Assessing Site-Specific Risk to Aquatic Biota from Multiple Exposure Pathways.  
Vertucci, F.A., Naddy, R.B., and Tillquist, H.T. 
ENSR, 4303 W. La Porte Ave., Fort Collins, CO. 

Abstract--Many risk assessments rely on ambient water quality criteria (AWQC) to evaluate potential risks to aquatic biota from contaminant exposure.  The AWQC are not intended to consider risk from sediment or dietary exposure pathways and few assessments account for site-specific bioavailability of metals.  Risks to a variety of aquatic biota from metals in the Upper Clark Fork River (UCFR), Montana were evaluated using assessment tools specific to each exposure medium- water, sediment, and diet.  Each medium was evaluated using extensive site-specific exposure data and, when possible, effects criteria that reflect site-specific metals bioavailability.  Risk from exposures to water column metals (As, Cd, Cu, Pb and Zn)  was evaluated by comparing observed dissolved metals concentrations (an average of long term monitoring) to chronic AWQC and a site-specific rainbow trout chronic toxicity reference value (TRV) developed by EPA based on testing UCFR water. (When environmental concentrations are below TRV’s, risks are usually assumed nominal.) Sediment metals exposure was evaluated using EPA draft Equilibrium Sediment Guidelines (ESGs--i.e., simultaneously extractable metals - acid-volatile sulfide and sediment porewater summed as toxic units) and using bulk Sediment Effects Concentrations (SEC) derived from toxicity tests with UCFR bed sediments.  Trout dietary exposures were addressed by comparing extensive benthic macroinvertebrate tissue data with dietary TRVs derived from selected literature.  Dissolved metals concentrations were generally lower than the water column TRVs for each metal.  Metals in pore water and bulk sediments were below ESGs and SECs.  Furthermore, benthic macroinvertebrate tissue concentrations did not exceed dietary TRVs.  These results predict nominal risk to most UCFR aquatic biota under observed conditions for each of the exposure pathways evaluated.