Sample results from ground water within the wetland area indicate that there are no significant impacts from the contamination found in the wetland, the embankment or the Area A tailings.

5.6.2 Soils

In the previous sections on Background Soils and Soil Cover (Section 5.5) it is made clear that impacts to the soils at the Site are minimal. Most contamination is in the form of tailings that were deposited within Area A and in some small areas within Area B. Migration of metals away from these small areas within Area B is extremely limited. Most of the small tailings deposits within Area B have been previously covered with topsoil. Any soils within Area B that have high concentrations of metals are included in the Study Area Boundary are addressed by the selected remedy.

SECTION 6

CURRENT AND POTENTIAL FUTURE LAND AND RESOURCE USES

This section describes the current and reasonably anticipated future land uses and current and potential beneficial ground and surface water uses at the Site.

Current Land Use

The Site is located in a rural area within a broad valley of mostly undeveloped rangeland within the Silver Creek Watershed, approximately two miles outside the Park City limits. The Deer Valley and Park City ski resorts sit at the top of the watershed and serve as recreational use areas for skiers in the winter and bikers/hikers in the warmer months. As Silver Creek passes through Park City and into the surrounding suburban areas, the land use is primarily residential and commercial, changing to recreational and agricultural in the areas surrounding Richardson Flat. Most of the land around the Site is undeveloped open space.

Mining activities at the Site ceased in 1982. Since that time, the Site has not been used and has remained open space. A small recreational trail skirts the Site along Silver Creek. There are a few small industrial operations in the vicinity of the Site, including a concrete plant on a nearby parcel. Park City and other resort-like residential developments are expanding in the general area, but none are closer than one mile away.

Reasonably Anticipated Future Land Use

The Site, and much of the surrounding area, is privately owned by UPCM. UPCM has consistently indicated a desire to retain title and limit future use to recreational activities at the Site. While no final decision has been made, uses that range from open space wildlife habitat to athletic fields are currently being discussed. Any type of recreational use is consistent with surrounding land uses, and both Park City and Summit County have indicated general agreement with recreational proposals. Park City is proactive in obtaining and preserving open space. There is no indication that higher uses of the land, such as residential, are reasonably foreseeable.

Ground and Surface Water Uses

The surface water features at the Site, including the south diversion ditch, the wetlands area below the embankment, the Site pond and Silver Creek are used as habitat by a limited number of vegetative species, fish, and wildlife. All of the surface water and shallow ground water on the Site eventually discharges to Silver Creek. Silver Creek is classified by the State of Utah as a potential drinking water source, a recreational use feature, a cold water fishery, and a potential irrigation source. At present, Silver Creek is used for irrigation and recreational fishing only, and no changes are expected. The State of Utah is considering issuing an advisory against fishing due to elevated metal levels in Silver Creek. Silver Creek is listed on the State’s Clean Water Act Section 303(d) list of impaired water bodies because zinc and cadmium levels exceed chronic standards for protection of aquatic wildlife.

Silver Creek has been impacted by the legacy of mining activities, though the remedial investigation confirmed that the Site is not, at present, a significant contributor of metals to the creek. The goal is to remediate the entire watershed, improving the ecological quality of the area, thereby allowing for continued beneficial use of the watershed and the Site by a variety of living organisms.

Ground water in the immediate area is used only for private wells, and no wells are known to be located within a half mile of the Site. Most area drinking water wells are finished in the deeper consolidated sedimentary rocks that can sustain aquifers and produce sufficient yields for culinary wells. In the Site area, these formations are very deep and are covered by the Keetley volcanics. The volcanic rocks are generally not suitable to sustain aquifers and serve as more of a confining unit. The shallow ground water at the Site is generally associated with the alluvial system of Silver Creek. This water is very high in solids and is also often contaminated due to water quality in Silver Creek and tailings that are present along the Creek in many areas. There are no known uses for this water at this time.

SECTION 7

SUMMARY OF SITE RISKS

A baseline human health risk assessment (BHHRA) and a baseline ecological risk assessment (BERA) were performed to evaluate the potential for adverse human health and ecological effects that might occur from exposure to Site-related contaminants. Current and future risks were estimated for the baseline scenario (i.e., risks that might exist if no remediation or institutional controls were applied). The BHHRA and the BERA aided in drafting the remediation goals by providing a basis for taking action at the Site. The Chemicals of Concern and the exposure pathways were also identified through these risk assessments.

7.1 HUMAN HEALTH RISK ASSESSMENT

7.1.1 Identification of Chemicals of Concern

The BHHRA identified two contaminants, lead and arsenic, as chemicals of potential concern (COPC’s) at the Site through a four step selection process. Risks to human health posed by exposure to these chemicals have been studied extensively through risk assessments completed at other Superfund sites in Utah and throughout the country. Currently, the Site has a soil cover that has a depth of 4 feet in some areas. Because of this soil cover, exposure pathways to these COPC’s are limited or interrupted. However, if the integrity of this soil cover were threatened in any way by forces of nature or human intervention, the exposure pathways could become complete. Because of the high human health risk associated with lead and arsenic, and because of the potential exposure to recreational Site visitors if a remedy were not in place, lead and arsenic were selected as chemicals of concern (COC’s) and risk drivers for the Site. The COC’s are summarized in Tables 7-1,7-2, and 7-3.

7.1.2 Exposure Assessment

The exposure assessment identifies scenarios through which people could be affected by the COCs in Site media and estimates the extent of exposure Site users could endure. The conceptual site model illustrates the media and exposure pathways that were evaluated in the BHHRA (Figure 5). Media selected for evaluation in the BHHRA were soil/tailings, surface water, sediment, and air particulates. Because land use will be limited to recreational visitors, two separate recreational use scenarios were considered. An evaluation of the exposure pathways is also presented in Figure 6.

Low intensity User

The first scenario includes low intensity users, such as hikers, bikers and picnickers, ranging in age from young children to adults. Exposure pathways evaluated were ingestion of soil/tailings, surface water and sediment, dermal exposure to surface water and inhalation of particulates in air.

High Intensity User

Scenario two includes high intensity users such as horseback riders, ATV users, dirt bikers and team sports players. High intensity users were assumed to exclude younger children and include teenagers and adults. The exposure pathways a high intensity user may be subjected to include ingestion of soil/tailings and inhalation of particulates in air.

7.1.3 Toxicity Assessment

The purpose of the toxicity assessment is to review and summarize the potential for each COC to cause adverse effects in exposed individuals. The toxic effects of a chemical generally depend on the inherent toxicity of a chemical, the route of exposure (ingestion, inhalation, and dermal), and the duration of exposure (subchronic, chronic or lifetime).

There is a positive relationship between dose (chemical intake through an exposure pathway), and adverse effect, so as dose increases the type and severity of adverse reponse also increases. Chemical toxicological information derived from either animal or human studies is used to estimate toxicity criteria which are numerical expressions between dose (exposure) and response (adverse health effects). Toxicity criteria are developed for the assessment of carcinogenic and non-carcinogenic health effects. Toxicity criteria include the EPA online Integrated Risk Information System (IRIS) and EPA’s Health Effects Assessment Summary Tables (HEAST).

Toxicity criteria for carcinogens are provided as cancer slope factors (CSF’s) in units of risk per milligram of chemical per kilogram of body weight per day (mg/kg-day). CSF’s are based on the assumption that no threshold exists for carcinogenic effects and that any dose is associated with some finite carcinogenic risk. The chemical-specific CSF is multiplied by the estimated chemical intake to provide an upper-bound estimate of the increased likelihood of cancer resulting from exposure to the chemical. This risk would be in addition to any background risk of developing cancer over a lifetime due to other causes. Consequently, the risk estimates in the BHHRA are referred to as incremental or excess lifetime cancer risks. Based on data from IRIS and other published data, arsenic is classified as a known human carcinogen (EPA weight of Evidence A). Table 7-4 shows the cancer toxicity criteria for ingestion of arsenic. Lead toxicity is evaluated using other methodologies such as the Integrated Exposure Uptake Biokinetic (IEUBK) model. Estimated blood lead levels are compared to target blood-lead concentrations to assess possible risks.

Toxicity criteria for noncarcinogens are provided as reference doses (RfDs) and represent the daily exposure to a chemical that would be without adverse effects, even if the exposure occurred continuously over a lifetime. The RfD is provided in units of milligrams per kilogram per day (mg/kg-day) for comparison with chemical intake into the body. Chemical intakes that are less than the RfD are not likely to be of concern even to sensitive individuals. Chemical intakes that are greater than the RfD indicate a possibility for adverse effects. Noncancer toxicity values for COCs for ingestion/dermal exposures are presented in Table 7-5.

EPA has not published toxicity criteria for lead. This is because available data suggest that there is no threshold for adverse effects even at exposure levels that might be considered background. Any significant increase in exposure above background levels could represent a cause for concern. Instead of evaluating risk using typical intake calculations and toxicity criteria, EPA has developed other methodologies for evaluating lead exposures. One such methodology is the Integrated Exposure Uptake Biokinetic (IEUBK) model, a computer model used to predict blood-lead levels in children exposed to lead from a variety of sources, including soil, dust, ground water, air, diet, lead-based paint, and maternal blood. Estimated blood-lead levels are compared to target blood-lead concentrations to assess possible risks. The IEUBK model is intended for use only for children up to the age of seven, as these are the most sensitive receptors to lead expos