Vulnerability Assessment of the Upper-Middle Pleistocene (qp2-3) Aquifer in Ho Chi Minh City, Vietnam Using GIS and Expanded DRASTIC Methods

The groundwater vulnerability assessment model (DRASTIC) for zoning sensitivity based on natural and objective factors of aquifers is currently used and applied widely. In this study, we have expanded the weights in the DRASTIC index through the Entropy weight-based technique and visualized them in association with GIS for assessing the vulnerability of the Upper-Middle Pleistocene aquifer in Ho Chi Minh City, Vietnam. The value of Entropy weights calculated from the collected dataset of 106 monitoring boreholes of the study area. The Entropy-DRASTIC results divided into three categories: 9.45% of the study area was in the high vulnerability zones, low and medium vulnerability zones are 21.5% and 69.05% of the total study area, respectively. The highly vulnerable area, which is shallow aquifer roofs and recharged significantly (directly from rains or runoff from the surface flows exposed to contaminants from runoff flows). Low vulnerability areas with clay cover are weak permeability and medium vulnerability area is a sizable transitional zone surrounding the high and low vulnerability. This result suggests that the DRASTIC index model in association with GIS is a useful tool to assist policymakers in formulating solutions for the use of groundwater resources. Importantly, this finding is useful to the local authorities in shaping regulations on the use and exploitation of groundwater resources in suburban areas, where the public water supply network is inadequate, constrained groundwater resource and exposed to contamination.


Introduction
Groundwater is not only affected by intrinsic natural factors of the aquifer, but also vulnerable to extraction activities and changes in surface land use [1].The issues of groundwater quality and vulnerability have drawn much attention from researchers, managers, and planners worldwide [2,3].
Besides the quality partition algebraical methods, the groundwater vulnerability assessed through the DRASTIC method [1].This method was developed by Aller et al. in the 1980s from a standardized system for assessing potential groundwater contamination with the premise that it was implemented by the United States Environmental Protection Agency (US EPA) [4].The purpose of in-depth study on objective factors, clarifying the natural and geological factors affecting the quality of groundwater and estimating the potential vulnerability of the aquifers based on the relative rating of hydrological parameters [5].Currently, the GIS-based DRASTIC index was widely adopted to visually partition the 1247 (2023) 012006 IOP Publishing doi:10.1088/1755-1315/1247/1/012006 2 vulnerability of aquifers, typically in Palestine [2], Jordan [6], India [7][8][9][10], Pakistan [11,12] and Iran [13].
The assessment of groundwater vulnerability through the DRASTIC index with a fixed weight is widely used in many studies but is not completely consistent with the actual natural conditions of different areas.In recent years, many research groups around the world have applied the DRASTIC index with different weights to suit the specific conditions of the study area, such as AHP, Entropy, Fuzzy, and GAs.The entropy approach is one of the weighting techniques used recently to assess the efficiency of the information and the dispersion of the data [14].The DRASTIC index with variable weights according to the Entropy method has been a latest popular application, typically in China [5,15], India [16] and Iran [17].Research have demonstrated the viability and suitability of Entropy weighting in the groundwater vulnerability assessment index.This new approach still pertains to seven DRASTIC parameters, but the weights are changed giving the results are improved and more rigorous.The Entropy weighted sensitive partitioning results explain the effect of hydrogeological conditions, soil structure, and topographical factors which directly relate to the potential risk of groundwater quality deterioration in the study area.
There is a fact that the DRASTIC index method has been applied for assessing groundwater vulnerability in Vietnam [18,19].The vulnerability is related to the migration and dispersion of contaminants into groundwater [20].However, such as vulnerability assessment focuses only on algebra-based calculations DRASTIC in which the weight of each parameter is usually fixed according to the original formula of Aller et al., and does not clearly show the distribution of each component parameter.
Although Ho Chi Minh City already has a widespread clean water delivery system, exploitation continues in businesses and households.The Upper-Middle Pleistocene aquifer in Ho Chi Minh Cityan exploitable groundwater source for various socio-economic development activitieshas been suffered from, many environmental problems due to an on-going increasing rate of urbanization and industrialization.Therefore, the aim of this present study is to zone the vulnerability of aquifer by using the DRASTIC method with a suitable set of Entropy weights, eflect the characteristics of the geological and hydrogeological characteristics of the area combining GIS-based platform to visualize the area.The findings of this study provide complete information on the vulnerability of groundwater quality in the study area, indicate the specific parameters in area that affect the index results.Facilitating the planning of appropriate areas and assisting decision-makers in developing a sustainable water resources management plan.

Study area
Ho Chi Minh City, which lies between 10° 10' and 10° 38' North latitude and 106° 22' and 106° 54' East longitude, is a city that is directly under the control of the central government.This area, which has a total size of around 2,095.2 square kilometers, serves as the focal point of Southeast Asia, the locomotive of the southern vital economic zone, the traffic hub connecting the provinces in the region, and the international gateway.
The pore aquifer of the Upper-Middle Pleistocene sediments (qp2-3) in the study area is an important aquifer, widely distributed throughout the inner HCMC with components of Upper-Middle Pleistocene formation (Q1 2-3 ).This is a shallow aquifer with many exposed areas in the Northwestern part (Cu Chi district) and the Eastern part (Thu Duc) and deeply buried in the southwest of the city.The aquifer thickness varies from 2 to 84m, with a depth of up to 120m, and an exploitation flow of up to 47,632,500m 3 /year according to the survey data of Ho Chi Minh City Department of Agriculture and Rural Development.The depth to water table of the aquifer varies on average of 53.40m, recharged by rainfall that seeps directly from the exposed area, surface runoffs, and from the Saigon River.The main petrographic composition of the Upper-Middle Pleistocene aquifer consists of fine to coarse sand, silty sand, mixed with gravel.Although the city's clean water supply system has expanded, it has not yet been able to keep up with demand for drinking, domestic purposes, and other activities (mainly industrial production).Therefore, the current situation of groundwater exploitation in the city is still happening.In addition, agricultural activities using fertilizers and pesticides are still quite common practices in the Northwestern area (Cu Chi district).The burying of urban wastes, groundwater extraction activities, and wastewater discharge also pose negative effects on regional groundwater quality.Data from several years of water quality monitoring by the Department of Natural Resources and Environment of Ho Chi Minh City show that District 12, Cu Chi District, and Go Vap District have significant values of Nitrate content.Therefore, the identification of vulnerable areas of the Upper-Middle Pleistocene aquifer in the study area is an urgent need for groundwater protection in order to ensure water security in the area.

Materials
The study area for this study is the Northwestern part and the inner center of Ho Chi Minh City (HCMC), which lies between 10°38'-11°10' North latitude and 106°22'-106°52' East longitude (except for Nha Be and Can Gio districts).Because these two districts are low-lying areas salt intrusion affected, no available groundwater source for exploitation, and no Upper-Middle Pleistocene aquifer monitoring wells.
A calculated data set was collected from 106 observation boreholes in the study area.The locations of observation wells are shown in Figure 1.The study area was mapped in the UTM 48N region; ArcGIS 10.4.1 software and the Inverse Distance Weight method (IDW) were applied.

DRASTIC method
The weighted entropy product and the evaluation of each parameter are added to determine the DRASTIC index, which is utilized to create the map.The DRASTIC index (Di) is used to create a vulnerability map in GIS according to the following formula:   =   .  +   .  +   .  +   .  +   .  +   .  +   .
Where, D, R, A, S, T, I, C correspond to depth to water table, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity; r and  represent the ratings and weights assigned to each parameter, respectively.In both cases, a high DRASTIC index value indicates a higher risk of contamination.

Entropy weight
According to the research by Wang Wei et al. [15], the Entropy weights are calculated as follows: According to the data, a matrix X based on the dataset of 106 observation wells in Ho Chi Minh City, with m values to be evaluated and n estimated parameters (m = 106; n = 7) can be constructed as follows: Where Data pre-processing is applied to eliminate the effects of different units, characteristic indices and quality categories.
After grading each evaluation index, the eigenvalue matrix of the evaluation index quota is obtained as follows: The proportion of value of the j th evaluation sample in i indicators: The information Entropy of the i th indicator is defined as follows: If   is all the same value, then the entropy value of each criterion is the maximum value (  = 1).If   is all 0, then   .ln (  ) is also 0 in value [21].
The weight of the Entropy of the i th indicator can be formulated as follows:

Entropy-weighted DRASTIC
The DRASTIC with Entropy weight is calculated according to the following formula: The spatial distribution maps of each component parameter are visualized by using Inverse Distance Weight (IDW) on the GIS platform for the purpose of constructing the best approximation surface to the existing data.The groundwater vulnerability zoning map is overlay using the results of the spatial distribution of each component parameter performed as the following formula [22]: Where, MDi is the total layers of component parameters.2).The ratings of the parameters are based on the geological dataset, petrographic units, and aquifer units instead of subjectively specified ratings [5].The groundwater vulnerability map of the Upper-Middle Pleistocene aquifer were based on local hydrogeological and geological characteristics in the Ho Chi Minh City.Seven characteristics are used to determine the vulnerability of groundwater in Ho Chi Minh City, including: Depth to water table (D), Net Recharge to the aquifer (R), Aquifer media (A), Soil media (S), Topography (T), Impact of vadose zone (I) and Hydraulic Conductivity (C).The DRASTIC component parameter layers are established on the basis of regional geological and hydrogeological parameters in the raster grid according to Aller's hierarchy [4].In ArcGIS software with a resolution of 30x30 m, point values are assigned from 1 to 10 depending on how it affects the ability to allow the contaminant to move into the aquifer.

Depth to water table
One of the crucial parameters defining the vulnerability of groundwater is the depth to water table (D), which in absolute value from the ground to the groundwater level and measures the distance that contaminants must travel before it reaches the groundwater level [4].Data on aquifer roof depth at wells is compiled from hydrogeological documents (Division for Water Resources Planning and Investigation for The South of Viet Nam) [25].The depth to water table in the study area is unevenly distributed (ranges from 0.5 to more than 80 m) from the Northwest to the South of the city (Figure 3a).The water table is divided into 7 levels with ratings from 1 to 10 (Table 2).The shallow aquifer with many exposed areas in the Northwestern part (Cu Chi district) and the Eastern part (Thu Duc), located from 0.5 to 30m below the surface (more than 16% of the entire area).The area with the depth to water table of the aquifer greater than 30m, distributed in the rest of the study area, occupying about 84% of the area.

Net Recharge
The annual amount of water that penetrates into the aquifer after infiltrating the soil surface is called the recharge (R) [11].The recharge value of aquifer is related to rainfall, surface runoff, and depth to water table of aquifer.The factors affecting the the replenishment of aquifers ground slope, rainfall, and soil permeability [23].In which the soil permeability is determined based on the soil map at 1:50,000 scale, the annual average rainfall provided by Southern Regional Hydrometeorological Center, and the terrain gradient obtained from the Digital Elevation Model (DEM) with a pixel value of 30m.According to Table 2, the area's aquifer recharge can be divided into two levels.About 39.44% of the study area has replenishment > 254mm is distributed in Cu Chi district in the Northwest and Thu Duc district in the East of the city.The remaining areas with a recharge of 51 mm accounted for 60.56% of the study area (Figure 3b).

Aquifer media
The aquifer media describes water-containing environment in the fractures or crevices of the rock.The alteration in the petrographic composition controls the rate of contaminant contact from the surface with the aquifer water which was assessed and assigned ratings according to Table 2.The vulnerability of groundwater is directly correlated with the size of the particle, the number of fractures or crevices, permeability, and the ability to control contaminants [24].This parameter is based on the report of engineering geology, and hydrogeological maps of the city at a scale of 1:50,000 [25] to create a vector map of the data in the study area.Then converted from vector to raster format in the GIS environment.The Upper-Middle Pleistocene aquifer is mostly made of sand, ranked as grains from fine to coarse, from 6 to 9, respectively.The study area includes four main assemblages of the aquifer composition, including fine-medium sand accounted for 4.63%, fine-coarse sand accounted for 7.53%, Fine to medium sand containing gravel and Medium to gravel-contained coarse sand accounted for 43.92% and 43.92% of the study area, respectively (Figure 3c).

Soil media
Soil media (S) controls on the migration of contaminants into aquifers [11].The thickness, texture, and structure of soil affect the rate that water infiltrates the soil's surface and how long it takes for contaminants to penetrate the soil.In this study, the soil map (1:50,000 scale) was prepared by Ho Chi Minh City Department of Agriculture and Rural Development provided has been used.The majority of the soils in the study area are clay and fine-grained soils, with rate of cover soil ranging from 1 to 6 according to Table 2 (Figure 3d).Highly permeable soils spreading to the North and center of Ho Chi Minh City, including sandy loam, loam, and clay loam at rates of 11.81%, 33.88%, and 10.05% of the area, respectively.Approximately 44.25% of the study area is covered by clay, which has structurally dominant and less permeable soils indicate a decreased ability of contaminants to infiltrate the surface layer.

Topography
The slope of the soil surface controls the ability of the contamination to be transported by runoff or to remain at the surface [4].In places with steep slopes, they are less affected by the high amount of overflow water and the small amount of seepage.Otherwise, the locations with low slopes tend to slow runoff, water will be kept on the surface for a longer period of time enough for pollutants seep into the aquifer.In this study, the terrain gradient map of the study area is generated through global Digital Elevation Model with a 30 m spatial resolution on a GIS platform.The study area is located in the transition zone between the Southeast region and the Mekong Delta, with a gentle slope, gradually decreasing from North to South and from East to West, classified into 5 different grades according to Table 2.The high proportion of flat areas in the study area with slope < 6% (about 81% of the study area).The areas with slope greater than 6% accounts for 19% of the area (Figure 3e).

Impact of Vadose zone
The vadose zone is the region between the soil cover and the groundwater level that is unsaturated or intermittently saturated [24].Vadose zone affects pollutant exposure similarly to aquifer composition, depending on the zone's permeability and the degradation characteristics of the environment [1].The highly permeability, resulting in a high degree of groundwater vulnerability.Using hydrogeological and and petrographic data from 106 wells in the study area [25], the unsaturated zone map was created in the GIS environment.The unsaturated zone of the permeation zone is mapped as shown in Figure 3f of age-river, river-sea sediments with regional geological units consisting of sand, gravel, and clay are ranked according to Table 1.The environment of the unsaturated zone presents around 75% of the study area with sand and gravel layer with considerable amount of silt and clay.Clay or silty clay layer occupies 19% of the study area, mostly in the East of Cu Chi district, close to the Saigon River.The remaining area is sand and gravel (about 6%), which is spread across the districts of Thu Duc, Cu Chi, and Binh Chanh.

Hydraulic Conductivity
The aquifer's composition is used to describe the permeability coefficient [1], controls the movement of contaminants into aquifers.As the permeability coefficient increases, the rate at which contaminants are transported into groundwater increases and vice versa [24].By using the IDW approach to interpolate data from 106 wells provided by Division for Water Resources Planning and Investigation for The South of Viet Nam [25], the hydraulic conductivity map was created.The permeability values of the Upper-Middle Pleistocene aquifer in the study area range from 4.43 to 16.57m/day, which is divided into two different grades according to Table 1.The majority of the study area has permeability coefficients between 12 and 27 m/day (92.08% of the area), and only 7.92% of the study area has permeability coefficients between 4 and 12 m/day (Figure 3g).

DRASTIC
The vulnerability map of the Upper-Middle Pleistocene aquifer in the study area (Figure 4a) is classified into three levels: low (100), medium (100-140), and high vulnerability (> 140).The periphery of the Northwest of the city (Cu Chi district) shown high vulnerability accounts for 0.41% of the area.The medium vulnerability zone accounts for 39.52% of the area, concentrated in the Cu Chi district and a part of the East of the city.On the other hand, low vulnerability was found in most of the central and Southern parts of the study area, accounting for 60.07% of the area.

Entropy-weighted DRASTIC
Vulnerability zoning values of the Upper-Middle Pleistocene aquifer in the study area is integrated with Entropy weights and ranting values of seven component data layers in GIS (Figure 4b).The DRASTIC index of the aquifer ranges from 2.55 to 7.16, and divided into three degrees of vulnerability: low, medium, and high levels.The areas in the Northwest and the East of the city show high vulnerability (6.001-8).The Western and the Southern parts of the city are low vulnerability areas (2-4), the central area and the remaining parts of the city are medium vulnerability (4.001-6).Generally, the weights of geological and hydrogeological features are typically fixed and used regardless of the unique characteristics of the study area for the classical DRATIC method.Therefore, after improving and expanding the weight of each component parameter in the DRASTIC index through the Entropy method.The result is more suitable for the regional characteristics based on the variability of each value in the dataset to replace classical weights.

Conclusion
The Upper-Middle Pleistocene aquifer in the Northwestern and central areas of Ho Chi Minh City (except Nha Be and Can Gio districts) is comprehensively evaluated by using the DRASTIC method and expanded DRASTIC in combination with GIS-based platform to visually partition the vulnerability of groundwater to locate latent sensitive areas.The Entropy-DRASTIC results are more appropriate and better reflect the local conditions and document conditions with the borehole distribution density.This weighting method reflects the better understanding the influence of depth to water table, rate of net recharge, and the composition of surface cover on the vulnerability of the local groundwater sources.
Both the DRASTIC and Entropy-DRASTIC methods have the same objective of identifying the most vulnerable areas.Zoning maps all show three levels of an aquifer's vulnerability to contaminants: low, medium, and high.According to Entropy-DRASTIC results show that 9.45% of the study area in the high vulnerability zone, the low and medium aquifer vulnerability is 21.50% and 69.05%, respectively.While the classic DRASTIC index classified into three (03) levels of low, medium, and high vulnerability of 0.41%, 39.52%, and 60.07% of the study area, respectively.The results show that Cu Chi and Thu Duc districts have high vulnerability index.The reason is that this area is characterized by a shallow depth to water table of aquifer, the predominant alluvial soil surface, gravelly sand, and the significant quantity of recharge.This allows contaminants to migrate into the aquifers.
The research results provide managers with a better understanding of groundwater quality in the Upper-Middle Pleistocene aquifers in Ho Chi Minh City.From there, the priority for building up a sustainable strategies for pollution management and aquifer protection.In addition to the results, further studies are needed in high vulnerability areas such as Cu Chi and Thu Duc districts, where dominated by agro-chemical applications.

Figure 1 .
Figure 1.Location of the study area and observation wells 2.3.Methods

Figure 2 .
Figure 2. Methodology for groundwater vulnerability assessment 3. Results and discussion 3.1.The weight Based on the data set of 106 observed boreholes in the study area, which have calculated the Entropy weighted set of the DRASTIC component parameters.The results of the Entropy weight clearly demonstrating their importance and significance in the study area (Table2).The ratings of the parameters are based on the geological dataset, petrographic units, and aquifer units instead of subjectively specified ratings[5].

Figure 3 .
Figure 3. Spatial distribution maps of seven parameters of the DRASTIC Index (Continued) (a) Depth to water table; (b) Net recharge; (c) Aquifer media; (d) Soil media; (e) Topography/ Slope; (f) Impact of vadose zone; (g) Hydraulic conductivity.3.3.Vulnerability Zoning with DRASTIC Component parameter layers are created by GIS interpolation that represents the spatial distribution density of each parameter, shown in Figures 3a to 3g.The ArcGIS geographic database was used to perform groundwater vulnerability by using the DRASTIC index with traditionally weights and expanded weight through the Entropy method.
. The ArcGIS geographic database was used to perform groundwater vulnerability by using the DRASTIC index with traditionally weights and expanded weight through the Entropy method.

Figure 4 .
Figure 4. DRASTIC index map (a) DRASTIC index map; (b) Entropy-DRASTIC index mapAccording to the vulnerability map using Entropy-DRASTIC approach, about 121,905 km 2 (9.45% of the area) of the city is classified as a high vulnerability zone, with DRASTIC value > 6. Concentrated in the Northwestern area (Cu Chi district) and the Eastern area of the city, coinciding with high permeability zones (surface lithology is mainly alluvial soil, gravelly sand), shallow water table of aquifer (1.5 -4.5m), high recharge and gentle slope.These factors accelerate contaminant transport and increase the risk of aquifer contamination.Low vulnerability account for 21.50% of the area (277.35km 2 ) with DRASTIC index < 4, including the Southwest and Southern parts of the city.This can be explained by the coating's impermeability, the buried aquifer depth and low recharge ability, but it is rarely impacted in shallow aquifers.The area with the medium vulnerability has the widest distribution, covering 69.05% of the area (890.745km 2 ), with DRASTIC values between 4.001 and 6, primarily composed of clay loam and loam in the central and Northwest parts of the city.

Table 1 .
.+   .+.+.+.+.+.(7)Aftercalculating the DRASTIC index with Entropy weight results, use the table to determine the DRASTIC value corresponding to the level of groundwater vulnerability for comparison and evaluation, specifically as the Table1below.Classification standards of groundwater vulnerability according to DRASTIC.

Table 2 .
Rating and weights of the parameters in the DRASTIC.