SBR- Content.pmd - INBO

8 - Agriculturepolluted with human waste, pesticides and other substances is not flushed into the sea. 104 Also theuse of river water to flush away intruding seawater reduces the availability of water for irrigation.At the end of the dry season, total flow in the coastal part of the delta is about 2000 m 3 , which wouldbe sufficient for irrigation purposes, except that approximately 1500 m 3 are required to wash awaythe saline water intruding from the sea. 105 However, recent MRC water quality testing shows thatsalinity levels have been decreasing across the basin, particularly in Northeast Thailand. 106 Thereasons for this are still unclear, but could include a general shift from small-holder to commercialisedsalt extraction that may be more efficient and result in lower levels of salt run-off.4.3 Environmental impacts of pesticide and fertiliser usePesticide and fertiliser use is another important means by which agricultural activity may impact theresources of the basin. While overall the rate of chemical application in the basin is comparativelylow, there are problems with improper handling and the use of banned pesticides. Farmers in Thailandand Viet Nam are generally aware of the potential dangers of pesticides, but such awareness is lowerin Lao PDR and Cambodia, where these products have only recently begun to be used on a largescale. 107 Persistent pesticides are banned in the riparian countries, but it is clear that residual andillegally imported stocks continue to be used because residues of DTT, Dieldrin, and similar chemicalshave been found in fish across the basin. 108 However, investigation into the build-up of pesticideresidues in the bodies of such fish hasrevealed that contaminant levels are wellbelow the maximum safety levels specifiedby the World Health Organisation. 109Pesticides banned elsewhere in the world are readilyavailable in some areas of the LMBPesticides can also cause environmentalproblems through build-up in the soil,toxicity to humans and the development ofresistance on the part of pests. The latter hasbeen held responsible for localised outbreaksof pests such as brown plant hopper. 110 Thereis currently little available data to indicatethe extent of such problems. The relationshipbetween pesticide use and fisheries is furtherdiscussed in Chapter Seven on fisheries.Because of high intensities of fertiliser use, the Korat Plateau in Thailand and the Mekong Delta areareas of ongoing concern for ground and surface water contamination. 111 In the delta there is concernover excessive fertiliser use affecting water quality and damaging integrated aquaculture operations.MRC water quality surveys have revealed an increase in levels of nutrients in the water. 112 There isa need for further systematic investigation of this issue, but it appears that in general, the use ofnitrogen, phosphate and potassium fertilisers is not at levels likely to cause significant damage toecosystems. 1134.4 Acid sulphate soilsThe conversion of acid-sulphate soils to agricultural use, especially in the Mekong Delta, is creatingmore agricultural land, but acid runoff is damaging other subsystems downstream such as the culturingof fish in rice fields. 114 Acid sulphate soils occur in the mangroves and grasslands of the delta,covering some 1.6 million ha, or 40 percent of its soils, mainly in the Plain of Reeds, the LongXuyen Quadrangle, and the Ca Mau Peninsula. 115 These soils are also characterised by high levelsof potentially toxic aluminium and poor phosphorous availability. The issue is mostly localised to171