Although water resources in the Mekong Delta are very abundant, the geological structure and topographical characteristics of this region as well as human activities are factors that have an impact on water quality and consequently are restricting the utilization of these resources. The
TABLE 2
Values of SAR and EC in the Mekong river
EC (dS/m) SAR
Min Max Min Max
Mekong (Taân Chaâu) 1.02 2.10 0.235 0.967 Bassac (Chau Doc) 1.19 2.88 0.269 0.850
acidity of the water, the salinity intrusion and other impacts from human activities are the main causes of the utilization constraints.
Acidic water
The acidity of the water in the Mekong Delta is a typical characteristic and this situation is due
to the occurrence of acid sulphate soil (ASS) and its content of pyrite (FeS2). Some 1.6 million
ha or 41.1 % of the Mekong Delta area is characterized by acid sulphate soil and this is particularly pronounced in some 0.55 million ha found in the Plain of Reeds and the Long Xuyen Quadrangle. A case of intermediate severity is also found in the Ca Mau Peninsula the acid sulphate soil is affected by saline water during the dry season. The acid sulphate soil is responsible for the acidification of the water through a the process described below.
During the dry season, the acid sulphate soil dries out and cracks form, giving way for oxygen to penetrate into the deeper layers of the soil. As a result, the oxidation of pyrite will occur and produce the substances responsible for the acidification process. Then at the beginning of the rainy season, the acidification products are washed out or leached into the water of canals where low pH values of about 4 or even less are measured as well as high concentrations of toxic ions. The oxidation of pyrite is quite a complicated process involving both chemical and microbiological reactions. The process involves several stages and it can be summarised by the following chemical equations (Dent, 1986):
FeS2 + 7/2 O2 + H2O = F e2+ + 2 SO 42- + 2H+ (1) Fe2+ + 1/4 O 2 + H+ = F e3+ + 1/2 H2O (2) Fe3+ + 3 H 2O = Fe(OH) 3 + 3 H+ (3)
substituting (2) and (3) into (1) gives the overall reaction
FeS2 + 15/4 O2+ 7/2 H2O = Fe(OH) 3+ 2 SO42- + 4 H+ (4)
dissolved Fe3+ can be reduced by FeS
2 to produce more acid into the stream
FeS2 + 14 Fe3+ + 8 H
2O = 15 Fe2+ + 2 SO42- + 16 H+ (5)
The released acid will react with aluminosilicate minerals to form soluble products such as aluminum sulphate and some secondary minerals such as kaolinite. These products will be washed out into the canal upon the first precipitation during the rainy season. Consequently,
the water in canals will be heavily polluted by high concentration of Al3+, Fe2+, Fe3+, and SO
42-
and a pH value as low as 3 or less.
As mentioned above, the Mekong Delta comprises three areas characterized by acid sulphate soil structure and they are delineated as follows (see Figure 1):
a. One area is the Plain of Reeds located in the northeastern part of the Mekong Delta, where 378,185 ha or about 50% of the total area of 738,000 ha are affected by acid sulphate soil. In the Tram Chim (Dong Thap province) area, the pH of the water is quite low at about below 4, it contains a high concentration of aluminum of about 50 mg/L and the duration of the acidic water period is very short. In Kien Binh (Long An province), on account of a depressed topography and poor drainage conditions the duration of the acidic water period is rather long (for some months) and the aluminum concentration in the water reaches about 30 mg/L. Acidic water is encountered all year around in the Bo Bo region because the acid sulphate soil covers a large area and there is a shortage of freshwater at the beginning of the rainy season to wash out the acidic water.
b. In the Ca Mau peninsula, the highest concentration of aluminum observed in the water is about 40 mg/L. The most severe cases of acidic water are found at Thoi Binh, Viõnh Thuan
on the Trac Bang canal. Every year at Thoi Binh, the canal water is acidic with pH values in the range of 3 to 4.5 for 4 months or 5 months (from June to October) [Figure 2]. Drainage conditions are poor and the duration of the acidic water period is prolonged.
c. Located in the northwestern part of the Mekong Delta, the Long Xuyen Quadrangle occupies an area of about 492,000 ha and regions with acid sulphate soil are concentrated between the Ba The and Tri Ton canals. The acidic water appears at the beginning of the rainy season at the end of the Vinh Te and Tam Ngan canals and lasts from May to August. Moreover, these areas have also been affected by saline water intrusion, thus the water is not only acidic but also saline.
Besides having low pH values and high contents of ions such as Al, Fe, and SO4, the acidic
water has also a rather high concentration of total Nitrogen with an observed maximum value of about 3.5 mg/L. Because the acid sulphate soil has a surface soil layer with a rather high nitrogen content, typically from 0.15-0.50%, at the beginning of the rainy season this nitrogen content gets washed out also into canal water. In contrast, the phosphorus content in the acidic water is low at about 0.2 mg/L as the phosphorus present in the acid sulphate soil reacts with aluminum and iron to form insoluble compounds.
The occurrence of acidic water is a natural phenomenon in the Mekong Delta that has caused great damages not only in terms of water quality but also in terms of ecosystem conservation. This situation has impact on the mangrove forest and its biota, on the sedimentation of suspended solids in the water, and on the health and living conditions of inhabitants using this water.
Salinity intrusion
In the Mekong Delta, the rainfall during the dry season is quite low with about 15% of the annual rainfall recorded during that period. For the months at the beginning and the end of the dry season (April and December) the rainfall is about 50 mm and about 10 mm for the other
FIGURE 2
Variations of pH in the Chac Bang canal in the Ca Mau peninsula
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
pH value
months in this season. The driest period of the year often occurs toward the end of April until the first days in May. This dry season and situations of salinity intrusion occur in areas those are far away from the source of the Mekong River and located along the coast. In the Mekong Delta, an area of more than 1.7 million ha is affected by salinity intrusion in the dry season. Salinity intrusion is one of main obstacles for appropriate living conditions as well as adequate water supply demand for agricultural development in the Mekong Delta, especially for the coastal areas.
The East and West seas are the two main sources for salt intrusion. The East sea has a semi- diurnal tidal regime and the tides raise and recede twice daily with a high amplitude of 300-350 cm. The water level of the tidal foot fluctuates strongly (160-300 cm), while that of the tidal peak fluctuates weakly (80-100 cm). Seawater from the East sea intrudes into the Mekong Delta soil through the East Vaico, West Vaico, Mekong and Bassac rivers. The West Sea tidal regime is a mixed one and the dominant diurnal tidal regime has an amplitude of about 80-100 cm, with little fluctuation of the tidal foot (20-40 cm) and a larger one for the tidal peak (60-80 cm). The seawater of the West Sea intrudes into fields through canals from the Ca Mau Cape to Ha Tien. The salinity intrusion problem is a more important with the East Sea than with the West Sea.
The salinity intrusion in the Mekong Delta can be linked to several main factors, namely meteorological conditions, tidal activity, changes of water level and flow in the estuary area, and other localized characteristics. This process is a very complicated one as the factors enumerated vary not only periodically and unpredictably, but also in time and space.
Considering natural conditions such as topography, water sources, and the existence of canal systems, the area affected by salinity intrusion in the Mekong Delta can be divided into the following sub-regions :
The Plain of Reeds with the East Sea acting as a source of salt water through the East Vicoand West Vico rivers.
The area between the Mekong and Bassac rivers impacted by the East Sea tide through rivermouths at Cua Tieu, Cua Ñai, Ba Lai, Ham Luong, Cung Hau, Co Chien, Ñinh An and Tran Ñe.
The Long Xuyeân Quadrangle with the West Sea as the source of saline water throughcanals Giang Thanh Creek to Cai San Canal.
The Ca Mau Peninsula with both the East and West seas providing the source of salinewater.
Some the areas mentioned above are impacted by salinity intrusion for 5 or 6 months per year and during that period rainwater is the sole source of freshwater source for local inhabitants. While in recent years a small increase rise in the extent of the salinity intrusion in main rivers has been observed on account of increases in the water consumption in their upstream area, generally the salinity intrusion phenomenon has not changed in that the maximum length of the salinity intrusion occurs in April when the upstream flow is lowest. Table 3 provides data about the distance of salinity intrusion in the main rivers during the period February to May (Anh et al., 1995).
Impacts caused by human activities
As mentioned previously, various human activities can have an impact on water quality and the main ones at play in the Mekong Delta are briefly described below.
a) The utilization of fertilizer has direct impact on water quality. Nitrogen and phosphorus are vital nutrients for plant growth and the application of fertilizers on soil to supply these nutrients is increasing day by day. The discharge of these nutrients into the water of rivers and canals can favor the growth of algae and seaweed and lead to eutrophication, a water quality problem. Data collected over a long period as part of the project Water Quality Monitoring Network in the Lower Mekong Basin have indicated an increase of the nitrogen concentration in recent years.
b) Improvements in crop production are obtained by the application of new technical and scientific knowledge and the use of pesticides to protect crop from pest is a particular case in point. According to an investigation of carried out by the Department of Plant Protection (Ministry of Agriculture and Rural Development), there are 242 substances that have been approved by the responsible agency for use as plant protection chemicals. Some of the pesticides that have been used include: organophosphorus pesticides (methyl parathion, basudin, etc.), organochlorine pesticides (tindan, 2,4 D, lindane, etc.) ad carbamate pesticides (bassa, carbofuran, etc.)
c) The results obtained during the Pesticide Monitoring Network in the Mekong Delta carried out in 1995-1996 have shown that some types of organochlorine pesticides were in use, such as DDT, hexachlorohexane (HCH), endosulfan and their derivatives, pentachlorobenzene, heptachlo, technozene, and hexachlorobenzene (HCB). However the measured concentration were low, in the range of 100-400 ng/L. Over a three year period (1995-1997), 37 fish samples obtained in the Mekong Delta were analyzed for organochlorine pesticides and the average concentration of total pesticide residue reported was less than 9 mg/kg.
d) Shrimp culture in fresh and saline water is quite common because of the high income it can generate. With an area of about 250,000 ha, the permanent saline region along the coast has been used for mangrove forest and aquaculture. Presently, the mangrove forest occupies an area of about 90,000 ha and the shrimp culture, about 100,000 ha. Some areas adjacent to the permanent saline zone are used for both agricultural production and shrimp farming. During the dry season these areas are used for blackish shrimp farming and in the rainy season an area of about 15,000 ha is used for rice cultivation. In ponds used for shrimp culture, the water is significantly polluted by the decay of organic matter contributed by shrimp food. The results of an investigation carried out in 1997 (Ngu, 1998) in such ponds located in the Binh Dai district, Ben Tre province, have shown that during the dry months the dissolved oxygen content was very much reduced and rather low (from 2.61 to 2.35 mg
O2/L) and the hydrogen sulfide (H2S) level at 0.019-0.079 mg/L. During the rainy season,
the H2S level was 0.019-0.079 mg/L. Unfortunately, the impact of these sources of wastewater
has not been evaluated carefully, but the damage of shrimp culture to the mangrove forest has been clearly observed.
TABLE 3
Monthly average length of salinity intrusion (1985 –1996) in main rivers (km) Limit of 4 g/L Limit of 1 g/L River
Feb Mar Apr May Feb Mar Apr May
Cua Tieu 23 32 37 32 43 51 59 56
Ham luong 22 30 34 26 46 51 57 54
Co Chien 22 31 35 27 44 48 55 51