§ç Minh L·ng¹, §ç Träng Sù², CAO DUY GIANG¹,
NGUYÔN HåNG QUANG, NGUYÔN TH¹c C­êng¹, PH¹M V¨N HµN¹

Up to now, research work on hydrogeochemistry of groundwater in the B¾c Bé coastal zone in general, and Nam §Þnh area in particular, has been paid attention by many organizations and scientists. But it was carried out only in the form of combination with the other forms of work therefore the study level is still restricted.

Recently, the research Project of "Hydrogeochemical features of groundwater in the B¾c Bé and North Trung Bé coastal zone" has just been completed, touching also upon the Nam §Þnh Province. This is one of the places concentrating the densest population (1,898,100 people) in comparison with provinces of the all country and there are many industrial branches and agricultural- sea product processing factories...

According to the research results the hydrogeochemical regions have been divided into three levels of total dissolved solids (TDS) equivalent to three water types: fresh (TDS < 1g/l); brackish (TDS: 1-3 g/l); and saline (TDS> 3 g/l). The hydrogeochemical subregions according to chemical composition (chloride, bicarbonate and mixed water) have been determined, at the same time the tendency of hydrogeochemical change of aquifers have been preliminarily set forth.

Most of fresh water types are of recent origin supplied by rainwater from the apex of the delta or by rivers to groundwater, and groundwater always moves toward the seaside.

In the area of Nam §Þnh Province, fresh-water is often distributed along the systems of Red River, Linh Cõ River, §¸y River and some other river systems creating concave curves directing the concave part toward the sea. It tends to move from inside to outside, from high site down to low site following the northwest-southeast direction advancing gradually to the seashore. According to a tide accumulating curve, the super fresh water lies in the upper side, the brackish water intercalated with fresh water in the lower side. Due to the influence of tide, the areas lying along two banks of large rivers are often intruded by sea water to create brackish water which has the general tendency to move gradually from river banks into the two sides according to a cross-section perpendicular to the river. The more it is far from the bank, the more total dissolved solids gradually decrease to create the fresh water.

According to the research data on hydroisopressure lines, these lines express always the tendency of groundwater advancing toward the sea. The saline/fresh interfacial boundary lines of the two aquifers also manifest the above features. These lines have the meandering shape transecting through the plain with the difference in two sides: the upper side consists of fresh-water, but the sea side - brackish and saline water. The hydrogeochemical horizontal cross-section of both aquifers in Nam §Þnh area also has similar features. However, in fact, there are also some excepting cases, those are the fresh water-bearing areas lying next to the seashore of Nam §Þnh - Th¸i B×nh Provinces running deeply into the plain; this phenomenon occurs fairly frequently in Holocene aquifer, with the bicarbonate-chloride chemical composition (such as in Giao Phong, Xu©n Thñy, west NghÜa Th¸i, south Trùc §¹o districts...). For the Pleistocene aquifer this typical phenomenon is expressed by a large fresh-water bearing area lying near the sea side, such as in H¶i HËu, NghÜa H­ng areas, and expanding to the vicinities.

According to hydrogeochemical research data (period of 1982-1987) the saline/fresh interfacial boundary has the following TDS: 1 g/l, forming a narrow band, taking the Luộc River as the boundary extending from H¶i Phßng through the Nam §Þnh Province to Trà Lý River. In the upper side of Luộc River there is fresh-water (TDS < 1 g/l), in its lower side toward the coast there is brackish water (TDS = 1-3 g/l) and saline water (TDS > 3 g/l). On the hydrogeochemical cross-section in the horizontal direction are formed two clearly zones, with the internal zone (directed toward the mainland) characterized by fresh-water, which is of calcium-sodium bicarbonate chemical composition (as in Giao Phong, NghÜa Th¸i, Trùc §¹o etc.); the external zone (directed toward the sea) characterized by brackish and saline-waters, which are of sodium chloride-bicarbonate or chloride chemical composition (as in NghÜa Hoµ, NghÜa Lîi, south Xu©n Ngäc, etc.)

Generally, brackish water has general tendency to intrude deeply into the mainland in the northwest-southeast direction. In Nam §Þnh area (Nam §Þnh City and Nam Trùc District) there is a fresh-water area of mushroom shape, having the saline/fresh water interfacial boundary curve directing the concave side toward brackish and saline water (toward the sea). Therefore fresh water in this area has the tendency to develop in the northwest-southeast direction and advancing gradually to the seaside. But in the other areas, intrusion of brackish and saline waters happens toward the southwest-northeast direction.

Generally, the changing tendency in chemical composition of groundwater fairly coincides with the change of TDS (in horizontal direction) from high to low topography, from inside to outside corresponding to the bicarbonate, bicarbonate-chloride to chloride water types.

In addition, there still are different points from the above mentioned general laws, these are the fresh-water bands of marine-eolian sediments lying along the seashore of Nam §Þnh Province and other coastal provinces. There, the sand banks, sand dunes are the highest relief in comparison with surrounding areas. Their groundwater level is sometime still higher than the sea water level; it proves that the water source of these formations can be only supplied by rainwater. While in the areas situated far from the sea, lying deeply in the mainland, exist brackish water lenses and bands distributed uncontinuously and unlawfully. This phenomenon can be regarded as buried types of water, that the unthorough fresh washed process still exists in the regressive period.

The characteristics of this aquifer is its large distribution and deep position beneath the ground surface, under a cover of fine-grained sediments (water-sealing). Therefore, they are often of stable regime in the depth, less influenced by the factors of topography, climate, hydrology and other artificial activities.

Generally, the chemical composition and TDS of Pleistocene aquifer are less changed complicatedly in comparison with overlying Holocene aquifer. The general widespread feature of this aquifer is the lawful change of chemical composition as well as TDS, it means from the seaside toward deeply the mainland in the order from the saline water of TDS > 3 g/l, with the chloride sodium chemical composition of water to the brackish water of TDS = 1-3 g/l with the sodium, calcium chloride-bicarbonate and sodium, calcium chloride, bicarbonate chemical composition and at last fresh water of TDS < 1 g/l with the sodium, calcium bicarbonate, chloride or sodium, calcium bicarbonate chemical composition. However in this aquifer there are also unlawful excepting cases, that is the existence of a relatively large fresh-water area (near 1000 km²) lying near the seashore of H¶i HËu, NghÜa H­ng areas, with the typical chemical composition of sodium bicarbonate. The presence of this large fresh-water mass can be originated from the very rich karstic fissure aquifer distributed in a large limestone massif which is developed with a gradually sloping and sinking deeply toward the sea. Thus the fresh-water source in it moves also in this direction and impacts directly to the groundwater source in front of it. This groundwater source existed from ancient time through the transgressive marine periods. In the regressive marine period, with a prolonged continental regime, the fresh wash process took place by the fresh-water source of §ång Giao Formation (T₂a ®g) to make this saline water mass transferring gradually into fresh and completely fresh water. At the same time the chemical composition of water also changes gradually from chloride to bicarbonate like the composition of groundwater in this limestone massif.

The tendency of hydrogeochemical change following the depth in Holocene and Pleistocene aquifer in Nam §Þnh area is also manifested in the hydrogeochemical cross section. The fairly complicated upper aquifer is composed of three types of water corresponding to the above said chemical composition in some places intercalated with brackish and saline waters. Though the lower aquifer contains also all three water types, but the chemical composition of water is widespread chloride; carbonate water is met mainly in H¶i HËu - NghÜa H­ng area.

The comparing results between the previous and present data indicate that the hydrogeochemical change in the interval of time from 10-15 years ago up to present time is not great, therefore the impacts from the ground surface to groundwater are not so intensive to make basic change of TDS and of groundwater chemical composition. During more than ten recent years, the very strong exploitation of groundwater in Nam §Þnh area has been leading to the lowering of groundwater level in some areas. If the topsy-turvy water exploitation situation is not examined in time lacking a close control a threat of change to bad tendency of both quality and quantity of groundwater will happen.

According to inadequate statistics to 1998, only in the southeastern area of Nam §Þnh Province there are over 60 thousand drilled wells exploiting mainly the fresh water mass belonging to Pleistocene aquifer; the total exploited water quantity is about 60,000 m³/day (Table 1).

Table 1. Quantity of UNICEF - typed wells in the southeastern area of Nam §Þnh Province

Due to the massive well drilling and water exploitation the aquifer becomes gradually exhausted. The monitoring result of level lowering in the wells of above districts has been showing that the change of water level is from very small to 3 m.

In the future, the need of water use to serve living and economic development will become more and more great. In order to meet this need, the quantity of exploited points certainly must be increased and the exploited capacity (of both surface water and groundwater) must be expanded. If the above said problems haven't been paid attention they will cause bad impacts and make retrograde the water sources. And humans will have to face to immeasurable consequences.

With regard to water pollution in Nam §Þnh area, there are also relatively clear manifestations. There is a place of concentration of many industrial branches such as textile, dye industries etc.. Therefore groundwater polluted manifestations as NO₂, NH_4, microorganism, cyanide, phenol, manganese, iron, aluminium and chromium, arsenic in some places have been discovered. On the other hand, a relatively great amount of fertilizer and plant-protecting chemicals are frequently used. Very economical activities of humans have been causing pollution to the surface water and groundwater sources in general.

In order to have sustainable environmental development there must have preventive measures against water pollution including artificial clean water recharge for aquifers.

Hydrogeochemical research aims to participate in the protection of groundwater resources. This is an indispensable duty and has practical significance for living conditions of the people's material and moral welfare, needing to be paid attention by the state. The research results have been showing that many UNICEF- typed wells and other water-exploited wells have manifestations of saline contamination and pollution. In order to restrict and to come to prevent the qualitative and quantitative retrograde manifestations of groundwater, we have to manage closely the drilling of the UNICEF-typed boreholes of small diameter at a spontaneous way. Due to the lack of knowledge about geology and hydrogeology of the study area and water drilling techniques this can lead to damage the aquifers. It is necessary to carry out the propaganda and education on the sense of responsibility of protecting the groundwater source for every body and every branch. The topsy-turvy and personal water exploitation is easy to get different risks. We have better to carry out concentrated water exploitation for planning the protection of the groundwater source and establishing the zone of hygiene precaution to guarantee the stable and durable water sources.

1. §ç Träng Sù, NguyÔn Träng HiÒn, 1997. Current status of the groundwater pollution in some important population and economic areas of the B¾c Bé Plain. Geology and Min. Res., 7. RIGMR, Hà Néi (in Vietnamese).
2. NguyÔn §«ng L©m, 1997. Groundwater resources in ViÖt Nam and capacity of its exploitation and use for water supply and environmental hygiene. Proc. Nat. Symp. on Groundw. Res. : 17-22. Hà Néi (in Vietnamese).
3. NguyÔn Kim Ngäc, 1991. Water supply problem serving the material economy and morale welfare of people in the Qu¶ng Ninh Mine area. Selected works Univ. Mining-Geol., Hà Néi (in Vietnamese).