Sediment quality analyses are conducted for specific reasons and most for some type of regulatory purpose. These purposes could include support for: sediment remediation, dredged material disposal, and sediment monitoring. One such sediment monitoring effort in the United States is to fulfill the requirements set forth in the Water Resources Development Act of 1992 (WRDA). This Act requires EPA to develop the National Sediment Quality Survey, a national evaluation of sediment quality in the United States. The first report was prepared in 1997 and described the incidence and severity of sediment contamination nationwide. The first update to this report was recently released (USEPA, 2004) and is designed to be a screening-level assessment for the identification of potentially contaminated sediments. While there are many challenges to assessing sediment contamination on a localized area, these are drastically magnified at a national scale. One of the biggest challenges was the reliance on existing sediment quality data (mostly with only a single line of evidence reported—sediment chemistry), not collected with the WRDA mandate in mind and providing a limited spatial coverage for most watersheds. Out of the nearly 20,000 stations evaluated in this report (using data from 1990 to 1999), 70% had data available for sediment chemistry only that precluded a weight-of-evidence approach at these stations. Therefore, sediment quality guidelines (SQGs—both empirical and mechanistic) were a primary tool for assessing sediment quality. Other sediment quality benchmarks used in this report included theoretical bioaccumulation potential (TBP), sediment toxicity, and tissue residues. To develop benchmarks for classifying sediments according to possible hazard, some assumptions were made that may not reflect site-specific conditions. While recognizing the inherent limitations of the data and the assessment approach, this is a viable approach to broad scale sediment assessments.
Why we evaluate sediments
Toxic sediments have contributed to a wide variety of environmental problems around the world. The observed effects include direct toxic effects to aquatic life, bio-magnification of toxicants in the food chain, and economic impacts. Evident effects include: loss of important fish and shellfish populations (USEPA, 1998); decreased survival, reduced growth, and/or impaired reproduction in benthic invertebrates and fish (USEPA, 2002a); and fin rot and increased tumor frequency in fish (Van Veld et al., 1990). More subtle effects resulting from contaminated sediments include: changes in composition of benthic invertebrate communities from sensitive to pollution-tolerant species; and decreases in aquatic system biodiversity (USEPA, 1998). Tolerant species may process contaminants in a variety of ways, and the resulting novel metabolic pathways and products may affect ecosystem functions such as energy flow, productivity, and decomposition processes (Griffiths, 1983). Loss of any biological community in the ecosystem can indirectly affect other components of the system. For example, if the benthic community is significantly changed, nitrogen cycling might be altered such that forms of nitrogen necessary for key phytoplankton species are lost and replaced with blue-green algae, capable of nitrogen fixation (Burton et al., 2002).
Contaminated sediments can also result in economic impacts, as well as affect aquatic and human health. These impacts can be seen from added cost to industry as some harbors contain contaminated sediments that are not suitable for open water disposal. Delays in being able to find an acceptable land disposal facility will result in difficulty for ships in navigating through the canals and harbors. Some ships will have to come in with lighter loads to accommodate the decreased water depths. This, along with other problems getting cargo into ports results in added transportation costs (Ireland and Ho, 2005).
Numerous regulations exist throughout the world that authorize regulatory programs to address contaminated sediments. A few of these involve support for dredged material disposal, support for sediment remediation activities and support for sediment monitoring.
For dredged material, it is estimated that about 400 million cubic yards of sediment is dredged annually in the United States to maintain more than 400 ports and 25,000 miles of navigation channel. It was estimated that between 5 and 10% of dredged material was not suitable for open water disposal (NRC, 1997). It has also been suggested that this number could be as high as 50% in the Great Lakes (David Cowgill, USEPA/GLNPO, Chicago, Illinois, pers. comm.).
In the United States, one of the most comprehensive authorities available to USEPA to obtain sediment clean-up is commonly referred to as Superfund. In 1997, USEPA developed and published guidance for conducting Ecological Risk Assessments (ERA) within the Superfund program (USEPA, 1997). Sediment toxicity tests are a commonly used tool in ecological risk assessments (ERAs) and are used to assist in the determination of an unacceptable risk due to sediment contamination.
Sediment monitoring is an important instrument in assisting the United States and Canada to address sediment quality under the Great Lakes Water Quality Agreement. The Governments of the United States and Canada have identified forty-three areas of concern (AOCs) in the Great Lakes basin. These AOCs have been designated as such for having impairments to various beneficial uses (such as restrictions on fish and wildlife consumption). Several of these beneficial use impairments can be related back to contaminated sediments. Since 1997, a little over three million cubic yards of contaminated sediments have been remediated in the Great Lakes.
The tools used to evaluate sediment quality
The five general categories of sediment quality measurements are sediment chemistry, sediment toxicity, community structure, tissue chemistry, and pathology (Power and Chapman, 1992). Sediment chemistry is used as a chemical benchmark intended to predict the observation of harmful biological effects (or lack of effects). The objective of a sediment toxicity test is to determine whether or not contaminated sediments are harmful to benthic organisms (ASTM 2002a; USEPA 2000). The structure of benthic invertebrate communities represents an important indicator of sediment quality conditions. These assessments provide a means for assessing chemical related effects associated with exposure to sediments in the assessment area (USEPA 1992a; 1992b; 2002b). Numerous studies have documented changes in the composition of benthic communities resulting from sediment contamination (i.e., Rosenberg and Wiens, 1976; Hilsenhoff, 1982, 1987; Clements et al., 1992). Contaminated sediments represent important sources of the substances that accumulate in aquatic food webs (Ingersoll et al., 1997). Because these contaminants can adversely affect aquatic-dependent wildlife species and/or human health, tissue chemistry represents an important ecosystem health indicator in sediment quality assessments (ASTM 2002b; USEPA 2000, 2002b). Pathology can be defined as looking at disease and the modifications in cellular function. Sediment quality conditions can be evaluated by looking at fish health as fish that are exposed to contaminated sediments can exhibit impaired health, such as liver tumor frequency (USEPA, 2002b).
Each category has strengths and limitations for a national-scale sediment quality assessment. EPA developed the National Sediment Inventory (NSI) database, a sediment database containing sediment quality data throughout the United States from 1980 through 1999. During development, it was noted that sediment toxicity, community structure, and pathology measures are less widely available than sediment chemistry and fish tissue data in the broad-scale electronic format EPA sought for this database (USEPA, 2004). Community structure measures, such as fish abundance and benthic diversity, and pathology measures are potentially indicative of long-term adverse effects; yet there are multitudes of mitigating physical, hydrologic, and biological factors that might not relate in any way to chemical contamination (USEPA, 2004). Sediment toxicity tests have evolved into effective tools that provide direct, quantifiable evidence of biological consequences of sediment contamination that can only be inferred from chemical or benthic community analysis (USEPA, 2000). Sediment chemistry measures alone might not accurately reflect risk to the environment, but as in the case of the NSI database, it is a very frequently collected sediment quality measurement. Although these measures alone might not always accurately reflect risk to the environment, sediment quality guidelines (SQGs) have been developed as numerical chemical concentrations intended to be either protective of biological resources, or predictive of adverse effects to those resources, or both (Wenning and Ingersoll, 2002). SQGs for assessing sediment quality have been developed that combine contaminant concentrations with measures of the primary binding phase to address bioavailability for certain chemical classes, under assumed conditions of thermodynamic equilibrium (Di Toro, Mahoney et al., 1991; Di Toro Zarba et al., 1991; Ankley et al., 1996; NYSDEC, 1998; Di Toro and McGrath, 2000). Other methods, which rely on statistical correlations of contaminant concentrations with incidence of adverse biological effects, also exist (Barrick et al., 1988; FDEP, 1994; Field et al., 1999, 2002; Ingersoll et al., 2001; Long et al., 1995; MacDonald et al., 1996). Increasingly, SQGs are used as informal benchmarks or aids to interpret sediment chemistry data, in some cases in spite of their narrative intent, for a variety of purposes (Wenning and Ingersoll, 2002).
Looking at sediment quality on a national scale
In an attempt to evaluate sediment quality nationwide, USEPA developed the National Sediment Quality Survey (NSQS) report for the United States Congress. This report is the result of the Water Resources Development Act (WRDA) of 1992 that directed the USEPA to conduct a comprehensive national survey of data regarding the quality of sediments in the United States. The first step in this task was to develop the NSI database (mentioned above). This database is a national compilation of readily available data that has been compiled to evaluate sediment contamination throughout the United States. It includes sediment chemistry, tissue residue, and toxicity data collected from 1980 through 1999 from more than 50,000 stations and contains about 4.6 million analytical observations (USEPA, 2004). The report evaluated sediment chemistry, tissue chemistry, and sediment toxicity data, taken at the same sampling station, individually and in combination using a variety of assessment methods. Because of the limitations of the available sediment quality measures and assessment methods, this identification of contaminated sediment locations is identified as a screening-level analysis (USEPA, 2004). A screening-level analysis typically identifies many potential problems that prove not to be significant upon further analysis.
Due to the complex nature of the reactions among different chemicals in different sediment types, in water, and in tissues, no single sediment assessment technique can be used to adequately evaluate potential adverse effects from exposure to all contaminants. Uncertainties and limitations are associated with all sediment quality evaluation techniques (USEPA, 2004). In conducting this type of approach, many limitations exist (both with the data used as well as the assessment methodology employed). With respect to the data itself, inherent in the diversity of data sources are contrasting monitoring objectives and scopes; which makes comparison of data from different data sets difficult. For example, several of the databases contain only information from marine environments or other geographically focused areas (USEPA, 2004). As this effort focused on existing data available in an electronic format, there was no control over the type of sediment data collected. The lack of consistency among the different monitoring programs in the suite of chemicals analyzed represents an area of uncertainty in this type of data evaluation. In developing the NSI database, it was noticed that certain databases contain primarily information describing concentrations of metals or pesticides, whereas others contain data describing concentrations of nearly every chemical monitored in all of the NSI data. Also, matching assessment parameters (e.g., sediment chemistry, sediment toxicity) were typically lacking. Out of the nearly 20,000 stations evaluated, approximately 70% had data available for sediment chemistry only. This makes it virtually impossible to conduct a weight-of-evidence approach that is commonly recommended in sediment quality assessments.
The lack of data also had an impact on the assessment methodology. Missing data (e.g., total organic carbon—TOC, acid-volatile sulfides-AVS) impacted the application of some important assessment parameters. TOC and AVS are essential pieces for interpreting bioavailability, and subsequent toxicity, of nonpolar organics and metal contaminants, respectively. Also, because the evaluation approach had to be applicable across the entire United States, various assumptions were made (such as assuming 1% TOC when none was reported). Additionally, only those chemicals for which sediment chemistry screening values are available were evaluated in the NSQS report analysis (USEPA, 2004). Therefore, the methodology could not identify contamination associated with chemical classes such as ionic organic compounds (e.g., alkyl phenols) and organometallic complexes (e.g., tributyl tin).
Currently, no single sediment assessment technique can be employed to adequately evaluate potential adverse effects from exposure of all contaminants. This is due to the complex nature of the reactions among different chemicals in different sediment types, in water and in tissues (USEPA, 2004). Uncertainties and limitations exist with all sediment quality evaluation techniques. While recognizing the inherent limitations of the data and the assessment approach, this is a viable approach to broad scale sediment assessments as it evaluates the potential impact on both human health and aquatic life. This approach applies assessment techniques using multiple-lines-of-evidence recommended by national experts (Ingersoll et al., 1997) using sediment chemistry, tissue residue, and toxicity tests. This allows for the use of the most sensitive endpoint to assess environmental impacts (USEPA, 2004). These tools have been applied across North America as well as in other parts of the world, and results of these applications have been published in peer-reviewed literature (USEPA, 2004).
Recognizing the strengths and limitations, the NSQS report provides a screening-level assessment outlining stations throughout the United States where the probability of adverse effects to human health and/or the environment exist. More information on the assessment approach as well as the findings can be seen in The Incidence and Severity of Sediment Contamination in Surface Waters of the United States. National Sediment Quality Survey: Second Edition (USEPA, 2004).
Any opinions expressed in this publication are those of the author and do not necessarily reflect the official positions and policies of the U.S. EPA. Any mention of products or trade names does not constitute recommendation for use by the U.S. EPA.