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Research about San Francisco Bay

Source-linked reports with geographic coverage including San Francisco Bay.

At least 307 records · Page 17Linked to original sources
USGS1993

Organic carbon sources and sinks in San Francisco Bay: variability induced by river flow

Sources and sinks of organic carbon for San Francisco Bay (California, USA) were estimated for 1980. Sources for the southern reach were dominated by phytoplankton and benthic microalgal production. River loading of organic matter was an additional important factor in the northern reach. Tidal marsh export and point sources played a secondary role. Autochthonous production in San Francisco Bay appears to be less than the mean for temperate-zone estuaries, primarily because turbidity limits microalgal production and the development of seagrass beds. Exchange between the Bay and Pacific Ocean plays an unknown but potentially important role in the organic carbon balance. Interannual variability in the organic carbon supply was assessed for Suisun Bay, a northern reach subembayment that provides habitat for important fish species (delta smelt Hypomesus transpacificus and larval striped bass Morone saxatilus). The total supply fluctuated by an order of magnitude; depending on the year, either autochthonous sources (phytoplankton production) or allochthonous sources (riverine loading) could be dominant. The primary cause of the year-to-year change was variability of freshwater inflows from the Sacramento and San Joaquin rivers, and its magnitude was much larger than long-term changes arising from marsh destruction and point source decreases. Although interannual variability of the total organic carbon supply could not be assessed for the southern reach, year-to-year changes in phytoplankton production were much smaller than in Suisun Bay, reflecting a relative lack of river influence.

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USGS1992

On the characteristics of local geology and their influence on ground motions generated by the Loma Prieta earthquake in the San Francisco Bay region, California

Strong ground motions recorded at 34 sites in the San Francisco Bay region from the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions. Peak horizontal acceleration and velocity inferred for sites underlain by “rock” generally occur on the transverse component of motion. They are consistently greater with lower attenuation rates than the corresponding mean value predicted by empirical curves based on previous strong-motion data. Theoretical amplitude distributions and synthetic seismograms calculated for 10-layer models suggest that “bedrock” motions were elevated due in part to the wide-angle reflection of S energy from the base of a relatively thin (25 km) continental crust in the region. Characteristics of geologic and geotechnical units as currently mapped for the San Francisco Bay region show that average ratios of peak horizontal acceleration, velocity and displacement increase with decreasing mean shear-wave velocity. Ratios of peak acceleration for sites on “soil” (alluvium, fill/Bay mud) are statistically larger than those for sites on “hard rock” (sandstone, shale, Franciscan Complex). Spectral ratios establish the existence of predominant site periods with peak amplifications near 15 for potentially damaging levels of ground motion at some sites underlain by alluvium and fill/bay mud. Average spectral amplifications inferred for vertical and the mean horizontal motion are, respectively, (1,1) for sites on the Franciscan Complex (KJf), (1.4, 1.5) for sites on Mesozoic and Tertiary rocks (TMzs), (2.1, 2.0) for sites on the Santa Clara Formation (QTs), (2.3, 2.9) for sites on alluvium (Qal), and (2.1, 4.0) for sites on fill/Bay mud (Qaf/Qhbm). These mean values are not statistically different at the 5% significance level from those inferred from previous low-strain data. Analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 km in the cities of San Francisco and Oakland.

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USGS1992

Trace element and organochlorine concentrations in California clapper rail (Rallus longirostris obsoletus) eggs

Trace contaminant concentrations were measured in eggs of endangered California clapper rails ( Rallus longirostris obsoletus ) collected from San Francisco Bay during 1975, 1986, and 1987, and in clapper rail eggs ( R. l. waynei ) from North Carolina during 1987. Residues of 10 detected organochlorines (OC) were low and decreased by 2 to 3-fold from 1975 to 1986–1987 for San Francisco Bay samples, whereas eggshell thickness remained unchanged and comparable to eggs collected prior to 1940. Mercury (Hg) concentrations in eggs from San Francisco Bay were higher than those from North Carolina, and comparable to concentrations associated with reproductive effects in other avian species. Selenium (Se) concentrations in eggs from San Francisco Bay were also higher than those from North Carolina, and were highest in a north San Francisco Bay marsh adjacent to an oil refinery. Individual Se concentrations were slightly lower than values associated with embryotoxicity in other rallids. We recommend additional research on the effects of these and other contaminants on California clapper rails and the restoration of rail habitat.

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USGS1992

Uptake of environmental contaminants by small mammals in pickleweed habitats at San Francisco Bay, California

Small mammals were live-trapped in pickleweed ( Salicornia virginica ) habitats near San Francisco Bay, California in order to measure the uptake of several contaminants and to evaluate the potential effects of these contaminants on the endangered salt marsh harvest mouse ( Reithrodontomys raviventris ). Tissues of house mice ( Mus musculus ), deer mice ( Peromyscus maniculatus ), and California voles ( Microtus californicus ) from nine sites were analyzed for chemical contaminants including mercury, selenium, cadmium, lead, and polychlorinated biphenyls (PCBs). Concentrations of contaminants differed significantly among sites and species. Mean concentrations at sites where uptake was greatest were less than maximum means for the same or similar species recorded elsewhere. Harvest mice ( Reithrodontomys spp. ) were captured only at sites where concentrations of mercury or PCBs were below specific levels in house mice. Additional studies aimed at the protection of the salt marsh harvest mouse are suggested. These include contaminant feeding studies in the laboratory as well as field monitoring of surrogate species and community structure in salt marsh harvest mouse habitats.

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USGS1992

Use of geochemical biomarkers in bottom sediment to track oil from a spill, San Francisco Bay, California

In April 1988, approximately 1500 m3 of a San Joaquin Valley crude oil were accidentally released from a Shell Oil Co. refinery near Martinez, Californa. The oil flowed into Carquinez Strait and Suisun Bay in northern San Francisco Bay Sediment and oil samples were collected within a week and analysed for geochemical marker compounds in order to track the molecular signature of the oil spill in the bottom sediment. Identification of the spilled oil in the sediment was complicated by the degraded nature of the oil and the similarity of the remaining, chromatographically resolvable constituents to those already present in the sediments from anthropogenic petroleum contamination, pyrogenic sources, and urban drainage. Ratios of hopane and sterane biomarkers, and of polycyclic aromatic hydrocarbons and their alkylated derivatives best identified the oil impingement. They showed the oil impact at this early stage to be surficial only, and to be patchy even within an area of heavy oil exposure.

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USGS1992

Bioaccumulation of hydrocarbons derived from terrestrial and anthropogenic sources in the Asian clam, Potamocorbula amurensis , in San Francisco Bay estuary

An assessment was made in Suisun Bay, California, of the distributions of hydrocarbons in estuarine bed and suspended sediments and in the recently introduced asian clam, Potamocorbula amurensis . Sediments and clams were contaminated with hydrocarbons derived from petrogenic and pyrogenic sources. Distributions of alkanes and of hopane and sterane biomarkers in sediments and clams were similar, indicating that petroleum hydrocarbons associated with sediments are bioavailable to Potamocorbula amurensis . Polycyclic aromatic hydrocarbons in the sediments and clams were derived mainly from combustion sources. Potamocorbula amurensis is therefore a useful bioindicator of hydrocarbon contamination, and may be used as a biomonitor of hydrocarbon pollution in San Francisco Bay.

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USGS1992

Hayward fault: Large earthquakes versus surface creep

The Hayward fault, thought a likely source of large earthquakes in the next few decades, has generated two large historic earthquakes (about magnitude 7), one in 1836 and another in 1868. We know little about the 1836 event, but the 1868 event had a surface rupture extending 41 km along the southern Hayward fault. Right-lateral surface slip occurred in 1868, but was not well measured. Witness accounts suggest coseismic right slip and afterslip of under a meter. We measured the spatial variation of the historic creep rate along the Hayward fault, deriving rates mainly from surveys of offset cultural features, (curbs, fences, and buildings). Creep occurs along at least 69 km of the fault's 82-km length (13 km is underwater). Creep rate seems nearly constant over many decades with short-term variations. The creep rate mostly ranges from 3.5 to 6.5 mm/yr, varying systemically along strike. The fastest creep is along a 4-km section near the south end. Here creep has been about 9mm/yr since 1921, and possibly since the 1868 event as indicated by offset railroad track rebuilt in 1869. This 9mm/yr slip rate may approach the long-term or deep slip rate related to the strain buildup that produces large earthquakes, a hypothesis supported by geoloic studies (Lienkaemper and Borchardt, 1992). If so, the potential for slip in large earthquakes which originate below the surficial creeping zone, may now be 1/1m along the southern (1868) segment and ≥1.4m along the northern (1836?) segment. Substracting surface creep rates from a long-term slip rate of 9mm/yr gives present potential for surface slip in large earthquakes of up to 0.8m. Our earthquake potential model which accounts for historic creep rate, microseismicity distribution, and geodetic data, suggests that enough strain may now be available for large magnitude earthquakes (magnitude 6.8 in the northern (1836?) segment, 6.7 in the southern (1868) segment, and 7.0 for both). Thus despite surficial creep, the fault may be ready for the recurrence of large earthquakes today. However, the timing (Williams, 1992) and size of future events may vary greatly due to uncertainties in the tectonophysical model assumed for the Hayward fault within the greater San Andreas fault system (Lisowski and Savage, 1992).

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USGS1991

On the observation, characterisation, and predictive GIS mapping of strong ground shaking for seismic zonation

Tragic earthquakes of the last decade in Mexico, Armenia, and the United States have re-emphasised the importance of local geologic site conditions in determining amounts of damage and consequent loss of life. Extensive data sets in the San Francisco Bay region on strong earthquake ground motions, the damage distributions from past earthquakes, and geologic materials provide the basis to quantify site condition effects for purposes of earthquake hazard mitigation. These observational data are reviewed and analysed to provide methodologies for the characterisation and predictive mapping of potential variations in strong ground shaking for seismic zonation. The methodologies are based on existing geologic maps. They provide a method for seismic zonation applicable to many urbanised seismic regions of the world.

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USGS1990

Arsenic in benthic bivalves of San Francisco Bay and the Sacramento/San Joaquin River Delta

Arsenic concentrations were determined in fine-grained, oxidized, surface sediments and in two benthic bivalves, Corbicula sp. and Macoma balthica , within San Francisco Bay, the Sacramento/San Joaquin River Delta, and selected rivers not influenced by urban or industrial activity. Arsenic concentrations in all samples were characteristic of values reported for uncontaminated estuaries. Small temporal fluctuations and low arsenic concentrations in bivalves and sediments suggest that most inputs of arsenic are likely to be minor and arsenic contamination is not widespread in the Bay.

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USGS1990

Temporally intensive study of trace metals in sediments and bivalves from a large river-estuarine system: Suisun Bay/delta in San Francisco Bay

Distributions in time and space of Ag, Cd, Cr, Cu, Pb and Zn were determined in fine-grained sediments and in the filter-feeding bivalve Corbicula sp. of Suisun Bay/delta at the mouth of the Sacramento and San Joaquin Rivers in North San Francisco Bay. Samples were collected from seven stations at near-monthly intervals for 3 years. Aggregated data showed little chronic contamination with Ag, Zn and Pb in the river and estuary. Substantial chronic contamination with Cd, Cu and Cr in Suisun Bay/delta occurred, especially in Corbicula, compared with the lower San Joaquin River. Salinity appeared to have secondary effects, if any, on metal concentrations in sediments and metal bioavailability to bivalves. Space/time distributions of Cr were controlled by releases from a local industry. Analyses of time series suggested substantial inputs of Cu might originate from the Sacramento River during high inflows to the Bay, and Cd contamination had both riverine and local sources. Concentrations of metals in sediments correlated with concentrations in Corbicula only in annually or 3-year aggregated data. Condition index for Corbicula was reduced where metal contamination was most severe. The biological availability of Cu and Cd to benthos was greater in Suisun Bay than in many other estuaries. Thus small inputs into this system could have greater impacts than might occur elsewhere; and organisms were generally more sensitive indicators of enrichment than sediments in this system.

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USGS1990

Influence of seasonal growth, age, and environmental exposure on Cu and Ag in a bivalve indicator, Macoma balthica, in San Francisco Bay

Temporal and spatial variations in Cu and Ag in the deposit-feeding clam Macoma balthica and in surficial sediments were analysed at 8 stations in San Francisco Bay at near-monthly intervals for periods ranging from 3 to 10 yr during 1977 to 1986. Strong seasonal variations in metal concentrations of M. balthica were associated with seasonal variations in soft tissue weight. Aperiodic fluctuations in metal concentration appeared to be driven by changes in metal content of the soft tissues. Metal content of clams of standard shell length was less variable than tissue metal concentration, and generally followed changes in the concentrations of Cu and Ag in the sediments. Correlations between metal content and sediment concentrations were improved when content was standardized to age rather than shell length. Metal content of M. balthica displayed few consistent temporal trends among stations, evidently reflecting different sources of input and complex hydrologic and geochemical processes affecting metal availability in San Francisco Bay. Increases in Cu and Ag were noted at several stations in South Bay during 1977 to 1980. A continuous 10 yr record at one of these stations showed that the 1977 to 1980 increase and the subsequent decline beginning in 1981 coincided with fluctuations in metal inputs from a nearby source.

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