GIS modelling of movement of agrochemicals and site selection for aquaculture in the Mekong delta, Vietnam
Long Kim Pham: Ph.D.
The development of aquaculture and agriculture creates conflicts.The Mekong delta is one of the largest deltas in the world, producing huge quantities of grains. In addition, it significantly contribute to global aquaculture production especially in Pangasius and Tiger shrimp. Agricultural chemicals (pesticides) used in paddy fields could cause many problems on aquaculture in term of organizing rearing sites, production and quality.
This study uses GIS to model the fate of pesticides and identify suitable sites for aquaculture to minimize the risk from agrochemicals. Pesticide use was investigated at 344 locations in the Vietnamese Mekong delta (VMD) (above). Ninety eight types of pesticide in 22 groups were found mainly in intensive rice farms in flooding areas and orchards in riverside zones. The general analysis (figures below) shows that the total volume of insecticide used is significantly high (63%), followed by fungicide (23%) and then equal use of herbicide and molluscicide (7%). Pesticides in the Carbamate and Conazole group had the highest percentage application rate, reaching over 2kg pure substance per hectare, followed by Organophosphates and Chitin Synthesis Inhibitors with over 1.5kg.
All pesticide data was collected and mapped for Mekong delta to show the distribution of current pesticide use (right). The complex channel system in the VMD and hydrological processes are the main factors which distribute persistent substances in the water bodies, with accumulation in riparian or coastal areas.
To simulate all these processes, mathematical models were built in GIS tools with 3 main objectives:
- Soil loss erosion and sediment yield calculation model (right above): To evaluate the amount of pesticide concentration in thesoil layer. Under the annual soil erosion process, pesticides existing in soil particles were transported and deposited elsewhere. The RUSLE model was used in this situation with input data of topography, landuse and land cover, cropping system, soil type and climate data. The final output shows the actual amount of pesticide deposition in fields and river basins (right middle).
- Runoff model: To evaluate the amount of pesticide loss in solute phase when runoff process takes place. This model was a combination between a pesticide property database and dynamic model. The volume of water contains a particular amount of pesticide and the total water loss caused by rainfall represents the concentration of pesticide movement. The accumulation of water runoff shows the deposition of pesticides (right bottom)
- Site selection for aquaculture: The toxicity of pesticides was used to classify areas for aquaculture. LD50 and LC50 are two common parameters which were applied to classify suitable site for fish farms and shrimp farms.
The study makes a wide framework evaluation of current agrochemical pollutants in the Mekong delta of Vietnam. Going further, it shows the impacts between agriculture and aquaculture and considers their harmonic development. Future research will propose re-arrangement of aquaculture in order to avoid the hazards from pesticides.