GROUNDWATER GEOPHYSICS

Introduction

Inland groundwater salinity in Uttar Pradesh assessed through geophysical logs

Aquifers in palaeo-channels of salinity infested Mathura & Agra districts

Resource finding in hard rock areas

Alternative sustainable aquifers for urban areas

Deep Aquifers in Ganga Basin

Introduction

             Central Ground Water Board, initiated the application of surface and borehole geophysics on regular basis in the seventies, for the scientific evaluation of the subsurface hydrogeological conditions required in pin-pointing of drilling sites and proper designing and construction of water well. It has made extensive use of both surface geophysical techniques as well as borehole geophysical logging techniques fully integrating them with hydrogeological and geomorphological investigations and worked as inter-disciplinary team of geoscientists in various aspects of groundwater exploration, environmental studies, support to crisis management and special applications. Groundwater geophysics has become an integral part of the exploration programme.

              The geophysical techniques have been used under diverse geological settings of the country, i.e., Archaean to Recent formations containing shallow as well as deep aquifers in the hilly terrain, piedmont area, alluvial plain, Deccan plateau, hard rock, desert and coastal tract to cater to the hydrogeological needs. The geophysical techniques have been integrated to assess the disposition of capable aquifers under vulnerable conditions as interspersed with saline water zones and the encroachment of saline and polluted groundwater.

               An effective and wide application has been made of the conventional surface electrical resistivity technique for source finding in the hard rocks. The approach has been improved by integrating resistivity sounding with gradient resistivity profiling and their non-conventional interpretations. Besides, other techniques like self potential, induced polarization, Mise-a-la-Masse of electrical method, magnetic, refraction seismic, electromagnetic-the horizontal loop and Very Low Frequency (VLF) have also been used. Board has also analysed the aeromagnetic, seismic and deep borehole data of other agencies for their effective utilization in groundwater exploration. So far, about 30,000 resistivity soundings and 7000 borehole logging have been conducted, besides the resistivity and magnetic profiling data in large quantity. Along with this, the feedback from drilling results and logging has resulted into a professional expertise in geophysical applications for area-specific groundwater information and transfer of technology.

              The Board has the geophysical capability of locating saturated fracture zones and mapping bedrock topography in hard rocks, mapping sea water intrusion in coastal tracts and fresh water lenses of island and inland groundwater salinity, locating freshwater aquifers in desert, mapping palaeo-channels of river, identifying direction of groundwater movement and seepage, locating spot and area for artificial recharge scheme, mapping pollution plume and locating leakage from landfill, locating groundwater zone for restoring water supply in earthquake, cyclone and drought affected areas and multi-parametric logging of borehole for well construction and regional geometry of aquifers.

              The Board has been actively associated with the preparation of ISO manuals on geophysical logging and surface surveys for groundwater. Besides the CGWB working manual on surface geophysical surveys and borehole logging prepared for the in-service trainees has become a popular ready-reference document for the groundwater geophysicists in the country. The Board has created a data base for country wide geophysical data storage/ retrieval and their analysis and reporting. So far 6 Refresher Course has been organised in which geophysicists of CGWB and several state agencies participated.

               CGWB, Northern Region, Lucknow has carried out more than 3000 resistivity soundings and 500 borehole loggings besides profiling with electrical and magnetic techniques. With the field-based experience of capabilities and limitations of techniques and their standardization through borehole logging and drilling information for nearly three-decades, evolving to tackle the groundwater problems in various terrains and the societal demands, CGWB has generated the area-specific geophysical information and packages for the survey planning and designing and the transfer of technology to meet the requirements.

Inland groundwater salinity in Uttar Pradesh assessed through geophysical logs

              Board has drilled more than 500 boreholes, in the depth range of 200 to 700m in Utter Pradesh. Interpretation of the geophysical logs of boreholes have unraveled a large tract of brackish/saline to marginally fresh groundwater in the Central Ganga Plain. The water quality deterioration is regionally correlatable all along 700 kms run from Meerut in northwest to Ghazipur in southeast. The deteriorated quality groundwater is found mostly sandwiched between impermeable clays with fresh water occurring above and below. As an exception in southern and southwestern fringe brackish/saline groundwater once sets in at the water table, continues up to bedrock occurring around 200m bgl. The maximum thickness of brackish/saline groundwater column is found in Ghaziabad, Aligarh, Agra, Mathura, Hathras, Firozabad, Etawah and Fatehpur - the most affected districts in proximity of the river Yamuna. The groundwater quality assessment from geophysical logs has been an invaluable information in targeting fresh water horizons for sustainable groundwater development.

Aquifers in palaeo-channels of salinity infested Mathura & Agra districts

            Mathura is the only district of the state where saline water, at large, starts right from the water table and continues up to the bedrock. The area was geo-electrically evaluated on the basis of more than 500 resistivity soundings. The fresh water occurrence is very rare and is in the form of near surface water pockets impregnated in saline water surroundings. These 30 to 50 m thick fresh water pockets totaling 250 sq,kms in area, are isolated and limited in extents, look like oasis of potable water. The geometry of these pockets is adequately defined and recommended for development up to an average depth of 50m. Iso-resistivity depth slicing and synthesis of the data helped identify the palaeo-channel of the river Yamuna in parts of Mathura and Agra. As a futuristic measure, precise locations for the purpose of artificial recharge are also identified to conserve and sustain the fresh groundwater bodies.

         Groundwater environs of Agra City is plagued by dual ills of saline water occurrence below 30 /40 mbgl as well as sporadic presence of fluorides and nitrates much above permissible limits. An exhaustive surface geophysical survey with the objective of combating the crisis has led to discover an area of 7 sq.kms located 10 kms southeast of the city limit on Fatehabad road, as worthy for groundwater development. The area stores a huge amount of fresh groundwater upto a depth of 80m, that could be free from undesirable amount of fluorides and nitrates owing to the regular flushing.

Resource finding in hard rock areas

         Surface geophysical surveys through electrical and magnetic methods for pin-pointing of drilling sites have been carried out in the hard rock terrain of Uttar Pradesh encompassing the districts of Banda, Hamirpur, Chitrakut, Mahoba, Jhansi, Lalitpur, Mirzapur, Allahabad, Varanasi and Sonbhadra. The combination of electrical resistivity sounding and gradient resistivity profiling and their non-conventional interpretation has been effective in delineating the high yielding fracture zones within 100m bgl.

Alternative sustainable aquifers for urban areas

         Due to almost doubling of population in every 20 years of all the major cities a continuous decline in water table has been set in. Hence the need has been felt to delineate alternative sustainable aquifers in and around the cities and further give a boost to device handy and economical ways of artificial recharge within the cities. The preferred area and the alternative worthy aquifers are defined through correlation of borehole geophysical logs for the districts Lucknow, Kanpur, Farrukhabad, Etah and Etawah.

Deep Aquifers in Ganga Basin

         More than 200 large-spread resistivity soundings have been carried out to delineate, hitherto undiscovered, deep fresh water aquifers in parts of Ganga basin. Thick pile of granular sediments has been identified along the Himalayan foothills of Uttar Pradesh. Also the existing seismic, deep resistivity, magnetotelluric and borehole log data from other agencies have been contextually interpreted to identify the fresh water aquifers upto a depth of 2000m bgl in the Recent alluvium and underlying Tertiaries- the Siwaliks.