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Pore-water silver concentration gradients and benthic fluxes from contaminated sediments of San Francisco Bay, California, U.S.A.
ELSEVIER
Marine Chemistry
56 ( 1997) 15-26
Pore-water silver concentration gradients and benthic fluxes from contaminated sediments of San Francisco Bay, California, U.S.A. Ignacio Rivera-Duarte Earth Science Department,
*, A. Russell Flegal
WIGS, Uniuersity of California, Santa Cruz, CA 95064, USA
Received 7 July 1995: accepted 8 October 1996
Abstract Estimates of the benthic fluxes of silver within San Francisco Bay substantiate the proposal that diagenetic remobilization is an important source of silver in surficial waters of the estuary (G.J. Smith and Flegal, 1993). Dissolved (< 0.45 p,m) silver concentrations in pore waters from relatively pristine and contaminated sediments from San Francisco Bay, as well as from sediments from Tomales Bay and Drakes Ester0 ranged from 4 to 4600 pM, with one exception (8900 PM). With the
exclusion of four exceptionally high concentrations (> 620 PM), the average (X f 1 sd) dissolved pore-water concentration of silver from these three embayments was 120 f 140 pM. The apparent molar partition coefficient of silver (log DAg.)in pore waters of benthic sediments (I 2 cm deep) within San Francisco Bay ranged from 3.7 to 5.8 (5.1 f 5.21, which was consistent with the previously reported partitioning of silver within the bay surface waters (G.J. Smith and Flegal, 1993) (log L&s. 5.3 k 0.3). Estimated diffusive benthic fluxes of silver were greatest at sites affected by waste water outfalls with relatively elevated concentrations of silver in the effluent. Upper limit estimates of the integrated net benthic flux of silver to South Bay (32-290 mol yr-‘> indicate that it could be 2.5-fold greater than the estimated fluvial input of dissolved silver (_ Ill mol yr-‘) to the estuary (G.J. Smith and Flegal, 1993). Keywords:
silver; sediment; pore water; sediment-water
interface;
1. 1. Introduction San Francisco Bay (Fig. 1) is an urban estuary (Nichols et al., 1986) with anomalously high concentrations of silver in its water (G.J. Smith and Flegal, 1993), sediments (Luoma and Phillips, 1988; Luoma et al., 1995) and benthic organisms (D.R. Smith et al., 1986; Luoma and Phillips, 1988; Luoma et al.,
* Corresponding
author.
03044203/97/$17.00 Copyright PI1 SO304-4203(96)00086-2
partition coefficients;
geochemical
cycle; San Francisco
Bay
1995). Total dissolved ( < 0.45 km) silver concentrations in the bay are as high as 250 pM (G.J. Smith and Flegal, 1993), which is approximately two orders of magnitude above baseline concentrations of silver in northeast Pacific coastal waters (l-2 pM; Saiiudo-Wilhelmy and Flegal, 1992). Particulate concentrations of silver in sediments from highly contaminated areas within the bay averaged 9 pmol kg-’ (Luoma and Phillips, 1988), which is an order of magnitude greater than both the average crustal abundance of silver (N 0.7 pmol kg-‘; Taylor and McLennan, 1985) and baseline concentrations (0.9
0 1997 Elsevier Science B.V. All rights reserved
16
I. Rivera-Duarte,
Fig. 1. Sediment pore-water sampling Bay and adjacent embayments.
locations
A. Russell Flegal/Marine
in San Francisco
pmol kg-’ ) in San Francisco Bay and Tomales Bay sediments (Luoma et al., 1995). Silver concentrations in tissues of some bend-tic organisms in the bay are equal to or greater than those of benthic organisms that have been measured in any other estuary in the U.S.A. or Europe (Luoma and Phillips, 1988; Luoma et al., 1995). For example, silver concentrations in the mussel, Mytilus californianus, range from 6.5 to 27 pmol kg-’ in the bay (D.R. Smith et al., 1986; Luoma and Phillips, 19881, which is one to two orders of magnitude greater than silver concentrations (N 0.37 pmol kg-‘) in mussels in the northeast Pacific (Goldberg et al., 1983). The anomalously high concentrations of silver in the San Francisco Bay estuary occur in its southern reach, or South Bay (Fig. 1). It is a physically distinct embayment, with a lagoon-type circulation dominated by tidal oscillations (L.H. Smith, 1987). This contrasts with the circulation pattern of the northern reach of the San Francisco Bay estuarine system, which is a partially mixed estuary dominated
Chemistry 56 (1997) 15-26
by riverine inflows from the Sacramento and San Joaquin rivers that discharge to the northeast Pacific through the Golden Gate (Conomos et al., 1985). The South Bay also receives approximately half (13,000-33,000 mol yr- ’ ) of the total amount of industrial and municipal silver (25,000-66,000 mol yr-‘) discharged to the entire estuary (Davis et al., 1991). While the high levels of dissolved silver in the South Bay appear to be primarily due to point source discharges of industrial silver and to the long residence times of water within the South Bay, it has been proposed that the remobilization of silver from benthic sediments may also contribute substantially to the seasonally high concentrations in South Bay waters (G.J. Smith and Flegal, 1993). That proposal was based on the temporal covariance of elevated silver concentrations in South Bay waters with those of other metals (Cu, Ni, and Zn) and nutrients (e.g., H,SiO,) that were associated with diagenetic remobilization processes @legal et al., 1991). The proposal has been supported by measurements of high silver concentrations in San Diego Bay waters (I 307 p M). Those high levels have been tentatively attributed to the remobilization of silver from benthic sediments, because there are no contemporary point source discharges of industrial silver to that semi-enclosed marine embayment @legal and Satiudo-Wilhelmy, 1993). That attribution has been further supported by recent measurements of dissolved lead concentration gradients and diffusive fluxes from pore waters of San Francisco Bay sediments (Rivera-Duarte and Flegal, 1994), which parallel the dissolved silver concentration gradients in pore waters of San Francisco Bay sediments that are presented in this paper. Consequently, the objective of this research is to quantify the benthic flux of silver in San Francisco Bay. Dissolved silver concentrations were measured in sediment pore waters, and then used to estimate the diffusive and irrigation-based benthic fluxes of silver to overlying waters in the estuary, as well as in two adjacent embayments. These preliminary estimates are compared with other previously reported inputs of silver to the San Francisco Bay from fluvial and anthropogenic sources. Those initial comparisons attest to the relative importance of benthic fluxes on the silver cycle in that estuary.
I. Rioera-Duarte, A. Russell Flegal /Marine Chemistry 56 (19971 15-26
2. Methods Cores (lo-cm width, 40-cm length) were collected directly from the sediments in locations within San Francisco Bay, Tomales Bay and Drakes Ester0 from May to August 1992 and May 1993 (Fig. 1; Table 1). These included four sites along the periphery of San Pablo Bay (Castro Cove in 1992; Inner Castro Cove, Outer Castro Cove and Pinole Point in 19931, three sites along the periphery of the South Bay (Oakland Harbor, Mayfield Slough and Coyote Creek in 19921, and two sites in adjacent embayments (Tomales Bay and Drakes Ester0 in 1992). Sediments at some of the locations within San Francisco Bay were contaminated by the products of the oil refinery industry (i.e. Castro Cove, Inner Castro Cove and Outer Castro Cove), municipal wastewater discharges (i.e. Mayfield Slough and Coyote Creek), or maritime activities (i.e. Oakland Harbor). Sediments at other locations (i.e. Pinole Point) were considered to be more representative of the less contaminated shallow areas in the northern reach of the San Francisco Bay estuary. Total dissolved ( < 0.45 p,m) pore-water samples were extracted from the sediments with a whole-core squeezer (Jahnke, 1988). The squeezer was modified
Table 1 General characteristics
in bay
San Pablo
out of
out of
Bay
Bay
Bay
no
no
no
subtidal
subtidal
intertidal
intertidal
yes
yes
yes 0.1-0.3 4 > 26 -
yes 0.1-0.6 7
Castro Cove
South
South
San Pablo
San Pablo
Bay
South/ Central
San Pablo
Bay
Bay
Bay
Bay
Bay maritime
oil
oil
oil
subtidal
intertidal
intertidal
yes
yes
99
97
90
78
0 8
22
22
waste water
Location w.r.t. tides Macro biota %TOC range
intertidal no
intertidal no
% Clay&silt
90
95
yes 1.0-3.0 87
0 6
0 22
26
(cm)
Drakes Ester0
Oakland Harbor
waste water
1.3-3.4
1.l-2.4
Inner Castro Cove
Bay
Mayfield Slough
Contaminated
Oxic/suboxic Suboxic/
for trace-element collections by using acid-cleaned PVC cores, an aluminum frame and an hydraulic jack to pressurize the sediment core from the bottom (Rivera-Duarte and Flegal, 1994). The extracts were evacuated with all-polypropylene syringes and acidcleaned Teflon filters (0.45 pm>. A sample from the nepheloid layer at the sediment-water interface at the top of the whole-core squeezer was collected immediately, and then samples from 10 depths (i.e. 1, 2, 4, 6, 8, 10, 14, 18, 22 and 26 cm) in the core were collected. At each sampling depth, the first 5 ml of pore water were used to condition both the syringe and filter and to determine ancillary parameters (i.e. NH:, gelbstoff, HPOi-, H,SiO,, Cl- and SO,‘->, as reported elsewhere (Rivera-Duarte and Flegal, 1994, 1997). The final l- 10 ml of pore water were aliquoted into acid-cleaned polyethylene bottles, acidified to pH < 2 with sub-boiling quartz distilled (2 X ) 6 N HCl, and stored for one month prior to elemental analyses. The extractions, as well as the preceding sample preparations and subsequent elemental analyses, were conducted in HEPA filtered air (Class 100) tracemetal clean working areas. Silver in the filtered solutions was preconcentrated with microtechniques (Rivera-Duarte and Flegal, 1996), which were based
of the sampled sites Coyote Creek
Location
17
1.3-2.3
Outer Castro Cove
0.8-1.4
1.9-3.2
anoxic (cm) A more detailed description of these characteristics w.r.t. = with respect to. % Clay&silt = average for whole sediment core.
is given in Rivera-Duarte
Pinole Point
and Flegal (1997).
0.1-1.1
Tomales
> 22 _
18
I. Riuera-Duarte,
A. Russell Flegal/Marine
on the ammonium l-pyrrolidine dithiocarbamate/diethylammonium diethyldithiocarbamate organic extraction procedures de(APDC~DDC) tailed by Bruland et al. (1985). The efficiency of the silver extraction was lOO%, based on quantitative recovery analyses of internal reference materials and spiked samples, and N 106% with respect to the silver concentration (47.9 + 2 PM) of CASS2 (G.J. Smith and Flegal, 1993). The coefficient of variation (CV) of replicate silver extractions was +6% with a 500 pM sample solution and &24% with CASS2. Total dissolved silver concentrations were measured by graphite furnace atomic absorption spectrometry (GFAAS) with stabilized platform techniques and the method of standard additions. The CV of replicate measurements was 5 10%. Procedural blanks ranged from 9 pg (0.08 pmol) to 31 pg (0.29 pm00 in N 50 ml of high-purity (1X Ma cm-‘> water. The limit of detection, defined as three times the standard deviation of the procedural blanks, was 0.37 pM. The sediment cores were frozen immediately after the initial pore-water extractions for subsequent particulate concentration measurements at corresponding depths. The sediments were analyzed for porosity (assuming a density of 2.65 g cme3 for the sediment). Near-total silver concentrations were measured by GFAAS following aqua regia digestions @legal et al., 1981). Sediment concentrations are reported as near-total, rather than total, because HF was not used. Still, the recoveries for silver in most sediments are nearly quantitative ( 2 90%) with aqua regia digestions @legal et al., 1981). Procedural silver blanks ranged from 10 to 760 pg, which was 5 1% of the particulate silver in the sediment samples. Silver concentrations (X f lsd) derived from concurrent analyses of standard reference sediments (BCSS-1, MESS-l, PACS-1 and MESS-2) from the National Research Council Canada were: 970 f 52 nmol kg-’ (n = 9) for BCSS-1; 1,100 f 120 nmol 15,000 f 3,300 nmol kg-’ (n=6) f or MESS-l; kg-’ (n = 5) for PACS-1; and 1,600 f 11 nmol kgg ’ (n = 3) for MESS-2. The measured silver concentrations are consistent with the certified silver concentration (1,669 + 185 nmol kg- ’ > for MESS-2 and with the suggested silver concentration (927 + 371 nmol kg- ’ ) for BCSS-I by the Marine Analytical Chemistry Standards Program (Ottawa, Canada).
Chemistry 56 (1997) 15-26
3. Results and discussion
3.1. Redox gradients The subtidal, surface sediments collected in San Francisco Bay are suboxic. This is illustrated by the pore-water concentration profiles of dissolved iron, manganese and nutrients in the cores, which indicate suboxic conditions in the surficial sediments of the seven locations in San Francisco Bay. These suboxic conditions extended to 6 cm in Coyote Creek, to 22 cm in Mayfield Slough, Outer Castro Cove and Pinole Point, to 26 cm in Oakland Harbor, to 8 cm in Castro Cove, and to 4 cm deep at Inner Castro Cove (Table 1). Other evidence indicates these suboxic conditions are the result of both biological and mechanical flushing of the sediments (Rivera-Duarte and Flegal, 1997). In contrast, the profiles of those redox parameters in cores from Tomales Bay and Drakes Ester0 exhibited oxic conditions to a depth of at least 26 cm (Table 1). The contrasting profiles of redox species between sediments in Drakes Ester0 and Tomales Bay with those in the San Francisco Bay are primarily attributed to differences in grain size and organic content (Table 1). Sediments in the cores from San Francisco Bay are predominantly silts and clays (< 62.5 pm) in grain size, which comprises 90% of the average mass fraction at Coyote Creek, 96% at Mayfield Slough, 87% at Oakland Harbor, 99% at Castro Cove, 97% at Inner Castro Cove, 90% at Outer Castro Cove, and 78% at Pinole Point. Silts and clays comprise only 7% of the average mass fraction at Drakes Ester0 and only 4% of that fraction at Tomales Bay. The associated permeability of the sediments from the latter two embayments is evidenced by the presence of oxic conditions down to 26-cm depth in the cores from both Tomales Bay and Drakes Ester0 (Rivera-Duarte and Flegal, 1997). Sediments within San Francisco Bay also have higher percent of total organic carbon (0.68-3.43%) than the sediments of Drakes Ester0 (0.06-0.61%) and Tomales Bay (0.12-0.32%) (Rivera-Duarte and Flegal, 1997). These pronounced variations in the composition and redox condition of sediments from the different locations limit the following comparisons between measured silver concentrations and esti-
I. Ricera-Duarte, A. Russell Flegal /Marine
mated fluxes in pore waters of San Francisco and adjacent embayments. 3.2. Dissohed
and particulate
Bay
silver concentrations
The pore-water concentrations of silver in sediments of San Francisco Bay and adjacent embayments were comparable to most previously reported concentrations of silver in other estuarine pore waters. These include pore-water concentrations of silver in Long Island Sound, which were reported to range from < 2000 to 9000 pM (Lyons and Fitzgerald, 1983). Conversely, the pore-water concentrations in San Francisco Bay sediments were an order of magnitude greater than the highest concentrations of dissolved silver in its surface waters, which range from I 6 to 250 pM (G.J. Smith and Flegal, 1993). The total dissolved ( < 0.45 pm) silver concentrations of pore waters from the three northeast Pacific embayments (San Francisco Bay, Tomales Bay and Drakes Estero) were < 4600 pM, with one exception (Table 2; Fig. 2). Excluding that anomalously high measurement and three other exceptionally high concentrations (1100, 4500 and 4600 PM) from a highly contaminated site in the extreme South Bay (Mayfield Slough), the total dissolved silver concentrations in pore waters of the three embayments ranged from 4 to 380 pM, with an average concentration of 120 f 140 pM (n = 55). While those four high concentrations may be due to sampling or analytical contamination, the highest of them (8900 pM in Tomales Bay) is similar to the highest previously reported pore-water concentration of silver (9000 PM), which was measured in sediments considered representative of natural conditions within Long Island Sound (Lyons and Fitzgerald, 1983). Consequently, the anomalously high silver concentrations in pore waters of Tomales Bay and Drakes Ester0 are tentatively attributed to their prevailing oxic condition and the relative smaller surface areas of their sediment grains. Those two factors would allow relatively high concentrations of silver to remain in solution as chloride complexes (Jenne et al., 1978; Lyons and Fitzgerald, 1983; Cowan et al., 1985; Luoma et al., 1995). Thermodynamic calculations indicate that silver is released at both the water-sediment interface and in zones with extreme anoxic conditions. This is shown
Chemistry 56 (1997) 15-26
19
by the pore-water concentration gradients of dissolved silver and the redox characteristics of the different sites (Rivera-Duarte and Flegal, 1997). Comparisons between sites indicate that silver is
Table 2 Total dissolved ( < 0.45 p,rn) silver concentrations (PM) in pore water, near-total silver concentrations (p,mol kg- ’ dry weight) in sediment, and apparent molar partition coefficients (DAg +) of silver in the San Francisco Bay estuary (Coyote Creek, Mayfield Slough, Oakland Harbor, Castro Cove, Inner Castro Cove, Outer Castro Cove and Pinole Point), Drakes Ester0 and Tomales Bay Depth
Ag
(cm)
(PM)
Ag (km01 kg- ’ dry weight)
D
-
log D,ig*
As
San Francisco Bay South Bay : Coyote Creek: N” 1 2 4 6 8 10 14 18 22
64 71 110 240 +zb n.d. ’ * 190 7 n.d.
5.8 1.4 1.2 1.3 1.2 1.3 0.72 0.41 0.84
82X 10’ 12x 10” 4.9x
10’
4.9 4.1 3.7
3.9 X 10’ 66X lo3
3.6 4.8
4.9x 11 x 103 4.3 X 13x lo3 0.2 x 0.1 x 3.3 x
3.7 4.0 3.6 4.1 2.3 2.1 3.5
Mayfield Slough: N 1 2 4 6 8 10 14 18
48 [llool 160 250 [45Z [46001 270
[181 5.4 1.7 1.1 1.1 0.9 1 0.56 0.88
10’ IO’ 10’ 10” 10’
Central Bay Oakland Harbor: N 1 2 4 6 8 10 14 18
27 12 n.d. n.d. n.d. n.d. n.d. 11 n.d.
7.1 7.4 7.4 7.5 7.9 7. 7 5.5 5.1
570x
101
5.8
500x
lo?
5.1
I. Riuera-Duarte, A. Russell Flegal/ Marine Chemistry 56 (1997) 15-26
20 Table 2 (continued) Depth
Ag
(cm)
(PM)
Table 2 (continued) Ag (km01 kgdry weight)
’
DA&? *
“??I DAM*
San Pablo Bay:
I 2 4 6 8 10 14 18 22 26
380 32 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 66 24
3.1 3.2 2.8 2.2 1.8 1.6 1.9 3.5 2.1 2.6
96 X 10’
5.0
32x 10’ 110x 10’
4.5 5.0
Inner Castro Cove: N 1 2 4 6 8 10 14 18
45 n.d. 43 88 n.d. 6 37 180 47
3.9 3.5 4.4 4.3 4.2 4.3 4.1
81 x lo3 51 x 10’
4.9 4.7
660X 10’ 120x 10’ 23x10’
5.8 5.1 4.4
Outer Castro Cove: N 1 2 4 6 8 10 14 18 22
40 n.d. 29 18 n.d. n.d. n.d. n.d. 33 n.d.
91 20 66 63 n.d. 10 130 15 9
(PM)
2.8 2.3 2.0 3.2 2.6 3.4 2.6 1.5 3.0
2.1 3.0 2.9 2.1 2.1 2.4 1.7 1.3
’
DAg*
78X lo3 110x 10’
47x 10’
4.9 5.1
[89001 620 53 300 440 190 310 250 130 4
480
Drakes
Ester-0 4 43 n.d. 140 5 28 320 310 12
0.14 0.13 0.15 0.13 0.10 0.12 0.10 0.11 0.10 0.10
0.08 0.36 0.44 0.26 0.17 0.18 0.11 0.11 0.09 0.14
[0.02x 0.2 x 2.7 x 0.4x 0.2 x 0.6 x 0.3 x 0.4x 0.8 x 26x 10’
PM). ’ Concentrations
10’1 10’ 10’ 10’ 10’ lo7 10’ 10’ 10’
t1.21 2.3 3.4 2.6 2.4 2.8 2.5 2.6 2.9 4.4
1.8X 10’
3.3
3.2 x 10’ 52X IO3 6.3 x 10’ 0.6 x 10”
3.5 4.7 3.8 2.1
0.3 x 10’ 8.2 x 10’
2.5 3.9
below the estimated
detection
limit ( < 0.37
limit of detection (L.O.D. =
3 x lsd,,,,,). Values in brackets arc considered suspect, and are not included in statistical averages.
as discussed
in text,
4.1
130x 103 45 x 10’ 46X lo3
5.1 4.7 4.1
210x 19x 110x 140x
5.3 4.3 5.0 5.1
103 10’ 10’ 103
log DAM*
Bay
N 1 2 4 6 8 10 14 18 22 26
N 1 2 4 6 8 10 14 18 22 26
Ag (Fmol kgdry weight)
a N indicates nepheloid layer. b Concentrations below the instrumental
Pinole Point: N 1 2 4 6 8 10 14 18
Ag
km) Tomales
Castro Cove: N
Depth
released at the sediment-water interface, which is presumably coupled with the diagenesis of labile organic matter and the formation of soluble silver chloride complexes (Jenne et al., 1978; Lyons and Fitzgerald, 1983; Cowan et al., 1985; Luoma et al., 1995). Silver is also released to pore water in extreme anoxic conditions (Inner Castro Cove), which is assumed to be the result of the formation of soluble silver sulfide complexes (Jenne et al., 1978; Lyons and Fitzgerald, 1983; Cowan et al., 1985). There are decreases in dissolved silver concentra-
I. Riuera-Duarte, A. Russell Flegal/Marine
Ag (PM) Coyote Creak
Ag (PM) Mayfield Slough
Ag (PM) Oakland Harbor
Castro Cove
Inner Castro C.
Outer Castro C.
~
/+f=
G&=
Pinole Point 200
0
0 FS 2 10 f
400
Tomales Bay
Drakes Ester0
0
0
600
1200
200
400
:
Chemistv 56 (1997) 15-26
21
was 18 pmol kg-’ in surface sediments (1 cm) at Mayfield Slough, which is in the southernmost portion of the South Bay where silver concentrations in sediments (i N 9 p.mol kg-’ ) are anomalously high (Luoma and Phillips, 1988). The particulate silver concentrations of cores from the locations in San Francisco Bay (Coyote Creek, Mayfield Slough, Oakland Harbor, Castro Cove, Inner Castro Cove, Outer Castro Cove and Pinole Point) were also markedly higher (up to two orders of magnitude) than those of cores from Tomales Bay and Drakes Estero, which ranged from 0.08 to 0.44 pmol kg-’ dry weight (0.16 & 0.09 kmol kg-’ dry weight, II = 20) (Table 2; Fig. 3). Bioturbation and mechanical mixing appeared to substantially influence the distribution of particulate silver in most of the sites within San Francisco Bay. The only sites where macrofauna was visually absent were Coyote Creek and Mayfield Slough (which are
. Ag (pmol kg-’ dry weight) Coyote Creek
Fig. 2. Pore-water concentration (PM) profiles of total dissolved ( < 0.45 km) silver (0) in San Francisco Bay and adjacent embayments. Values denoted by n are below the estimated limit of detection (L.O.D. = 3 X lsd b,anks). Note the different silver concentration ranges, from 0 to 1200 pM for Mayfield Slough and Tomales Bay, and from 0 to 400 pM for the other stations. The position of the oxic-suboxic front ( * - * * ) and the suboxicanoxic front (-- - -_) are from Table 1, as defined by RiveraDuarte and Flegal (1997).
0
depth in most of the cores (Fig. 2), which parallel decreases in dissolved manganese concentration under suboxic conditions (Rivera-Duarte and Flegal, 1997). The latter covariance is consistent with reports of the partitioning of silver in the manganese fraction of freshwater sediments (Borovec, 1993). Particulate silver concentrations measured in the sediment cores from San Francisco Bay (Table 2; Fig. 3) were consistent with previously reported measurements of particulate silver in the estuary (Luoma and Phillips, 1988). Silver concentrations in cores from the bay ranged from 0.47 to 7.9 pmol kg-’ dry weight (3.0 &-2.0 pmol kg-’ dry weight, n = S9), with one exception. The silver concentration
10
Mayfield Slough
Oakland Harbor
F fr 0
5
10
0
5
10
16
b b
b
l
:
.
l
Castro Cove 0
tions
5
5
10
0
Inner Castro C. 0
5
10
Outer Castro C. 0
10
l
:
with
5
l .
f l .
.
. b
iPinole Point 0
5
10
~
.
Tomales Bay
Drakes Ester0
0
0
5
10
5
10
i Fig. 3. Sediment concentration profiles of silver (pm01 kg-’ dry weight) in San Francisco Bay stations and adjacent embayments (Tomales Bay and Drakes Estero).
I. Riuera-Duarte, A. Russell Flegal/Marine
22
both located in highly contaminated areas), which were the only sites that had surficial peak concentrations of particulate silver (Fig. 3). These surface maxima contrasted with the other sites (Oakland Harbor, Castro Cove, Inner Castro Cove, Outer Castro Cove, and Pinole Point), where there were macrofauna and particulate silver concentrations were homogeneously distributed with depth in their sediments (Fig. 3). In contrast, the oxic surface sediments at Tomales Bay and Drakes Ester0 [which were also inhabited by macroorganisms, relatively free of industrial silver inputs, and primarily ( > 93%) sands] had relatively low and homogeneously distributed silver concentrations. 3.3. Partition ments
coeflcients
of siluer in wjkial
Chemistry 56 (1997) 15-26
log Dng* Sutflcialwatara
25 -f 30 i
0
a. San Francisco Bay
sedi-
The apparent molar partition of silver ( DAg*> between sediment (Cr,“. , mol kg-’ dry weight) and pore water (Cl:*, mol L- ‘) was calculated with the following equation from Beattie et al. (1993): D&‘=-
LAg
*
OI)W
(‘1
It is termed an “apparent” partition coefficient, because it is derived from a near-total extraction of silver from the sediments, rather than from a total extraction with HF. Consequently, the ratio is conservatively low. The log of the partition coefficient (log DAg L> varied between 2.1 and 5.8 (Table 2; Fig. 4) with an average of 4.9 f 5.2 (n = 48). That range and average did not include the one outlier of 1.2, which was derived from the anomalously high pore-water concentration. Since the silver concentration of the associated sandy sediments was relatively low (0.14 pmol kg-’ ) and the dissolved silver concentration of pore waters at that depth was anomalously high (8900 PM), the partition coefficient derived from those extreme values was not considered to be appropriate. The differences in partition coefficients between sediments from San Francisco Bay and adjacent embayments (Fig. 4) may be primarily due to differences in the specific surface areas of the sediments. As previously noted, the sediments from the San Francisco Bay cores were primarily silts and clays,
3Oi
b. Adjacent embayments
Fig. 4. Log partition coefficients (log DAs *) vs. depth in (a) San Francisco Bay locations: Coyote Creek (01, Mayfield Slough (m), Oakland Harbor (O), Castro Cove (01, Inner Castro Cove (v ), Outer Castro Cove (A) and Pinole Point ( q 1; and (b) Drakes Ester0 (0) and Tomales Bay ( n ). The range of log DAg. for suspended sediments in surficial waters of San Francisco Bay (4.3-6.1) was reported by G.J. Smith and Flegal (1993).
while those from Drakes Ester0 and Tomales Bay were primarily sands with relatively lower specific surface areas. The associated difference in the surface area of those sediments are reflected in their log DAM* , which ranged from 2.3 to 4.7 (3.8 f 4.1, n = 16) in Drakes Ester0 and Tomales Bay and from 2.1 to 5.8 (5.0 f 5.2, n = 32) in the stations located within San Francisco Bay (Fig. 4). The consistency of these data is exemplified by the log DAg * determined in pore water from surficial sediments (I 2 cm deep) within San Francisco Bay. Which ranged from 3.7 to 5.8 (5.1 f 5.2, n = 9). That range is comparable with the log DAg f in surficial waters of the estuary (G.J. Smith and Flegal,
1. Riuera-Duarte, A. Russell F&al/Marine
1993), which ranged from 4.3 to 6.1 (5.3 + 0.3). This consistency between the partitioning of silver in pore waters and surface waters evidence a continuum at the water-sediment interface. Notably, the only stations (Coyote Creek and Mayfield Slough) that had values of log DAp. lower than the range for surficial waters were those located nearby the discharges of wastewater treatment plants (Fig. 4). This corroborates reports that high concentrations of silver are associated with wastewater discharges and that silver may be used as a tracer of sewage in coastal waters (Saintdo-Wilhelmy and Flegal, 1992). 3.4. D$iisiue
benthic fluxes
ofsiher
The benthic flux of silver from the sediments due to molecular diffusion (J,, mol cm;* s-i), was evaluated from Fick’s first law of diffusion:
4&l
J, = -@D,---
(2)
dx
where e, is the porosity; D, is the sediment diffusion coefficient; and d[Ag]/dx is the change in concentration of dissolved silver with depth in the sediment (mol cmi4). Th e subscript b indicates that those distances are measured over the bulk sediment, and the subscript p indicates the pore water only. The
Chemistg 56 (1997) 15-26
d[Ag]/dx was conservatively calculated as the gradient between the nepheloid layer concentration and the maximum subsurface concentration of dissolved silver in pore waters at each location. The sediment diffusion coefficient was derived from the empirical relation (Iversen and Jorgensen, 1993): D, =
D,
T
Diffusive
(3)
1 + ?z( 1 - @)
The diffusion coefficient of silver at infinite dilution CD,) was estimated from the data for the diffusion coefficient of silver at infinite dilution as proposed by Li and Gregory (1974), and corrected for the average annual bottom water temperature in the surroundings of the sediment sampling locations (Caffrey et al., 1994). The average bottom water temperatures were 17°C in Coyote Creek and Mayfield Slough and 15°C by the other locations within San Francisco Bay. An average bottom water temperature of 17°C was assumed for the adjacent Tomales Bay and Drakes Estero. Both D, and the diffusive benthic flux of silver were then calculated for 15°C (D, 1.3X 10m5 cm; s-‘> and 17°C (D, 1.4 X lo-’ cm; SK’) in the corresponding locations. The constant y1 varies according with the sediment
Table 3 Estimated diffusive and diffusive plus irrigation benthic fluxes (mol m b2 day- ‘) of dissolved bottom water temperatures in the San Francisco Bay estuary Station
23
(< 0.45 pm) silver at the annual average
Diffusive + irrigation
(“0
(mol rn;’
San Francisco Bay: Coyote Creek Mayfield Slough Oakland Harbor Castro Cove Inner Castro Cove Outer Castro Cove Pinole Point
day-‘)
17 17 15 IS IS 15 15
0.38 X lO-9 3.4 x lo-9 -0.10 x lo-9 -2.1 x lo-9 0.06 X lO-9 -0.03 x lo-” -0.45 x lo-9
Tomales Bay
17
0.33 x 10-a
Drakes Ester0
17
0.20 x 10-9
(mol yr-
32 290 -8 - 120 4 -2 -27
’)
(mot m;’
day-‘)
-0.39 x lo-9 - 10 x lo-” 0.09 x 10-9 -0.34 x lo-” - 1.3 x lo-9
(mol yr
’)
32 290 -33 - 600 6 -20 -78
The fluxes were calculated between nepheloid layer concentrations and peak pore-water concentrations (Table 2), as described in the text. The three-zone model for irrigation fluxes (Hammond et al., 1985) was not applied to the data from Coyote Creek and Mayfield Slough for the reasons explained in the text. The flux (mol yr- ’ ) was conservatively estimated assuming benthic fluxes are limited to the shallower regions of the estuary as proposed by Hammond et al. (1985).
24
I. Riuera-Duarte.
A. Russell Flegal/Marine
grain size, i.e. n = 3 for silt-clay sediment and n = 2 for sandy sediment (Iversen and Jorgensen, 1993). Following the grain size analyses, a value of n = 3 was used for the stations located within San Francisco Bay, and II = 2 was used for both Drakes Ester0 and Tomales Bay. The resultant DS was conservatively calculated to range from 7.6 X 10m6 to 9.0 X 1O-6 cm; s-‘. The estimated diffusive benthic fluxes of silver were greater (0.38 and 3.4 X 10m9 mol rn;* day-‘) at the stations in the South Bay (Coyote Creek and Mayfield Slough) affected by wastewater discharges (Table 3). Again, this is consistent with the association of silver with wastewater discharges (Sai’mdoWilhelmy and Flegal, 1992). In contrast, the five less contaminated sites (Oakland Harbor, Castro Cove, Inner Castro Cove and Outer Castro Cove and Pinole Point) in the northern reach of the estuary, had either negligible diffusion of silver out of the sediment or diffusive benthic fluxes of silver into the sediments (Table 3). Therefore, these preliminary calculations indicate that benthic fluxes of silver are into the sediment in the northern reach of the San Francisco Bay estuary. Estimated diffusive benthic fluxes of silver in Drakes Ester0 (0.20 X 10e9 mol rn; 2 day- ’ ) and Tomales Bay (0.33 X 10e9 mol rn;* day-‘) were similar to those for some contaminated sites within South Bay. As previously mentioned, the relatively high dissolved silver concentrations in pore waters from those embayments appear to be, in part, due to the relatively large grain size of the sediment, the relatively low concentrations of particulate manganese, and the complexation of silver with chlorides. This suggests that the relatively high estimates of silver fluxes in those relatively pristine embayments are, at least partially, caused by the formation of soluble chloride complexes within their pore waters. 3.5. Benthic fluxes due to irrigation of the sediments Physical and biological processes can increase diffusive benthic fluxes several times (Hammond et al., 1985; Santschi et al., 1990; Aller and Aller, 1992; ten Hulscher et al., 1992; Caffrey et al., 1996), and bioturbation and mechanical mixing appears to influence the distribution of particulate silver gradi-
Chemistry 56 (1997) 15-26
ents within San Francisco Bay. Since the effects of irrigation could not be determined directly from the pore-water silver concentration gradients, the three zone irrigation model (Hammond et al., 1985) was applied to those gradients in order to estimate this effect. In that model: Fi=
4441 + C’hih;(
-QD,-----dx
C, - C;)
where hi is the thickness of zone i (cm,); ,Viis the irrigation rate constant (s - I >; C, the concentration in the nepheloid layer; and Ci is the average concentration in pore waters throughout zone i (mol cmi3>. The A’,estimated by Hammond et al. (1985) for a shoal station during summer (h; = 2 X 10e6 s- ’ ; h; = 0.52 X 10m6 s- ‘) were used in the calculations. The thickness of both zones were estimated from the pore-water concentration gradients of H,SiO, and NH,+, as described by Rivera-Duarte and Flegal (1997). The three zone irrigation model was not applied in the sediment cores of both Coyote Creek and Mayfield Slough, since no bioturbation was observed at those sites. The fluxes due to irrigation were added to the diffusive fluxes to approximate net benthic fluxes (Table 3). These estimates of the net benthic fluxes of silver are from 1.5- to lo-fold greater than the estimated diffusive benthic fluxes. The relative increases agree with those reported by Aller and Aller (1992) for the increase (2- to lo-fold) due to the activities of macroorganisms in sediments. Again, these calculations indicate benthic fluxes out of the sediments within South Bay, and benthic fluxes into the sediments within the northern reach of the San Francisco Bay estuary. 3.6. Estimated integrated net benthic jluxes Preliminary estimations of the integrated net benthic fluxes of silver indicate that they are a relatively important source of dissolved silver in the South Bay. Sediments in the South Bay are extensively contaminated with silver; as illustrated in Fig. 5 where most of the surficial sediment samples had acid labile (0.5 N HCl leacheates) silver concentrations of > 2.0 kmol kg-’ dry weight. An upper limit calculation of those fluxes was made with the assumption that the benthic fluxes of silver to overly-
I. Riuera-Duarte, A. Russell Flegal/Marine
Chemistry 56 (1997) 15-26
25
4. Conclusions These initial measurements and preliminary calculations substantiate the proposal that benthic sediments are a principal source of silver in San Francisco Bay waters (G.J. Smith and Flegal, 1993). Calculations of the integrated net benthic fluxes estimated from measurements of dissolved silver in pore waters from contaminated sediments in San Francisco Bay are markedly greater than the fluvial input of dissolved silver to the bay. These analyses substantiate the proposal @legal and Safiudo-Wilhelmy, 1993) that contaminated sediments may be an important source of some trace elements to surface waters in some semi-enclosed embayments.
Acknowledgements
Fig. 5. Particulate silver (pmol kg-’ dry weight) distributions in San Francisco Bay. Distributions of the silver concentrations (pm01 kg-’ dry weight) measured in 0.5 N HCl leacheates of surficial sediments, collected in San Francisco Bay in January 1992. Data from A.R. Flegal’s WIGS group at UCSC.
The criticisms and comments from Peggy Delaney, Joris Gieskes, Charles Gobeil, Jim Kuwabara, and the anonymous reviewers are greatly appreciated. This research was funded by the Research Enhancement Program for the San Francisco Bay/Delta of the Interagency Ecological Studies Program and the San Francisco Bay Estuary Project, and the San Francisco Bay Regional Water Quality Control Board.
References ing waters are limited to the shallower regions (5 4 m) of the estuary [232 X lo6 mz in South Bay and 164 X lo6 m* in San Pablo Bay (Fuller, 198211, as conservatively suggested by the silicate data of Hammond et al. (1985). The estimated net diffusive flux of dissolved silver to the South Bay was between 32 and 290 mol yr-’ (Table 3). This preliminary and highly qualified estimate indicates the net diffusive flux of silver could be up to 2.5 times the entire riverine input of dissolved silver ( N 111 mol yr-’ > in San Francisco Bay (G.J. Smith and Flegal, 1993). Moreover, the relative magnitude of the benthic fluxes of silver in San Francisco Bay is projected to become greater as point source discharges of industrial silver to the estuary are reduced, as they have in San Diego Bay @legal and Saiiudo-Wilhelmy, 1993).
Aller, R.C. and Aller, J.Y., 1992. Meiofauna and solute transport in marine muds. Limnol. Oceanogr., 37: 1018-1033. Beattie, P., Drake, M., Jones, .I., Leeman, W., Longhi, J., McKay, G., Nielsen, R., Palme, H., Shaw, D., Takahashi, E. and Watson, B., 1993. Terminology for trace-element partitioning. Geochim. Cosmochim. Acta, 57: 1605-1606. Borovec, Z.C., 1993. Partitioning of silver, beryllium and molybdenum among chemical fractions in the sediments from the Labe (Elbe) River in Central Bohemia, Czech Rep. GeoJournal, 29(4): 359-364. Bruland, K.W.. Coale, K.H. and Mart, L., 1985. Analysis of seawater for dissolved cadmium, copper and lead An intercomparison of voltammetric and atomic absorption methods. Mar. Chem., 17: 285-300. Caffrey, J.M., Cole, B.E., Cloern, J.E., Rudek, J.R., Tyler, A.C. and Jassby, A.D., 1994. Studies of the San Francisco Bay, California, estuarine ecosystem - Pilot regional monitoring results, 1993. U.S. Geol. Surv., Menlo Park, CA, Open-File Rep. 94-82, 412 pp.
26
I. Riuera-Duarte,
A. Russell Flegal /Marine
Caffrey, J.M., Hammond, D.E., Kuwabara, J.S., Miller, L.G. and Twilley, R.R., 1996. Benthic processes in South San Francisco Bay: The role of organic inputs and bioturbation. In: J.T. Hollibaugh (Editor), San Francisco Bay: The Ecosystem. Pac. Div., Am. Assoc. Adv. Sci., pp. 425-442. Conomos, T.J., Smith, R.E. and Gartner, J.W., 1985. Environmental setting of San Francisco Bay. Hydrobiologia, 129: l- 12. Cowan, C.E., Jenne, E.A. and Crecelius, E.A., 1985. Silver speciation in seawater: The importance of sulfide and organic complexation. In: A.C. Sigleo and A. Hattori (Editors), Marine and Estuarine Geochemistry. Lewis, Chelsea, MI, pp. 285-303. Davis, J.A., Gunther, A.J., Richardson, B.J., O’Connor, J.M., Spies, R.B., Wyatt, E., Larson, E. and Meiorin, E.C., 1991. Status and trends report on pollutants in the San Francisco estuary. San Francisco Bay-Delta Aquat. Habit. Inst., Richmond, CA, 240 pp. Flegal, A.R. and Saiiudo-Wilhelmy, S.A., 1993. Comparable levels of trace metal contamination in two semi-enclosed embayments: San Diego Bay and South San Francisco Bay, Environ, Sci. Technol., 27: 1934-1936. Flegal, A.R., Cutter, L.S. and Martin, J.H., 1981. A study of the chemistry of marine sediments and wastewater sludge. Calif. State Water Resour. Control Board, Moss Landing Mar. Lab., Moss Landing, CA, 69 pp. Flegal, A.R., Smith, G.J., Gill, GA., Saiiudo-Wilhelmy, S.A. and Anderson, L.C.D., 1991. Dissolved trace elements cycles in the San Francisco Bay estuary. Mar. Chem., 36: 329-363. Fuller, C., 1982. The use of Pb-210, Th-234 and Cs-137 as tracers of sedimentary processes in San Francisco Bay, California. MS. Thesis. University of Southern California, Los Angeles, CA, 251 pp. Goldberg, E.D., Koide, M., Hodge, V., Flegal, A.R. and Martin, J.H., 1983. U.S. Mussel Watch: 1977-1978 results on trace metals and radionuclides. Estuarine Coastal Mar. Sci., 16: 69-93. Hammond, D.E., Fuller, C., Harmon, D., Hartman, B., Korosec, M., Miller, L.G., Rea, R., Warren, S., Berelson, W. and Hager, SW., 1985. Benthic fluxes in San Francisco Bay. Hydrobiologia, 129: 69-90. Iversen, N. and Jorgensen, B.B., 1993. Diffusion coefficients of sulfate and methane in marine sediments: Influence of porosity. Geochim. Cosmochim. Acta, 57: 571-578. Jahnke, R.A., 1988. A simple, reliable, and inexpensive porewater sampler. Limnol. Oceanogr., 33: 483-487. Jenne, E.A., Girvin, D.C., Ball, J.W. and Burchard, J.M., 1978. Inorganic speciation of silver in natural waters - fresh to marine. In: D.A. Klein (Editor), Environmental Impacts of Artificial Ice Nucleating Agents. Dowden, Hutchinson and Ross, Stroudsburg, PA, pp. 41-61.
Chemistry 56 (1997) 15-26
Li, Y.-H. and Gregory, S., 1974. Diffusion of ions in seawater and in deep-sea sediments. Geochim. Cosmochim. Acta, 38: 703714. Luoma, S.N. and Phillips, D.J.H., 1988. Distribution, variability, and impacts of trace elements in San Francisco bay. Mar. Pollut. Bull., 19: 413-425. Luoma, S.N., Ho, Y.B. and Bryan, G.W., 1995. Fate, bioavailability and toxicity of silver in estuarine environments. Mar. Pollut. Bull., 31: 44-54. Lyons, W.B. and Fitzgerald, W.F., 1983. Trace metals speciation in nearshore anoxic and suboxic pore waters. In: C.S. Wong, EA. Boyle, K.W. Bruland, J. Burton and E.D. Goldberg (Editors), Trace Metals in Sea Water. NATO (N. Atlantic Treaty Org.) Mar. Sci. Ser., 9. Plenum, New York, NY, pp. 621-641. Nichols, F.H., Cloem, J.E., Luoma, S.N. and Peterson, D.H., 1986. The modification of an estuary. Science, pp. 567-573. Rivera-Duarte, I. and Flegal, A.R., 1994. Benthic lead fluxes in San Francisco Bay, California, USA. Geochim. Cosmochim. Acta, 58: 3307-3313. Rivera-Duarte, I. and Flegal, A.R., 1996. Microtechniques for the determination of nanomolar concentrations of trace elements in I 10 mL of sediment porewater. Anal. Chim. Acta, 328: 13-17. Rivera-Duarte, I. and Flegal, A.R., 1997. Porewater gradients and diffusive benthic fluxes from relatively pristine and contaminated sites in San Francisco Bay: Nutrients and redox species, In prep. Santschi, P., HGhener, P., Benoit, G. and Buchholtz-ten Brink, M., 1990. Chemical processes at the sediment-water interface. Mar. Chem., 30: 269-315. Safiudo-Wilhelmy, S.A. and Flegal, A.R., 1992. Anthropogenic silver in the Southern California Bight: A new tracer of sewage in coastal waters. Environ. Sci. Technol., 26: 21472151. Smith, D.R., Stephenson, M.D. and Flegal, A.R., 1986. Trace metals in mussels transplanted to San Francisco Bay. Environ. Tox. Chem., 5: 129-138. Smith, G.J. and Flegal, A.R., 1993. Silver in San Francisco Bay estuarine waters. Estuaries, 16: 547-558. Smith, L.H., 1987. A review of circulation and mixing studies of San Francisco Bay, California. U.S. Geol. Surv., Circ. 1015, 38 PP. Taylor, S.R. and McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 3 12 pp. ten Hulscher, Th.E.M., Mol. G.A.J. and Liiers, F., 1992. Release of metals from polluted sediments in a shallow lake: Quantifying resuspension. Hydrobiologia, 235/236: 97- 105.
Marine Chemistry
56 ( 1997) 15-26
Pore-water silver concentration gradients and benthic fluxes from contaminated sediments of San Francisco Bay, California, U.S.A. Ignacio Rivera-Duarte Earth Science Department,
*, A. Russell Flegal
WIGS, Uniuersity of California, Santa Cruz, CA 95064, USA
Received 7 July 1995: accepted 8 October 1996
Abstract Estimates of the benthic fluxes of silver within San Francisco Bay substantiate the proposal that diagenetic remobilization is an important source of silver in surficial waters of the estuary (G.J. Smith and Flegal, 1993). Dissolved (< 0.45 p,m) silver concentrations in pore waters from relatively pristine and contaminated sediments from San Francisco Bay, as well as from sediments from Tomales Bay and Drakes Ester0 ranged from 4 to 4600 pM, with one exception (8900 PM). With the
exclusion of four exceptionally high concentrations (> 620 PM), the average (X f 1 sd) dissolved pore-water concentration of silver from these three embayments was 120 f 140 pM. The apparent molar partition coefficient of silver (log DAg.)in pore waters of benthic sediments (I 2 cm deep) within San Francisco Bay ranged from 3.7 to 5.8 (5.1 f 5.21, which was consistent with the previously reported partitioning of silver within the bay surface waters (G.J. Smith and Flegal, 1993) (log L&s. 5.3 k 0.3). Estimated diffusive benthic fluxes of silver were greatest at sites affected by waste water outfalls with relatively elevated concentrations of silver in the effluent. Upper limit estimates of the integrated net benthic flux of silver to South Bay (32-290 mol yr-‘> indicate that it could be 2.5-fold greater than the estimated fluvial input of dissolved silver (_ Ill mol yr-‘) to the estuary (G.J. Smith and Flegal, 1993). Keywords:
silver; sediment; pore water; sediment-water
interface;
1. 1. Introduction San Francisco Bay (Fig. 1) is an urban estuary (Nichols et al., 1986) with anomalously high concentrations of silver in its water (G.J. Smith and Flegal, 1993), sediments (Luoma and Phillips, 1988; Luoma et al., 1995) and benthic organisms (D.R. Smith et al., 1986; Luoma and Phillips, 1988; Luoma et al.,
* Corresponding
author.
03044203/97/$17.00 Copyright PI1 SO304-4203(96)00086-2
partition coefficients;
geochemical
cycle; San Francisco
Bay
1995). Total dissolved ( < 0.45 km) silver concentrations in the bay are as high as 250 pM (G.J. Smith and Flegal, 1993), which is approximately two orders of magnitude above baseline concentrations of silver in northeast Pacific coastal waters (l-2 pM; Saiiudo-Wilhelmy and Flegal, 1992). Particulate concentrations of silver in sediments from highly contaminated areas within the bay averaged 9 pmol kg-’ (Luoma and Phillips, 1988), which is an order of magnitude greater than both the average crustal abundance of silver (N 0.7 pmol kg-‘; Taylor and McLennan, 1985) and baseline concentrations (0.9
0 1997 Elsevier Science B.V. All rights reserved
16
I. Rivera-Duarte,
Fig. 1. Sediment pore-water sampling Bay and adjacent embayments.
locations
A. Russell Flegal/Marine
in San Francisco
pmol kg-’ ) in San Francisco Bay and Tomales Bay sediments (Luoma et al., 1995). Silver concentrations in tissues of some bend-tic organisms in the bay are equal to or greater than those of benthic organisms that have been measured in any other estuary in the U.S.A. or Europe (Luoma and Phillips, 1988; Luoma et al., 1995). For example, silver concentrations in the mussel, Mytilus californianus, range from 6.5 to 27 pmol kg-’ in the bay (D.R. Smith et al., 1986; Luoma and Phillips, 19881, which is one to two orders of magnitude greater than silver concentrations (N 0.37 pmol kg-‘) in mussels in the northeast Pacific (Goldberg et al., 1983). The anomalously high concentrations of silver in the San Francisco Bay estuary occur in its southern reach, or South Bay (Fig. 1). It is a physically distinct embayment, with a lagoon-type circulation dominated by tidal oscillations (L.H. Smith, 1987). This contrasts with the circulation pattern of the northern reach of the San Francisco Bay estuarine system, which is a partially mixed estuary dominated
Chemistry 56 (1997) 15-26
by riverine inflows from the Sacramento and San Joaquin rivers that discharge to the northeast Pacific through the Golden Gate (Conomos et al., 1985). The South Bay also receives approximately half (13,000-33,000 mol yr- ’ ) of the total amount of industrial and municipal silver (25,000-66,000 mol yr-‘) discharged to the entire estuary (Davis et al., 1991). While the high levels of dissolved silver in the South Bay appear to be primarily due to point source discharges of industrial silver and to the long residence times of water within the South Bay, it has been proposed that the remobilization of silver from benthic sediments may also contribute substantially to the seasonally high concentrations in South Bay waters (G.J. Smith and Flegal, 1993). That proposal was based on the temporal covariance of elevated silver concentrations in South Bay waters with those of other metals (Cu, Ni, and Zn) and nutrients (e.g., H,SiO,) that were associated with diagenetic remobilization processes @legal et al., 1991). The proposal has been supported by measurements of high silver concentrations in San Diego Bay waters (I 307 p M). Those high levels have been tentatively attributed to the remobilization of silver from benthic sediments, because there are no contemporary point source discharges of industrial silver to that semi-enclosed marine embayment @legal and Satiudo-Wilhelmy, 1993). That attribution has been further supported by recent measurements of dissolved lead concentration gradients and diffusive fluxes from pore waters of San Francisco Bay sediments (Rivera-Duarte and Flegal, 1994), which parallel the dissolved silver concentration gradients in pore waters of San Francisco Bay sediments that are presented in this paper. Consequently, the objective of this research is to quantify the benthic flux of silver in San Francisco Bay. Dissolved silver concentrations were measured in sediment pore waters, and then used to estimate the diffusive and irrigation-based benthic fluxes of silver to overlying waters in the estuary, as well as in two adjacent embayments. These preliminary estimates are compared with other previously reported inputs of silver to the San Francisco Bay from fluvial and anthropogenic sources. Those initial comparisons attest to the relative importance of benthic fluxes on the silver cycle in that estuary.
I. Rioera-Duarte, A. Russell Flegal /Marine Chemistry 56 (19971 15-26
2. Methods Cores (lo-cm width, 40-cm length) were collected directly from the sediments in locations within San Francisco Bay, Tomales Bay and Drakes Ester0 from May to August 1992 and May 1993 (Fig. 1; Table 1). These included four sites along the periphery of San Pablo Bay (Castro Cove in 1992; Inner Castro Cove, Outer Castro Cove and Pinole Point in 19931, three sites along the periphery of the South Bay (Oakland Harbor, Mayfield Slough and Coyote Creek in 19921, and two sites in adjacent embayments (Tomales Bay and Drakes Ester0 in 1992). Sediments at some of the locations within San Francisco Bay were contaminated by the products of the oil refinery industry (i.e. Castro Cove, Inner Castro Cove and Outer Castro Cove), municipal wastewater discharges (i.e. Mayfield Slough and Coyote Creek), or maritime activities (i.e. Oakland Harbor). Sediments at other locations (i.e. Pinole Point) were considered to be more representative of the less contaminated shallow areas in the northern reach of the San Francisco Bay estuary. Total dissolved ( < 0.45 p,m) pore-water samples were extracted from the sediments with a whole-core squeezer (Jahnke, 1988). The squeezer was modified
Table 1 General characteristics
in bay
San Pablo
out of
out of
Bay
Bay
Bay
no
no
no
subtidal
subtidal
intertidal
intertidal
yes
yes
yes 0.1-0.3 4 > 26 -
yes 0.1-0.6 7
Castro Cove
South
South
San Pablo
San Pablo
Bay
South/ Central
San Pablo
Bay
Bay
Bay
Bay
Bay maritime
oil
oil
oil
subtidal
intertidal
intertidal
yes
yes
99
97
90
78
0 8
22
22
waste water
Location w.r.t. tides Macro biota %TOC range
intertidal no
intertidal no
% Clay&silt
90
95
yes 1.0-3.0 87
0 6
0 22
(cm)
Drakes Ester0
Oakland Harbor
waste water
1.3-3.4
1.l-2.4
Inner Castro Cove
Bay
Mayfield Slough
Contaminated
Oxic/suboxic Suboxic/
for trace-element collections by using acid-cleaned PVC cores, an aluminum frame and an hydraulic jack to pressurize the sediment core from the bottom (Rivera-Duarte and Flegal, 1994). The extracts were evacuated with all-polypropylene syringes and acidcleaned Teflon filters (0.45 pm>. A sample from the nepheloid layer at the sediment-water interface at the top of the whole-core squeezer was collected immediately, and then samples from 10 depths (i.e. 1, 2, 4, 6, 8, 10, 14, 18, 22 and 26 cm) in the core were collected. At each sampling depth, the first 5 ml of pore water were used to condition both the syringe and filter and to determine ancillary parameters (i.e. NH:, gelbstoff, HPOi-, H,SiO,, Cl- and SO,‘->, as reported elsewhere (Rivera-Duarte and Flegal, 1994, 1997). The final l- 10 ml of pore water were aliquoted into acid-cleaned polyethylene bottles, acidified to pH < 2 with sub-boiling quartz distilled (2 X ) 6 N HCl, and stored for one month prior to elemental analyses. The extractions, as well as the preceding sample preparations and subsequent elemental analyses, were conducted in HEPA filtered air (Class 100) tracemetal clean working areas. Silver in the filtered solutions was preconcentrated with microtechniques (Rivera-Duarte and Flegal, 1996), which were based
of the sampled sites Coyote Creek
Location
17
1.3-2.3
Outer Castro Cove
0.8-1.4
1.9-3.2
anoxic (cm) A more detailed description of these characteristics w.r.t. = with respect to. % Clay&silt = average for whole sediment core.
is given in Rivera-Duarte
Pinole Point
and Flegal (1997).
0.1-1.1
Tomales
> 22 _
18
I. Riuera-Duarte,
A. Russell Flegal/Marine
on the ammonium l-pyrrolidine dithiocarbamate/diethylammonium diethyldithiocarbamate organic extraction procedures de(APDC~DDC) tailed by Bruland et al. (1985). The efficiency of the silver extraction was lOO%, based on quantitative recovery analyses of internal reference materials and spiked samples, and N 106% with respect to the silver concentration (47.9 + 2 PM) of CASS2 (G.J. Smith and Flegal, 1993). The coefficient of variation (CV) of replicate silver extractions was +6% with a 500 pM sample solution and &24% with CASS2. Total dissolved silver concentrations were measured by graphite furnace atomic absorption spectrometry (GFAAS) with stabilized platform techniques and the method of standard additions. The CV of replicate measurements was 5 10%. Procedural blanks ranged from 9 pg (0.08 pmol) to 31 pg (0.29 pm00 in N 50 ml of high-purity (1X Ma cm-‘> water. The limit of detection, defined as three times the standard deviation of the procedural blanks, was 0.37 pM. The sediment cores were frozen immediately after the initial pore-water extractions for subsequent particulate concentration measurements at corresponding depths. The sediments were analyzed for porosity (assuming a density of 2.65 g cme3 for the sediment). Near-total silver concentrations were measured by GFAAS following aqua regia digestions @legal et al., 1981). Sediment concentrations are reported as near-total, rather than total, because HF was not used. Still, the recoveries for silver in most sediments are nearly quantitative ( 2 90%) with aqua regia digestions @legal et al., 1981). Procedural silver blanks ranged from 10 to 760 pg, which was 5 1% of the particulate silver in the sediment samples. Silver concentrations (X f lsd) derived from concurrent analyses of standard reference sediments (BCSS-1, MESS-l, PACS-1 and MESS-2) from the National Research Council Canada were: 970 f 52 nmol kg-’ (n = 9) for BCSS-1; 1,100 f 120 nmol 15,000 f 3,300 nmol kg-’ (n=6) f or MESS-l; kg-’ (n = 5) for PACS-1; and 1,600 f 11 nmol kgg ’ (n = 3) for MESS-2. The measured silver concentrations are consistent with the certified silver concentration (1,669 + 185 nmol kg- ’ > for MESS-2 and with the suggested silver concentration (927 + 371 nmol kg- ’ ) for BCSS-I by the Marine Analytical Chemistry Standards Program (Ottawa, Canada).
Chemistry 56 (1997) 15-26
3. Results and discussion
3.1. Redox gradients The subtidal, surface sediments collected in San Francisco Bay are suboxic. This is illustrated by the pore-water concentration profiles of dissolved iron, manganese and nutrients in the cores, which indicate suboxic conditions in the surficial sediments of the seven locations in San Francisco Bay. These suboxic conditions extended to 6 cm in Coyote Creek, to 22 cm in Mayfield Slough, Outer Castro Cove and Pinole Point, to 26 cm in Oakland Harbor, to 8 cm in Castro Cove, and to 4 cm deep at Inner Castro Cove (Table 1). Other evidence indicates these suboxic conditions are the result of both biological and mechanical flushing of the sediments (Rivera-Duarte and Flegal, 1997). In contrast, the profiles of those redox parameters in cores from Tomales Bay and Drakes Ester0 exhibited oxic conditions to a depth of at least 26 cm (Table 1). The contrasting profiles of redox species between sediments in Drakes Ester0 and Tomales Bay with those in the San Francisco Bay are primarily attributed to differences in grain size and organic content (Table 1). Sediments in the cores from San Francisco Bay are predominantly silts and clays (< 62.5 pm) in grain size, which comprises 90% of the average mass fraction at Coyote Creek, 96% at Mayfield Slough, 87% at Oakland Harbor, 99% at Castro Cove, 97% at Inner Castro Cove, 90% at Outer Castro Cove, and 78% at Pinole Point. Silts and clays comprise only 7% of the average mass fraction at Drakes Ester0 and only 4% of that fraction at Tomales Bay. The associated permeability of the sediments from the latter two embayments is evidenced by the presence of oxic conditions down to 26-cm depth in the cores from both Tomales Bay and Drakes Ester0 (Rivera-Duarte and Flegal, 1997). Sediments within San Francisco Bay also have higher percent of total organic carbon (0.68-3.43%) than the sediments of Drakes Ester0 (0.06-0.61%) and Tomales Bay (0.12-0.32%) (Rivera-Duarte and Flegal, 1997). These pronounced variations in the composition and redox condition of sediments from the different locations limit the following comparisons between measured silver concentrations and esti-
I. Ricera-Duarte, A. Russell Flegal /Marine
mated fluxes in pore waters of San Francisco and adjacent embayments. 3.2. Dissohed
and particulate
Bay
silver concentrations
The pore-water concentrations of silver in sediments of San Francisco Bay and adjacent embayments were comparable to most previously reported concentrations of silver in other estuarine pore waters. These include pore-water concentrations of silver in Long Island Sound, which were reported to range from < 2000 to 9000 pM (Lyons and Fitzgerald, 1983). Conversely, the pore-water concentrations in San Francisco Bay sediments were an order of magnitude greater than the highest concentrations of dissolved silver in its surface waters, which range from I 6 to 250 pM (G.J. Smith and Flegal, 1993). The total dissolved ( < 0.45 pm) silver concentrations of pore waters from the three northeast Pacific embayments (San Francisco Bay, Tomales Bay and Drakes Estero) were < 4600 pM, with one exception (Table 2; Fig. 2). Excluding that anomalously high measurement and three other exceptionally high concentrations (1100, 4500 and 4600 PM) from a highly contaminated site in the extreme South Bay (Mayfield Slough), the total dissolved silver concentrations in pore waters of the three embayments ranged from 4 to 380 pM, with an average concentration of 120 f 140 pM (n = 55). While those four high concentrations may be due to sampling or analytical contamination, the highest of them (8900 pM in Tomales Bay) is similar to the highest previously reported pore-water concentration of silver (9000 PM), which was measured in sediments considered representative of natural conditions within Long Island Sound (Lyons and Fitzgerald, 1983). Consequently, the anomalously high silver concentrations in pore waters of Tomales Bay and Drakes Ester0 are tentatively attributed to their prevailing oxic condition and the relative smaller surface areas of their sediment grains. Those two factors would allow relatively high concentrations of silver to remain in solution as chloride complexes (Jenne et al., 1978; Lyons and Fitzgerald, 1983; Cowan et al., 1985; Luoma et al., 1995). Thermodynamic calculations indicate that silver is released at both the water-sediment interface and in zones with extreme anoxic conditions. This is shown
Chemistry 56 (1997) 15-26
19
by the pore-water concentration gradients of dissolved silver and the redox characteristics of the different sites (Rivera-Duarte and Flegal, 1997). Comparisons between sites indicate that silver is
Table 2 Total dissolved ( < 0.45 p,rn) silver concentrations (PM) in pore water, near-total silver concentrations (p,mol kg- ’ dry weight) in sediment, and apparent molar partition coefficients (DAg +) of silver in the San Francisco Bay estuary (Coyote Creek, Mayfield Slough, Oakland Harbor, Castro Cove, Inner Castro Cove, Outer Castro Cove and Pinole Point), Drakes Ester0 and Tomales Bay Depth
Ag
(cm)
(PM)
Ag (km01 kg- ’ dry weight)
D
-
log D,ig*
As
San Francisco Bay South Bay : Coyote Creek: N” 1 2 4 6 8 10 14 18 22
64 71 110 240 +zb n.d. ’ * 190 7 n.d.
5.8 1.4 1.2 1.3 1.2 1.3 0.72 0.41 0.84
82X 10’ 12x 10” 4.9x
10’
4.9 4.1 3.7
3.9 X 10’ 66X lo3
3.6 4.8
4.9x 11 x 103 4.3 X 13x lo3 0.2 x 0.1 x 3.3 x
3.7 4.0 3.6 4.1 2.3 2.1 3.5
Mayfield Slough: N 1 2 4 6 8 10 14 18
48 [llool 160 250 [45Z [46001 270
[181 5.4 1.7 1.1 1.1 0.9 1 0.56 0.88
10’ IO’ 10’ 10” 10’
Central Bay Oakland Harbor: N 1 2 4 6 8 10 14 18
27 12 n.d. n.d. n.d. n.d. n.d. 11 n.d.
7.1 7.4 7.4 7.5 7.9 7. 7 5.5 5.1
570x
101
5.8
500x
lo?
5.1
I. Riuera-Duarte, A. Russell Flegal/ Marine Chemistry 56 (1997) 15-26
20 Table 2 (continued) Depth
Ag
(cm)
(PM)
Table 2 (continued) Ag (km01 kgdry weight)
’
DA&? *
“??I DAM*
San Pablo Bay:
I 2 4 6 8 10 14 18 22 26
380 32 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 66 24
3.1 3.2 2.8 2.2 1.8 1.6 1.9 3.5 2.1 2.6
96 X 10’
5.0
32x 10’ 110x 10’
4.5 5.0
Inner Castro Cove: N 1 2 4 6 8 10 14 18
45 n.d. 43 88 n.d. 6 37 180 47
3.9 3.5 4.4 4.3 4.2 4.3 4.1
81 x lo3 51 x 10’
4.9 4.7
660X 10’ 120x 10’ 23x10’
5.8 5.1 4.4
Outer Castro Cove: N 1 2 4 6 8 10 14 18 22
40 n.d. 29 18 n.d. n.d. n.d. n.d. 33 n.d.
91 20 66 63 n.d. 10 130 15 9
(PM)
2.8 2.3 2.0 3.2 2.6 3.4 2.6 1.5 3.0
2.1 3.0 2.9 2.1 2.1 2.4 1.7 1.3
’
DAg*
78X lo3 110x 10’
47x 10’
4.9 5.1
[89001 620 53 300 440 190 310 250 130 4
480
Drakes
Ester-0 4 43 n.d. 140 5 28 320 310 12
0.14 0.13 0.15 0.13 0.10 0.12 0.10 0.11 0.10 0.10
0.08 0.36 0.44 0.26 0.17 0.18 0.11 0.11 0.09 0.14
[0.02x 0.2 x 2.7 x 0.4x 0.2 x 0.6 x 0.3 x 0.4x 0.8 x 26x 10’
PM). ’ Concentrations
10’1 10’ 10’ 10’ 10’ lo7 10’ 10’ 10’
t1.21 2.3 3.4 2.6 2.4 2.8 2.5 2.6 2.9 4.4
1.8X 10’
3.3
3.2 x 10’ 52X IO3 6.3 x 10’ 0.6 x 10”
3.5 4.7 3.8 2.1
0.3 x 10’ 8.2 x 10’
2.5 3.9
below the estimated
detection
limit ( < 0.37
limit of detection (L.O.D. =
3 x lsd,,,,,). Values in brackets arc considered suspect, and are not included in statistical averages.
as discussed
in text,
4.1
130x 103 45 x 10’ 46X lo3
5.1 4.7 4.1
210x 19x 110x 140x
5.3 4.3 5.0 5.1
103 10’ 10’ 103
log DAM*
Bay
N 1 2 4 6 8 10 14 18 22 26
N 1 2 4 6 8 10 14 18 22 26
Ag (Fmol kgdry weight)
a N indicates nepheloid layer. b Concentrations below the instrumental
Pinole Point: N 1 2 4 6 8 10 14 18
Ag
km) Tomales
Castro Cove: N
Depth
released at the sediment-water interface, which is presumably coupled with the diagenesis of labile organic matter and the formation of soluble silver chloride complexes (Jenne et al., 1978; Lyons and Fitzgerald, 1983; Cowan et al., 1985; Luoma et al., 1995). Silver is also released to pore water in extreme anoxic conditions (Inner Castro Cove), which is assumed to be the result of the formation of soluble silver sulfide complexes (Jenne et al., 1978; Lyons and Fitzgerald, 1983; Cowan et al., 1985). There are decreases in dissolved silver concentra-
I. Riuera-Duarte, A. Russell Flegal/Marine
Ag (PM) Coyote Creak
Ag (PM) Mayfield Slough
Ag (PM) Oakland Harbor
Castro Cove
Inner Castro C.
Outer Castro C.
~
/+f=
G&=
Pinole Point 200
0
0 FS 2 10 f
400
Tomales Bay
Drakes Ester0
0
0
600
1200
200
400
:
Chemistv 56 (1997) 15-26
21
was 18 pmol kg-’ in surface sediments (1 cm) at Mayfield Slough, which is in the southernmost portion of the South Bay where silver concentrations in sediments (i N 9 p.mol kg-’ ) are anomalously high (Luoma and Phillips, 1988). The particulate silver concentrations of cores from the locations in San Francisco Bay (Coyote Creek, Mayfield Slough, Oakland Harbor, Castro Cove, Inner Castro Cove, Outer Castro Cove and Pinole Point) were also markedly higher (up to two orders of magnitude) than those of cores from Tomales Bay and Drakes Estero, which ranged from 0.08 to 0.44 pmol kg-’ dry weight (0.16 & 0.09 kmol kg-’ dry weight, II = 20) (Table 2; Fig. 3). Bioturbation and mechanical mixing appeared to substantially influence the distribution of particulate silver in most of the sites within San Francisco Bay. The only sites where macrofauna was visually absent were Coyote Creek and Mayfield Slough (which are
. Ag (pmol kg-’ dry weight) Coyote Creek
Fig. 2. Pore-water concentration (PM) profiles of total dissolved ( < 0.45 km) silver (0) in San Francisco Bay and adjacent embayments. Values denoted by n are below the estimated limit of detection (L.O.D. = 3 X lsd b,anks). Note the different silver concentration ranges, from 0 to 1200 pM for Mayfield Slough and Tomales Bay, and from 0 to 400 pM for the other stations. The position of the oxic-suboxic front ( * - * * ) and the suboxicanoxic front (-- - -_) are from Table 1, as defined by RiveraDuarte and Flegal (1997).
0
depth in most of the cores (Fig. 2), which parallel decreases in dissolved manganese concentration under suboxic conditions (Rivera-Duarte and Flegal, 1997). The latter covariance is consistent with reports of the partitioning of silver in the manganese fraction of freshwater sediments (Borovec, 1993). Particulate silver concentrations measured in the sediment cores from San Francisco Bay (Table 2; Fig. 3) were consistent with previously reported measurements of particulate silver in the estuary (Luoma and Phillips, 1988). Silver concentrations in cores from the bay ranged from 0.47 to 7.9 pmol kg-’ dry weight (3.0 &-2.0 pmol kg-’ dry weight, n = S9), with one exception. The silver concentration
10
Mayfield Slough
Oakland Harbor
F fr 0
5
10
0
5
10
16
b b
b
l
:
.
l
Castro Cove 0
tions
5
5
10
0
Inner Castro C. 0
5
10
Outer Castro C. 0
10
l
:
with
5
l .
f l .
.
. b
iPinole Point 0
5
10
~
.
Tomales Bay
Drakes Ester0
0
0
5
10
5
10
i Fig. 3. Sediment concentration profiles of silver (pm01 kg-’ dry weight) in San Francisco Bay stations and adjacent embayments (Tomales Bay and Drakes Estero).
I. Riuera-Duarte, A. Russell Flegal/Marine
22
both located in highly contaminated areas), which were the only sites that had surficial peak concentrations of particulate silver (Fig. 3). These surface maxima contrasted with the other sites (Oakland Harbor, Castro Cove, Inner Castro Cove, Outer Castro Cove, and Pinole Point), where there were macrofauna and particulate silver concentrations were homogeneously distributed with depth in their sediments (Fig. 3). In contrast, the oxic surface sediments at Tomales Bay and Drakes Ester0 [which were also inhabited by macroorganisms, relatively free of industrial silver inputs, and primarily ( > 93%) sands] had relatively low and homogeneously distributed silver concentrations. 3.3. Partition ments
coeflcients
of siluer in wjkial
Chemistry 56 (1997) 15-26
log Dng* Sutflcialwatara
25 -f 30 i
0
a. San Francisco Bay
sedi-
The apparent molar partition of silver ( DAg*> between sediment (Cr,“. , mol kg-’ dry weight) and pore water (Cl:*, mol L- ‘) was calculated with the following equation from Beattie et al. (1993): D&‘=-
LAg
*
OI)W
(‘1
It is termed an “apparent” partition coefficient, because it is derived from a near-total extraction of silver from the sediments, rather than from a total extraction with HF. Consequently, the ratio is conservatively low. The log of the partition coefficient (log DAg L> varied between 2.1 and 5.8 (Table 2; Fig. 4) with an average of 4.9 f 5.2 (n = 48). That range and average did not include the one outlier of 1.2, which was derived from the anomalously high pore-water concentration. Since the silver concentration of the associated sandy sediments was relatively low (0.14 pmol kg-’ ) and the dissolved silver concentration of pore waters at that depth was anomalously high (8900 PM), the partition coefficient derived from those extreme values was not considered to be appropriate. The differences in partition coefficients between sediments from San Francisco Bay and adjacent embayments (Fig. 4) may be primarily due to differences in the specific surface areas of the sediments. As previously noted, the sediments from the San Francisco Bay cores were primarily silts and clays,
3Oi
b. Adjacent embayments
Fig. 4. Log partition coefficients (log DAs *) vs. depth in (a) San Francisco Bay locations: Coyote Creek (01, Mayfield Slough (m), Oakland Harbor (O), Castro Cove (01, Inner Castro Cove (v ), Outer Castro Cove (A) and Pinole Point ( q 1; and (b) Drakes Ester0 (0) and Tomales Bay ( n ). The range of log DAg. for suspended sediments in surficial waters of San Francisco Bay (4.3-6.1) was reported by G.J. Smith and Flegal (1993).
while those from Drakes Ester0 and Tomales Bay were primarily sands with relatively lower specific surface areas. The associated difference in the surface area of those sediments are reflected in their log DAM* , which ranged from 2.3 to 4.7 (3.8 f 4.1, n = 16) in Drakes Ester0 and Tomales Bay and from 2.1 to 5.8 (5.0 f 5.2, n = 32) in the stations located within San Francisco Bay (Fig. 4). The consistency of these data is exemplified by the log DAg * determined in pore water from surficial sediments (I 2 cm deep) within San Francisco Bay. Which ranged from 3.7 to 5.8 (5.1 f 5.2, n = 9). That range is comparable with the log DAg f in surficial waters of the estuary (G.J. Smith and Flegal,
1. Riuera-Duarte, A. Russell F&al/Marine
1993), which ranged from 4.3 to 6.1 (5.3 + 0.3). This consistency between the partitioning of silver in pore waters and surface waters evidence a continuum at the water-sediment interface. Notably, the only stations (Coyote Creek and Mayfield Slough) that had values of log DAp. lower than the range for surficial waters were those located nearby the discharges of wastewater treatment plants (Fig. 4). This corroborates reports that high concentrations of silver are associated with wastewater discharges and that silver may be used as a tracer of sewage in coastal waters (Saintdo-Wilhelmy and Flegal, 1992). 3.4. D$iisiue
benthic fluxes
ofsiher
The benthic flux of silver from the sediments due to molecular diffusion (J,, mol cm;* s-i), was evaluated from Fick’s first law of diffusion:
4&l
J, = -@D,---
(2)
dx
where e, is the porosity; D, is the sediment diffusion coefficient; and d[Ag]/dx is the change in concentration of dissolved silver with depth in the sediment (mol cmi4). Th e subscript b indicates that those distances are measured over the bulk sediment, and the subscript p indicates the pore water only. The
Chemistg 56 (1997) 15-26
d[Ag]/dx was conservatively calculated as the gradient between the nepheloid layer concentration and the maximum subsurface concentration of dissolved silver in pore waters at each location. The sediment diffusion coefficient was derived from the empirical relation (Iversen and Jorgensen, 1993): D, =
D,
T
Diffusive
(3)
1 + ?z( 1 - @)
The diffusion coefficient of silver at infinite dilution CD,) was estimated from the data for the diffusion coefficient of silver at infinite dilution as proposed by Li and Gregory (1974), and corrected for the average annual bottom water temperature in the surroundings of the sediment sampling locations (Caffrey et al., 1994). The average bottom water temperatures were 17°C in Coyote Creek and Mayfield Slough and 15°C by the other locations within San Francisco Bay. An average bottom water temperature of 17°C was assumed for the adjacent Tomales Bay and Drakes Estero. Both D, and the diffusive benthic flux of silver were then calculated for 15°C (D, 1.3X 10m5 cm; s-‘> and 17°C (D, 1.4 X lo-’ cm; SK’) in the corresponding locations. The constant y1 varies according with the sediment
Table 3 Estimated diffusive and diffusive plus irrigation benthic fluxes (mol m b2 day- ‘) of dissolved bottom water temperatures in the San Francisco Bay estuary Station
23
(< 0.45 pm) silver at the annual average
Diffusive + irrigation
(“0
(mol rn;’
San Francisco Bay: Coyote Creek Mayfield Slough Oakland Harbor Castro Cove Inner Castro Cove Outer Castro Cove Pinole Point
day-‘)
17 17 15 IS IS 15 15
0.38 X lO-9 3.4 x lo-9 -0.10 x lo-9 -2.1 x lo-9 0.06 X lO-9 -0.03 x lo-” -0.45 x lo-9
Tomales Bay
17
0.33 x 10-a
Drakes Ester0
17
0.20 x 10-9
(mol yr-
32 290 -8 - 120 4 -2 -27
’)
(mot m;’
day-‘)
-0.39 x lo-9 - 10 x lo-” 0.09 x 10-9 -0.34 x lo-” - 1.3 x lo-9
(mol yr
’)
32 290 -33 - 600 6 -20 -78
The fluxes were calculated between nepheloid layer concentrations and peak pore-water concentrations (Table 2), as described in the text. The three-zone model for irrigation fluxes (Hammond et al., 1985) was not applied to the data from Coyote Creek and Mayfield Slough for the reasons explained in the text. The flux (mol yr- ’ ) was conservatively estimated assuming benthic fluxes are limited to the shallower regions of the estuary as proposed by Hammond et al. (1985).
24
I. Riuera-Duarte.
A. Russell Flegal/Marine
grain size, i.e. n = 3 for silt-clay sediment and n = 2 for sandy sediment (Iversen and Jorgensen, 1993). Following the grain size analyses, a value of n = 3 was used for the stations located within San Francisco Bay, and II = 2 was used for both Drakes Ester0 and Tomales Bay. The resultant DS was conservatively calculated to range from 7.6 X 10m6 to 9.0 X 1O-6 cm; s-‘. The estimated diffusive benthic fluxes of silver were greater (0.38 and 3.4 X 10m9 mol rn;* day-‘) at the stations in the South Bay (Coyote Creek and Mayfield Slough) affected by wastewater discharges (Table 3). Again, this is consistent with the association of silver with wastewater discharges (Sai’mdoWilhelmy and Flegal, 1992). In contrast, the five less contaminated sites (Oakland Harbor, Castro Cove, Inner Castro Cove and Outer Castro Cove and Pinole Point) in the northern reach of the estuary, had either negligible diffusion of silver out of the sediment or diffusive benthic fluxes of silver into the sediments (Table 3). Therefore, these preliminary calculations indicate that benthic fluxes of silver are into the sediment in the northern reach of the San Francisco Bay estuary. Estimated diffusive benthic fluxes of silver in Drakes Ester0 (0.20 X 10e9 mol rn; 2 day- ’ ) and Tomales Bay (0.33 X 10e9 mol rn;* day-‘) were similar to those for some contaminated sites within South Bay. As previously mentioned, the relatively high dissolved silver concentrations in pore waters from those embayments appear to be, in part, due to the relatively large grain size of the sediment, the relatively low concentrations of particulate manganese, and the complexation of silver with chlorides. This suggests that the relatively high estimates of silver fluxes in those relatively pristine embayments are, at least partially, caused by the formation of soluble chloride complexes within their pore waters. 3.5. Benthic fluxes due to irrigation of the sediments Physical and biological processes can increase diffusive benthic fluxes several times (Hammond et al., 1985; Santschi et al., 1990; Aller and Aller, 1992; ten Hulscher et al., 1992; Caffrey et al., 1996), and bioturbation and mechanical mixing appears to influence the distribution of particulate silver gradi-
Chemistry 56 (1997) 15-26
ents within San Francisco Bay. Since the effects of irrigation could not be determined directly from the pore-water silver concentration gradients, the three zone irrigation model (Hammond et al., 1985) was applied to those gradients in order to estimate this effect. In that model: Fi=
4441 + C’hih;(
-QD,-----dx
C, - C;)
where hi is the thickness of zone i (cm,); ,Viis the irrigation rate constant (s - I >; C, the concentration in the nepheloid layer; and Ci is the average concentration in pore waters throughout zone i (mol cmi3>. The A’,estimated by Hammond et al. (1985) for a shoal station during summer (h; = 2 X 10e6 s- ’ ; h; = 0.52 X 10m6 s- ‘) were used in the calculations. The thickness of both zones were estimated from the pore-water concentration gradients of H,SiO, and NH,+, as described by Rivera-Duarte and Flegal (1997). The three zone irrigation model was not applied in the sediment cores of both Coyote Creek and Mayfield Slough, since no bioturbation was observed at those sites. The fluxes due to irrigation were added to the diffusive fluxes to approximate net benthic fluxes (Table 3). These estimates of the net benthic fluxes of silver are from 1.5- to lo-fold greater than the estimated diffusive benthic fluxes. The relative increases agree with those reported by Aller and Aller (1992) for the increase (2- to lo-fold) due to the activities of macroorganisms in sediments. Again, these calculations indicate benthic fluxes out of the sediments within South Bay, and benthic fluxes into the sediments within the northern reach of the San Francisco Bay estuary. 3.6. Estimated integrated net benthic jluxes Preliminary estimations of the integrated net benthic fluxes of silver indicate that they are a relatively important source of dissolved silver in the South Bay. Sediments in the South Bay are extensively contaminated with silver; as illustrated in Fig. 5 where most of the surficial sediment samples had acid labile (0.5 N HCl leacheates) silver concentrations of > 2.0 kmol kg-’ dry weight. An upper limit calculation of those fluxes was made with the assumption that the benthic fluxes of silver to overly-
I. Riuera-Duarte, A. Russell Flegal/Marine
Chemistry 56 (1997) 15-26
25
4. Conclusions These initial measurements and preliminary calculations substantiate the proposal that benthic sediments are a principal source of silver in San Francisco Bay waters (G.J. Smith and Flegal, 1993). Calculations of the integrated net benthic fluxes estimated from measurements of dissolved silver in pore waters from contaminated sediments in San Francisco Bay are markedly greater than the fluvial input of dissolved silver to the bay. These analyses substantiate the proposal @legal and Safiudo-Wilhelmy, 1993) that contaminated sediments may be an important source of some trace elements to surface waters in some semi-enclosed embayments.
Acknowledgements
Fig. 5. Particulate silver (pmol kg-’ dry weight) distributions in San Francisco Bay. Distributions of the silver concentrations (pm01 kg-’ dry weight) measured in 0.5 N HCl leacheates of surficial sediments, collected in San Francisco Bay in January 1992. Data from A.R. Flegal’s WIGS group at UCSC.
The criticisms and comments from Peggy Delaney, Joris Gieskes, Charles Gobeil, Jim Kuwabara, and the anonymous reviewers are greatly appreciated. This research was funded by the Research Enhancement Program for the San Francisco Bay/Delta of the Interagency Ecological Studies Program and the San Francisco Bay Estuary Project, and the San Francisco Bay Regional Water Quality Control Board.
References ing waters are limited to the shallower regions (5 4 m) of the estuary [232 X lo6 mz in South Bay and 164 X lo6 m* in San Pablo Bay (Fuller, 198211, as conservatively suggested by the silicate data of Hammond et al. (1985). The estimated net diffusive flux of dissolved silver to the South Bay was between 32 and 290 mol yr-’ (Table 3). This preliminary and highly qualified estimate indicates the net diffusive flux of silver could be up to 2.5 times the entire riverine input of dissolved silver ( N 111 mol yr-’ > in San Francisco Bay (G.J. Smith and Flegal, 1993). Moreover, the relative magnitude of the benthic fluxes of silver in San Francisco Bay is projected to become greater as point source discharges of industrial silver to the estuary are reduced, as they have in San Diego Bay @legal and Saiiudo-Wilhelmy, 1993).
Aller, R.C. and Aller, J.Y., 1992. Meiofauna and solute transport in marine muds. Limnol. Oceanogr., 37: 1018-1033. Beattie, P., Drake, M., Jones, .I., Leeman, W., Longhi, J., McKay, G., Nielsen, R., Palme, H., Shaw, D., Takahashi, E. and Watson, B., 1993. Terminology for trace-element partitioning. Geochim. Cosmochim. Acta, 57: 1605-1606. Borovec, Z.C., 1993. Partitioning of silver, beryllium and molybdenum among chemical fractions in the sediments from the Labe (Elbe) River in Central Bohemia, Czech Rep. GeoJournal, 29(4): 359-364. Bruland, K.W.. Coale, K.H. and Mart, L., 1985. Analysis of seawater for dissolved cadmium, copper and lead An intercomparison of voltammetric and atomic absorption methods. Mar. Chem., 17: 285-300. Caffrey, J.M., Cole, B.E., Cloern, J.E., Rudek, J.R., Tyler, A.C. and Jassby, A.D., 1994. Studies of the San Francisco Bay, California, estuarine ecosystem - Pilot regional monitoring results, 1993. U.S. Geol. Surv., Menlo Park, CA, Open-File Rep. 94-82, 412 pp.
26
I. Riuera-Duarte,
A. Russell Flegal /Marine
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