Keywords

1 Introduction

The central coastal plains of Vietnam are narrow and small area plains that were filled by sediments of short and steep rivers cross the region, so central coastal aquifers of Vietnam in general and Ninh Thuan coastal aquifer in particular characterized by small and scattered in distributed areas and thickness. Saltwater intrusion in aquifers has become the biggest problem to human life in this area where limited surface water and groundwater are often the main sources of drinking water and agriculture activities. Besides the investigation to assess the status or occurrence of saltwater intrusion in the aquifer system, the assessment of saltwater vulnerability may also helpful for management of groundwater source in the area.

There are a number of methods to assess vulnerability caused by the continuous extent of seawater intrusion in coastal aquifers in which the GALDIT index model developed by Chachadi and Ferreira in 2001 was widely used over the world based on rating scores and weights of six distinct influencing hydrogeological parameters as follows: (1) the groundwater occurrence G; (2) the aquifer hydraulic conductivity A; (3) level of the groundwater level above sea level L; (4) the distance from shoreline D; (5) the impact of existing status of seawater intrusion in the area I; and (6) thickness of the mapped aquifer T. The weights of GALDIT parameters are assigned value from 1 (the least significant parameter) to 4 (the most significant parameter) from the statistics of survey results of scientists and experts (Chachadi and Ferreira, 2001).

In this study, an overview on occurrence of saltwater intrusion was carried out based on previous investigation results, geophysical surveys (VES measurements), and water sample analysis. Then, the GALDIT parameters weights were modified by analytic hierarchy process (AHP) method that is based on questionnaire survey results of over 50 hydrogeological experts in Vietnam, and the range of each factor rating is slightly changed to meet the condition of coastal unconsolidated Quaternary aquifers of Ninh Thuan province. Accordingly, GALDIT index mapping obtained with the factors values gathered from previous studies and this study, and the modified weights and parameter ratings.

2 Study Area

Ninh Thuan is located in the south central coast of Vietnam lying between latitude of 11o18'28" and 11o50'52" North, and longitudes of 108o45'19" and 109o13'54" East with total area of 3358 km2. It is bounded by Khanh Hoa, Lam Dong, and Binh Thuan provinces to the North, West and East respectively, and faces the sea in the East. The province has three types of topography as mountains, hills, semi-mountains, and coastal plain in which the hill and mountain occupy 77.6% of the total land with the elevation ranges from 200 to 1000 meters, and the coastal plain accounts only for 22.4% of the total province area. The province has a tropical monsoon climate characterized by hot dry and windy weather with temperature ranges between 24.7 and 29.1 °C. There are two distinct seasons, that is, the rainy season lasts for 3 months from September to November which contributes to total of 40% to 80% annual rainfall, and the dry season lasts from December to August. The average annual potential evaporation is 1844.1 mm (around 153.66/month) and the highest evaporation rate is in January (about 166.2 mm) while the average annual rainfall is about 1131.4 mm (i.e. about 50% evaporation); this makes Ninh Thuan one of the most arid regions of Vietnam making groundwater resources a cause of concern in this province.

The aquifer system of Ninh Thuan coastal plain consists of Quaternary unconsolidated aquifers, fracture igneous rock and sedimentary rock aquifers formed in between Middle Jurassic and Pleistocene era. The fracture aquifers have very low productivity and are distributed at great depth; limited study has been carried out to investigate the quality and quantity of those aquifers and they are not the subject of this study as well. The Quaternary unconsolidated aquifers consists of Pleistocene, Holocene, and undivided Quaternary aquifer in which the undivided Quaternary aquifer is distributed in transition zone between the mountainous area and the plain with distributed area of around 65 km2; the Holocene aquifer is widely distributed in Phan-Rang plain and along Cai river valley with total area of 315 km2, and the Pleistocene aquifer is found at Tan Hai ward, the southern area of Phan Rang plain, and from Phuoc Hoa to Quang Sơn ward with a total area of 364 km2 (Nguyen Truong Giang et al., 1998).

According to Ngo Tuan Tu and Nguyen Ton ( 2015 ), seawater was intruded to aquifer system of Ninh Thuan coastal plain due to current sea water throughout a long coastline of around 105 km and ancient seawater in depth aquifers through tectonic faults and groundwater exploitation activities. The distribution of saline groundwater depends on the aquifer system characteristics and the natural conditions: (i) the saline groundwater can be found over the whole aquifer system; (ii) saline groundwater can be found on top of aquifer system (Holocene aquifer); (iii) the saline groundwater can be found in the lower part of aquifer system (Pleistocene aquifer). Total complete saline groundwater area is around 280 km 2 , and partly saline groundwater area is around 420 km 2 .

In this study, the current saltwater intrusion status and the origin was investigated by 16 survey lines of vertical electrical sounding (VES) measurements in which 15 survey lines were arranged perpendicular to the coastlines in northwest-southeast direction; only survey line No.16 was arranged parallel to the coastlines in north-south direction. Forty -ive groundwater samples were collected during sampling trip for chemical analysis (both conventional and stable isotopic analysis). The ES investigation was arranged in the area of current saltwater intrusion; the results showed that the fresh–salt water interface moved toward the land at the distance up to 13 km from the shoreline and the saltwater covered almost length of all investigated lines of both Holocene and Pleistocene aquifers. The arrangement of VES survey lines, the fresh–salt (F-S) water interface and groundwater samples are presented in Fig. 45.1.

Fig. 45.1
figure 1

ES surveys arrangement, F-S interface, and water sampling locations in Ninh Thuan coastal plain

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One example of salt–fresh water interface analysis of the survey line No.1 can be seen in Fig. 45.2. It is clear that the saline water covered whole survey line No.1 except some part of the bedrock.

Fig. 45.2
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Salt–fresh water interface analysis for VES survey line No.1

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The chemical analysis showed that water samples in 40 out of 45 locations to be saltwater, and the origin of saltwater differs from place to place depending on the hydrogeological structures, groundwater flow regime; the origin can be from current saltwater intrusion found at the coastal area or from ancient seawater found in the inland area.

3 Saltwater Intrusion Vulnerability

According to Chachadi and Ferreira (2001), the final decision criteria of vulnerability to saltwater intrusion are based on GALDIT index value; the higher the vulnerability index value, the more vulnerable the aquifer is to seawater intrusion, and it was obtained by evaluating each factor score and summing them in the following formula:

$$ \mathrm{GALDIT}\ \mathrm{index}=\frac{\sum_{\mathrm{i}=1}^6\mathrm{WiRi}}{\sum_{\mathrm{i}=1}^6 Wi} $$
(45.1a)

or

$$ {GALDIT}_{Index}=\frac{\left({\mathrm{W}}_1\times G\right)+\left({\mathrm{W}}_2\times A\right)+\left({\mathrm{W}}_3\times L\right)+\left({\mathrm{W}}_4\times D\right)+\left({\mathrm{W}}_5\times I\right)+\left({\mathrm{W}}_6\times T\right)}{\sum \limits_{i=1}^6{\mathrm{W}}_i} $$
(45.1b)

where:

  • Wi is the weight and Ri is the importance rating of the ith factor.

  • G is the groundwater occurrence.

  • A is the aquifer hydraulic conductivity.

  • L is the level of the groundwater level above sea level.

  • D is the distance from shoreline.

  • I is the impact of existing status of seawater intrusion in the area.

  • T is thickness of the mapped aquifer.

3.1 Calculating the Weight of GALDIT Parameters

The weight of each GALDIT parameter was calculated using AHP method that was proposed by Thomas Saaty (1980) on the basis of pairwise relative evaluation of both the criteria and the decision-maker’s experience. The process of AHP method and its application to calculate GALDIT parameter for Ninh Thuan coastal aquifers are described as follows.

The criteria weight vector is calculated based on the pairwise comparison matrix A, the m x m real matrix with aij entry of the matrix A represents the importance with respect to the goal of the ith parameter relative to the jth parameter. In this study, pairwise comparisons were conducted to six GALDIT parameters with effect to seawater intrusion based on survey results of 56 hydrogeological scientists and experts in Vietnam (the pairwise comparison results was presented in Table 45.1), and accordingly the A matrix for six GALDIT parameters with effect to seawater intrusion was built, as presented in Table 45.2.

Table 45.1 Pairwise comparison results for GALDIT parameters of Ninh Thuan coastal aquifer
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Table 45.2 Pairwise comparison matrix for GALDIT parameters of Ninh Thuan coastal aquifer
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A normalized pairwise comparison matrix, Anorm, was constructed with each entry bjk a result of dividing each element in every column by the sum of that column as presented in eq. (45.2), and based on Anorm matrix, the weight of each parameter was calculated as Eq. (45.3):

$$ {b}_{jk}=\frac{a_{ij}}{\sum_{j=1}^m{a}_{ij}} $$
(45.2)
$$ {w}_i=\sum \limits_{j=1}^m{b}_{jk} $$
(45.3)

where: aij is the entry of matrix A; bjk is the entry of matrix Anorm. wi is weight of ith parameter; m is number of comparison parameter

The Anorm matrix and the weight of GALDIT parameters for Ninh Thuan coastal aquifer were calculated and presented in Table 45.3.

Table 45.3 The normalized pairwise comparison (Anorm) matrix and the calculated weight of GALDIT parameters for Ninh Thuan coastal aquifer
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To check the consistency of the calculated weight, the consistent index (CI) and RI (random index) ratio was calculated; If CI/RI < 0.1, the result is reliable. The CI is calculated by eq. (45.4) and RI values for small problems (m ≤ 10) are shown in Table 45.4.

$$ CI=\frac{x-m}{m-1} $$
(45.4)
Table 45.4 Values of the random index (RI) for small problems (Thomas Saaty, 1980)
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Where: x equal to average value of the weighted sum value divided to the calculated weight.

m is number of compared parameter.

The weighted sum value is sum of each value in the row of a multiplied weight matrix that is constructed by taking multiple of each calculated weight to the A matrix column.

The calculation of consistency for GALDIT parameter weight of Ninh Thuan aquifer is presented in Table 45.5.

Table 45.5 Multiplied weight matrix and consistency for the weights of GALDIT parameters
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As the CI = 0.079 < 0.1, the calculation of GALDIT parameter weight is reasonably consistent so that the result can be used to process calculation of GALDIT index for this study.

3.2 Vulnerability Map of Ninh Thuan Coastal Area

The indicator variables of ratings for GALDIT parameters were slightly modified based on the questionnaire surveys and summary with calculated weights as presented in Table 45.6.

Table 45.6 Calculated weights and ratings for GALDIT parameters
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Vulnerability map was constructed based on GALDIT index that was calculated using new GALDIT factor weight, and the data gathered from previous study (type of aquifer, hydraulic conductivity, height of groundwater level above mean sea level, distance from shore, thickness of the aquifer) and this study data (status of seawater intrusion) are the basis to assign the ratings of each GALDIT factor. The final result showed that the vulnerability of Ninh Thuan unconsolidated aquifers can be classified into three levels: the high vulnerability, medium vulnerability, and low vulnerability and the distribution is around 40%, 50 and 10% to the high, medium and low vulnerability, respectively. The high vulnerability was found in the central coastline and extends landward along with Cai river where the most concentrated population and economic activities are seen as shown in Fig. 45.3.

Fig. 45.3
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Vulnerability map of Ninh Thuan coastal aquifers

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4 Conclusions

Saltwater intrusion has always been a serious concern for coastal areas. Therefore, the need of mapping groundwater vulnerability to saltwater intrusion is important in order to plan and manage this precious resource. Six factors in GALDIT method incorporated allow us to assess, numerically rank and map areas from low to high vulnerability. For more precise vulnerability map, more data is required. For specific water use such as irrigation, industry etc. important rating is required to modify to suit this purpose.