GROUNDWATER ARTIFICIAL RECHARGE AND SALINIZATION PREVENTION AS A DROUGHT-FIGHTING MEASURE IN CENTRAL COASTAL AREAS OF VIETNAM.

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1 GROUNDWATER ARTIFICIAL RECHARGE AND SALINIZATION PREVENTION AS A DROUGHT-FIGHTING MEASURE IN CENTRAL COASTAL AREAS OF VIETNAM. 1. GENERAL Prof. Dr. Maria-Theresia Schafmeister Ernst Moritz Arndt University Greifswald, Germany. Dr. Nguyen Van Hoang Institute of Geology, National Center for Natural Sciences and Technology of Vietnam. Vietnam has a coastline of more than 2200 km. The central coastal areas of Vietnam are of high economic importance because they are plain areas where the main provincial political and economical organizations are located and through which the national highways and railways pass. Vietnam in general and its coastal areas in particular have a high rainfall. However, the coastal areas of Vietnam are the areas of most natural water-related hazards: floods during rainy seasons, and droughts during dry seasons. During the last 5 years the central Vietnam coastal areas experienced several sequential severe floods and droughts. Several large-scale projects related to the coastal areas have been carried in order to develop possible ways of fighting water shortages. One of the projects is the National Research Project: "Study of Methods for Diminishing Drought Hazards in the Central Coastal Plain Provinces of Vietnam" supported by the Government of Vietnam since 1999 and currently under completion. Groundwater artificial recharge and salinization elimination and prevention are some possible and effective methods since the aquifers in the project sites are mostly unconfined with good permeability and high infiltration capacity and the rainfall is also high. These methods can significantly increase the groundwater natural resources available for both domestic water supply and drought-fighting purposes. This is especially useful for rural areas, where the daily domestic water demand is not as high as in urban areas since the population is scattered. Besides, application of these methods would be a plus point for the Rural Socioeconomic Development Plan as they would improve the standard of living and solve prevalent social problems existing between the urban and rural areas, such as constructing water supply facilities and improving sanitary conditions in the rural area. In the recent plan, the government has pointed out that 58% of the population suffers from safe water shortage and has set up the aim to attain a service rate of 80% by the year 2005 for use of clean water and 85% by the year of 2010 (NRWSS: No.237/1998/QD-TTg). The possibility of recharging groundwater, salinization reduction and methods of maximal possible groundwater extraction for small-scale water supply plants have been considered and demonstrated through the calculation, numerical modeling and dynamic programming in this work. 2. GROUNDWATER SALINIZATION RESEARCH IN VIETNAM Although Vietnam has long coastal areas, surface water and groundwater salinization has not attracted due attention. This may be illustrated by the limited number of the related publications in this field. Usually, in groundwater resources investigations, the salinization problem is expressed only by the subdividing the area of concern into two regions of different mineralization levels: greater than 1 g/l and less than 1 g/l and by the recommendation that attention must be paid to possible groundwater salinization if the groundwater is to be developed in the area.

2 Nguyen Van Hoang (1987), using the relationship between the seawater intrusion length and the groundwater discharge into the sea, estimated the so-called dynamic groundwater resources of the Pleistocene confined aquifer in the Bacbo plain (North of Vietnam) in his trial work report. Dang Huu On in 1996 carried out the field testing for determination of effective porosity and therefore the real groundwater flow in the Ba Ria-Vung Tau groundwater pumping field for sea water intrusion prediction. In 1997 he evaluated the fresh water potential in the Da Nang-Hoi An coastal area. Similar work has been carried out by Nguyen Truong Giang (1992) for the future groundwater pumping field in Gio Linh-Quang Tri. More general fresh water potential in the Central Coastal area of Vietnam has been evaluated by Nguyen Van Dan, Vo Cong Nghiep, Dang Huu On and Nguyen Truong Giang in the booklets Groundwater potential in Northern and Southern coastal Plain of Vietnam (1996, 1998). In those works the 1g/l of total dissolved solids has been used as the limit between salt-water and freshwater. Ngo Ngoc Cat and Doan Van Canh (1998) determined the time when the saltwater would reach the pumping wells in Co Dao island, where the total pumpage is planned to be 1600 m 3 /day. Ngo Ngoc Cat and his co-workers (Ngo Ngoc Cat et al., 1998) described the sea-water intrusion situation and possibility in the Northern coastal area from Hai Phong city to Ninh Binh province. It can be seen that the sea-water intrusion problems have been studied using very simple approaches in Vietnam. Only few field tests have been carried out in this topic. Furthermore, the advantages of numerical methods have not duly been utilized in solving such problems. Only simple analytical formulas have been used to evaluate crudely simplified field conditions, that would not yield accurate results of analysis and prediction. Elsewhere the seawater intrusion problems have been intensively studied by highly technical approaches and simulated by numerical models of high level of accuracy and reality conditions. A review of the related world literature shows that the variety of seawater intrusion problems has been treated in the following topics: Causes of seawater intrusion (over groundwater pumpage in coastal areas, destruction of the natural plants in coastal areas); conditions related to salinization of soils and water; Effect of tides on groundwater regime and potential in coastal areas; Geochemistry in soil and groundwater salinization process; Origin of salinized groundwater; Natural and artificial processes de-salinization of salinized aquifers; Mechanisms of interaction between fresh water and salt water; Quantitative analysis and prediction of sea water intrusion: water balance method, Chlorine balance, Isotopes, numerical simulations (development of numerical codes that give the most accurate and closest results to natural conditions); Prevention methods from salinization and aluminous processes of agricultural soil, such as groundwater level control, change of vegetation etc. Optimal coastal groundwater exploitation on the view of groundwater salinization, i.e., coastal fresh groundwater evaluation; salinization elimination and prevention for the coastal pumping fields; Coastal fresh groundwater recharge; Techniques and GIS in groundwater salinization forecast and evaluation; Groundwater origin and its age.

3 3. MODELING ARTIFICIAL GROUNDWATER RECHARGE AND ADDITIONAL SALINIZATION PREVENTION BY UNDERGROUND SLURRY WALLS a Dynamic programming to determine the optimal groundwater pumping from an unconfined aquifer Using the finite element algorithms with a backward time difference scheme to solve the partial differential equation of 2-D unconfined groundwater flow in a plane with certain boundary conditions gives a system of equations with variables of water levels at time t: { Am B} h = C + (1) where: m - column matrix of water bearing thickness (L) (m is the difference between the water elevation and aquifer's bottom elevation), h - column matrix of water level at time t (L), A, B, C - matrices resulting from application of FEM algorithms to solve the flow equation with appropriate initial and boundary conditions. Figure 1: Block-scheme of DP for unconfined acquifers

4 All the matrices depend on the mesh form and size. Besides, matrix A depends on the permeability K, matrix B depends on time step t and storage coefficient, matrix C depends on the storage coefficient, water level at moment t- t and the pumping rates. The equations in the system (1) are non-linear. One of the methods of solving the system of non-linear equations is the Picard method: first assuming the thickness of the aquifer m gives a set of linear equations; second, solving this system of linear equations for the water level of the aquifer h. Putting the newer values of m calculated from the water level h into system (1) and solving it gives newer values of water level h. This process continues until required convergence of h has been reached. At the last convergence step a system of linear equations with the only unknowns of water level h can be formed. Dynamic programming (DP) applied to estimate the optimal pumping rates with given conditioning water levels in some locations in this time can be carried out as in case of a confined aquifer. The DP applied to confined aquifers is not to be described here and the details may be referred to Richard E. Bellman and Stuart E. Dreyfus, (1962), Daniel M. Murray and Sidney J. Yakowitz (1979), and Babs A., Makinde-Odusola and Miguel A. Marino (1989). The block-scheme of DP in unconfined aquifers can be described as in Figure 1. b Modeled area characteristics The area to be considered in this section is the coastal Song Luy (Luy river) delta in Binh Thuan province (Southern Nam Trung Bo region of Vietnam) (Figure 2). Lai Chau An Giang Ha Noi Binh Duong Tay Ninh Long An Dong Thap Lao Cai Son La Nghe An Binh Phuoc Dac Lac B¾c Ninh Ha Tinh Thanh Hoa Quang Binh Kon Tum Gia Lai Cao Bang Ninh Binh Lang Son Nam Dinh Quang Ninh Hung Yen Thai Binh Quang Tri Hue Da Nang Quang Nam Quang Ngai Binh Dinh Phu Yen Dong Nai Ba Ria-Vung Tau P A C I F I C O C E A N Khanh Hoa Lam Dong Ninh Thuan Binh Thuan (Study area) Ca Mau Ho Chi Minh Figure 2. Location map of study area showing most provinces of Vietnam This area is characterized by the lowest rainfall in Vietnam with annual precipitation of 700 mm, while at 60 km southwest of Song Luy the precipitation reaches 1200 mm/year.

5 Seventy percent of that low rainfall falls during three months August, September and October. The Luy River is one of two longest rivers in the Binh Thuan province and has a catchment area of 1900 km 2 and a flow of km 3 /year. Other streams are rather short and of low flows and usually dry during dry seasons. Surface water bodies are also very limited. Thus, water is a serious problem for the study area. Hydrogeological conditions of the area are also unfavorable. Most of the river's catchment area is of low to moderate elevation, the upper part of which consists of the Holocene aquifer (Q IV ). The area of this aquifer in the Luy river catchment is 70 km 2 with thickness varying from 2 m (upstream area) to 28 m (downstream area), in average 15 m. The remaining area with Holocene aquifer distribution has very low groundwater potential, that is insignificant for exploitation by well pumping fields. From the above-mentioned reasons it is clear that the fresh water development is an important issue for the area in the sense of both fresh water demand and fresh-water agricultural land development. It is possible to say that for an area of such unfavorable hydrogeological conditions, even with artificial improvement the centralized groundwater exploitation of high rate is impractical. However, considering the people's living standards, the limited water resources, the severe water shortage, the water and soil salinization and the low economic level of the area it can be stated that fresh water development is essential for the area, especially if it requires low investment measures. It is of more importance if taking the future population increase and the limited land reserve part of which is salinized and aluminous. Figure 3. Schematic aquifer diagram section and plan of modeled area

6 Firstly let us consider the required condition for the artificial fresh water recharge in the area. The total annual rainfall of 700 mm gives 70 m 3 over 100 m 2 of land, which is equivalent to 190 l/day. The water amount can be considered just enough for four persons in domestic use (if it is considered that the minimum standard is 50 l/day/inhabitant). The study area is of lower elevation compared with the surrounding area so actually in addition to the rainwater the water may flow into the area from the surrounding areas. Therefore the required fresh water potential to be used in recharge is available. The hydrogeological investigation has shown that the salt water intrusion into the Holocene aquifer has reached up to 2000 m to the area from the sea and Luy river's mouth. If we assume that this is a balanced condition, then the goal of increasing the fresh groundwater and eliminating the soil salinization in the salinized aquifer part can be reached by three methods (or at least the two first methods): 1) Setting an impervious underground dike on the sea side and Luy river's mouth's side; 2) Pumping the salinized water out from the aquifer, and 3) Increase of the effective rainwater infiltration into the aquifer and recharging ditches. Besides, other methods of decrease of evaporation from the land surface and from the groundwater surface may be carried out at the same time. The archive data have shown that the salinized water in the aquifer along the sea side and Luy river bank has a total dissolved solids amount between 1.62 g/l and 1.81 g/l. The average aquifer parameters can be selected as follows: water depth between 2.5m and 3m; total aquifer thickness 15m (water-bearing thickness between 12 m and 12.5 m), permeability 1.03 m/day. The cross section from the North to the South has a length of 8000 m, of which 2000 m in the South end is of salinized water (Figure 3). If an underground slurry wall is to be built (as shown in Figure 3) somewhere in the salinized aquifer part, the salt water may be discharged by: 1) Natural way (fresh water flow from the inside part of the aquifer makes the salinized water move out the aquifer over the slurry wall), or 2) Construction of ditch on the slurry wall's side to collect salt water and pump it out into the sea, or 3) Construction of pumping wells in the salinized water part and pump out the salinized water. The first method has the lowest investment but a very long remedial time, the second one has a higher investment and but a shorter remedial time and the third one has the most expensive and shortest remedial time. It is worthwhile to note that in the third method, the remedial wells may be used as water supply wells when the salinized water is over. c FEM settings From the previous paragraph the aquifer can be set up as follows: aquifer thickness is 15 m, initial water bearing thickness is 12 m, the permeability is 1.03 m/day, storage coefficient is 0.11, the width of the aquifer is 8000 m (0 x 8000 m) with the salinized water zone of 2000 m. The Northern and North-west boundaries of the aquifer are considered to be no-flow, natural Southern and South-Eastern boundaries are of prescribed head, but an underground slurry wall is to be designed for salt-water intrusion prevention so that they will be no-flow into the modeled area too. Mean sea water level is accepted as the aquifer initial water level for some safety level. It is to be determined the optimal pumping rates of the proposed pumping wells in a pumping field from a rectangular area 1040 m 1040 m. Preliminary evaluation has shown that the number of pumping wells should be about 8 and located in a circle of radius of 120m, the center of which is in the center of the modeled area. Finite element mesh and the location of the well are shown in Figure 4.

7 The alternatives of the model are: 1) Without a recharging ditch along all the surrounding boundaries and the effective spatial rainwater infiltration rate a) W S = 10 %; b) W S = 15 %, and 2) with recharging ditch and spatial infiltration rate: a) W S = 10 %; b) W S = 15 %. In the case of a recharging ditch, the ditch is 0.5 m-width and operating 270 days in the season of no rainfall and the ditch is filled with water two days in a week. During this time the spatial infiltration rate W S is zero: In the days when the ditch is filled with water the infiltration rate is W =0.4 m/day/(m 2 of the ditch's bottom, what is equivalent to flow rate Q = 0.2 m 3 /day/(1 m ditch's length). In average during the recharging season the infiltration rate from the ditch is W AVR = m/day/(m 2 of ditch's bottom), equivalent to inflow rate Q AVG = m 3 /day/(1m ditch's length). The spatial infiltration rate over the model area during 90 days of rainy season is: a) W S = m/day when the effective infiltration rate is equal to 10 % of rainfall, b) W S = m/day day when the effective infiltration rate is equal to 15 % of rainfall. d Results The permissible water level in the pumping wells have been selected as the middle of the aquifer over the whole simulation time. The simulation time step used in the modeling was 15 days. All the spatial and temporal boundary conditions have correspondingly been incorporated into the modeling. The results are the groundwater levels in all FEM nodes and pumping rates in all pumping nodes. Total simulation time was selected to be 5 years, a tentative time of a short-term domestic water supply of small scale centralized water plants, especially built up as a pilot example for the study area. Actual exploitation conditions during that time may be used for further needed adjustment of the relocation of pumping wells, number of pumping wells, changing conditions etc. Optimal pumping rates at half-year interval are given in Table 1 and graphically presented in Figure 5. Table 1. Results of optimal groundwater pumping estimation by DP Time (Year) Effective rainwater infiltration rate W S = 10 % Without recharging ditch With recharging ditch Without recharging ditch With recharging ditch Each well's average pumping rate (m 3 /day) Total 8 wells' pumping rate (m 3 /day) Each well's average pumping rate (m 3 /day) Total 8 wells' pumping rate (m 3 /day) Effective rainwater infiltration rate W S = 15 % Each well's average pumping rate (m 3 /day) Total 8 wells' pumping rate (m 3 /day) Each well's average pumping rate (m 3 /day) Total 8 wells' pumping rate (m 3 /day)

8 Distance (m) Node number Element number Distance (m) Pumping nodes: 99, 113, 123, 195, 207, 279, 289 & 303 All the boundary sides are: 1) No-flow if no recharging canal exists; 2) Of prescribeb flow= (m 3 /day/1m) when recharging canal exists Figure 4. Finite element mesh and boundary conditions Total pumping rate Q (m 3 /day) Effective spatial rainwater infiltration ratew S =15% Effective spatial rainwater infiltration ratew S =10% Time T (year) T with ditch with ditch no ditch no ditch Figure 5. Temporal total pumping rates determined by DP for different conditions

9 4. CONCLUDING REMARKS Many provinces of Vietnam are located along the sea coast. The fresh water resources in the area are very limited. The total rainfall, though high, is concentrated in a short time. Groundwater aquifers are usually unconfined and of limited sizes. Fresh water therefore is extremely important for the area. Measurements must be carried out in order to develop fresh water resources in the area, such as store rainwater, increase rainwater infiltration capability, recharge groundwater by filtrating ditches. The water resources increment would be useless if no measures are taken in water quality protection, especially the environmental sanitary conditions in the rural areas are not to be improved. The current rural sanitary conditions have caused the water quality degradation as many field surveys have proved. The hydrogeological conditions of the major area of the coastal areas and the rainfall conditions in the areas have shown the great potential in artificial rainwater storage and groundwater recharge. As the calculation results have shown, a land slot of only about 1 km 2 can provide from 133 m 3 /day to 300 m 3 /day with only 10 up to 15% of rainwater infiltration and short time recharge by ditch. The pilot design of pumping wells and recharge scheme would be useful to be realized and multiplied to other areas. The results of such works would be not only the technical issues of water resources development itself, but also be of a great practical socio-economic importance. From the model presented for area in Luy River's mouth, Binh Thuan province, the following points must be taken into consideration when groundwater development is carried in the coastal area. Although the aquifer is of limited size both in spatial extension and thickness, appropriate pumping field design and some artificial recharge may create a reasonable fresh water amount from 133 m 3 /day to 300 m 3 /day. Underground dike would be necessary for salt water intrusion prevention; The pumping rate would be increased 1.85 times if the spatial infiltration rate increases from 10 % to 15 % in case without recharging ditch and 1.49 times in case with recharging ditch. The recharging ditch increases the pumping rate 1.52 times in case with spatial infiltration rate W S = 10 %, and 1.32 times if W S = 15 %; Since a part of the modeled area contain salinized water the quality of pumped water must be observed for on time salt water discharging; Detailed salinity distribution in space and time, and therefore, the salt water cleaning can be determined by a mass transport model simulated together with the groundwater movement model if concrete model area and its initial salt water distribution and pumping scheme are specified. Measures must be carried out to protect the surface water and the waste disposal (both solid waste and waste water) since the aquifer is shallow and unconfined. This issue is of a special attention for the area where the villages of traditional craft are existing due to possible sources of toxic wastes. Acknowledgement: This work is a part of the Fundamental Research Program coded which has been supported by the Council of Natural Sciences Ministry of Sciences, Technology and Environment of Vietnam.

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