Herbicide sorption by immersed soils: stoichiometry and the law of mass action in support of predictive kinetics

The Longitudinal Profile of a Stream Contaminated With 2,4-D and its Effects on Non-Target Species

Pesticides can cause harmful effects to aquatic communities, even at concentrations below the threshold limit established as guidelines for the water bodies by environmental agencies. In this research, an input of the herbicide 2,4-dichlorophenoxyacetic acid (i.e., 2,4-D) was simulated under controlled conditions in a 500-m-long reach of a first-order tropical stream in Southeastern Brazil. Two water samplings at eight stations investigated the stream longitudinal contamination profile. The ecotoxicological effects were analyzed using Eruca sativa L. seed germination assays and the acute and chronic toxicity tests with the neotropical cladoceran Ceriodaphnia silvestrii. Physicochemical parameters of water quality were evaluated to characterize the study area and quantify 2,4-D concentrations along the stream to assess pesticide retention. The 2,4-D concentration was reduced by approximately 50% downstream in the samplings, indicating that the herbicide was retained along the stream. Moreover, C. silvestrii reproduction in long-term assays decreased approximately 50% in the stations with higher concentrations of 2,4-D than the laboratory control. After contamination, E. sativa L. showed a lower average root growth (1.0 cm), statistically different from the control (2.2 cm). On the other hand, similar growth values were obtained among the background and the most downstream stations. Our study highlighted the relevance of reviewing and updating herbicide guidelines and criteria to prevent possible ecological risks.

Pesticide and nitrate transport in an agriculturally influenced stream in Indiana

Agrochemicals can be transported from agricultural fields into streams where they might have adverse effects on water quality and ecosystems. Three enrichment experiments were conducted in a central Indiana stream to quantify pesticide and nitrogen transport dynamics. In an enrichment experiment, a compound solution is added at a constant rate into a stream to increase compound background concentration. A conservative tracer (e.g., bromide) is added to determine discharge. Water and sediment samples are taken at several locations downstream to measure uptake metrics. We assessed transport of nitrate, atrazine, metolachlor, and carbaryl through direct measurement of uptake length (S _w ), uptake velocity (V _f ), and areal uptake (U). S _w measures the distance traveled by a nutrient along the stream reach. V _f measures the velocity a nutrient moves from the water column to immobilization sites. U represents the amount of nutrient immobilized in an area of streambed per unit of time. S _w varied less than one order of magnitude across pesticides. The highest S _w for atrazine suggests greater transport to downstream ecosystems. Across compounds, pesticide S _w was longest in August relative to October and July. V _f varied less than one order of magnitude across pesticides with the highest V _f for metolachlor. U varied three orders of magnitude across pesticides with the highest U associate with sediment-bound carbaryl. Increasing nitrate S _w suggests a lower nitrate demand of biota in this stream. Overall, pesticide transport was best predicted by compound solubility which can complement and improve models of pesticide abundance used by water quality programs and risk assessments.

Pesticide regulations for agriculture: Chemically flawed regulatory practice

Two categories of pesticide soil models now exist. Government regulatory agencies use pesticide fate and transport hydrology models, including versions of PRZM.gw. They have good descriptions of pesticide transport by water flow. Their descriptions of chemical mechanisms are unrealistic, having been postulated using the universally accepted but incorrect pesticide soil science. The objective of this work is to report experimental tests of a pesticide soil model in use by regulatory agencies and to suggest possible improvements. Tests with experimentally based data explain why PRZM.gw predictions can be wrong by orders of magnitude. Predictive spreadsheet models are the other category. They are experimentally based, with chemical stoichiometry applied to integral kinetic rate laws for sorption, desorption, intra-particle diffusion, and chemical reactions. They do not account for pesticide transport through soils. Each category of models therefore lacks what the other could provide. They need to be either harmonized or replaced. Some preliminary tests indicate that an experimental mismatch between the categories of models will have to be resolved. Reports of pesticides in the environment and the medical problems that overlap geographically indicate that government regulatory practice needs to account for chemical kinetics and mechanisms. Questions about possible cause and effect links could then be investigated.

Online sequential-injection chromatography with stepwise gradient elution: a tool for studying the simultaneous adsorption of herbicides on soil and soil components

The adsorption of triazine herbicides simazine (SIM), atrazine (ATR), and propazine (PRO) as well as the metabolites deisopropylatrazine (DIA), deethylatrazine (DEA), and 2-hydroxyatrazine (HAT) on soil, humic acid, and soil modified with humic acidic was studied by sequential-injection chromatography with UV detection at 223 nm. An online monitoring system was assembled, which was composed of a tangential filter and a peristaltic pump for the circulation of the soil (25 g L(-1)) or humic acid (2.5 g L(-1)) suspensions. A stepwise gradient elution separated the compounds using three mobile phases whose compositions were 28, 40, and 50% (v v(-1)) methanol in 1.25 mmol L(-1) ammonium acetate buffer, pH 4.7. The sampling throughput was about six analyses per hour; the linear dynamic range was between 100 and 1000 μg L(-1) for all of the studied compounds. The detection limits varied from 9 μg L(-1) for ATR to 36 μg L(-1) for DEA. At contact times <2 h, humic acid was the material with a higher adsorptive capacity (from 1470 ± 43 μg g(-1) for DIA to 2380 ± 51 μg g(-1) for PRO). In soil, HAT exhibited the highest adsorption (23.8 ± 0.2 μg g(-1)). The presence of humic acid in the soil increased the adsorption of ATR (14 ± 1 to 23 ± 2 μg g(-1)) and PRO (21.5 ± 0.5 to 24.0 ± 0.2 μg g(-1)), decreased the adsorption of HAT (23.8 ± 0.2 to 18 ± 2 μg g(-1)), and did not affect DIA and DEA. The adsorption of SIM was negligible in all of the sorbents studied. Simazine is the herbicide with the greatest potential for leaching to water bodies followed by DEA and DIA.

Propanil in a Manitoba soil: an interactive spreadsheet model based on conventional chemical kinetics

An interactive spreadsheet model has been created for quantitative predictions of propanil sorption and reaction in a slurried Manitoba clay soil. Based on experimental values for the numbers of empty and filled sorption sites as reactants and products, the reaction mechanism has been described with conventional chemical kinetics. The on line HPLC μ extraction method revealed labile sorption, intraparticle diffusion, and a chemical reaction. Laidler's integral rate law for second order kinetics describes the labile sorption. Desorption, intraparticle diffusion, and the chemical reaction are all described by first order kinetics. The time dependent effects of initial concentration and amount of slurried soil can be predicted for sorption, intraparticle diffusion, and the amount of reaction product. Suggested applications include storm runoff and inputs for fate and transport hydrology models.