**2. Materials and methodology**

The aquifer mapping study under NAQUIM was taken up by Central Ground Water Board (CGWB) in 3 phases and completed in March 2022. One of the important aspects of aquifer mapping is data gap analysis because it determines the optimum data, which is required to be generated after considering the existing available data as per the aquifer mapping protocols. Without the availability of sufficient data, a true picture of the area cannot be generated [2]. The available data of the exploratory wells, groundwater level monitoring stations, micro-level hydrogeological data acquisition, and groundwater quality monitoring stations of CGWB were compiled and analyzed for adequacy. In addition to these, the data on groundwater monitoring stations and groundwater quality stations of the groundwater survey and development agency (GSDA) was also utilized [3]. After taking into consideration the available data on groundwater exploration, geophysical survey, monitoring, and quality, the data adequacy has been worked out considering the heterogeneity of aquifers in the area.

The thematic layers of geomorphology, soil, drainage, and land use-land cover were not available, hence considered as a data gap. The data gap analysis was done for major parameters namely, exploratory drilling data, micro-level hydrogeological data (acquisition from the key well inventory/establishment), groundwater monitoring data, and groundwater quality data. The data gap is arrived at by considering the requirement and the existence in the grid pattern whose details are as follows -


Considering the data gaps, feasibility and availability of locations, additional data on exploratory wells, micro-level hydrogeological well inventory, water level monitoring, and groundwater quality was generated (**Table 1**). The drilling data of 61 wells comprising exploratory wells, observation wells, and piezometer was used to prepare the subsurface lithology, hydrogeological cross sections, fence diagram, and aquifer maps. The water level data of 215 monitoring wells of CGWB and GSDA of Aquifer-I, and 55 wells of Aquifer-II of CGWB was utilized to depict the aquifer-wise water level


**Table 1.** *Data gap analysis.* scenarios during pre-monsoon and post-monsoon seasons. The long-term water level data for the period 2012–2021 was available for 185 monitoring wells out of 215 wells, and it was analyzed to depict the water level trends for Aquifer-I. The groundwater samples collected primarily during the pre-monsoon season from 128 dug wells, and 35 borewells were analyzed for physicochemical constituents *viz*., turbidity, pH, electrical conductivity, total dissolved solids, alkalinity, total hardness, calcium, magnesium, chloride, sulphate, nitrate, fluoride, and iron. The results of the analyses were used to determine the aquifer-wise groundwater quality of the area. The micro-level hydrogeological well inventory was done at 148 locations to have detailed information on the subsurface strata, water levels, aquifer type, and yields, etc.

Based on the existing and additional data generated, various spatial maps on water level, quality, and subsurface lithological sections, which included 2-D, 3-D, and fence diagrams, were prepared. This was followed by the preparation of aquifer maps on a 1:50,000 scale and the formulation of implementable management plans [2].
