Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

Carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, calcium, and iron are the principal elements used by living systems for structural tissues, replication, and energy-harvesting activities. These elements also are important components of the hydrosphere, the atmosphere, and lithosphere materials. Living organisms along with chemical, physical, and geological forces continuously redistribute these elements between living and nonliving reservoirs through processes referred to as biogeochemical cycles. The term biogeochemistry links contributions from biology, chemistry, and geology (including hydrology). Biogeochemistry is used here in a broader sense to include contributions from organic geochemistry to convey the molecular level interactions of organic matter (natural as well as anthropogenic) with the lithosphere, hydrosphere, and biosphere.

C, H, N, etc., occur in significant quantities in sediments, on sediment surfaces, in the water column, and in pore fluids. The chemistry of these elements influences or even controls important environmental processes. Some of these processes are: sorption/desorption of contaminants, exchange of chemical species among air-water-sediment reservoirs, and transformation of contaminants (biological, chemical, photochemical). Therefore, a detailed knowledge of the biogeochemical cycling of C, H, N, etc. is crucial to understanding the bioavailability of toxic elements or toxic compounds, and their natural geochemical variability in different environmental systems. To achieve a comprehensive evaluation of the environmental behavior of contaminants requires consideration of their behavior at three scales or levels - molecular, macroscopic, and ecosystem.

The Biogeochemistry Group in the Geologic Division at Reston is applying a diverse set of biogeochemical tools to address a wide range of problems spanning environmental geochemistry, ecosystem restoration, paleoenvironmental and paleoclimate studies, and resource studies. Field and laboratory oriented research utilizing state-of-the-art analytical techniques and innovative approaches has resulted in important advances toward the understanding of complex issues. Analytical techniques available for our investigations include: solid state 13C nuclear magnetic resonance spectrometry, gas chromatography, gas chromatography - mass spectrometry, liquid chromatography, liquid chromatography - mass spectrometry, elemental analysis, analytical pyrolysis, ion chromatography, isotope ratio mass spectrometry (C, H, N, O, S), compound specific isotope ratio mass spectrometry, and several inorganic analytical techniques. A select number of examples from ongoing and completed investigations are discussed below to illustrate the potential contributions of biogeochemistry to USGS sediment research:

Environmental Geochemistry: Landfilling is the most common method of disposal of municipal waste. Many older landfills were sited on alluvial deposits because of alluviumÕs low economic value and the availability of excavated sites after sand and gravel operations. The Norman landfill is a closed municipal landfill located on alluvium associated with the Canadian River in central Oklahoma. An increased understanding of the hydrologic and geochemical processes controlling the migration of organic and inorganic contaminants from the Norman landfill may be applicable to many sites across the United States. Investigations are currently under way to characterize the organic and inorganic constituents of the leachate plume that is believed to have moved downgradient from the landfill. The contamination of the shallow aquifer provides an excellent opportunity to study the spatial variability of biogeochemical processes and the resulting effects on the fate of degradable contaminants in the leachate plume. In addition, efforts are under way to characterize the aquifer material and its interactions with both landfill-derived substances as well as native organic and inorganic constituents. The Biogeochemistry Group's efforts are focusing on the nature of the nonvolatile dissolved organic matter (NVDOM) and its interaction with other components of the microbial/geochemical /hydrological system. Pyrolysis - gas chromatography - mass spectrometry is being utilized to examine the chemical structure of various fractions of NVDOM from groundwater. Studies are also underway to evaluate the interaction of the leachate plume with an adjacent wetland. Potential remediation of landfill leachate plumes by natural buffering wetlands is of interest to many municipalities.

Naturally occurring organic substances may also pose an environmental health hazard. Studies are currently underway to examine the link (if any) between Balkan Endemic Nephropathy (BEN), a fatal kidney ailment affecting thousands of people in the Balkans, and the leaching of toxic organic substances by groundwater from Pliocene lignites (low rank coals). The disease occurs in clusters of villages, some of which are located only a few km from villages unaffected by BEN. In preliminary work, we observed that wells in BEN villages are located hydrologically downstream from Pliocene lignite deposits (Finkelman et al., 1991). Analysis of well water from BEN and non-BEN villages showed that the well water from BEN villages had significant concentrations of polycyclic aromatic hydrocarbons with chemical structures similar to those of acetaminophens, compounds known to cause kidney problems with prolonged usage. Water extracts from Pliocene lignites also contained aromatic components similar to those observed in the well water. While this work does not prove a link between organic compounds leached from Balkan and other Pliocene lignites and BEN, it does provide nephrologists with a new hypothesis to explain the causes of this puzzling disease. Pliocene lignites occur worldwide, notably in Turkey and Asia, and BEN-like diseases may exist unrecognized in these areas.

Ecosystem Restoration: Ecosystems in south Florida are currently believed to be threatened by increasing urbanization, agricultural activity, and water management. Excess nutrients in runoff from agricultural areas are implicated in the alteration of the extant vegetation in the Everglades ecosystem. A dramatic decrease in the population of wading birds has also been noted. Excessive accumulation of mercury in fish has led to the issuing of hazard advisories and mercury poisoning has also been related to the death of Florida panthers and may be related to reproductive failure in panthers, alligators and wading birds. Changes in the ecology and chemistry of Florida Bay have also occurred due to decreases in the quantity and quality of freshwater entering the bay from the Everglades, and pollution and stress to the ecosystem from nearby urban areas and recreational boaters. Observed changes include a decline in seagrass and coral communities, excess nutrient load and algal blooms, and episodes of hypersalinity. C, N, P, and S play a crucial role in the environmental health of the ecosystems in south Florida. The utilization and biochemical transformations of these elements by microorganisms in sediments drive many of the important chemical reactions affecting other chemical species, especially metals. Both the nature of transformations of these elements and the rates of these transformations need to be better understood. Porewater and sediment geochemistry studies are being conducted in Florida to provide rates of recycling for C, N, P, and S. These rate estimates are being used to evaluate the residence times of the elements and provide wetland managers with information on the fate of uncontaminated wetland areas downstream. This information is also useful for determining the effectiveness and lifespan of constructed wetlands designed to mitigate nutrient pollution. In addition to porewater studies, information on organic-matter diagenesis is also needed for an understanding of C,N,P, and S cycling. The reactive forms of these elements largely enter sediments as organic matter, and are released as microbial decomposition products. The cycling and transformations of S in aquatic sediments are of particular interest because of high biochemical and geochemical reactivity of this element. Under anoxic conditions in sediments microbial sulfate reduction transforms sulfate to sulfide, which readily reacts with various metals to produce insoluble metal sulfides that are immobilized in the sediments. For mercury, an element of particular environmental concern in south Florida, microbial sulfate reduction is also the process by which Hg is converted to the methylated forms. The methylated forms are bioaccumulated and are potent neurotoxins. In south Florida and elsewhere bioaccumulated methyl-Hg poses a serious threat to the health of both humans and wildlife. We are currently conducting studies in south Florida using sulfur isotopes to track the sources of S which is entering the Everglades, and to examine the range of sulfate concentrations over which maximum methyl-Hg production occurs. Mercury occurs naturally in south Florida sediments at a moderate abundance level, yet is accumulated to very high levels in fish. Thus the biogeochemical conditions regulating mercury methylation and the principal mechanisms for bioaccumulation are critical factors in determining mercuryÕs impact on ecosystems. Regional variations and historical variations in concentration and accumulation rates of mercury and other metals of environmental concern in sediments are important factors that need to be better understood. Restoration to historical conditions requires a knowledge of what these ecosystem conditions may have been. We are also conducting investigations to determine regional present-day and historic variations for mercury and a number of other metals in sediment cores collected in south Florida.

Paleoenvironmental and Paleoclimate Studies: The use of biogeochemical methods in studies of sediment cores for developing paleoenvironmental and paleoclimatic models is a relatively new field, sometimes referred to as molecular stratigraphy. In conjunction with isotopic and paleontologic data, molecular data adds insights on the nature of paleoenvironmental change recorded in sediments. Alkenones (long chained ketones) are a group of molecules whose distribution patterns downcore have been used to delineate the variations in sea-surface temperature going back tens of millions of years. The Biogeochemistry Group is currently exploring the utility of another group of molecules, lignin phenols, as indicators of paleoenvironmental conditions. Lignin phenols are a group of 15 methoxy and dihydroxy phenols thought to be derived exclusively from the lignin of vascular plants. Studies of the concentration and distribution of lignin phenols in sediment cores may provide three types of information relevant to paleoenvironmental applications: (1) changes in the land plant community reflecting changes in climate (analogous and complementary to pollen studies), (2) changes in the amount of terrestrial runoff to a basin, reflecting rainfall patterns, and (3) changes in the relative proportions of vascular plant (land) production compared to aquatic productivity. Lignin phenols in sediments from a number of cores from Lake Baikal, Siberia have been examined to reconstruct paleoenvironmental conditions and changes in climate to 200,000 y B.P. (Orem et al. 1993). We currently are using lignin phenols to examine changes in the abundance and distribution of seagrass in Florida over the last 200 years. Results will provide information on whether the dieoff of seagrass observed during the last decade is a result of recent human impact on the region, or part of a long-term natural cycle.

Resource Studies: Biogeochemical approaches also are being applied to resource-related studies. Petroleum, natural gas, and coal are essentially organic in nature and many of the tools of the trade have been developed in organizations concerned with the identification of new sources of and estimation of the abundance of these resources. The association of these resources, as well as certain mineral resources, with sedimentary rocks has provided opportunities to study biogeochemical cycles at higher temperature and pressure than in the case of unconsolidated sediment. The thermal maturity of organic matter in sedimentary units often indicates whether petroleum has been produced. Variations in the composition and distribution of certain extractable marker organic compounds (biomarkers) are used to identify source beds for petroleum and migration pathways of petroleum. We have applied biomarker data to determine the thermal history of sedimentary rocks from the early Mesozoic Newark basin (Walters and Kotra, 1990). We also have studied changes in chemical structure of certain other biomarker compounds in a series of coals increasing in rank to understand the chemical transformations occurring during early stages of coalification (Hatcher et al., 1988).

REFERENCES

Hatcher, P.G., Lerch, H.E., Kotra, R.K., and Verheyen, T.V., 1988, Pyrolysis/gas chromatography/mass spectrometry of a series of degraded woods and coalified logs that increase in rank from peat to subbituminous coal: Fuel 67, 1069-1075

. Orem, W.H., Lerch, H.E., and Kotra, R.K., 1993, Lignin oxidation products in sediments from Lake Baikal: Indicators of Late Quaternary paleovegetation and paleoclimate change in north- central Asia: Geologiya I Geofizika (Russian Geology and Geophysics) 34, 89-100.

Walters C.C.,and Kotra, R.K., 1990, Thermal maturity of Jurassic shales from the Newark Basin, U.S.A.: Influence of hydrothermal fluids and implications to basin modeling: Applied Geochem. 5, 211-225.