Chemical Mixtures and
Pulsed Exposure Tests

Introduction Methods Results The Bottom Line

Introduction

Over the last two years, we have been investigating how the mixtures of contaminants in stormwater influences their toxicity.  The purpose of these experiments is to generate a more holistic understanding of how organisms may respond to pollutants in natural waters.  Because it is much easier to define the toxic levels of pollutants using tests of one chemical at a time, regulators have taken this approach when developing water quality criteria. But as chemicals interact in natural waters, their impacts on aquatic organisms can vary. This is because both the uptake and toxic effects of one chemical can be altered in the presence of another.  The nature of the toxic responses exhibited by organisms exposed to chemical mixtures can be additive, synergistic or antagonistic.

Suspended solids, such as grass clippings, leaves and suspended organic particulates, are a major component of stormwater runoff.  Over the past year, samples collected from stormwater entering the bay have been high in suspended solids.  In traditional concentration-response toxicity tests, such organic solids are introduced only in the form of food (YCT, a mixture of yeast, alfalfa, and trout chow), and the resulting suspended solid concentrations are much lower than those found in the stormwater runoff. The presence and the amount of these suspended solids may be of concern because the metals in test solutions may bind to the organic solids and then settle out of suspension, changing the concentration of bioavailable toxicant to which the organism is exposed.  Our previous results have also suggested that there may be a relationship between the age of the parents of the test organisms and the neonate’s ability to survive in toxicant-spiked test treatments.

Our research over the past two years, therefore, was designed to examine the effects of maternal age, as an indicator of neonate fitness, and the presence and amount of suspended food solids on survival and reproduction in the cladoceran, Ceriodaphnia dubia.  Our intent is to use this information to help improve the environmental realism of toxicity tests conducted in the laboratory, to increase control over variability in the lab, and through these improvements, to gain better insight into toxicity in natural waters.

We conducted our experiment using the common metal pollutants copper and zinc and the organophosphate insecticide diazinon.  All three of these contaminants have been found in our stormwater samples and are also common in other analyses of stormwater (Cole et al. 1984, Strunk 1992, Lenat and Crawford 1994, Ellis and Hvitved-Jacobson 1996, Sansalone and Buchberger 1997, Watzin et al. 2000, 2002, Hoffman et al. 2000).

Literature Cited

Cole, R.H., R.E. Frederick, R.P. Healy and R.G. Rolan. 1984. Preliminary findings of the priority pollutant monitoring project of the nationwide urban runoff program. J. Water Pollut. Cont. F. 56:898-908.

Ellis, J.B. and T. Hvitved-Jacobson. 1996.  Urban drainage impacts on receiving waters.  J. Hydraul Res. 34(6):771-783.

Hoffman, R.S., R.D. Capel, and S.J. Larson.  2000. Comparison of pesticides in eight U.S. urban streams.  Environ. Toxicol. Chem. 19(9):2249-2258.

Lenat, D.R, and J.K. Crawford. 1994. Effects of land use on water quality and aquatic biota of three North Carolinian Piedmont streams. Hydrobiologia. 294:185-199.

Sansalone, J.J. and S.G, Buchberger. 1997. Partitioning and first flush of metals in urban roadway stormwater. J. Environ. Eng. 123:134-143.

Strunk 1992. Case studies of variations in suspended matter transport in small catchments. Hydrobiologia. 235/236:247-255.

Watzin, M., A. McIntosh, A. Shambaugh, A. Pitt, A. Mahar, E. Brines. The Burlington Bay Project: Water quality and ecosystem health along the shores of Lake Champlain. Progress report April 30, 2002. Unpublished report.

Watzin, M, A. McIntosh, S. Beauvais, A. Shambaugh and A. Mahar. The Burlington Bay Project: Water quality and ecosystem health along the shores of Lake Champlain. Progress report February 15, 2000.  Unpublished report.

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