Naturally Occurring Background Levels of Arsenic in the Soils of Southwestern Oregon

241 Pages·2017·17.12 MB·English

by  Heather Ann Hurtado

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Heather Ann Hurtado
Portland State University
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Hurtado, Heather Ann, "Naturally Occurring Background Levels of Arsenic in the Soils of Southwestern
Oregon" (2015). Dissertations and Theses. Paper 2996.
https://doi.org/10.15760/etd.2997
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Naturally Occurring Background Levels of Arsenic
in the Soils of Southwestern Oregon

by
Heather Ann Hurtado

A thesis submitted in partial fulfillment of the
requirements for the degree of

Master of Science
in
Geology

Thesis Committee:
Scott F. Burns, Chair
Adam M. Booth
R. Benjamin Perkins

Portland State University
2016
Abstract
This study examines the natural background concentrations of arsenic in the soils
of southwest Oregon, using new samples in addition to data collected from previous
theses (Khandoker, 1997 and Douglas, 1999). The original 213 samples were run by
ICPAES with a reporting limit of 20 ppm, and only three samples had detected values.
The original samples were tested again (2013) at a detection limit of 0.20 ppm by
ICP-MS, as were 42 new samples (2013), to better ascertain the natural levels of arsenic
in undisturbed soils. The aim is to add to the existing DEQ data set, which has been used
to establish new regulatory levels based on natural levels in the environment that are both
safer and more economically viable than the former risk-based remediation levels (DEQ,
2013).
The maximum and mean concentrations, respectively, for each province (with
high formation map unit) are 85.4 and 21.99 ppm for South Willamette Valley (Tfee),
45.4 and 5.42 ppm for the Klamath Mountains (Jub), 11.9 and 2.76 ppm for the Cascade
Range (Tbaa), 10.6 and 5.15 ppm for the Coast Range (Ty), 2.32 and 1.29 ppm for the
Basin and Range (Qba) and 1.5 and 1.20 ppm for the High Lava Plains (Tmv).
In addition, the distribution and variance of arsenic in the A and B soil horizons is
assessed in this study; one of 18 new sites sampled for this study (distinguished with the
HH prefix), site HH11, was randomly chosen for this purpose. Site HH11 is an Inceptisol
soil above volcanic rock (KJdv map unit) located at 275 meters elevation in Douglas
County within the Klamath province. Five samples were taken from the A and from the B
i
horizons at site HH11. The mean and standard deviation, respectively, was 3.74 ± 0.44
for the A horizon and 4.53 ± 0.39 for the B horizon. The consistency and low deviation
within each horizon indicate that a single sample within a horizon is a good
representative of that horizon and supports the field methodology used in this study of
taking only one sample in the A horizon and one sample in the B horizon. The Wilcoxon
Rank-Sum test (p-value: 0.76) determined that A and B horizons for the 119 sites that had
data for both the A and B horizons were not statistically different. However, 47 sites
(39.5%) are higher in arsenic in the A horizon while 63 sites (52.9%) are higher in the B
horizon, with 9 sites (7.6%) having no value detected. This is strong support for sampling
in both the A and B horizons to ensure a sample is collected from the horizon with the
highest arsenic value, and critical for an accurate assessment of the maximum levels for
naturally occurring arsenic in soils.
Lastly, this study statistically examines six potentially important environmental
predictors of naturally occurring arsenic in southwestern Oregon: site elevation,
geomorphic province, mapped rock type and age, and sample soil order and color
(redness). A Classification and Regression Tree Model (CART) determined soil order,
elevation and rock type to be of significant importance in determining arsenic
concentrations in the natural environment. According to the regression tree, arsenic
concentrations are greater within Alfisol and Ultisol/Alfisol and Vertisol soil orders, at
lower elevations below 1,207 meters, and within soils from sedimentary, mixed
volcanic/sedimentary and unconsolidated rock types.
ii
The relationship between environmental factors and high levels of arsenic is
complex and research is ongoing. However, soil order is likely to be a significant factor
for predicting levels of arsenic since soil order is defined by its horizon development.
Negatively charged arsenic ions adsorb to positively charged minerals, commonly iron,
aluminum and magnesium oxides and clay. Any soils with a zone of mineral
accumulation (B horizon) — such as Alfisol, Ultisol and Vertisol soils have — are likely
to attract, retain and concentrate arsenic in the soil.
The relationship between elevation and rock type to arsenic level is less clear.
Arsenic values could be higher in lower elevations because weather and gravity have
transported arsenic-bearing sediments to lower topography where it accumulates, or a
result of more chemical processes in the more temperate climates at lower elevation, or is
correlated with other factors such as depositional regime of the rock. Rock type as a
predictive factor also requires more research; the degree of arsenic enrichment in rock
and arsenic released through physical and chemical weathering are likely to have a high
degree of local variability than will ultimately not allow definitive association of high
arsenic levels by rock type.
iii
Acknowledgements
Dr. Scott Burns, your encouragement and confidence has made the process of
creating this Master’s thesis much more enjoyable than I imagined it could be. Also, I
learned a lot from you! David Anderson from DEQ and Neil Morton from
GeoEngineers, Inc. were both invaluable resources for questions I had regarding the DEQ
data set. I thank Kassandra Lindsey and Aubrey Frimoth for being my post-surgery arms
and legs with their physical contributions (digging many holes) and can-do attitude in the
field. Classmates Traci Parker and Kimberly Yazzie joined me in playing with my data
for a class project, helped troubleshoot appropriate analyses for the data and traded ideas
for what the results may imply.
To my immediate family: thank you, parents, for emotional support and
perspective, always. I also very much appreciate my dog, Daisy, who has kept my toes
warm while snuggled under my desk during all of my higher education. My fantastic
husband, Josh, with his patience of steel, earns my lifetime of gratitude; he has sacrificed
many date-nights through both my Bachelor’s and Master’s education. Josh, without
your sense of humor and support, I would not have had the fun and focus that I am
grateful to have experienced through these degrees. Last but not least… Liam, welcome
to the world, my son!
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Table of Contents
Abstract ................................................................................................................................ i
Acknowledgements ............................................................................................................ iv
List of Tables ................................................................................................................... viii
List of Figures .................................................................................................................... ix
CHAPTER I: INTRODUCTION ........................................................................................ 1
1.1 PURPOSE AND SCOPE .......................................................................................... 2
1.2 STUDY AREA .......................................................................................................... 4
CHAPTER II: BACKGROUND ........................................................................................ 6
2.1 PHYSIOGRAPHIC PROVINCES OF OREGON .................................................... 6
2.2 GEOLOGIC HISTORY OF SOUTHWEST OREGON ........................................... 6
2.3 CLIMATE ............................................................................................................... 25
2.4 VEGETATION ....................................................................................................... 29
CHAPTER III: BACKGROUND WORK ON THE CONCENTRATION OF ARSENIC
IN SOILS .......................................................................................................................... 30
3.1 SOILS ...................................................................................................................... 30
3.1.1 SOIL DEFINITION .......................................................................................... 30
3.1.2 SOIL PROFILES AND HORIZON NOMENCLATURE ............................... 30
3.1.3 FIELD SOIL CHARACTERISTICS AND PROPERTIES ............................. 32
v
3.1.4 SOIL CLASSIFICATION ................................................................................ 36
3.2 SOURCES OF NATURALLY OCCURRING ARSENIC ..................................... 38
3.3 GEOCHEMISTRY: ARSENIC SPECIES AND TRANSPORTATION ............... 39
3.4 PREVIOUS WORK: ARSENIC IN SOILS IN SOUTHWEST OREGON ........... 41
CHAPTER IV: METHODOLOGY .................................................................................. 43
4.1 GEOGRAPHIC INFORMATION SYSTEMS ....................................................... 43
4.2 FIELD METHODS ................................................................................................. 44
4.3 LABORATORY METHODS ................................................................................. 45
4.4 GROUPINGS FOR STATISTICAL ANALYSES ................................................. 46
CHAPTER V: RESULTS ................................................................................................. 50
5.1 ARSENIC CONCENTRATIONS IN SOUTHWEST OREGON .......................... 50
5.2 DISTRIBUTION OF ARSENIC IN A AND B SOIL HORIZONS ....................... 51
5.3 DISTRIBUTION OF ARSENIC IN SOUTHWEST OREGON BY PROVINCE . 56
CHAPTER VI: DISCUSSION ......................................................................................... 61
6.1 LEVELS OF ARSENIC IN SOUTHWEST OREGON SOILS ............................. 61
6.1.1 DISTRIBUTION OF ARSENIC IN A AND B HORIZONS .......................... 61
6.1.2 COMPARISON BETWEEN DEQ AND PSU DATA SETS .......................... 61
6.2 RELATIONSHIP OF ARSENIC WITH ENVIRONMENTAL FACTORS .......... 64
6.2.1 REGRESSION TREE....................................................................................... 64
vi
6.2.2 FOREST REGRESSION .................................................................................. 66
6.2.3 RELATIONSHIP OF ARSENIC TO SOIL ORDER....................................... 67
6.2.4 RELATIONSHIP OF ARSENIC TO ELEVATION ....................................... 69
6.2.5 RELATIONSHIP OF ARSENIC TO ROCK TYPES ...................................... 71
6.2.6 SUMMARY OF TOP THREE PREDICTORS................................................ 73
CHAPTER VII: CONCLUSIONS .................................................................................... 78
CHAPTER VIII: FUTURE WORK.................................................................................. 81
REFERENCES CITED ..................................................................................................... 82
APPENDIX A: Stratigraphy of Provinces within Southwest Oregon .............................. 85
APPENDIX B: Description of Geologic Units by Province ............................................ 90
APPENDIX C: Site and soil Pit Images for HH Sites .................................................... 106
APPENDIX D: Soil Data ................................................................................................ 121
A Horizon .................................................................................................................... 121
B Horizon .................................................................................................................... 125
APPENDIX E: Sampling Site Characteristics ................................................................ 129
APPENDIX F: Analysis Categories Data ....................................................................... 135
APPENDIX G: Apex Labs Sample ID and Control Data .............................................. 139
vii
List of Tables
Table 1. Historic and new maximum limits of arsenic established by DEQ. ..................... 3
Table 2. Description of geologic units sampled within Klamath Mountain Province. ..... 10
Table 3. Description of geologic units sampled within the Coast Range. ........................ 15
Table 4. Description of geologic units sampled within the Cascade Range Province.. .... 19
Table 5. Description of geologic units sampled within the South Willamette Valley. ..... 20
Table 6. Description of geologic units sampled within the Basin and Range Province. .. 23
Table 7. Description of geologic units sampled within High Lava Plains Province. ....... 25
Table 8: Soil Horizon Nomenclature. ............................................................................... 33
Table 9: Typical arsenic concentrations in rocks and unconsolidated sediments. ............ 39
Table 10. Description of rock type categories. ................................................................. 49
Table 11: Comparison of A and B horizon center of data ................................................ 53
Table 12. Arsenic concentrations for samples collected in the A horizon ........................ 54
Table 13. Arsenic concentrations for samples collected in the B horizon. ....................... 55
Table 14. Comparison of Site 11 A and B horizons center of data .................................. 56
Table 15. Summary statistics for the DEQ and PSU data sets.......................................... 59
Table 16. Background calculation for the DEQ and PSU data sets .................................. 60
Table 17. Forest regression seeds. .................................................................................... 67
Table 18: Summary of statistical tests run on the regression tree for soil order. .............. 69
Table 19: Summary of statistical tests run on the regression tree for elevation ............... 71
Table 20: Summary of statistical tests run on the regression tree for rock type ............... 72
Table 21. Summary of results for arsenic level and three important factors by province. 80
viii