BÙI HäC¹, Lª THÞ LÀI²_,
MARIA-THERESIA SCHAFMEISTER³,
PH¹M KHÁNH HUY¹, ®ç v¨n b×NH¹

Nam §Þnh Province is an important economic center in the east of the B¾c Bé Delta. In the past years, in this area, with more and more increasing demand of clean water for life, and industry, there were many research projects of different authors, such as Lª ThÞ Lµi, §oµn V¨n C¸nh, Ph¹m Quý Nh©n, NguyÔn V¨n DÇn … partly solving queries on reserves and hydraulic conditions of groundwater. Many problems as age and origin of groundwater have not been answered and investigated thoroughly yet. In this paper, basing on recent research results of isotopic measurements, which have been applied for following isotopic elements: ¹⁸O, ²H, ³H, ¹³C, ¹⁴C, ³²Si, ³⁴S, ²²²Rn, ²²⁶Rn, and on theoretical models of authors such as Bïi Häc, Hoµng §¾c Lùc and Vò ThÞ Kim TuyÕn, we will try to solve those remaining problems.

Most recently (Spring 2003) new samples numbered as DV01, DV02a, DV02b, DV03a, DV03b, and DV04 were used for comparing with former results of Hoàng §¾c Lùc. Two samples DV01 and DV04 collected in Xu©n Tr­êng District and in Kim S¬n (Ninh B×nh Province) were used for giving us a larger view of groundwater not only in the observation system, but also in neighbouring areas. All isotopic samples were sampled according to technique standard. Sampling sites are displayed in figures 1 and 2.

  T, D and ¹⁸O samples

- Tritium samples are sampled in 0.5 l bottles, and D and ¹⁸O samples were sampled in from 0.1 to 0.3 l bottles, then they are covered tightly to avoid atmospheric influences. During sampling, storing and transport processes it is necessary to limit the influence of other water kinds and evaporation.

- To sample ¹⁴C it is necessary to prepare chemical substances and equipment as follows: 100 l of container, 5 l of Ba(OH)₂ (200 g Ba(OH)₂.8H₂O dissolved in 5 l of H₂O), catalyst Stipix - A40, H₂SO₄ and other means for measuring pH, CO₂, HCO₃^-, Eh .

- ¹⁴C water sample and Ba(OH)₂ solution are mixed with 100 l of container, then waiting for solution precipitated completely (about 2 hours) we can mix 1 to 2 catalyst drops to decrease the precipitation time. Finally all BaCO₃ precipitation and solution is gathered in a 5 l can and covered tightly. During the sampling process it is necessary to limit the penetration of air in water sample.

On the Earth, the oceans are the biggest reservoir of stable isotopes. Therefore, the measured values of stable isotopes in seawater are used as standard values. These standards are called as SMOW (Standard Mean Ocean Water).

If R is the ratio of the heavy isotope to the light one, then the relative fractionation is expressed in denotation as

 Results are expressed as deviation in per thousand (⁰/₀₀) parts.

If the value of d is positive, the sample is enriched in the heavy isotope relative to the standard, a negative sample is isotopically light [4].

Based on the results of global investigation of the International Atomic Energy Agency/World Meteorological Organization (IAEA/WMO), scientists determine a linear relationship between d D and d ¹⁸O in rainwater; this relationship can be expressed by the following formula:

This linear function is known as the Global Meteoric Water Line (GMWL). Continental precipitation samples will tend to group near to this line. Precipitation falling in areas of lower temperatures or at higher latitudes will have lower d D and d ¹⁸O values. Naturally, oceanic water will fall below the meteoric water line as it is isotopically enriched. Deviations from the meteoric water line can be interpreted as being caused by precipitation that occurs during a warmer or colder climate than at present or by geochemical interaction occurring during underground passage [16].

Radioactive isotope tritium is formed by the effect of cosmic rays on the nuclei of nitrogen and oxygen atoms in the uppermost layer of the atmosphere, or by nuclear weapon experiments and nuclear reaction piles.

These nuclear reactions are displayed in the following equations:

- Piston flow model

- Exponential function model

 T_1/2: half-life of ¹⁴C (5730 years)

C_O: initial content of ¹⁴C before percolating into aquifer (pmc)

C_t: content of ¹⁴C measured in water sample (pmc)

t_M: mean age of groundwater in aquifer (year)

t : age of groundwater (year)

l : radioactive decay constant

  (8)

The analyses of stable isotopes ¹⁸O, ³H and D was performed by Atom Institute in Bombay, Indian, Hydrography-Isotope Laboratory of IAEA in Vienna, Austria by spectrometer method, and at UFZ -Halle-Leipzig, Germany. The analysis of ¹⁴C content in the groundwater is carried out by Institute of Archeology in Hµ Néi, ViÖt Nam with equipments named TRICARB 2770 TR/SL, CANBERRA, USA.

Table 1. Analytic results of stable isotopes in Nam §Þnh - Ninh B×nh area

Table 2. Analytic results of ¹⁴C in Nam §Þnh - Ninh B×nh area

Table 3. Analytic results of major compounds and selected contaminants
in ground water in Nam §Þnh - Ninh B×nh area

To explain the origin and relation of groundwater we established the following graphs

According to figures 3 and 4, we see that almost all of values are below the Global Meteoric Water Line. The content of d D and d ¹⁸O in the groundwater varies from -35.9 to -59.6 ^o/_oo SMOW, and from -8.46 to -5.33 ^o/_oo SMOW, respectively. They indicate that there is no penetration of seawater into aquifers until the depth of 200 m.

 According to above figures, we can define two main problems as follows:

- The original content of ¹⁴C isotope varies from 70 to 110%

- There is supplementation of young water into the aquifer at the depth from 100 to 200 m (110a, 109a, 109b wells).

The relationship-figure between value of ¹⁴C and d ¹⁸O in figure 5 shows that:

- From 4000 to 1000 years ago in the Holocene period, the d ¹⁸O values in the groundwater had a rising tendency. From 1000 years ago, the content of d ¹⁸O has tended to decline. This figure also shows the climate changes in the Holocene period, at which the changes in sea level also occurred about 6000 years ago.

- Although the TDS increases, d ¹⁸O values vary from -8.46 to -5.24. This shows that the groundwater in aquifer has salt penetration, but the d ¹⁸O values do not vary so much with TDS.

The main results of using isotopic methods to study groundwater in Nam §Þnh province helped us to understand, and solve several remaining problems at previous stage as origin and age of groundwater. An average age of groundwater is from 4930 years old to present, and groundwater has meteoric and penetrated origins.

This application has been opening a new method to study hydrogeological characteristics and solve other matters not only in Nam §Þnh Province but also in other areas in ViÖt Nam, where classical methods cannot answer thoroughly about hydrogeological characteristics.

1. Bïi Häc, 1997. Isotopic methods for Hydrogeology. Textbook for master training. Hanoi University of Mining and Geology.

2. Hoàng §¾c Lùc. Application of isotopic technique to groundwater investigation in the Rea River Delta. Summation of technology and science themes at ministry level of 2000-2001 of Ministry of Technology Science and Environment. Kept in Hanoi Institute of nuclear energy.

 3. Division of Hydrogeology and Engineering Geology of South ViÖt Nam . Documents on geology and hydrogeology and map of hydrogeology and cross-sections in Nam §Þnh areas.

4. C. W. Fetter, 1993. Applied Hydrogeology. Third edition Prentice Hall.

5. H. Craig, 1961. Standard for reporting concentrations of deuterium and oxygen-18 in natural waters. Science, 133 : 1833-1834 and Isotopic variations in meteoric waters. –Science, 133 : 1720-1703