Table 1.

Factors to consider when selecting between ex situ or in situ application of PSMs

	Approach
Factor	Ex situ	In situ
Ability to estimate equilibrium Cfree	Laboratory conditions can be controlled to better attain equilibrium.	Uncertainty can occur; need to use PRCs, multiple polymer thicknesses, or time series sampling to confirm equilibrium. Time series interpretation can be impacted by temporal changes in the field.
	Comparison to independent confirmatory methods (e.g., air bridge) can be applied.
Spatial scale (e.g., to differentiate between biologically active zones and underlying sediments or contaminant migration through a cap)	Sediments are frequently composited and/or homogenized to avoid concentration variability caused by vertical and horizontal spatial heterogeneity.	Fine-scale spatial (vertical and horizontal) patchiness in concentrations can be measured (e.g., identify gradients).
	Coring followed by passive sampling in intact cores can maintain spatial characteristics if not influenced dramatically by site dynamics.	Best approach to capture field conditions.
Contaminant depletion	Mixing (e.g., tumbling of sample) during equilibration period is used to limit localized depletion.	Contaminant depletion may occur in the zone around samplers; use of multiple polymer thicknesses or time series analysis may be used to evaluate depletion.
Statistical design	Multiple treatments and replication are possible; hypothesis testing can be performed.	Multiple treatments, replication, and hypothesis testing are possible, but logistically challenging and expensive.
Ease of experimentation	Experiments are simpler to perform under laboratory conditions.	Expense, achieving experimental and statistical design goals, safety concerns, weather, adverse site conditions, and vandalism.
Ability to capture field conditions (e.g., currents, tidal cycles, groundwater intrusion, sediment-water column fluxes, bioturbation, temperature and salinity change)	Laboratory conditions are frequently standardized, but can be altered to attempt to replicate some field conditions.	Best approach for capturing field conditions.

PRCs = performance reference compounds; PSMs = passive sampling methods.