Ecological Evaluation Technical Guidance - State of New Jersey

al., 1996; USEPA 1994a, 2003). The equilibrium partitioning (EqP) model,widely used for many years, measures bioavailability by calculating a porewater concentration. It is assumed that equilibrium exists between thecontaminants sorbed to the bulk sediment (organic carbon) and the sedimentpore water as expressed in following equation:C W = C S / f OC K OCSediment pore water concentrations (C W ) are derived through a calculationwhereby the bulk sediment concentration (C S ) is divided by the fraction oforganic carbon (f OC ) times the organic carbon partition coefficient (K OC ).Toxicity in sediments can be estimated by comparing the derived pore waterconcentration to effects concentrations previously measured in water-onlyexposures (e.g. USEPA Ambient Water Quality Criteria).While the use of EqP is considered to provide a more accurate measure ofbioavailability (toxicity) than simply screening bulk sediment data againstscreening values, more recent advancements in pore water sampling havemade it possible to measure site-specific bioavailability at an even greateraccuracy. Many of these methods have the advantage of directly measuringpore water, and thus reduce the inherent uncertainty associated withcalculating a pore water concentration via a model. Details regardingsediment pore water sampling techniques can be found in Appendix F.Groundwater to Surface Water Discharges:Contaminated groundwater that discharges to surface water also has thepotential to impact the pore water present in the interstitial space of sediment.Historically, measuring chemicals in groundwater that discharges to surfacewater was evaluated via groundwater monitoring wells positioned along theshoreline, through mass-balance equations designed to model dischargeconcentrations, or through the analysis of grab surface water samples.However, these methods do not accurately characterize the pore watercontaminant levels in the sediment in the biotic zone where the majority of thebenthic organisms reside.More recently, methods have been developed to sample tidal and subaqueousgroundwater discharges to a water body (Chadwick and Hawkins, 2008;Chadwick et al., 2003; Duncan et al., 2007a, b). These tools include intertidalseep sampling, piezometers, and diver-deployed diffusion samplers. Thereferences provide a detailed description of the tools and measures applicableto measuring COPECs in groundwater and pore water. Additionally, theUSEPA has released a document stressing the importance of evaluating thisecologically significant zone: Evaluating Ground-Water/Surface-WaterTransition Zones in Ecological Risk Assessments (USEPA, July 2008).Diffusion bags have also been used for the collection of pore water fromsediments when the groundwater to surface water pathway is of concern. Thediffusion bags are deployed and are allowed to equilibrate over time. Thebags are then collected and the water within the bags is analyzed for theEcological Evaluation Technical Guidance Document 59Version 1.2 8/29/12