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In the United States, there is an absence of federal guidance related to deriving and applying background concentrations at contaminated sediment sites. This absence has resulted in significant variability, uncertainty, and disagreement regarding how representative background concentrations of chemicals of concern should be derived for these sites. The present article discusses important considerations in the derivation of representative background concentrations to be used in the evaluation of contaminated sediment sites. Specifically, a thorough understanding of a site is critical to selecting the background reference areas from which representative background concentrations can be derived, representative background concentrations should account for contributions from those background chemical inputs (natural and anthropogenic sources) that will continue affecting the site even after remediation, perceived outliers should not be eliminated from the background data set just because they are the highest or lowest values, and geochemical evaluation of trace metals is a useful tool for deriving representative background concentrations. On a site‐specific level, representative background concentrations are critical for putting site‐related risk into context, developing a cost‐effective and technically feasible remedial approach, understanding the potential for recontamination, and ensuring long‐term remedy success. In a broader context, clear guidance from the United State Environmental Protection Agency (USEPA) for deriving and applying background concentrations for contaminated sediment sites would help promote national consistency in site assessment and remedy decision making. Integr Environ Assess Manag 2019;00:000–000. © 2019 The Authors. Integrated Environmental Assessment and Management Published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC)

At many sites, long-term monitoring (LTM) programs include metals as chemicals of concern, although they may not be site-related contaminants and their detected concentrations may be natural. At other sites, active remediation of organic contaminants in groundwater results in changes to local geochemical conditions that affect metal concentrations. Metals should be carefully considered at both types of sites, even if they are not primary contaminants of concern. Geochemical evaluation can be performed at LTM sites to determine if the monitored metals reflect naturally high background and, hence, can be removed from the analytical program. Geochemical evaluation can also be performed pre-and post-treatment at active remediation sites to document the effects of organics remediation on metals and identify the processes controlling metal concentrations.Examples from both types of sites are presented in this article. O c 2008 Wiley Periodicals, Inc. INTRODUCTIONMetal concentrations in groundwater samples can be natural, reflect anthropogenic input, or be indirectly affected by organic contaminants and/or their treatment. The mechanisms controlling metal concentrations under natural conditions are well established (Appelo & Postma, 2005;Drever, 1997;Hem, 1985;Stumm & Morgan, 1996). One mechanism of particular importance is trace-element adsorption on suspended minerals. In neutral-range pH waters lacking high concentrations of complexing anions (such as fluoride), detections of aluminum above approximately 1,000 micrograms per liter (µg/L) indicate the presence of suspended clay minerals; similarly, under neutral-range pH and oxic conditions, detections of iron above approximately 1,000 µg/L indicate the presence of suspended iron oxide minerals (Hem, 1985). Trace elements have affinities to adsorb on the surfaces of these suspended clay and oxide minerals, depending in part on the charges of the element species and sorptive surfaces.In the absence of contamination, the concentrations of the trace element versus aluminum (representing clays) or trace element versus iron (representing iron oxides) usually covary in a given data set, when adsorption of the trace element on suspended particulates is the dominant process. Under reducing conditions, however, such covariance is not always observed, at least for iron and associated trace elements that are 99Geochemical Evaluation of Metals in Groundwater at Long-Term Monitoring Sites and Active Remediation Sites redox active. Iron oxides become soluble as the redox potential drops below a threshold value, and this increases the dissolved concentrations of iron and the dissolved concentrations of trace elements that were adsorbed on the iron oxide surfaces. Detailed discussion of the effects of suspended particulates and reductive dissolution on elemental concentrations, as well as the methodology for evaluating these effects, is provided in Thorbjornsen and Myers (2007).Iron oxides become soluble as the redox potential drops below a threshold value, and this increases the di...