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HEAVY METALS IN CLAMS AND SEDIMENTS FROM MORRO BAY
Masters Thesis Presented to the faculty of
California Polytechnic State University, San Luis Obispo
In partial fulfillment Of the requirements for the degree of
Masters of Science in Civil and Environmental Engineering
By Jennifer Pehaim
June 2004
iii
APPROVAL PAGE
Title: Heavy Metals in Clams and Sediments from Morro Bay
Author: Jennifer Pehaim
Date Submitted: June 2004
Dr. Yarrow Nelson, Advisor
Dr. Samuel A. Vigil, Committee Member
Dr. Tom Ruehr, Committee Member
iv
ABSTRACT Heavy Metals in Clams and Sediments from Morro Bay
Morro Bay on the Central Coast of California is an impacted estuary, currently listed by
the State Water Resources Control Board under the Clean Water Act as a 303(d) – Impaired Water Body for metals, pathogens, and sedimentation and siltation (SWRCB 2002 Staff Report). Although the sediment concentrations of some metals are elevated in the Morro Bay estuary, the bioavailability of these metals to aquatic organisms living in the estuary is unknown.
A study of metal contamination of clams and the surrounding sediments collected from the estuary was conducted to evaluate the potential impacts and bioavailability of these metals. An additional purpose of this study was to determine if sediment metal concentrations could be used as a reliable predictor of metal concentrations in clams. From the metal concentrations measured, acceptable consumption levels for five metals (As, Cd, Cr, Pb, and Ni) were calculated from the Food and Drug Administration (USFDA) Guidance Documents for Metals in Shellfish.
The two clam species found in the bay were Macoma secta and Macoma suda. The clams and surrounding sediments were collected in the bay over three days at five different sites and were analyzed for nine different elements (As, Cd, Cr, Cu, Pb, Ni, V, Zn, and Fe) using inductively coupled plasma (ICP) analysis (EPA Method 6010).
Of the metals tested, total chromium and nickel are of greatest concern. Total chromium sediment concentrations (average 62.8 mg/kg) exceeded the NOAA Threshold Effects Level (TEL) of 52.3 mg/kg, and clam concentrations (average 37.0 mg/kg) greatly exceeded the Median International Standards (MIS) of 1.0 mg/kg. No correlation was found between total chromium concentrations in clams and total chromium concentrations in sediments. For some of the sampling sites, the total chromium concentrations were higher in the clams than in the sediments, and at other sites the opposite was true. Nickel sediment (average 79.9 mg/kg) and clam concentrations (average 25.6 mg/kg) exceeded NOAA benchmark values for sediments, but no tissue benchmark value could be found for nickel. Nickel did not bioaccumulate in the clams as evidenced by considerably lower concentrations in the clams compared to the sediments.
Clam tissue concentrations of total arsenic exceeded both the USFWS limit of 0.25 mg/kg and the OEHHA 1.0 mg/kg benchmark. However, measured total arsenic concentrations were very close to the detection limit, therefore arsenic is probably not a concern in Morro Bay. The concentration of zinc in clams was higher than the MIS (70 mg/kg) at the mouth of Chorro Creek site (Site 2). However, sediment and tissue zinc concentrations were below the TEL and MIS values for zinc at all other sites. Higher zinc concentrations were observed in the clam tissues compared to the zinc concentrations in the sediments, indicating zinc might bioaccumulate in these two species of clams.
Tissue metal concentrations for clams collected near the mouth of Chorro Creek (Site 2) were high for cadmium, total chromium, nickel, and zinc. Clam tissue metal concentrations for South Middle Bay (Site 5) appear to be unusually high for total chromium, nickel and vanadium. Sediment and clam metal concentrations were low at all sites for total arsenic, copper, lead, and
v
vanadium. Metal concentrations were lower in clams than in sediments at all sites for total arsenic, cadmium, copper, nickel, lead, and vanadium.
Consumption of clams from Morro Bay at normal consumption rates should be safe with regards to total arsenic, cadmium, lead, and nickel concentrations. However, total chromium measured in the clam tissue may not be safe to eat at the USFDA Levels of Concern if clams from Morro Bay are consumed at normal shellfish consumption levels. Consumption of more than 25 g per week of clams in Morro Bay could place a 60 kg person over the USFDA Level of Concern for chromium consumption.
vi
ACKNOWLEDGMENTS
I would like to express my deepest gratitude to my thesis advisor and professor Dr.
Yarrow Nelson in the Environmental Engineering Department for his expertise and immense
patience with me through the course of this project. His hard work and diligence made this
project possible. A special thanks to Dr. Tom Ruehr in the Soil Science Department for his
insightful discussions and for his different approach to my project. Thank you to my professor
and thesis board advisor, Dr. Sam Vigil, for his work in this project.
A special thanks to the Morro Bay National Estuary Program (NEP) for their generous
mini-grant, which made this project possible.
Thank you to the staff at Department of Fish and Game, Moss Landing, especially Gary
Ichikawa for answering my relentless questions and opening their facilities for my use.
I would also like to express my deepest gratitude to my parents and husband for their
vigilance, guidance, and support. All of you are my rock in the turbulent seas of life and you all
played an important role in my achievements and success in life. Thank you.
vii
TABLE OF CONTENTS
List of Tables ................................................................................................................................ ix
List of Figures ..................................................................................................................................x
List of Abbreviations ..................................................................................................................... xi
Chapters
Chapter 1. Introduction..............................................................................................................1
Chapter 2. Background..............................................................................................................3
Bioaccumulation Factors .........................................................................................4
Competitive Metal Sorption.....................................................................................4
Colloid Influences on Metal Transport ....................................................................5
Colloid Influences on Bioavailability ......................................................................7
Soil Amendments and Chelating Agents .................................................................8
Chapter 3. Procedures..............................................................................................................10
Sample Collection..................................................................................................12
Sample Preparation ................................................................................................12
Sample Homogenization and Digestion.................................................................13
Sample Analysis.....................................................................................................13
Sample Calculation ................................................................................................14
Quality Assurance..................................................................................................14
Chapter 4. Results....................................................................................................................16
Sampling Site Characteristics and Observed Clam Populations............................16
Metal Concentrations in Clams and Sediments .....................................................17
Spatial Variation of Metal Concentrations in Clam Tissue and Sediments...........24
Human Consumption Levels..................................................................................35
Bioaccumulation Concentration Factor .................................................................39
Chapter 5. Conclusion .............................................................................................................40
Chapter 6. Works Cited ...........................................................................................................43
viii
APPENDICES
A1 Sample Collection Method
A2 California Department of Fish and Game (CADFG) Sample Collection
and Preparation Procedure
B1 Sample Preparation and Digestion Methods
B2 California Department of Fish and Game (CADFG) Methods
C Results from Creek Environmental Laboratories
D Detailed Description of Sampling Sites
E National Oceanic Atmospheric Administration (NOAA) Screening
Quick Reference Tables (SQuIRT)
ix
TABLES
Table 1. Sampling Locations by Number ............................................................................10
Table 2. Quality Control Duplication Analysis ...................................................................15
Table 3. Sample Composite Preparation..............................................................................18
Table 4. Clam Tissue Metal Concentrations measured by ICP ...........................................19
Table 5. Sediment Metal Concentrations measured by ICP ................................................20
Table 6. Clam Tissue and Sediment Metal Concentrations Averaged by Site ....................21
Table 7. Summary of Clam Tissue (wet) and Sediment (dry) Metal Concentrations..........23
Table 8. NOAA SQuIRT Marine Sediment Increasing Predicted Toxicity Gradient ........24
Table 9. Recommended Human Consumption Levels.........................................................36
Table 10. Quantity of Clams Safe for Consumption Below USFDA Levels of Concern......37
Table 11. Bioaccumulation Concentration Factors................................................................39
x
FIGURES
Figure 1. Map of Moro Bay Sampling Sites .........................................................................11
Figure 2. Metal Concentrations Averaged by Metal Over All Sites .....................................22
Figure 3. Total Arsenic Concentration in Clam Tissues and Sediment ................................25
Figure 4. Cadmium Concentration in Clam Tissues and Sediment ......................................26
Figure 5. Total Chromium Concentration in Clam Tissues and Sediment ...........................28
Figure 6. Copper Concentration in Clam Tissues and Sediments.........................................29
Figure 7. Iron Concentration in Clam Tissues and Sediments ..............................................30
Figure 8. Lead Concentration in Clam Tissues and Sediments.............................................31
Figure 9. Nickel Concentration in Clam Tissues and Sediments ..........................................32
Figure 10. Vanadium Concentration in Clam Tissues and Sediments ....................................33
Figure 11. Zinc Concentration in Clam Tissues and Sediments .............................................34
xi
LIST OF ABBREVIATIONS
AET Apparent Effects Threshold (NOAA)
DDWQA Decision Document of Water Quality Assessment
CADFG California Department of Fish and Game
ERL Effects Range – Low (NOAA)
ERM Effects Range Median (NOAA)
USFDA Food and Drug Administration
ICP Inductively Coupled Plasma
MIS Median International Standards
MRCA Market Research Corporation of America
MTRL Maximum Tissue Residue Level(s) (State Mussel Watch Program, 2000)
NOAA National Oceanic Atmospheric Administration
OEHHA California Office of Environmental Health Hazard Assessment
PEL Probable Effects Level (NOAA)
RWQCB Regional Water Quality Control Board
SQuIRT Screening Quick Reference Tables
SWRCB State Water Resources Control Board
TEL Threshold Effects Level (NOAA)
USFW U.S. Fish and Wildlife
1
CHAPTER 1 INTRODUCTION
Morro Bay on the Central Coast of California is an impacted estuary, currently listed by
the State Water Resources Control Board (SWRCB) under the Clean Water Act as a 303(d) –
Impaired Water Body for metals, pathogens, and sedimentation and siltation (SWRCB 2002
Staff Report). Recent analyses by the Regional Water Quality Control Board (RWQCB) indicate
concentrations of several toxic metals in Morro Bay sediments are considerably higher than the
National Oceanic and Atmospheric Administration (NOAA) Apparent Effects Threshold (AET)
(Duffield, 2001, personal communication). The sediment concentrations of nickel (Ni), total
chromium (Cr) and aluminum (Al) are up to six times higher than the AET values for these
metals.
Although the sediment concentrations are elevated, the bioavailability of these metals to
aquatic organisms living in the estuary is unknown. Since clams are benthic filter feeders, they
are a meaningful indicator of the bioavailability of toxic metal contamination in the estuary
(Luoma et al., 1983). These animals ingest metal-enriched particles directly (Luoma et al.,
1983), thereby giving an indication of the bioaccumulation ability of metals. Therefore, a study
of metal contamination of clams and the surrounding sediments collected from the estuary was
conducted as an appropriate means of evaluating potential impacts of metal loadings from the
watershed plus residual metal contaminants in sediments. An additional purpose of this study
was to determine if sediment metal concentrations could be used as a reliable predictor of metal
concentrations in clams. Clams and surrounding sediments collected in the bay over three days
at five different sites were analyzed for nine different elements (As, Cd, Cr, Cu, Pb, Ni, V, Zn,
and Fe) using Inductively Coupled Plasma (ICP) analysis (EPA Method 6010). The clam and
2
sediment results were compared to NOAA metal standards. From the metal concentrations
measured in clams, an acceptable consumption levels for five metals (As, Cd, Cr, Pb, and Ni)
were calculated from the Food and Drug Administration (FDA) Guidance Documents for Metals
in Shellfish.
Oysters were originally considered for this study due to their ease in collection.
However, because the only oysters in the bay are located in an oyster farm, their spatial
distribution in Morro Bay is limited. Since a spatial relationship of metals was desired in this
study, it was decided native clams found in the bay would satisfy the necessary components for
this study. The two species found in the bay and used in the analyses were Macoma secta and
Macoma suda.
3
CHAPTER 2 BACKGROUND
Marine sediments of the world are increasingly contaminated with heavy metals and
other contaminants due to a history of industrial discharges and urban runoff. Marine sediments
are an important environmental component when considering the fate and transport of metals
within a watershed. The behavior and distribution of metals in marine sediments is influenced
by hydrodynamics, anthropogenic discharges, and biogeochemical processes (Zwolsman et al.,
1997). Marine bivalves (clams and mussels) have long been employed as pollution biomonitors
in coastal environments. This is due to their intimate contact with the contaminated sediments
and exceedingly high pumping activity and their responses are often proportional to ambient
pollutant concentrations (Wang and Guo., 2000).
Numerous factors need to be considered in marine environments when assessing
bioaccumulation of metals in bivalve animals. Various conditions (organic content, pH, and
presence of sulfide) are considered potential metal sinks. An important sink of metals are
hydrous iron oxides (a large part of oxidized sediments) acting by binding heavy metals to the
surface of sediments (Luoma and Bryan, 1978). The strength of metal binding to the particulates
contributes to the availability of the metal. Strongly bound metals are less available, and weakly
bounded metals are more available (Luoma, 1983). However, bound metals may be bioavailable
depending on the feeding and biogeochemical characteristic of the organism. No universal way
to assess the bioconcentration factors to benthic organisms exists for several reasons. The two
most important factors include differences between organisms and inconsistent chemistry
between metals and metal interactions. Redox cycles, such as those caused by tidal action
4
increased the metal availability to organisms by interfering with the metal species equilibrium
(Simpson et al., 2002).
Bioaccumulation Factors
Several studies have developed mathematical relationships to predict bioaccumulation of
metals in benthic organisms. In one recent experiment, the influences of metal bioavailability of
natural colloids to marine bivalves were studied using artificially contaminated sediments radio-
labeled metals (Wang and Guo, 2000). Their accumulation index was defined as the
radioactivity of metals in whole individual bivalve (dpm) divided by radioactivity of metals in
the water column (dpm/L). This index includes shell uptake by adsorption or absorption and
cannot be used to indicate the absolute uptake of metals to the tissues. Assimilation efficiencies
were calculated by Griscom et al. 2000, but were not clearly defined. Assimilation efficiencies
differed more widely between metals than among geochemical conditions for a single metal.
The maximum differences in metal assimilation efficiencies were approximately two-fold from
high organic content and low organic content. In addition, the direction (positive or negative) of
influence total organic content had on bioavailability assimilation efficiencies was not consistent
among metals or among species tested (Griscom et al., 2000).
Competitive Metal Sorption
Competitive sorption between different metals was observed in several studies. In one
recent study, zinc limited the uptake of cadmium in plants and animals (Brown et al., 2002a,
Brown et al., 2002b). This was due to the excessive amounts of zinc compared to cadmium
found at mine waste sites in ratios greater than 100 to 1 (Brown et al., 2002a). These high levels
of zinc created iron deficiencies in plants, causing yellowing of young leaves. Similar levels of
5
zinc created copper deficiencies in ruminant animals (cow, goat, deer, elk), but the deficiencies
were easily reversed during the span of the study with no apparent injury to the animal (Brown et
al., 2002b). Copper and chromium uptake from wastewaters by biomass consisting of the algae
from C. vulgaris were mutually inhibited by the presence of the other metals (Aksu and Açikel,
1998). Uptake inhibition was a function of pH; removal of copper was maximized at higher pH
(pH = 4) for copper and maximized at lower pH (pH = 2) for chromium (Aksu and Açikel,
1998). Such competitive sorption is likely to occur in marine sediments. The presence of one
metal in excess over the other metals will influence to some degree the uptake of each metal in
benthic organisms. For instance, the presence of iron may compete with lead in binding or
transport sites, thus decreasing the uptake of lead (Luoma and Bryan, 1978). Therefore,
comparing the concentration between all metals will yield an additional component needing to be
included in any bioaccumulation model.
Colloid Influences on Metal Transport
Metals are generally considered to be immobile in most soils due to various metal
binding processes (Luoma, 1983). However, the presence of colloid-bound metals as
resuspended soil particles is related to the soil metal concentrations and is an important factor for
metal transport. The presence of colloids enhances metal transport between 50 and 90 %, mainly
due to colloid-metal binding, and secondly due to cotransportation mechanisms (Karathanasis,
1999). Metal cotransport is dependent on the colloidal composition and the characteristics of the
metal (Karathanasis, 1999). An inverse relationship exists between sorption energy and surface
coverage (Karathanasis, 1999). Given this relationship, sorption affinities can be higher for the
colloids than for the soil matrix. Therefore, increased metal transport by preferentially sorbing
or desorbing metals from colloids is expected. Another study supported this relationship by
6
reporting the colloidal desorption was the primary process responsible for metal concentration
increase (Cantwell et al., 2002).
Several geochemical factors need to be considered when addressing the influence of
colloids on metal transfer. Factors include soil characteristics, colloid characteristics, pH,
organic carbon (OC), redox potential (Eh), tidal action, the presence of organisms in the
sediments, acid volatile sulfides (AVS), and dissolved oxygen (DO) (Karathanasis, 1999; Wang
and Guo, 2000; Simpson et al., 2002; Griscom et al., 2000; and Cantwell et al., 2002). Increased
colloidal surface area and charge, pH, and OC generally increase metal transport (Karathanasis,
1999 and Wang and Guo, 2000). In contrast, large colloid size and Fe- and Al-oxyhydroxides
present in the sediment were generally inhibiting for metal transport (Karathanasis, 1999). Grain
size regulated the amount of sediment resuspended to the water column, with the finer particles
(silt and clay fraction) increasing metal transport (Cantwell et al., 2002). Anaerobic and aerobic
cycles under simulated tidal action increased the flux of zinc to the overlying water due to
increased colloidal concentrations (Simpson et al., 2002). In addition, disturbances from benthic
organisms increased the colloidal concentration, thus increasing the zinc flux (Simpson et al.,
2002). Bioturbation of sediments can remove surface bacterial coatings and expose new surface
sites for adsorption of metals (Griscom et al., 2000). Acid volatile sulfides (AVS) react strongly
with metals to form insoluble metal sulfides, thus removing them from the water column (Lee et
al., 2000). Colloids with a high AVS concentration will increase metal transport due to the
increased mobility of the colloids. Dissolved oxygen and redox potential generally limit metal
transport. Various properties are influenced by dissolved oxygen concentrations, such as AVS
and redox potential. Therefore, DO is an important component to consider when determining
metal transport.
7
Colloid Influences on Bioavailability
The presence of colloids can increase the bioavailability of the metals to organisms
through two possible mechanisms. The first involves direct ingestion of the colloids. Colloidal
particles represent an important food source for deposit and suspension feeding benthic
organisms (Griscom et al., 2000). Uptake of metals from these particles is a function of the
particle metal concentration, feeding rates, and biogeochemical factors (Griscom et al., 2000).
The geochemical composition of the colloid amplifies the bioavailability of silver to deposit-
feeding clams. Manganese oxides increased the accumulation of silver 100 times more rapidly
than amorphous iron oxides (Luoma and Jenne, 1977). In addition to direct metal ingestion of
metal-bound colloids, colloids have the potential to release metals into the dissolved phase.
Uptake of metals from the dissolved phase is the second possible method. Aquatic colloids were
operationally defined as particles in the size fraction between 1 nm and 0.2 µm by Wang and
Guo, (2000) but Cantwell et al. (2002) defined colloids as particles retained on a one-micron
filter (≥ 1 µm). As illustrated by the differing definition of colloids, inaccurate recognition of
colloidal phase could be credited with the observed increase in bioavailability, the dissolved
metal phase or the colloid-bound metal phase.
Benthic organisms bioaccumulated substantial amounts of Cd, Ni, and Zn from sediments
when simultaneous extracted metals (SEM) were only a fraction of AVS, most notably for
cadmium (Lee et al., 2000). Although sulfide binds with metals to form insoluble metal sulfides,
feeding characteristics of the organism can increase the bioaccumulation from direct ingestion of
the colloids. Bioavailability of metals to benthic organisms followed changes in particulate
metal concentrations. However, metal concentration will not always predict bioavailability in
nature, as cautioned by Lee et al (2000). In addition, acid solubility alone was an inadequate
8
predictor of the bioavailability of metals in the gut of bivalves (Griscom et al., 2000; Luoma,
1983). In contrast to the study by Lee et al. (2000), cadmium uptake was not significantly
increased by colloidal concentration (Wang and Guo, 2000), because most Cd was partitioned
into the dissolved phase. In contrast to conventional thinking, anoxic conditions did not
significantly reduce metal uptake. Metals were bioavailable to benthic organisms under anoxic
conditions (Wang and Guo. 2000). The differences may be due to the different feeding methods
of the bivalves studied. Therefore, simple SEM/AVS correlations are not likely to be adequate
for predicting metal uptake by different species of benthic organisms in different environments.
Soil Amendments and Chelating Agents
Soil amendments (in-situ) and chelating agents (ex-situ) influence the availability of
metals for uptake by organisms. Biosolid amendments (municipal wastewater sludge) with
added alkaline byproducts significantly reduced metal availability and toxicity to organisms
(Brown et al., 2002a, Brown et al., 2002b). Metal availability in the soils decreased sufficiently
to be safe for plant and animal habitation (Brown et al., 2002a, Brown et al., 2002b). The
addition of capping materials over marine sediments reduced the zinc flux compared to no
application (control) samples (Simpson et al., 2002). The clean native sediment cap was the
most effective in reducing zinc flux. Capping materials were recommended to control metals
due to metal sulfide formation (Simpson et al., 2002). Four ex-situ extraction agents were
investigated for their ability to remove metals from contaminated sites (Steele and Pichtel, 1998).
Using EDTA (ethylenediaminetetraacetic acid) as an extraction species yielded better lead
removal rates than did ADA (N-2(acetamido)-iminodiacetic acid), PDA (pyridine-2,6-
dicarboxylic acid), and HCl for lead. However, HCl was a more efficient extractant for cadmium
(Simpson et al., 2002). Chelating agents have the added bonus of allowing recovery of the
9
metals for reuse or sale and the chelating reagent can be recovered for reuse in the process. In
addition, bioavailability was independent of the concentration of metals complexed with EDTA
or NTA compared to uptake from free metal ion concentrations. Depending on the extent and
characteristics of contamination in marine sediments, these and other methods may be a feasible
method for remediation of contaminated sediments by decreasing bioavailability of metals.
10
CHAPTER 3 PROCEDURES
The sampling locations were in Morro Bay estuary in Central California. Five sampling
sites were chosen for clam and sediment collection to correspond to the same sites of previous
sediment sampling by Shanta Duffield at the RWQCB (Figure 1). The original sites included
Site 1 - Front Bay, Site 2 - Chorro Creek Mouth, Site 3 - Los Osos Creek Mouth, Site 4 - Back
Bay, and Site 5 - South Middle Bay. During collection, the Back Bay was determined to be
unsafe for collection because of deep mud, resulting in its abandonment as a collection site. A
second site in the middle of the bay (Site 4 - North Middle Bay) was chosen to maintain the
number of sites to five. The site numbers and locations are noted in Table 1 below. The days
chosen for sampling were April 29, May 4, and May 5, 2002, to take advantage of the negative
tides during daylight hours. Kayaks were used for transportation across the main channel and up
tributaries in the bay.
Table 1: Sampling Locations by Number
Site Number Site Location
1 Front Bay
2 Chorro Creek Mouth
3 Los Osos Creek Mouth
4 North Middle Bay
5 South Middle Bay
11
Figure 1: Map of Morro Bay Sampling Sites*
*Map of Morro Bay Sampling Sites: This map shows the clam and sediment sampling locations in Morro Bay. Sites labeled with an “X” are the sites used previously for RWQCB sediment sampling. Circled numbers are the sampling sites used in this study.
12
Sample Collection
The clams analyzed were collected using hand methods (gloved hands, plastic spades,
and digging). Approximately 50 grams of surrounding sediment was collected from the same
depth the clams were collected. The sediment and clams were double-bagged separately in
gallon-sized Ziploc™ bags and labeled with site name and the date of collection. Contamination
sources were minimized as much as possible through the practice of cleaning the collection
equipment with 10% nitric acid, detergent, and deionized water between sampling sites. After
collection, the samples were transported to the laboratory in coolers with ice. After collection
and initial rinsing of the clams, the clam and the sediment samples were frozen until digested and
analyzed. Refer to Appendix A1 for the complete Sample Collection Method as amended from
the California Department of Fish and Game’s (CADFG) procedure Sample Collection and
Preparation Procedure, Appendix A2.
Sample Preparation
Inside the clean room laboratory at Moss Landing, CADFG, individual clams were
separated into three groups by species for each sampling site, as the sample allowed. Due to the
variations in the collection of individuals, the number and weight of each of the composites
differed. The total tissue weight collected in each composite was not critical because an equal
amount of homogenized tissue was used in the digestion step. The objective was to collect
sufficient tissue to obtain a representative sample for each population. The clams were rinsed
with deionized water to remove any remaining sediment outside or inside the shell. All of the
soft tissue was scraped out of the shell with a scalpel and placed into pre-weighed labeled jars.
Each shell length was measured by closing the clamshell around a ruler and recording the widest
length. The equipment was washed between each group with soap, tap water rinse, and a
13
deionized water rinse. The clams were frozen until the homogenization step. The shells from
the individuals used and any remaining individuals were bagged, labeled, and frozen.
Sample Homogenization and Digestion
The frozen clams were homogenized using a hand-held Tissue TearorTM homogenizer
Model 398 (Biospec Products, Inc.) to a smooth consistency with no chunks. Between each
sample, the hand-held homogenizer was washed using a six-step process (Appendix B).
Approximately equal amounts of homogenized clam paste were digested in concentrated nitric
acid in a microwave digester. The digestion program consisted of a 15 minute controlled warm
up to 195 ºC, 20 minutes at this temperature, and with a 20 minute cool down process at the end.
Once the microwave digested samples cooled and returned to surrounding pressure, each sample
was placed into pre-weighed, labeled, and acid-cleaned plastic bottles. Care was taken to entrain
droplets on the walls of the digestion vessels to ensure complete recovery of the sample. Each
bottle was diluted with MiliQ water to a volume of approximately 20 mL. Refer to Appendix B1
for the complete Sample Preparation and Digestion Methods as amended from the CADFG at
Moss Landing Methods, Appendix B2.
Any remaining samples not digested or used were kept frozen and saved for future
reference. The digested clam solutions produced from the week spent in Moss Landing were
stored in the refrigerator until analyzed.
Sample Analysis
A selected number of these clam digestion solutions, along with the corresponding
sediment sample, were sent to Creek Environmental Laboratory (Creek) in San Luis Obispo in
June 2002, for Inductively Coupled Plasma (ICP) analysis using EPA Method 6010. Both the
clam and sediment results were reported on a wet basis from Creek Environmental Laboratory.
14
Blind duplicates of one clam sample and one sediment sample were sent to Creek Environmental
Laboratory for quality control purposes.
Sample Calculation
Two sample calculations are shown below. The first calculation demonstrates how to
calculate metal concentrations in clam tissue using the metal concentration measured. The
second calculation demonstrates the conversion of the sediment metal concentration on a wet
basis to sediment metal concentration to on a dry basis. Sediment results were changed from a
wet basis to a dry basis using reported moisture content to correlate comparisons with threshold
values.
Clam Metal Conc = (metal concentration measured)(volume of digested sample) (composite body mass)
Clam Metal Conc = (0.011 mg Cd/L)(20.254 mL)(1 L/1000 mL) = 0.172 mg Cd (1.293 g body mass)(1 kg/1000 g) kg body mass
Sediment Metal Conc (dry) = (Metal Conc (wet)) / (1 - % Moisture/100)
Sediment Metal Conc (dry) = (47 mg Cr/kg sediment) / (1 – 0.26) = 63.5 mg Cr kg sediment
Quality Assurance
Duplication of analysis was performed for Site 4 (North Middle Bay) for quality
assurance purposes. All of the duplicate analyses matched closely (Table 2), except the duplicate
for vanadium in clam tissues. The values reported for clam tissue vanadium concentrations were
2.07 and 22.3 mg/kg, a factor of ten difference. All other metals were within 1% for clam tissue
concentrations and within 15% for sediment concentrations.
15
Table 2: Quality Control Duplication Analysis
Clam Results Percent Difference
Sediment Result (dry)
Percent DifferenceMetal
mg/L % mg/kg %
Cadmium ND ND
Cadmium (duplicate) ND 0
ND 0
Total chromium 1.1 52
Total chromium (duplicate) 1.1 0
54 4
Nickel 0.85 64
Nickel (duplicate) 0.86 1
75 15
Vanadium 0.13 22
Vanadium (duplicate) 1.4 90
22 0
Iron 48 9,000
Iron (duplicate) 48 0
10,000 11
* ND – no detect
16
CHAPTER 4 RESULTS AND DISCUSSION
Sampling Site Characteristics and Observed Clam Populations
During sampling, a variety of conditions were encountered at the five sampling sites.
Chorro Creek (Site 2) and Los Osos Creek (Site 3) mouths were similar in the type of sediment
observed. Both had gray sandy material with little or no surrounding vegetation. The surface of
the sediment above the riverbed on Los Osos Creek was softer compared to the same area on
Chorro Creek, but both were firm compared to the three other sites. The clams were collected at
a depth between 6 to 8 inches and no other marine life was observed in the vicinity. However,
the number and species of clams found at both creek mouths differed greatly (Table 3). Chorro
Creek had a large concentration of the smaller (approximately 26 mm) clam M. secta, whereas
the clams collected from Los Osos Creek mouth were the larger (approximately 36 mm) clam M.
suda and occurred in smaller clusters.
Site 1 (Front Bay) and Site 4 (North Middle Bay) were similar in the type of sediment
observed and clams collected. Both sites were soft, and appear to be high in organic content, and
had a very strong sulfur odor. Due to the softness of the sediment, walking upright was difficult
to the point where moving around on hands and knees in a wetsuit was required. The sediment
contained two layers within the depth the clams were collected. The top layer was
approximately one inch thick, brown in color, and contained a variety of vegetation. The second
layer appeared to have a higher organic content, was black in color and more viscous than the top
layer. At these two sites, other marine life was seen, including oysters, crabs, shrimp, snails, and
other bivalve species. At the Front Bay site, the large (36mm) clam M. suda was found in small
groups of 3 to 4 individuals at approximately 6 inches deep. Similarly, at the North Middle Bay
17
site, the same species of clam was found in smaller quantities as Site 1 at approximately the same
depth. Broken shells of clam, oyster, and other bivalve organisms were found at Site 4.
Site 5 (South Middle Bay) is similar to the creek mouth sites (Site 2 and Site 3) in the
characteristics of the sediment. The firm brown sandy material was firm enough to walk on
above the level of the channel as it was for the creek mouth sites. In addition, the vegetation was
sparse, but contained small amounts of organic material below the surface. At this site, some
empty and broken clam, oyster, and other bivalve shells were found. This site is characterized
by a meager amount of clams, as demonstrated by the number of individuals collected (Table 3).
A detailed description of the sampling site conditions can be found in Appendix D.
The average body weight per individual for M. suda and M. secta was 2.12 and 0.82
grams with an average shell length at the widest point of 36 and 26 mm, respectively (Table 3).
Metal Concentrations in Clams and Sediments
The metal concentrations determined in clam tissue and sediment using ICP analysis by
Creek Environmental Laboratory is reported in Table 4 and Table 5. The results from Creek are
found in Appendix C. For calculation purposes, no detection values were entered as half of the
not detected levels. These values are marked with an asterisk to distinguish the no detection
values apart from the other results. Clam tissue metal concentration is expressed in mg of metal
per kg of body mass (wet) and the sediment results are expressed in mg of metal per kg of
sediment (dry). Sediment results were changed from a wet basis to a dry basis to correlate
comparisons with threshold values, as indicated on Figures 2-11.
Table 3: Sample Composite Preparation Composite #1 Composite #2 Composite #3 Composite #4 Composite #5 A B C A B C D A B A B C A
Species M.suda M.suda M.secta M.secta M.secta M.secta M.suda M.suda M.suda M.suda M.suda M.suda M.suda Number of individuals 10 10 11 27 27 26 4 6 6 9 8 8 4
Initial wt (g) 41.31 40.66 41.16 41.27 41.05 41.23 41.18 41.63 41.66 41.19 41.24 41.75 41.42 Final wt (g) 58.34 59.57 52.42 63.81 59.77 60.53 52.24 55.47 57.24 61.52 54.87 56.53 49.36
Net wt (g) 17.03 18.91 11.26 22.54 18.72 19.30 11.06 13.84 15.58 20.33 13.63 14.78 7.94 Ave wt/individual (g) 1.70 1.89 1.02 0.83 0.69 0.74 2.77 2.31 2.60 2.26 1.70 1.85 1.99
Ave shell length (mm) 33 32 27 26 25 26 32 40 41 37 34 36 38 Stdev shell length (mm) 9.1 6.7 2.6 2.0 1.6 1.3 19 12 12 10 8.7 4.8 6.8
Shell lengths (mm) 48 37 29 29 26 27 40 51 53 58 47 43 47 39 40 30 27 27 26 55 50 51 30 28 40 37 28 27 27 25 26 23 16 39 45 43 25 32 35 23 34 27 28 26 26 18 39 44 32 42 39 31 36 34 26 21 25 25 41 30 28 39 31 25 41 26 26 26 25 18 22 36 27 39 32 34 27 24 28 27 30 24 31 24 19 27 28 26 25 48 36 33 47 28 23 26 27 26 29 30 29 33 28 26 28 26 25 26 25 28 24 27 25 23 24 25 24 26 22 26 26 26 24 25 26 26 27 25 25 26 26 25 26 23 26 26 28 22 26 24 24 25 26 22 26 26 22 22 28 24 26 27 26 24 28 25
Average weight per individual for M. suda = 2.12 grams M. secta = 0.82 grams Average shell length per individual for M. suda = 36 mm M. secta = 26 mm
18
19
Table 4: Clam Tissue Metal Concentrations measured by ICP
Extract Conc (wet) Body Mass Final Volume
Metal Conc. in
tissue (wet)Species Site Sample Metal
mg/L g mL mg/kg M.secta Front Bay 1 Cadmium 0.011 1.293 20.254 0.172 M.secta Front Bay 1 Total chromium 0.91 1.293 20.254 14.3 M.secta Front Bay 1 Nickel 0.65 1.293 20.254 10.2 M.secta Front Bay 1 Vanadium 0.051 1.293 20.254 0.799 M.secta Front Bay 1 Iron 21 1.293 20.254 329 ִ M.suda Chorro Mouth 2 Total arsenic 0.11 1.265 20.000 1.74 M.suda Chorro Mouth 2 Cadmium 0.031 1.265 20.000 0.490 M.suda Chorro Mouth 2 Total chromium 6.6 1.265 20.000 104 ִ M.suda Chorro Mouth 2 Copper 0.18 1.265 20.000 2.85 M.suda Chorro Mouth 2 Lead 0.022 1.265 20.000 0.348 M.suda Chorro Mouth 2 Nickel 4.3 1.265 20.000 67.9 ִ M.suda Chorro Mouth 2 Vanadium 0.17 1.265 20.000 2.69 M.suda Chorro Mouth 2 Zinc 5.8 1.265 20.000 91.7 M.suda Chorro Mouth 2 Iron 75 1.265 20.000 1,186 ִ M.suda Osos Mouth 3 Total arsenic 0.097 1.341 19.984 1.45 M.suda Osos Mouth 3 Cadmium 0.0025* 1.341 19.984 0.0373M.suda Osos Mouth 3 Total chromium 0.40 1.341 19.984 5.96 M.suda Osos Mouth 3 Copper 0.25 1.341 19.984 3.73 M.suda Osos Mouth 3 Lead 0.010* 1.341 19.984 0.149 M.suda Osos Mouth 3 Nickel 0.43 1.341 19.984 6.41 M.suda Osos Mouth 3 Vanadium 0.11 1.341 19.984 1.64 M.suda Osos Mouth 3 Zinc 2.2 1.341 19.984 32.8 M.suda Osos Mouth 3 Iron 40 1.341 19.984 596 ִ M.suda N Middle Bay 4 Cadmium 0.0025* 1.256 20.011 0.0398M.suda N Middle Bay 4 Total chromium 1.1 1.256 20.011 17.5 M.suda N Middle Bay 4 Nickel 0.85 1.256 20.011 13.5 M.suda N Middle Bay 4 Vanadium 0.13 1.256 20.011 2.07 M.suda N Middle Bay 4 Iron 48 1.256 20.011 765 ִ M.suda S Middle Bay 5 Cadmium 0.0025* 1.246 19.976 0.0401M.suda S Middle Bay 5 Total chromium 3.9 1.246 19.976 62.5 M.suda S Middle Bay 5 Nickel 2.6 1.246 19.976 41.7 M.suda S Middle Bay 5 Vanadium 1.2 1.246 19.976 19.2 M.suda S Middle Bay 5 Iron 56 1.246 19.976 898 ִ M.suda N Middle Bay (Duplicate) 4 Cadmium 0.0025* 1.256 20.011 0.0398M.suda N Middle Bay (Duplicate) 4 Total chromium 1.1 1.256 20.011 17.5 M.suda N Middle Bay (Duplicate) 4 Nickel 0.86 1.256 20.011 13.7 M.suda N Middle Bay (Duplicate) 4 Vanadium 1.4 1.256 20.011 22.3 M.suda N Middle Bay (Duplicate) 4 Iron 48 1.256 20.011 765 ִ
*No detection are reported as half of the detection limit
20
Table 5: Sediment Metal Concentrations measured by ICP
Wet Conc Metal Conc. in Sediment
(dry wt basis) Site Sample Metal Percent Moisture
mg/kg mg/kg Front Bay 1 Cadmium 26 0.15* 0.203 Front Bay 1 Total chromium 26 47 63.5 Front Bay 1 Nickel 26 61 82.4 Front Bay 1 Vanadium 26 19 25.7 Front Bay 1 Iron 26 8,700 11757 ִ Chorro Mouth 2 Total arsenic 26 1.5* 2.03 Chorro Mouth 2 Cadmium 26 0.15* 0.203 Chorro Mouth 2 Total chromium 26 48 64.9 Chorro Mouth 2 Copper 26 6.9 9.32 Chorro Mouth 2 Lead 26 1.6 2.16 Chorro Mouth 2 Nickel 26 63 85.1 Chorro Mouth 2 Vanadium 26 19 25.7 Chorro Mouth 2 Zinc 26 18 24.3 Chorro Mouth 2 Iron 26 8,600 11622 ִ Osos Mouth 3 Total arsenic 23 1.5* 1.95 Osos Mouth 3 Cadmium 23 0.15* 0.195 Osos Mouth 3 Total chromium 23 40 51.9 Osos Mouth 3 Copper 23 4.1 5.32 Osos Mouth 3 Lead 23 0.5* 0.649 Osos Mouth 3 Nickel 23 44 57.1 Osos Mouth 3 Vanadium 23 16 20.8 Osos Mouth 3 Zinc 23 14 18.2 Osos Mouth 3 Iron 23 6,700 8701 ִ N Middle Bay 4 Cadmium 28 0.15* 0.208 N Middle Bay 4 Total chromium 28 52 72.2 N Middle Bay 4 Nickel 28 64 88.9 N Middle Bay 4 Vanadium 28 22 30.6 N Middle Bay 4 Iron 28 9,000 12500 ִ S Middle Bay 5 Cadmium 22 0.15* 0.192 S Middle Bay 5 Total chromium 22 34 43.6 S Middle Bay 5 Nickel 22 41 52.6 S Middle Bay 5 Vanadium 22 16 20.5 S Middle Bay 5 Iron 22 6,300 8077 ִ N Middle Bay (Duplicate) 4 Cadmium 33 0.15* 0.224 N Middle Bay (Duplicate) 4 Total chromium 33 54 80.6 N Middle Bay (Duplicate) 4 Nickel 33 75 112 ִ N Middle Bay (Duplicate) 4 Vanadium 33 22 32.8 N Middle Bay (Duplicate) 4 Iron 33 10,000 14925 ִ *No detection are reported as half of the detection limit
21
Metal concentrations for clam tissues and sediment, averaged over all sites are
summarized in Table 6 and shown in Figure 2. The highest metal concentrations in both
sediment and clams were observed for total chromium, nickel, vanadium, and zinc, excluding
iron. Total arsenic and cadmium concentrations were similar in clam tissue and sediment, all at
low concentrations. In contrast, total chromium, copper, lead, nickel, and vanadium exhibited
over twice the metal concentrations in the sediment compared to the clam tissue. In the reverse,
zinc had three times higher metal concentrations in the clam tissue compared to the sediment.
To achieve a useful representation of the relationships between averaged metal concentrations,
iron was not included in this figure due to the high values measured. The results as presented in
Figure 2 are averaged over all of the sites by metal, however the bar graph does not properly
address spatial variations or whether the measured concentrations are detrimental. The metal
concentrations for clams tissue and sediments are described in more detail in Table 7.
Table 6: Clam Tissue and Sediment Metal Concentrations Averaged by Site
Metal Clam Ave (mg/kg)
Clam Standard Deviation
Sediment Ave (mg/kg)
Sediment Standard Deviation
Total Arsenic 1.59 0.208 1.99 0.0558
Cadmium 0.137 0.181 0.204 0.0113
Total Chromium 37.0 38.5 62.8 13.4
Copper 3.29 0.622 7.32 2.83
Lead 0.248 0.141 1.41 1.07
Nickel 25.6 24.3 79.7 21.9
Vanadium 8.12 9.86 26.0 5.00
Zinc 62.2 41.7 21.3 4.34
Iron 756 ִ 287 ִ 11264 ִ 2531 ִ
62.8
79.7
62.2
21.326.0
1.41
7.32
0.2041.99
37.0
0.2
8.1
25.6
3.290.137
1.59
0
10
20
30
40
50
60
70
80
90
Arsenic Cadmium Chromium Copper Lead Nickel Vanadium Zinc
Met
al C
once
ntra
tion
(mg/
kg)
Sediment AverageClam Average
Figure 2: Metal Concentrations Averaged by Metal Over All Sites *Error bars are calculated using the Standard Error.
22
23
Table 7: Summary of Clam Tissue (wet) and Sediment (dry) Metal Concentrations
Clam Results Sediment Result (dry) Site Sample Metal
mg/kg mg/kg Chorro Mouth 2 Total arsenic 1.74 2.03 Osos Mouth 3 Total arsenic 1.45 1.95 Front Bay 1 Cadmium 0.172 0.203 Chorro Mouth 2 Cadmium 0.490 0.203 Osos Mouth 3 Cadmium 0.0373 0.195 N Middle Bay 4 Cadmium 0.0398 0.208 N Middle Bay (Duplicate) 4 Cadmium 0.0398 0.224 S Middle Bay 5 Cadmium 0.0401 0.192 Front Bay 1 Total chromium 14.3 63.5 Chorro Mouth 2 Total chromium 104 . 64.9 Osos Mouth 3 Total chromium 5.96 51.9 N Middle Bay 4 Total chromium 17.5 72.2 N Middle Bay (Duplicate) 4 Total chromium 17.5 80.6 S Middle Bay 5 Total chromium 62.5 43.6 Chorro Mouth 2 Copper 2.85 9.32 Osos Mouth 3 Copper 3.73 5.32 Chorro Mouth 2 Lead 0.348 2.16 Osos Mouth 3 Lead 0.149 0.649 Front Bay 1 Nickel 10.2 82.4 Chorro Mouth 2 Nickel 68.0 85.1 Osos Mouth 3 Nickel 6.41 57.1 N Middle Bay 4 Nickel 13.5 88.9 N Middle Bay (Duplicate) 4 Nickel 13.7 112 ִ S Middle Bay 5 Nickel 41.7 52.6 ִ Front Bay 1 Vanadium 0.799 25.7 Chorro Mouth 2 Vanadium 2.69 25.7 Osos Mouth 3 Vanadium 1.64 20.8 N Middle Bay 4 Vanadium 2.07 30.6 N Middle Bay (Duplicate) 4 Vanadium 22.3 32.8 S Middle Bay 5 Vanadium 19.2 20.5 Chorro Mouth 2 Zinc 91.7 24.3 Osos Mouth 3 Zinc 32.8 18.2 Front Bay 1 Iron 329 . 11757 ִ Chorro Mouth 2 Iron 1,186 . 11622 ִ Osos Mouth 3 Iron 596 . 8701 ִ N Middle Bay 4 Iron 765 . 12500 ִ N Middle Bay (Duplicate) 4 Iron 765 . 14925 ִ S Middle Bay 5 Iron 898 . 8077 ִ
24
Spatial Variation of Metal Concentrations in Clam Tissue and Sediments
The spatial variability in metal concentrations for both clams and sediments are shown
for each metal in Figures 3 through 11. The values for clam tissue and sediment metal
concentrations are plotted by the sampling location for each metal separately with no-detect
levels expressed as half of the detection limit and are italicized for reference. Results for each
metal are described individually. Observed metal concentrations at each site are compared to
benchmark values. Sediment values were compared to standards reported by NOAA (page 2 in
the Screening Quick Reference Tables (SQuiRT), Appendix E). Clam tissue values were
compared to benchmarks reported by the FDA’s Center for Food Safety and Applied Nutrition
Study (1993) and from pages 11 and 12 of Part H: Decision Document of Water Quality
Assessment for San Diego Creek and Newport Bay (DDWQA).
Table 8: NOAA SQuIRT Marine Sediment Increasing Predicted Toxicity Gradient
Abbreviation Effect Level
TEL Threshold Effects Level
ERL Effects Range – Low
PEL Probable Effects Level
ERM Effects Range Median
AET Apparent Effects Threshold
Total arsenic concentrations in the clam tissue and sediment were similar at the two creek
mouth sites (Figure 3). Total arsenic concentrations were only determined by ICP at these two
sites due to budget constraints. The sediment concentrations were both below the detection
level, and therefore below all of the NOAA benchmark levels. The clam tissue total arsenic
25
concentrations exceeded the U.S. Fish and Wildlife (USFW) benchmark of 0.25 mg/kg (page 11
of Part H), and slightly exceeded the California Office of Environmental Health Hazard
Assessment (OEHHA) value of 1.0 mg/kg (page 12 of Part H). However, since the arsenic
concentrations measured are near the detection limit (0.05 mg/L), this is probably not
meaningful. Sediment total arsenic concentrations were always slightly larger than the tissue
arsenic concentrations, signifying arsenic may not bioaccumulate for the two clam species
studied.
1.95
2.03
1.45
1.74
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
Metal Concentration (mg/kg)
SedimentClam
Sediment Limits(NOAA Values)TEL = 7.24ERL = 8.2PEL = 41.6ERM = 70
Tissue Limits:OEHHA Value1.0 mg/kgUS Fish & Wildlife0.25 mg/kg
Chorro Creek
Osos Creek
USFW OEHHA TEL
Figure 3: Total Arsenic Concentration in Clam Tissues and Sediment
26
Cadmium concentrations were analyzed at all five sites (Figure 4). All of the sediment
values and most of the clam tissue values were below the detection limit. Cadmium was
detected above the detection limit in only two clam tissue samples, at Front Bay (Site 1) and at
Chorro Creek (Site 2). The cadmium concentration in clam tissue at the mouth of Chorro Creek
(Site 2) of 0.490 mg/kg was considerably higher than the cadmium concentrations of the other
samples, but was below the 1.0 mg/kg Maximum Tissue Residue Level (MTRL) from State
Mussel Watch Program, 2000 value for cadmium in seawater (on page 12 of Part H).
0.203
0.195
0.192
0.490
0.203
0.224
0.2080.03980.0398
0.0373
0.172
0.0401
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Metal Concentration (mg/kg)
SedimentClam
Sediment Limits(NOAA Values)TEL = 0.676ERL = 1.2PEL = 4.21ERM = 9.6
Tissue Limits:MTRL1.0 mg/kg
Chorro Creek
Osos Creek
Front Bay
N Middle Bay
S Middle Bay
TEL
Figure 4: Cadmium Concentration in Clam Tissues and Sediment
27
Total chromium distribution in tissue and sediment samples for all five sites is shown in
Figure 5. The sediment chromium concentrations are in the range of the two lowest benchmark
limits. The mouth of Chorro Creek sediment chromium value is above the ERL (81 mg/kg), Site
5 sediment concentration was above the TEL (52.3 mg/kg), and the remaining three sites had
sediment chromium concentrations below the TEL (Figure 5). All of the tissue chromium
concentrations were above the 1.0 mg/kg Median International Standards (MIS), indicating the
observed metal concentrations in the tissues might be a problem with regards to total chromium
loading on the estuary with the highest tissue concentration at the mouth of Chorro Creek (Site
2). No apparent correlation was observed between total chromium concentrations in clams and
total chromium concentrations in sediments. For some sites, sediment concentrations were
higher, while for other sites, the clam tissues had higher total chromium concentrations (Figure
5).
28
63.5
64.9
51.9
80.6
62.5
72.2
43.6
17.5
14.3
104
5.96
17.5
0.0 20.0 40.0 60.0 80.0 100.0 120.0
Metal Concentration (mg/kg)
SedimentClam
Chorro Creek
Osos Creek
Front Bay
N Middle Bay
S Middle BaySediment Limits(NOAA Values)TEL = 52.3ERL = 81PEL = 160.4ERM = 370
Tissue Limits:MIS Value1.0 mg/kg
TEL ERLMIS
Figure 5: Total Chromium Concentration in Clam Tissues and Sediment
29
The copper values measured are low for both tissue and sediment (Figure 6). The largest
sediment value (9.32 mg/kg) was approximately half of the TEL for this metal (18.7 mg/kg).
Likewise, the tissue values were a fifth of the USFW benchmark of 15 mg/kg. Copper values
were determined by ICP only at two sites for budgetary reasons. Sediment copper
concentrations were always larger than the tissue copper concentrations, signifying copper may
not bioaccumulate for the two clam species studied.
9.32
5.32
2.85
3.73
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
Metal Concentration (mg/kg)
SedimentClam
Chorro Creek
Osos Creek
Sediment Limits(NOAA Values)TEL = 18.7ERL = 34PEL = 108.2ERM = 270
Tissue Limits:USFW Value15 mg/kg
USFW
Figure 6: Copper Concentration in Clam Tissues and Sediment
30
Iron concentrations in the clams and sediments were consistently lower than the NOAA
value (Figure 7). The iron concentrations were much higher in the sediments than in the clams,
as expected. A large portion of soil is composed of iron oxides, as clearly shown in Figure 7.
Since iron is not considered toxic, only guidelines can be found from NOAA for sediments and
no tissue limits could be found. Sediment iron concentrations were always larger than the tissue
iron concentrations, signifying iron may not bioaccumulate for the two clam species studied.
11757
11622
8701
12500 14925
8077
329
596
765
898
1,186
765
0 5000 10000 15000 20000
Metal Concentration (mg/kg)
SedimentClam
Chorro Creek
Osos Creek
Front Bay
N Middle Bay
S Middle Bay
Sediment Limits(NOAA Values)AET 220,000 N
Tissue Limits:None found
Figure 7: Iron Concentration in Clam Tissues and Sediment
31
Figure 8 shows the spatial variations for lead at the two creek mouth sites. The lead
concentrations at Los Osos Creek (Site 3) for both sediment and clam tissue were below the
detection limits. The sediment lead concentration was well below all of the NOAA limits. The
tissue value measured at the mouth of Chorro Creek (Site 2) was below the MIS limit of 2.0
mg/kg. It appears lead may not bioaccumulate in the two clam species studied (apparent from
the sediment lead concentrations were always higher than the tissue lead concentrations). Note,
only two sites were sampled for lead with ICP. Based on previous sediment analyses, lead was
not expected to be notable and was not considered critical. Consequently, only two samples
were analyzed.
0.649
2.16
0.149
0.348
0.0 0.5 1.0 1.5 2.0 2.5
Metal Concentration (mg/kg)
SedimentClam
Chorro Creek
Osos Creek
Sediment Limits(NOAA Values)TEL = 30.24ERL = 46.7PEL = 112.18ERM = 218
Tissue Limits:MIS Value2.0 ppm
MIS
Figure 8: Lead Concentration in Clam Tissues and Sediment
32
Overall, the sediment nickel concentrations were higher than in the clam tissue (Figure
9). For the sediment nickel concentrations, all of the values were above the ERM of 51.6 mg/kg.
The highest nickel concentration for clam tissue was 68.0 mg/kg for clams collected at the mouth
of Chorro Creek (Site 2). This is well above the NOAA Effects Range Median (ERM) for
sediments, however no tissue benchmark limit could be found for comparison. Three of the clam
tissue values were below the NOAA Threshold Effects Level (TEL) for nickel in sediments. The
South Middle Bay clam tissue value was close to the NOAA Probable Effects Level (PEL) at a
value of 41.7 mg/kg for sediment. Nickel appears not to bioaccumulate in the two species of
clams studied (as apparent from the sediment concentrations being larger than the tissue
concentrations).
57.1
88.9
52.6
68.0
112
85.1
82.4
13.5
10.2
13.7
6.41
41.7
0.0 20.0 40.0 60.0 80.0 100.0 120.0Metal Concentration (mg/kg)
SedimentClam
Chorro Creek
Osos Creek
Front Bay
N Middle Bay
S Middle Bay
Sediment Limits(NOAA Values)TEL = 15.9ERL = 20.9PEL = 42.8ERM = 51.6
Tissue Limits:None Found
PEL ERMERLTEL
Figure 9: Nickel Concentration in Clam Tissues and Sediment
33
No correlation existed between clam tissue and sediment concentration for vanadium
(Figure 10). The sediment vanadium concentrations were clustered together with values between
19.2 to 32.8 mg/kg. The vanadium clam tissue values were highest at Site 5 (South Middle Bay)
and Site 4 (North Middle Bay). The other three sites had small clam tissue concentrations
compared to the sediment values. Note, one vanadium value was a factor of ten larger than the
other in the duplicate clam tissue concentrations at Site 4 (Table 2). Similar to iron, only
guidelines were given by NOAA, and not benchmark limits because vanadium is a natural
component of soil. All tissue and sediment vanadium concentrations were below the lowest
NOAA benchmark. No tissue guidelines or benchmark limits could be found.
25.7
25.7
20.8
20.5
0.799
2.69
1.64
22.330.6
32.82.07
19.2
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Metal Concentration (mg/kg)
SedimentClam
Chorro Creek
Osos Creek
Front Bay
N Middle Bay
S Middle Bay
Sediment Limits(NOAA Values)AET = 57 N
Tissue Limits:None Found
Figure 10: Vanadium Concentration in Clam Tissues and Sediment
34
Sediment zinc concentrations were well below the sediment limits for zinc by a factor of
six for both sites. The tissue zinc concentration at Chorro Creek mouth (91.7 mg/kg) was above
the MIS tissue value (70 mg/kg), however the zinc tissue concentration was below the MIS limit
at Osos Creek mouth (32.8 mg/kg). Zinc may bioaccumulate in these two species of clams as
indicated by the considerably higher metal concentration in the clam tissues compared to the
metal concentrations in the sediments. Unfortunately, only samples from two sites were
analyzed by ICP, making it difficult to confirm this observation. Only two samples were
analyzed by ICP for zinc since it was not considered to be notable at the onset of the study, and
due to budget constraints.
24.3
18.2 32.8
91.7
0 20 40 60 80 100Metal Concentration (mg/kg)
SedimentClam
Chorro Creek
Osos Creek
Sediment Limits(NOAA Values)TEL = 124ERL = 150PEL = 271ERM = 410
Tissue Limits:MIS Value70 mg;kg
MIS
Figure 11: Zinc Concentration in Clam Tissues and Sediment
35
The mouth of Chorro Creek (Site 2) tissue metal concentrations was high for cadmium,
total chromium, nickel, and zinc. These high metal concentrations could be caused by
discharges into the watershed upstream of the estuary. Runoff from Camp San Luis Obispo, the
state penitentiary, or other anthropologic activities are possible sources of the metals observed
(Duffield, 2000). The SWRCB cites the sources of metal contamination to be surface mining,
nonpoint sources, boat discharge, and vessel wastes.
Human Consumption Levels
To determine safe levels of consumption of these clams for the general public, Table 9
was created to summarize background and maximum consumption levels from the Guidance
Documents for Metals in Shellfish (Guidance Documents) from the Center for Food Safety &
Applied Nutrition at the FDA (USFDA, 1993a, 1993b, 1993c, 1993d). Guidance Documents
have been published for five out of the nine metals considered in this study (As, Cd, Cr, Pb, and
Ni). Consumption guidelines from the World Health Organization (WHO) are included in Table
9 for comparison. For additional information consult WHO (1983) and WHO/FAO (1989).
36
Table 9: Recommended Human Consumption Levels
Background Consumption
Level
WHO/FAO 1989
WHO tolerable daily
intake
USFDA Levels of Concern Metal
µg/person/day* µg/kg/week mg µg/person/day*
Other Entity Recommendations
As 30 15 130 130 WHO 1983 Tolerable Daily Intake = 0.05 mg/kg
Cd 10 7 55 55
Cr 30 -100 ? ? 200 National Academy of Science Cr(III) = 50 - 200 µg/person/d
Pb 5 - 10 ? ? 6, 15, 25, 75**
Ni 120 no set limit no set limit 1200 EPA Oral Dose of 20 µg/kg/d and NOAEL of 5 mg/kg/d
* for a 60 kg person ** depending on population 6 µg/person/d ages 0 - 6 15 µg/person/d children over 7 25 µg/person/d pregnant women 75 µg/person/d adults
The frequency of shellfish consumption was characterized by the Market Research
Corporation of America (MRCA) 14-Day Survey (5 Year Menu Census, 1982-87) (MCRA,
1988). The MRCA reported 4.8% of the surveyed population consumed molluscan bivalves.
Calculated within the Guidance Documents, the average intake of molluscan bivalves is
estimated to be 12 grams/day for adults (male and female) between 18 and 44 years of age. This
equals approximately 84 grams/week for an average adult who eats shellfish. For the average
clam size in this study, this was approximately 40 M. suda clams and 102 M. secta clams per
week.
Table 10 summaries the FDA Levels of Concern, average tissue concentration, and the
maximum number of clams eaten per week to exceed the Levels of Concerns for each metal.
Example calculations used to find the Level of Concern and Threshold Consumption Levels are
listed in Table 10. Level of Concern was calculated using the USFDA Level of Concern value
37
and subtracting the USFAD Background Level to obtain a metal concentration value accounting
for the metal exposure not from food sources. This value was then used to calculate a Threshold
Consumption Level appropriate for the metal concentrations measured in the clams from Morro
Bay.
Level of Concern = [(130 µg As/person/day) – (30 µg As/person/d)](7 d/wk) = 700 µg As person/wk
Threshold = (700 µgAs/person/wk)(kg clam/1.59 mg As)(mg/1000 µg)(1000g/kg) = 440 g clam person/wk
Table 10: Quantity of Clams Safe for Consumption Below USFDA Levels of Concern
Metal USFDA Levels of Concern
USFDA Background
Level
Average Tissue
Concentration
Threshold Consumption
Level
µg/person/day* µg/person/day* mg/kg g clam/week
As 130 30 1.59 440
Cd 55 10 0.137 2,300
Cr 200 65 37.0 25.5
Pb 75 7.5 0.248 1,900
Ni 1200 120 25.6 295
* for a 60 kg person
Total Arsenic was treated as all inorganic arsenic in the Guidance Document due to the
high toxicity of inorganic arsenic compared to organic arsenic. This was considered to be a
conservative assumption with regards to the toxicity of arsenic. The total arsenic measured in
the clams from Morro Bay was 1.59 mg As/kg clam body weight. Using the USFDA Level of
Concern for arsenic (subtracting out the USFDA Background arsenic level) the threshold amount
38
for a 60 kg person to eat was calculated to be 440 grams of clams per week. Since this quantity
greatly exceeded the average consumption of 84 grams of clams per week a person consumes
each week, consumption of clams from Morro Bay should be safe with regards to arsenic
concentration.
The cadmium measured in the clams from Morro Bay was 0.137 mg Cd/kg clam body
weight. Calculating the USFDA Level of Concern for cadmium, the threshold amount for a 60
kg person to eat was calculated to be 2,300 grams of clams per week. This quantity of clams
exceeds by more than two orders of magnitude the average personal consumption of 84 grams
per week. Therefore, consuming clams from Morro Bay at the average consumption rate should
be safe with regards to cadmium concentrations.
With a measured total chromium value of 37.0 mg Cr/kg clam body weight, the
calculated weekly threshold amount a 60 kg person could eat is approximately 25.5 grams of
clams. This quantity of clams is below the 84 grams/week an average person consumes each
week. Therefore, caution should be exerted when considering consuming clams from Morro
Bay; it may not be safe to eat with regards to chromium concentration.
The lead measured in the clams from Morro Bay was 0.248 mg Pb/kg clam body weight.
Using the USFDA Level of Concern for lead and subtracting out the USFDA Background lead
level, the threshold amount a 60 kg person could eat was calculated to be 1,900 grams of clams
per week. Since this quantity far exceeds by two orders of magnitude the amount the average
person consumes each week, consumption of clams from Morro Bay should be safe with regards
to lead concentration.
The last metal listed in Table 10 (nickel) had an average concentration of 25.6 mg Ni/kg
clam body weight. The calculated weekly threshold value for a 60 kg person is 295 grams of
39
clams. Since this quantity of clams exceeds the amount the average person consumes each week
(84 grams of clams per week), consuming clams from Morro Bay may be safe with regards to
nickel concentration.
Bioaccumulation Concentration Factor
Bioaccumulation concentration factors (BCFs) shown in Table 11 were calculated from
dividing tissue metal concentrations by the corresponding sediment metal concentrations. BCFs
for all metals (except for zinc) were less than 1, indicating these metals may not bioaccumulate
under the conditions found in Morro Bay for the clams species studied. Zinc has a BCF of
almost three (BCF = 2.93), indicating zinc bioaccumlates three fold for the species studied from
Morro Bay.
Table 11: Bioaccumulation Concentration Factors
Tissue Metal Concentration
Sediment Metal Concentration Metal
mg/kg mg/kg
Bioaccumulation Concentration Factor
Total Arsenic 1.59 1.99 0.80
Cadmium 0.137 0.204 0.67
Total Chromium 37.0 62.8 0.59
Copper 3.29 7.32 0.45
Lead 0.248 1.41 0.18
Nickel 25.6 79.7 0.32
Vanadium 8.12 26.0 0.31
Zinc 62.2 21.3 2.93
Iron 756 ִ 11264 ִ 0.07
40
CHAPTER 5 CONCLUSION
Of all of the metals tested, total chromium, nickel, and zinc are of greatest concern
according to clam and sediment benchmarks established by NOAA, OEHHA, USWFS, FDA,
WHO, and others.. With chromium, sediments exceeded NOAA benchmark value and clam
concentrations greatly exceeded the MIS value. Additionally, nickel sediment and clam
concentrations exceeded NOAA benchmarks value, although no tissue benchmark value could be
found for nickel. The clam concentration of zinc was higher than the MIS at one site (the sample
collected at the mouth of Chorro Creek). For arsenic, clam tissue concentrations exceed both the
USFW and OEHHA benchmark values. However, all measured arsenic concentrations were
near the detection limit, and are therefore, probably not a concern.
Three-fold higher zinc concentrations were observed in the clam tissues compared to the
zinc concentrations in the sediments, indicating zinc might bioaccumulate in these two species of
clams. However, sediment and tissue zinc concentrations were below the TEL and MIS values
for zinc. Simulated tidal action increased the zinc flux to the overlying water column due to
increased colloidal concentration (Simpson et al., 2002). In addition, benthic organisms
increased the zinc flux (Simpson et al., 2002). Therefore, zinc likely bioaccumlates in the
conditions found in Morro Bay. The clam concentrations of cadmium, copper, lead, and
vanadium were below relevant benchmark values.
Risk associated with the consumption of these two clam species with regards to the five
metals (As, Cd, Cr, Pb, and Ni) based on published values in the Guidance Documents for metals
in Shellfish by the FDA is minimal at average molluscan bivalve consumption levels, except for
total chromium. An average person (60 kg) consuming clams from the Bay would consume a
41
quantity of total chromium possibly posing a health threat. In addition to the average population,
total chromium levels in the clams represent a health risk to high-risk populations (fishermen and
their families) who consume larger quantities of shellfish. Chromium (III) has been documented
as an essential trace element for humans. The National Academy of Sciences recommends a
daily intake level of Cr (III) of 50-200 µg/day. This upper value is the Level of Concern used for
the calculations. However, levels of total chromium measured in clams from Morro Bay exceed
this health level. Therefore, the average person should exercise caution when consuming clams
from Morro Bay with regards to the person’s total chromium consumption.
Both chromium and nickel are contained in serpentine geological formations common in
the Morro Bay watershed. Erosion of these formations (natural and unnatural) is likely to
contribute to high nickel and chromium concentrations.
Uptake of metals by benthic organisms is influenced by many factors (metal chemistry,
sediment chemistry, type of organism, colloid concentrations, chelating agents, and others), and
it is nearly impossible to predict bioaccumulation using simple models with a single or few
indicators. The results in this study support this. No correlation was discovered between
sediment concentration and clam tissue concentration in Morro Bay.
All of the metals studied did not appear to bioaccumulate, except with zinc. Sediment
metals concentrations, such as chromium, nickel, and vanadium, were higher than the tissue
concentrations. In contrast, zinc tissue metal concentrations were almost three times higher
compared to the sediment metal concentration.
Spatial relationships existed within this study (the mouth of Chorro Creek had higher
values). With added data on metal concentrations for both the clam tissue and for the sediments,
42
more detailed descriptions could be made concerning the relationships between each metal, the
site location, and the benthic organism.
With more data, a bioaccumulation factor measuring the weight of metal(s) per kg of
clam body mass can be attempted for the species studied from Morro Bay. Additional studies
recommended are to compare metal concentrations in the shell of the organism to the soft tissues
and sediments. A complete study would entail the collection and measurement of metals in the
water column, sediments, soft and hard tissues of the benthic organisms studied.
43
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