GROUNDWATER BUDGETING OF NARI AND GAJ FORMATIONS AND GROUNDWATER MAPPING SITE SUB-DIVISION, DISTRICT WEST, KARACHI, PAKISTAN

Abstract

The groundwater depletion is an emerging problem worldwide due to changes in climate and an increase in urbanization. Two major water-bearing formations Nari of Oligocene and Gaj of Miocene age were used as study area which was exposed near Karachi, Pakistan. These formations consist of limestone and sandstone with intercalation of thick layers of mudstone. This research work is divided into two parts the first part composed of groundwater budgeting by using a classical equation. The inflow of groundwater in the formations was calculated by thermo-pluviometric data of the study area and water loss of Hub Dam. The potential of evapotranspiration (P.E.T) was calculated by the Thornthwaite method, matrix effect by the coefficient of infiltration potential (C.I.P) and runoff by the coefficient of infiltration potential weighted with the slope (C.I.P.S). The precipitation on the exposure of the formations was estimated by the ArcGIS integration method. The groundwater inflow from Hub Dam was estimated by using twenty years of annual water loss data by removing P.E.T. The total mean annual inflow of groundwater in the formations was 2414.12 US Gallons per Second (G.P.S). The annual mean outflow was estimated by calculation of groundwater usage of industries and domestic purposes and the mean annual groundwater outflow was 5562.61 US G.P.S. and an annual deficit of groundwater was 3148.5 US G.P.S, which make it a total discharge of 56%. The second part of the thesis composed of validating the groundwater budget by developing groundwater maps, a graph of the past twenty year’s water table and their comparison with the Standard Precipitation Index (S.P.I). For making groundwater maps Direct Current Electrical Resistivity (D.C.E.R) and static water level data from existing industrial wells were used, followed by ArcGIS integration. The D.C.E.R data indicates A-Type and K-Type sub-surface with high resistivity in the three-layer model of IPI2win, which shows the dry sub-surface of existing ground level to 200 m with hard rocks. The average water table of residential areas in 2019 was 60 m and in industrial areas was 130 m. The oscillation of the groundwater table of the last twenty years and the deficit of groundwater budget show an alarming condition for the future. If the same scenario persists, then by 2025, the water table will decline up to 140 m.