The effects of sediment-associated cadmium on aquatic organisms employing continuous-flow bioassays.

Abstract

A 21-day single pass continuous-flow sediment bioassay system was used to examine: (1) The toxicity and bioaccumulation of sediment-associated cadmium on midge larvae (Chironomus tentans); (2) The effect of midge bioturbation on the resuspension of sediment-associated cadmium and its subsequent uptake from water by fathead minnows (Pimephales promelas); (3) The impact of sediment storage time on cadmium bioavailability; and (4) The dominant cadmium bioavailable form. Midges were subjected to four concentrations of cadmium spiked sediment; while, fish were exposed only to the water overflowing from the midge sediment chambers. The midge bioturbation increased the bioconcentration factor in fish; but, no statistically significant adverse effect on growth rate was determined. No measurable change in cadmium water concentration was observed. The results of four separate experiments run after designated storage periods revealed that cadmium concentrations did not significantly change in respective levels with time. However, significant differences in midge toxicity (p = 0.001) and bioaccumulation factors (p $<$ 0.001) were observed with storage periods of 1 to 120 days (LD$\sb{50}$s of 0.33(NC) to 4.38 (1.46, 25.87) mg Cd/Kg with 95% C.I.). No significant differences were observed in the bioconcentration factors of fish or in the growth of the fish (p = 0.526) and midges (p = 0.127). Based on these results, contaminated sediments storage should be standardized to assure comparable bioaccumulation and toxicity data. Cadmium partitioning to sediment was estimated from both sorption and desorption studies. The partition coefficient was determined at apparent equilibrium via the Freundlich isotherm (K$\sb{\rm f}),$ because of a better fit to observed curvilinear isotherms. The high value of desorption compared to sorption suggests that the bioavailability of sediment-associated cadmium would be overestimated when based solely on sorption experiment. The employed continuous-flow bioassay is superior to the existing static and recirculating systems because: (1) Long-term exposure can be accommodated; (2) No waste accumulation over time; (3) Water quality can be maintained; (4) Different flow rates can be employed; (5) Fate and form of the bioavailable contaminant can be determined; (6) Facilitates multi-species exposures; and (7) Bioturbation influences can be investigated.