Toxicity Evaluation of Diazinon Contaminated Leaf Litter

Diazinon (O,O-diethyl O-[2-isopropyl-6-methyl-4pyrimid-inyl] phosphothiate) is an organophosphate pesticide withwidespread use on a variety of agricultural crops, such asfruit trees, corn, and tobacco (Burkepile et al., 2000).Approximately 6 million kg of diazinon are appliedannually in the United States and, as a result, are a potentialcontributer to nonpoint source contamination of aquaticenvironments (Banks et al., 2003). Within these aquaticenvironments, decaying plant materials and leaf litter canbe a prominent physical feature, especially during autumn(Webster and Benﬁeld, 1986; Federle and Ventullo, 1990).These materials have extensive surface areas conducive tosorption of soluble organic contaminants (Pierce et al.,1977). This allows for the potential entry of these com-pounds into detrital food webs (Odum and Drifmeyer,1978) resulting in potential effects on aquatic ecosystems(Forrow and Maltby, 2000).Despite the extensive presence of detrital materialwithin some aquatic systems, such as wetlands, marshes,small streams, and heavily canopied rivers, ponds, andlakes, limited research or information exists on the inﬂu-ence of pesticide-contaminated detritus on aquatic eco-systems (Odum et al., 1969; Swift et al., 1988; Harrahyet al., 1994). With increasing use of constructed wetlandsto mitigate nonpoint source contamination, there is agrowing need to assess the role of detritus in understandingthe fate and effects of various contaminants on nontargetaquatic organisms.This study presents an assessment of leaf litter con-tamination in a constucted wetland and a toxicity evalua-tion using a standard laboratory bioassay with thefreshwater test organism, Hyalella azteca Saussure.Materials and MethodsLeaf litter was simulated using postabcission Norwaymaple leaves (Acer platanoides) contained within polyesterbags having 0.5 cm diameter mesh openings. Approxi-mately 20 g (dry weight) of leaf litter was added to eachbag, and approximately 25 g of stainless steel weights wereadded to prevent ﬂoating and to ensure all leaf litter re-mained submerged. Five leaf-litter bags were placed to-gether in the center of each of three cells within aconstructed wetland designed for the mitigation of agri-cultural contaminant runoff (e.g., sediment, pesticides,nutrients, etc.). Wetland cells included a sediment retentionpond (SRP), primary wetland cell (1 cell), and a secondary(2 , ﬁnishing) wetland cell located adjacent to BeasleyLake in Sunﬂower County, Mississippi, USA. The con-structed wetland was dosed with 0.16 mg/L diazinon (ac-tive ingredient) as Diazinon 4E , and 403 mg/L suspendedsediment (as a carrier), simulating a 1.3-cm rainfall eventand runoff from a 14 ha agricultural ﬁeld. Rainfall eventwas based on climatological data from the Beasley Lakeweather station, whereas the contributing area (14 ha) is theactual drainage area feeding the constructed wetland. Oneleaf-litter bag was collected from each wetland cell at 8 h,48 h, 7 d, 15 d, and 27 d after initial dosing.Upon collection, leaf-litter samples were dried and ana-lyzed for diazinon within 48 h after drying. Analyticalchemistry was conducted according to Bennett et al. (2000),using a Hewlett-Packard 6890 gas chromatograph (SantaClara, California,USA) equipped with dual HP 7683 ALS