The 'Fate of Pesticides in paddy rice systems'''': Two case studies
In the last decades, high population growth combined with an increasing rice demand have led to an intensification of rice production. Paddy rice is the dominant production system of rice worldwide, where fields are temporarily flooded. Paddy field farming is practiced in Cambodia, Bangladesh, China, Taiwan, India, Indonesia, Japan, North Korea, South Korea, Malaysia, Myanmar, Nepal, Pakistan, the Philippines, Sri Lanka, Thailand, Vietnam and Laos as well as in the USA and Europe (in Piedmont in Italy and in the Camargue in France). Increasing levels of rice yield are achieved by the introduction of high-yielding, short-duration varieties in association with a wider use of agrochemicals, particularly pesticides which are applied to prevent, mitigate or destroy pests . The amount of agrochemicals used has increased dramatically and their toxic nature has raised concern about environmental impact and effects on human health. Numerous monitoring studies have showed that agriculture is responsible for pollution in surface and groundwater as well as in rivers and lakes. As paddy rice fields are part of larger aquatic systems, the spread of pesticides through water-bodies constitutes a particular problem. It is therefore a crucial task to quantify and predict pesticide losses into water-bodies and their environmental impact. This in turn may help in adapting pesticide and water management practices.
The following two studies, one from Thailand and one from Vietnam, highlight important aspects of pesticide use in paddy rice systems.
In the following case studies the fate of pesticides commonly used in paddy rice production has been analysed. The use of pesticides has increased, partly due to an export-oriented economy which has triggered an increase of production in the remote mountainous regions of northern Vietnam as well as in the large rice-growing areas of the Mekong and the Red River delta. An increase of pesticide use has also been taking place in Thailand. In the case of Vietnam, agrochemicals are likely to be the main source of non-point pollution agents of surface and groundwater.
Paddy rice fields create a “water continuum” of water bodies, and as a consequence higher pesticide loss with drainage water is to be expected. It is called a water continuum because there is a continuous flow of water from one water-body to another, moving through a canal system or different production ponds (e.g. rice and fish) and finally discharging in a river. Fish cultivation represents an additional source of income and food production for many households. In this system, the discharge water from the rice field flows into the fishponds where the fish are raised .
The transfer of agrochemicals from the target area to surface water is mainly determined by surface and subsurface runoff . This occurs when rain intensity is exceeding the infiltration capacity (e.g. in sloping land or low hydraulic conductivity of the soil after the dry season), surface run-off being the main transport mechanism. This holds true in paddy rice systems as well as in other typologies of agricultural fields.
After application, the chemical compounds undergo several transformation processes. The ones which mainly affected the pesticides in the study site are: 1. biodegradation and 2. absorption into the soil matrix. Both processes affect the pesticide concentration in the water phase, which influences transport, the ecological impact on the environment and bioavailability.
The behavior and the transport processes in paddy rice fields are influenced by the properties of the pesticide itself (e.g. solubility and biodegradability), by water management practices (e.g. water-holding period and continuous and intermittent automatic irrigation), by application (misapplication) practices, as well as by the soil properties (e.g. organic C content and clay content).
The biodegradability of an agrochemical is characterised by “half-life time”, which is the time needed for 50% of the pesticide mass to degrade. The absorption strength in these studies is measured by the distribution coefficient Kd which is the ratio of the sorbed phase concentration to the dissolved concentration of a pesticide in equilibrium. That means the higher the Kd the stronger the pesticide is sorbed and the less mobile it is in the soil matrix. The transport velocity of the pesticide in the soil is slower than the flow velocity of the water, due to the virtue of sorption.
Table 1 Physico-chemical properties of seven pesticides investigated by the Uplands Programme1)
|| Log Koca
|| Half-life time in water
|| Chemical class
|| L kg-1
|| Chlorinated hydrocarbon
|| > 50a
1) Uplands Programme of the University of Hohenheim, a Footprint PPDB , b PAN database, c Howard 
In addition, the amount of pesticides on the market with different formulation has dramatically increased. As a result analytical methods for detecting and quantifying also have to be adapted to the current active compounds in the agrochemicals.
Understanding the fate of pesticides in the paddy rice agricultural system, as opposed to the non-flooded field system, is particularly important because pesticides easily enter the water phase and are transported far away from the application site. This leads to an increase in contamination of surface and groundwater, both used for domestic purposes. Furthermore, the soil can only influence the amount of pesticides in the soil if the pesticides are in close contact with the soil particles. Understanding the fate of pesticides in remote areas, where good pesticide management practices have not yet been established and the use has increased, is very important.
All described findings are in the papers referenced here.      .
Case study I: Fate of pesticides in paddy rice - fishpond farming systems in mountainous regions of northern Vietnam
The study area was the Son La province, northern Vietnam. In this mountainous area subsistence-oriented agriculture systems prevail, on the plateaus paddy rice is cultivated, while on the upland fields and steep slopes maize and cassava are the major crops. The nearby reservoir guarantees irrigation and two rice-growing seasons a year (one irrigated and one rain-fed). Pesticides are used almost exclusively in the paddy rice fields.
The samples to assess pesticide load of four commonly applied pesticides (imidacloprid, fenitrothion, fenobucarb, dichlorvos) in the stream water were manually collected every 10 days. Additionally water from the wells was sampled to assess the pesticide level in the groundwater.
The concentration of the agrochemicals detected in the stream (sample site) was mainly hypothesised to follow two different hydrological flow mechanisms. The first concentration peak, associated with the discharge peak, was attributed to surface run-off. The second, deleted in time, was attributed to preferential interflow. Interflow is defined in hydrology as the lateral movement of water from an unsaturated zone and enters a stream before becoming groundwater. Interflow is the movement that takes place when water infiltrates into the surface of the soil, but with increasing depth the infiltration speed is reduced. Lateral flow might particularly increase on hill-slope sites , but it also occurs within the bounds of a paddy rice field. Preferential interflow is the path that the water takes first, because it is easier to access (larger pores in the soil for instance, or because of slope inclination).
It was found that the higher the solubility of the compounds the higher the runoff losses. Many pesticides enter the paddy rice field's surface water shortly after application. Since the ponds are arranged in cascades and connected by channels or outlets the receiving stream is contaminated by the substances transported by surface runoff. However, the management practices also influence the final pollution of the surface water, in particular the drainage system. In this regard, it was found that when the water remains in the ponds for a time span of 8 to 10 days the amount of agrochemicals is substantially reduced.
Despite the different chemical compositions the persistence of chemicals in the paddy water was found to be relatively short, but risk to the fishes raised in the following ponds cannot be excluded and the use of agro-chemicals cannot be considered as safe under current management conditions.
The environmental problems caused by pesticides to the mountainous regions of Vietnam have to be taken seriously into consideration, especially as surface and groundwater are both reused for domestic purposes.
See also: Fate of pesticides in paddy rice farming systems in NWVietnam
All findings are described in the following referenced papers   .
Case study II: Fate of pesticides in mountainous regions of northern Thailand
In Thailand the dynamics of pesticides in rivers were studied and quantified during three runoff events. The study illustrates the processes involved in the transport of the pesticide from crop land to river.
Water samples were analysed for seven different pesticides (atrazine, chlorothalonil, chlorpyrifos, cypermethrin, dichlorvos, α- and β-endosulfan). Only dichlorvos was below the detection level but the remaining six were found in the stream water. On the whole pesticides with a low sorption coefficient were transported during the runoff peaks, whereas pesticides with a higher sorption rate were particularly influenced by sporadic sub-surface flow components (e.g. preferential interflow) which are other important transportation pathways.
The assumption was therefore made that sorption rate influences the travel distance and the concentration in the river of the pesticides. This holds true if strongly sorbing pesticides have time to interact with the soil. During preferential flow this time is very short and therefore direct transfer of the pesticide to the surface water is very likely.
At the beginning of the rainy season, when soils are still dry, the interflow pathways are not well connected and potential pesticide leaching is low. On the other hand, when the soils are wetter preferential interflow is enhanced and pesticides are leached.
Maximum concentration of many pesticides is found in rivers during rainfall and for several hours afterwards. In the case analysed the pesticide peaks were in the allowed range permitted by the Thai quality standards but exceeded the European limits for pesticides in surface water and the Canadian water quality guidelines for protection of aquatic life. Pesticides showed a highly dynamic concentration pattern. Extreme and short events (about one hour) were recorded after the beginning of the runoff and after a longer retention phase. (Pesticides are dissolved by previous rainy events and sorb to the walls of the macropores. The water of a later rain event travels through the macropores and leaches these sorbed pesticides.)
Conventional eco-toxicological tests do not consider the stress factor to which the aquatic ecosystem is exposed by the described short and multiple-pulsed extreme events. Therefore, it is advisable to consider pesticide dynamics in surface water when doing risk assessments or eco-toxicological tests, in order to better assess their effects on the environment, non-target organisms and human health.
The authors underline the necessity that “in tropical areas sampling schemes with a high temporal resolution are needed to adequately assess the pesticide contamination of rivers. Otherwise, extreme situations may remain unsampled“.
All findings are described in the following referenced papers   .
Run-off and preferential flow are the main transport patterns of pesticides in paddy rice systems into surface and ground water. Soil is naturally able to attenuate the effect of contaminants and the larger share of the pesticide mass is usually not transported into the deeper horizons of the soil.
The shorter the time from the application of the pesticide to the first surface run-off event (e.g. first rains) the higher the risk that a pesticide will be transported from the application site to the surface waters. Additionally, due to the rapid transport (preferential flow), the positive attenuation “service” of the soil (biodegradation) is limited.
To conclude, the current management practices of pesticide application in paddy rice fields and the combined paddy rice-fishpond systems are harming the quality of surface and ground water and are not environmentally safe, with possible implications for human health.
It is particularly important to consider, while preparing the sampling procedure, that pesticides are sorbed to the walls of the macropores and during a later rain event the water leaches these sorbed pesticides. Therefore, a high temporal resolution of the sample scheme is needed to record all the pesticide peaks in the stream.
Risk assessments should also consider not only the total pollutant transported into the water but also the pesticide dynamics (described as short and multi-pulsed extreme events).
In the field:
The current pesticide management strategies are harming the environment, the cultivated crops and fishes as well as human health.
Particular attention has to be given to the dosage of the pesticide in the field, and this needs to be respected, which is often not the case.
In particular, where the water flows into fish ponds after it was part of the paddy rice system, it is preferable to use pesticides with a low “half-life time” (time needed for 50% of the pesticide mass to degrade) and a high Kd (stronger sorption and it is less mobile in the soil matrix) in order to reduce the effect of the chemicals on the raised fishes.
The pesticides level of the water was strongly reduced after about 10 days in the pond, therefore a strategy to reduce the risk imposed on carp and crustacean species would be to exchange the water in the fish ponds at the latest 8 to 10 days after application of the pesticide.
Application of the pesticides should be done, when possible, ahead of the rains in order to reduce surface run-off of the pesticides away from the application site and to guarantee sufficient time for the sorption processes to take place.
1 Riz de Camargue, Silo de Tourtoulen, Riz blanc de Camargue, Riz et céréales de Camargue". Riz-camargue.com. Retrieved 2013-04-25. http://www.riz-camargue.com/pages-uk/moisparmois.html
3 Capri, E, and D. Karpouzas. 2008. Pesticide risk assessment in rice paddies: Theory and practice. Elsevier, Amsterdam
4 Anyusheva, M.,Lamers, M., Van Vien Nguyen, N., and Streck, T., 2012. Fate of Pesticides in Combined Paddy Rice–Fish Pond Farming Systems in Northern Vietnam. J. Environ. Qual.41:515-525.
5 Brown, C.D., and W. van Beinum. 2009. Pesticide transport via sub-surfacedrains in Europe. Environ. Pollut. 157:3314–3324.
6 Kahl, G., J. Ingwersen, P. Nutniyom, S. Totrakool, K. Pansombat, P. Thavornyutikarn, and T. Streck. 2008. Loss of pesticides from a litchi orchard to an adjacent stream in northern Thailand. Eur. J. Soil Sci. 59:71–81
7 Footprint PPDB 2011. The footprint pesticide properties database. Agriculture and Environmental Research unit (AERU), University of Hertfordshire, page cited 28 April 2011, available from: http://sitem.herts.ac.uk/aeru/footprint/en/index.html
8 PAN 2008. Pesticide Action Network. Pesticide database, page cited 20 April 2007, available from http://pesticideinfo.org/
9 Howard PH 1991. Handbook of environmental fate and exposure data for organic chemicals: volume III: pesticides. Lewis Publishers, Chelsea
10 Anyusheva, M., Lamers M., Schwadorf, K., and Streck. T., 2011. Analysis of pesticides in surface water in remote areas in Vietnam: Coping with matrix effects and test of long-term storage stability. Int. J. Environ. Anal. Chem. 92:7, 797-809.
11 Lamers, M., Anyusheva, M., Van Vien Nguyen, N., Nguyen, L. and Streck, T., 2012. Pesticide pollution in Surface- Groundwater by paddy rice cultivation: A case study from northern Vietnam. Clean – Soil, Air, Water 2011, 39:4, 356–361
12 Gut, T., Lamers, M., Van Vien, N., Streck T., 2012. Fate of pesticides in paddy rice farming systems in NW Vietnam. International Conference “Sustainable Land Use and Rural Development in Mountain Areas”. Hohenheim, Stuttgart, Germany, 16-18 April 2012
13 Froehlich, H., Ingwersen, J., Schmitter, P., Lamers, M., Hilger, T. and Schad, I., 2013. Water and matter flows in mountainous watersheds of Southeast Asia: Processes and implications for management. In: Sustainable Land Use and Rural Development in Southeast Asia: Innovations and Policies for Mountainous Areas. Springer Environmental Science and Engineering 2013, 109-148
14 Sangchan, W., Hugenschmidt, C., Ingwersen, J., Schwadorf, K., Thavornyutikarn, P., Pansombat, K., Streck, T., 2012. Short-term dynamics of pesticide concentrations and loads in a river of an agricultural watershed in the outer tropics. Agr Ecosyst Environ 158:1–14.
15 Müller, K., Deurer, M., Hartmann, H., Bach, M., Spiteller, M., Frede, H.G., 2003. Hydrological characterization of pesticide loads using hydrograph separation at different scales in German catchment. J. Hydrol. 273: 1–17
16 Ward A., and Trimble S., 2004. Environmental Hydrology. Boca Raton: CRC Press. p.122. ISBN 1566706165
Further reading and external links
http://npic.orst.edu/ National Pesticide Information Center (NPIC) Information about pesticide-related topics.
http://www.dropdata.org/RPU/pesticides_MoA.htm Pesticide Modes of action (International Pesticide Application Research Centre)
http://www.beyondpesticides.org/dailynewsblog/?p=373 Source of information on pesticide hazards, least-toxic practices and products, and on pesticide issues. Website has Daily News Blog relating to pesticides
http://ceqg-rcqe.ccme.ca/ Canadian Water Quality Guidelines for the Protection of Aquatic Life
http://www.pcd.go.th/info_serv/en_reg_std_water.html Thai Water Quality Standards
http://www.alanwood.net/pesticides/class_pesticides.html Compendium of Pesticide Common Names: Classified Lists of Pesticides. Lists of pesticide names by type.
http://www.pesticideinfo.org/ Pesticide Action Network. Compilation of multiple regulatory databases into a web-accessible form.
http://www.examinetics.com/ProfessionalResources/Pesticides/ Pesticide pathfinder. Information about pesticide use in the workplace and links to U.S. regulatory information.
http://www.ams.usda.gov/AMSv1.0/ams.fetchTemplateData.do?template=TemplateC&navID=PesticideDataProgram&rightNav1=PesticideDataProgram&topNav=&leftNav=&page=PesticideDataProgram&resultType=&acct=pestcddataprg USDA Pesticide Data Program, tracking residue levels in food
http://www.snellsci.com/ Snell Scientifics Pesticide Development Lab General Pesticide Development Information
Pesticide regulatory authorities:
http://www.pesticides.gov.uk/guidance/industries/pesticides UK Pesticides Safety Directorate
http://ec.europa.eu/food/plant/plant_protection_products/index_en.htm European Commission pesticide information
http://www.epa.gov/pesticides/ United States Environmental Protection Agency Office of Pesticides Program
http://iaspub.epa.gov/apex/pesticides/f?p=CHEMICALSEARCH:1:5980154726512::NO:1:: US EPA Pesticide Chemical Search