Assessment of the exposure level to saltwater intrusion in the context of climate change at Dong Nai province to 2030

The study aimed to assess the exposure level (E) to saltwater intrusion (SI) in the context of climate change (CC) at Dong Nai province to 2030. The results serve to assess vulnerability due to this phenomenon. The research scope included 57 wards in Bien Hoa city, Long Thanh, and Nhon Trach districts where have been facing to SI. Results showed that: (i) The high exposure area (E ≥ 75): near Long Tau, Dong Tranh, Go Gia rivers and Thi Vai river downstream; (ii) The medium – high exposure area (50 ≤ E < 75): Thi Vai river upstream, Nha Be, and Long Tau rivers; (iii) The medium – low exposure area (25 < E < 50): Dong Nai river above the confluence with Saigon river about 10 km and the area between Dong Mon and Thi Vai rivers; (iv) The low exposure area (E ≤ 25): the entire of Bien Hoa city, a part of Long Thanh district located near Buong river, part 4 of Dong Nai river, and Thi Vai river upstream.The exposure level tends to increase over time (2020, 2030) and under CC scenarios. The differences between the current E and that in 2030 are relatively small: 8.6, 1.96, and 2.71 in Bien Hoa, Long Thanh, and Nhon Trach, respectively. Thus, effects of climate change and sea level rise to the exposure level to SI are not really clear in the period 2014–2030. However, the increase in exposure index partly reflects the challenges for local governments and communities in response to SI and CC.


INTRODUCTION
In the context of climate change (CC) is increasingly going strong, rivers' flow have been greatly influenced, indirectly affecting the saltwater intrusion (SI), thereby, altering water quality, affecting most of relevant activities in the riparian areas. Recently, many studies on CC consider SI as one of the most concern issues, especially in the estuaries and coastal areas [1][2][3][4][5] Dong Nai province is in Dong Nai -Saigon River basin, despite the landlocked area (the nearest point from the sea about 9 km), but with the characteristics of water reserve distribution (approximately 20 % in the dry season ) and a semi-diurnal tide, rivers here are still at the high risk of SI. In recent years (monitoring data from 2007 to 2014), the salinization at Dong Nai province was negatively changed, salinity had increased significantly, especially from March to May. Thus, the SI at Dong Nai province is increasingly serious and should be concerned. Many studies on SI were carried out in downstream Dong Nai River [6][7][8][9], however, mostly focused on assessing salinization current, simulating and warning, etc. In other words, the vulnerability to SI in the context of CC have not been forecasted and fully assessed yet.
It is very important to assess completely vulnerability to SI in the context of CC of different regions or sectors -to be considered in the relationship among the level of exposure (E), sensitivity (S) and adaptive capacity (AC). That provides a basis for planning suitable policies, strategies, and coping measures in each specific condition, contributes to minimize risks, and ensures the sustainable development of concerned socio-economic sectors. Accordingly, this study aims to evaluate the exposure level to SI in the context of CC at Dong Nai till 2030 by the index method, detailed to communes /wards in the investigated areas (Bien Hoa city, Long Thanh and Nhon Trach districts), serving the vulnerability assessment to SI in the context of CC.

Index method
Index method was applied to quantify the exposure level of the SI with criteria such as highest salinity, salinity fluctuation, salinity duration, etc. E index was calculated by the formula: = ∑ ( * ) −1 (1), where: Ei: the value of standardized Ei; wi: priority weight of respective Ei The exposure levels to SI in the context of CC were devided into 4 levels ( Table 1) as a basis for comparison and evaluation of E among considered communes/wards.

Data process method
This method was used to analyse salinity simulation results, and then process concerned criteria: the highest salinity, the lowest salinity, salinity fluctuation, salinity duration, etc.

Spatial interpolation method
From the calculated results of salinity, this method was applied to calculate exposure levels to SI for different investigated areas. Inverse Distance to a Power gridding method and Surfer 10.0 were chosen. Fixed-search radius of 30-60 km was applied. The barrier was the boundary of river basins.

Professional adjustment
30 experts in the field of environment, hydrology, water resources, etc. from 17 prestigious universities and instituteswho have knowledge and experience on CC, SI, and investigated area were consulted to determine indicators and respective weights representing exposure levels to SI at Dong Nai province. Consistency is assured when CR ≤ 10 %. Results of professional adjustment was synthesized by multiplication average method.

Standardized method
The variables have different units and proportions, accordingly, it should be standardized (from 0 -100) before calculating. In this study, based on the interpolation results (raster), the standardization was carried out by ArcGIS 10.2 using the tool Raster Calculator.

GIS method
This method was used to exploit the data, perform calculations (standardize and calculate synthetic index), mapping, show data after being processed in order to visualize the calculation results. ArcGIS 10.2 and Map Info 11.0 were used.

Inherited method
The study inherits simulation results of SI in the context of CC at Dong Nai province [10]. Accordingly, the exposure indicators were statistically analysed include: The highest salinity of a year: the highest salinity at each calculated node.
Salinity fluctuation of the saltiest month: was calculated by the difference between the highest and the lowest salinity in April.
Salinity duration (above 1 and 4 ‰): rate of time that salinity was above 1 and 4 ‰ in the dry season (from 05 th Jan to 30 th May) with a total simulation time of 3480 hours.
The output nodes of calculation results used to calculate exposure index are shown in Fig. 1

Indicators and weights
By professional adjustment method and AHP, indicators for assessing exposure levels to SI in the context of CC were established as in Table 2. Trang 258

E index to SI in the context of CC
Component exposure index (E.dm, E.dd and E.tg) and total exposure index (E) were calculated for different scopes (district/city and ward/commune), periods (current, 2020, and 2030), and CC scenarios (low -B1, medium -B2, and high -A1FI emission scenarios).

E index to salinity (E.dm)
Calculating results, given in Fig. 2, showed that: currently, communes/wards significantly exposed to salinity include Phuoc Thai (Long Thanh district), Phuoc An, Long Tho, Phuoc Khanh, Vinh Thanh (Nhon Trach district), mainly located in Thi Vai, Dong Tranh, and Go Gia riversides. Phuoc An (Nhon Trach district) has the highest exposure level to salinity (92.07) while that in Cam Duong (Long Thanh district) is the lowest (0.1). In the period of 2020-2030, corresponding to CC scenarios, the highest exposure level to salinity tends to gradually increase, but not significantly. Fig. 3 showed E index to salinity fluctuation (the saltiest month). It could be inferred that currently, 2/57 wards had high level of E.dd: Phu Huu (80.46) and Phuoc Khanh (83.56) (Nhon Trach district), located in Long Tau and Dong Tranh river areasthe area with great salinity fluctuation. The area with lowest E.dd is Cam Duong (0.23) (Long Thanh district). In the period 2020-2030, the CC scenarios were taken into account, E.dd transformations would be complex in the investigated zone: relatively stable and tend to decrease at Nhon Trach district, rapidly increase at Bien Hoa city (highmedium level in 2030).

E index to salinity duration (E.tg)
E index to salinity duration was shown in Fig. 4. Currently, Bien Hoa city is hardly exposured with this index -consistent with the current situation of SI in this area. E.tg in Long Thanh district is relatively complex (the average E.tg is 23.96 -low level): 2/15 wards with high exposure (near Thi Vai river); 2/15 wards with medium-high exposure; the others with low exposure. At Nhon Trach district, average E.tg is 70.20 (medium-high level): the number of wards with low, medium-low, medium-high, and high exposure are 3/12, 2/12, 3/12, and 4/12, respectively. In the period 2020-2030, E.tg index tends to increase gradually over time and under CC scenarios. However, similar to E.dm index, the increase is relatively small.

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The total exposure index to SI (E) Fig. 5 presents the current exposure index to SI at the investigated area. It could be divided into 4 regions.
The high exposure region (> 75): near Long Tau, Dong Tranh, Go Gia rivers, and Thi Vai river downstream -where have high salinity (the highest one is more than 25 ‰), large salinity fluctuation (15-28 ‰), and long salinity duration (salinity is more than 1 ‰ in the whole calculated time); including Phuoc Khanh, Phuoc An, Vinh Thanh (Nhon Trach district).
The medium-high exposure region (50-75): near Thi Vai, Nha Be, and Long Tau rivers. Although the same exposure level was found (medium-high), each area has its own characteristics: (i) Thi Vai river upstream area: including Long Tho (Nhon Trach district), Long Phuoc, Phuoc Thai, Tan Hiep, Phuoc Binh (Long Thanh district), has relatively high salinity (20-25 ‰) but stable salinity fluctuation (1-3 ‰); (ii) Nha Be river area: including Phu Huu, Phu Dong (Nhon Trach district), has medium-high salinity (the highest one is about 10-15 ‰) and salinity fluctuation (around 7.5-12 ‰). Considering E index in district/city scope, Bien Hoa city (12.07-20.84) and Long Thanh district (23.96-24.82) had low exposure level to SI while medium-high (62.55-65.26) at Nhon Trach district. Figure 6 shows the total E index to SI in the context of CC in 2020 and 2030. Overall, although the level of exposure to SI tends to increase over time and under CC scenarios, the differences are not significant due to relatively short calculation period, effects of CC and sea level rise to SI thereby are not really clear as compared to the current situation. Besides, E index to SI tends to increase over the time (2020, 2030) and under CC scenarios.