A. Russell Flegal

Mercury speciation in the San Francisco Bay estuary

In order to understand the biogeochemistry of mercury in the San Francisco Bay estuary, California, we undertook a 2-year study of sediments and surface waters to investigate sources of mercury to the estuary and the processes that control its distribution. Over several seasons, unfiltered and filtered (<0.45 μm) surface water samples were collected from 26 sites in the estuary and analyzed for total mercury (HgT), monomethylmercury (MMHg), and ancillary water quality parameters. Concentrations of mercury in unfiltered surface water UHgT ranged from 0.73 to 440 pM, and were well correlated with suspended particulate matter (SPM). Concentrations of UHgT were typically highest in the winter and spring, when precipitation, fluvial discharges, and SPM were greatest. Although the concentrations of UHgT appear elevated with respect to other contaminated estuaries, this is largely explained by the high SPM in this system. The average concentration of mercury in suspended particles was 1.8±0.6 nmol g−1, and was similar to the concentration of HgT in sediments (1.1±0.7 nmol g−1), especially when normalized to grain size (%<63 μm; 1.7±0.7 nmol g−1), demonstrating the preferential resuspension of fine-grained sediment, and the importance of wind and tidal driven resuspension in controlling UHgT concentrations. Concentrations of dissolved gaseous mercury averaged 0.90±0.88 pM, and the estuary was a net source of mercury to the atmosphere of 40–240 kg year−1. The concentrations of surface water MMHg ranged from <0.050 to 2.3 pM, and appeared to be controlled by input from the Sacramento–San Joaquin delta in the northern reach and by wastewater inputs in the extreme southern reach. Sediment mercury concentrations were significantly correlated with the fraction of fine-grained material present and the concentrations of total organic carbon. Concentrations of MMHg in the sediments ranged from 0.5 to 5.0 pmol g−1, with an average of about 2 pmol g−1; the highest concentrations were found in areas with detectable pore water sulfide, high nutrient input from wastewater, and low salinity. Although the mercury in this system is principally attributed to contamination from historic mining activity, concentrations were similar to those of other large, urbanized estuaries that are primarily contaminated with recent industrial inputs.

DISTRIBUTION OF COLLOIDAL TRACE METALS IN THE SAN FRANCISCO BAY ESTUARY

The size distribution of trace metals (Al, Ag, Cd, Cu, Fe, Mn, Ni, Sr, and Zn) was examined in surface waters of the San Francisco Bay estuary. Water samples were collected in January 1994 across the whole salinity gradient and fractionated into total dissolved (<0.2 μm colloidal (10 KDa–0.2 μm) and < 10 kDa molecular weight phases. In the low salinity region of the estuary, concentrations of colloidal A1, Ag, and Fe accounted for ≥84% of the total dissolved fraction, and colloidal Cu and Mn accounted for 16–20% of the total. At high salinities, while colloidal Fe was still relatively high (∼40% of the dissolved), very little colloidal Al, Mn, and Cu (<10%) and no colloidal Ag was detectable. Colloidal Zn accounted for <3% of the total dissolved along the estuary, and colloidal Ni was only detectable (<2%) at the river endmember. All of the total dissolved Cd and Sr throughout the estuary consisted of relatively low molecular weight (<10 kDa) species. The relative affinity of metals for humic substances and their reactivity with particle surfaces appear to determine the amounts of metal associated with colloids. The mixing behavior of metals along the estuary appears to be determined by the relative contribution of the colloidal phase to the total dissolved pool. Metals with a small or undetectable colloidal fraction showed a nonconservative excess (Cd, Cu, Ni, and Mn) or conservative mixing (Sr) in the total dissolved fraction, relative to ideal dilution of river water and seawater along the estuary. The salt-induced coagulation of colloidal A1, Fe, and Cu is indicated by their highly nonconservative removal along the salinity gradient. However, colloidal metals with low affinity for humic substances (Mn and Zn) showed conservative mixing behavior, indicating that some riverine colloids are not effectively aggregated during their transport to the sea. While colloidal metal concentrations correlated with dissolved organic carbon, they also covaried with colloidal Al, suggesting that colloids are a mixture of organic and inorganic components. Furthermore, the similarity between the colloidal metal:A1 ratios with the crustal ratios indicated that colloids could be the product of weathering processes or particle resuspension. Distribution coefficients for colloidal particles (Kc) and for large, filter-retained particles (Kd) were of the same magnitude, suggesting similar binding strength for the two types of particles. Also, the dependence of the distribution coefficients on the amount of suspended particulate matter (the so-called particle concentration effect) was still evident for the colloids-corrected distribution coefficient (Kp+c) and for metals (e.g., Ni) without affinity for colloidal particles.

Organic complexation and total dissolved trace metal analysis in estuarine waters: comparison of solvent-extraction graphite furnace atomic absorption spectrometric and chelating resin flow injection inductively coupled plasma-mass spectrometric analysis

The measured concentrations of cadmium, cobalt, copper, nickel, lead, zinc, and manganese in acidified (pH 0.961, simple linear regression) to the dissolved organic carbon (DOC) concentration of the samples. Good agreement between the two methods for cobalt and copper was achieved after ultraviolet (UV) digestion of the acidified samples. Samples collected from the South Bay of the San Francisco Estuary with high DOC showed the greatest difference for cobalt, copper, and nickel which is tentatively attributed to complexation with humic material for copper and cobalt and strong synthetic chelating agents such as ethylenediaminetetraacetic acid (EDTA) for nickel. This is consistent with previous studies on copper, nickel and cobalt complexation in this region. We recommend UV digestion of acidified estuarine samples prior to multi-element analysis by chelating resin flow injection ICP-MS methods.

A synthesis of lead isotopes in two millennia of European air

Four airborne particulate records from ombrotrophic peat bogs in southern Norway, extending back 300 years, have been measured for chronology, lead concentration, and lead isotope composition. Since southern Norway receives an airborne lead signal that accumulates emissions from the European continent, the trend in the four bog records can be used to correlate previously reported measurements from France, Switzerland, England, and Greenland that cover different ranges of time. When these are compiled, the integrated European record that emerges spans the last 2300 years of human influence on lead in the air over Europe and suggests human control of lead in airborne particulates over the entire period. From 366 BC through the first half of the 20th century, lead isotopic compositions in European air have fallen within the range of compositions in European ore bodies. Since 1950, isotopic compositions have been beyond the range in those ore body compositions and have fallen within the array of lead isotope compositions typical of gasoline from western industrial nations (a mixing line between US and Australian lead in gasoline). The overlap between the European record and the range in modern European air suggests an average isotopic composition of 206 Pb= 207 Pb ca. 1.13 and of 208P b= 207 Pb ca. 2.41 in air over Europe during the last 20 years.

Response of lead cycling in the surface Sargasso Sea to changes in tropospheric input

Lead and its stable isotopes have been analyzed in surface water samples (0–600 m) and trapped particles collected at the Bermuda Atlantic Time Series Station (U.S. Joint Global Ocean Flux Study) in April and November 1989. These results are compared with wet atmospheric lead deposition as determined from precipitation continuously collected in Bermuda since August 1988 as pan of the Atmosphere-Ocean Chemistry Experiment program. Despite an expected seasonal variability, lead concentrations in surface waters have clearly decreased by 30 to 40% since 1979 in response to a corresponding decline by a factor of 5 to 8 of the tropospheric deposition. This result is corroborated by stable lead isotope measurements with 206Pb/207Pb ratios which are significantly less radiogenic (1.18–1.20) in the first 500 m of the 1989 profile than those measured in 1984 (1.20–1.21). This isotopic shift reflects changes of lead ore supply in the United States as well as a relative increase of the Eurafrican contribution to lead input in the northwest Atlantic that is likely due to the reduction of lead emissions from gasoline consumption in North America. This fast response of lead to interannual variations of the tropospheric input into surface waters is related to its efficient bioreactivity, as demonstrated with a sediment trap deployed at 150 m in April 1989. Sediment trap results show the rapid penetration of lead into the first 200 m associated with large particles during a period of high plankton activity. Retrospective isentropic air mass trajectories in three dimensions coupled with the precipitation events collected in Bermuda in 1988–1989 show that 30 to 40% of the annual lead deposition originates from the trade easterly meteorological regime. This input is clearly evidenced with lead isotopic signals observed in surface waters (0–100 m) in April and November 1989. We show that lead accumulated in the seasonal mixed layer (0–50 m) reflects this atmospheric input. Taking into account the isotopic signal measured in this mixed layer (1.178±0.001) as well as the respective contribution of the temperate westerlies and trade easterlies to atmospheric lead deposition to the Sargasso Sea, we calculate that the isotopic signature from the Eurafrican regions is 1.155 (±0.004). Based on these results, we calculate that the actual 206Pb/207Pb average ratio in surface waters of the Sargasso Sea is 1.188 (±0.004).

Silver in San Francisco Bay Estuarine Waters

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 μm) 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−1. The contamination is most evident in the south bay, where wastewater discharges of silver are on the order of 10 kg d−1 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 (≥160 d), and revealed the apparent seasonal benthic fluxes of silver from anthropogenically contaminated sediments. These were conservatively estimated to average ≈16 nmol m−2 d−1 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−2 d−1, with an annual input of approximately 540 kg yr−1 of silver to the system. This dwarfs the annual fluvial input of silver during the study period (12 kg yr−1) and is equivalent to approximately 10% of the annual anthropogenic input of silver to the estuary (3,700–7,200 kg yr−1). 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 (Kd≈105).

Temporal variations in lead concentrations and isotopic composition in the Southern California Bight

Lead concentrations in surface waters of the Southern California Bight appear to have decreased threefold (from >170 to <60 pM) since they were initially measured by Clair Patterson and his associates in the 1970s. The decrease parallels a threefold decline in anthropogenic inputs of industrial lead to the bight over the past two decades. Moreover, mass balance calculations indicate that the primary source of lead to the bight now is upwelling. This is evidenced by the isotopic compositions of surface waters in the bight, which are most characteristic of Asian industrial lead aerosols (0.4793 [le] [sup 206]Pb/[sup 208]Pb [le] 0.4833) deposited in oceanic waters of the North Pacific. While the decrease in surface water lead concentrations in the bight reflects the reduction in industrial lead emissions from the United States, the isotopic compositions of surface waters in the southern reach of the bight reflect a concurrent increase in industrial lead emissions from Mexico (0.4852 [le] [sup 206]Pb/[sup 208]Pb [le] 0.4877). The isotopic composition ([sup 208]Pb/[sup 207]Pb [approximately] 2.427) of elevated lead concentrations of surface waters in San Diego Bay indicate that lead is being remobilized from contaminated sediments within that bay.

Current needs for increased accuracy and precision in measurements of low levels of lead in blood

Needs for accurate and precise measurements of low levels of lead in blood are discussed. Current methodologies are sufficient for measuring blood lead (PbB) levels greater than 0.5 microM, but the accuracy and precision of those methodologies are inadequate for measuring lower (less than 0.5 microM) PbB concentrations. These analytical limitations are primarily due to contamination bias incurred during sample collection, processing, and analyses. Consequently, trace metal-clean procedures should be adopted to elucidate the contribution of environmental lead to the PbB levels of the U.S. population and to estimate the threshold(s) of subclinical lead toxicity.

Trace element distributions in coastal waters along the US-Mexican boundary: relative contributions of natural processes vs. anthropogenic inputs

Abstract Trace element concentrations (Pb, Cd, Mn, Fe, and Zn) were measured along four surface water transects across the continental shelf off Baja California, to evaluate the magnitude of heavy metal contamination in the coastal waters along the US-Mexican boundary. These initial measurements of trace elements in Mexican neritic waters revealed offshore concentration gradients, with the highest levels in coastal waters with high salinities and nutrient concentrations. There were also longshore gradients, with lower concentrations in the southern locations. Although the relative enrichment of metals detected at nearshore stations along the US-Mexican border appeared to correspond to wastewater discharges in that area, these trace metal enhancements were found to be primarily associated with physical oceanographic processes (upwelling and advection), rather than anthropogenic inputs. This was demonstrated both by metal-nutrient correlations and multivariate statistical analyses. Mass balance calculations also indicated that about 1% of Cd, 9% of Zn, and 29% of Pb were from urban discharges within the area.

Benthic lead fluxes in San Francisco Bay, California, USA

Porewater concentration gradients indicate relatively large benthic fluxes of Pb from sediments in the San Francisco Bay estuary. Gradients in total dissolved (<0.45 μm) Pb concentrations in sediment porewaters, which range from 0.07–19.2 nM, parallel gradients in ammonia and dissolved Fe in sediment cores from the bay. Corresponding Fickian diffusive fluxes range from 2.6 × 10−9 moles m−2 d−1 to 3.1 × 10−8 moles m−2 d−2 in anoxic surface (<2 cm) sediments along the periphery of the estuary. These indicate the net diffusive benthic flux of Pb from sediments in San Francisco Bay (3–31 moles d−1) is at least an order of magnitude greater than the fluvial input of dissolved Pb to the estuary (0.2 moles d−1) during low flow periods. Moreover, estimates of the total benthic Pb flux, which were based on Hammond et al. (1985) irrigation benthic flux model, are two- to six-fold greater (6–186 moles d−1) than the estimates of diffusive fluxes. Therefore, the total benthic flux of Pb from the bays sediments may be within an order of magnitude of the total anthropogenic flux of Pb to the San Francisco Bay estuary (965–8,410 moles d−1).