Development of a Sediment Remedial Action Objective for Tributyltin

Transcription

1 Public Works and Government Services Canada (PWGSC) Esquimalt Graving Dock Site, Esquimalt Harbour, Victoria BC Development of a Sediment Remedial Action Objective for Tributyltin

2 Site Context Esquimalt Harbour Constance Cove EGD Waterlot (owned by PWGSC) Historical Contamination June 11,

3 An organo-metal; Bioaccumulative in tissue; Tributyltin (TBT) A Unique Contaminant Less suited to standard toxicity tests because of uptake duration; Highly toxic to some aquatic life (reproductive/growth effects observed); No Canadian sediment quality guideline available; Regional contaminant (found throughout Georgia basin). The RAO strategy for other substances (based on sediment chemistry) is poorly suited to TBT need a different approach. June 11,

4 Context from Other Jurisdictions Puget Sound Dredge Disposal Analysis (PSDDA) program in Washington State. Sediment screening level of 73 µg/kg dw. Problems: High uncertainty in the technical derivation procedures; Background contamination. Meets PSDDA screening value Above PSDDA screening value June 11,

5 Purpose of TBT Assessment Evaluate effectiveness of other RAOs for addressing risks of co-occurring TBT contamination; and Identify whether sediment management units may require intervention that lie beyond the remedial footprint identified for other substances. Principles: 1. Apply risk assessment framework; 2. Acknowledge local background contamination; 3. Acknowledge bioaccumulation pathway for TBT; 4. Consider site-specific sediment properties; 5. Consider relevant biological endpoints. June 11,

6 Threshold Derivation Procedure Tissue Threshold Determine tissue threshold(s) protective of benthic community Bioaccumulation Relationship Determine linkage between tissue and sediment, considering organic carbon and lipid content Sediment Threshold Calculate sediment threshold in organic carbon units (µg/kg OC) Site-Specific Application Convert to dry weight using site-specific sediment TOC (µg/kg dry weight TBT) June 11,

7 Adverse Responses of TBT to Marine Invertebrates (Growth, Mortality, Reproduction) June 11,

8 Survival Endpoints in Invertebrates June 11,

9 Growth Endpoints in Invertebrates 0.13 µg/g 0.50 µg/g June 11,

10 Reproductive Endpoints 0.13 µg/g 0.50 µg/g June 11,

11 Tissue TBT Threshold Summary An upper threshold (0.50 µg/g TBT ww) June 11, 2012 A lower threshold (0.13 µg/g TBT ww) Protective of survival, growth, and reproduction for most organisms. 11 Protective of all organisms, including sensitive animals such as gastropods (sterilization due to imposex)

12 Bioaccumulation Relationships Can be derived from laboratory uptake experiments; Can also be developed from tests conducted at similar sites (e.g., Vancouver Shipyards, Harbor Island Superfund Site. 30 µg/g OC (lower-bound) 100 µg/g OC (upper bound) June 11,

13 Site Specific TBT Threshold Synoptic TOC data not available for all sampling points Dry weight TBT threshold desirable The mean, median, and geomean of TOC for both the Waterlot and Constance Cove are all approximately 2.5% The corresponding dry weight sediment TBT thresholds are: 750 µg/kg dry weight (0.75 µg/g dw); 2,500 µg/kg dry weight (2.5 µg/g dw). Negligible risk Low risk Moderate to high risk June 11,

14 Lines of Evidence Evaluation 1. Absolute risks Comparison to site-specific TBT sediment thresholds. 2. Relative risks Profile of near-field elevations of TBT versus regional reference conditions. 3. Local contaminant distributions Spatial variations in sediment TBT concentrations, and linkage to the PWGSC water lot uses. June 11,

15 Maximum TBT Concentrations in Surface Sediments of Constance Cove (top 10 cm) Negligible risk Low risk Moderate to high risk June 11,

16 Cumulative Distribution Plots (Pre- and Post- Remediation) Diminishing returns beyond EGD remediation footprint Remediation addresses most of the moderate-to-high risk TBT Scale of EGD Waterlot Scale of Constance Cove Sediment TBT (µg/g dry) Cumulative Area (sq. m.) Cumulative Area (sq. m.) June 11,

17 What about those hot spots? Limited areas of elevated TBT contamination remain above the 2.5 µg/g dw threshold, even with remediation for other RAOs Constraints to additional remediation: 1. Delineation difficult; 2. TBT more heterogeneous at distance from point source; 3. Influence of non-site related sources (other regional sources); 4. cost and short-term disruption to resident biota. June 11,

18 Conclusions Tissue-based TBT assessment more reliable method for deriving a sediment effects threshold. Most of benthic community (bivalves, crustaceans, polychaetes) protected using dry weight TBT of 2.5 µg/g. Most areas relevant to sensitive gastropod habitat (e.g., whelks) are nearshore areas already part of remediation design. Limited benefit to the environment in terms of considering additional sediment TBT contamination for removal (i.e., beyond the currently proposed remediation footprint). The tiered risk framework provides for potential refinement of the uncertainties in our assessment, but future study (if conducted) should be linked to regional-scale efforts to evaluate TBT. June 11,