BIOGEOCHEMISTRY OF MONOMETHYL MERCURY IN SAN FRANCISCO BAY ESTUARY. A Dissertation KEY-YOUNG CHOE

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1 BIOGEOCHEMISTRY OF MONOMETHYL MERCURY IN SAN FRANCISCO BAY ESTUARY A Dissertation by KEY-YOUNG CHOE Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY May 2002 Major Subject: Oceanography

2 BIOGEOCHEMISTRY OF MONOMETHYL MERCURY IN SAN FRANCISCO BAY ESTUARY A Dissertation by KEY-YOUNG CHOE Submitted to Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved as to style and content by: Gary A. Gill (Chair of Committee) Luis A. Cifuentes (Member) Bruce E. Herbert (Member) Peter H. Santschi (Member) Wilford D. Gardner (Department Head) May 2002 Major Subject: Oceanography

3 iii ABSTRACT Biogeochemistry of Monomethyl Mercury in San Francisco Bay Estuary. (May 2002) Key-Young Choe, B.S., Pusan National University; M.S., Pusan National University Chair of Advisory Committee: Dr. Gary A. Gill An accurate cross-flow ultrafiltration technique was developed to isolate colloidal material for the determination of the phase speciation of monomethyl mercury (MeHg) in natural waters. This technique was applied to the estuarine waters of San Francisco Bay (SFB) to elucidate the importance of colloidal material in the biogeochemical cycling of MeHg in estuaries. Surface water samples were collected from the SFB estuary in September - October 2000 and March Mercury (Hg) (inorganic mercury + MeHg) and MeHg concentrations were measured in unfiltered, filter-passing (< 0.45 µm), colloidal (1 kda 0.45 µm), and (truly) dissolved (< 1 kda) fractions. Particulate Hg was the dominant phase species (88 ± 7 %, n = 29) in unfiltered water. Particulate MeHg averaged 52 ± 18 % (n = 29) of the total MeHg in unfiltered water. A significant portion of the filterpassing Hg (49 ± 17 %, n = 21) and MeHg (46 ± 18 %, n = 21) were associated with colloidal material. Strong correlations were observed between colloidal Hg and colloidal organic carbon, and between colloidal MeHg and colloidal organic carbon, in the filter-

4 iv passing fraction. These relationships suggest that organic material is an important transport medium of the filter-passing Hg and MeHg in the SFB estuary. Benthic flux chamber studies demonstrated that sediment-water exchange is an important source of MeHg in the SFB estuary and Delta regions. Fluxes for both Hg and MeHg were generally higher in May than in September or February. The Cosumnes River, Franks Tract, Franks Tract marsh, and Prospect Slough had higher MeHg fluxes (90 ± 18 pmol/m 2 /day, n = 6) compared with Little Holland and Sherman Island (5 ± 37 pmol/m 2 /day, n = 3) in May 2000 and May 2001, suggesting a MeHg source in the upper Delta region. Evidence of colloidally associated MeHg was observed in interstitial porewater of SFB Delta sediments. Both absolute and relative colloidal MeHg concentrations were significantly higher in upper (0 2 cm) sediments (32 ± 9 %, n = 8) compared to deeper sediments (18 ± 16 %, n = 16).

5 v ACKNOWLEDGMENTS I would like to express my deepest appreciation to Dr. Gary A. Gill, Chairman of the Advisory Committee, for allowing me an opportunity to study at Texas A&M University, and for his encouragement, guidance and support throughout the entire research. I would also like to thank my committee members, Dr. Peter H. Santschi for his endless patience while answering my many questions and his thoughtful insights into the science of my research, and Drs. Luis A. Cifuentes, Bruce E. Herbert and GCR Dr. Claudia L. Barton for their constructive suggestions and guidance for my dissertation. My sincerest appreciation goes to Ron Lehman for his incredible help in the field and in the laboratory over the last five years. There are numerous friends in the Laboratory for Oceanographic and Environmental Research. I wish to thank; Dr. Laodong Guo and Dr. Liang-Saw Cat Wen for their assistance in my early experimental work, Dr. Chin Chung Hung, Dr. Degui Tang, Dr. Kent Warnken, Seunghee Han and Kathy Schwehr for sharing their experiences and knowledge, Mary Howley, Sheri Shaklovitz, Miriam Barranco, Amy Bratcher, Becky Hunt, Aimee Jacobs, Lori Muller, Cris Schmidt, Adam Thurlow and Dr. Ivano Vassura for their assistance with sample collection. I am also indebted to Dr. Kenneth Coale, Wes Heim and John Haskins at the Moss Landing Marine Laboratory, Dr. Mark Stephenson, Amy Byington, Nisse Goldberg, Myah Gunn and Sean Mundell at the California Department of Fish and Game, and Dr. Chris Foe at the California Regional Water Quality Control Board. The

6 vi dissertation would not have been possible without their tremendous efforts in San Francisco Bay. This research was supported by grants from the CALFED Bay-Delta Mercury Project, the Texas Sea Grant, Texas Institute of Oceanography Graduate Fellowship, and Texas Higher Education Coordinating Board. I also received financial support through teaching assistantships from Texas A&M University at Galveston. Special thanks to my friend, Dr. Joon-Hyung Ahn who provided encouragement to me throughout my research. Finally, this dissertation is dedicated to my parents, my wife, Jung-Ah, and two boys, Jeremy Seungwoo and Brian Jiwoo, thanks for their unconditional and infinite love, prayers and sacrifice over the last two thousand days.

7 vii TABLE OF CONTENTS ABSTRACT... ACKNOWLEDGMENTS... TABLE OF CONTENTS... LIST OF TABLES... LIST OF FIGURES... iii v vii ix xi CHAPTER I INTRODUCTION... 1 Hypothesis... 7 Objectives... 8 Study Area... 8 Organization of the Dissertation II ISOLATION OF COLLOIDAL MONOMETHYL MERCURY IN NATURAL WATER USING CROSS-FLOW ULTRAFILTRATION TECHNIQUES Introduction Materials and Methods Results and Discussion Conclusions III DISTRIBUTION OF PARTICULATE, COLLOIDAL AND (TRULY) DISSOLVED MERCURY IN SAN FRANCISCO BAY ESTUARY Introduction Materials and Methods Results Discussion Conclusions... 81

8 viii CHAPTER IV DISTRIBUTION OF PARTICULATE, COLLOIDAL AND (TRULY) DISSOLVED MONOMETHYL MERCURY IN SAN FRANCISCO BAY ESTUARY Introduction Materials and Methods Results Discussion Conclusions V SEDIMENT-WATER EXCHANGE OF MERCURY AND MONOMETHYL MERCURY IN ESTUARINE AND DELTA REGIONS OF SAN FRANCISCO BAY Introduction Materials and Methods Results and Discussion Conclusions VI PHASE DISTRIBUTION OF ORGANIC CARBON AND MONOMETHYL MERCURY IN SAN FRANCISCO BAY DELTA SEDIMENT POREWATERS Introduction Materials and Methods Results and Discussion Conclusions VII SUMMARY AND CONCLUSIONS REFERENCES APPENDIX VITA

9 ix LIST OF TABLES TABLE 2.1 Permeation and retention coefficients of organic macromolecules MeHg, Hg and OC concentrations in different fractions of natural water samples and mass balance recoveries Particle-water partitioning coefficients for MeHg and Hg. All values are mean ± 1 SD (n = 3, unless specified otherwise) Sampling location and salinity in September 2000 and March Retention coefficients of organic macromolecules by 1 kda membranes Suspended particulate matter (SPM), chlorophyll-a and filter-passing (< 0.45 µm) nutrient concentrations in San Francisco Bay Particulate (> 0.7 µm), filter-passing (< 0.45 µm), colloidal (1 kda 0.45 µm), and (truly) dissolved (< 1 kda) organic carbon concentrations in San Francisco Bay Unfiltered, particulate (> 0.45 µm), filter-passing (< 0.45 µm), colloidal (1 kda 0.45 µm), and (truly) dissolved (< 1 kda) Hg concentrations in San Francisco Bay Mercury flux estimates in the northern reach of San Francisco Bay Unfiltered, particulate (> 0.45 µm), filter-passing (< 0.45 µm), colloidal (1 kda 0.45 µm), and (truly) dissolved (< 1 kda) MeHg in San Francisco Bay estuary Particle-water partitioning coefficients in natural waters Monomethyl mercury flux estimates in the northern reach of San Francisco Bay

10 x TABLE 5.1 Sampling locations and sample collection matrix Sediment-water exchange fluxes of Hg and MeHg in the San Francisco Bay Delta (pmol/m 2 /day) Retention coefficients (R C ) of standard organic macromolecules with a 3 kda Amicon microconcentrator Filter-passing, colloidal and (truly) dissolved organic carbon concentrations in interstitial porewater (µm) Filter-passing, colloidal and (truly) dissolved MeHg concentrations in porewater (pm) Diffusive fluxes calculated using the MeHg concentration gradients in porewater (pmol/m 2 /day)

11 xi LIST OF FIGURES FIGURE 1.1 The cycling of Hg in natural environments. Dotted lines between the (truly) dissolved Hg and the particulate Hg represent the colloidal pumping processes suggested by Honeyman and Santschi (1989) The San Francisco Bay estuary and surface water sampling sites The Delta region of San Francisco Bay and sampling sites for sediment-water exchange studies A schematic diagram showing the sample processing procedure and operational terms used in this study Discrete (!) and integrated (") permeate concentration (left panels) and the retention coefficient of HMW organic macromolecules based on the integrated permeate concentration (right panels) as a function of CF. Vitamin B-12 (1350 Da) was used for the ultrafiltration with the 1 kda membrane and the 20 kda dextran (19500 Da) with the 10 kda membrane. The integrated permeate concentration was calculated using Σ(mass of organic macromolecules)/σ(permeate volume) and the retention coefficient was calculated using (1 - [integrated permeate]/[initial]) Discrete (!) and integrated (") permeate concentration (left panels) and the retention coefficient of LMW organic macromolecules based on the integrated permeate concentration (right panels) as a function of CF. Rhodamine 6G (480 Da) was used for the ultrafiltration with the 1 kda membrane, and vitamin B-12 and the 4 kda dextran (4400 Da) with the 10 kda membrane Discrete (!) and integrated (") permeate concentration (left panels) and the retention coefficient of MeHg in an estuarine water based on the integrated permeate concentration (right panels) as a function of CF. The solid line was calculated using a permeation model. Error bars represent 1 SD of MeHg concentrations (n = 3)... 30

12 xii FIGURE 2.5 Sorption of MeHg spiked in DI water and an estuarine water with and without preconditioning. The experiments with DI water were conducted in duplicate. Error bars represent 1 SD (n = 3) Effect of storage time on MeHg concentration in the permeate and retentate fractions of a filtered estuarine sample with the 1 kda (upper panel) and 10 kda (lower panel) ultrafiltration systems. Error bars represent 1 SD (n = 3) Flow rates of Sacramento River monitored at Freeport, CA (near Sacramento) and San Joaquin River monitored at Vernalis, CA (near Tracy) (a) in and (b) in The arrows represent for the sampling periods Distribution of SPM, chlorophyll-a and filter-passing (< 0.45 µm) nutrients as a function of salinity in September October 2000 (") and March 2001(# northern reach; # South Bay) Organic carbon distributions as a function of salinity. (a) particulate (> 0.7 µm), (b) filter-passing (< 0.45 µm), (c) colloidal (1 kda 0.45 µm), (d) (truly) dissolved (< 1 kda) OC and (e) percent of colloidal OC in the filter-passing pool in September October 2000 (") and March 2001(# northern reach; # South Bay) Mercury distributions as a function of salinity. (a) total, (b) particulate (> 0.45 µm), (c) filter-passing (< 0.45 µm), (d) colloidal (1 kda 0.45 µm), (e) (truly) dissolved (< 1 kda) Hg and (f) percent of colloidal Hg in the filter-passing fraction in September October 2000 (") and March 2001(# northern reach; # South Bay). r N, Sep represents the correlation coefficient for the northern reach data in September October Particulate and filter-passing (< 0.45 µm) Hg distributions as a function of salinity in the interval. Open symbols represent low flow periods, gray symbols high flow periods, and blank symbols South Bay... 64

13 xiii FIGURE 3.6 Long-term changes in total and filter-passing Hg in Sacramento River (river water end-member) and in the extreme South Bay. Arrows represent sampling periods under high flow condition The relationship between Hg and particulate material in the northern reach (" low flow condition; # high flow condition) and in South Bay (!). r N, r S, r N+S represent the correlation coefficients in the northern reach, South Bay and in the entire bay, respectively Correlations between (a) unfiltered and filter-passing and (b) filterpassing and colloidal Hg. r N+S refers to the correlation coefficient in the entire bay Correlations between Hg and organic carbon in filter-passing, colloidal and (truly) dissolved fractions. r Mar 01 refers to the correlation coefficient in March Particle-water partitioning coefficients of Hg as a function of SPM measured in and r N+S refers to the correlation coefficient in the entire bay Particle-water partitioning coefficients of Hg as a function of salinity Monomethyl mercury distributions as a function of salinity. (a) total, (b) particulate (> 0.45 µm), (c) filter-passing (< 0.45 µm), (d) colloidal (1 kda 0.45 µm), (e) (truly) dissolved (< 1 kda) Hg and (f) percent of colloidal MeHg in the filter-passing fraction in September October 2000 (") and March 2001(# northern reach; # South Bay). r N, Sep and r N, Mar represent the correlation coefficients for the northern reach data in September October 2000 and March 2001, respectively The relationship between MeHg and particulate material in September - October 2000 (") and March 2001(# northern reach; # South Bay). r N and r S represent the correlation coefficients in the northern reach and South Bay, respectively... 92

14 xiv FIGURE 4.3 Correlations between (a) unfiltered and filter-passing and (b) filterpassing and colloidal MeHg in September - October 2000 (") and March 2001(# northern reach; # South Bay). r N+S refers to the correlation coefficient in the entire bay Correlations between MeHg and organic carbon in filter-passing (< 0.45 µm), colloidal and (truly) dissolved (< 1 kda) fractions in September - October 2000 (") and March 2001(# northern reach; # South Bay) Particle-water partitioning coefficients of MeHg as a function of SPM Particle-water partitioning coefficients of MeHg as a function of salinity. The lower panel includes the South Bay data MeHg to Hg concentration ratio (%) in surface water of the San Francisco Bay estuary (both the northern reach and South Bay) Depth profiles of porewater Hg (> 0.45 µm) at selected sites in the San Francisco Bay Delta. Mercury concentrations at 0 cm represent those measured in the overlying water. For the left bottom panel, the data for May and September of 2000 refer to Little Holland and February and May of 2001 to Prospect Slough Depth profiles of porewater MeHg (< 0.45 µm) at selected sites in the San Francisco Bay Delta. Monomethyl Hg concentrations at 0 cm represent those measured in the overlying water. For the left bottom panel, the data for May and September of 2000 refer to Little Holland and February and May of 2001 to Prospect Slough Sediment-water exchange fluxes of Hg in the San Francisco Bay Delta. F C s refer to fluxes measured with benthic chamber deployments and F D s refer to diffusive fluxes estimated using interfacial concentration gradients (CC = Columbia Cut; CR = Cosumnes River; FT = Franks Tract; FTM = Franks Tract marsh; LH = Little Holland; PS = Prospect Slough; SI = Sherman Island; WS = White Slough)

15 xv FIGURE 5.4 Sediment-water exchange fluxes of MeHg in the San Francisco Bay Delta. F C s refer to fluxes measured with benthic chamber deployments and F D s refer to diffusive fluxes estimated using interfacial concentration gradients (CC = Columbia Cut; CR = Cosumnes River; FT = Franks Tract; FTM = Franks Tract marsh; LH = Little Holland; PS = Prospect Slough; SI = Sherman Island; WS = White Slough) Comparison of time course measurements of MeHg from light chambers deployed at the Cosumnes River, Franks Tract marsh and Prospect Slough in February 2001 and May Error bars and symbols represent the range of duplicate measurements and averages, respectively. The unit of a slope is pm/hour Comparison of MeHg fluxes from light/dark chambers deployed at Franks Tract, Little Holland and Sherman Island in September Error bars and symbols represent the range of duplicate measurements and averages, respectively. The unit of a slope is pm/hour hour benthic flux chamber deployment at Franks Tract. Error bars and symbols represent the range of duplicate measurements and averages, respectively The percentage of the chamber flux (F C ) attributed to diffusive flux (F D ) in May 2001 (CR = Cosumnes River; FT = Franks Tract; FTM = Franks Tract marsh; PS = Prospect Slough; SI = Sherman Island; WS = White Slough) Sampling sites in Franks Tract (FT = ordinary Franks Tract site; FTM = Franks Tract marsh site; FTM, FT1, FT2, FT3 = transect stations). Time series samples were collected between FTM and FT1 on May 19, Time series of Hg and MeHg measurements at the Franks Tract marsh site in May Mercury and MeHg concentrations in a surface water transect in Franks Tract

16 xvi FIGURE 5.12 Depth profiles of sediment MeHg at selected sites in the San Francisco Bay Delta. For the left bottom panel, the data for May 2000 refer to Little Holland and February and May of 2001 to Prospect Slough Particle-water partitioning coefficient for MeHg at selected sites in the San Francisco Bay Delta region. For the left bottom panel, the data for May 2000 refer to Little Holland and February and May of 2001 to Prospect Slough Retention coefficients of organic macromolecules Depth profiles of organic carbon in different size fractions in interstitial porewaters. Interstitial waters were collected by slicing and centrifuging (sliced) and by extraction of pore fluid using nitrogen pressure (squeezed) Depth profiles of MeHg in different size fractions in interstitial porewaters. Filter-passing MeHg concentrations at 0 cm represent those measured in the overlying water Correlations between MeHg and OC concentrations in (a) filterpassing (b) colloidal and (c) (truly) dissolved fractions Depth profiles of inorganic Hg in different size fractions in interstitial porewaters. Inorganic Hg is determined by the difference between Hg and MeHg MeHg to Hg concentration ratios (%) in porewaters in the filterpassing fraction