Journal of Atmospheric & Earth Sciences Category: Agriculture Type: Research Article

Identification of Artificial Groundwater Recharge System near Ayyankulam Village of Tirunelveli, Tamilnadu, India Using Electrical and Magnetic Survey

Mathiazhagan Mookiah1*, Srinivas Yasala2, Saravanan Sakthivel3, Chandrasekar Nainarpandian4, Ramji Vaidhyanathan5 and Swaster Robert6
1 Assistant Professor (T), Centre for Geo Technology, Manonmaniam Sundaranar University, Tirunelveli-12, India
2 Professor and Head, Centre for Geotechnology, Manonmaniam Sundaranar University, Tirunelveli-12, India
3 Technical Assistant (S.G.), Centre for Geotechnology, Manonmaniam Sundaranar University, Tirunelveli-12, India
4 Vice Chancellor, Manonmaniam Sundaranar University, Tirunelveli-12, India
5 Assistant Professor, Centre for Water Resources, Anna University, Chennai-25, India
6 Post Graduate Student, Centre for Geotechnology, Manonmaniam Sundaranar University, Tirunelveli-12, India

*Corresponding Author(s):
Mathiazhagan Mookiah
Assistant Professor (T), Centre For Geo Technology, Manonmaniam Sundaranar University, Tirunelveli-12, India
Email:mathiazhagan1987@gmail.com

Received Date: Aug 24, 2023
Accepted Date: Sep 15, 2023
Published Date: Sep 22, 2023

Abstract

A geophysical investigation was conducted in Ayyankulam village, Tirunelveli District, Tamil Nadu, and India to identify the groundwater flow pattern of a recharge well using the magnetic and electrical resistivity method. Magnetic data (using a proton precession magnetometer) were collected at a gridded point distance of 300 metres around the recharge well, after which the data was processed and geomagnetic (IGRF) and diurnal corrections were applied for interpretation. The Schlumberger method was used to gather 12 vertical electrical sounding (VES) data points, while the Wenner method was used to collect two profiling data points. 1X1D software was used to analyse the sounding and profiling data (i.e., apparent resistivities and thicknesses) of various layers. The lithology data in the study area and the resulting VES results were compared, and good agreement between both sets of data was found. The interpretation of the results reveals that the study area has a maximum of three layers. The majority of the VES curves are H types, representing successive sedimentary formations in this area. Approximately 1.2 to 1.5 m3/sec of water flows into the recharge well for 50 days. A water level indicator was used to measure groundwater levels in open wells and bore wells as well as delimit groundwater dynamics in the area. ArcGIS 10.3 was used to digitise various thematic maps, such as magnetic maps, resistivity maps, and water level maps. From the spatial map, the groundwater flow direction was delineated on the south and south-western sides of this study area may have volumetric increase in whole dimensions due to the gradual increase in thickness of limestone deposits towards the coast. The trends of the lineaments and the topography indicate the ground water flow towards the east coast and may have volumetric increase in whole dimensions due to the gradual increase in thickness of limestone deposits towards the coast.

Keywords

Groundwater; Recharge Well; Groundwater flow pattern; Magnetic method; Resistivity Method

Introduction

An open agricultural well in Ayyankulam was widely reported to recharge estimated 1,500-2,500 liters of water every second for several weeks without overflowing. The recharge water for this well was from the excess overflow of an adjacent minor irrigation tank due to the record monsoon rains in Nov-Dec 2021. The well became a local attraction and was called a "miracle well" since typical wells would fill and overflow in a matter of hours at such recharge rates [1]. Surface drainage and stream flow pattern were decided and controlled by the topography and the subsurface geology. Water generally takes the least resistive path while its selection of flow direction; deeper geological formations and its characteristics control the drainage thus provides the clue about the hydrogeology of the area. Erodable limestone, dolomite and marble formations provide a way for the subsurface flow of water with typical subsurface geological conditions. Many researches were conducted about the dissolution of calcareous rocks and karstification across the globe and because of its hydrological and constructional importance. Karst is considered as the effective source of water storage and supplies 40 to 50% of the World’s drinking water needs [2-5]. Typical geological signatures are visible associated with the complex structural features such as karst. Non-invasive geophysical methods such as electrical resistivity survey, electrical tomography, and seismic methods are preferable for mapping Karst topography [6-8]. The exceptional high velocity groundwater flow is observed in some wells at Aayankulam village and was focused with interest because that faster flow is only possible at Karst geology. Interestingly no previous records of karst topography are identified within the southern terrain. Electrical resistivity survey with a grid pattern of close intervals was conducted to get detailed picture of the geological features in small scale. Lineaments are the geological features control the flow of subsurface water within the particular region and can be effectively used as the potential indicator of flow direction [9]. It is necessary to examine the location and distribution of groundwater taking into account both its vertical and areal extents. It also requires identifying the geologic zones that are significant for groundwater as well as their structure in terms of their capacity to hold and yield water. Deep water-bearing zones can be directly recharged with water using recharge or injection wells, which are subsurface groundwater recharge techniques. Only in locations where there is a substantial impermeable layer between the soil surface and the aquifer that has to be recharged are recharge wells appropriate. They are also useful in areas where land is limited. The material encasing the aquifer can be used to cover recharge wells. If the material is not yet solidified, a screen can be inserted into the injection zone [10]. Due to its high degree of heterogeneity, which is frequently characterised by distinctive features such as cavities and sinkholes contained in a landscape with great spatial variability of weathering, karst is typically complex hydrogeologically. While it is difficult to characterise such heterogeneity using traditional hydrogeological techniques, geophysical technologies may provide insight into important elements that govern the hydrological behaviour of a karst aquifer [11]. Most of the research mainly used to identify the recharge well dynamics used the geochemistry (isotope) method. Groundwater recharge is characterised by the sources, circulation patterns, and geochemical evolution along flow pathways. The ability of the aquifer to store surface and subsurface flows has been evaluated hydrodynamically. [12-14]. Environmental tracers were utilized by to characterize the groundwater flow patterns [15]. [16-18] connected groundwater flow paths by combining groundwater and geochemical modelling with stable isotope analysis. 

The main field of the Earth is perturbed by various combinations of induced and remnant magnetization found in magnetic rocks [19]. According to research done by [20-21], locations with high elevation have comparatively high magnetic susceptibility. Zones with high magnetic susceptibility are likely to be younger and have less secondary porosity, which would allow for easier groundwater recharge. The groundwater potentiality and subsurface hydrogeological setting were investigated by using an integrated methodology based on a combination of remote sensing and geophysical techniques (Aero magnetics and VES) [22]. This was done to aid decision-makers in their upcoming development plans. [23] Assessed the preferred flow pathways at each location and contrasted the various sites through MARS. A groundwater investigation in regions, where the hard rock aquifer system is significantly impacted by weathering was done in 2021 by this strategy is better for evaluating groundwater resources than more conventional ones [24]. [25] Aimed to identify surface and subsurface structures, define their effects on groundwater flow direction, and measure the thickness of the groundwater aquifers (accumulation) in their study region. [26] Concluded that overburden thickness calculation and structure mapping were needed for groundwater potential evaluation in a typical basement complex terrain, which could be reliably done using the magnetic approach. Have proven Electrical resistivity imaging to be a successful and affordable method for mapping the near-surface karst development, particularly the sites of the prospective water flow pathways, due to the resistivity variations between the water-filled karst and the bedrock. According to [27]. In comparison to other infiltration basins that are separated from their respective extraction wells by fine-grained materials, some portions of the infiltration basins have hydraulic linkages to the extraction wells through preferential flow routes depending on the electrical resistivity data. [28] Identified various flow systems and hydraulic traps and mapped resistivity differences. [29] Investigated the spatial and temporal mixing patterns in karst conduit and matrix fluids under base flow and high flow conditions. Using a built-in MLR recharge model, the groundwater recharge rate evaluation was properly assessed on a regional level [30]. In a typical calcareous valley, a preferential flow and solute transport channel were identified [31]. In an effort to investigate the distribution of conduit flow pathways in a tiny freshwater lens inside the karst aquifer system, [32] adopted an inter-disciplinary integrated approach. To locate and describe the faults, discontinuities, and water research of the fractured and limestone aquifers, [33-37] used ERT techniques. In order to choose a potential aquifer recharge site, provided baseline geophysical and geochemical results. Hence, it is imperative to investigate the groundwater flow direction and the recharge of an artificial recharging structure by studying its geology or geophysics through any technology. One such artificial recharge well present in the Ayyankulam is taken up for the present study, as the recent past flow pattern is highly mysterious. The mystery is that the well failed to fill despite five days of strong water inflow from the Nambiyar canal. Based on these facts, the main objectives of this study are: (1) to delineate subsurface layers through the electrical resistivity method; (2) to delineate subsurface variation through the magnetic method; (3) to determine groundwater fluctuation through periodical groundwater level monitoring; and (4) to identify the recharge well flow pattern connecting with resistivity data, magnetic data, groundwater level, and existing lithological information along the study area. 

  • Study Area 

In this research, the study area is in and around the Ayyankulam Recharge Well of Thangammalpuram Village, Radhapuram Block, Tirunelveli District, which is a subbasin of Nambiyar (Figure 1a). The Nambiyar River is the major river that runs from west to east through the study area, and it is a seasonal river. As surface water supplies have been limited, groundwater has become the primary source for the drinking and irrigation water sectors. The tail end of the Nambiyar basin was chosen for this research. Aeolian soil, sandstone, limestone, sedimentary rock, and base rock dominate the subsurface of the study area. The area of the study area is 45 km2 and lies between the geographical coordinates of Latitude 8°15'15" N - 8°19'30" N and Longitude 77°49'30" E - 77°54'00" E. The eastern boundary is bounded by the Bay of Bengal. 

 The climate in this study area is generally hot and semi-arid. The nearest rain gauge station is located in Thisyanvillai, and it was used for the current study. The study area's monthly maximum and minimum temperatures are 38 °C and 20 °C respectively. The months of March, April and May have the highest temperatures, while November, December and January have the lowest. The annual normal rainfall recorded at Thisyanvillai rain gauge station is 900 mm, which comprises three seasons: the south-west monsoon, the north-east monsoon, and the transitional period. 

Laterite deposits are found as patches in most of the study region observed as porous, weathered formation. The Charnockites of Achaean age act as the base rock with less fractures and joints and considering the regional geology, charnockites is noted as the part of southern granulite terrain. Crystalline limestone was also identified at north of the coastal boundary of tertiary age 7. Bodies of charnockites, linear (trending NW) and lenticular in shape occur within the khondalite belt with varying dimensions. A few outcrops of hard marine sandstone and shell limestone with intercalations of pebble beds of Miocene age, unconformable overlie the basement rocks. The sandstone and limestones are often encountered in most of the inner and coastal part of the study region.

 Figure 1a: Study area location map.

Material And Methods

  • Data 

The primary data location is shown in Figure 1b. The water levels from an open well and a bore well were checked every fifteen days. The Wenner (2-profile) and Schlumberger electrode arrangement (12 VES) methods were employed to acquire resistivity data. 100 total magnetic data points were gathered from the study region at a grid of 300 m x 300 m, as shown in Figure 2. Lithology information was gathered from the open well and bore well already on or near the site. The artificial recharge well in Ayyankulam is shown in Figure 3. The well is 3 m in length and 2 m in width (rectangular well). The system tank for the Nambiyar subbasin is the Thangamalpuram tank. It is the tank for the nearby recharge well, and both are connected through a canal. The canal is 1.5 m deep and 3 m wide. 1.2 to 1.5 m3/sec of water have flown into this well in 50 days. 

The Figure 4 shows the subsurface lithological information and sample at nearby VES 6. Hard rock formations are available on the north and north-eastern sides of the open well, as confirmed by lithology. Similarly, limestone formation is available on the south and south-western sides, and through the open well and bore well information, it is confirmed.

 Figure 1b: Primary data illustrating the study area.

 Figure 2: Grid point survey for Magnetic survey along the study area.

 Figure 3: Artificial Recharge System at Ayyankulam Well.

 Figure 4: Lithology of the study area.

  • Magnetic Method 

Magnetic Survey

A magnetic survey is a procedure that uses a magnetometer to measure the geomagnetic field over a certain area. Surveys document the spatial variation in the strength of the Earth's magnetic field. They may measure the total magnetic field strength using two geographically separated sensors, or they may measure the gradient of the magnetic field using two spatially separated sensors. In order to create a grid of data points, the magnetic measurements are made using magnetometers along profiles or a set of parallel profiles. This technique is often employed in magnetic surveys. A proton precession magnetometer was used to collect magnetic data from 100 sites in the study region (Table 1). The magnetometers used have the same sensitivity. For measuring diurnal variation, one is fixed at the base station, and the other is fixed in the research region for the survey. A certain time interval can be chosen for the base station magnetometer to record. Crosschecking the readings from the two magnetometers in the same location and at the same time for identical values is essential before moving further with the research. An east-west alignment is used for the sensor head. Every 10 minutes, base station readings are recorded. The stations should be placed far from any large metallic objects (such as reinforced concrete walls, electrical lines, cars, and metropolitan areas), as these objects might occasionally prevent data from being available within the survey grid in ground magnetic surveys. The range of the sources and the area to be examined are key factors in station separation. At any given time, three readings should be recorded there. Measurements must be taken until the difference between these three readings is within two gammas. The time of recording is also indicated because it will be necessary to account for diurnal variations. The main drawback of a ground magnetic survey is how challenging and time-consuming it is. High data resolution is a benefit, and field correction of survey data errors is also possible. Without processing, the magnetic data cannot be utilised. In general, it has been discovered that the processed findings acquired are well correlated with geological features. The International Geomagnetic Reference Field (IGRF) anomaly map was produced after the data underwent the necessary modifications, such as diurnal variations. 

S. No

Latitude in degree decimal

Longitude in degree decimal

Total Magnetic Field

IGRF Correction

Diurnal Correction

1

8.29179343

77.8628831

40872.67

122.67

120.67

2

8.29185199

77.8547204

40854

104

100.34

3

8.29189093

77.8492786

40763.33

13.33

10.33

4

8.2919298

77.8438367

40852.67

102.67

96.67

5

8.29194921

77.8411158

40844.33

94.33

88

6

8.28902451

77.8710261

40833

83

78

7

8.28383777

77.8383365

40882

132

132

8

8.28645098

77.8519604

40841

91

90.34

9

8.28643151

77.8546813

40825

75

80.33

10

8.2890441

77.8683053

40823

73

68

11

8.29181297

77.8601622

40881

131

128.34

12

8.29183249

77.8574413

40859.67

109.67

106.01

13

8.29187147

77.8519995

40672.33

-77.67

-82.67

14

8.29191038

77.8465577

40846.67

96.67

90.67

15

8.2919686

77.8383948

40838.33

88.33

82.33

16

8.29198797

77.8356739

40836

86

80

17

8.2838184

77.8410574

40861.67

111.67

105

18

8.28647043

77.8492395

40848.67

98.67

102.67

19

8.28641202

77.8574022

40825

75

80.33

20

8.28906366

77.8655844

40836.84

86.84

91.5

21

8.29177388

77.865604

40863

113

109.67

22

8.29467887

77.8384143

40856.33

106.33

106.33

23

8.29465948

77.8411353

40875.33

125.33

130.33

24

8.29992409

77.862942

39270

-1480

-1483.33

25

8.29994364

77.8602211

39591.67

-1158.33

-1157.33

26

8.29990451

77.865663

40363.67

-386.33

-381.67

27

8.28379901

77.8437783

40865

115

108.33

28

8.28648986

77.8465187

40853.67

103.67

108

29

8.28639252

77.8601231

40826.67

76.67

42.34

30

8.28908321

77.8628635

40840.17

90.17

75.17

31

8.2917543

77.8683249

40846.67

96.67

96.67

32

8.29462063

77.8465772

40814.33

64.33

66.66

33

8.29464006

77.8438562

40842.67

92.67

97.67

34

8.2998653

77.8711049

39640

-1110

-1109.34

35

8.29996318

77.8575001

39583.33

-1166.67

-1161.67

36

8.29988492

77.8683839

40003

-747

-742.34

37

8.2837796

77.8464992

40867.67

117.67

115

38

8.28650927

77.8437978

40855

105

109.33

39

8.28637299

77.8628439

40825

75

40.67

40

8.28910274

77.8601426

40840

90

75

41

8.29173471

77.8710458

40849

99

97.34

42

8.29458171

77.8520191

40726

-24

-29.66

43

8.29460118

77.8492981

40755.33

5.33

10.99

44

8.29715511

77.8710852

40458

-292

-298.66

45

8.2999827

77.8547792

39222

-1528

-1527

46

8.3028291

77.8357517

39684.67

-1065.33

-1064.67

47

8.28376018

77.84922

40859.67

109.67

109.34

48

8.28652867

77.8410769

40863.67

113.67

123

49

8.28635345

77.8655648

40828.33

78.33

110.99

50

8.28912225

77.8574218

40846

96

117

51

8.29469825

77.8356933

40855.67

105.67

104.01

52

8.29454272

77.8574609

40853.67

103.67

102.67

53

8.29456222

77.85474

40722.67

-27.33

-28.33

54

8.30011881

77.8357322

40198.67

-551.33

-555.66

55

8.30009943

77.8384532

40176

-574

-575

56

8.30008002

77.8411742

40185

-565

-566

57

8.28374073

77.8519409

40874.67

124.67

124.34

58

8.28654804

77.8383559

40853.33

103.33

101

59

8.28633389

77.8682856

40823.67

73.67

74.67

60

8.28914175

77.8547009

40838.5

88.5

87.84

61

8.29450365

77.8629028

40758

8

10.33

62

8.29452319

77.8601819

40572.33

-177.67

-175.34

63

8.29735033

77.8438757

40773

23

13.35

64

8.3000022

77.8520582

39210

-1540

-1544

65

8.30002168

77.8493372

39838.67

-911.33

-913.99

66

8.30006059

77.8438952

40223.33

-526.67

-529

67

8.28372127

77.8546618

40883.33

133.33

132

68

8.2865674

77.835635

40892

142

142.66

69

8.28631431

77.8710064

40814

64

63.67

70

8.28916122

77.85198

40853

103

103.17

71

8.29446451

77.8683446

40726

-24

-24

72

8.29448409

77.8656237

40773

23

24.66

73

8.29733089

77.8465967

40798.33

48.33

50.66

74

8.2971943

77.8656433

40345.67

-404.33

-407.66

75

8.29717471

77.8683643

40058.83

-691.17

-694.13

76

8.30004115

77.8466162

39772

-978

-980.66

77

8.28370179

77.8573826

40914

164

162.67

78

8.28360411

77.8709868

40877.67

127.67

128.33

79

8.28927768

77.8356544

40841.67

91.67

89.01

80

8.28918068

77.8492591

40859.67

109.67

108.67

81

8.29740853

77.8357128

40785.33

35.33

38.66

82

8.29444491

77.8710655

40761.33

11.33

14.66

83

8.29731143

77.8493177

40718.33

-31.67

-36.33

84

8.29723342

77.8602015

40652.33

-97.67

-101

85

8.29721387

77.8629224

40132.83

-617.17

-618.83

86

8.3028097

77.8384727

39613.33

-1136.67

-1136.67

87

8.28368229

77.8601035

40875.67

125.67

128.33

88

8.28362368

77.868266

40857

107

107.66

89

8.28925832

77.8383754

40834.67

84.67

85.33

90

8.28920012

77.8465382

40845.84

95.84

96.5

91

8.29738915

77.8384338

40785.67

35.67

38.67

92

8.29736975

77.8411547

40764.67

14.67

14.01

93

8.29729196

77.8520386

40769

19

21

94

8.29727246

77.8547596

40775.33

25.33

17.33

95

8.29725295

77.8574805

40730

-20

-27.66

96

8.30279029

77.8411937

40850.5

50.5

40.51

97

8.28366277

77.8628243

40871

121

118.67

98

8.28364324

77.8655451

40864.33

114.33

117.33

99

8.28923894

77.8410963

40826.67

76.67

75.34

100

8.28921954

77.8438172

40845.5

95.5

96.34

 Table 1: Magnetic field data along the study area.

  • Resistivity Method

Resistivity Survey 

Wenner electrode arrangement 

The Wenner array is made up of four collinear, equally spaced electrodes, as depicted in Figure 5. Typically, the outer two electrodes are current (source) electrodes, while the inner two electrodes are potential (receiver) electrodes. While maintaining an equivalent spacing between each electrode, the array spacing expands around the array midpoint. The Wenner array is also extremely sensitive to near-surface homogeneities, which can skew deeper electrical responses. Two profiles were conducted in this investigation. The readings are taken from west to east.

 Where, ρa is apparent resistivity; it is electrode spacing; V is voltage in volts and I is current in amperes.

Figure 5: Schematic diagram of Wenner method.

Schlumberger electrode arrangement

The Figure 6 depicts the Schlumberger array, which consists of four collinear electrodes. The two outer electrodes are current (source) electrodes, while the two inner electrodes are potential (receiver) electrodes. The potential electrodes are installed in the centre of the electrode array with a small spacing, usually less than one-fifth the spacing between the current electrodes. During the survey, the current electrodes are separated further, while the potential electrodes remain in the same position until the observed voltage becomes too small to measure. Typically, the current electrodes are expanded six times every decade.

Where, ρa is apparent resistivity; AB is current electrode spacing in meter; MN is potential electrode spacing in meter; I is current in amperes and V is voltage in volts. 

In this investigation, VES was conducted at 12 points spread over the area of the recharge well. The resistivity field data collection is shown in Figure 7.

 Figure 6: Schematic diagram of Schlumberger method.

 Figure 7: Resistivity field survey with M.Sc. Students in the month of January 2022.

  • Water Level Indicator

A calibrated wheel attached to the counter mechanism of a water level indicator or dip metre is lowered into the well over a sensor attached to the end of a rope wound on a winch. On the winch, a panel metre shows full deflection and makes a buzzing sound when the sensor makes contact with the water's surface. The distance from the ground to the water level can be determined using a metric scale on the counter. It may be used to assess the water level in both open wells and bore wells, and it is simple to use. There were 15 open wells and 19 bore wells that were monitored every 15 days.

Results And Discussion

  • Magnetic data analyses 

It is necessary to do magnetic data reduction to eliminate all sources of magnetic fluctuation from the observation other than those brought on by subsurface magnetic influences. Two corrections were made in this study, including a diurnal correction and an International Geomagnetic Reference Field (IGRF) correction. 

  • Total magnetic field 

The magnetic or absolute value, of the magnetic field vector as displayed in Figure.8 is the magnetic total field. The magnetic total field, which is measured in units of nanotesla, describes the strength or intensity of the magnetic field (nT) (Figure 8).

 Figure 8: Spatial variation of Total Magnetic Field.

  • IGRF Correction 

The geomagnetic correction is the magnetic equivalent of the latitude correction in gravity surveying, and it removes the effects of a geomagnetic reference field from survey data. The most exact approach to geomagnetic correction is the use of the International Geomagnetic Reference Field (IGRF), which represents the undisturbed geomagnetic field in terms of a large number of harmonics and incorporates temporal components to account for secular variation, as shown in Figure 9. Due to the IGRF's complexity, corrective measures must be calculated using a computer. It should nonetheless be highlighted that the IGRF is flawed because the harmonics employed are based on data from a sparse collection of scattered magnetic observatories. As it extrapolates the spherical harmonics obtained from observatory data forward, the IGRF is also prognostic. As a result, the IGRF can be drastically off in locations far from observatories. A magnetic field's components and values can be used to approximate the geomagnetic reference field. In this case, the IGRF value average of X, H, and F components, such as 40750 nT, was used. The Figure 10 depicts the spatial IGRF variation map.

 Figure 9: IGRF value of study area.

Source: http://www.geomag.bes.ac.uk/models_compass/igrf_calc.html

 Figure 10: Spatial variation of IGRF Correction.

  • Diurnal variation correction 

The effects of diurnal variation can be removed using a variety of techniques. Based on the time of observation, the variations in the base values are spread across the readings at stations taken over the day. The map of the spatial diurnal correction is shown in Figure 11. Based on the magnetic survey, it was possible to see a pattern where a very low magnetic field was taken into account as a very highly discontinuous karstic feature, or a very high degree of solution opening, which was then possibly followed by a highly discontinuous karstic feature, or a high degree of solution opening. Additionally, there are numerous abandoned bore wells in this region, which can lead to two outcomes: first, highly to very highly discontinuous groundwater, and second, deeper vertical groundwater flow.

 Figure 11: Spatial variation of Diurnal correction.

  • Resistivity Data Analyses

Wenner method 

In this investigation, two lateral resistivity variations were found: one was close to the recharge well and the other was farther away; both were in west-to-east orientation. In this profile line one, certain resistivity smooth values are obtained between 400 and 1000 metres, which is a roughly discontinuous area. Additionally, a slight rise was seen on the second profile line at 500 and 1500 metres, signifying the potential location of bore wells, as shown in Figure 12.

 Figure 12: Horizontal variation of resistivity (Profile 1 & 2).

Schlumberger method 

In total, 12 vertical electrical sounding (VES) (Table 2) locations were investigated. The percentage relative root mean square (RMS) errors and interpretive models for each VES station provide a quantitative assessment of the method's quality. The number of layers encountered in soundings ranged from two to three.

Sl. No.

R1

T1

R2

T2

R3

T3

R4

RMS Error (%)

VES1

114.3

8

19.7

76.4

2138.7

­­­­­­­-

­­­­-

8.95

VES2

94

5.6

4.3

4.3

24

­­­­­­-

­­­­-

6.11

VES3

97

6.4

17.6

36.4

836

­­­­­­­-

­­­­-

9.69

VES4

256.3

15

12.5

26.6

122.7

­­­­­­­-

­­­­-

5.66

VES5

593.7

4.1

4.1

34.1

369.4

­­­­­­­-

­­­­-

7.19

VES6

316.5

2.4

98

60.3

13.6

31

2930

7.32

VES7

263

6.8

18.6

18

1594

­­­­-

­­­­-

8.91

VES8

1175.5

8.7

39.3

­­­­-

­­­­-

­­­­-

­­­­-

18.32

VES9

89

4.1

8.5

5.1

41.8

21.6

10.6

6.54

VES10

70.7

2

8

19.2

72.1

­­­­-

­­­­-

4.03

VES11

29.1

7.8

14.3

47.1

84.3

­­­­-

­­­­-

3.2

VES12

4965

3

24.5

8

1760

­­­­-

­­­­-

16.18

Table 2: Geo-electrical Parameters of Vertical Electrical Sounding points.

Qualitative Interpretation of VES Curves

In the qualitative interpretation approach, the number of layers and their resistivity are calculated using the field curve's shape as a guide. It provides details on the number of layers present, their continuity over the entire region, and the degree of homogeneity or heterogeneity present in each layer. The field resistivity sounding curves with two and three layers in the research area are shown in Figures 13a&b. This graph mostly contains H-type curves. The possible groundwater indication in the study area is reflected in it.

 Figure 13a: VES curves from VES 1 to VES 6.

 Figure 13b: VES curves from VES 7 to VES 12. 

Quantitative interpretation of VES

To validate the findings, the quantitative interpretation of the VES curve was connected and compared with the lithology based on the existing open well depth. The VES and lithology data match quite well up to this depth, as shown in Figure 14. Each layer's resistivity range and layer thickness (quantitative information) were found and are shown. VES 8 got a minimum of two layers, while VES 6 and 9 got a maximum of four layers, with the remaining layers all being three. The Figure 15a shows the spatial map of resistivity and layer thickness. The variation in the top layer of soil is shown on the layer 1 resistivity spatial map. Seashore sand and Aeolian soil both have resistivities that are greater than 400 ohm-m. A resistivity value of less than 50 ohm-m is produced by the clay type. In contrast to sandy clay, which produces values between 200 and 400 ohm-m, silty clay produces values between 50 and 200 ohm-m. Greater depth was found on the southern side of the recharge well, where the thickness of layer 1 varied from less than 5 to more than 11 metres. In the study region, different colours depict changes in layer thickness. 

Resistivity from the second layer was similarly divided into five groups. This was found to be less than 10 ohms in clay formation. In sandstone formations, 10 to 40 ohms were noted. Over 40 ohm-m indicates the presence of weathered sandstone. The second layer's thickness was determined to be deeper in the south-west direction. The eastern side has the lowest layer thickness. Resistivity spatial variation was only displayed in the third layer, as shown in Figure 15b. In this map, the calcareous limestone formation is under the recharge well and in the nearby south region (dark blue). But the hard rock formation is to the east of the recharging well (dark blue) (greater than 800 ohms). Due to deep bore wells and abandoned bore wells, the solution opening extends in green and yellow (from less than 100 ohm-m to 400 ohm-m). 400 ohm-m to 800 ohm-m was seen in limestone regions that had little erosion.

 Figure 14: Correlation of subsurface geology with resistivity depth profile at VES 6 & VES 9.

 Figure 15a: Spatial Resistivity variation and Thickness variation of layer 1 & 2.

 Figure 15b: Spatial variation of Resistivity of layer 3. 

  • Spatial Groundwater fluctuation mapping 

 A spatial groundwater fluctuation map of an open well for various time steps in mean sea level (msl) is shown in Figure 16. There are five different categories on this map. It is rather obvious that the forms of the nearby recharge wells vary continuously every fifteen days. An open well's water level is a dynamic representation of shallow aquifer groundwater storage. Irrigation tanks and infiltration techniques can refill shallow aquifers. However, the input from the recharge well abruptly stopped, causing the water levels in the nearby monitoring wells to plummet as a response. The Figure 17 displays a groundwater bore well fluctuation map for a number of time steps in mean sea level (msl). On this map, there are five different categories. The patterns of the neighbouring recharge wells are constantly fluctuating every 15 days, which is highly obvious. The fluctuating water level in the bore wells represents semi-confined or confined aquifer groundwater storage. Semi-confined and confined aquifers can be filled by vertical flow (leakage) from shallow aquifers and abandoned bore well systems as shown in Figure 18. However, when the recharge well's input abruptly stopped, the water levels in the monitoring wells dropped as a result.

 Figure 16: Spatial maps of Open well groundwater level at various time steps. 

Figure 17: Spatial maps of bore well groundwater level at various time steps.

 Figure 18: 3D view projection of Ayyankulam Recharge well system.

  • Discharge

The Table 3 displays the various locations in this study area where bore well discharge was measured. Most of the time, it is used for irrigation for 5 to 10 hours, occasionally longer. However, Q1, Q4, and Q5 values above 13 m3/hr suggest some relationship between the bore well and the recharge well. When the inflow to the Q6 recharge well was interrupted, it no longer functioned as a potential well, and the water level dropped. Hard rock terrain had a flow rate of less than 1 m3/hr. 

ID

Latitude

Longitude

Discharge (m3/hr)

Q1

8° 17' 04.88"

77° 50' 40.01"

14

Q2

8° 17' 00.27"

77° 51' 05.69"

5.6

Q3

8° 16' 58.21"

77° 51' 36.19"

0.72

Q4

8° 17' 28.66"

77° 50' 43.73"

14.5

Q5

8° 17' 50.89"

77° 50' 44.45"

13.8

Q6

8° 17' 59.87"

77° 50' 18.37"

6.3

Q7

8° 18' 52.91"

77° 51' 01.53"

0.8

Q8

8° 18' 14.46"

77° 51' 13.49"

0.9

Table 2: Pumping rate at the study area.

Conclusion

In this study, the water flow direction of a recharge well was identified using the magnetic method, a resistivity metre, and a water level indicator. The Ayyankulam recharge well receives water from the Nambiyar canal and Thangammalpuram irrigation tank. The magnetic method's results indicate a south-to-southwest direction of flow. Also, the resistivity method gives a similar direction of flow, but in this result, some quantitative depths are observed in the same directions. Twelve VES were performed to study the aquifer's structural and hydro-geophysical settings. The most common recorded curve type was an H-type curve. The main water-bearing layer was considered to be saturated unconsolidated sedimentary (sand) in the deeper aquifer (confined aquifer) and shallow sandstone/limestone aquifer (unconfined aquifer), and their resistivity values were calculated. From December 2021 to February 2022, groundwater levels were measured on a regular basis with a water level indicator. Readings were taken from both bore wells and an open well that are present in and around the Ayyankulam recharge well, i.e., approximately 45 km2, using a water level indicator. In this study area, the groundwater level fluctuated between 5 and 10 m (msl). Throughout the region, water is used for domestic and agricultural purposes. The presence of an unconfined aquifer flow in the southwest direction was found in the survey. However, to the south and southwest of the recharge well, some abandoned bore wells are not in use because they were filled with a solution of weathered limestone or sand, and some bore wells are still working properly. These bore wells aid in recharging the deeper aquifer. It is water from the recharge well that accelerates the recharge of confined aquifers via bore wells to the south. From the pumping tests conducted, it is found that groundwater discharge from the pumping well near VES 7 may take 14 cubic metres of water per hour in this research region, but groundwater discharge from the pumping well near VES 8, which is to the southeast of the recharge well, can take only 0.72 cubic metres of water per hour. Deeper aquifer recharges from the Ayyankulam recharge well have been found in the VES 7 area. This implies that the water is pouring through the Ayyankulam recharge well from the south and southwest directions. 

The study region is considered as soft rock zone because of the resistivity high limited up to 250 Ωm. below the top layer the intermediate resistive layer is observed with 70 Ωm as average resistivity and that can be interpreted as stiff kaolinite clay layer. Wherever the laterite patch is, absent in that places clay layer is observed that can be delineated based on the lateral resistivity variations between the consecutive sounding points. Thus whenever the dissolution canal dimension is increased it is capable to discharge huge amount of fresh water collected and accumulated from all the vein formations. If properly planned, this karst water can be utilized for satisfying public water needs and for irrigation because already the region facing severe water crisis is summer.

Recommendations

  • The study will reveal the natural recharge system relevant with lithological characterization to quantify the Groundwater system at each location along the study area. It will discriminate and evaluate the rate of improvement for groundwater quality in particular area. 
  • The data on rain water supply and create the natural recharge system in particular area will be useful to assess the erosion pattern in subsurface lithological pattern and to identify the impact of sea-water intrusion in coastal regions. 
  • This interpretation will be an eye-opener to understand the natural recharge system in other similar areas in Tamilnadu.

Acknowledgement

I wish to thank Dr. S.Rajakumar, Mr. S Suriyarajan, Mr. J Vignesh, Mr. J S Jhon Algin, and Mr. Ahamed Kabeer Rahmathullah, M.Sc. graduate students from the Centre for Geotechnology, Manonmaniam Sundaranar University, Tirunelveli, for their support in collecting high-quality field data during the field survey. I would also like to thank the registrar of M.S. University for allowing me to do this research independently.

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Citation: Mookiah M, Yasala S, Sakthivel S, Nainarpandian C, Vaidhyanathan R, et al. (2023) Identification of Artificial Groundwater Recharge System near Ayyankulam Village of Tirunelveli, Tamilnadu, India Using Electrical and Magnetic Survey. J Atmos Earth Sci 7:42.

Copyright: © 2023  Mathiazhagan Mookiah, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


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