Potential Toxicity of Pesticides in Midwestern Rivers  

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Potential Toxicity of Pesticides in Midwestern Rivers

Problem
Small fractions, estimated at <1 to 2% of the pesticides applied to Midwestern cropland are lost from fields and enter nearby streams during rainfall events. In many cases aquatic organisms are exposed to mixtures of chemicals, which may lead to greater non-target risk than that predicted based on traditional risk assessments for single chemicals. Relatively little research has been directed at determining the risk of environmental mixtures of pesticides to non-target aquatic organisms.

Methods
One approach involves computing a toxicity index using actual measurements of pesticide residues in stream water and published estimates of acute toxicity. We evaluated the potential toxicity of environmental mixtures of 5 classes of pesticides using concentrations from water samples collected from sites (fig 1) on 76 Midwestern streams during late spring or early summer runoff events in 1998.

Figure showing stream sites sampled in 1998 pesticide reconnaissance
Figure 1

The 5 classes of pesticides examined are: (1) ALS-inhibiting herbicides (including sulfonylureas, sulfonamides, and imidazolinones), (2) triazine herbicides, (3) chloroacetamide herbicides, (4) organophosphate insecticides, and (5) carbamate pesticides. Acute toxicity data are primarily from the EPA’S ECOTOX database, but other sources were also used. Published EC50 values for two aquatic plants (duckweed, Lemna gibba; and green algae, Selenastrum capricornutum), and LC50 values for two aquatic vertebrates (bluegill sunfish, Lepomis macrochirus; and chorus frogs, Pseudacris triseriata) are used as toxicity metrics in this study (fig 2). Potential chronic effects of low-level pesticide exposures or the non-cancer or mutagenic properties of pesticide exposures are not address in this study.

Image showing 5 classes of pesticides examined
Figure 2

Toxicity index (TI) values are calculated as the concentration of a compound in the sample divided by the EC50 or LC50 of that compound for an aquatic organism. All non-detects were treated as zeros for this calculation. Individual TI values were summed for all pesticides in a pesticide class. When an EC50 or LC50 value was not available (for example there was no reported EC50 value for imazaquin on duckweed), the mean value for the pesticide class was used. If less than 50% of the pesticides in a class had acute toxicity estimates, then a mean value was not calculated and a TI was not calculated for the pesticide with missing data.

The TI values are summed within a pesticide class and for all classes to determine additive pesticide class and total pesticide toxicity indices. There is some debate over the validity of summing TI values for classes of compounds with different modes of action. Studies of mixtures of chemicals have generally concluded that an additive model is appropriate for estimating the toxicity of mixtures of chemicals with the same mode of action. However, for mixtures of chemicals with differing modes of action, additive models may overestimate the mixture toxicity and independent action models may be more accurate. Some recent research has also documented synergistic (more than additive) toxicity of mixtures of pesticides from different classes. Acute toxicity estimates for herbicide transformation products were not found in any of the listed sources. There are limited studies that show that herbicide transformation products may or may not be as toxic as their corresponding parent compounds. For this study, primary herbicide transformation products where estimated to be one-half as toxic as the parent herbicides.

TI values greater that 1.0 indicate probable toxicity of a class of pesticides to the subject aquatic organism. TI values greater than 0.5 indicate potential toxicity, while TI values greater than 0.1 indicate limited toxicity.

Results

The percentage of samples with TI values greater than 0.1, 0.5 and 1.0 for each class of pesticide and for total pesticides were calculated and are shown in figure 3. None of the samples had probable, potential, or limited toxicity from any of the five pesticide classes to bluegill sunfish.

Less than 10 percent of the samples showed probable or potential toxicity from ALS inhibitors to three of the four organisms (fig. 3A). There was no toxicity data for frogs.

Less than 10 percent of the samples showed probable or potential toxicity from triazine herbicides to any of the four organisms (fig. 3B). A few samples had potential or limited toxicity from triazines to frogs. For this calculation, the bullfrog (R. Catesbeiana) with a reported LC50 of 410 ug/L was used as the test organism. LC50ís for triazines on chorus frogs were not found. Reported LC50 values of atrazine on other frog species ranged from 220 to 127,000 μg/L.

More than 15% of samples showed potential toxicity from chloroacetamides to duckweed and 10% had TI values of 1.0 or more (fig. 3C). There was no chloroacetamide toxicity data for frogs.

Only a few samples had limited or potential toxicity from organophosphate insecticides to frogs (fig. 3D), all as a result of the occurrence of chlorpyrifos. Chlorpyrifos toxicity to frogs is highly species dependent, but data for chorus frogs was not be found. For this calculation the American toad (Bufo americanus) with an LC50 of 1 ug/L was used. Other reported LC50 values for chlorpyrifos on frogs ranged from 10 to 3,000 ug/L. EC50 values for most organophosphates to duckweed or green algae were not be found.

None of the samples had probable, potential, or limited toxicity from carbamates to the four organisms (fig. 3E). There were no carbamate toxicity data for frogs and limited data for duckweed and green algae.

The TI values for the 5 classes of pesticides were summed to estimate a total pesticide TI values (fig. 3F). Duckweed was the most susceptible organisms investigated; 17% of the samples had total pesticide TI values for duckweed greater than 1.0 and 27% had TI values greater than 0.5. For green algae, 8% of the samples had total pesticide TI values greater than 1.0, and 15% had TI values greater than 0.5. Only 1% of samples had total pesti-cide TI values for frogs greater than 0.5, but there was limited LC50 data for frogs.

The effects of herbicide transformation product occurrence on the potential toxicity of stream water to aquatic organisms have not been well studied. TI values for triazine and chloroacetamide herbicides and for total pesticides were recalculated using the assumption that herbicide degradates were one-half as toxic as their parent compounds to the four aquatic organisms.

The addition of the triazine degradates had only a minor effect on the percentage of samples with TI values for duckweed or green algae that were greater than 0.1, 0.5, or 1.0 (figs 3B and 3G).

However, accounting for the chloroacetamide degradates added substantially to the percentages of samples with TI values for duckweed and green algae that were greater than 0.1 0.5 or 1.0 (figs. 3C and 3H).

The effect on the total pesticide TI values was also substantial. Twenty-six percent of the samples had total pesticide plus degradate TI values for duckweed greater than 1.0, and 50% had TI values greater than 0.5 (fig. 3I). Ten percent of the samples had total pesticide plus degradate TI values for green algae greater than 1.0, and 23% had TI values greater than 0.5.

Figure 3A showing ALS inhibitors 1998
Figure 3A
Figure 3B showing Triazines 1998
Figure 3B
Figure 3C showing Chloroacetamides 1998
Figure 3C
Figure 3D showing Orgaophosphates 1998
Figure 3D
Figure 3E showing Carbamates 1998
Figure 3E
Figure 3F showing total pesticides 1998
Figure 3F
Figure 3G showing Triazines and degradates 1998
Figure 3G
Figure 3H showing Chloroacetamde degradate 1998
Figure 3H
Figure 3I showing total pesticides degradates 1998
Figure 3I
 


Conclusions
Three conclusions can be drawn from the results of this investigation. First, water in Midwestern streams during spring and early summer runoff events can contain pesticides in sufficient quantities to be toxic to non-target aquatic organisms. In this study, the concentrations of some pesticides did exceed concentrations thought to affect aquatic plants. Still, this data set may underestimate potential effects of pesticides on aquatic systems in smaller streams because peak concentrations are generally inversely related to stream size. Second, accounting for herbicide transformation products can substantially increase the estimated toxicity of stream water to aquatic plants. More information is needed on both the occurrence of herbicide and insecticide transformation products in streams and their toxicity to aquatic organisms. Finally, the quality of this analysis is limited by the lack of acute toxicity data for many of the pesticide-organism combinations.

Reference
This report is based on a poster presented at the 2001 Annual SETAC meeting and an article published in the journal "Water & Science Technology."

Battaglin, W.A. and Fairchild, J. 2001. Potential toxicity of pesticides measured in Midwestern streams to aquatic organisms. abstract. Society of Environmental Toxicology and Chemistry 22nd Annual Meeting Abstract Book, p. 204.
Battaglin, W.A. and Fairchild, J. 2002. Potential toxicity of pesticides measured in Midwestern streams to aquatic organisms. Water Science and Technology 45 (9):95-103.

 

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