Introduction
The
Bay Protection and Toxic Cleanup Program (BPTCP) was established
by the California State Legislature in 1989 with four major goals:
- Provide
protection of present and future beneficial uses of the bay and
estuarine waters of California.
- Identify
and characterize toxic hot spots.
-
Plan for toxic hot spot cleanup or other remedial actions.
-
Develop prevention and control strategies for toxic pollutants
that will prevent creation of new toxic hot spots or the perpetuation
of existing ones within the State's bays and estuaries.
These
goals are being addressed by each of California's coastal Regional
Water Quality Control Boards. The San Francisco Bay Regional Board's
(Regional Board) activities under the BPTCP have included completion
of the Pilot Regional Monitoring Program as a precursor to the current
Regional Monitoring Program (RMP), continued participation in the
RMP, completion of a fish tissue study that identified contaminant
concentrations sufficient to trigger a health advisory on consumption
of Bay fish, and completion of baywide sediment assessments to identify
toxic hot spots. The sediment quality assessments have been described
in two recently released reports: Evaluation and Use of Sediment
Reference Sites and Toxicity Tests in San Francisco Bay, and Sediment
Quality and Biological Effects in San Francisco Bay (Hunt et al.,
1998a, 1998b). Both are available from the Bays and Estuaries Unit,
Division of Water Quality, State Water Resources Control Board.
Together they describe a phased approach using reference site comparisons
and a suite of biological and chemical measurements to screen numerous
sites in the region and provide information that can be used by
the Regional Board to identify locations requiring cleanup, source
control, or other remedial action.
The
objectives, methods, and findings of these studies are summarized
here. Major parts of the reference site study were described in
the RMP 1995 Annual Report (SFEI, 1996), so the present summary
will focus on the results of reference envelope statistical analyses
that used reference site data to calculate tolerance limits for
comparison with test site results. In addition to toxicity tolerance
limits, tolerance limits for concentrations of sediment-associated
chemicals were also evaluated and reported to the Regional Board,
and the results of that analysis are also briefly summarized below.
Reference
Site Study
Study
Objectives
To
date, the primary focus of the BPTCP has been the identification
of toxic hot spots, which can be defined as localized areas where
elevated concentrations of toxic pollutants are found in association
with adverse biological impacts. Implicit in the definition of a
toxic hot spot is the assumption that pollution in a localized area
is worse than in surrounding areas, either in the same water body
or in the region where the hot spot exists. The goal of the San
Francisco Bay sediment reference site study was to adequately characterize
ambient conditions in the Bay to provide a standard against which
to compare measurements from sites being investigated as possible
hot spots. However, since program goals are to manage the State's
bays and estuaries to promote environmental quality, it is not sufficient
to simply characterize the "average" condition of a water
body, but instead the goal of the study was to characterize the
"optimal ambient conditions" currently existing. Therefore,
the study focused on the identification and evaluation of sediment
reference sites, the least polluted fine-grained sediment sites
that could be found in San Francisco Bay with reasonable sampling
effort. Reference site evaluations were based on criteria established
by reviewing relevant scientific literature and consulting with
the BPTCP Scientific Planning and Review Committee.
To
meet this goal and to support continuing BPTCP investigations, the
study focused on four objectives:
- Identify
and evaluate sediment reference sites in San Francisco Bay.
- Evaluate
appropriate sediment toxicity test methods for use in San Francisco
Bay.
- Evaluate
a statistical method (the "reference envelope approach")
that uses toxicity test data from reference sites to establish
relative standards against which to compare results from test
sites.
- Investigate
the use of toxicity identification evaluations (TIEs) in determining
the causes of toxicity at sites with both high and low concentrations
of measured pollutants.
The
results of investigations to address objectives 1, 2, and 4 were
discussed in the RMP 1995 Annual Report (SFEI, 1996). But the statistical
method used to calculate reference envelope tolerance limits underwent
significant re-evaluation to address issues regarding the effects
of combined spatial and temporal variation, and tolerance limit
results were not available at that time. Therefore, the evaluation
of the reference envelope approach is summarized below.
Reference
Sites and Reference Envelope Approach
The
study evaluated data from five specified reference sites in San
Francisco Bay, plus data from three RMP sites that satisfied the
reference site criteria (see Figure
4.20). The sites were Island #1 and Tubbs Island (in San Pablo
Bay), Paradise Cove (in Central San Francisco Bay), and a northern
and southern site in the South Bay. Three stations (field replicates)
were established at each of these sites. The RMP sites used in the
reference envelope calculations were Pinole Point (in San Pablo
Bay), Horseshoe Bay (in Central San Francisco Bay), and San Bruno
Shoal (in the South Bay). Surveys were conducted during three separate
seasons, late summer 1994 and late winter/early spring 1994 and
1995. The RMP sites were sampled in winter and summer from 1993
to 1997. A total of 61 reference site samples were used to establish
a population of reference site toxicity values (the "reference
envelope") that could be used to determine tolerance limits
against which to compare the results of test sites in future sediment
toxicity surveys. This statistical method is described briefly below.
The
"reference envelope" approach was developed to provide
an appropriate statistical method for determining whether conditions
at test sites were significantly worse than those in the surrounding
area. This objective is different from that of determining absolute
sample toxicity. Rather than comparing results of test samples with
laboratory controls using laboratory replicate variance as the statistical
test variance component, the reference envelope method establishes
tolerance limits based on test results from reference site samples.
Tolerance limits are calculated to identify samples significantly
more toxic than a chosen proportion of the reference site distribution,
and statistical significance is determined using variation among
reference site results. In this way, the method considers all relevant
sources of variation that could affect comparisons between sites,
such as variation in time and space, the interaction of time and
space components, and variation between replicates (the error term).
If natural factors such as grain size vary among reference sites
or between surveys, then the effects of these factors are accounted
for in the analysis. Any additional toxicity is assumed (statistically)
to be caused by anthropogenic constituents of the test sample.
Results
of the Reference Envelope Evaluation of Toxicity Data
As
described in the report, the calculation of tolerance limits was
affected by a number of factors, including data distribution, occurrence
of outliers, method of calculation, and reference envelope "p"
values. The "p" value is the proportion of the reference
site distribution selected for the tolerance limit. For example,
a "p" value of 10 would set the tolerance limit such that
any sample with a test result below the limit would be as toxic
or more toxic than the worst 10% of samples expected in the water
body characterized by the reference sites.
Tolerance
limits were highest when calculated from data with high mean values
and low variability among reference sites. The sea urchin embryo/larval
development test in porewater had the highest tolerance limits.
For example, the tolerance limit for sea urchin larval development
in pore water at a "p" value of 10 was 94.3% (Table
4.3). Porewater samples exhibiting lower rates of larval development
would be considered in the worst 10% of the reference distribution,
or lower. Such high tolerance limits are indicative of consistently
high reference site values, but do not necessarily indicate that
the level of response was biologically significant. In such cases,
we would recommend deferring to a "detectable difference"
criterion based on test minimum significant difference (MSD) values
(such as described by Thursby et al., 1997). On the other hand,
data sets with relatively low values and high variability often
produced tolerance limits that were very low or negative. Toxicity
test standards below zero clearly have no utility, and these data
cannot be used in this approach. Solid-phase sediment tests using
the amphipods Eohaustorius and Ampelisca had tolerance limits ranging
from 55% to 78% of control values (for "p" values of 1
to 20; Table 4.3).
As
mentioned above, this study also evaluated three methods for calculating
tolerance limits. Two of the methods were appropriate for studies
in which all data are collected at the same time. These two methods
used conventional formulae and statistical tables. The third method
was appropriate for the BPTCP program, which analyzed samples collected
from multiple sites at multiple times. This method required extensive
development for the study, and relied on bootstrap simulations in
the calculation of tolerance limits.
Appropriate
application of the reference envelope approach and the resulting
tolerance limits will depend on professional judgment in determining
the quality of the reference database, selection of "p"
values, and suitability to the goals of the investigation. This
method can effectively distinguish impacted sites from optimal ambient
conditions if those conditions are well characterized and the assumptions
of the method are met. Reference site databases with less than about
six values probably cannot produce acceptable tolerance limits,
and tolerance limits based on less than twenty reference site values
should be applied with caution. In some cases, entire water bodies
may be polluted to the extent that optimal ambient conditions are
not a sufficient standard for comparison, and other methods would
need to be applied to measure and improve environmental quality.
Results
of this study indicate that the reference sites evaluated were not
pristine, but had relatively low concentrations of pollutants, and
probably approximated optimal ambient conditions for fine-grained
sediments in San Francisco Bay. Many of the toxicity test protocols
produced distributions of reference site data that could be used
to calculate reasonable toxicity tolerance limits. Successful application
of this information for monitoring activities will require continued
sampling of reference sites coincident with monitoring surveys,
and thoughtful selection of reference envelope "p" values,
based on careful consideration of data quality and study objectives.
Results
of the Reference Envelope Evaluation of Chemistry Data
Tolerance
limits were calculated for a number of chemicals, based on the distribution
of sediment chemical concentrations measured at reference sites
in San Francisco Bay (Smith, 1997, report to the Regional Board).
The chemical tolerance limits were calculated to provide 95% certainty
that measured concentrations exceeding the tolerance limit would
be as high or higher than expected of the highest 15% of samples
from reference sites. This reflects the "p" value of 0.85
selected by the Regional Board staff when they derived threshold
values for ambient concentrations of these chemicals in their assessments
of test sites (Gandesbery and Hetzel, 1998). Concentrations above
the tolerance limits could therefore be assumed to be elevated relative
to ambient conditions in the Bay. No assumptions were made about
the relationship between the tolerance limit concentrations and
their potential for biological effects; the chemical tolerance limits
were simply descriptive of chemical concentrations found at reference
sites.
These
chemical tolerance limits were not used in the identification of
toxic hot spots, but they were listed in the San Francisco Bay BPTCP
report. Two points regarding the chemical tolerance limits are worth
noting here. First, for the majority of chemicals for which San
Francisco Bay reference tolerance limits were derived, the tolerance
limits were much lower than concentrations at a similar percentile
of the BPTCP statewide database, and were also much lower than concentrations
usually associated with biological effects, as indicated by ERM
(Effects-Range Median) values. Second, the nickel concentration
at the 85th percentile of the San Francisco Bay reference
site distribution (the tolerance limit) was higher than the 90th
percentile for all BPTCP samples statewide, many of which were collected
to characterize potentially polluted sites. The elevated nickel
concentrations throughout the Bay are probably the result of local
geologic abundance and human-enhanced transport of this element,
though localized nickel concentrations may also be due to municipal,
industrial, or urban non-point sources.
Studies
to Identify Toxic Hot Spots
The
focus of BPTCP sediment monitoring in San Francisco Bay has been
to conduct sediment quality assessments in several phases: 1) previous
information on water and sediment quality was evaluated by reviewing
approximately 100 relevant reports; 2) a large number of Bay and
wetland sites were surveyed in the Pilot Regional Monitoring Program
(PRMP), which also included a methods validation study along a pollution
gradient; 3) the reference site study evaluated appropriate sediment
reference sites and toxicity tests; 4) approximately 127 stations
from throughout the region (selected on the basis of previous information
and PRMP results) were screened for sediment toxicity and/or chemistry;
and 5) a number of sites that exhibited toxicity and/or elevated
chemistry were resampled for additional biological and chemical
analyses to confirm previous results. This confirmation survey incorporated
three components commonly known as the sediment quality triad: toxicity
testing, chemical measurement, and benthic community analysis. Additional
samples were collected at selected confirmation sites to estimate
the bioavailability of sediment-associated chemicals.
Study
Design
During
the screening phase of the study, 127 stations that had been identified
in previous investigations were screened for sediment toxicity.
Since funding constraints precluded comprehensive assessments at
each screening site, toxicity testing was used as the primary screening
tool. Toxicity tests were used because they are direct, precise
indicators of the integrated effects of sediment contaminants, and
they provide information about biological impacts of pollutants,
information difficult to discern solely from chemical measurements.
Generally, two toxicity tests were used at each screening site:
a solid-phase sediment test with benthic amphipods, and a sediment
porewater test using developing embryos of sea urchins. As methodological
improvements were incorporated during the study, some screening
samples were tested with sea urchins exposed to the sediment-water
interface (SWI), rather than porewater.
After
reviewing the screening data and information from previous studies,
twelve sites were resampled during the confirmation phase of the
study. These sites were analyzed with the sediment quality triad,
including two toxicity tests, sediment chemistry, and benthic community
analysis. Ten samples from these sites were also analyzed for bioaccumulation,
using 28-day laboratory exposures with the clam Macoma nasuta. A
total of 46 samples were screened for a broad suite of trace metal
and organic compounds, and a total of 143 samples were analyzed
for mercury and PCBs, chemicals that were identified as elevated
in fish tissues in the Bay (SFBRWQCB et al., 1995) and were the
subject of a fish consumption health advisory. An additional 15
sites were resampled and tested with sea urchin larvae in sediment-water
interface exposures, because their screening samples exhibited toxicity
only in sea urchin porewater tests that were accompanied by elevated
sulfide or ammonia concentrations.
In
order to provide additional information about potential toxic hot
spots, linear transects (gradients) were sampled at some confirmation
sites to evaluate relationships between sediment chemistry and biological
effects. Phase I sediment toxicity identification evaluations (TIEs)
were conducted at two sites, and an abbreviated sediment-water interface
TIE was conducted at a third site to investigate possible causes
of sediment toxicity.
Results
of Sediment Assessments
Through
the screening and confirmation process, this study identified several
highly polluted locations that exhibited adverse biological effects.
The study also indicated that 21% of all samples tested were toxic
to amphipods, 31% of porewater samples were toxic to sea urchin
embryos, and 33% were toxic to sea urchin embryos exposed at the
sediment-water interface. Statistical analyses indicated a number
of chemicals that were both correlated with biological effects and
found at concentrations exceeding sediment quality guideline values.
A number
of sites had numerous chemicals with concentrations above sediment
quality guideline values and significant biological effects. These
sites were categorized based on the magnitudes of chemical concentrations
and effects. The sites exhibiting highest chemical concentrations
and greatest biological effects included: Stege Marsh, Mission Creek,
Islais Creek, Point Potrero (notable for extremely high PCB and
mercury concentrations), Pacific Drydock, Castro Cove, Peyton Slough,
and San Leandro Bay.
Principal
components analyses (PCA) indicated that sediment quality guideline
quotient means (ERMQs) and the number of chemicals exceeding guideline
values both covaried negatively with biological indicators (increasing
concentration of chemical mixtures associated with decreasing biological
function). Individual chemicals or chemical classes identified by
PCA that also exceeded guideline values and were significantly correlated
with adverse biological effects included: total chlordanes and 2-methylnaphthalene
(with amphipod toxicity); cadmium, copper, silver, and zinc (with
sea urchin porewater toxicity); and cadmium, copper, and zinc (with
sea urchin SWI toxicity).
Sediment
quality guidelines (such as ERMs) have been derived empirically
from a large number of studies nationwide to indicate chemical concentrations
often associated with adverse biological effects. The use of guideline
values allows simple comparisons of sample concentrations to those
observed in numerous other studies. This comparison is useful for
perspective, but does not necessarily indicate that chemicals with
concentrations above guideline values are responsible for any observed
impacts. Only site-specific investigations, using TIEs and other
toxicological methods, can determine causal relationships. In the
present study, numerous chemicals were found at concentrations exceeding
guideline (ERM) values. Of these, chlordanes, PCBs, DDTs, PAHs,
dieldrin, copper, mercury, lead, and zinc were commonly found above
ERMs. Hexachlorobenzene and chlorpyrifos, for which ERM values have
not yet been derived, were often found at concentrations above the
90th percentile of the statewide BPTCP sediment chemistry
database. Combined concentrations of chemical mixtures were high
at many sites, with 9 sites having mean ERM quotients above the
95th percentile of the statewide BPTCP database.
In
tests of ten samples from the Bay, exposed clams accumulated elevated
tissue concentrations of nine chemicals or chemical classes: copper,
lead, total chlordanes, total DDTs, dieldrin, total PCBs, LMW PAHs,
HMW PAHs, and total PAHs. The identification of these chemicals
was dependent on the particular samples tested, the analyte list,
the physiology of the clam Macoma nasuta, and the 28-day exposure
period of the laboratory tests.
The
data provided in the report represent a significant body of information
to assist in management efforts to identify and remediate toxic
hot spots in San Francisco Bay. A number of sites were identified
as having elevated pollutant concentrations and severe biological
impacts. Determination of spatial extent and development of information
relevant to pollutant source control will require additional investigation
at many sites. A number of other sites demonstrated elevated chemical
concentrations without severe acute toxicity, and still other sites
had toxic sediment without having elevated concentrations of measured
chemicals. These sites may warrant further studies of chronic effects
and/or investigations to determine the likely causes of observed
biological impacts.
References
Gandesbery,
T. and F. Hetzel. 1998. Ambient concentrations of toxic chemicals
in San Francisco Bay sediments. Staff report, Regional Water Quality
Control Board, San Francisco Bay Region, Oakland, CA.
Hunt,
J.W., B.S. Anderson, B.M. Phillips, J. Newman, R.S. Tjeerdema, M.
Stephenson, H.M. Puckett, R. Fairey, R.W. Smith, and K. Taberski.
1998a. Evaluation and use of sediment reference sites and toxicity
tests in San Francisco Bay. Final report. State Water Resources
Control Board, Sacramento, CA.
Hunt,
J.W., B.S. Anderson, B.M. Phillips, J. Newman, R.S. Tjeerdema, K.
Taberski, C.J. Wilson, M. Stephenson, H.M. Puckett, R. Fairey, and
J. Oakden. 1998b. Sediment quality and biological effects in San
Francisco Bay. Final report. State Water Resources Control Board,
Sacramento, CA.
SFBRWQCB,
SWRCB, and CDFG. 1995. Contaminant levels in fish tissue from San
Francisco Bay. Final report. San Francisco Bay Regional Water Quality
Control Board, Oakland, CA. 155p.
SFEI.
1996. 1995 Annual Report: San Francisco Estuary Regional Monitoring
Program for Trace Substances. San Francisco Estuary Institute, Oakland,
CA.
Smith,
R.W. 1997. Sediment criteria project ambient analysis report. Final
report. San Francisco Bay Regional Water Quality Control Board,
Oakland, CA.
Thursby,
G.B., J. Heltshe, and K.J. Scott. 1997. Revised approach to toxicity
test acceptability criteria using a statistical performance assessment.
Environ. Toxicol. Chem. 16:13221329.
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