Abstract: The plain area of Tianjin could be classified into three parts: piedmont plain, central plain, and coastal plain, which are characterized by abundant groundwater resources. With the development of the society and the exploration of groundwater resources by human beings, groundwater in Tianjin plain has become overloaded. In this study, we used an improved DRASTIC model to evaluate groundwater vulnerability in the Tianjin plain area. Seven indices affecting groundwater vulnerability were selected, the respective weights of which were determined using the analytic hierarchy process. The evaluation was conducted based on the spatial analysis functions of GIS software. The results showed that groundwater vulnerability in the plain area of Tianjin is predominantly categorized as medium and high vulnerability, accounting for 64.01% of the evaluated area. The groundwater vulnerability exhibits a spatial pattern of higher values in the south and lower values in the north. Areas with low and relatively low groundwater vulnerability show distinct zonal distribution, accounting for 27.02% of the total area, mainly distributed in the northern part of the plain area of Tianjin. These areas are characterized by generally deep groundwater levels and thick aquifer layers. Higher vulnerability areas accounted for 8.97% of the evaluated area and are mainly distributed in the northeast of Ninghe District, the six districts of the central city, the southwest of Wuqing District, and the southeast of Binhai New Area. Our results indicate that the overall groundwater vulnerability in the plain area of Tianjin is relatively high and significantly influenced by groundwater depth, aquifer thickness, and the lithology of vadose zone. We present an evaluation method suitable for groundwater vulnerability in coastal plain area. By incorporating aquifer thickness and phreatic water salinity index, the assessment of groundwater vulnerability in the Tianjin plain was optimized. Our results provide a scientific basis for ecological management of groundwater and key zoning strategies for groundwater pollution prevention and control in Tianjin plain area.

Key words: DRASTIC, Tianjin, GIS, groundwater vulnerability, analytic hierarchy process