Study on the impact of tsunami on shallow groundwater from Portnova to Pumpuhar, using geoelectrical technique - south east coast of India

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1 Indian Journal of Marine Sciences Vol. 37(2), June 2008, pp Study on the impact of tsunami on shallow groundwater from Portnova to Pumpuhar, using geoelectrical technique - south east coast of India S. Chidambaram 1, AL. Ramanathan 2, M.V. Prasanna 2, D.Lognatan 2, T.S. Badri narayanan 3, K. Srinivasamoorthy 2 & P. Anandhan 2 1 Department of Earth Sciences, Annamalai University, Annamalai Nagar, Tamilnadu, India 2.School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India 3 #97, Agraharam street, annaikaran chattram (P.O), Coleroon, Tamilnadu , India Received 11 January 2008, revised 22 May 2008 Andaman Sumatra tsunami has caused distress to humanity and natural environment. Damage on the natural system has to be assessed scientifically for sustainable development. Geoelectrical studies are the realistic approach for comparing behavior of aquifer before and after tsunami. A study has been conducted on a shallow aquifer in the coastal region to assess salinity variation due to the impact of tsunami. Significant variations were observed in apparent resistivity values, due to percolation of sea water into shallow aquifers. Transformation of curve types has been noted in few regions in various aquifer depths. Changes in the formation resistivity and formation factor have also been noticed, which indicate salinity increase in aquifers. Geoelectrical cross section of the aquifer shows that perched water lens identified has also been affected by tsunami stress. Keywords: Tsunami, Apparent Resistivity, Shallow aquifers, Tamilnadu. Introduction Most tsunamis are caused by underwater earthquakes in which fault slippage moves huge segments of oceanic crust vertically, displacing water column over it. Off the coast in deep water, the initial height may be only a meter or two, even for major tsunamis. But as they travel from deep to shallow waters, waves slow down, their wavelength shortens, and their height increases dramatically causing distress to environment.. Tamil Nadu is the most severely affected state in India than the other states due to Tsunami of December 12, This coastal region has an extensive agricultural area depending on the groundwater for irrigation. The presence of estuaries and lagoonal back water also causes the ground water to become saline 2. Tsunami waves encroached the coastal ecosystem and changed the geomorphology, sediment characteristics and water quality 3,4 Deeper aquifers of coastal environment are fragile and are under the threat of salt water intrusion due to over exploitation. This becomes more serious when shallow aquifer is also contaminated by saline waters due to invasion and percolation of sea water by 2 Corresponding Author: alr0400@mail.jnu.ac.in the giant waves. The groundwater samples were collected and resistivity surveys were conducted, before tsunami from this study area as a part of a Ph.D thesis. Hence an attempt has been made in bringing out the variation in salinity in the groundwater before and after tsunami. The geoelectrical properties of the aquifer have been studied in the shallow aquifers of the study area to achieve the above mentioned objective. Study area The study covers from Parangipettai to Pumpuhar along the coastal Tamilnadu from 79 70' E to 79 86' E and 11 12' N to 11 51' N (Fig 1). The Vellar and Coleroon are major rivers flowing in the study area which form an Estuary with marshy mangrove environment at Pichavaram. Small river Pattinatharu flows in southern part of study area near Pumpuhar. Maximum temperature ranges between C and minimum temperature ranges from C. The area receives maximum precipitation from the north-east monsoon (53%) followed by south-west monsoon (33%) and average precipitation is mm/yr. Quaternary formation of the study area consists of fluvial, fluvio-marine and marine

2 122 INDIAN J. JUNE. SCI., VOL. 37 NO. 2, JUNE 2008 Table 1 Geological succession of the study area Era Age Formations Lithology Quaternary Recent to Alluvium & Soils, alluvium Sub-Recent Laterite and coastal sands, Clays, kankar and Laterite Figure 1 Location map of the study area. Tertiary UNCONFORMITY Mio-Pliocene Cuddalore Sandstones pebble bearing grits, UNCONFORMITY Argillaceous and Clays (variegated) with lignite seams and pebble beds. sedimentary facies. It includes various types of soils, fine to coarse grained sands, silts, clays, latrite and lateritic gravels. Fluvial sediments occupy flood plains, Vellar and Coleroon Rivers. It consists of mainly sands, sandy loams or clayey loams. Irregular mounds of 10 to 15 m height are prominent feature due to wind action near Porto Novo. Geological succession is shown in Table 1. Bathymetry contour intravels are less in north than southern pat of the study area indicating steepness of the region. Water table in the study area ranges from 5 20 mgbl during pre-monsoon and between 2-10 mbgl during post monsoon. Similar changes caused by changes in hydrostatic pressure due to earth quake induced changes in crustal volumetric strain were recorded in Hyderabad pluton 5.. Water level observations for certain locations in the study area (Fig 2) after tsunami indicate that there has been a considerable increase in water level at certain location accompanied by fall in certain regions. A study on the permeability of the alluvial aquifers 6 in the shallow aquifers of the study area reveals that the yield of these wells ranges from l for drawdown ranging from m. Transmissivity is also in higher ranges from m 2 /day. Movement of water from one region to other is also governed by hydraulic conductivity (K) ranging from m/d. Materials and Methods The Geophysical survey was conducted by DDR3- resistivity meter, adopting Schlumberger electrode array with maximum spacing (AB/2) in the range of 2 90m. Pre-tsunami resistivity was carried to a depth of 50m depth during August, 2004 and post-tsunami survey was done in February 2005 at different locations (Fig 3). Interpretation of resistivity curves was done by curve matching technique using standard graphs 7 and master curves 8 for determining thickness and resistivity of corresponding layers. Impact of tsunami was noted to be higher in the shallow aquifers 9,10. Hence the geophysical survey was restricted to shallow aquifers. Results and discussion Geoelectrical characteristics Interpretation of resistivity sounding data indicates that shallow subsurface in study area can be broadly classified into four groups of geoelectrical models Type I,II,III & IV, Corresponding to A, K, H, Q curves of Zhondy et al (1974). Table 2A and 2B, shows distribution and characteristics of curves for pre and post-tsunami. Model I The first model is A type curve obtained by geoelectrical survey at sites 16, 14, 11, 10, 3,4, 8, 7 in

3 CHIDAMBARAM et al.: IMPACT OF TSUNAMI ON SHALLOW GROUNDWATER 123 Figure 2 Groundwater levels of the study area for pre-tsunami (Nov 2004) and post-tsunami (Jan 2005). these region ranges from m. Water samples collected from these locations have specific conductance ranging from µs in posttsunami. Figure 3 Vertical electrical sounding locations of the study area post-tsunami and location 1 in pre-tsunami (Fig 4A and 4B) the model represents increase of resistivity values with depth (ρ 1 < ρ 2 < ρ 3 ).The deeper layer having high resistivity values than the shallow. Top layer thickness ranges from (1-10m), which is composed of sandy alluvium and middle layer have a thickness of (2.5m to 3.5m). Static water level of Model II Results from K-type curve by geoelectrical survey in sites 2, 12, 6 in post-tsunami and 2, 3, and 4 in pretsunami (Fig 5A and 5B), indicates higher resistivity layer is sandwiched between more conductive (or less resistive) top and bottom layers (i.e.) ρ 1 < ρ 2 >ρ 3. The top layer corresponds to dry silty sand with vadose water, having an average thickness of 2.7m (Table 3A and 3B); the middle layer is composed of mostly medium to coarse sand with average thickness of 17 m. This is the major aquifer unit tapped by boreholes in study area. The bottom layer may be clayey or sandy layer with high conductive water in the pores. The static water level of these regions ranges from m. Electrical conductivity of water was 1400 µs in pre-tsunami and 1186 µs in post-tsunami. Model III The third geoelecrical model represents H type curve. They are noted in locations 9,5 in post- Tsunami and 5, 6, 7, 8 in pre-tsunami (Fig 6A and

4 124 INDIAN J. JUNE. SCI., VOL. 37 NO. 2, JUNE 2008 S. No Location Table 2A Resistivity survey values Pre-tsunami I Layer II Layer III Layer Resistivity Thickness Resistivity Thickness Resistivity Thickness Curve type 1 Kuliyar A 2 M.G.R Thittu K 3 Kavarapattu K 4 Perunthotum K 5 Palayar H 6 Killar H 7 Kulaiyar H 8 T.S pettai H 9 Thoduvai Q 10 Pumpuhar Q Table 2B Resistivity survey values Post-tsunami 1st Layer 2nd Layer 3rd Layer S.No Location Thickness Resitivity Thickness Resitivity Thickness Resitivity Curve type 1 Kulaiyar A 2 Thirumullaivasal A 3 Killai A 4 T.S.Pettai A 5 Melmuvakkarai A 6 Puthukkuppam A 7 Portonova A 8 Thandavankulam A 9 M.G.R Thittu K 10 Poombugar K 11 Kottayamedu K 12 Nayakkan Kuppam H 13 Palaiyar H 14 Manampadi Q 15 Kilperumpalam Q 16 Perunthotam Q 17 Thiruvenkadu Q 18 Neithalvasal Q 19 Vazhuthalaikudi Q 20 Pitchavaram Q 6B).Top layer shows higher resistivity and corresponds to dry coarse sand about 5 m thick. Middle layer is relatively conductive (p1>p2<p3), with lower resistivity values ranging from 1.7 to 27 Ωm, which corresponds to saturated sand (fine medium grained) about 12m thick. The underlying formation are also aquiferous in nature, composed of saturated medium grained sands with still higher resistivities of Ωm. Average specific conductance and static water level of the locations in post-tsunami are1890 µs and 3.12 m respectively. Model IV The fourth geoelectrical model results from Q type

5 CHIDAMBARAM et al.: IMPACT OF TSUNAMI ON SHALLOW GROUNDWATER 125 sounding curve obtained at 1, 17, 18, 19,20,15,13 in post-tsunami and 9 and 10 in pre-tsunami; (Fig. 7A Figure 5A Curve type K : Pre-tsunami Figure 5B Curve type K : Post-tsunami Figure 4A Curve type A : Pre-tsunami Figure 4B Curve type A : Post-tsunami and 7B).The model is characterized by decrease in resistivity, ρ 1 >ρ 2 >ρ 3, from high values of top layer to lower values in the subsequent layers. Water samples collected from these region ranges from depth of 3.45 mbgl, with average specific conductance of 1890 µs in pre-tsunami and 2581 µs in post-tsunami. Samples from shallow aquifer are comparatively fresh and deeper ones have saline nature indicating, lesser resistivity and shows evidence of saltwater intrusion. The higher variation of resistivity is observed in the present study, this may be attributed to the period of study (post-tsunami) as it was conducted during summer, February, Hence the salinity levels are more. High resistivity in pre-tsunami may be due to

6 126 INDIAN J. JUNE. SCI., VOL. 37 NO. 2, JUNE 2008 Table 3A Resistivity survey values from different curve types for Pre-tsunami Curve type Thickness Resistivity Thickness Resistivity Thickness Resistivit y A type Average p1<p2<p K type Average p1<p2>p H type Average p1>p2<p Q type Average p1>p2>p K type Maximum K type Minimum H type Maximum H type Minimum Table 3B Resistivity survey values from different curve types for Post-tsunami Curve type Thickness Resistivity Thickness Resistivity Thickness Resistivity A type Average p1<p2<p K type Average p1<p2>p H type Average p1>p2<p Q type Average p1>p2>p A type Mamimum A type Minimum Q type Mamimum Q type Minimun fresh water incorporated immediately after SWM, August It s noted that models II & IV are characteristic of coastal sites 11 with fresh groundwater and average specific conductance <100 µcm -1. Geoelectrical model IV is marked by saltwater intrusion with average conductance of 1800 and 2581 µcm -1 in pretsunami and post-tsunami, respectively. Representation of Q type curves are suspected for saltwater intrusion at deeper depth. Focusing on the impact of tsunami on shallow aquifers the quality at the depths is more considered. Hence the region of low ρa values reflecting high saline waters, with curve types may be A or K type believed to be due to tsunami suspect. Where, Q type and H type can be changed over to the above said tsunami suspects by reduction of ρa values in shallow aquifers (Fig 8A and 8B). Resistivity of the formation depends on the property of the fluid and medium. The electrical resisitivity sounding curves are characterized by spatial variability due to inhomogenity of subsurface. In a sedimentary region of Gawhati in Assam, the layers of Resistivity range from Ωm is characterized by fine sand and Clay and that of Ωm is demarcated by coarse sand with good water potential 12 In general the variation of resistivity is affected by three main reasons; presence of clay content, massiveness of rock and conductivity of fluid medium. Lesser resistivity was noted in certain regions mainly due to either presence of clay content or fluid with high conductivity 13 in the formation. Three basic types of classifications can be proposed based on the interrelationship between the aquifer matrix and pore fluid. Type A: Formation with intergranular porosity without clay content similar to non-shaly sandstone when saturated with fresh water has intermediate resistivity. Type B: Formation with intergranular porosity and clay content similar to shaly sandstone, when saturated with fresh water they have low resistivity values because the clays conduct electronic as well as electrolytic process. Type C: Formation with fractures, jointed and unweathered formations with no clay content with intergranular porosity, when saturated with fresh

7 CHIDAMBARAM et al.: IMPACT OF TSUNAMI ON SHALLOW GROUNDWATER 127 water they have relatively high resistivity. In addition to these three types 14 a fourth type D, was introduced, where the formation has high conductive water in the medium. This creates a drop conditions. The type II conditions are mainly based on properly of medium and type D is based on fluid content. Hence, when there is a variation in the water quality of the fluid in the matrix, type D can be Figure 6A Curve type H: Pre-tsunami Figure 6B Curve type H: Post-tsunami in the resistivity value due to the high conductivity values of the fluid in the medium. The drop in resistivity values in the formation is either due to the presence of Type B or Type D Figure 7A Curve type Q: Pre-tsunami Figure 7B Curve type Q: Post-tsunami adopted. Where the tsunami water stagnated along topographic lows they have infiltrated and contaminated the aquifer medium which has resulted in the decrease of resistivity values. Since the impact is noted in the shallow depths, the study has been concentrated on shallow aquifers in 3

8 128 INDIAN J. JUNE. SCI., VOL. 37 NO. 2, JUNE 2008 m, 5 m, 10 m and 15 m. Resistivity study of the formation for the shallow depth of 3 m indicate pretsunami samples range from moderate to saline, with in 50% of locations showing moderate salinity. After Figure 8A Curve transformations from Q to K type Figure 8B Curve transformations from H to A type the event almost all locations show salinity reflexes from resistivity values. Formation factor is obtained by ratio between formation resistivity and water specific conductance 15. It is suggested that a value of 4 is for fresh water aquifer, almost all samples in post event falls in saline nature and 6 samples of pretsunami falls in this category. EC for rest of the samples in the Pre tsunami was not available. Curves were drawn with the maximum ρa values for each depth. The resistivity curves of Pre-tsunami (Fig 9A) indicate that these values are ranging from Ωm with an average of Ωm. The curves were also drawn with minimum ρa values at different depth. Since the region fall in coastal tract minimum values observed are also lesser ranging between 0.41 and 0.52 Ωm. A curve was also drawn for the average ρa values for different depth and similar variations for maximum ρa values were also observed. Highest and lowest ρa values were noted in 3m depth. Similar curves were drawn for the post tsunami resistivity readings. Curve with maximum ρa values Figure 9A Resistivity values for shallow aquifer: Pre-tsunami Figure 9B Resistivity values for shallow aquifer: Post-tsunami of post tsunami (Fig 9B) ranges from Ωm. The average ρa curve has values between 2.1 and 5.2 Ωm. The minimumρa curve, notably has a lesser values than that of the pre-tsunami ranging from Ωm. Considering the specific depth as layer the curve drawn with maximum ρa values from each depth represent a Q type curve, the curve drawn with minimum ρa values represent, a H type curve and the average ρa values represent A type curve. The average curve of pre-tsunami represents Q type curve, and after tsunami, due to the subsequent percolation of saline waters into the aquifers the curve type has changed. The curve of post tsunami drawn with minimum ρa values also clearly shows in deeper aquifer the ρa values are higher than shallow, more saline water at 5m depth are noted than 3m at certain locations, may be due to movement of neighboring saline water down gradient or fast downward migration of saline water in that specific location.

9 CHIDAMBARAM et al.: IMPACT OF TSUNAMI ON SHALLOW GROUNDWATER 129 Resistivity values of pre-tsunami (Fig 10A) for these depth indicates in shallow depth have low resistivity ranges (0.1-5 Ωm) and few location have higher resistivities >40 Ωm, which is vise versa in A B Figure 10A Vertical distribution of resistivity for shallow aquifer: Pre-tsunami Figure 10B Vertical distribution of resistivity for shallow aquifer: Post-tsunami deeper regions (i.e. 15 m) but in certain location deeper depths also have low ρa values. In post tsunami scenario (Fig 10B) samples of shallow depths show low resistivity ranges and the deeper depths have higher ρa values. Hence, it is clear that after tsunami, shallow ground waters have been contaminated by saline waters in many locations. In general, the sandy aquifer of low resisitivity values of 10 Ωm is an indicative of salt water intrusion 16,15,17,18. However, higher values (in the order of 25 Ωm) were also used to explain the salt water intrusion 11. Spatial distribution of apparent Resistivity for 3m Spatial distribution of 3m ρa values of the pretsunami (Fig 11A) shows value range from <0.3 to >81.3 Ωm. The entire region can be categorized into four resisitivity zones. Two with low resisitivity values (Zone II and Zone IV) and two with higher resisitivity values (Zone I and Zone IV). Zone I is noted near the mouth of river Coleroon in northern part of the study area and zone II is between Thirumullaivasal and Pumpuhar. Zone III is located in between Vellar and Coleroon rivers. Zone IV lies between Zone I and Zone II. Apparent reisitivity values of 3m (Fig 11B) in post-tsunami scenario shows the similar trend of pre-tsunami, but resistivity value of entire region has reduced significantly. There is a considerable increase in the spatial distribution of resistivity in Zone II due to the invasion of tsunami waves, this may be explained by: Figure 11A Apparent resistivity for 3 m: Pre-tsunami. Figure 11B Apparent resistivity for 3 m: Post-tsunami. 1. The bathymetry of the region is gentler in southern part of study area and comparatively steeper in northern part. This has helped the giant waves to prograde easily inland. 2. There are a number of distributaries channels for Pattinatharu river, which has aided movement of wave

10 130 INDIAN J. JUNE. SCI., VOL. 37 NO. 2, JUNE 2008 inland and later to get percolated into alluvium causing salinity in shallow aquifers. The invaded sea water has been locked inland which has been subsequently infiltered. Geoelectrical section Location and thickness of fresh water lens obtained by the Geoelectical Cross Section (GECS). GECS was used to delineate fresh water zones of island aquifer of Lakshwadeep and thematic contour map was prepared, depicting the variation in thickness of fresh water (e.g. Ajay Kumar, 1966). Similar attempt made to draw the GECS for the section along A-A of the pre-tsunami and post-tsunami show a similar trend, but the resistivity values has been considerably decreased after tsunami (Fig 12A and 12B). It has been observed that higher resistivity values are colonized in the form of a lens at a depth of 5 8 m. It indicates that there may be presence of a perched aquifer. Towards north, due to the higher density saline water, the resistivity values have decreased significantly. The same scenario has been maintained after tsunami also, but the shape of the lens has been shifted to ellipsoidal because of recharge of denser Figure 12A Cross section of A-A : Pre-tsunami. Figure 12B Cross section of A-A : Post-tsunami. saline water from the distributary channels. It is also noticed that the freshwater at the southern part has been contaminated with the saline water, thus the resistivity values increases with depth. Conclusion The impact of tsunami on the shallow aquifer reveals that Q type of T.S.Pettai has been changed to K type and H to A in certain locations. The formation resistivity of shallow depths confirms that almost at all the locations show salinity reflexes in the resistivity values. The formation factor explains large samples after tsunami are in saline nature. Considering the specific depth as layer in posttsunami, curve drawn with maximum ρa values represent a Q type, minimum represent H type and the average represents A type curve. In the post tsunami, most of the samples of the shallow depths have low resistivity ranges and the deeper depths have higher ρa values. It is also inferred that in the spatial distribution of 3m resistivity, there is a considerable increase in the region occupied by low resistivity values in Zone II after the event, this is mainly due to gentle bathymetry at the south and also because of rivers with more number of distributaries. A fresh water lens has moved further north, after tsunami due to dense saline water compression from south (Thirumullaivasal). The lesser value of this lens may also indicate that the saline water has contaminated this perched aquifer. It is also inferred that the tsunami impact on the aquifer was more in the south than in the north. The present study explains that the shallow aquifers of this region are susceptible and respond to the changes to the natural forces. Reference 1 Pradip Kumar Pal. Impact of Earthquake on Geohazard Management. Everyman s Science, 4 (2005): Chidambaram S, Ramanathan AL, Anandhan P, Srinivasamoorthy.K & Prasanna. M.V. A 3. comparative study on the coastal surface and ground water in and around Puduchattiram to Coleroon, Tamil nadu. Special issue in International journal of Ecology and environment sciences, 31,3 (2005a) Prasad M.B.K & Ramanathan AL. Tsunami impact over the biogeochemistry of Pichavaram mangrove waters. Appl. Geochem 2006 (Submitted). 4 Chidambaram S. Water quality assessment in tsunami affected coastal areas of Tamilnadu Portonovo to Pumpuhar. DST Report, 2005b. 5 Muralideran D, Rolland Andradge G, Laxminarayanan T, Swathi T & Swetha T. The imprint of 26 December 2004

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