Sediment is a major repository for many of the more persistent chemicals that are introduced into surface waters and provides habitat for many aquatic organisms. In the aquatic environment, most anthropogenic chemicals and waste materials including toxic organic and inorganic chemicals eventually accumulate in sediment. Mounting evidences exists of environmental degradation in areas where EPA Water Quality Criteria are not exceeded, yet organisms in or near sediments are adversely affected (EPA 1994a). Although certain chemicals are highly sorbed to sediment, these compounds may still be available to the biota. Contaminated sediments may be directly toxic to aquatic life or can be a source of contaminants for bioaccumulation in the food chain. EPA recently completed a National Sediment Inventory and reported that 26% of the samples in a national database were categorized as having a high probability of causing adverse effects and an additional 49% of the samples were categorized as having an intermediate probability of causing adverse effects (EPA 1996a). Concentrations of contaminants in sediment may be several orders of magnitude higher than in the overlying water; however, bulk sediment concentrations are not always strongly correlated to bioavailability. Because relationships between concentrations of contaminants in sediment and their bioavailability are poorly understood, determining effects of contaminants in sediment on aquatic organisms requires controlled toxicity and bioaccumulation tests (Ingersoll et al. 1997a).
Researchers at MSC have been working with other groups in developing a variety of standard methods for assessing the toxicity of contaminants associated with marine and freshwater sediments using amphipods, midges, polychaetes, oligochaetes, mayflies, or cladocerans (Ingersoll 1991; USEPA 1994a,b; ASTM 1996a-f; Environment Canada 1996a,b). These standard methods suggest several potential endpoints that can be monitored in sediment tests including survival, growth, behavior, or reproduction. Usually, survival of test organisms in 10- to 14-d exposure is the endpoint most typically reported. These short-term exposures which monitor only effects on survival can be used to identify high levels of contamination; however, these exposures may not be able to discriminate among marginally contaminated sediments (Kemble et al. 1994).
Additional research is being conducted with the EPA to develop methods for assessing sublethal effects of contaminants associated with sediments in 42-day exposures with the amphipod H. azteca (Ingersoll et al. 1997b) and the midge C. tentans (Benoit et al. 1997). Endpoints monitored in these tests include survival, growth, and reproduction of H. azteca and survival, growth, emergence, reproduction, and egg viability of C. tentans. These methods are currently undergoing round-robin testing to evaluate inter-laboratory precision and to develop test acceptability requirements.
The USGS has been monitoring the upper Mississippi River (UMR) since 1987 to document the fate and transport of contaminated sediments. The UMR is that part of the river upstream of the confluence with the Ohio River at Cairo, Illinois and consists of a series of 26 navigational pools created by a lock and dam system extending from Minneapolis, Minnesota to St. Louis, Missouri. The navigational pools are shallow lake-like areas which trap and store large quantities of fine grained sediments during normal river flows. Concern with the redistribution of the river sediments arose after the flood of 1993. A project was designed to evaluate the current status of sediments in the UMR by: (1) measuring the concentrations of contaminants in sediments of the UMR, (2) evaluating the toxicity of sediments collected from the river, (3) determining the bioaccumulation of contaminants from UMR sediments using both field-collected and laboratory-exposed oligochaetes, (4) and determining the benthic community structure in fine-grain sediments from within the river.
In the summer of 1994, sediment samples and benthic organisms were collected from 24 of the navigational pools in the river and from one pool in the Saint Croix River. Two types of sediment samples were collected from the pools. One sediment sample was a composite sample of 15 to 20 sediment grabs along one to five transects across the downstream one-third of each pool (B samples). The other sediment sample was a composite sample from one station along one transect within each pool (C samples). These latter stations were selected based on historical chemistry data and the potential for the sample to contain a large number of oligochaetes. Samples were not collected from the main navigation channels. Whole-sediment toxicity tests were conducted for 28 days with the amphipod H. azteca measuring effects on survival, growth and sexual maturation. Toxicity tests were conducted with both the B and C sediment samples. Bioaccumulation of contaminants from sediments was evaluated using field-collected oligochaetes and 28 day bioaccumulation studies conducted in the laboratory with the oligochaete L. variegatus. Bioaccumulation tests were conducted with C sediment samples from 13 of the 24 possible individual C sediment samples. Benthic community assessments were conducted on all 24 of the C samples. In addition, the Triad approach was used to assess the overall status of the UMR by integrating data from sediment chemistry, laboratory toxicity tests, and benthic community measurements.
Survival of H. azteca in laboratory exposures to sediments collected from the UMR was significantly reduced in only one sediment sample relative to both a control and reference sediment. Growth of amphipods was also reduced in only one sediment sample. Sexual maturation was not significantly reduced in any treatments. Significant correlations were not observed between survival, growth, or sexual maturation and any of the physical or chemical sediment characteristics. Using sediment chemistry and the Effect Range Medium (ERM), 96% of the samples were classified as non-toxic (i.e., measured chemical concentrations rarely exceeded ERMs). Classifications using ERMs and sediment chemistry were consistent with the biological results from the H. azteca toxicity tests. Winger and Lasier (1997) evaluated sediment toxicity with H. azteca in the lower Mississippi River from Cairo, Illinois to New Orleans, Louisiana and observed toxicity in only one set of samples (upstream from Memphis, Tennessee).
In the bioaccumulation tests, concentrations of contaminants were relatively low in oligochaetes collected from the pools and in oligochaetes exposed to the sediments in the laboratory. Organochlorine pesticides were generally below detection in sediment and tissue samples. Only aliphatic and polycyclic aromatic hydrocarbons (PAHs) and total polychlorinated biphenyls were frequently measured above detection limits in oligochaete tissue and in sediment samples. Concentrations for a specific contaminant in laboratory-exposed and field-collected oligochaetes were similar within a station. About 90% of the paired PAH concentrations in laboratory-exposed and field-collected oligochaetes were within a factor of three of one another. Concentrations of PAHs in oligochaetes collected from the pools or exposed in the laboratory to sediments from the UMR averaged about 1000 times less than tissue concentrations measured in oligochaetes from other sites within the U.S.
The benthic community was dominated (>80%) by oligochaetes and chironomids in 14 of the 23 sediment samples from the UMR and the one sediment sample from Saint Croix River. Total abundance values of benthic invertebrates in these samples ranged from 250/m2 to 22,389/m2 and were comparable to previously reported values for the UMR. The frequency of chironomid mouthpart deformities was only 3% which is consistent with the incidence of mouthpart deformities from uncontaminated sediments. Correlations between benthic measures, sediment chemistry, or other abiotic parameters exhibited few strong or significant correlations indicating benthic communities are most likely controlled by factors independent of contaminant concentrations in sediment.
Results of the Triad analysis indicated that 88% of the samples from the UMR were classified as not impacted based on sediment chemistry, laboratory toxicity, and benthic measures. These results are consistent with the bioaccumulation study in which concentrations of contaminants in tissue were less than other sites across the United States that have been studied (Ingersoll et al. 1995). In addition, pools in about the lower third of the river had lower sediment contaminant concentrations, less accumulation of contaminants in tissue, and greater taxa richness. The results of this study indicate that the UMR is not severely contaminated relative to other sites that have been studied. Reductions in productivity of the river would not likely be related to chemical contamination but rather other perturbations such as channelization, sedimentation from surface runoff, or long-term changes in the natural flow conditions of the river due to lock and dam construction. This study only conducted a partial assessment of the UMR sediments and included no assessment of river water. Also this study was a one-time assessment and did not evaluate temporal or spatial variability of sediment contamination within the pools. Any future research on, or management of, the Upper Mississippi River should evaluate the limitations of this study.
Three types of SQGs were calculated for H. azteca and for C. riparius: (1) Effect Range Low (ERL) and Effect Range Median (ERM), (2) Threshold Effect Level (TEL) and Probable Effect Level (PEL), and (3) No Effect Concentration (NEC; analogous to Apparent Effect Thresholds). The SQGs were calculated using: (1) dry-weight concentrations, (2) dry-weight concentrations normalized to total organic carbon concentrations (for non-ionic organics), or (3) dry-weight concentrations normalized to acid volatile sulfide concentrations (for divalent metals). SQGs were calculated primarily for total metals, simultaneously extracted metals, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons. The ranges of concentrations in sediment were too narrow in the database to adequately evaluate SQGs for butyltins, methyl mercury, polychlorinated dioxins and furans, or chlorinated pesticides.
Historically, the predictive ability of SQGs has been evaluated by determining the frequency of exceeding individual SQGs in independent datasets. In addition to evaluating the frequency of exceeding SQGs, we also evaluated the predictive ability of SQGs using a toxic quotient approach. A toxic quotient for ERMs was calculated for each sample by summing the concentration of each chemical divided by the ERM for that chemical. The mean ERM quotient was then calculated by dividing the sum ERM quotient by the total number of ERMs evaluated.
Good correspondence was observed between the proportion of ERMs exceeded and the mean ERM quotient. The frequency of samples classified as toxic was highest at a proportion of ERMs exceeded of >0.4 or at a mean ERM quotient of >2. About 60 to 80% of the sediment samples in the database were correctly classified as toxic or not toxic depending on type of SQG evaluated. ERMs and ERLs were generally as reliable as paired PELs and TELs at classifying both toxic and non-toxic samples in the database. Reliability of the SQGs in terms of correctly classifying sediment samples was similar between ERMs and NECs; however, ERMs minimized Type I error (false positives) relative to ERLs and minimized Type II error (false negatives) relative to NECs. Correct classification of samples was improved by using only the most reliable individual SQGs for chemicals (i.e., those with a higher percentage of correct classification).
Calculating SQGs using dry-weight concentrations vs SQGs calculated using sediment concentrations normalized to TOC concentrations for PAHs and total PCBs resulted in similar correct classification of toxicity and similar Type I and Type II error. The range of TOC concentrations in the database was relatively narrow compared to the ranges of contaminant concentrations. Therefore, normalizing dry-weight concentrations to a relatively narrow range of TOC concentrations had little influence on relative concentrations of contaminants among samples.
The SQGs were calculated from toxicity tests with field-collected samples. If a chemical concentration exceeds an SQG generated using data from these tests with field-collected samples, it does not necessarily mean the chemical caused the observed effect. Rather, the SQG is the concentration of a chemical that is associated with the effect. Field-collected sediments typically contain complex mixtures of contaminants. Additional information is needed to identify the specific contaminants that were actually responsible for the toxicity. Confirmation of sediment toxicity due to individual or groups of contaminants can be determined by using Toxicity Identification Evaluation (TIE) procedures or by conducting toxicity tests with spiked sediments (Besser et al. 1997). Once the probable cause(s) of sediment toxicity has been identified, better decisions can be made regarding remediation options.
Future collaborative efforts involving all four USGS Divisions could lead to the development of methods for assessing the bioavailability, geochemistry, sources, and distribution of contaminants associated with sediments. The goal of this research would be to assimilate the biological, hydrological, and geological information necessary for the assessment and management of contaminated sediments. The end result of this research would be an improved understanding of the importance of contaminants in sediment relative to other factors influencing ecosystems.
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American Society for Testing and Materials. 1996c. Standard guide for conducting 10-day static sediment toxicity tests with marine and estuarine amphipods. E 1367-92. In Annual Book of ASTM Standards, Vol. 11.05, Philadelphia, PA. pp. 769-794.
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Ingersoll, C.G., P.S. Haverland, E.L. Brunson, T.J. Canfield, F.J. Dwyer, C.E. Henke, and N.E. Kemble. 1996. Calculation and evaluation of sediment effect concentrations for the amphipod Hyalella azteca and the midge Chironomus riparius. J. Great Lakes Res. 22:602-623.
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Contribution from the USGS