Reconnaissance Soil Geochemistry at the Riverton 

Uranium Mill Tailings Remedial Action Site, Fremont 

County, Wyoming

Open-File Report 2012–1257

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012 

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Suggested citation:

Smith, D.B., and Sweat, M.J., 2012, Reconnaissance soil geochemistry at the Riverton Uranium Mill 

Tailings Remedial Action Site, Fremont County, Wyoming: U.S. Geological Survey Open-File Report 

2012–1257, 23 p.
iii

Contents

Abstract ......................................................................................................................................................... 1

Introduction.................................................................................................................................................... 1

Sample Collection .......................................................................................................................................... 3

Sample Preparation and Analytical Methods ................................................................................................. 3

Quality Assurance and Quality Control.......................................................................................................... 5

Estimation of Risk.......................................................................................................................................... 6

Ecological Soil Screening Levels ............................................................................................................... 6

Human Health Soil Screening Levels......................................................................................................... 7

Estimation of Background.............................................................................................................................. 8

Results........................................................................................................................................................... 9

Discussion ................................................................................................................................................... 19

Acknowledgments ....................................................................................................................................... 19

References Cited.......................................................................................................................................... 19

Appendix.....................................................................................................................................................link

Figures

Figure 1. Location of 19 soil samples collected from the Riverton Uranium Mill Tailings Remedial 

Action Site, Fremont County, Wyoming......................................................................................................... 2

Figure 2. Box-and-whisker plots showing the variation of (a) As, (b) Ba, (c) Cd, (d) Co, (e) Cu, 

(f) Mn, (g) Ni, (h) Pb, (i) Sb, (j) Se, (k) V, and (l) Zn in soils from the Riverton Uranium Mill Tailings 

Remedial Action Site, Fremont County, Wyoming, and in soils from the 160-sample Statewide dataset. 

The outer limits of the lower and upper whiskers on the plots represent the 10th and 90th percentiles, 

respectively. The “X” between the two box-and-whisker plots in each figure represents the median for 

six soil samples from the Wyoming Statewide dataset that are located within the Wind River drainage ..... 11

Figure 3. Box-and-whisker plots showing the variation of (a) U and (b) Th in soils from the Riverton 

Uranium Mill Tailings Remedial Action Site, Fremont County, Wyoming, and in soils from the 160-sample 

Statewide dataset. The outer limits of the lower and upper whiskers on the plots represent the 10th and 

90th percentiles, respectively. The “X” between the two box-and-whisker plots in each figure represents 

the median for six soil samples from the Wyoming Statewide dataset that are located within the Wind 

River drainage ............................................................................................................................................. 18
iv

Tables

Table 1. Elements determined by ICP–MS and ICP–AES.......................................................................... 4

Table 2. Ecological soil screening levels (Eco-SSL) as established by the U.S. Environmental 

Protection Agency ......................................................................................................................................... 7

Table 3. Human health soil screening levels for industrial soils (U.S. Environmental Protection 

Agency, 2011) ............................................................................................................................................... 8

Table 4. Human health soil screening levels for industrial soils (U.S. Environmental Protection 

Agency, 2011). [mg/kg, milligrams per kilogram; NA, not available] .............................................................. 9

Table 5. Statistical summary for 41 elements in 0–5 cm soils from the Riverton UMTRA Site ................. 10
1 

Reconnaissance Soil Geochemistry at the Riverton 

Uranium Mill Tailings Remedial Action Site, Fremont 

County, Wyoming

By David B. Smith and Michael J. Sweat

Abstract

Soil samples were collected and chemically analyzed from the Riverton Uranium Mill 

Tailings Remedial Action Site, which lies within the Wind River Indian Reservation in Fremont 

County, Wyoming. Nineteen soil samples from a depth of 0 to 5 centimeters were collected in 

August 2011 from the site. The samples were sieved to less than 2 millimeters and analyzed for 

44 major and trace elements following a near-total multi-acid extraction. Soil pH was also 

determined. The geochemical data were compared to a background dataset consisting of 160 soil 

samples previously collected from the same depth throughout the State of Wyoming as part of 

another ongoing study by the U.S. Geological Survey. Risk from potentially toxic elements in 

soil from the site to biologic receptors and humans was estimated by comparing the 

concentration of these elements with soil screening values established by the U.S. Environmental 

Protection Agency. All 19 samples exceeded the carcinogenic human health screening level for 

arsenic in residential soils of 0.39 milligrams per kilogram (mg/kg), which represents a one-inone-million cancer risk (median arsenic concentration in the study area is 2.7 mg/kg). All 19 

samples also exceeded the lead and vanadium screening levels for birds. Eighteen of the 19 

samples exceeded the manganese screening level for plants, 13 of the 19 samples exceeded the 

antimony screening level for mammals, and 10 of 19 samples exceeded the zinc screening level 

for birds. However, these exceedances are also found in soils at most locations in the Wyoming 

Statewide soil database, and elevated concentrations alone are not necessarily cause for alarm. 

Uranium and thorium, two other elements of environmental concern, are elevated in soils at the 

site as compared to the Wyoming dataset, but no human or ecological soil screening levels have 

been established for these elements.

Introduction

The effects of soil composition on human and ecological health is well documented (see, 

for example, Anastasia and Kender, 1973; Rehab and Wallace, 1978; Kaplan and others, 1990; 

Ainsworth and others, 1991; Angle and Chaney, 1991; Lamersdorf and others, 1991; Jiang and 

Singh, 1994; Spurgeon and others, 1994; Lehoczky and others, 1996; Oliver, 1997; Abrahams, 

2002; Plumlee and Ziegler, 2003; Pierzynski and others, 2005). Soil can be a pathway for 

potentially toxic elements of natural or anthropogenic origin to enter the human or animal body 

through ingestion, inhalation, or dermal absorption and to enter plants by absorption through root 

tissues. Low concentrations of nutrients or elevated concentrations of toxic elements in soil may 

control the vegetation species that can grow in the soil (Gough and others, 1989; Oze and others, 

2004). 
2 

On August 11, 2011, 19 soil samples were collected from the Riverton Uranium Mill 

Tailings Remedial Action (UMTRA) Site about 4 kilometers southwest of Riverton, Wyoming 

(fig. 1). The Riverton UMTRA Site is located on the Wind River Indian Reservation (Eastern 

Shoshone and Northern Arapahoe Tribes) and covers an area of approximately 10.4 square 

kilometers. The land is currently owned by Chemtrade Refinery Services (U.S. Department of 

Energy, 2011). The mill was constructed in 1958 and closed in 1963. During this period of 

operation, the mill processed approximately 800,000 metric tons of uranium and vanadium ore 

(U.S. Department of Energy, 1995, 2011). Solid waste material was transferred to a tailings pile 

covering about 29 hectares. Between 1988 and 1990, the mill tailings and contaminated soils 

were removed from the site. However, the Eastern Shoshone and Northern Arapahoe Tribes 

continue to have concerns about groundwater and soil contamination at the site. 

The purpose of this investigation was to determine the geochemical variation for 44 

chemical elements in soils of the UMTRA site and to compare these soil concentrations with 

available ecological and human health soil screening levels established by the U.S. 

Environmental Protection Agency. To put the concentrations observed at the UMTRA site into a 

more regional context, the site-specific geochemical variation was compared to the variation 

observed for the entire State of Wyoming as determined from samples collected at 160 sites 

throughout the State in 2008 as part of the North American Soil Geochemical Landscapes Project 

(Smith and others, 2011, 2012).

Figure 1. Location of 19 soil samples collected from the Riverton Uranium Mill Tailings Remedial Action 

Site, Fremont County, Wyoming.
3 

Sample Collection

Nineteen samples were collected from sites selected to provide relatively even coverage 

throughout the UMTRA site. The site was mapped using land-ownership coverages in ArcGIS, 

and the Chemtrade property (former mill-tailings site) was excluded to determine the surface 

area available for sampling. A pixilated randomized selection process was then run to determine 

sampling locations. Land ownership was determined for five sites in each pixel and access 

requested for the first site in each pixel. If access was denied or not available (not able to contact 

owner), then the next site owner was contacted. This process continued until access was obtained 

or until all sites for a pixel were exhausted. For this sampling effort, access was obtained from 

the first or second property owner in all cases. 

At each site, a sample was collected from a depth of 0–5 centimeters (cm). This particular 

sample medium was selected because this near-surface material is the part of the soil with which 

humans and most animals come into contact most often. Geochemical information for this layer 

is critical for evaluation of a soil pathway through which potentially toxic elements may enter the 

bodies of both humans and animals. It also is considered to be the portion of the soil most likely 

to indicate the influence of uranium milling at the site.

Sample Preparation and Analytical Methods

Samples were air dried at ambient temperature, disaggregated, and sieved through a 2-

millimeter (mm) stainless steel screen. Material less than 2 mm in size was crushed to less than 

150 micrometers (μm) in a ceramic mill and thoroughly mixed to ensure homogeneity. Prepared 

samples were sent to a U.S. Geological Survey (USGS) contract geochemical laboratory for 

major and trace element analysis.

Concentrations of the elements aluminum (Al), calcium (Ca), iron (Fe), potassium (K), 

magnesium (Mg), sodium (Na), sulfur (S), titanium (Ti), silver (Ag), barium (Ba), beryllium 

(Be), bismuth (Bi), cadmium (Cd), cerium (Ce), cobalt (Co), chromium (Cr), cesium (Cs), 

copper (Cu), gallium (Ga), indium (In), lanthanum (La), lithium (Li), manganese (Mn), 

molybdenum (Mo), niobium (Nb), nickel (Ni), phosphorus (P), lead (Pb), rubidium (Rb), 

antimony (Sb), scandium (Sc), tin (Sn), strontium (Sr), tellurium (Te), thorium (Th), thallium 

(Tl), uranium (U), vanadium (V), tungsten (W), yttrium (Y), and zinc (Zn) were determined by 

inductively coupled plasma–atomic emission spectrometry (ICP–AES) by a method similar to 

Briggs (2002) and inductively coupled plasma–mass spectrometry (ICP–MS) by a method 

similar to Briggs and Meier (2002). A sample of 0.25 grams (g) was decomposed using a mixture 

of hydrochloric, nitric, perchloric, and hydrofluoric acids at a temperature between 125 and 150 

degrees Celsius (oC). 

The four-acid digestion results in an effectively total dissolution of most mineral 

constituents in soil. However, it does not fully dissolve some of the more refractory or resistant 

minerals. Examples of such incomplete dissolution include Ba in barite, Cr in chromite, Ti in 

rutile, Sn in cassiterite, Al in corundum, and rare earth elements in monazite (Briggs, 2002). An 

aliquot of the digested sample was aspirated into the ICP–AES instrument and the ICP–MS 

instrument and the concentrations of the optimal elements were determined. The ICP–AES 

method is best for the major elements, sulfur, and elements with relatively high concentrations 

not requiring a low detection limit. The ICP–MS method is optimal for trace elements requiring 

lower limits of determination near or below their crustal abundance and elements not determined 

by ICP–AES. The lower limits of determination (LLD) are listed in table 1.
4 

Table 1. Elements determined by ICP–MS and ICP–AES. 

[LLD, lower limit of determination; ICP–MS, inductively coupled plasma–mass spectrometry; ICP–AES, 

inductively coupled plasma–atomic emission spectrometry; %, percent; mg/kg, milligrams per kilogram]

Element Analytical method LLD

Aluminum ICP-AES 0.01%

Calcium ICP-AES 0.01%

Iron ICP-AES 0.01%

Potassium ICP-AES 0.01%

Magnesium ICP-AES 0.01%

Sodium ICP-AES 0.01%

Phosphorus ICP-AES 50 mg/kg

Titanium ICP-AES 0.01%

Silver ICP-MS 1 mg/kg

Barium ICP-AES 5 mg/kg

Beryllium ICP-MS 0.1 mg/kg

Bismuth ICP-MS 0.04 mg/kg

Cadmium ICP-MS 0.1 mg/kg

Cerium ICP-MS 0.05 mg/kg

Cobalt ICP-MS 0.1 mg/kg

Chromium ICP-AES 1 mg/kg

Cesium ICP-MS 5 mg/kg

Copper ICP-AES 0.5 mg/kg

Gallium ICP-MS 0.05 mg/kg

Indium ICP-MS 0.02 mg/kg

Lanthanum ICP-MS 0.5 mg/kg

Lithium ICP-AES 1 mg/kg

Manganese ICP-AES 5 mg/kg

Molybdenum ICP-MS 0.05 mg/kg

Niobium ICP-MS 0.1 mg/kg

Nickel ICP-AES 0.5 mg/kg

Lead ICP-MS 0.5 mg/kg

Rubidium ICP-MS 0.2 mg/kg

Sulfur ICP-AES 0.01%

Antimony ICP-MS 0.05 mg/kg

Scandium ICP-MS 0.1 mg/kg

Tin ICP-MS 0.1 mg/kg

Strontium ICP-AES 0.5 mg/kg

Tellurium ICP-MS 0.1 mg/kg

Thallium ICP-MS 0.1 mg/kg

Thorium ICP-MS 0.2 mg/kg

Uranium ICP-MS 0.1 mg/kg

Vanadium ICP-AES 1 mg/kg

Tungsten ICP-MS 0.1 mg/kg

Yttrium ICP-MS 0.1 mg/kg

Zinc ICP-AES 1 mg/kg
5 

Mercury (Hg) was determined by treating 0.1 g of sample with a mixture of nitric and 

hydrochloric acids and heating at 110 °C for 30 minutes. Once cooled, solutions of sulfuric acid, 

potassium permanganate, and potassium persulfate were added followed by sodium chloridehydroxylamine sulfate. The final solution was reduced by stannous chloride and analyzed by

atomic absorption spectrometry (AAS). This method is a modification of that published by the 

U.S. Environmental Protection Agency (1994). The LLD is 0.02 milligrams per kilogram 

(mg/kg).

Selenium (Se) was determined by heating 0.25 g of sample at 175 °C with a combination 

of nitric, hydrofluoric and perchloric acids. After the solution was cooled, hydrochloric and nitric 

acids were added, then the solution was heated and cooled again. The sample was diluted and 

analyzed using hydride-generation AAS in a method similar to Hageman and Brown (2002). The 

LLD is 0.2 mg/kg. For analysis of arsenic (As), the sample was fused in a mixture of sodium 

peroxide and sodium hydroxide at 750 °C. The fused mixture was then dissolved in hydrochloric 

acid and analyzed by hydride-generation atomic absorption spectrometry in a method similar to 

Hageman and Brown (2002). The LLD is 0.6 mg/kg. 

Splits of the material less than 2 mm in size were provided to the Colorado State 

University Soil, Water, and Plant Testing Laboratory for determination of soil pH on a saturated 

soil paste using the methods described by Richards (1954). 

Quality Assurance and Quality Control

Quality assurance (QA) is mainly the concern of the analytical laboratory. The various 

components of the QA plan include standard operating procedures, instrument logs, training 

records, data acceptance and rejection criteria, and laboratory audits. Unlike the unquantifiable 

QA element, the quality control (QC) element measures, by an analytical method, the accuracy 

and precision of the data produced. The accuracy and precision are established through the 

analysis of reference materials (RMs) and sample replicates, respectively. The 19 samples from 

the Riverton UMTRA Site were processed along with thousands of soil samples collected as part 

of the North American Soil Geochemical Landscapes Project (Smith and others, 2011, 2012) and 

were subject to the same QA/QC protocols used in that project as described below. 

The samples taken through the analytical process in the USGS contract laboratories 

received QC checks on three separate levels. The first-level QC assessment was made by the 

USGS contract laboratory. In the next level, quality was assessed by the USGS QC officer, 

followed by a third-level assessment by the USGS principal investigator for the Riverton 

UMTRA Site soil geochemistry study. 

The USGS contract laboratory is accredited to the International Organization for 

Standardization/International Electrotechnical Commission (ISO/IEC) 17025 standard, which 

includes both QA and QC protocols. The QC is monitored by analyzing a RM with every batch 

of 48 samples. The RM most often used is a syenite rock standard (SY-3) developed by the 

Canadian Centre for Mineral and Energy Technology (Govindaraju, 1989). Shewhart Control 

Charts (Taylor, 1987) are generated for the RM analyses and reviewed with every report as part 

of the internal quality audits.

The accuracy for elements determined by ICP–MS and ICP–AES was considered 

acceptable if recovery was within the range of 85–115 percent at five times the LLD. The 

accuracy for Hg and Se was considered acceptable if recovery was within 80–120 percent at five 

times the LLD.
6 

At the second tier, the USGS QC officer assessed precision and accuracy on the basis of 

five RMs that were inserted between every batch of 50 samples. The soil RMs used in this study 

were SRM 2709 and SoNE-1. SRM 2709, a soil from the San Joaquin Valley of central 

California, is a certified RM available from the National Institute of Standards and Technology 

and is used to assess both accuracy and precision (National Institute of Standards and 

Technology, 2003). The acceptance criteria for accuracy are the same as those used by the 

contract laboratory. 

SoNE-1 is a USGS in-house, noncertified soil RM prepared specifically for the North 

American Soil Geochemical Landscapes Project. It was collected from the Sharpsburg Soil 

Series (Natural Resources Conservation Service, 2008) in Lancaster County, Nebraska, and is 

used in this study to assess only precision. The precision for elements determined by ICP–MS 

and ICP–AES, and for total carbon (C) and carbonate C was considered acceptable if the 

calculated relative standard deviation (RSD) of duplicate samples is no greater than 15 percent. 

The precision for Hg, As, and Se was considered acceptable if the calculated RSD of duplicate 

samples was no greater than 20 percent. 

The USGS principal investigator for the Riverton UMTRA Site soil geochemistry study 

initiated the final QC tier, which included two blind SoNE-1 RMs inserted in each batch of 18 

samples. In general, one RM was inserted into the first half of the batch and one RM into the 

second half. The acceptance criteria for accuracy and precision are those stated in the previous 

paragraph. 

The Colorado State University Soil, Water, and Plant Testing Laboratory maintains its 

own rigorous QC program for soil pH determinations including periodic calibration of the pH 

electrode with standard solutions. 

Estimation of Risk

In this report, risk to biologic receptors and humans from potentially toxic elements in 

soil within the Riverton UMTRA Site is estimated by comparing the concentration of these 

elements with soil screening values established by the U.S. Environmental Protection Agency 

(USEPA).

Ecological Soil Screening Levels

Ecological Soil Screening Levels (Eco-SSLs) are “concentrations of contaminants in soil 

that are protective of ecological receptors that commonly come into contact with and (or) 

consume biota that live in or on the soil” (U.S. Environmental Protection Agency, 2003). 

USEPA has established Eco-SSLs for many of the soil contaminants that are frequently of 

ecological concern for plants and animals at hazardous waste sites. These contaminants include 

the following 15 trace elements that occur naturally in soil: Ag, As, Ba, Be, Cd, Cr, Co, Cu, Mn, 

Ni, Pb, Sb, Se, V, and Zn (U.S. Environmental Protection Agency, 2006, 2005b, 2005c, 2005d, 

2005e, 2008a, 2005f, 2007a, 2007b, 2007c, 2005g, 2005a, 2007d, 2005h, 2007e, respectively). 

Eco-SSLs have been derived separately for four groups of biological receptors: plants, soil 

invertebrates, birds, and mammals (table 2). Complete details on how the Eco-SSLs were derived 

are given in U.S. Environmental Protection Agency (2003). Eco-SSLs are considered to be 

protective of terrestrial ecosystems and are intended to be applied at the screening stage of an 

ecological risk assessment. For the current investigation, the values are used to identify those 

trace elements of potential concern in soils of the Riverton UMTRA Site. 
7 

As seen in table 2, Eco-SSL for Cr have been established for Cr(III) and Cr(VI). The 

chemical analysis performed on the Riverton UMTRA Site samples in this study resulted in total 

Cr concentrations and did not yield separate concentration values for the two oxidation states. 

For this reason, Cr will not be considered further in this report. 

Table 2. Ecological soil screening levels (Eco-SSLs) as established by the U.S. Environmental Protection 

Agency. 

[Ag, silver; As, arsenic; Ba, barium; Be, beryllium; Cd, cadmium; Co, cobalt; Cr(III), trivalent chromium; Cr(VI), 

hexavalent chromium; Cu, copper; Mn, manganese; Ni, nickel; Pb, lead; Sb, antimony; Se, selenium; V, vanadium; 

Zn, zinc; NA, not available; mg/kg, milligrams per kilogram] 

Eco-SSL Eco-SSL Eco-SSL Eco-SSL

Element Plants (mg/kg) Soil invertebrates (mg/kg) Birds (mg/kg) Mammals (mg/kg)

Aga 560 NA 4.2 14

Asb 18 NA 43 46

Bac NA 330 NA 2000

Bed NA 40 NA 21

Cde 32 140 0.77 0.36

Cof 13 NA 120 230

Cr(III)g NA NA 26 34

Cr(VI)g NA NA NA 130

Cuh 70 80 28 49

Mni 220 450 4300 4000

Nij 38 280 210 130

Pbk 120 1700 11 56

Sbl NA 78 NA 0.27

Sem 0.52 4.1 1.2 0.63

Vn NA NA 7.8 280

Zno 160 120 46 79

a U.S. Environmental Protection Agency (2006); b U.S. Environmental Protection Agency (2005b); c U.S. 

Environmental Protection Agency (2005c); d U.S. Environmental Protection Agency (2005d); e U.S. Environmental 

Protection Agency (2005e); f U.S. Environmental Protection Agency (2005f); g U.S. Environmental Protection 

Agency (2008a); h U.S. Environmental Protection Agency (2007a); i

U.S. Environmental Protection Agency 

(2007b); j

 U.S. Environmental Protection Agency (2007c); k U.S. Environmental Protection Agency (2005g); l

U.S. Environmental Protection Agency (2005a); m U.S. Environmental Protection Agency (2007d); n U.S. 

Environmental Protection Agency (2005h); o U.S. Environmental Protection Agency (2007e)

Human Health Soil Screening Levels

EPA has developed regional screening levels based on human health risk for both 

residential and industrial soils (U.S. Environmental Protection Agency, 2011) as shown in tables 

3 and 4, respectively. A detailed discussion of the human health soil screening level concept is 

found in U.S. Environmental Protection Agency (1991; 1996a, b; 2002; 2008b). For some 

elements, two screening levels have been established, each of which corresponds to fixed levels 

of risk. One of the screening levels represents a one-in-one million (10–6) cancer risk 
8 

(carcinogenic screening level), and the other represents a noncarcinogenic hazard quotient (HQ) 

of 1. The HQ is defined as the estimated (maximum) concentration of a specific toxic element 

divided by a screening benchmark regarded as a threshold of toxicity (generally a No-Adverse 

Effects Level) (U.S. Environmental Protection Agency, 2008b). If the HQ is greater than 1, then 

harmful effects are likely due to the element in question. If the HQ is less than 1, then harmful 

effects are not likely. Generally for residential soils, if a substance causes both carcinogenic and 

noncarcinogenic effects, the 10–6 carcinogenic risk will result in a more stringent screening level.

Table 3. Human health soil screening levels for industrial soils (U.S. Environmental Protection Agency, 2011).

[Ag, silver; As, arsenic; Ba, barium; Be, beryllium; Cd, cadmium; Co, cobalt; Cu, copper; Mn, manganese; Ni, 

nickel; Pb, lead; Sb, antimony; Se, selenium; U, uranium; V, vanadium; Zn, zinc; mg/kg, milligrams per kilogram; 

NA, not available]

Human health screening level Human health screening level

Element carcinogenic (mg/kg) noncarcinogenic (mg/kg)

Ag NA 390

As 0.39 22

Ba NA 15,000

Be 1,400 160

Cd 1,800 70

Co 370 23

Cu NA 3,100

Mn NA 1,800

Ni 1,300 1,500

Pb NA 400

Sb NA 31

Se NA 390

U NA 230

V NA 390

Zn NA 23,000

Estimation of Background

For most investigations of soil geochemistry at potentially hazardous sites, an estimation 

of the background concentrations of elements in soil is made by collecting a very limited number 

of samples outside the boundaries of the site. Given that true geochemical background for a 

given element is not just one concentration value but rather a distribution of concentration values 

(Matschullat and others, 2000; Reimann and Filzmoser, 2000; Reimann and Garrett, 2005; 

Reimann and others, 2005); this approach is considered by the authors to be wholly inadequate.

For this investigation, a dataset of 160 samples of soils from depths of 0 to 5 cm collected 

throughout the entire State of Wyoming by the North American Soil Geochemical Landscapes 

Project (Smith and others, 2011, 2012) was used to determine the regional-scale geochemical 

background variation to provide context within which to consider the data from the Riverton 

UMTRA Site.
9 

Results

Table 5 presents a statistical summary for 41 elements in 0- to 5-cm soils from the 

Riverton UMTRA Site. The elements Ag, Cs, and Te are not included in this table because most 

concentration values are below the LLD. A complete listing of the geochemical data (44 

elements and soil pH) is shown in appendix 1. Box-and-whisker plots allow a visual 

representation of the data distribution for each of the elements showing at least one exceedance 

of the Eco-SSLs or the human health soil screening levels (fig. 2). Figure 2 also provides a 

comparison of the UMTRA data with the 160-sample Statewide dataset. Given the history of 

uranium milling at the site, U and Th are elements of potential environmental concern, and their 

box-and-whisker plots are shown in figure 3. There are no established EcoSSLs for either 

element. The noncarcinogenic human health screening level (residential soils) for U is 230 

mg/kg and for industrial soils is 3,100 mg/kg, both far exceeding any concentrations found at the 

Riverton UMTRA Site.

Table 4. Human health soil screening levels for industrial soils (U.S. Environmental Protection Agency, 2011). 

[Ag, silver; As, arsenic; Ba, barium; Be, beryllium; Cd, cadmium; Co, cobalt; Cu, copper; Mn, manganese; Ni, 

nickel; Pb, lead; Sb, antimony; Se, selenium; U, uranium; V, vanadium; Zn, zinc; mg/kg, milligrams per kilogram; 

NA, not available]

Human health screening level Human health screening level

Element carcinogenic (mg/kg) noncarcinogenic (mg/kg)

Ag NA 5,100

As 1.6 260

Ba NA 190,000

Be 6,900 2,000

Cd 9,300 800

Co 1,900 300

Cu NA 41,000

Mn NA 23,000

Ni 64,000 20,000

Pb NA 800

Sb NA 410

Se NA 5,100

U NA 3,100

V NA 5,200

Zn NA 310,000
10

Table 5. Statistical summary for 41 elements in 0–5 cm soils from the Riverton UMTRA Site. 

[Al, aluminum; Ca, calcium; Fe, iron; K, potassium; Mg, magnesium; Na, sodium; S, sulfur; Ti, titanium; As, 

arsenic; Ba, barium; Be, beryllium; Bi, bismuth; Cd, cadmium; Ce, cerium; Co, cobalt; Cr, chromium; Cu, copper; 

Ga, gallium; Hg, mercury; In, indium; La, lanthanum; Li, lithium; Mn, manganese; Mo, molybdenum; Nb, niobium; 

Ni, nickel; P, phosphorus; Pb, lead; Rb, rubidium; Sb, antimony; Sc, scandium; Se, selenium; Sn, tin; Sr, strontium; 

Th, thorium; Tl, thallium; U, uranium; V, vanadium; W, tungsten; Y, yttrium; and Zn, zinc; n = 19; UMTRA, 

Uranium Mill Tailings Remedial Action; MAD, median absolute deviation; %, percent; mg/kg, milligrams per 

kilogram; <, less than]

Element Units Minimum Median Maximum MAD

Al % 3.57 5.73 10.0 0.62

Ca % 1.30 2.15 7.39 0.89

Fe % 0.72 1.95 3.36 0.22

K % 1.36 1.97 3.54 0.21

Mg % 0.22 0.98 1.88 0.25

Na % 0.84 1.51 2.65 0.24

S % 0.01 0.03 0.3 0.01

Ti % 0.07 0.21 0.36 0.03

As mg/kg 1.5 2.7 7.3 1.2

Ba mg/kg 551 870 1,780 208

Be mg/kg 0.8 1.2 2.2 0.1

Bi mg/kg 0.05 0.12 0.23 0.03

Cd mg/kg <0.1a 0.2 0.3 0.1

Ce mg/kg 30 67.8 136 20.2

Co mg/kg 2.5 7.8 14.7 1.3

Cr mg/kg 7 50 104 9 

Cu mg/kg 4.1 10.6 20.3 2.1

Ga mg/kg 9.01 13.5 25.2 1.5

Hg mg/kg 0.01 0.01 0.06 0.00

In mg/kg <0.02b 0.03 0.04 0.00

La mg/kg 16 38.4 77.2 11.9

Li mg/kg 7 15 29 4 

Mn mg/kg 208 410 1,410 71

Mo mg/kg 0.21 0.56 3.36 0.15

Nb mg/kg 3 7 11.9 0.7

Ni mg/kg 4.7 21.2 40.3 3.0

P mg/kg 320 750 1,350 148

Pb mg/kg 12.5 22.3 35.7 5.5

Rb mg/kg 48.3 63.2 111 9.8

Sb mg/kg 0.14 0.32 0.59 0.09
11

Table 5. Statistical summary for 41 elements in 0–5 cm soils from the Riverton UMTRA Site.—Continued

[Al, aluminum; Ca, calcium; Fe, iron; K, potassium; Mg, magnesium; Na, sodium; S, sulfur; Ti, titanium; As, 

arsenic; Ba, barium; Be, beryllium; Bi, bismuth; Cd, cadmium; Ce, cerium; Co, cobalt; Cr, chromium; Cu, copper; 

Ga, gallium; Hg, mercury; In, indium; La, lanthanum; Li, lithium; Mn, manganese; Mo, molybdenum; Nb, niobium; 

Ni, nickel; P, phosphorus; Pb, lead; Rb, rubidium; Sb, antimony; Sc, scandium; Se, selenium; Sn, tin; Sr, strontium; 

Th, thorium; Tl, thallium; U, uranium; V, vanadium; W, tungsten; Y, yttrium; and Zn, zinc; n = 19; UMTRA, 

Uranium Mill Tailings Remedial Action; MAD, median absolute deviation; %, percent; mg/kg, milligrams per 

kilogram; <, less than]

Element Units Minimum Median Maximum MAD

Sc mg/kg 2 6.1 10.8 0.6

Se mg/kg <0.2c 0.2 2.5 0.1

Sn mg/kg 0.9 1 2 0.1

Sr mg/kg 216 332 647 105

Th mg/kg 4.6 15.3 31.9 6.8

Tl mg/kg 0.3 0.4 0.6 0 

U mg/kg 1.2 2.6 11.3 0.6

V mg/kg 16 49 82 6 

W mg/kg 0.1 0.6 0.9 0.1

Y mg/kg 7 12.6 21.4 1.2

Zn mg/kg 16 47 74 6 

a One sample had a Cd concentration of <0.1 mg/kg. This value was

replaced by 0.05 mg/kg for calculation of the MAD.

b One sample had an In concentration of <0.02 mg/kg. This value was

replaced by 0.01 mg/kg for calculation of the MAD.

c Eight samples had Se concentrations of <0.2 mg/kg. These values were

replaced by 0.1 mg/kg for calculation of the MAD.

Figure 2 (following six pages). Box-and-whisker plots showing the variation of (A) As (arsenic), (B) Ba

(barium), (C) Cd (cadmium), (D) Co (cobalt), (E) Cu (copper), (F) Mn (manganese), (G) Ni (nickel), (H) 

Pb (lead), (I) Sb (antimony), (J) Se (selenium), (K) V (vanadium), and (L) Zn (zinc) in soils from the 

Riverton Uranium Mill Tailings Remedial Action (UMTRA) Site, Fremont County, Wyoming, and in soils 

from the 160-sample Statewide dataset. The outer limits of the lower and upper whiskers on the plots 

represent the 10th and 90th percentiles, respectively. The “X” between the two box-and-whisker plots in 

each figure represents the median for six soil samples from the Wyoming Statewide dataset that are 

located within the Wind River drainage. Eco-SSL, ecological soil screening level; mg/kg, milligrams per 

kilogram.
12
13
14
15
16
17
18

Figure 3. Box-and-whisker plots showing the variation of (A) U (uranium) and (B) Th (thorium) in soils 

from the Riverton Uranium Mill Tailings Remedial Action (UMTRA) Site, Fremont County, Wyoming, and 

in soils from the 160-sample Statewide dataset. The outer limits of the lower and upper whiskers on the 

plots represent the 10th and 90th percentiles, respectively. The “X” between the two box-and-whisker 

plots in each figure represents the median for six soil samples from the Wyoming Statewide dataset that 

are located within the Wind River drainage. mg/kg, milligrams per kilogram.
19

Discussion

Within the UMTRA site, all 19 samples exceed (1) the carcinogenic human health 

screening level for As in residential soils (0.39 mg/kg) and (2) the Pb and V Eco-SSLs 

(ecological soil screening level) for birds (11 and 7.8 mg/kg, respectively). Eighteen of the 19 

samples exceed the Mn Eco-SSL for plants (220 mg/kg), 13 of the 19 samples exceed the Sb 

Eco-SSL for mammals (0.27 mg/kg), and 10 of 19 samples exceed the Zn Eco-SSL for birds (46 

mg/kg). However, to provide perspective, these exceedances are found in soils at most locations 

in the 160-sample Wyoming Statewide soil database. Therefore, elevated concentrations alone 

are not necessarily cause for alarm.

Elements of most environmental concern to the tribes are U, V, Ni, Zn, and Th. Elements 

Ni, Zn, U, and Th are enriched in soils at the site compared to the Wyoming Statewide soil 

database (figures 2G, 2L, 3A, and 3B, respectively). The former mill site processed both U and V 

(U.S. Department of Energy, 2011), so it might be expected that both elements would be 

enriched at the site with respect to the Wyoming Statewide soils dataset; however V is depleted 

compared to the Wyoming soil dataset (fig. 2K). 

Of interest to the tribes is how these elements in the soils are transported and 

incorporated into vegetation, biota, and water. The principal surface uses are growing hay, 

grazing, and subsistence gardening. The Little Wind River runs along the south institutionalcontrol boundary of the UMTRA site and is used for recreation and subsistence purposes. 

Previous investigations by the Wind River Environmental Quality Commission have found U 

concentrations in sediments of an oxbow lake as high as 9.23 mg/kg (Shakespeare and Goggles, 

2010), similar to the highest soil concentrations found in this study. Concentrations of As, Ni, 

and V in oxbow lake sediments also were similar to soil concentrations found in this study. This 

could imply that either eolian transport and (or) surface runoff are the principal transport 

mechanisms for moving soils toward the Little Wind River. If the former is a principal transport 

mechanism, then long-term exposure to dust and airborne particulates possibly could present 

human health concerns. 

Perhaps of greater interest to the tribes is the subsurface mobilization and transport of 

these elements through groundwater. A recently completed retrospective analysis of groundwater 

quality at the UMTRA site (Anthony Ranalli, written commun.) indicates that subsurface 

mobilization is not a primary consideration in groundwater, although these elements are present 

in groundwater at detectable concentrations. 

Acknowledgments

Samples were collected with the assistance of Armando Ramirez.

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