Spatial Analysis Of Groundwater Quality Using Geophysical And Geochemical Approach In Uthal, Balochistan

Abstract

Uthal is located in Lasbela District in Pakistan's Balochistan province. It has an arid to semi-arid climate, and the main source of fresh water for domestic and agricultural purposes is groundwater. To determine the spatial distribution of groundwater and its quality, an electrical resistivity survey was carried out using the schlumberger array. The PASI Electrical Resistivity Imaging System was utilized to examine 25 stations in the study area, to investigate a depth of 150 meter. IPI2win software was used for curve matching to assess the measured geoelectrical characteristics. The Dar Zarrouk parameters were computed after identifying five geoelectrical layers. The resistivity of the topmost, thinnest layer of unconsolidated material ranged from 600 to 2600 Ωm. The resistivity of the second layer of unconsolidated sand and gravel ranged from 50 to 11500 Ωm; that of the third layer of dry sand and clay was up to 200 Ωm; that of the fourth layer showed a thick freshwater zone with a resistivity between 75 and 100 Ωm; and that of the layer after that showed a thick layer of sand and clay with fresh water had a resistivity of between 48 and 90 Ωm. The Dar Zarrouk parameters estimated for longitudinal unit conductance (S), ranges from 0.1 to 1.65 mho, transverse unit resistance (T), which is larger than 25000 Ωm2, longitudinal resistance (RS), which ranges from 50 to 1350 Ωm, and transverse resistivity (RT), which ranges from 22 to 1150 Ωm. The ERS data and Dar Zarrouk characteristics identified the freshwater zones in the subsurface. Small pockets of saline water were found in few near surface zones due to evaporation owing to the arid and semiarid surface conditions. There has been no indication of seawater intrusion, from the ERS data and Dar Zarrouk characteristics. To confirm the findings of electrical resistivity data, groundwater samples from 100 sites were collected from the study area. Chemical analyses were performed for major cations and anions present in water. In addition, to study the characteristic of water, trace element analysis was also performed on the samples. The major cations and anions of the study area are, Sodium (Na) (50.7 to 1858 mg/l), Chloride (Cl) (6.2 to 2150 mg/l), Potassium (K) (1 to 34 mg/l), Calcium (Ca) (19 to 857 mg/l), Magnesium (Mg) (5.77 to 281 mg/l), Bicarbonates (HCO3) (58 to 860 mg/l), Sulfate (SO4)(44 to 4411 mg/l) & NO3 (48 to 854). The trace elements are, Aluminum (Al) (5 to 756 µg/l), Chromium (Cr) (0.5 to 114 µg/l), Cadmium (Cd) (0 to 24 µg/l), Chromium (Cu) (0.05 to 354 µg/l), Manganese (Mn) (2 to 675 µg/l), Nickle (Ni) (0 to 412 µg/l), Lead (Pb) (1.5 to 79 µg/l), Zinc (Zn) (5 to 801 µg/l) and Arsenic (As), Cobalt (Co), & Titanium (Ti) below detection limits. Estimates were made for the physical parameters; alkalinity, total dissolved solids, and electric conductance parameters. The concentration of main cations, anions, and trace elements are within the permissible limits as per WHO standards, except for a few sample points. The sedimentary rocks and the mineralization of the Bela Ophiolites exposed in the study area are the main sources of these cations, anions, and trace elements. The study integrated the Dar zarrouk parameters, as calculated from the electrical resistivity values (ρ), and the corresponding Total Dissolved Salts (TDS) values. The study area has been divided into four zones namely, very good potential (TDS ˂ 500 mg/l & ρ from 200 to 1500 Ωm), good potential (TDS from 501 to 1000 mg/l & ρ from 100 to 200 Ωm), fair potential (TDS from 1001 to 2000 mg/l & 20 to 100 Ωm) and poor potential zones (TDS ˃ 2000 mg/l & ρ ˂20 Ωm). The current study has provided the quantitative legend of the area. This legend can be quantified and applied to surrounding semi-arid to arid of the area. This is valid for reservoir modeling and changes in hydrofacies and trend analysis for good water management practices.