DELINEATING THE BOUNDARY BETWEEN FRESH AND BRACKISH GROUNDWATER IN NAM ĐỊNH COASTAL AREAS BY GEOPHYSICAL METHODS

NGUYỄN TRỌNG VŨ¹, TĂNG ĐÌNH NAM², ANDREAS WELLER³

¹Institute of Geophysics, VAST, 18 Hoàng Quốc Việt, Hà Nội.
²Institute of Geology and Mineral Resources, Thanh Xuân, Hà Nội
³Institute of Geophysics, Clausthal University of Technology, Germany,
Arnold-Sommerfeld-Str. 1, D-38678 Clausthal-Zellerfeld, Germany.

Abstract: Saltwater intrusion into aquifers is a serious problem for coastal areas where a huge amount of water is extracted. To study the problem of saltwater intrusion, the knowledge of the boundary between fresh and brackish groundwater is of fundamental interest. Using the results of 30 vertical electrical soundings, five electromagnetic profiles measured by very low frequency method and additional measurements of the electrical conductivity of water in wells, this paper presents the geological structure of water-bearing formations and delineates the boundary between fresh and brackish groundwater in Giao Thuỷ and Xuân Trường districts of Nam Định Province. The study indicates that an apparent resistivity value of 50 Wm represents the limit between fresh and brackish groundwater-bearing formations.

I. OVERVIEW

Nam Định Province is located in Red River delta with the most dense population (about 3000-4000 people per km²). A 72 km long coastline with three large estuaries (Ba Lạt, Lạch Giang and Đáy) forms the southeastern boundary of the province. The province in the coastal plain needs a huge amount of water supply. The estimated water consumption will increase at 160,000m³ per day till 2010 [2]. Due to the rapid growth of population, the progressing urbanization, and the uncontrolled exploitation, surface water becomes more and more polluted. Groundwater will become the main source to cover the increasing water demand.

Nam Định Province has to face the problems of saltwater intrusion, especially in coastal areas. The monitoring and forecast of the progressing saltwater intrusion are very difficult tasks. To study the intrusion problem, the water-bearing formations have to be identified and the boundary between fresh and brackish water has to be determined. This paper presents the latest results of geophysical surveys that aimed at a delineation of the boundary between fresh and brackish water in Giao Thuỷ and Xuân Trường districts in Nam Định Province.

II. METHODOLOGY

1. Vertical electric sounding

In order to achieve the objective of the study, a network of vertical electric sounding (VES) was used to investigate the coastal area in Xuân Trường and Giao Thuỷ districts. VES method is usually applied to determine the layered structure of a more or less horizontally stratified subsurface. The theoretical fundamentals of VES method are summarized in geophysical textbooks e.g. [1]. The method uses two electrodes (A and B) to feed a low frequency current into the ground. Two other electrodes (M and N) are used to measure the voltage difference. The depth of penetration depends on the spacing between the electrodes. The used Schlumberger configuration is characterized by a small distance between the electrodes M and N and a larger spacing between the outer pair of current electrodes A and B. The two pairs of electrodes are placed along a line symmetrically with regard to a common point of reference O which indicates the station of the VES. The apparent resistivity r_a of the subsurface can be calculated by the formula.

(1)

with DU being the voltage between the electrodes M and N, I the injected current, and K the configuration factor which depends on the distances between all the electrodes A, B, M, and N.

2. Very low frequency (VLF) profiling

The VLF instrument used for this project is the WADI, manufactured by ABEM Instruments, AB, Sundbyberg, Sweden. The instrument utilizes the electromagnetic components of VLF waves produced by VLF transmitters around the world. The VLF transmitters are used by military personnel to communicate with submarines. The frequency of the VLF wave emitted from these transmitters ranges from 15 to 30 kHz. These VLF waves can propagate several thousands of kilometers around the Earth. The depth of penetration depends on the frequency and the resistivity of the subsurface. The emitted electromagnetic wave induces secondary currents when it encounters a structure of low electrical resistivity compared to the surrounding rock such as a saltwater-bearing formation. A secondary magnetic field is created by the secondary currents. The instrument records the ratio (as a percentage) of the strengths of the vertical and horizontal magnetic field components at the ground surface.

III. HYDROGEOLOGICAL SITUATION

In Nam Định Province, many water-bearing formations can be found in the shallow subsurface, but some of them contain brackish groundwater or the water quantity is not abundant [3]. The most significant of them are fresh water lenses within the Pleistocene aquifer located in the coastal areas of province. The Pleistocene (qp) aquifer is distributed all over the area of investigation and it is not exposed on the surface. The lithology consists mainly of quartz sand, gravel and pebble. The aquifer covers directly the Neogene sediments. The depth of this aquifer is about 80-90 m in the central part of the province and rises gradually at 60-70 m in coastal areas. The thickness of the aquifer increases from inland to the sea, reaching 30-40 m in the coastal areas. The water table of this confined aquifer does not change significantly during the seasons [6]. Due to groundwater exploitation, the water table of this aquifer decreases significantly. According to the “Yearly reports on actual state of groundwater” from 1996 to 2006 which are compiled by the Hydrogeological Division, the groundwater level at national monitoring wells in Hải Hậu district decreased about 6 m [4].

An aquifuge containing marine sediments (mQ₁^3bvp) and alluvial-marine sediments (amQ₁^3bvp) consisting mainly of clay and clayey sands covers the Pleistocene aquifer. The Holocene aquifer is found above that aquifuge. It can be divided into the Lower Holocene (qh₁) aquifer and Upper Holocene (qh₂) aquifer. The Lower Holocene aquifer is a confined aquifer which consists of fine-grained sand and clayey sand sediments. The Upper Holocene aquifer is an unconfined aquifer consisting of sandy clay and clay sediments. The geological unit Q₂^1-2hh₂ is inserted between the Lower and Upper Holocene aquifers as an aquifuge. Most Holocene aquifers contain brackish water [5,6].

IV. RESULTS AND DISCUSSION

1. Area of investigation

According to Đoàn Văn Cánh et al. [3] and Nguyễn Văn Độ et al. [6], brackish water is intruded in almost all Holocene aquifers. For the Pleistocene aquifer, freshwater lenses only exist in Hải Hậu, Nghĩa Hưng and part of Giao Thuỷ and Xuân Trường districts. It can be concluded that the boundary between fresh and brackish water is located in Giao Thuỷ and Xuân Trường Districts. Therefore in this study, geophysical surveys were carried out in Giao Thuỷ and Xuân Trường Districts (Fig. 1).

Figure 1. VES stations and VLF profiles in the area of investigation.

2. Data acquisition and interpretation

To separate the geological structures in the area of investigation, 30 VES were performed using Schlumberger configuration with the electrode spacing AB reaching up to 800 m. A current transmitter made in Việt Nam was applied that provides a current strength from several hundred mA to 2000 mA. The voltage between the electrodes M and N was measured by the ABEM instrument SAS 300C. VES data were interpreted for each station separately by IPI2WIN software.

To delineate the boundary between fresh and brackish water, five sections with a total length of 16 km of VLF profiles were measured. The distance of measured points along the profiles was 20 m. Along VLF profiles, the electrical conductivity of groundwater was also measured with a distance of sample collection of approximately 500 m. All VES stations and VLF profiles were shown in the map of the area of investigation (Fig. 1).

Generally, the VES curves can be separated into two curve types: one is measured in fresh groundwater area while the other is measured in brackish groundwater area. VES curves can be interpreted using five to six layers providing resistivity and thickness of each layer. The results of several VES stations can be combined to a profile providing a resistivity depth section. Fig. 2b shows the resistivity section which compiles the results of 1D interpretation of VES stations D21, D20, D30, D22, D25, D26, and D19. Fig. 2a shows a pseudo cross-section displaying the measured apparent resistivity as a function of half the separation between the current electrodes A and B (AO = AB/2).

Considering the value of apparent resistivity of the investigated geological structures, fresh groundwater can be found at a depth of more than 70 m in the area of VES stations D21, D20, and D30. The values of apparent resistivity reach around 100 Wm in this part of the section (red color in Fig. 2a) which corresponds to the Pleistocene aquifer. The other part of this profile exhibits brackish groundwater with apparent resistivity values below 50 Wm. For the Holocene aquifer, there is only an area around VES station D22 with an apparent resistivity of 80 Wm which might be caused by a fresh groundwater filling. Brackish groundwater should be assumed in the Holocene aquifer from the center to the end of the profile. The resistivity survey confirms that most of the Holocene aquifer is filled with brackish groundwater.

Figure 2. (a) Distribution of apparent resistivity (pseudo cross-section) along a profile across Xuân Trường district; (b) Resistivity cross-section compiling the 1D inversion results of several VES stations.

VLF data were acquired at five profiles (see Fig. 1). Fig. 3 shows a filtered curve of VLF data corresponding to a depth of 60 m from VES station D30 to VES station D27. The first section of this profiles from 0 to 1700 m is located on a fresh groundwater area. The VLF value at this section only is ±10%. The section from 1700 m to the end of the profile is characterized be values of up to three times higher compared with first section (±30%). From the VLF data, we can conclude that the boundary between fresh and brackish groundwater is located at position 1700 m of the profile.

Along the VLF profile, the electrical conductivity of water in groundwater wells is determined. The depth of the wells is about 100 m. The measured values of electrical conductivity of groundwater s_w along the profile are compiled in the graph of Fig. 4 that shows a good agreement with the VLF data in Fig. 3. The boundary between fresh and brackish water is well resolved by both surveys.

Figure 3. Filtered data of VLF profile in Xuân Trường district.

Figure 4. Electrical conductivity along VLF profile in Xuân Trường district.

Another profile is investigated from Quất Lâm to Ngô Đồng town of Giao Thủy dictrict (see Fig. 1). The geoelectrical cross-sections, which combine VES station D1, D2, D3, D4, D7, D5, D8, and D10, are shown in Fig. 5a and Fig. 5b. Considering the apparent resistivity, we can identify the boundary between fresh and brackish groundwater for Holocene aquifer between VES stations D4 and D7. For the Pleistocene aquifer, it is located between VES stations D2 and D3.

Figure 5. (a) Apparent resistivity distribution along profile from Quất Lâm to
Ngô Đồng, Giao Thuỷ dictrict; (b) Resistivity cross-section compiling the 1D inversion results of several VES stations.

The apparent resistivity measured with an electrode spacing between A and B of 100 m corresponds to an approximate depth of penetration of 12 m [7]. This spacing is chosen to represent the apparent resistivity of the Holocene aquifer. Fig. 6 shows the distribution of the apparent resistivity measured with this spacing in the whole area of investigation. Using the resulting map, the areas where fresh groundwater can be exploited from the aquifer are identified by apparent resistivity values larger than 50 Wm.

Figure 6. Apparent resistivity distribution at a depth of 12 m (AB = 100 m).

The apparent resistivity of the Pleistocene aquifer shown on Fig. 7 is represented by the values measured with an electrode spacing AB of 800 m. Fresh groundwater was found only in areas of the aquifer where the apparent resistivity value exceeds 50 Wm. The dotted line in Fig. 7 marks the boundary between fresh and brackish groundwater. Fresh groundwater can be exploited in areas indicated by yellow to red color while the brackish groundwater is expected in areas of green to yellow colors.

Figure 7. Apparent resistivity distribution at depth of 100 m (AB = 800 m).

V. CONCLUSIONS

Since the formation resistivity is to a large extend influenced by the water salinity, fresh and brackish water-bearing formations can well be separated by geoelectrical methods. Vertical electrical soundings and VLF profiling have proven to be appropriate tools to delineate the boundary between fresh and brackish groundwater.

VES provides the layered structure beneath the sounding station. A single VES enables the identification of the depth sequence of aquifers and aquifuges. Several aquifers can be investigated in one survey. A profile or a map of the results of a network of VES stations provides useful information on the distribution of fresh and brackish water in both the Holocene and Pleistocene aquifers. The study has shown that even the apparent resistivity measured with a certain spacing that corresponds to the depth of the aquifer can be used to identify fresh water zones. In the area of investigation, an apparent resistivity value of 50 Wm has proven to be an appropriate choice to separate fresh and brackish groundwater-bearing formations.

VLF is only sensitive to a single depth interval. The boundary between fresh and brackish water in the Pleistocene aquifer could be well resolved by VLF profiling. The electrical conductivity data of water samples acquired at wells along the same profile confirmed the VLF results.

Acknowledgements

We would like to thank Dr. Lê Thị Lài (Institute of Geological Sciences, VAST Hà Nội) and Dr. Jörn Kasbohm (Greifswald University) who organized the funding of the field work in Nam Định Province. We also thank Dr. Nguyễn Như Trung (Institute of Ocean Geophysics) who provided the geoelectrical equipments.

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