Marc Lamers

Fate of Pesticides in Combined Paddy Rice–Fish Pond Farming Systems in Northern Vietnam

During the last decades, high population growth and export-oriented economics in Vietnam have led to a tremendous intensification of rice production, which in turn has significantly increased the amount of pesticides applied in rice cropping systems. Since pesticides are toxic by design, there is a natural concern on the impact of their presence in the environment on human health and environmental quality. The present study was designed to examine the water regime and fate of pesticides (fenitrothion, dimethoate) during two consecutive rice crop seasons in combined paddy rice-fish pond farming systems in northern Vietnam. Major results revealed that 5 and 41% (dimethoate), and 1 and 17% (fenitrothion) of the applied mass of pesticides were lost from the paddy field to the adjacent fish pond during spring and summer crop seasons, respectively. The decrease of pesticide concentration in paddy surface water was very rapid with dissipation half-life values of 0.3 to 0.8 and 0.2 d for dimethoate and fenitrothion, respectively. Key factors controlling the transport of pesticides were water solubility and paddy water management parameters, such as hydraulic residence time and water holding period. Risk assessment indicates that the exposure to toxic levels of pesticides for aquaculture (, ) is significant, at least shortly after pesticide application.

Pesticide transport simulation in a tropical catchment by SWAT

The application of agrochemicals in Southeast Asia is increasing in rate, variety and toxicity with alarming speed. Understanding the behavior of these different contaminants within the environment require comprehensive monitoring programs as well as accurate simulations with hydrological models. We used the SWAT hydrological model to simulate the fate of three different pesticides, one of each usage type (herbicide, fungicide and insecticide) in a mountainous catchment in Northern Thailand. Three key parameters were identified: the sorption coefficient, the decay coefficient and the coefficient controlling pesticide percolation. We yielded satisfactory results simulating pesticide load dynamics during the calibration period (NSE: 0.92-0.67); the results during the validation period were also acceptable (NSE: 0.61-0.28). The results of this study are an important step in understanding the modeling behavior of these pesticides in SWAT and will help to identify thresholds of worst-case scenarios in order to assess the risk for the environment.

Analysis of pesticides in surface water in remote areas in Vietnam: Coping with matrix effects and test of long-term storage stability

During the last years, the increased use of pesticides and growing awareness of associated environmental and health problems have led to the implementation of various monitoring programmes in South-East Asia. The introduction of numerous new active ingredients and commercial pesticide formulations in connection with reports on pesticide-related health problems strongly indicate that the analytical procedures should be tested and evaluated for currently used pesticides. Coping with matrix effects and ensuring pesticide stability when samples are taken in remote areas are paramount. In the present study, we tested an analytical method that targets nine currently used pesticides in surface water in northern Vietnam. The method consists of solid phase extraction, storage at −18°C in the adsorbed state, and capillary gas chromatography with nitrogen-phosphorus-detection of five insecticides (dichlorvos, fenobucarb, dimethoate, fenitrothion, and chlorpyrifos), three fungicides (chlorothalonil, metalaxyl, and edifenphos) and one herbicide (atrazine). We evaluated the potential analytical bias caused by matrix effect and investigated its possible causes. We also tested the long-term stability (up to 9 months) of pesticides adsorbed to Carbopack SPE cartridges when stored at temperatures below −18°C. Adopting a matrix-matched calibration technique considerably improved the recovery values of seven of the nine tested pesticides. At spiking levels of 0.1 µg L−1 and 1 µg L−1 and after storage of 119 days at −18°C, recovery values of these pesticides ranged from 67% to 107% and from 67% to 155%, respectively. For the remaining two pesticides recovered at 53–55% at both spiking levels – dichlorvos and chlorothalonil – the method could still be useful for semi-quantitative analysis or as a screening tool. Even though the general recommendation is to minimise storage time to reduce pesticides degradation, our results showed that storage times up to nine months can be adopted for atrazine, metalaxyl, fenitrothion, and chlorpyrifos.

Modelling the fate of pesticides in paddy rice–fish pond farming systems in northern Vietnam

BACKGROUND In Vietnam, paddy rice fields have been identified as a major non-point source of pesticide pollution of surface- and groundwater which is often directly used for domestic purposes. One strategy to assess the risk of pesticide pollution is to use process-based models. Here, we present a new model developed for simulating short-term pesticide dynamics in combined paddy rice field-fish pond farming systems. The model was calibrated using the Gauss-Marquardt-Levenberg algorithm and validated against measured pesticide concentrations of a paddy field-fish pond system typical for northern Vietnam. RESULTS In the calibration period, model efficiencies were 0.82 for dimethoate and 0.87 for fenitrothion. In the validation period, modelling efficiencies slightly decreased to 0.42 and 0.76 for dimethoate and fenitrothion, respectively. Scenario simulations revealed that a field closure period of 1 day after pesticide application considerably reduces the risk of pond and surface water pollution. CONCLUSION These results indicate that the proposed model is an effective tool to assess and evaluate management strategies, such as extended field closure periods, aiming to reduce the loss of pesticides from paddy fields.

Pesticide-contaminated feeds in integrated grass carp aquaculture: toxicology and bioaccumulation.

Effects of dissolved pesticides on fish are widely described, but little is known about effects of pesticide-contaminated feeds taken up orally by fish. In integrated farms, pesticides used on crops may affect grass carp that feed on plants from these fields. In northern Vietnam, grass carp suffer seasonal mass mortalities which may be caused by pesticide-contaminated plants. To test effects of pesticide-contaminated feeds on health and bioaccumulation in grass carp, a net-cage trial was conducted with 5 differently contaminated grasses. Grass was spiked with 2 levels of trichlorfon/fenitrothion and fenobucarb. Unspiked grass was used as a control. Fish were fed at a daily rate of 20% of body mass for 10 d. The concentrations of fenitrothion and fenobucarb in pond water increased over time. Effects on fish mortality were not found. Fenobucarb in feed showed the strongest effects on fish by lowering feed uptake, deforming the liver, increasing blood glucose and reducing cholinesterase activity in blood serum, depending on feed uptake. Fenobucarb showed increased levels in flesh in all treatments, suggesting bio-concentration. Trichlorfon and fenitrothion did not significantly affect feed uptake but showed concentration-dependent reduction of cholinesterase activity and liver changes. Fenitrothion showed bioaccumulation in flesh which was dependant on feed uptake, whereas trichlorfon was only detected in very low concentrations in all treatments. Pesticide levels were all detected below the maximum residue levels in food. The pesticide-contaminated feeds tested did not cause mortality in grass carp but were associated with negative physiological responses and may increase susceptibility to diseases.

Agricultural Pesticide Use in Mountainous Areas of Thailand and Vietnam: Towards Reducing Exposure and Rationalizing Use

A change in land use from the growing of upland rice to the cultivation of cash crops has increased the level of use of synthetic pesticides in the mountainous areas of Thailand and Vietnam. Although this increase has occurred generally across both countries, it has been especially prevalent in mountainous areas. The objective of this chapter is to describe the challenges faced when wishing to reduce the risks caused by the use of agricultural pesticides in mountainous areas, both from an economic and a biophysical point of view. Building on case studies from Thailand and Vietnam, we show how the potential risk of pesticide use is related to the limited experience farmers have in handling pesticides, and the hydrological relationships between highland and lowland areas.

Water and Matter Flows in Mountainous Watersheds of Southeast Asia: Processes and Implications for Management

The mountainous ecosystems of Southeast Asia have experienced a tremendous increase in human activity over the last few decades. An integral part of land use change and land use intensification in the area is the changes in water and matter cycling that have taken place, a situation which calls for increased attention to be paid to resource management, particularly given the environmental vulnerability of the region. Building on four case studies that describe the consequences of such change and provide implications for resource management with regard to flooding, nutrient stocks and fluxes, and the fate of pesticides, we develop a general perspective on current matter and water related issues. This perspective will require us to integrate (1) knowledge on water and matter related processes, (2) the spatial and temporal scales on which they are active and through which they connect inside a watershed, and (3) their physiographic (site and land use specific) domains.