Wildfire Impacts on Soil Physical Properties: a 3-Year Assessment for The
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Wildfire Impacts on Soil Physical Properties: A 3-year assessment for the 2016 Erskine Fire, CA By Cindy Rodriguez A Thesis Submitted to the Department of Geological Sciences California State University, Bakersfield In Partial Fulfillment for the Degree of Master of Science in Geology Spring 2021 Copyright By Cindy Rodriguez 2021 Wildfire Impacts on Soil Physical Properties: A 3-year assessment for the 2016 Erskine Fire, CA By Cindy Rodriguez This thesis has been accepted on behalf of the Department of Geological Sciences by their supervisory committee: _____________________________________________________________________________ Dr. Junhua Guo, Committee Chair _____________________________________________________________________________ Dr. William Krugh _____________________________________________________________________________ Dr. Eduardo Montoya iii ACKNOWLEDGEMENTS Thanks to the CSUB Center for Research Excellence in Science and Technology (CREST) program for fully supporting me financially and making it possible for me to attend school and conduct this research. I want to thank my advisor Dr. Guo, for his guidance, support, and patience throughout each stage of my thesis research. Thank you, Dr. Krugh and Dr. Montoya, for your input and revising my thesis, and being part of my committee. Thanks to Elizabeth Powers and Sue Holt for maintaining laboratory supplies, providing door access, and their positive attitudes. To my friends, thank you for all the happy distractions to rest my mind outside of my research. Special thanks to Toni Ramirez, to whom I worked closely throughout my time at CSUB. Thanks for the laughs, advice, and words of encouragement. I would also like to acknowledge my cat Lexie for her companionship through the writing process. Most importantly, I would like to thank my parents and sister for their continued support throughout my academic journey. Their love and emotional support helped me push through difficult times. This would not be possible without them. iv ABSTRACT The 2016 Erskine Fire burned 48,020 acres in a portion of the southern Sierra Nevada, creating unburned or very low, low, moderate, and high burn severity patterns that allowed us to quantify the differences in soil alterations as a function of burn severity. Haake et al. (2020) investigated the same area and found that soil properties changed according to burn severity. In order to investigate the potential recovery state for the burned soils, we re-collected soil samples from the same site two years later. The physical properties of soils were explored using liquid limit (LL), plastic limit (PL), X-Ray diffractometry (XRD), total organic carbon content (TOC), grain size analysis, and direct shear tests. Results from this study reveal that low burn severity (LBS) and moderate burn severity (MBS) soils had the highest LL and PL. High burn severity (HBS) soils had a very low plasticity index (PI) value due to the low abundance of clay minerals. These results are different from the previous work done by Haake et al. (2020), which could be attributed to the timing of post-fire soil sampling. Shear results reveal variable results under the normal loads of 0.5 tons per square foot (TSF) and 1.0 TSF; and an increasing trend in shear strength with increasing burn severity under the normal load of 2.0 TSF. HBS, MBS, and very low burn severity (VLBS) soils exhibit cohesion values of zero and 0.2 for LBS soils. Shear strength and cohesion values differ from the results of Haake et al. (2020). Differences in results might be due to modifications in the tests. XRD analysis reveals a lower abundance of clay minerals (smectite and chlorite/kaolinite) in the HBS soils. TOC content was highest in HBS and MBS soils and lowest in LBS soils. This result differs from the previous study by Haake et al. (2020), where TOC increased with increasing burn severity. Differences in the TOC may be related to the high vegetation density in the sampling locations during the recovery time. Grain size results of this study show clay-sized particles were less abundant in moderate and high burn v severities. Similarly, grain size results from Haake et al. (2020) revealed that clay-sized particles were most abundant in lower burn severity soils. Less abundant clay- and silt-sized particles in the high burn severity soils could be attributed to high temperature exposure from the fire. Previous studies exploring the impacts of fire on soil properties have produced variable results, suggesting that soil property alterations due to fire is different from case by case. vi TABLE OF CONTENTS ACKNOWLEDGEMENTS ........................................................................................................... iv ABSTRACT .................................................................................................................................... v LIST OF FIGURES ..................................................................................................................... viii LIST OF TABLES ......................................................................................................................... ix INTRODUCTION .......................................................................................................................... 1 BACKGROUND ............................................................................................................................ 3 Fire Behavior .............................................................................................................................. 3 Soil Burn Severity Classification ................................................................................................ 4 Soil properties, vegetation, and slope stability ........................................................................... 6 Post-fire debris flows .................................................................................................................. 9 MATERIALS AND METHODS .................................................................................................. 11 Study Area ................................................................................................................................ 11 Sample Collection and Processing ............................................................................................ 12 Atterberg Limits ........................................................................................................................ 13 Direct Shear Strength ................................................................................................................ 15 Mineralogy ................................................................................................................................ 16 Total Organic Carbon ............................................................................................................... 18 Grain Size Distribution ............................................................................................................. 18 RESULTS ..................................................................................................................................... 20 Atterberg Limits ........................................................................................................................ 20 Shear Test.................................................................................................................................. 21 Mineralogy ................................................................................................................................ 21 Total Organic Carbon ............................................................................................................... 24 Grain Size.................................................................................................................................. 24 DISCUSSION ............................................................................................................................... 26 CONCLUSION ............................................................................................................................. 32 REFERENCES ............................................................................................................................. 34 FIGURES ...................................................................................................................................... 41 TABLES ....................................................................................................................................... 67 APPENDICES .............................................................................................................................. 78 vii LIST OF FIGURES Figure 1. Fire Behavior Triangle .................................................................................................. 41 Figure 2. Four States of Soil ......................................................................................................... 42 Figure 3. Geologic Map of the Kern River Valley and surrounding region ................................. 43 Figure 4. Soil Map of the Erskine Fire Perimeter ......................................................................... 44 Figure 5. Images of the sampling locations .................................................................................. 45 Figure 6. Map of the Erskine Fire Perimeter and surrounding region .......................................... 46 Figure 7. Erskine Fire Burn Severity