Deriving sediment Interstitial Water Remediation Goals (IWRGs) for the protection of benthic organisms from direct toxicity
Citation:
Burkhard, L., Dave Mount, AND R. Burgess. Deriving sediment Interstitial Water Remediation Goals (IWRGs) for the protection of benthic organisms from direct toxicity. Battelle’s Conference on Remediation and Management of Contaminated Sediments, New Orleans, LA, January 09 - 12, 2017.
Impact/Purpose:
Not Applicable
Description:
Background/Objectives. Passive sampling is becoming a frequently used measurement technique at Superfund sites with contaminated sediments. Passive sampling measures the concentrations of freely dissolved chemicals (Cfrees) in the sediment interstitial water. The freely dissolved chemical is a good surrogate for and a very practical means for estimating the concentrations of bioavailable chemical in the sediments. Building from this approach, a methodology is proposed to derive sediment Interstitial Water Remediation Goals (IWRGs) for the protection of benthic organisms from direct toxicity using Cfrees measured with passive sampling.Approach/Activities. In the early 2000s, EPA developed and released Equilibrium Partitioning Sediment Benchmarks (ESBs) for a series of chemicals. ESBs are intended to be chemical concentrations below which unacceptable toxicity to benthic organisms does not occur. The ESBs (expressed with the units of ug/g OC) were derived using the equations:ESB= K_OC×FCV where K_OC=0.00028+0.983K_OWThe KOC is the organic carbon normalized sediment-water chemical partition coefficient, FCV is the Final Chronic Value from EPA’s ambient water quality criteria for the protection of aquatic life, and KOW is the n-octanol/water partition coefficient for the chemical. At a specific site, the remedial goal (CS:ESB µg/kg-dw) in sediment are then derived using the site-specific fraction of organic carbon in the sediment (fOC:SS) at the site:C_(S:ESB)=ESB×f_(OC:SS)In practice, a challenge for the ESB approach is that the KOC values in field sediments often differ from those calculated from KOW. The IWRG approach uses the same scientific understanding incorporated into the ESB methodology, but addresses the KOC issue by basing the assessment directly on passive sampling measurements of Cfree in site sediments. In equation form, the remedial goals (CS:IWRG µg/kg-dw) in sediment are derived using site-specific values:C_(S:IWRG)=K_(OC:SS)×f_(OC:SS)×FCV where K_(OC:SS)= ((C_S⁄f_(OC:SS) ))⁄C_free The KOC:SS is the site-specific organic carbon normalized sediment-water chemical partition coefficient, and CS is the concentration in the bulk sediment (ug/kg-dw).Results/Lessons Learned. The scientific underpinnings of the methodology will be provided along with illustrative examples for sediments with one toxicant and with a mixture of toxicants. Consistency between passive sampling measurements and sediment toxicity test results is important. When consistency is demonstrated, one can be reasonably assured that the causes of toxicity to benthic organisms in the sediment have been correctly identified and that the developed IWRGs for the toxicants will be protective of the benthic organisms at the site. The methodology will be published as EPA guidance (EPA 600/R-15/289).