SBR- Content.pmd - INBO

3 - Water in the Mekong Basin4. Water qualityAt Kratie, the chemical composition of the Mekong is remarkably similar to the mean composition ofworld river waters (Table 2). The composition changes upstream, particularly between Kratie andPakse, reflecting the influence of the large tributaries flowing in from eastern Cambodia which drainthe Eastern Highlands. The upper Mekong flowing from Yunnan shows the influence of limestone inthe upper catchment, with relatively high concentrations of calcium, which are only slightly diluted bythe large tributaries in northern and central Lao PDR. Concentrations of nitrogen are well below theworld average (Table 2). However, concentrations of total phosphorus are relatively high (Table 3).These concentrations are almost certainly associated with suspended solids in the water.Table 2.Mean composition of river waters of the world, Asia and three sites on the Mekong.Data for the world and Asia from Wetzel (1983)Ca 2+ Mg 2+ Na + K + Fe 3+ 2-SO 4Cl - SiO 2-NO 3HCO 3- 2-CO 3Asia 18.4 5.6 5.5 3.8 0.01 8.4 8.7 11.7 0.7 79World Average 15 4.1 6.3 2.3 0.67 11.2 7.8 13.1 1 58Mekong (Chiang Saen) 28.0 6.6 9.9 1.9 0.06 17.8 8.8 11.4 0.3 109Mekong (Pakse) 23.1 4.6 6.7 1.6 0.18 17.1 8.0 12.2 0.1 83Mekong (Kratie) 15.5 4.1 6.9 1.4 1.21 9.3 5.0 11.4 0.1 62Note: Data from the Mekong are means of the MRC monthly measurements from 1985-1999 forChiang Saen and Pakse and from 1992-1999 for Kratie.Source: Wetzel 1983For all the parameters listed in Table 3 and for total phosphorus, there was no appreciable differencebetween mean and median values at any of the three sites. This indicates that there were fewextreme values (either high or low) within the data set. Where extreme values are present, themedian and mean values tend to deviate from each other.Values for Total Suspended Solids (TSS) did show appreciable differences between the mean andmedian values (Table 3), with the means substantially higher than the median values. This resultsfrom a relatively small number of very high TSS measurements during high flow events and is acommon phenomenon in rivers. It is likely that the difference in pattern between TSS and the TotalP, which is normally associated with suspended particles, is due to elevated TSS levels being causedmainly by an increase in larger particles, fine sand and upwards. Phosphorus is mainly associatedwith fine particles such as clay, so elevated TSS concentrations caused by larger particles would notbe accompanied by major increases in Total P.Table 3.Mean and median values for total suspended solids and total phosphorus from samplescollected at Chiang Saen, Pakse and KratieTSS (mgL -1 ) Total P (mgL -1 )mean median mean medianMekong (Chiang Saen) 397 222 0.06 0.05Mekong (Pakse) 215 130 0.04 0.04Mekong (Kratie) 122 74 0.02 0.0223