Charles S. Helling

Behavior of Pesticides in Soils

Publisher Summary The behavior of pesticides in soils has been the subject of research long before pollution became a byword. In recognition that soil is the ultimate sink for most widely used pesticides, and given the impetus of recent public awareness of the quality of our environment, the past decades have marked much progress in the understanding of the fate and behavior of pesticides in soils. A strong case for the continued use or precipitate abandonment of agricultural pesticides is inappropriate. It is reasonable, however, to note the vitally important role pesticides have assumed in increasing the quantity and quality of foodstuffs, timber, and ornamental plants; in improving animal health; and in combating certain diseases transmitted to man. This chapter reviews the behavior of pesticides in soils from the standpoint of processes affecting pesticides (physicochemical and metabolic), the effect of pesticides on the soil microbiota, and the implications of these processes on persistence, bioactivity, and plant uptake. Adsorption, the most influential process affecting pesticides in soils, depends on both soil and pesticide properties. Other significant soil factors include total surface area, water content, temperature, and pH. Pesticide properties that are relevant include overall chemical character and configuration, dissociation constant, water solubility, charge distribution, and molecular size. Movement of the pesticides occurs by leaching, volatilization, or runoff. The importance of photodecomposition as a process degrading pesticides in soils is uncertain. Generally, photolysis occurs more readily for compounds in solution, with soil inhibiting the reaction.

Persistence and leaching of atrazine, alachlor, and cyanazine under no-tillage practices

Abstract Soil residues of atrazine, alachlor, and cyanazine were measured in no-till corn plots that had received annual herbicide applications from 1981–1985. Treatments were as paired combinations of herbicides at recommended rates. Three cores were collected from 3–4 replicated plots per treatment; sampling occurred late in 1983, then periodically during the growing season in 1984 and 1985. Persistence up to 6 weeks after herbicide application was in the order: atrazine > cyanazine > alachlor. Sampling after ca. 6 weeks gave the order atrazine > alachlor > cyanazine. Atrazine and alachlor were found to the lowest sampled depths in 1983 (1–1.5 m) and 1984 (0.4–0.5 m). Cyanazine was not detected below 0.3 m. In 1985, atrazine leached to ≥0.3–0.5 m by 40 days, but no subsoil residue was detected at 139 days. Alachlor and cyanazine had virtually disappeared by 40 days.

Movement of s-triazine herbicides in soils

The movement of an organic pesticide1 placed onto or within the soil may influence both its effectiveness and its potential as a contaminant in adjacent soil, water, or air. Movement may occur while in solution or adsorbed on migrating soil particles, or by transfer as vapors. Volatilization, the latter, is discussed in another part of this volume (Jordan et al. 1970). The two general processes of non-gaseous movement, mass transfer and diffusion, will be considered within the soil profile; movement at the soil surface will be discussed separately.

Impact of four pesticides on the growth and metabolic activities of two photosynthetic algae

The acute toxicity was determined for soil algae Chlorella kesslerei and Anabaena inaequalis, exposed to pesticides lindane, pentachlorophenol (PCP), isoproturon (IPU), and methyl parathion (MP). Toxicity markers included growth inhibition, chlorophyll biosynthesis, and total carbohydrate content, as a function of dose and time. Concentration response functions (EC50) were estimated by probit data transformation and weighted linear regression analyses. Lindanes toxicity to Chlorella increased sharply with time (EC50=7490, 10.3, 0.09 mg L−1; 24, 48, 72 h), but remained nearly constant through 72 h with Anabaena (8.7−6.7 mg L−1; 24–72 h). PCP at low concentrations stimulated algal growth and chlorophyll a production, an effect reversed at higher doses. Anabaena was less tolerant of PCP and MP than was Chlorella. The 96-h static EC50 values for Chlorella were: 0.003, 34, 0.05, and 291 mg L−1 for lindane, PCP, isoproturon, and MP, respectively; for Anabaena, these were 4.2, 0.13, 0.21, and 19 mg L−1. Carbohydrate production responses were similar to those of cell density (growth) and chlorophyll biosynthesis, with MP having the lowest adverse impact. The overall relative toxicity among the four tested pesticides was: for Chlorella, lindane>IPU≫PCP≫MP; and for Anabaena, PCP>IPU>lindane>MP. The results confirm that toxicants such as these pesticides may affect individual (though related) species to significantly different degrees.

Groundwater residues of atrazine, alachlor, and cyanazine under no-tillage practices

Abstract Groundwater from no-till corn plots treated with atrazine, alachlor, and cyanazine was analyzed for residues of these herbicides over a 3-year period. Detectable levels of atrazine, alachlor, and cyanazine were found in 75, 18, and 13% of the recovered samples, respectively. Maximum residue levels were 5.9, 3.6, and 1.0 μg L −1 for atrazine, cyanazine, and alachlor, respectively. Rapid vertical transport to the shallow unconfined groundwater (ca. 1 m depth), as well as substantial lateral subsurface flow, was indicated.

Evaluation of molecular connectivity as a predicitive method for the adsorption of pesticides by soils

Abstract The use of molecular connectivity (MC), a topological description of organic compounds, was evaluated for the estimation of adsorption of organic pesticides by soils. Adsorption data were evaluated for over 40 pesticides from all classes of commonly used pesticides. The predictive ability of MC for such a heterogeneous group was rather low, but the data for two homogeneous groups showed that the MC indices can be used with a high degree of accuracy for predicting the adsorption of members of such a group. Correlation of the pesticides’ solubility and octanol‐water partition coefficient with MC indices also was low.

Evaluation of a fiber optic immunosensor for quantitating cocaine in coca leaf extracts

A fiber optic evanescent fluoroimmunosensor was used to rapidly detect and quantitate coca alkaloids as cocaine equivalents in leaf extracts of five Erythroxylum species. A monoclonal antibody (mAb) made against benzoylecgonine (BE), a major metabolite of cocaine, was immobilized covalently on quartz fibers and used as the biological sensing element in the portable fluorometer. Benzoylecgonine-fluorescein (BE-FL) was used as the optical signal generator when it bound to the fiber. If present, cocaine competed for the mAb and interfered with the binding of BE-FL, thereby reducing the fluorescence transmitted by the fiber. Calibration curves were prepared by measuring (over 30 s) the rates of fluorescence increase in the absence, or presence of cocaine. Ethanol or acid extracts of dry coca leaves were assayed by this fiber optic biosensor, gas chromatography and a fluorescent polarization immune assay. Biosensor values of cocaine content of leaves from five Erythroxylum species were not significantly different from gas chromatography values, but had higher variance. The biosensor assay was rapid and did not require cleanup of the crude leaf extracts. Cocaine in acid extracts was reduced significantly after 4 weeks at 23 degrees C and after 3 weeks at 37 degrees C. Fibers (mAb-coated), stored at 37 degrees C in phosphate-buffered solution (0.02% NaN3), gave stable responses for 14 days.

Reactions of pesticides in soils

Once a pesticide has destroyed the target organism, the compound has served its intended function and may remain as a residue in air, water, and soil. Residual pesticides in soils create two problems: (1) at nonphytotoxic concentrations pesticides or their metabolites may be absorbed into plants and eventually enter the food chain at levels below established tolerances; or (2) at phytotoxic levels (in the case of herbicides) they may injure or destroy subsequent crops. The latter situation is further confounded by recent evidence that other pesticides may interact with herbicides to cause synergistic phytotoxic effects or extend their persistence.

Soil-catalyzed oxidation of aniline

Abstract Aniline partially degraded in sterile soil to azobenzene, azoxybenzene, phenazine, form=anilide, and acetanilide. Nitrobenzene, p -benzoquinone, and unidentified species were possible products; substantial bound residues may also have formed. Soil-catalyzed conversion of aniline or [ d 5]aniline seems evidenced by 6-24X more product recovery in sterile soil than in sterile water alone, a process inhibited by Na2S2O4. Freundlich adsorption constants showed: azobenzene > azoxybenzene > phenazine > aniline.

Physicochemical and structural relationships of organic chemicals undergoing soil- and clay-catalyzed free-radical oxidation

This paper discusses soil- and clay-catalyzed oxidations in order to develop general guidelines for predicting the occurrence of these reactions. In this study, 93 organic chemicals that undergo soil- and clay-catalyzed oxidation were analyzed to identify similarities in (a) molecular structure and (b) physicochemical properties. Based on this analysis these chemicals were divided into four groups: Group 1 - aromatic chemicals that contain electron-withdrawing and weak electron-donating fragments (they have a lower water solubility limit of 200 ppm); Group 2 - aromatic chemicals that contain electron-withdrawing fragments and a very strong electron-donating fragment (lower water solubility limits is 112 ppm); Group 3 - aromatic chemicals that contain only electron-donating fragments (lower water solubility limit is 29 ppm); and Group 4 - aromatic chemicals that contain extensive conjugation (in general, these chemicals have a lower water solubility limit in the very low parts-per million to high parts-per-billion range). A hypothetical relationship between the lower solubility limit and chemical structure to chemical reactivity is proposed using simple kinetic theory. Finally, the applicability of the solubility and structure characteristics of the four groups as guidelines for predicting the occurrence of free-radical oxidation is discussed.