Most trace elements and many anthropogenic organic compounds are known to associate with sediments. For this reason, some researchers have recognized that studies of water quality that include hydrophobic constituents must necessarily include studies of sediment chemistry. Bradford and Horowitz (1982, p. 1) note that, "The strong association of numerous toxic chemicalsboth organic (such as PCB´s, DDT, Mirex, and Kepone) and inorganic (such as arsenic, mercury, cadmium, and lead)with sediment means that much of the downstream transport of these materials can not be detected or evaluated solely through the sampling and analysis of water." The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program has accepted this view and incorporated routine sampling of streambed sediments into the NAWQA design (Gilliom and others, 1995). The NAWQA Program also is conducting a study, with support from the USGS National Research Program, to identify historical trends in concentrations of hydrophobic constituents using paleolimnological analyses of reservoir sediment cores (Callender and Van Metre, 1994; Van Metre and others, 1996). This paper briefly describes these two sampling programs and presents selected findings from the trends study.
The objective of the NAWQA streambed-sediment sampling is to describe the occurrence and spatial distribution of hydrophobic contaminants in streams. Streambed-sediment sampling is done in two phases. The occurrence survey calls for sampling about 15-25 sites in each NAWQA study unit, with the objective of identifying target constituents and their importance to water-quality in the study unit. Relative importance is determined by the magnitude of constituent concentrations and the extent of their occurrence. The spatial distribution survey adds improved geographic coverage, with particular emphasis on assessment of priority constituents identified in the occurrence survey. Twenty to 30 sites typically are sampled for the spatial distribution survey (Shelton and Capel, 1994).
Streambed-sediment samples are collected using consistent methods to allow interpretations of data Nationally. Fine-grained surficial sediments are obtained from several depositional zones and composited to yield a sample assumed to represent average conditions. Samples are processed through a 2.0-millimeter stainless-steel-mesh sieve for organic contaminate analysis and a 63-micrometer nylon-cloth sieve for trace-element analysis (Shelton and Capel, 1994). Field sieving is done to reduce the natural variability among samples caused by variations in particle size, thus allowing more informative comparisons among samples. Organic carbon and grain-size distributions are measured, allowing for normalization of concentrations with respect to these variables to facilitate comparisons among samples.
Sample laboratory analyses are also done using consistent methods. Elemental concentrations are determined on concentrated-acid digests using atomic emission spectrometry-inductivity coupled plasma (ICP-AES); concentrations of silver and cadmium are determined on concentrated-acid digests using graphite furnace atomic adsorption (GFAA); concentrations of mercury are determined by cold-vapor atomic adsorption (CVAA); and concentrations of arsenic, antimony, and selenium are determined by hydride generation (HA) (Fishman and Friedman, 1989). Organochlorine compounds and semivolatile organic compounds (including polycyclic aromatic hydrocarbons (PAH´s)) are measured in organic-solvent extracts using a dual capillary-column gas chromatograph with dual electron capture detector. The method uses a soxlet extraction with dichloromethane and methanol followed by gel permeation and adsorption chromatographic fractionation (Foreman and others, 1994).
Sediment sampling also is being used by the NAWQA Program to identify trends. One of NAWQA´s three objectives is, "to define long-term trends (or lack of trends) in [our Nation´s] water quality" (Leahy and others, 1990, p. 1). Water-quality trends are of interest for at least three reasons: First, trends can indicate the effects of large investments in water-pollution control; second, trends can provide a warning of degradation in water quality; and, third, trends can improve our understanding of relations between human and natural environmental factors and water quality. A common approach to determining water-quality trends in rivers is to apply statistical tests to historical data; however, historical water-quality data have many limitations. These include lack of data, different sampling and analytical methods, numerous measurements below detection levels, and uncertain accuracy. These limitations are particularly severe when dealing with hydrophobic constituents.
A NAWQA study to investigate the use of reservoir sediment cores to evaluate historical trends in hydrophobic constituents has been underway since 1992. The study is using paleolimnological techniques to describe natural and anthropogenic changes in water quality in drainage basins of reservoirs. The techniques includes radiochemical dating of sediment cores; measurement of major, minor, and trace elements, chlorinated organic compounds, and PAH's; and interpretation of various sedimentological and geomorphic properties in cores. In one setting, historical changes in diatoms and pollen also were evaluated (Bradbury and Van Metre, 1997).
Sediment coring has long been used to document historical changes in water quality in natural lakes (for example, Eisenreich and others, 1989). Documenting historical changes in water quality in rivers using cores from reservoirs is less common (for example, Callender and Robbins, 1993; Callender and Van Metre, 1994). Important differences between reservoirs and natural lakes affect the sampling and interpretation of sediment cores. Sedimentation rates for most natural lakes vary from 0.05 to 10 millimeters per year. In contrast, reservoirs rates range from 10 to 200 millimeters per year. Higher sedimentation rates tend to minimize post-depositional changes caused by diagenesis and bioturbation, thus making interpretation of trends more direct.
Sediment cores were collected from 17 reservoirs in the eastern and central United States from 1992 to 1996 (fig. 1), 12 with support from NAWQA. The smallest reservoir sampled, Lake Anne in Virginia, has a drainage area of 2.3 square kilometers; the largest, International Falcon Reservoir on the Rio Grande, has a drainage area of 575,000 square kilometers and a storage capacity of 3,290 million cubic meters. Results from White Rock Lake in Dallas, Tex., illustrate the method for an urban setting (Van Metre and others, 1996; Van Metre and Callender, 1997). White Rock Lake was impounded in 1912. Its 259-square-kilometer drainage was predominantly agricultural until the early 1950´s when urban Dallas began to encroach. By 1990 the drainage was 72 percent urban with a population density of 1,290 people per square kilometer. Sediment cores were collected in 1994 and again in 1996. Trends in sediment cores reflect extensive urban development following World War II, historical use and elimination of lead in gasoline, historical use and subsequent restriction of manmade organic compounds (DDT, PCB´s, chlordane), and continuing increases in anthropogenically derived PAH´s (fig. 2).
|Figure 1. Locations of NAWQA study units and reservoirs sampled from 1992 to 1996.|
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|Figure 2. Trends in sediment cores from White Rock Lake, Dallas, Tex. (Benzo(a)pyrene was measured in a core collected in 1996; other plots are from a 1994 core).|
To be of value to NAWQA at a National scale, general patterns of trends regionally or as a function of environmental settings need to be identifiable. General patterns of change are indicated by at least two findings: One is a relation between cultural enrichment factor (CEF) for lead and population density of seven of the drainage basins (fig. 3). CEF is a ratio of peak lead concentration to background concentration. Peak concentrations occurred during the mid-1970´s, when use of leaded gasoline was greatest. The CEF is therefore equilivant to the magnitude of the increasing trend from pre-development to the mid-1970´s (Callender and Van Metre, 1996).
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|Figure 3 Maximum lead enrichment in sediment cores versus population density of drainage basin.|
The second general pattern observed is that rates of decrease in concentrations of cesium-137, total DDT, and PCB´s since the bans or restrictions on their use are similar among reservoirs (Van Metre and Callender, 1996). First-order rate models fit to data from eight reservoirs from about 1970 to the early 1990´s indicate mean half-lives of about 10 years. The similarity in rates of decrease of these three compounds probably is caused by similarities in source history and environmental behavior. All three were widely distributed in the environment, primarily during the 1950´s and 1960´s, are persistent (DDT was summed with its metabolites and cesium-137 was decay-corrected), and are hydrophobic. Additionally, the mean rate model of these reservoirs compares well with historical rates of change in fish samples collected by the National Contaminant Biomonitoring Program (Schmitt and others, 1990) for a similar part of the United States (fig. 4).
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|Figure 4. Decrease in PCB´s and DDE in sediment cores and fish.|
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Van Metre, P.C., and Callender, E., 1997, Identifying water-quality trends from 1912Ð94 using dated sediment cores from White Rock Lake Reservoir, Dallas, Texas: Journal of Paleolimnology 17, p. 239-249.
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