Silver in San Francisco Bay Estuarine Waters

Estuaries

Vol. 16, No. 3A, p. 547-558

September

1993

Silver in San Francisco Bay Estuarine Waters GEOFFREY J. SMITH A.

RUSSELL

FLEGAL

Institute of Marine Sciences University of Calijomia Santa Cruz, Calfomia 95064 ABSTRACT: Spatial gradients of silver concentrations in the surface waters of San Francisco Bay reveal substantial anthropogenic perturbations of the biogeochemical cycle of the element throughout the estuarine system. The most pronounced perturbations are in the south bay, where dissolved (~0.45 pm) silver concentrations are as high as 250 pM. This is more than one order-of-magnitude above baseline concentrations in the northern reach of the estuary (6 PM) and approximately two orders-of-magnitude above natural concentrations in adjacent coastal waters (3 PM). The excess silver is primarily attributed to wastewater discharges of industrial silver to the estuary on the order of 20 kg d-l. The contamination is most evident in the south bay, where wastewater discharges of silver are on the order of 10 kg d-’ and natural freshwater discharges are relatively insignificant. The limited amount of freshwater flushing in the south bay was exacerbated by persistent drought conditions during the study period. This extended the hydraulic residence time in the south bay (2160 d), and revealed the apparent seasonal benthic fluxes of silver from anthropogenically contaminated sediments. These were conservatively estimated to average z 16 nmol m-* d-’ in the south bay, which is sufficient to replace all of the dissolved silver in the south bay within 22 d. Benthic fluxes of silver throughout the estuary were estimated to average = 11 nmol m-* d-l, with an annual input of approximately 540 kg yr-’ of silver to the system. This dwarfs the annual fluvial input of silver during the study period (12 kg yrrl), and is equivalent to approximately 10% of the annual anthropogenic input of silver to the estuary (3,700-7,200 kg yrr’). It is further speculated that benthic fluxes of silver may be greater than or equal to waste water fluxes of silver during periods of intense diagenic remobilization. However, all inputs of dissolved silver to the estuary are efficiently sorbed by suspended particulates, as evidenced by the relatively constant conditional distribution coefficient for silver throughout the estuary (K, z 105).

periods in 1989. Episodic drought conditions are a natural feature of the area, but the impacts of recent droughts have been exacerbated by systematic increases in the diversion and moderation of freshwater discharges to the estuary (Nichols et al. 1986). As a result, both natural and anthropogenic factors contributed to the low flow conditions (130330 m3 s-l) that persisted throughout the study period from October 1988 to September 1989, with the exception of an ephemeral flow of 1,100 ms s-r in March 1989 (California Department of Water Resources unpublished data). This extended the hydraulic residence time to ~60 d in the northern reach and 1160 d in the southern reach (Smith 1987). It precluded the seasonal hydraulic flushing of the south bay, which is caused by the intrusion of high flow discharges (1 ,OOO-10,000 m3 SK’) from the northern reach of the estuary (Peterson et al. 1989). It also increased the relative amount of wastewater discharged to the south bay to at least one order of magnitude above the freshwater runoff into the south bay during the study period. Consequently, the following data are considered to be representative of only one of several possible hydrological conditions, especially in the south bay. That assumption is based on reports of anomalous conditions associated with previous low flow

Introduction STUDY AREA The study was conducted in the San Francisco Bay estuarine system, which originates in the Sacramento River-San Joaquin River Delta (Fig. 1). It is both a naturally complex estuary (Conomos 1979) and a severely impacted urban estuary (Nichols et al. 1986). The system is composed of three distinct areas, which are characterized by their unique hydrographic and geographic features: the northern reach, which extends from the fresh waters of the Sacramento and San Joaquin rivers to the intermediate salinity waters of San Pablo Bay, the central bay that connects with the Pacific Ocean, and the southern reach (Conomos 1979). The later region, which is referred to as the south bay, is essentially a high salinity [ 220 practical salinity units (psu)] lagoon with negligible natural freshwater inputs (1 x 10” m3 yr-I) from local tributaries, compared to the sum of freshwater inputs (2.1 x 1016 m3 yr-‘) to the entire system (Cheng and Gartner 1984). The south bay is also the area that is most impacted by anthropogenic inputs (5 x 10” m3 yr-‘) from wastewater discharges (Davis et al. 1991). The impact of those wastewater discharges was amplified by a protracted drought, which developed in 1987 and extended through the sampling 0

1993 Estuarine Research Federation

547

0160-6347/93/03A0547-12$01.50/O

548

G. J. Smith and A. Ft. Flegal

periods. These include observations of atypical nutrient and biological cycles during earlier droughts (Conomos 1979; Cloern et al. 1983,1985; Scheme1 et al. 1984; Cloern and Nichols 1985; Nichols 1985; Peterson et al. 1985, 1986). As previously reported (Flegal et al. 1991), atypical nutrient distributions were correlated with other trace element (Cd, Co, Cu, Fe, Ni, and Zn) distributions during the sampling period for this study. Consequently, this report is based primarily on comparisons with analyses of complementary data in that preceding report (Flegal et al. 1991). Anthropogenic Inputs of Silver to San Francisco Bay A summary of point source loadings of silver to San Francisco Bay (Davis et al. 199 1) indicates that comparable amounts of industrial silver are discharged into the northern (6.2-9.9 kg d-l) and southern (3.9-9.7 kg d-r) regions of the estuary. Point source loadings of silver in the northern reach include discharges into the Sacramento River-San Joaquin River Delta (52.6 kg d-l), Suisun Bay (0.31 .l kg d-l), central bay (1.0-2.4 kg d-l), and San Pablo Bay (2.3-3.8 kg d-r). The relatively high inputs of industrial silver to the latter bay are attributed to discharges of wastewater effluents with high silver concentrations (> 100 pg 1-l) from a relatively small (3 x 10’ 1 d-l) municipal treatment plant into the Napa River. Point source loadings of silver in the southern reach include discharges into the upper south bay (2.2-7.0 kg d-l), where there are major outfalls off Oakland and San Francisco; the middle south bay (0.2 kg d-l); and the lower south bay (1.5-1.6 kg d-l), where there are major wastewater outfalls off Palo Alto and San Jose. Details on those inputs are provided in the report by Davis et al. (1991). There has been no comparable report that quantifies nonpoint source inputs of industrial silver to the estuary (i.e., urban runoff, nonurban runoff, and atmospheric deposition), but those inputs are considered to be relatively insignificant. This is based on calculations of natural and anthropogenic fluxes of silver to the Southern California Bight (Saiiudo-Wilhelmy and Flegal 1992), which demonstrate that wastewater discharges account for essentially all (>95%) of the industrial silver in those coastal waters. Our assumptions that nonpoint source inputs of industrial silver to the San Francisco Bay estuary are insubstantial (X5%) are qualified by the United States Environmental Protection Agency report (1983) that silver concentrations of urban runoff are high (0.2-0.8 pg 1-l). Therefore, estimates of industrial silver inputs to the San Francisco Bay estuary may be conservatively low.

Fig. 1. Index map of the San Francisco Bay estuary showing station location numbers during April, August, and December 1989.

Silver Contamination in San Francisco Bay Sediments and Benthic Organisms Anomalously high concentrations of silver in San Francisco Bay sediments delineate substantial inputs of industrial silver to the estuary over a protracted period (Luoma and Cloern 1982; Luoma and Phillips 1988; Luoma et al. 1985, 1990). Spatial gradients of silver concentrations in sediments positively correlate with their proximity to point source loadings. Total silver concentrations (derived from aqua regia digestions) of sediments in the south bay are as high as 5 /*g g-l (dry weight) in areas adjacent to wastewater outfalls (Luoma and Cloern 1982). Dilute acid (0.5 N HCl) labile concentrations (dry weight) of silver in sediments range from 0.04 Fg g-l in San Pablo Bay to 0.4 pug g-l in the south bay; and they increase up to 0.9 sediments of pug g-l in the highly contaminated Islais Creek, which drains into the south bay (AbuSaba, Rivera and Flegal, unpublished data). [The weak acid leach provides a measure of the bioavailable concentration of silver in sediments (Jenne and Luoma 1977), and is consistent with the extraction used to measure trace element concentra-

Dissolved Silver in San Francisco Bay

549

tions associated with acid volatile sulfides (DiToro et al. 1990).] In summary, total silver concentrations of sediments in the estuary are more than one order-of-magnitude (60 fold) above the average abundance (0.080 pg g-l) of silver in the continental crust (Taylor and McLennan 1985), and concentrations of bioavailable silver in sediments in the south bay are more than two orders-of-magnitude (225 fold) above baseline concentrations of bioavailable silver in sediments in the San Francisco Bay estuary. Comparable levels of silver contamination are found in benthic organisms within the south bay, where concentrations are at least one order of magnitude above the baseline concentrations of estuarine organisms (Bradford and Luoma 1980; Cain and Luoma 1985, 1990; Luoma et al. 1985, 1990; Smith et al. 1986; Luoma and Phillips 1988). Silver concentrations in tellenid clams (1Clacoma balthica) range from 7 pg g-l to 117 pg g-l (dry weight) within the estuary, and concentrations in the south bay have been reported that exceed concentrations of clams in 37 European estuaries (Luoma and Cloern 1982). Silver concentrations in mussels (Myths edulus and 111.calzjkianus) from the south bay (Luoma and Cloern 1982; Smith et al. 1986) are also greater than or equal to those observed in clams from 63 locations surveyed in North American estuaries and northeast Pacific coastal waters (Goldberg et al. 1983). This indicates that the extent of silver contamination in benthic organisms within the south bay may be greater than or equal to that of any other estuary (Luoma and Cloern 1982; Luoma and Phillips 1988).

have limited primary productivity, reduced species diversity of the benthos, and contributed to the decline in fisheries throughout the estuary (Luoma and Cloern 1982). However, there have not been sufficient data to determine whether dissolved silver concentrations in the San Francisco Bay approach toxic threshold concentrations. Thermodynamic models of silver speciation in San Francisco Bay (Jenne et al. 1978; Cowan et al. 1985) used silver concentration data (Girvin et al. 1978) that were not intercalibrated or published in a refereed journal. In fact, we are not aware of any original data on silver concentrations in estuarine water in any refereed publication. [Manuscripts on trace element cycles in Texas estuaries that include silver data are in press (Morse et al. 1992; Benoit et al. 1993). As a result, our limited understanding of the biogeochemical cycle of silver in estuaries is based on analyses of sediments and benthic organisms, thermodynamic models, and laboratory studies with elevated silver concentrations or radiotracers (“OrnAg) that may not mimic estuarine conditions. Therefore, this study was designed to measure the partitioning of dissolved (co.45 pm) and particulate (~0.45 pm) silver within the San Francisco Bay estuary and to compare those distributions with the limited amount of data on silver concentrations in seawater and with the biogeochemical cycles of silver predicted by other techniques.

Silver Toxicity in San Francisco Bay

Near surface (” 1 m) samples were collected for measurements of total dissolved (co.45 pm) and total (unfiltered) silver concentrations at 27 locations in the estuary (Fig. 1). Table 1 lists the station numbers, location names, and latitude and longitude for the collections, which were replicated in April, August, and December 1989. The samples were collected aboard the R/V Scrutiny (United States Bureau of Reclamation) with a Teflon@ (PFA) and C-F1 ex@ peristaltic pumping system that had been cleaned with high purity reagents (subboiling quartz-distilled HNO, and HCl). The intake was suspended 1 m below the surface with an aluminum pole that was extended 5 m upstream of the ship’s drift. Acid-cleaned polypropylene filter cartridges (0.45 pm) were attached to the outlet of the pumping system for all filtered water samples. The cartridges were then removed, and the unfiltered water samples were collected in a similar manner. This sampling system has been intercalibrated with other trace metal sampling systems (i.e., teflon-coated Go-Flo’s@ and the Caltech deep wa-

The high levels of silver contamination in the south bay are of concern because silver is extremely toxic to some organisms. Silver ions competitively inhibit the synthesis of copper plastocyanins in green plants, eukaryotic algae, and cyanobacteria. It is also extremely toxic to microorganisms and marine invertebrates (Bryan 1971). This is attributed to the rapid uptake of silver ions on biological surfaces, the rapid intracellular accumulation of siiver, the affinity of silver for complex organic constituents, and its resultant interference with copper metabolism. As a consequence of the high concentrations of silver in the south bay and its acute toxicity to some organisms, it is recognized as one of the most probable causes of toxicity in the estuary (Luoma and Phillips 1988). Metal-induced stress among benthos in the south bay has been specifically associated with elevated silver concentrations in the sediments (Luoma and Phillips 1988). It has also been proposed that elevated silver concentrations may

Materials and Methods SAMPLE COLLECTION AND STORAGE TECHNIQUES

550 TABLE

G. J. Smith and A. R. Flegal 1.

Station numbers, locations, salinities, and total suspended solids for April, August, and December Total

Station Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Salinity Location

Name

Extreme South Bay Dumbarton Bridge Redwood Creek San Bruno Shoals Hayward Flats San Francis0 Airport San Leandro Channel Hunters Point Berkeley Flats Golden Gate Alcatraz Island Angel Island ‘San Rafael Bridge Nearshore San Rafael Bridge Channel San Pedro Point Petaluma River Pinole Shoal Channel Pinole Shoal Nearshore Benicia Bridge Pacheco Creek Grizzly Bay Port Chicago Honker Bay Stake Point Chipps Island New York Slough Sacramento River

Latitude North

37.29 37.30 37.33 37.37 37.38 37.37 37.45 37.43 37.50 37.49 37.50 37.50 37.55 37.55 37.59 38.02 38.03 38.01 38.02 38.02 38.06 38.03 38.04 38.03 38.02 38.01 38.03

Lon itude v$ est

122.05 122.07 122.11 122.17 122.13 122.20 122.18 122.20 122.20 122.28 122.25 122.23 122.24 122.26 122.26 122.24 122.19 122.19 122.08 122.05 122.02 122.01 121.56 121.57 121.55 121.51 121.51

ter sampler), as previously detailed (Flegal and Stukas 1987). All samples were aliquoted into acidcleaned polyethylene (LDPE) bottles. The samples were acidified to pH 1.5-2.0 with sub-boiling quartz-distilled (2 X) HCl (Q-HCl) for at least 1 mo prior to extraction. Our initial treatment raises an important qualification on the terminology used in this report, in addition to the obvious qualification of using “total dissolved” to characterize silver concentrations of solutions that were passed through a 0.45 pm filter. Silver concentrations of the unfiltered samples might be more appropriately termed “near-total,” rather than “total” concentrations, because the measurements were not preceded by a rigorous digestion with aqua regia and HF. This is demonstrated by the incomplete recoveries of silver in dilute acid extractions (0.5 N HCl) of certified reference sediments (BCSS-1, MESS-l, and PACS-1; National Research Council of Canada) and San Francisco Bay sediments (Abu-Saba, Rivera, and Flegal unpublished data). They might also be termed “biologically available,” which is the term applied to concentrations of marine sediments that are extracted with dilute (0.5-1.0 N) HCl (Jenne and Luoma 1977; Luoma and Bryan 1982; Luoma and Phillips 1988). For simplicity, we have retained the term “total” concentration throughout this re-

April

Suspended

(psu) December

24.7 25.0 25.0 25.6 25.8 25.9 25.9 26.9 27.1 28.8 28.0

29.9 30.4 31.3 31.4 31.4 31.4 31.5 31.4 30.5 32.2 31.6

24.7 27.9 22.2 21.0 17.5 17.9 17.5 9.2 8.6 3.0 1.4 0.7

30.0 31.2 27.7 28.1 22.6 20.4 22.3 11.7 10.7 6.5 5.4 3.4 2.5 1.8 0.7 0.2

April

August

30.1 30.4 30.6 30.5 30.5 30.7 29.4 30.4 29.8 32.1 32.3

78.3 18.9

14.2 15.9 19.1 21.7 106.9 9.0 5.2 5.4 18.3 3.9 3.0

29.8 31.3 28.7 25.4 26.5 14.3 14.0 11.6 10.6 9.4

8.0 12.0 17.8 111.1 12.8 25.3 21.5 35.8 71.7 63.1 96.6

7.9 6.4 6.2

46.8 45.4 54.5

port with the qualification vatively low. SAMPLE

Solids

(mg kg-‘)

August

0.4 0.2 0.2

1989.

9.6 13.0 8.4 13.8 2.7 9.1 3.8 !:Z

17.5 5.1 8.0 6.9 29.8 16.7

79.4 24.4 17.6 10.0 10.4 18.9 47.5 8.5 11.1 11.1 !.Z 11:7 20.3 34.8 52.3 17.4 10.5 19.9 35.8 12.4 13.5

that it may be conser-

PRECONCENTRATION

ANALYTICAL

13.1 5.9 29.7 18.1 13.8 28.0 10.2 10.2 13.9

December

AND

TECHNIQUES

Silver was preconcentrated using a modified ammonium 1-pyrrolidine dithiocarbamate/diethylammonium diethyldithiocarbamate (APDC/ DDDC) organic extraction. The procedure has been described by Bruland et al. (1985), and has been used to measure silver concentrations in seawater (Martin et al. 1983; Saiiudo-Wilhelmy and Flegal 1992; Flegal, Saiiudo-Wilhelmy, and Scelfo unpublished data; Flegal, Scelfo, Safiudo-Wilhelmy, and Smith unpublished data). Silver concentrations were measured by graphite furnace atomic absorption spectrometry (GFAAS) with stabilized temperature platform furnace techniques and the method of standard additions. Precision for replicate analyses was typically within 5%. Concurrent analyses of a coastal seawater standard reference material (CASS-2, no certified silver concentration given; National Research Council of Canada) provided a silver concentration of 48 rt 2 pM (n = 3) (Flegal, Saiiudo-Wilhelmy, and Scelfo unpublished data; Flegal, Scelfo, Safiudo-Wilhelmy, and Smith unpublished data). The average method detection limits for April, August, and December 1989 cruis-

551


TABLE 2. Total dissolved (CO.45 pm), unfiltered, suspended particulate and log distribution April, August, and December 1989. Suspended

station NWW ber

: 3 4 5 Is 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Dissolved

Silver

(PM) Decem-

Location

Extreme South Bay Dumbarton Bridge Redwood Creek San Bruno Shoals Hayward Flats San Francisco Airport San Leandro Hunters Point Berkeley Flats Golden Gate Alcatraz Island Angel Island San Rafael Bridge Nearshore San Rafael Bridge Channel San Pedro Point Petaluma River Pinole Shoal Channel Pinole Shoal Nearshore Benicia Bridge Pacheco Creek Grizzly Bay Port Chicago Honker Bay Stake Point Chipps Island New York Slough Sacramento River

April August ber 35 24 39 31 26 50 35 31 26 28 22 33 27 25 29 25 28 29 25 28 23 24 22 24 21 23

69 85 172 224 243 198 143 118 39 36 56 56 36 61 62 70 57 52 23 13 11 BDL BDL BDL BDL 6 6

Total

15 15 15

150 130 170

es were 2.3, 8.5, and 4 pM, respectively. The average procedural blanks for each extraction ranged from less than 1 pM to 4.8 pM. Samples from stations 26 and 27 for August 1989 with dissolved (X0.45 pm) silver concentrations of 6 pM were extracted with a method detection limit of 3.3 pM. Suspended particulate silver concentrations were derived by subtracting the dissolved silver concentrations from the total silver concentrations and dividing by the suspended load at each location on each cruise. ANCILLARY MEASUREMENTSAND MULTIVARIATE ANALYSES

The silver data in this report were obtained from samples analyzed for other trace element and nutrient concentrations which have been reported previously (Flegal et al. 1991). The preceding report included dissolved (~0.45 pm) concentrations of cadmium, cobalt, copper, iron, nickel, and zinc, which were measured with the trace metal clean procedures employed for the silver analyses. It also included dissolved nutrient (P0,3-, H,Si04 and NO,- + NO,-) an d c hl orophyll a concentrations, which were measured with the standard procedures detailed by Parsons et al. (1984); salinity concentrations, which were measured with an induc-

Particulate (egg-‘)

310 270 420 480 370 230 330 230 40 120 160 180 130 140 270 250 180 100 110 100 70 110 90 80 60 60

930 510 310 290 220 490 310 550 130 90 150 100 120 120 200

values for silver during

Silver

Log Distribution Coefficient (log Kd)

Decem-

D.XlXIl-

25 30 18 22 18 22 15

;: 30 18 24 29 34 28 37

(PM)

April August ber 710 160 160 130 120 130 110 70 70 30 30 120 60 60 50 300 70 120 100 270 240 190 280

80 89 89 116 126 133

Silver

coefficient

April

0.93 0.78

August

1.83 1.25 1.40 1.27

ber

April

1.16 1.86 1.35 1.88 0.98 2.04

5.40 5.48

200 270 90 110 80 50 170

1.12 0.78 1.03 0.59 1.55 0.53 0.06 0.17 1.23 0.44 0.32 0.13 0.27 0.35 0.39 0.37 0.73 0.33 0.28 0.29

0.82

0.29 0.84

5.53 5.46 5.29 5.20 5.67 5.28 4.31 4.85 5.55 5.18 5.08 4.62 4.99 5.07 5.10 5.13 5.38 5.12 5.05 5.08

100 70 60

0.29 0.26 0.29

0.79 0.57 0.42

0.26 0.48 0.36

5.06 5.05 5.08

2.06 1.80 4.24 1.13 0.10 2.29 2.21 0.89 0.93 1.22 0.72 1.25 0.64 1.77 0.32

1.09 1.27 0.70 1.23 1.13 1.29 0.85 0.86 0.54 0.49 0.55

AUgW

December

5.39 5.14 4.88 4.72 4.98 5.07 5.52 5.43 4.40 5.58 5.57 5.36 5.15 5.26 4.98 5.30 5.42 6.10 5.44

5.13 5.29 5.15 5.18 4.86 5.15 5.15 5.59 5.56 5.69 5.56 5.55 5.45 5.33 5.30 5.18 5.37 5.09 5.70

5.95 5.84

5.22 5.49 5.36

tive salinometer calibrated with IAPSO standard seawater; and temperature, dissolved oxygen, conductivity, and pH, which were measured in situ with portable meters. The salinity data have been reproduced for this report in Table 1, which also includes the concentrations of total suspended solids (TSS). Results and Discussion Results of the silver analyses for total dissolved (~0.45 pm), total (unfiltered), and suspended particulate silver concentrations are listed in Table 2. The conditional distribution coefficients (log Kd) for silver are also listed in that table. All of the data are reported by sampling stations for each of the cruises in April, August, and December 1989. Data for complementary parameters have been reported previously (Flegal et al. 1991). TOTAL DISSOLVED(~0.45 PM) SILVER CONCENTRATIONS The lowest dissolved (co.45 trations (16 PM) were found

pm) silver concen-

in the low salinity (15 psu) waters near the confluence of the Sacramento-San Joaquin Delta (stations 2 l-27) in August 1989. Although they were anomalous for estuarine waters of San Francisco Bay, they were

552

G. J. Smith

and

A. R. Flegal

of silver in similar to the natural concentration oceanic surface waters of the northeast Pacific (l2.4 PM) (Martin et al. 1983) and the baseline concentration of silver in neritic surface waters of the Southern California Bight (3 PM) (Safiudo-Wilhelmy and Flegal 1992). This indicates that silver concentrations at some of the low salinity locations in the San Francisco Bay estuary may approach natural concentrations. However, most of the dissolved silver concentrations in the estuary were substantially higher than natural concentrations in the marine environment. The average concentration in the estuary (49.9 * 49.2 PM) was greater than the highest concentration in contaminated waters of the Southern California Bight (39 PM), which were enriched (I 90%) with industrial silver from wastewater discharges (Saiiudo-Wilhelmy and Flegal unpublished data). The highest silver concentration in San Francisco Bay (243 PM) approached the highest concentration (3 10 PM) in contaminated waters of San Diego Bay (Safiudo-Wilhelmy and Flegal 1992). These comparisons indicate that industrial inputs account for at least half (~50%) of the dissolved silver throughout the San Francisco Bay estuary, most (180%) of the dissolved silver in the central bay, and essentially all (199%) of the dissolved silver in the most contaminated areas of the south bay. TOTAL DISSOLVED (CO.45 PM) SILVER CONCENTRATION GRADIENTS

Temporal and spatial variations of total dissolved (co.45 pm) silver concentrations in the estuary (Table 2) are illustrated in Fig. 2. The concentrations are plotted as a function of salinity with different characters to distinguish samples collected in April (0), August (V), and December 0. Samples collected from the northern reach (stations 9-27) are distinguished from samples collected from the south bay (stations 1-8) with open and solid characters, respectively. This is because the silver data evidenced two distinct biogeochemical regimes within the San Francisco Bay estuarine system. Northern

Reach

Total dissolved (co.45 pm) silver concentrations in the northern reach were relatively constant during the initial and final collection periods, and were elevated during the intermediate period (Fig. 2). The concentrations ranged from 21 pM to 33 pM with a mean concentration of 26 * 3 pM in April (1989); from 15 to 37 pM with a mean concentration of 23 f 7 pM in December (1989); and from 2

200

-

WI

125-

0

loo-

& I-

.

150

z

a

.-

175-

: rnv

75

.

?

-

50.

ai

O 0

-

.

"

5

6&n% .

B ” 10

g&A;

:

@ ” 15

SALINITY

” 20

” 25

” 30

35

(psu)

Fig. 2. Total dissolved(~0.45 Mm)silver concentrations(PM) versus salinity(practical salinityunits) for April (0), August @), and December (0) 1989. South San Francisco Bay stations l8 shown with filled symbols.

reflect temporal and spatial differences in point source discharges, biogeochemical scavenging, benthic fluxes, and hydraulic mixing within the northern reach. They also suggest mixing in the central bay with more contaminated waters from the south bay, which is discussed in the following section. While the distributions were relatively linear along the salinity gradients in the northern reach, mass balance calculations indicate small net internal source(s) and sink(s) of silver in August 1989. [The assumptions and limitations of the mass balance calculations are discussed in Flegal et al. 199 1.1 These show a net loss of silver in the north bay (-0.09 kg d-l) an d a net gain of silver in the central bay (+0.52 kg d-l) during that period. The loss is tentatively attributed to an excess of biogeochemical scavenging and the gain is tentatively attributed to an excess of anthropogenic inputs. The latter may reflect either direct inputs from wastewater discharges or indirect inputs from benthic fluxes of silver from contaminated sediments, as discussed in the following section. The excess silver in the Central Bay is primarily attributed to point source discharges of silver to the central bay and adjacent waters of San Pablo Bay and (upper) south bay. The amount of industrial silver in those discharges is estimated to range from 5.5 kg d-l to 13.2 kg d-l, which represents approximately half (5 l-67%) of the anthropogenic loading of silver (lo-19 kg d-l) to the entire estuary (Davis et al. 1991). However, those inputs must be rapidly scavenged, because the measurable silver excess only occurred during one of the three sampling periods and constituted less than one tenth of the amount of industrial silver discharged into the area daily.


South Bay The two-fold increase of silver concentrations in the central bay in August 1989 was negligible compared to the ten-fold increase in concentrations in the south bay during the same period. Silver concentrations ranged from 24 pM to 50 pM, with a mean concentration of 34 & 8 pM, in April 1989; from 25 pM to 126 pM, with a mean concentration 91 f 35 pM, in December 1989; and from 69 pM to 243 pM, with a mean concentration 157 + 64 pM, in August 1989. The spatial and temporal variations of silver concentrations were consistent with those of other trace elements and nutrients in the south bay. The latter reflected elevated inputs from numerous sources (e.g., wastewater discharges, surface runoff, benthic fluxes, and upwelling) and limited dilution from natural freshwater discharges (Flegal et al. 199 1). Although wastewater discharges are generally considered to be the primary source of excess silver in the estuary, the seasonal enrichment of silver in the south bay may reflect inputs from contaminated benthic sediments. This was suggested by similarities in the temporal variations of dissolved silver (Fig. 2) and dissolved silicate concentrations (Fig. 2 in Flegal et al. 199 1) in the south bay, where benthic fluxes of silicate are relatively substantial (Hammond et al. 1985). This is consistent with the highly significant (p < 0.0 1, simple linear regression) correlation of silver with silicate in oceanic waters, which demonstrates that silver is cycled with biogenic particulate matter (Flegal et al. 1992). It is also consistent with the covariance of cobalt and zinc with silicate in the south bay (Flegal et al. 1991). Correlations for simple, linear regressions between total dissolved silver and dissolved silicate concentrations in the south bay (stations l-8) were relatively low in April (r = 0.20), August (r = 0.64), and December (r = 0.3 1); but silver was most closely correlated with silicate in multivariate statistical analyses using the program SYSTAT (Wilkinson 1987). [Assumptions and limitations of these analyses have been summarized previously (Flegal et al. 199 l).] Loadings of dissolved silver (0.690) and silicate (0.963) were high in the principal component (58% total variance) in factor analyses of numerous constituents (TSS, salinity, and dissolved Ag, Cd, Hg, N02- + NOs-, P049-, Pb, SiOZ2-) in the south bay (stations l-8) during the three sampling periods. Dissolved silver was also most closely associated with dissolved silicate in the corresponding dendrogram derived with the average linkage method. This covariance indicates that the diagenic remobilization of silver from contaminated sediments contributed to the anomalously high concentrations of silver in the south bay.

553

ESTIMATED BENTHIC FLUXES OF DISSOLVED SILVER The amount of silica remobilized from benthic sediments in the south bay was measured by Hammond et al. (1985). Their analyses indicated that benthic silica fluxes average 5.6 + 1.1 mmol m-2 d-l, which is comparable to silica fluxes in Narragansett Bay (6.6 mmol m-2 d-l; Nixon 1981) and Chesapeake Bay (7.4 mmol m-* d-l; D’Elia et al. 1983). Those fluxes are sufficient to replace the standing stock of silica in the water column within 17 d in the shoals of the south bay, which represent 80% of its surface area, during a low flow period (