عنوان مقاله [English]
The increase in human activities in recent decades, the increase in population, the development of agriculture and industries has caused a sharp increase in water consumption and, as a result, the quantitative and qualitative reduction of underground water resources and created a vulnerable environment, and exposed underground water as a natural resource to industrial and agricultural pollutants. At present, a significant part of the country's water consumption is provided by underground water sources, which are mainly open aquifers and have higher vulnerability. Assessing the vulnerability and pollution of the aquifer is necessary for the management, development and allocation of land use, how to monitor the quality, prevention and protection of groundwater pollution. During the vulnerability assessment process, attention is paid to transfer and flow models in saturated and unsaturated areas, and the effect of physical processes of water movement and the path related to pollution transfer, feeding and penetration depth in pollution transfer are simulated and the distribution of sensitive or vulnerable areas is evaluated.
The area studied in this research, Mashhad plain aquifer, is quantitatively in a critical prohibited situation, and qualitatively, it has faced serious challenges. Therefore, the general purpose of this research is to identify and analyze the qualitative vulnerability of the Mashhad plain aquifer, which causes the potential of vulnerability to pollution to be determined more accurately.
An open aquifer is spread throughout the Mashhad plain, which is not the same in terms of discharge. The Mashhad plain aquifer with an area of 2700 square kilometers is located in the geographical coordinates of 58°29' to 59°56' east longitude and 35°58' to 37°03' north latitude in the catchment area of the Kashafroud river. The main drain of this region is the Kashafroud river, which flows from the northwest to the southeast in the Mashhad plain. Various formations from the first to the Quaternary period can be seen in this area.
In this research, the vulnerability assessment of the aquifer was done with SINTACS and GODS models, and the ArcGIS environment was used to analyze the parameters and prepare the vulnerability map.
The SINTACS model includes seven hydrogeological parameters, which are: (S) groundwater depth, (I) net nutrition, (N) effect of unsaturated zone, (T) soil type, (A) aquifer environment, (C) hydraulic conductivity, (SV) topography (slope), (w) weight and (r) related rank to each of the parameters. A relative weight is assigned to each of the parameters, which indicates the relative impact of each characteristic on the transfer of pollution in groundwater. In this method, the vulnerability index is obtained according to equation 1:
equation (1): SINTACS Index=S_Or S_Ow+I_r I_w+N_r N_w+T_r T_w+A_r A_w+C_r C_w+S_Vr S_Vw ∑_i^7▒〖I_SINTACS=P_i×W_i 〗
In the GODS method, four parameters of: (G) aquifer type, (O) Characteristics of unsaturated zone, (D) depth of underground water and (S) type of surface soil texture are used. The GODS vulnerability index is obtained from the product of the parameters and based on equation 2:
equation (2): I_v=G×O×D×S
Results and Discussion
To prepare the vulnerability map of the aquifer, the layers of each model were combined in the ArcGIS and the final map was obtained. Then the area of each floor was calculated. According to the SINTACS model, the Mashhad plain aquifer is divided into five zones with very low vulnerability (0.44%), low (25.57%), moderate (28.58%), high (2.79%), very high (42.61%). Also, based on the results of the GODS model, the study area is divided into five zones with very low vulnerability (0.93%), low (31.11%), moderate (11.45%), high (1.56%) and very high (54.95%).
The validation results of the vulnerability maps showed that both SINTACS and GODS models are highly accurate in zoning the vulnerability of the Mashhad plain aquifer, so that the correlation coefficient of the vulnerability maps with the qualitative index of TDS is (0.91) in the SINTACS model and (0.85) in the GODS model.
In both models, the largest area of the Mashhad plain aquifer in terms of vulnerability is located in the zone with very high risk, while the smallest area is located in the zone of very low vulnerability and then in the zone of medium vulnerability. In general, the vulnerability of the Mashhad plsin aquifer increases from the southeast to the northwest, and then the vulnerability decreases from the central areas of the range to the extreme northwest.
The reason for this can be attributed to the direction of the underground water flow, which is from east to west, as well as the high water table in these areas. Also, clearly, areas with medium, high and very high vulnerability are compatible with the uses of water agriculture, gardens, population and industrial centers. In terms of the type of sediments in the aquifer environment, sandy and sandy ranges show moderate to high vulnerability due to higher permeability, While there is the lowest level of vulnerability in clay and silty areas. The results of the current research can be used in environmental assessments and analysis of various pollutions and can be used as a basis for management decisions.
Keywords: Vulnerability, Mashhad Plain aquifer, SINTACS, GODS