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In the evaluation of the risk of pesticide leaching to groundwater, the soil surface is usually assumed to be level, although important crops like potato are grown on ridges. A fraction of the water from rainfall and sprinkler irrigation may flow along the soil surface from the ridges to the furrows, thus bringing about an extra load of water and pesticide on the furrow soil. A survey of the literature reveals that surface-runoff from ridges to furrows is a well-known phenomenon but that hardly any data are available on the quantities of water and pesticide involved. On the basis of a field experiment with additional sprinkler irrigation, computer simulations were carried out with the Pesticide Emission Assessment at Regional and Local scales model for separate ridge and furrow systems in a humic sandy potato field. Breakthrough curves of bromide ion (as a tracer for water flow) and carbofuran (as example pesticide) were calculated for 1-m depth in the field. Bromide ion leached comparatively fast from the furrow system, while leaching from the ridge system was slower showing a maximum concentration of about half of that for the furrow system. Carbofuran breakthrough from the furrow system began about a month after application and increased steadily to substantial concentrations. Because the transport time of carbofuran in the ridge soil was much longer, no breakthrough occurred in the growing season. The maximum concentration of carbofuran leaching from the ridge-furrow field was computed to be a factor of six times as high as that computed for the corresponding level field. The study shows that the risk of leaching of pesticides via the furrow soil can be substantially higher than that via the corresponding level field soil.

The sorption of nine pesticides to the aquatic macrophytes Chara globularis, Elodea nuttallii, and Lemna gibba was studied. A batch equilibrium method was used to study the sorption at five concentration levels to fresh shoots of the macrophytes. The results for the herbicides atrazine and linuron were described by nonlinear Freundlich equations, with Freundlich exponents ranging from 0.53 to 0.60. The results for the other compounds showed almost linear sorption isotherms, with Freundlich exponents ranging from 0.9 to 1.1. The highest sorption was measured for chlorpyrifos, with sorption coefficients ranging from 1,660 to 2,150 L/kg. Sorption coefficients for C. globularis tended to be lower than those for the other two macrophytes. Correlation (R(2) = 0.80) was found for the relation between the sorption coefficient (K(d)) of six pesticides and their solubility in water (S). The equation log K(d) = 3.20 - 0.65 log S can be used for a first estimate of the sorption coefficient of a pesticide to aquatic macrophytes.http://link.springer-ny.com/link/service/journals/00244/bibs/37n3p310.html

A sampling programme was set up to measure 1,3-dichloropropene and methyl isothiocyanate in air in a region with intensive agricultural use of these soil fumigants. In two consecutive autumns, 6-hour air samples were taken at two locations in that region with charcoal tubes using automatic samplers. Most (81%) of the 6-hour concentrations of 1,3-dichloropropene measured in both years were below 3.2 μg m−3 and a few percent could not be measured with a detection limit of around 0.3 μg m−3. Only 4% of the 6-hour concentrations exceeded 10 μg m−3, almost all of which were measured at a location where a field just upwind of the measuring site had been treated. For methyl isothiocyanate, 73% of the 6-hour concentrations of both years could not be measured with a detection limit in the two years of 1 and 2 μg m−3, respectively. A small fraction (3%) of the concentrations were in the range of 3.2 to 10 μg m−3 and only 1% exceeded 10 μg m−3.

Water flow and pesticide transport in the soil of fields with ridges and furrows may be more complex than in the soil of more level fields. Prior to crop emergence, the tracer bromide ion and the insecticide carbofuran were sprayed on the humic-sandy soil of a potato field with ridges and furrows. Rainfall was supplemented by sprinkler irrigation. The distribution of the substances in the soil profile of the ridges and furrows was measured on three dates in the potato growing season. Separate ridge and furrow systems were simulated by using the pesticide emission assessment at regional and local scales (PEARL) model for pesticide behavior in soil–plant systems. The substances travelled deeper in the furrow soil than in the ridge soil, because of runoff from the ridges to the furrows. At 19 days after application, the peak of the bromide distribution was measured to be in the 0.1–0.2 m layer of the ridges, while it was in the 0.3–0.5 m layer of the furrows. After 65 days, the peak of the carbofuran distribution in the ridge soil was still in the 0.1 m top layer, while the pesticide was rather evenly distributed in the top 0.6 m of the furrow soil. The wide ranges in concentration measured with depth showed that preferential water flow and substance transport occurred in the sandy soil. Part of the bromide ion distribution was measured to move faster in soil than the computed wave. The runoff of water and pesticide from the ridges to the furrows, and the thinner root zone in the furrows, are expected to increase the risk of leaching to groundwater in ridged fields, in comparison with more level fields.

The transformation of [1- 14 C]-1,2-dichloropropane in three water-saturated sandy subsoil materials (sampled at a depth of between 1.2 and 2.3 m) was studied in laboratory incubations at 10 °C. In the first material (methanogenic with a redox potential of 0.0–0.2 V and a pH of 7) no measurable transformation took place during the first 156 days but after 608 and 712 days more than 96% of the dose had been transformed into one or more highly volatile products. Analysis of the headspace above this subsoil material after 1059 days indicated that propene and propane are probably transformation products and excluded chlorinated hydrocarbons with one, two or three C atoms as significant transformation products. In the two other subsoil materials (with redox potentials of 0.0–0.4 V and 0.3–0.6 V and pH values between 4 and 5) no measurable transformation took place during the whole incubation period (1056 days).

Methods of sampling and analysis have been developed to measure concentrations of the soil fumigants 1,3-dichloropropene and methyl isothiocyanate (formed from metham-sodium) in air. We tested these methods under laboratory and field conditions. Air samples were taken with glass tubes containing charcoal as adsorbent. The charcoal was extracted with acetone and subsamples of the extracts were injected into a gas Chromatograph with a capillary column. 1,3-dichloropropene was determined by an electron capture (EC) detector and methyl isothiocyanate by a nitrogen-phosphorus (NP) detector. Concentrations of these fumigants in 40 L of air as low as 0.2 μg m−3 and 1.0 μg m−3, respectively, could be determined. A study on the influence of storage temperature and time on the recovery from charcoal showed that charcoal tubes can be stored for up to 4 d at −20 °C. A test program on the breakthrough of the charcoal tubes when sampling at different flow rates and air humidity revealed no significant breakthrough from the first to the second (check) tube. Similar results were obtained from sampling the air during fumigation of the soil in a greenhouse and in two fields.

When studying the effect of mixtures of toxic substances on soil organisms, attention must be paid to peculiarities in exposure to mixtures as opposed to that of single toxicants. The fungicides carbendazim and iprodione compete in the adsorption to soil. The presence of iprodione reduced the adsorption of carbendazim by 30%, while carbendazim reduced the adsorption of iprodione by 70%. Iprodione had little effect on the transformation rate of carbendazim in soil. However, carbendazim retarded the transformation of iprodione in soil by 26%. The concentration of the fungicides in pore water was found to be substantially higher for mixtures than when a fungicide alone was present in the soil. The effect of the additional fungicide on the concentration is especially apparent in the period following the first 1 to 2 weeks of the incubation. The inclusion of copper in the mixture has little additional effect on the concentration of the fungicides in pore water.

Volatilisation of pesticides from plants is one of the main pathways for their emission to the environment. A simplified computation model was set up to simulate this volatilisation, including penetration into plants and photochemical transformation as competing processes. Previous wind tunnel experiments using plants sprayed with C-14-labelled fenpropimorph were simulated using the model. Volatilisation could be simulated by diffusion through a laminar air-boundary layer, with a thickness in the range of 0.5-1.0 mm. Rate coefficients of 1.7-4.8 d(-1) had to be used to simulate the penetration of fenpropimorph into different plant species. The rate of phototransformation was lowest when the incoming air stream was filtered through activated carbon, thus minimising the formation of hydroxyl radicals by sunlight. The simulations enabled us to estimate model parameters that could neither be derived from laboratory studies nor could be obtained with pesticide (non-labelled) in the field.

This paper shows the results of experiments (adsorption, degradation and mobility) carried out to investigate the risk of pirimicarb leaching to groundwater, after its application to integrated fruit growing systems. Soil batches were collected from experimental fruit farms in Numansdorp and Zeewolde (The Netherlands). The distribution coefficients and the degradation rate coefficients for Numansdorp and Zeewolde soils, respectively, show a higher adsorption and degradation rate in the Zeewolde soil than in the Numansdorp soil. By leaching of pirimicarb in two soil columns, it was found that pirimicarb mobility is lower in the Zeewolde soil columns than in the Numansdorp ones but, in both cases, it is much lower than it would be expected from their adsorption coefficients. Pirimicarb is detected in the first effluents, however, the highest concentration remained in the upper 10 cm of the soil columns. A computation model simulating uniform water flow and instantaneous adsorption desorption equilibrium is not suitable to describe the leaching of pirimicarb in structured loamy soils at relatively high water flow-rates.

The transformation rates of methyl isothiocyanate and 1,3-dichloropropene were studied in four water-saturated sandy subsoil materials collected between 2 and 4 m below the soil surface, using laboratory incubation systems at 10 degrees C. In three samples, transformation of methyl isothiocyanate was rapid (< 10% remaining after 30 days), whereas in the fourth, a half-life of ~ 35 days was found. In the transformation of the (Z)- and (E)-isomers of 1,3-dichloropropene in the same materials, half-lives between 16 and 64 days were found. Incubation in systems where the subsoil materials had been exposed unintentionally to temperatures up to 30 degrees C before adding the compounds, resulted in much lower transformation rates for methyl isothiocyanate (half-lives ranging from 150 to 570 days) and in only slightly lower rates for (Z)- and (E)-1,3-dichloropropene (half-lives ranging from 38 to 76 days). (Abstract retrieved from CAB Abstracts by CABI’s permission)

With a computer model, pesticide behavior in soil after spring application to a sandy loam field with a potato crop, was simulated. Special attention was paid to the risk of leaching through the upper meter of soil in catchment areas. Unsaturated water flow resulting from rainfall was modeled in some detail. Uptake of water and solute by the developing root system of the annual crop was included in the model. The computations were carried out for hypothetical pesticides with first-order decomposition rate constants in soil of 0.03, 0.01, 0.003, and 0.001 day–1 at 20°C. Adsorption decreased with increasing soil depth and the adsorption coefficient for the top layer ranged from 0.0 to 10.0. For 20 combinations of decomposition rates and adsorption strengths, the extent of leaching from a top layer 1 m thick was computed. With a decomposition rate constant at 20°C of 0.03 day–1 or higher, leaching was extremely low; with 0.01 days–1 leaching was 1.7% of the dosage or lower. For compounds with a high persistence and mobility, leaching from the soil ranged up to about 10% of the dosage or more. Besides decomposition, uptake by plants was an important factor in reducing leaching, particularly for the weakly adsorbed and comparatively persistent compounds.

The fumigant 1,3-dichloropropene was injected into the soil of three bulb fields in the summer. At various times after injection, the contents of the (Z) and (E) isomers (formerly called thecis andtrans isomers) were measured in the unsaturated and saturated zones down to a 3 m depth. Within a month, the contents decreased to less than 0.2 mg/kg and they only gradually declined further in the subsequent months. Occasionally, very low contents (less than 1 μg/kg) of the (Z) and (E) isomers were measured in the groundwater zone.