Ellen V. Axtmann,
U.S. Geological Survey,
3215 Marine Street,
Boulder, Colorado 80303
Determinations of metal concentrations in suspended and bed sediments are useful in understanding processes that affect metal contamination in rivers and estuaries. Metal concentrations in sediments can be influenced by variations in sediment texture, composition of sediment, reduction/oxidation reactions, adsorption/desorption, and physical transport or sorting, as well as anthropogenic metal inputs. As a combined result of the influence of the above processes, fluctuation of the metal concentrations of sediments occurs in time and space; and bioavailability from sediments changes. The relative importance of individual processes is determined by the geochemistry of the specific metal or metalloid and the characteristics of the ecosystem.
A variety of strategies are available for improving the comparability of metal determinations in sediments, improving understanding of anthropogenic contributions to concentrations, and defining deviations from background concentrations (Luoma, 1990). Distribution of sediment contamination at local and regional scales in San Francisco Bay were the subject of a series of studies in the 1970's and 1980's (Luoma et al, 1990; Thomson et al, 1984). Temporally intensive (seasonal or near-monthly) determinations of metal concentrations in suspended and bed sediments are presently underway in Suisun and San Pablo Bays, the landward reach of San Francisco Bay. These studies examined the interacting influences on metal concentrations of geologic inputs, anthropogenic contamination, riverine inflows and geochemical processes (for example, desorption/adsorption at the freshwater/seawater interface). Sediment data are also being employed in studies of metal bioavailability to resident species.
Historic trends in contamination are being determined in dated sediment cores from San Francisco Bay (Hornberger et al, submitted). Concentrations of metals that occurred prior to modern expansion of anthropogenic activities can be a source of uncertainty in interpreting sediment contamination. The core studies offer an opportunity to identify pre-disturbance contaminant concentrations in the Bay. Mining, industry, agriculture and urban development each appear to be the primary sources of specific contaminants. Results to date show that concentrations of most contaminants reached their maxima ~1980. Concentrations of several contaminants have declined since 1970-1980 with the greatest reductions occurring nearest sources of input. For example, concentrations of Pb and Hg have declined to 30 - 40% of their historic maxima near sources in North Bay. Bans on chemicals, advanced waste treatment and economic shifts away from heavy industry have effectively reduced contamination in San Francisco Bay, but significant problems remain unsolved. Ongoing monitoring of surface sediments show that elevated concentrations of most contaminants remain widespread throughout the Bay.
Since 1986, bed sediments (and suspended sediments) have been employed to study processes affecting the fate of metal contamination in high gradient cobble bottom rivers in Montana (Moore et al, 1991; Axtmann and Luoma, 1991; Lambing, 1991; Hornberger et al, in review) . Bed sediments in cobble-bottom rivers vary widely in texture and geochemical characteristics, affecting interpretations of metal concentrations. Consideration of grain size effects, spatial variability and temporal change are especially important. In the Clark Fork River, Montana, sediments are sampled at low flow every year. Changes in the contamination gradient are compared among years across a range of flow regimes. The goals of these studies include determinations of hydrologic influences on year-to-year variability in contamination, and evaluation of the effects of upstream remediation on contamination. Earlier studies showed that downstream dilution of metal contamination from mining activities was evident over hundreds of km; but spatial trends were difficult to detect on scales of tens of km because of variability. A basin-area model was employed to predict and understand the influence of tributary inputs on the contamination gradient (Axtmann et al, in press). Monthly sampling was conducted over four years at four stations to assess degree and cause of variability within a year. Larval stages of aquatic insects, a vector whereby upper trophic level organisms such as trout may be exposed to metals, were collected with sediments as bioindicators of metal contamination. General trends in insects correspond to trends in sediments, indicating that both are effective indicators of contamination in the river. Although year-to-year variability is large, analysis of metal concentrations in suspended sediments, bed sediments and filter-feeding insects suggest remediation may be beginning to reduce contamination in the upstream-most reaches of the river system. Sediment contamination and tissue residues of metals are also general indicators of the effects of metals. Species richness in the Clark Fork increases downstream as metal contamination in sediments and biota decline (McGuire, 1988). Additionally, there is a significant inverse correlation with the number of mayfly taxa present at a site and the concentration of Cd in bed sediments.
The question of metal bioavailability from sediments is a long-standing impediment to determining metal effects in ecosystems. At the USGS, field and laboratory studies have been developed to address this question (for example, Luoma and Bryan, 1978; Decho and Luoma, 1991). Recent advances evaluate bioavailability from a kinetic model of bioaccumulation that incorporates site-specific field data and laboratory-derived characterizations of bioaccumulation processes for individual species. This modeling approach allows investigators to take advantage of geochemically robust concepts such as equilibrium partitioning. It also allows assessments that are site-specific and more biologically robust than models based purely on geochemistry. An important example is the experimental and field observation which show significant contaminant uptake via ingestion for nearly all contaminants. Uptake via ingestion is affected by food type, feeding rate and the specific digestive processes of the species making this pathway complex; but multiple, additive pathways of uptake must be considered in any viable model of bioavailability. One study in progress is assessing sources and bioavailability of Se in San Francisco Bay. Previous work showed that particulate organo-Se in diatoms exposed to selenite was assimilated with >80% efficiency by a variety of bivalves (Luoma et al, 1992; Wang et al, 1996). Elemental Se precipitated in sediments by microbial dissimilatory reduction was less bioavailable (absorption efficiency 23%), but was not permanently sequestered away from the food web. Uptake of Se from solution by bivalves was extremely slow; and direct dissolved uptake was an irrelevant source of exposure. Recent proposals suggest models based upon this knowledge could be used to predict the fate and effects of proposed Se inputs to San Francisco Bay from agricultural waste waters, and identify the most important gaps in understanding.
Another recent study is comparing effects of digestive "strategies" in different bivalves on metal bioaccumulation; and adverse effects of metals on digestion and food processing. Bioaccumulation models are also being developed to study the factors that determine concentrations of Cd observed in bivalves in nature. It appears that under some circumstances, Cd exposure of benthos occurs primarily via ingestion of contaminated food materials while in others (low salinity) dissolved uptake dominates. When bivalves ingest sediments, bioavailability may be greater from the living component of the sediment (benthic microflora; bacteria) than from Cd bound to non-living surfaces. Yet, many sediment models and sediment bioassays do not include living components. Comparisons of Cd/AVS may explain Cd toxicity in simplistic laboratory experiments. However, better understanding of particulate and solute geochemistry at the microhabitat scale, feeding characteristics, and digestive physiology will also be necessary for accurate generalizations about Cd toxicity from sediments in nature. Ongoing studies are approaching questions involved in understanding particulate and solute geochemistry at biologically relevant scales in sediments as well as feeding characteristics and digestive physiology. The goal is to develop accurate generalizations about Cd toxicity from sediments in nature and increase appreciation of how bioaccumulation is affected by the myriad of biological and geochemical processes that can be operative in nature.
Field studies have clearly shown that exposures to contaminants in nature are more variable and complex than can be fully simulated in experiments. Thus flexible, site-specific bioaccumulation models are probably necessary to provide the information about the dose of contaminants that animals can experience in an environment. Knowledge of dose is critical to understanding contaminant effects from sediments. Recent advances indicate that tissue-residue- based interpretation of metal effects (predicting effects of metals from bioaccumulation) could be a viable approach to further advancing a field-relevant understanding of adverse effects of sediment-bound contaminants.
Axtmann, E.V., and S.N. Luoma. 1991. Large-scale distribution of metal contamination in the fine-grained sediments of the Clark Fork River, Montana, U.S.A. Applied Geochemistry, 6: 75- 88.
Axtmann, E. V. , D.J. Cain and S. N. Luoma, 1997. The effect of tributary inflows on the distribution of trace metals in fine-grained bed sediments and benthic insects of the Clark Fork River, Montana Environ. Sci. Technol. (In press).
Decho, A. W., and S. N. Luoma 1994. Humic and fulvic acids: Sink or source in the availability of metals to the marine bivalves Potamocorbula amurenis and Macoma balthica. Marine Ecol. Prog. Ser. 108: 133-145.
Hornberger, M.I., S.N., Luoma, A. van Geen, C.F. Fuller, and Anima, R. Submitted. Historical trends of trace metals in the sediments of San Francisco Bay, California.Hornberger, M.I., J.H. Lambing, S.N. Luoma and E.V. Axtmann. In review. Spatial and temporal trends of trace metals in water, bed sediment, and biota of the upper Clark Fork basin, Montana: 1985-1995.
Lambing, J.H. 1991. Water-quality and transport characteristics of suspended sediment and trace elements in streamflow of the upper Clark Fork basin from Galen to Missoula, Montana, 1985- 1990. U.S. GeologicalSurv. Water Resources Investigation Report 91-4139, 73 p.
Luoma, S.N. 1990. Processes affecting metal concentrations in estuarine and coastal marine sediments. In: Heavy Metals in the Marine Environment edited by R. W. Furness & P. S. Rainbow, CRC Press, Inc. Boca Raton, p. 51- 66.
Luoma, S. N., and G. W. Bryan. 1978. Factors controlling availability of sediment-bound lead to the estuarine bivalve Scorbicularia plana. J. Mar. Biol. Ass. UK, 58, 793-802.
Luoma, S. N., R. Dagovitz, and E. Axtmann. 1990. Temporally intensive study of trace metals in sediments and bivalves from a large river-estuarine system: Suisun Bay/Delta in San Francisco Bay. Sci. Total Environ. 97/98: 685-712.
Luoma, S.N., and G.W. Bryan. 1981. A statistical assessment of the form of trace metals in oxidized estuarine sediments employing chemical extractants. Sci. Total Environ. 17: 165-196.
Luoma, S.N. and N.S. Fisher. Uncertainties in assessing contaminant exposure from sediments: Bioavailabililty. In Ecological Risk Assessment of Contaminated Sediments, Edited by G. Biddinger & T. Dillon, SETAC Press, Pensacola, FL, in press.
Luoma, S. N., C. Johns, N.S. Fisher, N.A. Steinberg, R.S. Oremland and J. Reinfelder, 1992. Determination of selenium bioavailability to a benthic bivalve from particulate and solute pathways. Environ. Sci. Technol. 26: 485, 1992.
McGuire, D.L., 1988. A synopsis of Clark Fork River macroinvertebrate studies through 1986 and a proposed long-term macroinvertebrate monitoring program. Prepared for the Montana Water Quality Bureau, 13p.
Moore, J.N., and S.N. Luoma. 1990. Hazardous wastes from large-scale metal extraction. Environ. Sci. and Tech. 24: 1278-1285.
Moore, J.N., S.N. Luoma, and D. Peters. 1991. Downstream effects of mine effluents on an intermontain riparian system. Can. J. Fish. Aquat. Sci. 48: 222-232.
Thomson, E. A., S.N. Luoma, C.E. Johansson and D.J. Cain, 1984. Comparison of sediments and organisms in identifying sources of biologically available trace metal contamination: Water Resources Research, v. 18, p. 755 - 765.
Wang, W., N.S. Fisher and S.N. Luoma, 1996. Kinetic determinations of trace element bioaccumulation in the mussel Mytilus edulis. Marine Ecology Progress Series, 140:91-113.
Contributions from Other Federal Agencies
Contribution from the USGS