Assessing changes in saltwater intrusion in main rivers of Dong Nai province and the adaptive capacity of the communities

Saltwater intrusion, which is a natural process, is strongly influenced by climate change, and almost negatively affects the lives and livelihoods of communities. This work aims at assessing changes in saltwater intrusion in main rivers located in Dong Nai province. The results indicated that Long Thanh and Nhon Trach districts have been affected by saltwater intrusion with the highest salinity concentrations of about 26-28 ‰. The salinity level during the dry season is higher than that in the rainy one and the salinity boundaries of 1 ‰ and 4 ‰ gradually extend towards inland water. Besides, adaptive capacity (AC) to saltwater intrusion in Bien Hoa city, Long Thanh and Nhon Trach districts was surveyed and assessed by the AC indices. By assessing AC of local government and communities against 19 criteria, Nhon Trach district showed its high AC meanwhile AC of Long Thanh and other districts were moderate and low, respectively. This research also indicates limitations of each locality, an important basis for proposing solutions to improve AC to saltwater intrusion for the communities.


INTRODUCTION
In the context of climate change (CC), rivers are greatly affected by the fluctuations of temperatures, rainfall, sea levels, etc. including saltwater intrusion (SI). SI alters water quality, as the result, most relevant activities, such as agriculture, farming, aquaculture, services, etc. of the coastal area are impacted. There have been many worldwide studies on SI and SI influences. In Vietnam, researches in this field have begun since 1960s by monitoring salinity in the Red river and Mekong river delta [1]. Dong Nai Province, which is in the Dong Nai -Sai Gon river basin, has high density river and high risk of saltwater intrusion due to being affected by semidiurnal tide in combination with the characteristics of water reserve distribution (20 % in the dry season). From 2007 to 2010, the salinity pervading into Dong Nai river increased significantly [2], which will largely influence socio-economic of Dong Nai Province in general and regions affected by SI in particular. Thence, the study aims at accessing the saltwater intrusion process of main rivers of Dong Nai province (2010-2014) and adaptive capacity to saltwater intrusion of communities by adaptive capacity index method, which provides the basis for vulnerability assessment, constructs appropriate solutions for adaptation, and contributes to ensure the local sustainable development

METHODS
Methodological research framework is shown in Fig. 1.

Data collection method
It is used for collecting the relevant data to assess the saltwater intrusion process and adaptive capacity. Salinity data were gathered from the Center of Environmental Monitoring and Technique, Dong Nai province. The socioeconomic and environmental data were collected from the Department of Environment and Natural Resources, Department of Agriculture and Rural Development, People's Committees, etc. Salinity monitoring locations in Dong Nai river and Thi Vai river are presented in Fig. 2.

Sociological investigation method
It was conducted via questionnaires for assessing adaptive capacity to saltwater intrusion of communities, office managers and communalward authorities at Bien Hoa city, Long Thanh and Nhon Trach provinces. The sample size for the communities was based on Yamen's formula (1967 -1986) with 94 % of the reliability. Therefore, the total sample size was 278, which was distributed equally to nine wards. For the office managers, the total sample size was 45 (5 officers/ward, 3 surveyed wards/province). For authorities, all 57 People's Committees were surveyed in the research scope.

Expert method
To improve the adaptive capacity indicators and to determine weight of each criterion, questionnaires were built and 30 experts studying environment, climate change, and saltwater intrusion from many universities, institutes/ research centers were consulted.

Assessing adaptive capacity by indices
Adaptive capacity is a degree to which the system can decrease damage caused by negative effects of saltwater intrusion or can take advantages of opportunities presented by positive effects [4]. The adaptive capacity to saltwater intrusion depends on many different factors determined by adaptive capacity index (AC). Table  1 demonstrates criteria for assessing adaptive capacity to saltwater intrusion, constructed by literature review and expert methods. The proportion of teachers / pupils AC.cq.10

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The criteria have their own scales; hence, it needs normalizing their values from 0 to 100 based on the relationship between the criteria and adaptability [5]. Weights of criteria (group) and component criteria were identified by expert method. Adaptability index was calculated based on the normalized values and the weight of each criterion. Adaptability in each area was assessed by using the scale in Table 2 [6] before being used to establish map of adaptability by GIS.

Salt intrusion process of Dong Nai river and Thi Vai river
It is obvious that tides in accordance with the ability to shift salinity from Tri An lake have significant influenced on the salinity at monitoring stations. In the 3 rd segment of Dong Nai river (SW-DN-8, SW-DN-9, SW-DN-10, SW-DN-11, SW-DN-12, SW-DN-13, SW-DN-14), the salinity is increasing downstream and tend to increase in the last two years (Fig 3). In 2011, that Dong Nai was affected by drought resulted in the declining salinity shifting capability of the Tri An reservoir, so the average salinity was higher than those of other years. The average salinity of the 3 rd segment is under 0.1 ‰, meeting standard of A1-QCVN 08:2008 [7]; therefore, this segment can be used in daily life activities, irrigation, and aquaculture. The 4th segment of Dong Nai river (SW-DN-15, SW-DN-16, SW-DN-17) has higher salinity, average value fluctuated between 0.1 -2.7 ‰, higher than the standard of B1-QCVN 08:2008. It cannot be used in daily life activities and irrigation. Monitoring stations on Thi Vai river (SW-TV1, SW-TV2, SW-TV3, SW-TV4, and SW-TV5) have recorded high salinity, annual average value varied from 5.3 to 26.6 ‰. The salinity was increasing downstream (Fig 4). The salinity decreased in the rainy season but not significantly among monitoring stations (2012-2014). According to standard of B1-QCVN-08:2008, with current salinity, Thi Vai river cannot be used for household water supply purposes and agricultural irrigation, resulted in troubles to local citizens.

Assessing salt intrusion adaptability
Adaptability is accessed based on many factors: quantitative (statistical numbers about officials, income, etc.) and qualitative (such as salt water intrusion awareness in the context of climate change, etc.). Survey results in 3 areas (Bien Hoa city, Long Thanh district, and Nhon Trach district) showed that 76 % of the officials and approximately 64 % of the residents have certain insights of salt intrusion, however, the ratio of those that have full understanding about salt intrusion is very low (<10 %). On the basis of AC indicators (Table 1), the relevant data were collected, normalized to 0-100 scale and presented in Table 3. Applying expert and AHP methods, weights of group criteria and component criteria are identified in relation to the characteristics of the studied area (Table 4).
AC index of each criterion was calculated based on the normalized values (Table 3), component weight of each criterion and group weight ( Table  4). The result is presented in Table 5. Then, salt water intrusion adaptability indices are summarized and presented in Table 6. The AC index of three locals are in medium-low level, in which. The Nhon Trach district has the highest adaptability, followed by Long Thanh district and Bien Hoa city. Each area has its own natural and socioeconomic characteristics; hence, its salt water intrusion adaptability is also different. Nhon Trach has high community awareness of salt intrusion awareness, number and quality of constructions to prevent salinity, and large budget for salinity prevention, etc. Therefore, its adaptability is higher than the rest. Long Thanh has its own strength of high saltwater intrusion community awareness, with programs/plans to prevent salinity, and the diversity of brackish aquatic, etc. Conversely, there still exists some limitations such as poor water storage capacity, limited number of annually dredged canals and salt preventing constructions, low diversity of livelihoods, etc. Bien Hoa has higher number of monitoring stations, livelihoods diversity, and information accessibility than those of Long Thanh and Nhon Trach, though, community awareness of saltwater intrusion is still low. Moreover, being lack of plans/programs to support people in preventing salinity, to build salinity preventing constructions, and to improve small number of salt-tolerant crops and livestock, etc. is its limitation to adapt with the saltwater intrusion.

CONCLUSION
Assessing result of saltwater intrusion process showed the increase in salinity at monitoring stations on Dong Nai river and Thi Vai river in Dong Nai province during the period of 2010-2014. Monitoring stations on Dong Nai river recorded the increase in salinity downstream. Saltwater intrusion process on Thi Vai River was the most serious in the province with the highest salinity level (>30‰, 2012).
Saltwater intrusion AC assessed by indices indicated that Nhon Trach district has the highest adaptability, followed by Long Thanh district and Bien Hoa city. These three investigated locals are in the range of medium-low AC level. Some factors, such as officials' and residential community's low awareness of saltwater intrusion, the poor quantity and quality of salinity preventing constructions as well as weak supporting solutions from the government have resulted in their limited adaptability to saltwater intrusion.