Transport and degradation of pesticides in a biopurification system under variable flux Part II: A macrocosm study

Transport of bentazone, isoproturon, linuron, metamitron and metalaxyl were studied under three different flows in macrocosms. The aim was to verify the observations from Part I of the accompanying paper, with an increase in column volume and decrease in chemical and hydraulic load. Very limited breakthrough occurred in the macrocosms for all pesticides, except bentazone, at all flows.From batch degradation experiments, it was observed that the lag time of metamitron and linuron decreased drastically in time for all flows, indicating a growth in the pesticide degrading population. This in contrast to isoproturon and metalaxyl, where an increase in lag time could be observed in time for all flows. From the batch degradation experiments, it could be concluded that the influence of flow on the lag time was minimal and that the inoculation of the pesticide-primed soil had a little surplus value on degradation.     

Biomassbed: a biological system to reduce pesticide point contamination at farm level

A potential method for cleaning water from point-source pollution by organic compounds is using biological reactors. In this study, four reactors were tested for their ability to retain and degrade pesticides. The pesticides tested were the insecticide chlorpyrifos, the fungicide metalaxyl and the herbicide imazamox. The reactors were filled with differing mixtures of vine-branch, citrus peel, urban waste and public green compost. The reactor volume was 188 l. Forced circulation of the contaminated solution was programmed to decontaminate the solution. Both retention and degradation of the compounds by the reactors was studied. Chlorpyrifos was the best retained, due to its physico-chemical characteristics, while only one substrate effectively retained metalaxyl and imazamox (citrus peel+urban waste compost). Degradation of the pesticides in the reactors was faster than published values for degradation in soil. The half-life of all pesticides in the reactors was less than 14 days, compared to literature values of 60-70 days in soil. The combined retention and fast degradation make the biofilter a feasible technique to reduce spill-related and point environmental contamination by pesticides. The technique is most effective against persistent pesticides, while for mobile pesticides, the efficiency can be improved with several passages of the contaminated solution through biofilters.     

Influence of different abiotic and biotic factors on the metalaxyl and carbofuran dissipation

Metalaxyl and carbofuran dissipation was studied in response to different factors (soil bacterial communities, light irradiation, presence of an inorganic culture medium and presence of soil) and combinations of these factors in short-term experiments (48 h). The soil microbial communities have no effect on metalaxyl or carbofuran dissipation in the time scale employed. Light irradiation and soil promote metalaxyl and carbofuran dissipation by photodegradation and adsorption, respectively. However, photodegradation has a stronger effect on metalaxyl and carbofuran dissipation than the adsorption of the pesticides in the soil. The addition of the culture medium have no direct effect on pesticide dissipation, degradation by microbial communities or adsorption but its presence greatly increased photodegradation.     

Volatility of TCDD and PCB from soil

Volatilisation fluxes of 2,3,7,8-tetrachlorodibenzodioxin and Aroclors 1242, 1254 and 1260 were estimated from the model of Jury et.al., developed for compounds with similar physical properties. The influence of soil water content, soil organic matter and water evaporation was investigated. Increasing soil organic matter decreased the volatilisation flux of all four chemicals. Water evaporation was accompanied by increased fluxes, implying that the chemicals were susceptible to wicking. The trends observed were in qualitative agreement with field and laboratory studies. The predicted fluxes serve as a basis for comparing their volatilisation behaviour against pesticides and other homologues, and for the construction of models describing the environmental fate of these compounds.     

Characterizing pesticide sorption and degradation in macro scale biopurification systems using column displacement experiments

The efficiency of biopurification systems to treat pesticide-contaminated water was previously studied in microcosms. To validate the obtained results, macrocosm systems were set-up. Four pesticides (linuron, isoproturon, bentazone, and metalaxyl) were continuously applied to ten different organic substrate mixes. Retention of the pesticides was similar and in some cases slightly lower in the macrocosms compared to the microcosms. Differences in retention between the different mixes were however minimal. Moreover, the classification of the retention strength of the pesticides was identical to that observed in microcosms: linuron > isoproturon > metalaxyl > bentazone. Monod kinetics were used to describe delayed degradation, which occurred for isoproturon, metalaxyl and bentazone. No breakthrough of linuron was observed, thus, this pesticide was appointed as the most retained and/or degraded pesticide, followed by isoproturon, metalaxyl and bentazone. Finally, most of the matrix mixes efficient in degrading or retaining pesticides were mixes containing dried cow manure.     

Influence of biotic and abiotic factors on dissipating metalaxyl in soil.

Under laboratory condition, dissipation of metalaxyl in sterile and non-sterile soils, its sorption behaviour and fate in presence of light have been studied. The half-life value of metalaxyl was found in the range of 36-73 d in non-sterile soil. 5.3-14.7% dissipation was observed due to abiotic factors other than light. Metalaxyl was found photostable in soil showing half-life of 188- 502 h under simulated sunlight. In adsorption study, a non-linear relationship between concentration of metalaxyl and its adsorption into soils was observed. Estimated koc value increased as organic carbon content decreased. Adsorption and desorption kD values ranged between 53.5 and 151.1.     

Leaching and sorption of neonicotinoid insecticides and fungicides from seed coatings

Seed coatings are a treatment used on a variety of crops to improve production and offer protection against pests and fungal outbreaks. The leaching of the active ingredients associated with the seed coatings and the sorption to soil was evaluated under laboratory conditions using commercially available corn and soybean seeds to study the fate and transport of these pesticides under controlled conditions. The active ingredients (AI) included one neonicotinoid insecticide (thiamethoxam) and five fungicides (azoxystrobin, fludioxonil, metalaxyl, sedaxane thiabendazole). An aqueous leaching experiment was conducted with treated corn and soybean seeds. Leaching potential was a function of solubility and seed type. The leaching of fludioxonil, was dependent on seed type with a shorter time to equilibrium on the corn compared to the soybean seeds. Sorption experiments with the treated seeds and a solution of the AIs were conducted using three different soil types. Sorption behavior was a function of soil organic matter as well as seed type. For most AIs, a negative relationship was observed between the aqueous concentration and the log K_oc. Sorption to all soils tested was limited for the hydrophilic pesticides thiamethoxam and metalaxyl. However, partitioning for the more hydrophobic fungicides was dependent on both seed type and soil properties. The mobility of fludioxonil in the sorption experiment varied by seed type indicating that the adjuvants associated with the seed coating could potentially play a role in the environmental fate of fludioxonil. This is the first study to assess, under laboratory conditions, the fate of pesticides associated with seed coatings using commercially available treated seeds. This information can be used to understand how alterations in agricultural practices (e.g., increasing use of seed treatments) can impact the exposure (concentration and duration) and potential effects of these chemicals to aquatic and terrestrial organisms.     

Modelling pesticide volatilization after soil application using the mechanistic model Volt'Air

Volatilization of pesticides participates in atmospheric contamination and affects environmental ecosystems including human welfare. Modelling at relevant time and spatial scales is needed to better understand the complex processes involved in pesticide volatilization. Volt'Air-Pesticides has been developed following a two-step procedure to study pesticide volatilization at the field scale and at a quarter time step. Firstly, Volt'Air-NH₃ was adapted by extending the initial transfer of solutes to pesticides and by adding specific calculations for physico-chemical equilibriums as well as for the degradation of pesticides in soil. Secondly, the model was evaluated in terms of 3 pesticides applied on bare soil (atrazine, alachlor, and trifluralin) which display a wide range of volatilization rates. A sensitivity analysis confirmed the relevance of tuning to K_h. Then, using Volt'Air-Pesticides, environmental conditions and emission fluxes of the pesticides were compared to fluxes measured under 2 environmental conditions. The model fairly well described water temporal dynamics, soil surface temperature, and energy budget. Overall, Volt'Air-Pesticides estimates of the order of magnitude of the volatilization flux of all three compounds were in good agreement with the field measurements. The model also satisfactorily simulated the decrease in the volatilization rate of the three pesticides during night-time as well as the decrease in the soil surface residue of trifluralin before and after incorporation. However, the timing of the maximum flux rate during the day was not correctly described, thought to be linked to an increased adsorption under dry soil conditions. Thanks to Volt'Air's capacity to deal with pedo-climatic conditions, several existing parameterizations describing adsorption as a function of soil water content could be tested. However, this point requires further investigation. Practically speaking, Volt'Air-Pesticides can be a useful tool to make decision about agricultural practices such as incorporation or for the estimation of overall pesticide volatilization rates, and it holds promise for time specific dynamics.     

Enhanced retention of linuron, alachlor and metalaxyl in sandy soil columns intercalated with wood barriers

A study has been made of the effect a reactive barrier made of pine (softwood) or oak (hardwood) wood intercalated in a sandy soil column has on the retention of linuron, alachlor and metalaxyl (pesticides with contrasting physicochemical characteristics). The leaching of pesticides has been carried out under a saturated flow regime and breakthrough curves (BTCs) have been obtained at flow rates of 1 mL min⁻¹ (all pesticides) and 3 mL min⁻¹ (linuron). The cumulative curves in the unmodified soil indicate a leaching of pesticides >80% of the total amount of compound added. After barrier intercalation, linuron leaching decreases significantly and a modification of the leaching kinetics of alachlor and metalaxyl has been observed. The theoretical R factors increased ∼2.6-3.3, 1.2-1.6-fold, and 1.4-1.7-fold and the concentration of the maximum peak decreased ∼6-12-fold, 2-4-fold and 1.2-2-fold for linuron, alachlor and metalaxyl, respectively. When considering the three pesticides, significant correlations have been found between the theoretical retardation factor (R) and the pore volume corresponding to the maximum peaks of the BTCs (r = 0.77; p < 0.05) or the total volume leached (r = −0.78; p < 0.05). The results reveal the efficacy of reactive wood barriers to decrease the leaching of pesticides from point sources of pollution depends on the type of wood, the hydrophobicity of the pesticide and the adopted water flow rate. Pine was more effective than oak in decreasing the leaching of hydrophobic pesticide linuron or in decreasing the maximum peak concentration of the less hydrophobic pesticides in soils. Efficacy of these wood barriers was limited for the least hydrophobic pesticide metalaxyl.     

Evaluation of sorption-desorption processes for metalaxyl in natural and artificial soils

The main process controlling soil-pesticide interaction is the sorption-desorption as influenced by active soil surfaces. The sorption phenomena can influence translocation, volatility, persistence and bioactivity of a pesticide in soil. The present investigation was conducted on natural and artificial soils in order to enumerate the effect of soil components such as montmorillonite and ferrihydrite on the sorption behaviour of the fungicide metalaxyl and if sorption-desorption of the chiral pesticide affects the enantiomeric ratio. The sorption-desorption characteristics of metalaxyl were investigated by batch equilibration technique in a natural soil, two artificial soils, and in pure montmorillonite and ferrihydrite. After extraction, pesticide residues were analyzed by conventional and chiral chromatography using tandem mass spectrometry. A K_dSorp (2.3–6.5) suggests low level sorption of metalaxyl with an appreciable risk of run-off and leaching. Thus, metalaxyl poses a threat to surface and ground water contamination. Furthermore, desorption tests revealed a hysteretic effect (H ≤ 0.8) in natural and artificial soils. Significant amount of metalaxyl was found tightly bound to the adsorbents without desorbing readily after desorption cycle. Desorption of 22–56% of the total amount of the retained metalaxyl was determined. This study reveals that an artificial soil derived from different soil constituents can be used to assess their influence on sorption/desorption processes. The present investigation showed that both montmorillonite and ferrihydrite play a significant role in the sorption of metalaxyl. The sorption doesn't influence the enantiomeric ratio of racemic metalaxyl.     

The effect of initial concentration, co-application and repeated applications on pesticide degradation in a biobed mixture

A 180 d laboratory experiment was conducted to investigate the degradation rates of chlorpyrifos (10 and 50 mg kg(-1)) and metalaxyl (100 mg kg(-1)) separately and co-applied in a biomix constituted by topsoil, vine-branches and urban-waste-garden compost. The effect of repeated application of metalaxyl was also investigated. Microbial biomass-C (MBC) content and metabolic quotient (qCO(2)) were measured to evaluate changes in microbial biomass size and activity induced by the presence of the two pesticides. Degradation rate decreased with increasing concentration of chlorpyrifos in all treatments. Metalaxyl half-life was significantly reduced in co-application with chlorpyrifos indicating a synergic interaction between the two pesticides in favour of enhanced degradation rate for metalaxyl but not for chlorpyrifos. Furthermore, repeated application resulted in a sharp reduction of metalaxyl half-life from 37 d after first application to 4 d after third application. MBC content was negatively influenced by the addition of pesticides but it started to recover immediately, in both separate and co-applied treatments, reaching the control value when pesticide residues were about 50% of the initial concentration. The qCO(2) reached a steady-state after about 20 d in separately applied and 40 d in co-applied treatments, indicating a tendency to arrive at a new metabolic equilibrium. In conclusion, the biomix tested has been shown to degrade pesticides relatively fast and to have a microbial community that is varied enough to allow selection of those microorganisms able to degrade metalaxyl and chlorpyrifos.     

Use of coal to retard pesticide movement in soil

Abstract

Three different coals and an activated carbon were mixed with prescribed amounts of a sandy loam soil and added to soil columns to test their ability to retard pesticide movement. The pesticides chosen were prometon, prometryn, 2,4‐D, carbofuran, dinoseb, fenamiphos, and two oxidation products of fenamiphos, fenamiphos sulfoxide, and fenamiphos sulfone. These compounds were chosen to represent different chemical classes of pesticides and because they were considered to have a high potential for transport in soils. All the coals were more effective in retaining the pesticides than the soil, however, some were more effective than others. One of the coals was the most effective in retaining the majority of the pesticides with an overall retention of 94.7% in a 4:1 soil/coal ratio compared to the soil only with a retention of 48.5%. The moisture content of the coal appears to have a positive correlation with the ability of the coal to retain the pesticides under the conditions used for this experiment.     

Characterizing pesticide sorption and degradation in microscale biopurification systems using column displacement experiments

Biopurification systems treating pesticide contaminated water are very efficient, however they operate as a black box. Processes inside the system are not yet characterized. To optimize the performance, knowledge of degradation and retention processes needs to be generated. Therefore, displacement experiments were carried out for four pesticides (isoproturon, bentazone, metalaxyl, linuron) in columns containing different organic mixtures. Bromide, isoproturon and bentazone breakthrough curves (BTCs) were well described using the convection-dispersion equation (CDE) and a first-order degradation kinetic approach. Metalaxyl and linuron BTCs were well described using the CDE model expanded with Monod-type kinetics. Freundlich sorption, first-order degradation and Monod kinetics coefficients were fitted to the BTCs. Fitted values of the distribution coefficient K_f,column were much lower than those determined from batch experiments. Based on mobility, pesticides were ranked as: bentazone > metalaxyl - isoproturon > linuron. Based on degradability, pesticides were ranked as: linuron > metalaxyl - isoproturon > bentazone.     

Elimination of racemic and enantioenriched metalaxyl based fungicides under tropical conditions in the field

The elimination has been studied of racemic and enantioenriched metalaxyl applied as an emulsifiable concentrate and wettable powder combined with copper in a Cameroonian field site. The kinetics of the degradation/dissipation of metalaxyl and its acid metabolite were investigated using reversed phase HPLC-MS/MS, while the enantiomeric ratios were measured by HPLC-MS/MS using a Chiralcel OD-H HPLC column. Some soil enzymes activities were determined concurrently for 120d. The elimination of racemic metalaxyl was shown to be enantioselective, with the R-enantiomer being degraded more slowly than the S-enantiomer. Dissipation followed approximate square root first-order kinetics (R>0.98) without lag phases. The enantiomers of metalaxyl have different elimination rates, with half-lives ranging from only 0.8 to 1.5 days. After application to soil, the elimination of metalaxyl in the copper containing formulation was slower. The activities of acid phosphatase, alkaline phosphatase, and alkaline glucosidase were monitored throughout the experiments. No significant influence of metalaxyl and copper could be observed on these parameters. The significantly shorter half-life values of all forms of metalaxyl under field conditions, compared to the previously reported laboratory derived ones, may have implications for the plant disease control with these fungicides in tropical rainforest areas.     

Key parameters and practices controlling pesticide degradation efficiency of biobed substrates

We studied the contribution of each of the components of a compost-based biomixture (BX), commonly used in Europe, on pesticide degradation. The impact of other key parameters including pesticide dose, temperature and repeated applications on the degradation of eight pesticides, applied as a mixture, in a BX and a peat-based biomixture (OBX) was compared and contrasted to their degradation in soil. Incubation studies showed that straw was essential in maintaining a high pesticide degradation capacity of the biomixture, whereas compost, when mixed with soil, retarded pesticide degradation. The highest rates of degradation were shown in the biomixture composed of soil/compost/straw suggesting that all three components are essential for maximum biobed performance. Increasing doses prolonged the persistence of most pesticides with biomixtures showing a higher tolerance to high pesticide dose levels compared to soil. Increasing the incubation temperature from 15 °C to 25 °C resulted in lower t(1/2) values, with biomixtures performing better than soil at the lower temperature. Repeated applications led to a decrease in the degradation rates of most pesticides in all the substrates, with the exception of iprodione and metalaxyl. Overall, our results stress the ability of biomixtures to perform better than soil under unfavorable conditions and extreme pesticide dose levels.     

Minimal pesticide-primed soil inoculum density to secure maximum pesticide degradation efficiency in on-farm biopurification systems

Addition of pesticide-primed soil containing adapted pesticide degrading bacteria to the biofilter matrix of on farm biopurification systems (BPS) which treat pesticide contaminated wastewater, has been recommended, in order to ensure rapid establishment of a pesticide degrading microbial community in BPS. However, uncertainties exist about the minimal soil inoculum density needed for successful bioaugmentation of BPS. Therefore, in this study, BPS microcosm experiments were initiated with different linuron primed soil inoculum densities ranging from 0.5 to 50 vol.% and the evolution of the linuron mineralization capacity in the microcosms was monitored during feeding with linuron. Successful establishment of a linuron mineralization community in the BPS microcosms was achieved with all inoculum densities including the 0.5 vol.% density with only minor differences in the time needed to acquire maximum degradation capacity. Moreover, once established, the robustness of the linuron degrading microbial community towards expected stress situations proved to be independent of the initial inoculum density. This study shows that pesticide-primed soil inoculum densities as low as 0.5 vol.% can be used for bioaugmentation of a BPS matrix and further supports the use of BPS for treatment of pesticide-contaminated wastewater at farmyards.     

Porous cups for pesticides monitoring in soil solution — Laboratory tests

In this study, preliminary tests were conducted aiming to validate the use of ceramic porous cup for collecting soil water samples and monitoring pesticides contents, as usually made for nitrates. Interactions between porous cup and pesticides were examined under different experimental conditions for three herbicides (atrazine, isoproturon, 2,4-D) and one insecticide (carbofuran).

The results showed that ceramic was not inert for pesticides : as much as 80% of the applied pesticide could be retained during the flowing of the first tenth milliliters of solution. Interactions were attributed to sorption and “screening” of molecules by the porous walls and were related to the ionic character of pesticides. However, retention was not irreversible, since pesticides were quickly released by rinsing with distilled water.

After these tests, porous ceramic cups could be considered as suitable samplers for pesticide determinations in soil solution, contingent on gaining further informations about soil - porous cup - pesticide interactions.     

Element fluxes through European forest ecosystems and their relationships with stand and site characteristics

This paper describes a European wide assessment of element budgets, using available data on deposition, meteorology and soil solution chemistry at 121 Intensive Monitoring plots. Input fluxes from the atmosphere were derived from fortnightly or monthly measurements of bulk deposition and throughfall, corrected for canopy uptake. Element outputs from the forest ecosystem were derived by multiplying fortnightly or monthly measurements of the soil solution composition at the bottom of the root zone with simulated unsaturated soil water fluxes. Despite the uncertainties in the calculated budgets, the results indicate that: (i) SO4 is still the dominant source of actual soil acidification despite the generally lower input of S than N, due to the different behaviour of S (near tracer) and N (strong retention); (ii) base cation removal due to man-induced soil acidification is limited; and (iii) Al release is high in areas with high S inputs and low base status.     

Dynamic flux chamber measurement of gaseous mercury emission fluxes over soils: Part 2—effect of flushing flow rate and verification of a two-resistance exchange interface simulation model

Both field and laboratory tests demonstrated that soil Hg emission fluxes measured by dynamic flux chamber (DFC) operations strongly depend on the flushing air flow rates used. The general trend is an increase in the fluxes with increasing flushing flow rates followed by an asymptotic approach to flux maximum at sufficiently high (optimum) flushing flow rates. This study indicates that the DFC measurements performed at low flushing flow rates can underestimate Hg emission fluxes over soils, especially Hg-enriched soils. High flushing flow rates therefore are recommended for accurate estimation of soil Hg emission fluxes by DFC operations. The dependence of DFC-measured soil Hg emission fluxes on flushing flow rate is a physical phenomenon inherent in DFC operations, regardless of DFC design and soil physical characteristics. Laboratory tests using DFCs over different soils confirmed the predictions of a two-resistance exchange interface model and demonstrated the capability of this model in quantitatively simulating Hg emissions from soils measured by DFC operations.     

Sorption kinetics and its effects on retention and leaching.

Sorption of pesticides to substrates used in biopurification systems is important as it controls the system's efficiency. Ideally, pesticide sorption should occur fast so that leaching of the pesticide in the biopurification system is minimized. Although modeling of pesticide transport commonly assumes equilibrium, this may not always be true in practice. Sorption kinetics have to be taken into account. This study investigated the batch sorption kinetics of linuron, isoproturon, metalaxyl, isoxaben and lenacil on substrates commonly used in a biopurification system, i.e. cow manure, straw, willow chopping, sandy loam soil, coconut chips, garden waste compost and peat mix. The first-order sorption kinetics model was fitted to the observed pesticide concentrations versus time resulting in an estimated kinetic rate constant alpha. Sorption appeared to be fast for the pesticides linuron and isoxaben, pesticides which were classified as immobile, while less mobile pesticides displayed an overall slower sorption. However, the substrate does not seem to be the main parameter influencing the sorption kinetics. Coconut chips, which is a substrate with a high organic matter content showed slow sorption for most of the pesticides. The effect of different estimated alpha values on the breakthrough of pesticides through a biopurification system was evaluated using the HYDRUS 1D model. Significant differences in leaching behavior were observed as a result of the obtained differences in sorption kinetics.