Geophysical Insights: Targeting Freshwater Aquifers in Coastal Sedimentary Formations | InformativeBD

 

S. Sundara Moorthy, and N. Jawahar Raj, from the different institute of india. wrote a Research Article about, Geophysical Insights: Targeting Freshwater Aquifers in Coastal Sedimentary Formations. entitled, Audio magneto telluric and geoelectric signatures of un-consolidated coastal sedimentary formations-an integrated geophysical approach for targeting freshwater phreatic aquifers. This research paper published by the Journal of Biodiversity and Environmental Sciences(JBES). an open access scholarly research journal on Biodiversity. under the affiliation of the International Network For Natural Sciences| INNSpub. an open access multidisciplinary research journal publisher.

Abstract

For exploring groundwater resources in any terrain, employing vertical electrical sounding [VES] technique is the common practice. This study pertains to comparing and correlating Audio magneto telluric [AMT] profiling with 1D vertical electrical sounding to prospect the groundwater potential pocket for an irrigation project in an unconsolidated coastal sedimentary terrain for targeting freshwater phreatic aquifers in saline environment. The area falls in Chinnamanulur village of Kilvelur block in Nagappattinam district, Tamil Nadu, South India. A borewell drilled to a depth of 150 feet was not successful as good sand aquifers could not be met. The area of investigation is with an extent of about twenty acres of agricultural land. There are two shallow tubewells of about 16 feet depth and with these two sources only water need is managed. The TDS [Total dissolved solids] of this well is 385 PPM [parts per million]. An attempt has been made to prospect the freshwater aquifers by an integrated geological, hydrogeological and geophysical approach with combined geoelectric and AMT sounding/scanning. Three two-dimensional audio magneto telluric imaging followed by three 1 dimensional vertical electrical soundings [1D VES] were conducted. A shallow sand aquifer zone could be explored and one pilot borehole was drilled engaging calyx drill to a depth of 30 meters. A good medium to coarse grained sand with a thickness of 10 meters was encountered from 18 to 28 meters below ground level. As the soil samples and water were tasted frequently for every two meters, found to be good without any brackish/salinity taste and no marine gastropod shells were encountered in soil samples, a tubewell of 178 mm diameter was bravely constructed to a depth of 29m without conducting electrical well logging. The well was developed and completed after 4 days. As the attempt, findings and results were encouraging and positive; the tubewell was constructed successfully and fruitfully.  The discharge of well is reported to be 63 liters per minute [LPM] and the quality of water is also good and fresh. It is recommended to test the water sample physiochemically tested for suitability for agriculture. 

Read more : Crafting Sustainability: Indigenous Artisanship in Yantok Chair Production | InformativeBD

Introduction

The Nagappattinam district and terrain is generally predominant with marine origin sediments with lot of in situ groundwater quality problems. Due to quality issues agriculturists did not take much risk in deep drilling to tap groundwater. The very shallow- shallow aquifers are generally good and fresh in nature. The deep and very deep aquifers are brackish to saline in general. It is reported that the tubewell constructed up to a depth of 37 meters for village water supply is with a TDS of 1480 PPM in the nearby vicinity. A comprehensive protected drinking water supply schemes [CWSS] have been implemented almost for all the rural villages of Nagappattinam district from Kollidam river mainly due to many quality issues and one among is, the excess iron and TDS contents of groundwater.

Geological and hydrogeological background The entire Nagappattinam district is plain terrain with a gentle slope towards east. The district falls in both Cauvery and Vennar sub basins. The terrain of the district is sedimentary underlain by various geological formations ranging in age from Pliocene to the recent alluviums and dominated by semi consolidated sedimentary Quaternary formations consisting of fluvial, fluvio-marine and marine facies. The Litho units are soils, fine to coarse grained sands, silts, clay, laterites, gravels etc. The geomorphic units being alluvial plains, deltaic plan and flood plains. Flood plain deposits are found near the river courses.

The major geological formations are alluviums, fluvio marine shale, silt, marine sands sandy clay. The geomorphic setup is the result of action of the major rivers, fluctuation in the sea level, tidal effects of Bay of Bengal and forces of wind. Depositional regime comprises of a coastal plain under marine influence, a flood plain of fluvial regime with an intermixing section of both fluvial and marine influences. Sand dunes and beach ridges are very common along the sea coast. Major part of the district is covered by black clayey soils. The district forms part of the Cauvery River basin and is drained by a network of rivers like Cauvery, Kollidam, Arasalar, Vettar and Pamniyar.

South- and south-central parts are drained by the distributaries of Vennar viz, Vettar, Pamniyar etc. The drainage pattern is sub dendritic.

Ground water occurs in all the geological formations ranging in age from lower Miocene to Recent, under unconfined, semi confined and confined conditions. The occurrence and movement of ground water are controlled by climate, topography, geomorphology, geology and structures etc. Groundwater is developed by shallow filter points for domestic, shallow tubewells of 2 to 3 numbers are coupled for agricultural purposes which are the common practice. The district receives maximum rainfall during Northeast monsoon period i.e. October to December. The average district rainfall is 1405.8 mm. In the post-monsoon period, the average water level ranges between 1.1 to 3.3 m and in the premonsoon period it varies from 2.1 to 5.5 m. iron and TDS are the two major parameters that renders most of the sources as non-potable. As per the central water commission [CWC] categorization of blocks, six blocks come under poor and saline category and of which Kilvelur, Keezhaiyur, Nagappattinam blocks are in this category.

Geophysical exploration Geophysical methods can be used to measure the spatial distribution of the physical properties of the subsurface specifically related to the depth of aquifer and its quality, and the subsurface lithology, structure. Geophysical methods, however, do not directly determine water quality or the geologic units. These must be interpreted from the distribution and magnitude of the physical properties interpreted from geophysical surveys. The application of Geophysical techniques to explore ground water is referred as Ground water geophysics or Hydro geophysics. Essentially the geophysical methods comprise of measurement and interpretation of signals from or induced physical phenomena generated as a result of the spatial changes in or more physical properties of sub terrain formation. The electrical resistivity is an intrinsic function of groundwater chemistry, and the degree of saline intrusion can be readily interpreted. Electrical methods also have the advantages of being non-intrusive, economical, and are relatively fast. DC resistivity methods can be used to measure the bulk resistivity of the subsurface. Bulk resistivity represents the resistivity of the entire subsurface, including both the solid and liquid phases. Electrical resistivity is the property which controls the amount of current that passes through a rock when a potential difference is applied, given by Ohms law, V=IR, (V) voltage, (I) current, (R) resistance. The resistivity of pore fluid depends upon the concentration of ions in the fluid. Saline water has high concentrations of total dissolved solids, mostly sodium and chloride ions, which are highly conductive. Therefore, water with high salinity has very low resistivity. DC resistivity introduces electrical currents into the ground through current electrodes in contact with the soil. The resulting electrical potential (voltage) is measured between two potential electrodes. 

The resistivity is defined as the resistance offered by a unit length of a substance of a unit area to the flow of electric current when the voltage is applied at the opposite faces (Fig. 1&2). 

The inverse of resistivity is termed as conductivity and the inverse of resistance is called conductance. Resistivity of a geological formation may be considered as a function of moisture content (porosity and water saturation) and salinity of the saturating water. The rocks containing high concentration of metallic minerals and saline water may show resistivities less than 1 ohmm to a few ohm-m. The resistivity of a formation can be used to know the nature of the formation and distinguish the saline and fresh water zones (Fig. 3).

Audidiomagneto telluric imaging Audidiomagneto telluric imaging is a geophysical technique coming under Electromagnetic method, based on theories of magneto telluric sounding method, MT method, AMT method, frequency sounding method, Natural Electrical Prospecting method and so on. It monitors abnormal variation of electrical resistivity, which is generated by different geological bodies.

Reference

Abderahman A. 2019. Magnetotelluric deep into groundwater exploration. B.Sc. Research Essay, Helwan University, Faculty of Science, Geology Department.

Badrinarayanan TS, Sundara Moorthy S, Jawahar Raj N. 2022. Geo-electric approach to mapping of groundwater contamination of shallow and deep coastal aquifers – a case study. IJRAR 9(1), February 2022. www.ijrar.org (E-ISSN 2348-1269, P-ISSN: 2349-5138).

Badrinarayanan TS. 2010. True resistivity contours to explore the shallow phreatic aquifer in coastal plains of Padur, Kanchipuram district, Tamil Nadu. Unpublished paper.

Badrinarayanan TS. A report on geoelectrical investigation to delineate the fresh and saltwater interface and subsurface lithology in the tsunami-affected coastal villages of Nagappattinam district, Tamil Nadu, under UGC-aided project: “A long-term comparative study of groundwater quality and modeling in tsunami-affected areas of Nagappattinam district, Tamil Nadu under UGC project.” Department of Civil Engineering, Annamalai University, Annamalainagar, Tamil Nadu, India.

CGWB. 2008. Central Ground Water Board, Southeastern Coastal Region, Chennai. District Groundwater Brochure, Nagapattinam District, Tamil Nadu. November 2008.

Chidambaram S, Ramanathan AL, Vasudevan S, Manivannan R. 2008. Study on the impact of tsunami on shallow groundwater from Portonovo to Poompuhar using geoelectrical techniques, Southeast coast of India. Indian Journal of Marine Sciences 37(2), 121–131.

Elango L, Gnanasundar D. 1999. Groundwater quality assessment of a coastal aquifer using geoelectrical techniques. Journal of Environmental Hydrology 7, Paper 2, January 1999.

Ezhisaivallabi. Adappa Watershed. Ph.D. Thesis.

Operation Manual of ADMT-300S. Natural Electrical Prospecting Instrument Water Detector, Shanghai Aidu Energy Technology Co. Ltd., China. Website: www.aiduny.com.

Ramanujachari KR, Gangadhar Rao T. 2005. A monograph on groundwater exploration by resistivity, SP, and well-logging techniques.

Ravindran AA, Kingston JV, Premshiya KH. 2020. Mitigation-dredging in seabed geotechnical engineering study using marine 2D ERI and textural characteristics in Thengapattanam Harbour, South India. Geotechnical and Geological Engineering: An International Journal. ISSN 0960-3182.

Senthil Kumar GR, Badrinarayanan TS, Ahilan J. Delineation and correlation of fresh and saltwater interface of shallow coastal phreatic aquifer by surface and subsurface geoelectrical investigations.

Source : Audio magneto telluricand geoelectric signatures of un-consolidated coastal sedimentary formations-an integrated geophysical approach for targeting freshwater phreatic aquifers