ارزیابی آسیب‌پذیری آبخوان‌های چندگانه با استفاده از روش‌های DRASTIC، SINTACS

 
Evaluation of multiple aquifer vulnerability using of DRASTIC, SINTACS frameworks
 
 
1. Introduction
Nowadays, the issue of freshwater supply is the main concern of researchers in the arid and semi-arid countries like Iran. Available water resources are necessary to be protected against pollution due to arid climate and frequent droughts and lack of water resources. One of the ways to protect groundwater against pollutants is to identify the areas with high potential of pollution. The quality of groundwater and surface water resources of Varzeqan plain can also be affected by the wastewater from the Sungun Copper Mine, which is impounding in tailing dam the north of Varzeqan plain. The groundwater flow direction is from west to east which thought to coincide with the surface water flow direction. The theory of groundwater vulnerability was first introduced in the 1960s in France to create an alertness of groundwater contamination. Several studies were carried out in different parts of the world to assess the pollution and vulnerability of groundwater. Many approaches have been developed to evaluate aquifer vulnerability. They include process-based methods, statistical methods, and overlay and index methods. One of the most common frameworks for assessing the vulnerability is the DRASTIC and SINTACS. The DRASTIC framework was developed by the National Groundwater Association (NGWA) in collaboration with the United States Environmental Protection Agency (USEPA). The SINTACS method used in this study was developed to evaluate relative groundwater pollution vulnerability using seven hydrogeologic parameters. In this paper, to assessment aquifer intrinsic vulnerability, a point count system framework called SINTACS and DRASTIC was selected, Also, in this study, the Corrected SINTACS framework has been introduced for the first time in assessing the vulnerability of confined aquifer.
 
2. Materials & Methods
Hydrogeological, geological characteristics of the study area
The results of geophysical study and exploratory wells, piezometer and observation logs prepare evidence to identify two types of aquifers: alluvium aquifer including unconfined (with approx. 150 km²) and confined (with approx. 57 km²) aquifers. Confined aquifers surrounding by the unconfined aquifer and hence multiple aquifers and just alluvium aquifer located in the plain. The aquifers within the study area are a complex system and comprise two confined aquifers, surrounded by one unconfined aquifer. Varzeqan alluvium aquifers classified into three main hydraulic conductivity sections (i) High hydraulic conductivity (about 90 m/d) area in the eastern part of the plain composed of coarse-grain size sedimentary; and (ii) Moderate hydraulic conductivity (about 25 m/d) area in the central part of the plain composed of sand, gravel, and silt grains; and (iii) Low hydraulic conductivity (about 8 m/d) area in the western part of the plain composed of  silt, clay and slightly sand.
 
DRASTIC framework
 
DRASTIC is an acronym of seven hydrogeological parameters comprises depth of groundwater (D), net recharge (R), aquifer media (A), soil media (S), topography (T), impact of vadose zone (I) and hydraulic conductivity (C). Each of these parameters is assigned a rate and a weight according to their relative importance. To prepare a vulnerability map, the required data of various parameters are given to ArcGIS software package in order to perform GIS processing, then, the vulnerability map is prepared. The overall DRASTIC vulnerability index (DVI) is calculated using Eq. 1. As follows,
 
DVI = D_wD_r + R_wR_r+ A_wA_r + S_wS_r + T_wT_r + I_wI_r + C_wC_r                                                                     (1)
 
Where D, R, A, S, T, I, and C represent the DRASTIC parameters, and the r and w subscripts correspond to the rates and weights respectively.
 
SINTACS framework
 
SINTACS is an updated model of DRASTIC frameworks and includes seven parameters such as water table depth (S), net recharge (I), unsaturated zone (vadose zone) (N), soil media (T), aquifer media (A), hydraulic conductivity (C) and topographic slope (S).The SINTACS is similar in type and number of parameters to the DRASTIC model, but the rates and weights of this framework are different from the DRASTIC. Vulnerability index is calculated using Eq. 2.
 
I_SINTACS= ∑⁷ _i=1 P_i*W_i                                                                                                                (2)
 
Where I_SINTACS is the SINTACS vulnerability index, P_i is the rating of each parameter and W_i is the relative weight of SINTACS parameters.
 
Corrected SINTACS framework
 
In this study, Corrected SINTACS framework is presented for the first time to assess the vulnerability of confined aquifer that mechanism of this framework is derived from DRASTIC in the evaluation of confined aquifer and how to rate and assign the weight of the proposed framework, such as SINTACS in unconfined aquifer. DRASTIC and SINTACS differ from Corrected SINTACS for confined aquifer in the process of preparing the layers and the difference in rating for this framework.
 
3. Discussion of Results
 
Results of DRASTIC, SINTACS and Corrected SINTACS frameworks
 
The objectives formed for the present study involve carrying out intrinsic vulnerability assessment using DRASTIC and SINTACS frameworks to priorities area according to their vulnerability to contamination. After preparing the raster layers of the frameworks, these layers were combined using the Raster Calculator command in the ArcGIS software and final vulnerability map was obtained. The vulnerability index based on DRASTIC in unconfined aquifer, varied in the range of 91 to 160 and for confined aquifer of 48 to 93, That's unconfined aquifer 33, 59, 8% of the study area is located in areas with low, medium and high vulnerability, respectively and for confined aquifer, 26%, 53%, 21%, respectively, are located in areas with low, medium and high vulnerability. The results of the evaluation with SINTACS framework for unconfined aquifer showed a number between 123-188, based on this, 12, 53 and 35% of the plain are located in areas with low, medium and high vulnerability, respectively. In assessing the confined aquifer using corrected SINTACS, the vulnerability index range was between 80-133.
 
Validation
 
In order to find out more accurately and compare the frameworks in this study, the correlation coefficient between vulnerability maps and nitrate data was calculated. Nitrate concentration data were used to validate the frameworks used in the study area separately for both types of aquifers, that is for unconfined aquifer, the results show a relative correlation with the value of R² equal to 0.59 and 0.24 for DRASTIC and SINTACS, respectively. The value of the correlation coefficient in confined aquifer, for Corrected SINTACS and DRASTIC is 0.55 and 0.3, respectively. The CI between vulnerability maps and nitrate concentration were used for validation of the frameworks. The results show that DRASTIC and Corrected SINTACS have higher CI for the unconfined and confined aquifer, respectively. To more accurately assess and select the best vulnerability assessment framework for both types of aquifer, the (CI) method was used.
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4. Conclusions
 
The purpose of this study was to determine the intrinsic vulnerability in multiple aquifer of Varzeqan plain using DRASTIC and SINTACS frameworks and finally, to compare the efficiency between them. In this study, simultaneous assessment of the intrinsic vulnerability of two aquifers confined and unconfined of Varzeqan plain and the determination of areas with contamination potential based on the hydrogeological conditions of the study area has been investigated. In this study, Corrected SINTACS has been proposed to assess the vulnerability of confined aquifer. Varzeqan multiple aquifer vulnerability index indicates that this aquifer is classified the capability of the contamination potential is in the medium to high range that in the unconfined aquifer due to the low water table depth, lower thickness of the unsaturated region and high hydraulic conductivity and for the confined aquifer due to the thickness of the impermeable layer. To assess the ability of frameworks in both aquifer, in fact, (R²) and (CI) were used to validate the frameworks.
The results showed that DRASTIC with higher correlation coefficient (R²) and correlation index (CI) values than SINTACS values is the better framework to vulnerability assessment of unconfined aquifer in Varzeqan plain. However, Corrected SINTACS have higher performance than DRASTIC for confined aquifer. In determining the vulnerable points of the confined aquifer, the most important factor is the thickness of the impermeable layer (clay and silt), whatever this layer is thicker, the possibility of contaminant transfer is less and as a result, the vulnerability index values will decrease.