Title of Thesis

Groundwater management using vertical electrical sounding survey and tubewell auditing at farmers’ fields

Author(s)

Pervaiz Sikandar

Abstract
Groundwater quality is deteriorating due to ongoing heavy pumping of groundwater in Punjab, Pakistan. A geoelectrical resistivity survey study using Vertical Electrical Soundings (VES) were conducted at Chaj Doab (land between river Jehlum and Chenab, Pakistan) and Rachna Doab (land between river Chenab and Ravi, Pakistan), with the objectives to locate low salinity groundwater aquifer layers for tubewell installation, tubewells auditing and to categorize different low salinity groundwater zones. The resistivity meter (ABEM Terrameter SAS 4000, Sweden) was used to collect the VES data by employing Schlumberger electrode configuration, with the half current electrodes spacings (AB/2) ranging from 2 to 180 m and the potential electrodes (MN) from 1 to 40 m. The field data were interpreted in terms of resistivity and the corresponding thickness of various sub-surface layers using “Interpex IX1D” computer software. This research study was completed in three phases to achieve its specific objectives in each phase.In the first phase six sites were selected to with three sites in each Doab. A total of ten VES surveys were conducted with three soundings at one site, two at other site and the rest of four sites were surveyed by single VES each. The outputs of the VES interpretation were verified using borehole data at all these six sites. The bore hole data of lithological and groundwater samples were collected from ground surface to 30 m depth with an interval of 1.5 m at all these sites. The correlation between VES data and borehole data was found to be in close agreement with each other. The combination of the VES data with the borehole data provided useful information on subsurface hydrogeologic conditions for tubewell installation and for further geoelectrical studies in the next phase. It was observed that the groundwater in the aquifers was fresh having EC < 1.5 dS/m with geoelectrical resistivity greater than 45 -m, between 25 to 45 -m was marginally-fit and less than 25 -m was un-fit for irrigation. The results also showed that the effective depth of the current penetration ranged between 1/8th to 1/2 of the outer current electrode spacings (AB/2).Than in the second phase six more sites were selected with three sites in each Doab to audit the salinity of groundwater extracted by existing tubewells. These sites were also subjected to VES surveys near to existing tubewells to find the depths and thickness of various subsurface layers having different salinities of groundwater. A total of 21 wells were audited in this study. Out these 21 tubewells, 8 were in un-fit aquifers layer, 7 in marginally fit aquifers, 4 in fit groundwater aquifers and two partially in fresh-saline layers.Nine tubewells are working fine and twelve need careful attentions. Out of these twelve tubewells three tubewells could be abandoned as only be used for shandying.Two tubewells were using extra energy by deep extraction of groundwater without any quality benefit. The other seven can be usefully converted from saline and marginally-fit to fresh groundwater wells by changing their depths.In the third phase, a total of ninety VES interpreted data spatially distributed over Chaj and Rachna Doabs was used, including ten of the 1st and six of the 2nd soundings with 43 in Chaj and 47 in Rachna Doab. The outputs (VES interpretation) of subsurface layers with resistivities and thickness were presented in contour maps and 3-D views by using SURFER software.The zones with low salinity groundwater either in shallow or deep aquifer were differentiated from brackish and saline groundwater.A total of 102 groundwater samples from nearby hand pumps and tubewells at different depths were also collected to verify previously developed correlation (1st phase) and to get more closed approximation between the aquifer resistivity of VES and the Electrical Conductivity (EC) of the groundwater over the study area with R2=0.86.

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