Ecological bioavailability of permethrin and p,p'-DDT: toxicity depends on type of organic matter resource.

Hydrophobic organic contaminants readily partition from aqueous to organic phases in aquatic systems with past research largely focusing on sediment. However, within many aquatic systems, matrices such as leaf material and detritus are abundant and ecologically important, as they may represent a primary exposure route for aquatic invertebrates. The objectives of the present study were to examine partitioning and toxicity to Hyalella azteca among permethrin and p,p'-DDT contaminated sediment, leaf, and a sediment-leaf mixture. Log organic carbon-water partitioning coefficients ranged from 4.21 to 5.82 for both insecticides, and were greatest within sediment and lowest in coarse leaf material. H. azteca lethal concentrations for 50% of the population (LC50s) ranged from 0.5 to 111µgg(-1) organic carbon, and were dependent on the matrix composition. The variation in sorption and toxicity among matrices common within stream ecosystems suggests that the ecological niche of aquatic organisms may be important for estimating risk of hydrophobic pesticides.

Determining modifications to bifenthrin toxicity and sediment binding affinity from varying potassium chloride concentrations in overlying water.

Bifenthrin, a current-use pyrethroid insecticide, has been repeatedly identified as a major contributor to toxicity in urban and residential stream sediment. Within an urban stream multiple stressors exist. However, other than pesticides, the influence of secondary stressors on bifenthrin toxicity has not been studied. The goal of this project was to study how dissolved ions, based on the model salt KCl, influence bifenthrin toxicity. The presence of these dissolved ions could influence bifenthrin toxicity either through joint action as a secondary toxicant or through changing the partitioning or bioavailability of bifenthrin between the sediment matrix and overlying water or pore water. The first objective was to determine if mixtures of bifenthrin and KCl, a commonly utilized reference toxicant, display additive toxicity to the benthic invertebrates Hyalella azteca and Chironomus dilutus using concentration addition and independent action mathematical models. The second objective of the present study was to examine how KCl dissolved in the overlying water influences partitioning and bioavailability of a pyrethroid (bifenthrin). Joint toxicity of bifenthrin and KCl was less than predicted by both concentration addition and independent action models. However, both models predicted the joint toxicity within a factor of two. Partitioning of bifenthrin was not significantly influenced by KCl concentrations based on K(oc) determinations and desorption to Tenax beads. This indicates that the fate and bioavailability of bifenthrin are not likely different in aquatic environments with varying dissolved ion concentrations. Therefore, the toxicological interaction that results in the antagonistic joint action between bifenthrin and KCl is likely due to the physiological effects of exposure to hypertonic solutions of KCl rather than alterations to bifenthrin bioavailability.

Identification and evaluation of pyrethroid insecticide mixtures in urban sediments.

Organochlorine, organophosphorous, and pyrethroid insecticides frequently have been detected together as mixtures in stream sediments. To simplify mixture analyses, additive toxic responses usually are assumed but rarely are confirmed, especially for compounds with similar modes of action. The first objective of the present study was to screen a database of 24 different pesticides and 94 urban-stream sediment samples collected throughout central and northern California (USA) to identify compounds and partial mixtures that dominated sample toxicity to Hyalella azteca. Pyrethroids and chlorpyrifos were the most toxicologically relevant compounds in terms of detection frequency, contribution to overall sample toxicity, and co-occurrence in the most common mixture patterns. Organochlorine insecticides were the least toxicologically relevant compounds, with only a small percentage of samples exceeding predefined screening values. The second objective was to confirm that mixtures of type I and type II pyrethroids display additive responses. Ten-day sediment toxicity tests of binary pesticide mixtures were conducted using H. azteca as the test organism. Observed dose-response curves were compared to those predicted from concentration-addition and independent-action models. Model deviation ratios (MDRs) were calculated at the median effect level to quantify the magnitudes of deviation between observed and predicted curves. Whereas the concentration-addition model adequately predicted toxicity for all the pyrethroid mixtures (MDRs within a factor of two), dose-response values deviated from additivity enough to warrant further investigation.

Partitioning and matrix-specific toxicity of bifenthrin among sediments and leaf-sourced organic matter.

Synthetic pyrethroids readily partition from the aqueous to the solid phase in aquatic systems. Previous work has focused on pyrethroid partitioning to sediment matrices. Within many aquatic systems, however, other carbon-containing materials are present and can be critically important to certain invertebrate species and ecosystem functioning. For example, some invertebrates readily process leaf material, and these processes may represent an additional route of contaminant exposure. To our knowledge, estimates for partitioning of pyrethroids to these nondissolved organic matter matrices and associated toxicity have not been examined. The objectives of the present study were to examine variation in organic carbon (OC)-based partition coefficient (K(OC)) among three size fractions of particulate organic matter from sugar maple (Acer saccharum) leaf litter and sediments for the pyrethroid insecticide bifenthrin and to examine variation in toxicity to Hyalella azteca among bifenthrin-bound organic matter matrices and sediment. Log K(OC) of [(14)C]bifenthrin was greatest within sediment (6.63+/-0.23; mean +/- standard deviation throughout) and lowest in coarse particulate leaf material (4.86+/-0.03). The H. azteca median lethal concentration was 0.07, 0.11, and 0.15 microg/g OC for leaf material, sediment, and a 50% mix of leaf and sediment, respectively. Nonoverlapping 95% confidence intervals occurred between the leaf treatment and the leaf-sediment treatment. This pattern was supported in an additional experiment, and at 0.22 microg/g OC, H. azteca survival was greater in the leaf-sediment mixture than in sediment or in leaf material alone (F=29.5, p<0.0001). In systems that contain sediment and leaf material, both greater partitioning of bifenthrin to the sediment fraction and preferential use of leaf substrates may drive H. azteca survival.