Mekong delta water resources assessment studies

Upper Miocene aquifer (n13) Aquifer n13 has not yet been studied in detail. There are very few boreholes drilled through this aquifer. It is very highly-confined aquifer, not outcropped on the surface. The upper part consists of silt, weathered silty clay, is at a depth from 257.9 to 364.1m, and a thickness varying from 11.7 ÷ 24.1m; The lower part consists of compacted fine to coarse sand, is at 260.6 to 377.9m, having a thickness from 40.1 to 71.5m.

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line does not change (coefficient = 0 and angular coefficient p = 0.982). - Saltwater intrusion in 2007was not high; in My Tho the maximum conductivity value is 1680 PS/cm in April, with a lower salinity of about 1.3 g / l. In recent years, the saltwater intrusion has minor fluctuations. The year 2004 is the year with the highest saltwater intrusion. - Silt content on the main streams and the internal canals tends to increase during the period 2002-2007. - Organic matter, nitrogen and phosphorus content: the data show an increase in periods with heavy rains (July). During the dry season, the nutritional composition of canals is higher than common on the mainstream river courses. In the period 2002-2007, some components tend to show an increase (nitrogen) while others are somewhat lower, such as phosphorus. - The content of organic matter (BOD5) is quite low, however COD components tend to increase in recent years (the period 2002-2007). Dissolved Oxygen was low in the channels in the dry season. - Water in the main streams is of good quality (and meets TCVN 5942:1995 standards) based up on most water quality indices observed, except for suspended solids. Water in most smaller canals was polluted. All water bodies have good quality for fresh aquatic life, except for a few months with impact of acid drainage from acid sulphate soils in the Long Dinh area. - Some local stations were polluted by acid water in May and June by acid soil leakage during the early rainy season. 30 Figure 12: pH in 2008 at some stations Figure 13: pH in different water resources in 2002-2008 For the period 2002-2007, the pH value of water in the main rivers and canals varies not much, only by about 0.5 units (Figure 12.) Values of SAR (sodium absorption ratio) of the Mekong branches as well as for canals (Cai San, Nguyen Tan Thanh, Tri Ton) is quite low, less than 5 units during the rainy season, during the dry season usually less than 10 (except at My Tho). This means water is suitable as irrigation water for agriculture, according to the Vietnamese Standard TCVN 6673-2000 (with SAR <10). In 2008, the variability of the mineral chemical composition (through electrical conductivity) of the main river branches (except for My Tho station, since this was affected by saltwater intrusion) and the internal canals did not fluctuate much (about 10 to 20 mS / m (0.06 g / l to 1.3 g / l). Stations Long Dinh, and Kien Binh and Bridge No13 had high conductivities that increased from 40 to 70 mS / m in the last months of the dry season and the start of the rainy season. 31 Figure 14: EC in fields and main streams in 2008 Figure 15: Salinity at Mӻ Tho, 2002- 2008 In the period 2002-2008 the salinity intrusion in My Tho was weak, while the most strong saltwater intrusion occurred in 2004. The highest concentration of silt in the major rivers in August is approximately 500 mg/l. Major rivers normally have a higher concentration of silt (TSS) as compared to the smaller canals; this is increasing during the rainy season and low during the dry season. TSS in the channels is higher in the rainy season then in the dry season possibly due to boats movements. Trends of the TSS on the main river branches in the period 2002-2008 shows that the concentration of TSS is increasing, but this rise is very small. Suspended solids in the flow, especially in the flood season, are a huge natural resource distributed with the flood waters, leading to annual accretion of the soils in the Mekong Delta region. This is a very positive impact of the floods. But for areas where people have to use surface water (rivers, canals) for domestic water supply (viz. cooking, bathing, etc.), the water quality is not good as the turbidity is too high. (Vietnamese standard: TCVN 5942:1995 requirements: TSS content source A is 20 mg / l, so water in the rainy season almost never meets the requirements for water supply activities). This creates difficulties for people living in the flooded areas. 32 Figure 16: TSS in fields and rivers in 2008 (right) and TSS in rivers in 2002- 2008 (left) In 2008, the concentration of total nitrogen (T-N) with wide range from 0.2 mg / l to 1.7 mg / l. Figure 17: T-N in river courses and canals in (data for 2008). Nitrogen composition varies in a range below 1.5 mg NO2-&NO3- per litre, which is lower than the limit for good surface water (quality level A of TCVN 5942- 1995, the so-called limit A TCVN5942-1995 with NO2-=10 mg/l). Ammonium concentration is less than 0.05 mg/l, (equivalent to NH3 below 0.01 mg/l, which is smaller than limit A TCVN5942-1995 with NH3= 0.06 mg/l). 33 In the period 2002-2008, these components tend to increase, but their content is low compared with standard. Figure 18: NH4+ and NO2&3- (data for 2002-2008) Figure 19: T-N in 2002-2008 & T-P in 2002-2008 Organic matter in water was determined by using the two indices BOD (biochemical oxygen demand) and COD (chemical oxygen demand). Observed data in 2008 show high BOD with 4 mg/l, above the limit A TCVN5942-1995, at My Thuan on the Tien river and in Can Tho on the Hau river. Other stations have low BOD. 34 Figure 20: BOD5 and COD (data for 2008). Surface water quality in the Mekong delta, in addition to the impact of wastewater resources from population living in concentrations along rivers and canals, is now also affected by emissions from sources of industrial production. However, the level and scale differs within the area. Every day there is a large amount of wastewater produced that is discharged into the environment. Aquaculture is strongly developed in the Mekong delta, and along with it the problem of wastewater, sludge waste of aquaculture ponds and from seafood processing facilities. Every year the agricultural sector also used to 2 million tons of chemical fertilizers and 500,000 tons of plant protection pesticides. All this waste sources impact the quality of water resources in the Mekong Delta. Also incidental accidents have caused serious environmental and economic damages. Figure 21: BOD5 in 2002-2008 and COD in 2002-2008 35 In most monitoring locations this year, total coliform and E. Coli is not high. The highest Coli. value was in My Tho on the Tien River (90,000 MPN/100 mL) in October 2008. In the channels, the highest value is at Long Dinh 24,000 MPN/100ml in May. These appear to exceed the threshold value of source A (5000 MPN/100 ml), TCVN 5945:1995 Observed coliform values are low in most monitoring points, while the value of E. Coli is very low, indicating the microbial contamination is not high. The chances for significant microbial contaminations of the larger river branches is very low because the flow volume is large. Table 11: Coliform in 2008 (MPN/100 ml) Source Average Minimum Maximum Middle Canal 2920.036 0 24.000 930 +ҰU river 1412.864 23 4.300 700 TIӄN river 7450.606 90 90.000 900 36 CHAPTER 4. HYDROGEOLOGY AND GROUNDWATER RESOURCES 4.1 Geology 4.1.1 Tectonics and Faulting Plate tectonics and related faulting and folding have played an important role in the origin of the Mekong River, the development of its course and the formation of the sedimentary basins now containing the unconsolidated sediments of the delta. The dominant tectonic direction is north-east south-west (NE-SW) causing steps in the bottom of the sedimentary basin which increase towards the coastline. The secondary tectonic directions are north-west south-east and north-south. The NW-SE direction is clearly dominant near the Bassac river, the Mekong river and the Vam Co rivers. The combination of the NE-SW and NW-SE directions caused several basement blocks near the mouth of the Bassac river which subsided to large depths (3,000 m below surface level in the south-east of Tra Vinh province). The N-S direction is only present in the west of the Mekong Delta. Faulting dominated the formation of the Mekong Delta, but is at present not active. The faults have influenced to some extent the thickness and continuity of the clay layers. This has proved to be important in the separation of groundwater from different aquifers. 4.2.2 Stratigraphy The stratigraphy of the Mekong delta is based on the latest results of geological studies done by Division for Geological Mapping for the South of Vietnam (DGMSVN, former Geological Division 6). The most recent study “Research geological structure and classification of N-Q stratigraphy in the Mekong Delta” produced geological maps at scale 1:500,000 based on detailed cross-sections and geological borehole data. Neogene and Quaternary deltaic sediments are grouped into formations having different geological ages. For each formation, sediments are further classified into sub- formations having different types of sedimentary origins 4.2. Hydrogeology and groundwater resources 4.2.1. The aquifer system in Mekong Delta The hydrogeology of the Mekong Delta is complex. The sedimentation regime, the sea level transgressions and regressions and active faulting during deposition of the sediments has caused large variations in aquifer and aquiclude dimensions and water quality. There are eight distinct aquifers in the delta subsurface, namely; 37 x the Holocene aquifer (qh); x the Upper Pleistocene aquifer (qp3); the Upper-middle Pleistocene aquifer (qp2-3); x the Lower Pleistocene aquifer (qp1); x the Middle Pliocene aquifer (n22); x the Lower Pliocene aquifer (n21); x the Upper Miocene aquifer (n13) and x the Upper- Middle Miocene aquifer (n12-3). Figure 22 shows a representative section through the subsurface of the delta, with the eight different aquifer systems. A more detailed description of the various aquifers and their groundwater potential is presented in Appendix 2. The aquifer system in the Mekong Delta MD has an artesian basin structure. The deepest part of the basin bottom is between the two rivers (Tien and Hau rivers) and rises in north-eastern east, northern and north-western directions. Except for the Holocene aquifer, the productivities of all aquifer varies from medium to high (from 1 to greater than 5 l/s). Each aquifer normally consists of two parts, the upper part is composed of silt, clay or silty clay, with no water bearing capacity; the lower part is composed of fine to coarse sand, gravel and pebble, with a medium to high water bearing capacity. In the deep aquifers such as n22, n21, n13, groundwater is of high temperature (from 32 to 390C). Mineralised water or hot water can be found in some places such as Long Dien, Nhon Trach, Tra Vinh, Vinh Long ... In the eastern part of the delta, groundwater is fresh and recharged by rainfall. In the western part, fresh groundwater exists in limited areas, the recharge mechanism is not well clarified. . 38 Figure 22. Cross – section III-III from the north-east to the southwest, parallel along the coast, approximately 15 km from the delta coastline 39 4.2.2. Ground water Quality In the Mekong Delta, groundwater quality issues are mainly related to the salinity and only occasionally related to other components, like heavy metals. Chemical components to determine the salinity concentration are the Total Dissolved Solids (TDS) and, in case no TDS data is available, chloride (Cl). These components have been monitored over the whole region for every geological layer for more than 20 years. Additional data is retrieved from Vertical Electrical Sounding (VES) measurements and interpretations, and showing the salt-fresh groundwater interface at depth. This is correlated with borehole logs. Hydrochemistry of Mekong Delta groundwater The hydrochemistry of the delta groundwater is very complex due to several sea level transgression and regression periods in the past and human influence more recently. In the western and northern delta groundwater is predominately fresh. In coastal areas groundwater is generally saline. In other areas fresh and saline ground water tend to mix in horizontal as well as vertical directions. Below the characteristic hydro-chemical features for the different stratigraphical layers in 4 regions (see Figure 23) are described. Figure 23. Hydrogeological - Groundwater zoning in the Mekong Delta 40 Dong Thap Muoi or Plain of Reeds (Between Vam Co Dong and Mekong Rivers): There is a considerable exchange of fresh and salt ground water between layers in this area. Saline water of the type Cl-Na is found in the eastern and western part and increases from the top downwards. An enrichment of Ca in groundwater is encountered resulting from salinization. The fresh groundwater of the type HCO3- Na-Mg is slightly enriched by Na, which is typical for desalinization. In the heart of this area there is a large reserve of fresh groundwater for water supply. However, the groundwater of the deep aquifers (n22 and n21) contains considerable arsenic concentrations, from 24.9 to 35.9Pg/l and up to 56.8Pg/l, detected in My Tho and Tan An Town; Between Mekong and Bassac River: In the north-western part the whole profile is saline while in the south-eastern part the whole profile is fresh of the type HCO3-Na The fresh groundwater shows enrichment of Na, which is due to desalinization by rainwater. The saline groundwater reveals enrichment of Ca, which is the result of saline water intrusion of seawater. In the north-western part the fresh water is of the type HCO3-Ca-Mg. There is probably a considerable amount of exchange of saline and fresh water between the different layers; Long Xuyen Quadrangle: Brackish groundwater is common and occupies almost the whole profile. The chloride content of the brackish water is lower compared to other regions. There is enrichment of Ca. Fresh water is found only in the north-east where the land surface is relatively high; Ca Mau Peninsula: Fresh water in the Pleistocene and Pliocene aquifers are of the types HCO3-Cl-Na and HCO3-Cl-Na-Mg and show signs of enrichment by Na, which is the result of desalinization by rain water in the past. The TDS of the fresh groundwater is usually higher than that of water in eastern delta. Locally some traces of nitrite are found. Saline water in the Holocene in the north is the result of saline water intrusion in the past. In the area of Ca Mau town the brackish water is fossil, because it shows no enrichment of Ca. In general the groundwater of the Holocene aquifer in the western delta has a relative high SO4 content and low pH value due to the presence of acid-sulphate soils in the area. Furthermore, Pleistocene and Pliocene aquifers often show signs of pollution, which decreases with depth. According to a recent arsenic study, arsenic concentration in groundwater of most aquifers is below the limit of 50Pg/l. Some noticeable arsenic concentration were detected locally from the n22 and n21 aquifers at great depth (200-300m). However, this study does not exclude the occurrence of arsenic pollution in the Mekong Delta,as the number of samples for determining arsenic is not considered representative. Further studies are required. Saline groundwater distribution in the MD In general, the origin of saline groundwater in all stratigraphical layers in the subsurface of the delta is related to the marine environment during sedimentation and the frequent flooding of the delta by seawater during interglacial (transgression) periods. Flushing of seawater by infiltration of fresh water during glacial periods 41 with lower sea levels was never completely, leaving large areas filled with brackish and saline ground water. Salinization and desalinization processes, responsible for the saline water in the delta subsurface, takes place even today. Combining all the information available on salinity and processes contributing to salinization and desalinization of groundwater, the following conclusions are derived regarding groundwater quality: x Fresh water in all layers is found in mountainous areas where recharge of groundwater is obvious from groundwater level maps; x Brackish water is found in layers near and between the major rivers. During the most recent transgression, the rivers facilitated intrusion of the seawater having free access to the aquifer below the sandy riverbed. So far, the influence of infiltrating river water on the groundwater quality has been limited, due to an actual recharge that is much less than the potential recharge. Increased pumping near and between rivers could cause further salinization or desalinization of the upper layers. This means, salinization by upstream migration of seawater along rivers in the coastal areas and desalinization by fresh river water infiltration upstream; x In areas along the coast where old dune deposits are found at the surface, complicated patterns of fresh and saline ground water are found, like in the Tra Vinh region. Here, infiltration of rainfall takes place in accordance with the higher water potentials in the dune ridges; x In the Plain of Reeds, north of the Mekong River, brackish to saline ground water predominates the Pleistocene layers with low groundwater levels. The deeper Pliocene layers with higher groundwater levels contain fresh water, indicating that there is a hydraulic isolation between the two formations. Studies show that this fresh water originates from a recharge area 170m higher than the Pleistocene one, which should correspond with the north- western extension of the delta and outcrops of the formation in Cambodia. These layers were already desalinized during the early Pleistocene period; x In the Ca Mau Peninsula, south of the Hau River, the deeper Pliocene layers show isolated areas with fresh groundwater in between generally brackish to saline groundwater. Around Ca Mau and Soc Trang this fresh water is exploited for drinking water. More geological and geophysical investigations could reveal more isolated fresh water zones; x The water quality of shallow wells in the southern part of the delta is often fresh, most probably due to surface water infiltration. 42 x In the west and north-west of the delta; isolated rock outcrops occur (see Chapter 1). Aquifers and aquicludes are thinning out towards the rocky outcrops, which facilitates intrusion of fresh and saltwater from the surface and mixing of water from different aquifers. At micro-scale, around these outcrops, the hydrochemistry is even more complex. Summarizing, the salt and fresh water in the MD is subject to many natural and artificial processes that alter the salt concentration continuously and consequently the salt and fresh water distribution is complex and heterogeneous. 4.2.3. Groundwater reserves Results of groundwater reserve calculations by the Division for Water Resources Planning and Investigation for the South of Vietnam provide reserves assessments as given in Table 14. Total groundwater reserves in the Mekong Delta are in the order of 26,754,764 m3/day. Of which, 4,045,095 m3/day is dynamic groundwater reserves, and 22,709,669 m3/day is static reserves. Table 12. Results of calculation of static reserves (m3/day) Aquifer Gravity reserves Elastic reserves Sum (static reserves) qp3 1,066,570 188,547 1,185,117 qp2-3 5,122,630 401,126 5,523,756 qp1 4,288,296 576,991 4,865,288 n22 3,909,452 81,596 3,991,048 n21 4,651,992 19,015 4,671,007 n13 2,461,638 11,816 2,473,454 Sum 21,500,578 1,209,991 22,709,669 Table 13. Results of calculation of dynamic reserves (m3/day) Dynamic reserves In aquifers Sum Inflow from rainfall to aquifers qp3,qp2-3, n22 3,860,850 Natural inflow into aquifers at boundary qp3,qp2-3,qp1, n22 101,095 Inflow from rivers to aquifers qh, qp3 83,150 Sum 4,045,095 43 4.2.4. Present groundwater utilization According to survey data of Hydrogeological Sub-division 806 in 2007, there are an estimated 465,230 groundwater abstraction wells, and abstraction with a total amount of 1,229,031 m3/day (Table 16). This concerns mostly shallow dug wells that exploit only the upper Holocene and Pleistocene aquifers. Water exploitation in the main deeper aquifers is as follows: In aquifer qp3 and qp2-3: 588 wells, occupied 59,6%. In aquifer qp1 and n22: 164 wells, occupied 16,6%. In aquifer n21: and 195 wells, occupied 20,0%. In aquifer n13: and 38 wells, occupied 3,8%. In general, groundwater in the Mekong Delta is used to serve domestic, industrial purposes and is partly used for aquaculture purposes. 44 Table 14: Groundwater utilization in the Mekong Delta Urban supply Large rural supply Small rural supply No Province Wells Total amoun (m3/day) Number Total amoun (m3/day) Aquifer Depth (m) Number Total amoun (m3/day) Aquifer Depth (m) Number Total amoun (m3/day) Aquifer Depth (m) 1 Trà Vinh 88.923 147.301 8 32.210 qp2-3 100-134 102 8.515 - 98-134 88.813 106.576 - 98-134 2 Sóc Trăng 50.111 100.090 12 31.903 - - 109 8.199 qp2-3 - 49.990 59.988 qp2-3 - 3 %ҥc Liêu 88.741 63.681 1 15.165 qp2-3 qp1 n22 106-138 152-168 245 65 8.612 qp2-3 qp1 80-142 146-154 88.675 39.904 - - 4 Cà Mau 67.185 134.657 13 46.326 qp2-3 qp1 n22 90-111 206-260 132 7.883 qp2-3 qp1 n22 - - - 67.040 80.448 qp2-3 5 &ҫn Thѫ 22.643 64.638 - - - - 396 37.942 qp2-3 82-114 22.247 26.696 - - 6 9ƭnh Long 6.258 8.705 - - - - 4 1.200 - - 6.254 7.505 - 7 +ұu Giang 29.656 50.045 - - - - 225 14.728 qp2-3 62-118 29.431 35.317 qp2-3 - 8 TiӅn Giang 1.029 37.695 8 21.148 n21 303-307 78 15.415 n22 n21 n13 253-260 253-347 342-464 943 1.132 - - 9 Ĉӗng Tháp 3.213 44.723 8 17.760 - - 165 23.315 qp1 n22 n21 - - - 3.040 3.648 - - 10 An Giang 4.971 71.917 2 44.930 n22 245-300 6 770 qp2-3 n22 - - 4.963 26.217 qp2-3 22-80 11 BӃn Tre 2.063 6.683 17 3.342 - - 20 910 - - 2.026 2.431 - - 12 Kiên Giang 96.950 328.970 1 6.240 - - 49 19.464 - - 96.900 303.266 - - 13 Long An 3.487 169.956 27 35.953 - - 1.079 78.147 - - 2.381 55.856 - - Total amount 465.230 1.229.061 97 254.977 2.430 225.100 465.703 748.984 45 CHAPTER 5. WATER DEMAND AND WATER BALANCE 5.1 METHODOLOGY Water in the Mekong Delta is used for a wide variety of purposes: irrigation for agricultural crops, fresh water for fresh water aquaculture, animal husbandry, poultry, supply of domestic use and industry. Total water demand also includes surface water evaporation and bare land evapotranspiration. Water demand for crop is calculated on the basis of area and growing season for the crops. Water needs of other sectors such as fisheries, livestock, poultry, cattle and people's livelihood and industry will define the total water demand in the Mekong delta region. The calculated water needs were made for the 120 irrigation subdivisions. The results have been calculated using documentation on land use in 2005, the progress of sowing and harvesting crops and the Mekong Delta provinces, also salt water and fresh water supply figures in 2005 and on the basis of earlier water balance calculations done by SIWRP in 2004. The water demand was calculated for categories: Paddy, crops, forestry, freshwater fisheries, domestic use, loss due to evaporation on the canal and bare land. Where: - The demand for water for rice, annual crops, perennial crops is calculated using CROPWAT-FAO model. - The demand for water for livestock by three categories: cattle: 20 liters / head /day; pig: 40 liters / head /day; chickens, ducks: 2 liters / head /day - The demand for water for forest: 0.15 l/s/ha - Water loss on bare land: 0,05 l/s/ha - Water for freshwater fisheries: surface water evaporation estimated 5 mm/day. - Water for population: urban 65 l/person/day; rural 45 l/person/day (according to Vietnamese standard TCXDVN33:2006). - Water for industry: 45 m3/ha/day (TCXDVN33:2006). - Time step for calculations: assuming 10 days. 5.2 RURAL, URBAN AND INDUSTRIAL WATER DEMAND Water demand is calculated for all regions including areas with fresh water and salt-affected areas. This is performed for the 120 sub-irrigation areas presented in Figure below. Table 23 and table 24 summarize the total water demand flow and volume of each month the whole Mekong Delta. The calculated data show the water demand for agriculture is the largest, accounting for 68% of the total water demand. The months of January and February require the largest amounts of water use due to the lack of rain. 46 Table 15: Monthly water flow demand Monthly water demand (m3/s) Month Rice Crops Poultry Fishery Forestry Population & Industry Total I 764.56 129.71 2.51 31.90 52.95 18.93 1000.56 II 834.95 161.06 2.51 31.90 35.62 18.93 1084.97 III 522.17 174.77 2.51 31.90 19.53 18.93 769.80 IV 450.29 162.39 2.51 31.90 19.46 18.93 685.48 V 440.44 63.76 2.51 21.27 26.32 18.93 573.23 VI 396.33 24.71 2.51 - 72.01 18.93 514.49 VII 225.11 31.08 2.51 - 86.17 18.93 363.79 VIII 86.21 20.24 2.51 - 82.16 18.93 210.05 IX 33.83 4.32 2.51 - 53.04 18.93 112.63 X 16.04 13.66 2.51 - 63.62 18.93 114.75 XI 110.27 56.11 2.51 10.20 63.46 18.93 261.49 XII 372.64 108.74 2.51 27.05 56.86 18.93 586.73 Average 354.40 79.21 2.51 15.51 52.60 18.93 523.16 Table 16: Monthly water volume demand Monthly water demand (million m3) Month Rice Crops Poultry Fishery Forestry Population & Industry Total I 2,048 347 7 85 142 51 2,680 II 2,020 390 6 77 86 46 2,625 III 1,399 468 7 85 52 51 2,062 IV 1,167 421 7 83 50 49 1,777 V 1,180 171 7 57 70 51 1,535 VI 1,027 64 7 - 187 49 1,334 VII 603 83 7 - 231 51 974 VIII 231 54 7 - 220 51 563 IX 88 11 7 - 137 49 292 X 43 37 7 - 170 51 307 XI 286 145 7 26 164 49 678 XII 998 291 7 72 152 51 1.572 Average 11,089 2,483 79 487 1,663 597 16,398 Ratio 68% 15% 0% 3% 10% 4% 100% 47 Figure 24: Map of 120 sub-irrigation areas. 48 5.3 WATER FOR NAVIGATION The minimum depth of water that is required for navigation in the dry season (LAD, m) is used as an indicator for the planning needs of the waterways. For the main channels to ensure access for ships from 200-300 DWT, requires a LAD from 2.5 to 3.0 m. For the main rivers to ensure access for ships from 1000-5000 DWT, requires a LAD from 4.0 to 7.0 m. Table 17: Water requirements for navigation/waterway transportation Project Route Distance (km) Ship size (DWT) LAD (m) TP.HCM-Kiên Lѭѫng Te canal – Dem market -Vam Co Dong... Rach Gia – Ha Tien canal 297,8 300 3,0 TP.HCM-Kien Luong- Ba Hon Ong Lon- Cay Kho canal - Sӓi canal - Rach Gia – Ha Tien canal - Ba Hon canal 320,8 300 3,0 TP.HCM-Ca Mau-Nam Can Te canal –Ong Lon canal - Can Giuoc river - Ganh Hao river –Bay Hap canal- Tac Nam Can canal 393,3 300 3,0 Moc Hoa – Ha Tien Upward Vam Co river - Cai Bat canal- Hong Ngu canal... Ha Tien 183,5 200 2,5 Tan Chau-Hong Ngu- Cua Tieu VN-CPC boundary-Tan Chau-Dong Thap-Vinh Long-Ben Tre-Cua Tieu 260,4 3.000 6,0 Rach Gia-Ca Mau- Ong Doc river TX. Rach Gia-Ca Mau- Ong Doc river 182,6 1.000 4,0 Tien river From VN-CPC boundary to the sea 260,4 5.000 7,0 Hau river From VN-CPC boundary to the sea 228,0 5.000 7,0 Ham Luong river Tien river -Ham Luong river 86,0 1.000 4,0 Quan Lo - Phung Hiep Phung Hiep-Hau Giang-Quan Lo - Phung Hiep canal- Ca Mau 104,5 300 3,0 Go Dau-Vam Co Dong river - Xoai Rap river Ben Soi -Vam Co Dong river-Go Dau- Duc Hue - Ben Luc-Xoai Rap 189,0 3.000 6,0 Moc Hoa-Xoai Rap river Moc Hoa-Vam Co Tay river-Tân An-Xoai Rap river 163,5 1.000 4,0 5.4 PRESENT WATER BALANCE Based on the amount of surface water available and water demand for the various user functions as calculated above, a preliminary balance for the entire Mekong Delta region is based on the principle of: total water supply volume, with subtracted the amount of water that is used each month. Surface water supply equals the total water volume of the two stations Tan Chau and Chau Doc with the frequency of 80%, 50% and 20%. For water consumption the situation in 2005 is used. 49 Table 18: Present water balance Month Sub-basin Level of Frequency Unit 1 2 3 4 5 6 7 8 9 10 11 12 Annual Supply – 80% Mil.m3 17329 9577 6918 5989 8056 16257 32953 55687 62951 63397 42290 28527 366162 Supply – 50% Mil.m3 20536 11844 8390 6924 10328 22699 42177 62745 68040 66297 47917 33353 405207 Supply – 20% Mil.m3 24338 14648 10174 8006 13243 31693 53984 70697 73540 69331 54293 38996 448415 Demand 2005 Mil.m3 2680 2625 2062 1777 1535 1334 974 563 292 307 678 1572 16398 Balance – 80% Mil.m3 14649 6952 4856 4212 6521 14923 31979 55124 62659 63090 41612 26955 349764 Balance – 50% Mil.m3 17856 9219 6328 5147 8793 21365 41203 62182 67748 65990 47239 31781 388809 Mekong Delta Balance – 20% Mil.m3 21658 12023 8112 6229 11708 30359 53010 70134 73248 69024 53615 37424 432017 Source Balance = (Supply-Demand). Supply calculated in section 3.3.2. Demand = Calculated in section 3.4.1 50 According to figures calculated in the above table, the volume of water resources for the whole of the Mekong Delta water is sufficient, so there is excess water in each month. In the dry months from January to May the water demand forms 20-30% of the total flow from upstream, while in the rainy months the total water demand accounts for only 0-6% of the total water volume. In an average year water demand accounts for about 5% of the total. It can be concluded that on the basis of these figures, most water still flows out to the sea. Actually, water shortages do occur for sub-regions due to lack of water storage facilities and salinity control structures. In addition water for environmental purposes – ecosystems, and water for controlling salinity intrusions should be considered as well. Comparison of water volumes through Tan Chau (Tien river) and Chau Doc (Hau river) with the total of the two stations at My Thuan and Can Tho in the dry season shows that the Plain of Reeds region and the use of freshwater in the Long Xuyen Quandrant covers about 20-25 % of the total freshwater volume of Mekong Delta region. Approximately 75-80% of the freshwater flows through the Mekong Delta, to Can Tho and My Thuan, and a large amount of this flows to the sea. The Ca Mau Peninsula and the coastal zone of the Plain of Reeds and South Muang Thit can use less fresh water due to saltwater intrusion in the estuaries. 51 CHAPTER 6. ISSUES TO BE SOLVED In the field of groundwater, many subjects are worthwhile to be studied further. In brief, these subjects are discussed below: x Mobilisation of arsenic from sediments in the Mekong Delta. From the literature study of arsenic in the Mekong Delta sediments, it becomes clear that the delta has a high potential for the development of arsenic. A study to this extent should focus on the presence of arsenic in several soils, the simulation of different environments at small scale level in a laboratory to study the mobilisation of arsenic from sediments and following this perhaps a full scale experiment in the field. x Isotope analysis (C-14 dating) so far has mainly concentrated on the absolute age of groundwater in the top aquifers of the Mekong Delta and with this its general flow patterns. More isotopes can be tested providing more specific information about recharge from rainwater, river water, etc. A project could be set up to determine the origin of water in selected areas. x Groundwater chemistry. This subject has not been studied in detail. Subjects to be studied in this context are the cation exchange capacities of the clays, the ion balances in the clays and the relation with the results of the isotope analysis. x Groundwater reserve assessments. Present procedures for assessment of groundwater reserves in Vietnam are not acceptable in terms of sustainability. They are not considered feasible and realistic. Modern literature is now available on groundwater reserve calculations. This literature can be studied by a panel of specialists and new guidelines for reserve calculations can be formulated. x In certain areas the groundwater levels are dropping. A study on the application of suitable artificial recharge methods could be useful for sustainable groundwater development in the delta. x Groundwater demand studies. Based on the population per province, growth rates, present number of wells and availability of other water resources, an estimate of the future water demand from groundwater is necessary. Based on the location of the inhabitants, wells were planned and designed in outline and the total number of required wells may be determined. Based on these 52 prognoses, potential locations of wells and well fields should be investigated by modeling. The result could form the basis for the water supply master planning for the entire Mekong Delta. 53 CHAPTER 7. CONCLUSIVE REMARKS The Mekong Delta has abundant water resources, but these are mainly concentrated in the six-month rainy season. The amount of water resources in the Mekong Delta is affected by the upstream flow to the Delta, rainfall, regulation of Great Lake (Ton Le Sap), tidal regimes, wind and sea level rise. These factors must be considered in assessing water resources in the Mekong Delta. The quality of the water resources in the delta is affected by floods, salinity intrusions, acid drainage water, alkaline soils, agro-chemicals, industrialization waste discharges, and ships navigation. There is flooding over an area of about 1.4- 1.9 million ha in the upper area of the delta. Salinity intrusion occurs over an area of about 1.2-1.6 million ha in the coastal areas with saline density of over 4g/l. There are widespread acid-sulfate soils and the spread of acid soil drainage water occurs over an area of about 1.0 million ha in the lower delta plains. These processes may create, locally a shortage of fresh water for production and domestic uses over an area of about 2.1 million ha in areas far from rivers, and close to the coast. The situation will become more critical as a consequence of climate change and its impact on the upstream flow regimes, different rainfall and weather patterns in the Mekong Delta itself and threats from sea level rise. Water shortages occur annually in Ca Mau Peninsula due to lack of diversion canals and water control structures. The present alternation of paddy fields and shrimp ponds constitute a difficult to regulate water works system. The Plain of Reeds is the most critical area for flood control due to large overland flood flow. The flood control plan has to deal with trans-boundary issues. Future agricultural and other economic sectoral development plans for the Mekong delta should be formulated with the additional objective to save water. Because climate change effects, sea level rise, upstream agricultural development, upstream water diversions and unfavourable upstream reservoir operation will cause more acute water shortages and more salinity intrusion for the Mekong delta. A quantitative water resources assessment is feasible on the basis of currently available data, viz.: x Long-term meteo-hydrological data and rainfall data are available from the Southern Meteo-hydrological Station. x Salinity intrusion data are observed at coastal stations. x Ten year water quality data are available at SIWRP. x Upstream meteo-hydrological data are available in the database of the Mekong River Commission (also available at SIWRP). x Meteo-hydrological stations are distributed throughout the Mekong delta. Time series data are long and good quality. x There are few flow monitoring stations (only 5 stations on the main Mekong courses at Tan Chau, Chau Doc, Vam Nao, My Thuan and Can Tho). For about 20 54 years time series data are available. In addition, SIWRP have carried out 2-week flow monitoring at all Mekong river mouths to assess dry flow distribution in main streams. x Overland flood flow in the upper parts of the Plain of Reeds and the Long Xuyen Quadrant is observed in recent years. x Flood and dry flow in canals can be simulated using existing calibrated hydraulic model VRSAP by SIWRP. 55 REFERENCES [1] MRCs-DSF (2004), WUP-A Decision support framework Version 1.0.0; Modeler’s grant access for Dang Thanh Lam of SIWRP. [2] SIWRP (2002), Study on water balance for MD for sustainable development strategy of the Mekong water resource, Main report. [3] SIWRP (2005), Integrated water resources planning for MD. [4] SIWRP (2008), Annual report on water quality monitoring for the lower Mekong basin. [5] To Van Truong (2005), Research study on Flood analysis, flood forecasting and flood control for ‘living with flood’ on demand in the MD. [6] (2010), ‘Climate change and sea level rise scenarios for Vietnam’ issued in 2009 by MONRE in PDF format. [7] State government (2006), Decision No.84/2006/QĈ-TTG. [8] GROUND WATER STUDY MEKONG DELTA, FINAL REPORT 2001. [9] NGUYÊN HUY DUNG, Research geological structure and classification of N-Q stratigraphy in MD, 2004. 56 APPENDIX 1: Water demand assessment source materials Document/ Database/ Model/ Material Author/ Owner/ User Contents/ Descriptions MRC-DSF Owner: MRC and Three line agencies VNMC, SIWRP, IMHR The MRC Decision Support Framework, or DSF, is a tool for managing and sharing observed and modelled datasets for the Mekong river basin. These data can be time series, spatial or other miscellaneous data. The datasets are contained with the DSF Knowledge Base database, which is packaged with the DSF software. All of these data can then be utilised to investigate the behaviour of the river basin and, thus, facilitate the decision- making process over how to react to future impacts on and changes to the basin. The Decision Support Framework contains a Main Interface and a series of associated tools. These applications are as follows: Main DSF Interface Impact Analysis Tools Time-Series Plotting Tool Probability Exceedence Tool Event Analysis Tool Low Flow Analysis Tool MQUAD DSF Model Interfaces DSF SWAT Interface DSF IQQM Interface DSF ISIS Interface Study on water balance for MD for sustainable development strategy of the Mekong water resource Owner: MARD Author: SIWRP The main report contains information as follows - Assessment of existing water utilization in the MD. - Computing water demand by 2010. - Computing upstream impacts on monthly flow with development of hydropower and irrigation. - Computing existing and future water balance. 57 Integrated water resources planning for MD Owner: MARD Author: SIWRP The content of main report consists of: - Analysis of characteristics of MKD meteorology, hydrogeology and water resources . - Assessment of welfare of the people, social and economical situation of the Mekong Delta to 2004. - Assessment the opportunities and demand for development of MKD, consisting of development orientation of agriculture, forestry, aquaculture, transportation, and construction. - Calculation of water demand for agriculture, aquaculture, transportation and domestic water supply at present 2005 and to 2010. - Formulation of water resources development projects. - Analysis of hydrology, hydraulic, cost estimation of construction of water resources works according to the planned development projects. - Choice of the list of water resources works for investment for the periods of 2006-2010 and 2011-2020. Research study on Flood analysis, flood forecasting and flood control for ‘living with flood’ on demand in the MD. Owner: MOST Author: Dr. To Van Truong, SIWRP The content of main report consists of: - Investigation of control flood and living with flood models; of production model in flood areas; Assessment of impacts of infrastructure development to the flow of MKD. - Analysis of basic data on topography, meteo-hydrology of MKD; - Building the methodology on recognition of flood in MKD and the methodology to control, manage and live with floods in MKD; - Calculation of parameters to recognise river floods; building the technology to recognise river floods in MKD and 58 application of the technology to recognise the level of flood peak in 2003-2004; - Upgrading the data on topography, boundary conditions and building a hydraulic model to simulate the floods in 2000, 2001 in MKD; - Building the technology to recognise floods in MKD and its application to forecast; - Building the series of maps of floods in MKD according to frequencies using GIS technology; - Researching of measures to control floods, to construct infrastructure, to develop economy, to protect environment, and to control floods for living with flood in MKD; Climate change and sea level rise scenarios for Vietnam Owner & Author: MONRE Report of MONRE on Climate change and sea level rise scenarios for Vietnam 59 Appendix 2: Description of Aquifer systems in the Mekong Delta 60 Holocene aquifer (qh) The Holocene aquifer occurs at the surface of almost the whole delta with an area of 40,000km2 (see Figure 33), and consists of the following sediments: - Lower – middle Holocene sediments (qh1-2), consisting of clayed silt, fine sand and organic matters. - Alluvial and coastal sediments (qh2-3), consisting of silt, clay, fine to medium sand to form sandy dunes ( indicative of ancient sea shores). - Sediments (qh3), consisting of clayed silt and fine sand, accumulated in river valley. Figure 25. Hydrogeological map of the Holocene aquifer. Sediments formulated in the period of sea regression (qh2-3) are characterized by sand dunes bearing fresh groundwater. These sand dunes are located in Mo Cay, Ba Tri, Tra Cu, Long Toan districts of Tra Vinh province. Its lithology consists of fine to medium sand, having a thickness of 5 to 10m. 61 The boreholes placed in this aquifer are mainly shallow boreholes, with depths from several meters to 30 m, having yields from 0.1 to 2.0 l/s and groundwater levels from 0.5 to 3.0m. Fresh groundwater (TDS<1 g/l) is distributed in an area of 5,816 km2, between the Tien and Hau rivers and in Tra Vinh, Tien Giang provinces (see Figure 33). Upper Pleistocene aquifer (qp3) The Upper Pleistocene aquifer is distributed widely throughout the whole Mekong Delta, and is mainly overlain by the Holocene aquifer. It only surfaces in the northeastern part of the delta, and consists of the formations Cu Chi and Moc Hoa, having alluvial and marine-alluvial origins. This is a weakly confined aquifer. The aquifer can be divided into two parts. The top part is an aquitard layer, consisting of silt, clay or silty clay, having a depth to the top of 10.1 to 37.3m and a thickness from 12.1 to 20.6m. The lower part is an aquifer, consisting of fine to coarse sand, having a depth to the top from 22.2 to 57.8m and a thickness from 9.4 to 22.4m. Fresh groundwater is distributed in an area of 8,541 km2, located in Vinh Hung (Long An province); My Tho (Tien Giang province); Tieu Can, Cau Ngang (Tra Vinh province) (see Figure 26). In summary, the upper Pleistocene aquifer qp3 is found in wide areas, but it has complicated hydrogeological conditions. Groundwater is fresh in some areas. It is suitable to abstract water for domestic use. 62 Figure 26. Hydrogeological map of Upper Pleistocene aquifer Upper-middle Pleistocene aquifer (qp2-3) Upper middle aquifer (qp2-3) distributes widely in the whole MD, is mainly overlain by upper Peistocene aquifer, just outcrops in some place such as An Giang province. Its lithology is composed of sediments of alluvial, marine alluvial and marinal origines, consisting of pebble, gravel, sand, silt, clayey silt and clay. The depth to the top and the thickness of the aquifer varies in space and on cross-sections. It is weakly confined aquifer. The aquifer can be divided into two parts. The top part is an aquitard layer, consisting of silt, clay or silty clay, having the depth to the top of 31.6 to 81.7m and the thickness from 3.6 to 13.5m. The lower part is an aquifer, consisting of fine to coarse sand, having the depth to the top from 44.2 to 85.5m and the thickness from 17.3 to 56.2m. 63 Figure 27. Hydrogeological map of upper- middle Pleistocene aquifer There are several hydraulic windows where groundwater in this aquifer has direct hydraulic relationship with groundwater in qp3 aquifer overlain. Fresh groundwater distributes on the areas of 21,798 km2, located in Tra Vinh, Bac Lieu and Ca Mau provinces (see figure 27). Fresh groundwater is of good quality and large amount. Lower Pleistocene aquifer (qp1) Aquifer qp1 distributes on the whole MD. It is a confined aquifer. Sediments in the lower Pleistocene aquifer have mainly alluvial origin, although that in Ca Mau province has marine origin. The aquifer is a confined aquifer and can be divided into two parts. The top part is an aquitard layer, consisting of clayey silt, clay or silty clay, having the average depth to the top from 65.8 to 141.6 m and the average thickness from 7.9 to 15.9m. The lower part is an aquifer, consisting of fine to coarse sand, having the average 64 depth to the top from 73.7 to 157.6 m and the average thickness from 14.2 to 43.9m. The depth to the top of the aquifer inclines in direction from north and west to the southern east. Figure 28. Hydrogeological map of lower Pleistocene aquifer There are several hydraulic windows where the groundwater in this aquifer has direct hydraulic relationship with groundwater in qp2-3 aquifer . Fresh groundwater distributes on the areas of 17,918 km2, located in Hochiminh city and Can Tho, Kien Giang, Bac Lieu, Ca Mau provinces (see Figure 28). This groundwater is now being abstracted for water supply in such city and province. Middle Pliocene aquifer (n22) Aquifer n22 distributes widely on the whole MD, it is composed of sediments having alluvial, marine-alluvial and marine origins. 65 The aquifer is a confined aquifer and can be divided into two parts. The top part is an aquitard layer, consisting of clayey silt, silty clay, having the average depth to the top from 79.7 to 202.7 m and the average thickness from 4.6 to 20.6m. The lower part is an aquifer, consisting of fine to coarse sand, having the average depth to the top from 84.3 to 223.1 m and the average thickness from 29.9 to 57.1m. Figure 29. Hydrogeological map of middle Pliocene aquifer Fresh groundwater distributes in two big areas with a total of 19,000 km2, one is located in the northern part of Tien river, and the other is located in Can Tho, Bac Lieu and Ca Mau provinces (see figure 29). Lower Pliocene (n21) Aquifer n21 is not outcropped on the surface, and is composed of sediments of alluvial, marine-alluvial origins. It is highly-confined aquifer and can be divided into two part. The top part is an aquitard layer, consisting of clayey silt, silty clay, having the average depth to the 66 top from 196.5 to 279.2 m and the average thickness from 11.8 to 20.5 m. The lower part is an aquifer, consisting of fine to coarse sand, having the average depth to the top from 209.4 to 296.5 m and the average thickness from 40.9 to 45.5m Figure 30. Hydrogeological map of lower Pliocene aquifer Fresh groundwater distributes in areas of 16,198 km2, located in Dong Thap, Long An, Hochiminh, Can Tho, Bac Lieu and Ca Mau (see figure 30). Upper Miocene aquifer (n13) Aquifer n13 has not yet been studied in detail. There are very few boreholes drilled through this aquifer. It is very highly-confined aquifer, not outcropped on the surface. The upper part consists of silt, weathered silty clay, is at a depth from 257.9 to 364.1m, and a thickness varying from 11.7 ÷ 24.1m; The lower part consists of compacted fine to coarse sand, is at 260.6 to 377.9m, having a thickness from 40.1 to 71.5m. 67 Figure 31. Hydrogeological map of upper Miocene aquifer Fresh groundwater distribute on areas of 7,612km2, located in Hochiminh, Dong Thap provinces (see Figure 31). Upper- middle Miocene aquifer (n12-3) Aquifer n12-3 is deepest confined aquifer in MD, overlain on the bed-rocks. There are not many boreholes drilled through this aquifer (see figure 32). At the two boreholes HG1 and CL1, the depth to the top and bottom of the aquifer and the lithological components are as following: At boreholes HG1: From 508 to 602m, is an aquitard layer, consisting of clay and sandy silt. From 602 to 798m is an aquifer layer, consisting of compacted fine to medium sand At boreholes CL1: From 724,6 to 914m, is an aquitard layer, consisting of clay and silty clay. From 914 to 1000m, an aquifer layer, consisting of pebble and gravel. 68 Figure 32. Hydrogeological map of upper – middle Miocene aquifer

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