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Historical
Trends of Metals in the Sediments of San Francisco Bay, California:
Core
data from San Pablo Bay, Grizzly Bay, Richardson Bay, and
Central Bay
by Michelle I. Hornberger,
Samuel N. Luoma, Alexander van Geen, Christopher Fuller, and
Roberto Anima, USGS
based on article published in
Marine
Chemistry, 1999. V. 64, pp 39-55.
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Methods:
Field and Sample Preparation and Analytical Procedures
All gravity cores were collected from the
R/V David Johnston using a corer with a 363 kg weight sound.
The cores were 9 cm in diameter and ranged from 0.5 - 2.5
meters in length. The core barrel was steel, with a polybutyrate
liner. In addition to the 1990-91 sampling, a gravity core
and box core were obtained from the mouth of Richardson Bay
in August, 1992 (RB92-3) (Fuller et al., 1998). Comparison
of isotope profiles (Fuller et al., 1998) and organic contaminant
distributions (Venkatesan et al., 1998) between the box core
and the surface sediments of the gravity core verified that
no significant loss of surface materials or distortion of
surface profiles occurred during gravity coring (Crusius and
Anderson, 1991). The core from Tomales Bay was collected in
1993 using a diver-operated piston corer (Sansone et al.,
1994).
After collection the cores were X-rayed,
split into a working half and an archive half, and stored
in a cold room (2-3°C) until sampling. Sand/silt ratio
was determined on all samples. Sediment samples were wet-sieved
using an acid-cleaned nylon-mesh screen into a tared 100 ml
beaker to <64 µm in ultra-clean deionized water and
dried at 70°C.
The <64 µm sediments were analyzed
for metals in all cores (these are the data reported here,
unless otherwise noted); bulk analyses were conducted on selected
samples. Sieving effectively reduces the most important grain
size biases that can affect comparisons (Salomons and Forstner,
1984; Luoma, 1990). Each sediment sample was homogenized using
a mortar and pestle, split into 0.5 g replicate aliquots,
and placed into a scintillation vial. For the weak-acid digest,
two replicate 0.5 g sediment aliquots were digested at room
temperature for two hours in 0.6 N HCl. The sample was filtered
with a 0.45_m filter and analyzed by Inductively Coupled Argon
Plasma Emission Spectroscopy (ICAPES). For near-total metal
analyses, replicate sub-samples from each horizon and procedural
blanks were digested using the concentrated nitric acid reflux
method described by Luoma and Bryan (1981). Sediment aliquots
of approximately 0.5 g were placed into 22 ml scintillation
vials. Ten ml of concentrated trace metal grade nitric acid
was added to each, a reflux bulb was placed on the vial, and
the sample was left at room temperature overnight. Samples
were then refluxed at 150°C for approximately one week,
until clear. Reflux bulbs were removed and the samples were
evaporated to dryness. The residue was reconstituted in 0.6
N trace metal grade hydrochloric acid, then filtered through
0.45 _m filters. Decomposition with concentrated nitric acid
reflux is comparable with procedures previously employed on
Bay sediments (San Francisco Bay Estuary Inst., 1994). It
is indicative of metals sufficiently mobile to be of potential
toxicological interest, but it has the disadvantage of not
providing a complete dissolution of the sediment.
Total decomposition was conducted on a full
suite of samples from RB92-3 and SP90-8, and on selected samples
from CB90-12, in order to compare trends to those observed
by near total decomposition. One ml of concentrated HClO4
and 2 ml of concentrated HF were added to sub-samples of 0.2
g, with selected replicates, in a Teflon vial. The samples
were placed on an aluminum heat block preset at 110°C,
and taken to dryness. One ml of HClO4 was added
and then ultra-clean deionized water added to bring the Teflon
vial to half full. Samples were returned to the hot plate
for evaporation, cooled, and reconstituted to 10 ml in 0.6
N HCl. The vials were capped and heated at 90°C for 1
hour.
Samples for Hg analyses were reacted at
100°C in aqua regia followed by 10% nitric/dichromate
reconstitution; 3% NaBH4 (in 1% NaOH) was added
as a reductant before analysis by cold vapor AAAS.
Concentrations of Al, Cr, Cu, Fe, Mn, Ni,
V, and Zn in the sediment were analyzed by ICAPES, after careful
correction for peak interferences in the sediment digest matrix.
Concentrations of Ag were analyzed by Graphite Furnace Atomic
Absorption Spectroscopy (GFAAS) using Zeeman background correction
with calibration by the method of standard additions. Lead
concentrations were analyzed by flame AAS. The ICAPES was
profiled and standardized according to normal operating procedures,
then a quality control (QC) standard was run every 10-15 samples
to ensure consistent performance of the instrument. Procedural
blanks were analyzed as an unknown, but no blank subtraction
was necessary. The instrument limit of detection (LOD) and
limit of quantitation (LOQ) were determined by 10 or more
analyses of a standard blank (0.6 N HCl) throughout each analytical
run (Keith et al., 1983). All data reported here fall above
the LOQ. If readings from replicate values of a solution were
of low precision (relative standard deviation >10%), the
readings were not used.
Recoveries from standard reference materials
(SRM Sediment Standard 1646 and 2709) are reported. Because
Pb analyses by ICAPES had an uncorrectable bias from Al, Pb
(HNO3 digest) was analyzed by AAS. Recoveries of
Pb from SRM 2709 were low in HNO3. As a second
test of recoveries, Pb in selected horizons of core sediments
were analyzed by both AAS (HNO3 digest) and isotope
dilution by mass spectrometry of totally decomposed sediments.
These two methods compared within 5% in both uncontaminated
and contaminated horizons, suggesting a high fraction of Pb
recovery in San Francisco Bay sediment. |
