Passive sampling methods for contaminated sediments: risk assessment and management

Abstract

This paper details how activity-based passive sampling methods (PSMs), which provide information on bioavailability in terms of freely dissolved contaminant concentrations (Cfree ), can be used to better inform risk management decision making at multiple points in the process of assessing and managing contaminated sediment sites. PSMs can increase certainty in site investigation and management, because Cfree is a better predictor of bioavailability than total bulk sediment concentration (Ctotal ) for 4 key endpoints included in conceptual site models (benthic organism toxicity, bioaccumulation, sediment flux, and water column exposures). The use of passive sampling devices (PSDs) presents challenges with respect to representative sampling for estimating average concentrations and other metrics relevant for exposure and risk assessment. These challenges can be addressed by designing studies that account for sources of variation associated with PSMs and considering appropriate spatial scales to meet study objectives. Possible applications of PSMs include: quantifying spatial and temporal trends in bioavailable contaminants, identifying and evaluating contaminant source contributions, calibrating site-specific models, and, improving weight-of-evidence based decision frameworks. PSM data can be used to assist in delineating sediment management zones based on likelihood of exposure effects, monitor remedy effectiveness, and, evaluate risk reduction after sediment treatment, disposal, or beneficial reuse after management actions. Examples are provided illustrating why PSMs and freely dissolved contaminant concentrations (Cfree ) should be incorporated into contaminated sediment investigations and study designs to better focus on and understand contaminant bioavailability, more accurately estimate exposure to sediment-associated contaminants, and better inform risk management decisions. Research and communication needs for encouraging broader use are discussed.

Keywords: Bioavailability; Contaminated sediments; Passive sampling methods; Risk assessment; Risk management.

Figure 1

Conceptual site model showing transport and exposure pathways of sediment-associated contaminants at a macroscale. (A) Air–water exchange. Contaminant transport through air deposition into the water and volatilization from the water into the air. (B) Sediment–water flux. Diffusive and active transport processes of contaminants across the sediment–water interface, including biological processes that assist or retard transport (e.g., bioturbation, bioirrigation, development of biological secretions, biofilms). (C) Land–water exchange. Surface runoff from contaminated soil, erosion of contaminated soil particles. Deposition of contaminated water and sediment onto land during high-water or flood events. (D) Point-source inputs. Storm water discharge, combined sewer overflows, sanitary sewer overflows, industrial effluents and outfalls, spills. (E) Groundwater discharge. The discharge of groundwater contaminated from land-based sources. (F) Surface water transport. Movement of surface water containing dissolved, colloidal, and particulate-associated contaminants. (G) Sediment transport. Movement of both clean and contaminated sediment particles through bed-load and suspended-sediment transport processes. (H) Bioaccumulation. Contaminant bioaccumulation through contact with dissolved-phase contaminants and the ingestion of contaminated sediment and prey (trophic transfer).

Figure 2

Conceptual site model showing transport and exposure pathways of sediment-associated contaminants at a microscale. Positions where contaminant flux is indicated are candidate locations for measurement of freely dissolved contaminant concentrations (C_free) using passive sampling methods (PSMs).

Figure 3

Fate and transport processes subject to modeling at contaminated sediment sites. From USEPA (2009). Estimates of freely dissolved contaminant concentrations (C_free) using passive sampling methods (PSMs) are expected to reduce uncertainty in the dissolved chemical term.