Delineation of freshwater zones in the shallow coastal aquifers of Ernakulam-Chettuva region, Central Kerala, India using electrical resistivity methods


 Freshwater zones in the shallow aquifers extending from Ernakulam to Chettuva region, central Kerala coast has been studied using electrical resistivity methods. Seasonal salinity patterns and hydrochemistry in the shallow aquifers in the region also has also been studied. The coastal zone is made up of shore parallel ridges and runnels formed by alternate marine trangressive and regressive events. Vertical Electrical Sounding (VES) was carried out at 33 locations in the area extending from the beach to approximately up to 10 km from the shoreline. The subsurface lithology and freshwater layers in the aquifer system was delineated from the resistivity model. The saline zones are intervening with fresh water zones at different depths along the coast. The salinity in the aquifer system seems to be either due to brackish water ingress or paleo-salinity. 107 dug wells from the study area were also monitored during 2008–2010 period in pre-monsoon, monsoon and post-monsoon seasons. The water samples collected in the post-monsoon 2008 was analysed for the physico-chemical parameters viz., pH, temperature, EC, total alkalinity, salinity, turbidity, TDS, chloride (Cl−), total hardness, Ca2+, Mg2+, Na+, K+, CO32−, HCO3−, Cl−, SO42−, NO3−N and Fe2+. The pH of the water samples varied from 4.47–8.32 in the post-monsoon season and 3.5–9.21 in the pre-monsoon and 5.55–9.05 in the monsoon season. The EC values ranges from 58-4500 µ mhos/cm in the post-monsoon, 59-3753 µ mhos/cm in the pre-monsoon and 51-2637 µ mhos/cm in the monsoon season. TDS values ranges from 35-2700 mg/L in the post-monsoon, 38-21190 mg/L in the pre-monsoon and 32-1668 mg/L in the monsoon season. Hill-Piper diagram indicated Ca-HCO3 is the dominant water type followed by Na-Cl type and Na-HCO3 type. Other water types are Ca-Cl, Mg-HCO3, Mg-Cl, Ca-SO4 and MgSO4.


Introduction
Groundwater salinization is extensive and represents a special category of groundwater pollution especially in the coastal regions. Most of the freshwater salinization along the coastal areas and oceanic islands are due to the encroachment of seawater through surface and subsurface pathways (Gingerich and Voss 2005;Post 2005;Trabelsi et al. 2007). Seawater intrusion has received wide attention in the most populated coastal areas since almost 50% of the world's total population lives within 60 km of the shoreline (Chen et al. 1997;Steinich et al. 1998;Izuka and Gingerich 1998;Salama et al. 1999;Radhakrishna, 2001;Edet and Okereke 2001;Lee et al. 2002;Cardona et al. 2004;Post 2005;Kallioras et al. 2006;Sherif et al. 2006;Bridge and Allen 2006;Feseker 2007;Tweed et al. 2007). Even though saline intrusion is a natural process, it becomes an environmental problem due to excessive pumping of fresh water from an aquifer reduces the water pressure and intensi es the effect, drawing salt water into new areas. When freshwater levels drop, the intrusion can proceed further inland until reaching the pumped well (Chandler and McWhorter 1975;Reilly and Goodman 1987;Kukillaya et al. 2004;Khan and Sapna, 2004;Pareek et al. 2006). There are different causes for the salinization of fresh water zones of aquifers; which mainly depends on local hydrogeological and geomorphic characteristics. Presence of salinity in coastal aquifers can be detected by geophysical and geochemical methods. Electrical resistivity of water bearing rocks largely depends on the amount of water saturation, the chemical composition and temperature, and the distribution of water. Groundwater columns in the shallow coastal aquifers can be characterized into fresh water, brackish water and saline water using electrical resistivity methods, employing appropriate electrode con guration. Electrical resistivity techniques were used by many researchers for the delineation of fresh-saline water interface (Ebraheem et al. 1997;Hopkins and Richardson 1999;Tronicke et al. 1999;Edet and Okereke 2001;Lee et al. 2002;Wilson et al. 2006;Bataynch 2006;Sherif et al. 2006;Morrow et al. 2010;Mario et al. 2011;McInnis et al. 2013;Befus et al. 2014;Whitman and Yeboah-Forson 2015;Bahija et.al. 2018).
The problem of salinization of coastal aquifers in Kerala has been mainly reported from Ernakulam, Thrissur and Alleppey districts (CGWBKR 2007). In Ernakulam, seawater incursion is severe around at Chellanum and Vypin coasts. It is also found along Azhikode-Kodungallur and Vatanapally-Chavakkad-Punnayurkulam area of the coast in Thrissur district. The problem has become severe by excessive groundwater draft -Kodungallur in Thrissur district and Paraur of Ernakulam district are over-exploited and semi-critical blocks respectively (CGWBKR 2007). Brackish water intrusion through surface water bodies and tidal inlets is a major cause of increasing salinity in the lowland areas viz., Thannermukkam bund and Cherthala-Alleppey coast in Alleppey district, at Enamakkal saltwater regulator and Kole land basin in Thrissur district (Lakshminarayanan and Siva Prasad Rao 1988;Vatakkepat and Narasimha Prasad 1991;Priju et al. 2018).
The study area is part of central Kerala coast from Ernakulam (Vypin) in the south to extending upto Chettuva in the north. Geomorphologically the area is part of the back barrier-lagoon complex system anked by Lakshadweep Sea in the western side and Vembanad lagoon and Canolly canal in the eastern side. The Vembanad backwaters and Thrissur Kol wetlands lies in the eastern side of the area drained by four rivers viz., Periyar, Chalakudy, Karuvannur and Keecheri-Puzhakkal carrying large volume of sediment load in this region. The coastal barrier land is mainly made up of sand to clayey-silty sand deposits (strandplain) with alternate ridge-runnel topography (Narayana and Priju 2006). Geologically the Warkalli and Vaikom beds (Tertiary age) underlie the strandplain deposits followed by the crystalline basement (Charnockites and hornblende biotitie-gneiss). Quilon series, a layer of lime shell bearing marine clay deposit is found pinching out within the Warkalli beds. Both Warakalli and Vaikom beds are fresh water bearing formations and consist of sandstone and conglomerate respectively. Periyar River is the major river with a length of 244 km and basin area of 5398 km 2 . Chalakudy River has a length of 130 km with a basin area of 1704 km 2 . Karuvannur river has a length of 48 km with a basin area of 1054 km 2 . Keecheri-Puzhakkal river has a length of 80 km with a total basin area of 635 km 2 .
Geologically the study area is mainly covered with paleobeach deposits (Guruvayur formation) and younger beach deposits (Kadappuram formation). It is intervened by uvial (Periyar formation) to uviomarine deposits (Viyyam formation) in the northern parts of the study area. The eastern parts are covered by Charnockites and biotite gneiss with alterites (laterites).

Materials And Methods
Geo-electrical sounding (vertical electrical sounding) was carried out along eight east-west cross-shore traverses (A-A¢, B-B¢, C-C¢, D-D¢, E-E¢, F-F', G-G' and H-H') perpendicular to the coast line during premonsoon season ( Figure 3). The survey was conducted applying Schlumberger electrode con guration using CRM 500 Aquameter with a maximum electrode separation (AB/2) of 100 m. VES survey was carried out at 33 stations and the apparent resistivity data was interpreted using ZondIP software. The seasonal water quality parameters (pH, EC, TDS, salinity and alkalinity) measured from the shallow dug wells during post-monsoon 2008, pre-monsoon 2010 and monsoon 2010 season. Laboratory analysis of the post-monsoon 2008 water samples were carried out (cations, anions, total hardness, carbonate, bicarbonate, iron, nitrate and turbidity). Water type of the post-monsoon 2008 samples were found out employing Hill-Piper diagram.

Results And Discussion
3.2 Water quality variations 3.2.1 pH pH shows alkaline nature in the coastal side of the post-monsoon 2008 well water samples. It is becoming acidic towards eastern side of the study area. Acidic to neutral nature is increasing in the monsoon and pre-monsoon water samples (Figure 4). In the post-monsoon season (2008), the pH of the water samples varies from 4.47 to 8.32. pH varies from 5.55 to 9.05 in the monsoon season (2010) and varies from 3.5 to 9.21 in the pre-monsoon season (2010).

Temperature
In-situ measurements shows the temperature of the pre-monsoon well water samples is higher compared to monsoon ( Figure 5). In the pre-monsoon 2010 season, the temperature of water samples varied from 25.33ºC to 33.71ºC. In the monsoon season the temperature of the water samples varied from 24.88ºC to 29.51ºC.

Total Dissolved Solids (TDS)
The TDS values are higher in the post-monsoon 2008 well water samples towards the southern part of the study area. Generally the TDS values are higher in the monsoon water samples compared with postmonsoon ( Figure 6). The TDS of the water samples varied from 34.8 mg/L to 2700 mg/L in the postmonsoon 2008 season. It varies from 38 mg/L to 21190 mg/L in the pre-monsoon 2010. TDS values ranges from 32 mg/L to 1668 mg/L in the monsoon 2010 season.

Electrical Conductivity
The EC values are comparatively higher in the southern part of the study area compared to north in the post-monsoon 2008 (Figure 7). The EC values of the well water samples ranges from 58µS/cm to 4500µS/cm in the post-monsoon 2008 season. It varies from 51 µS/cm to 2637 µS/cm in the monsoon season. EC values ranges from 59µcm to 3753 µS/cm in the pre-monsoon 2010 season. Higher EC values are seen in the shoreward water samples in the monsoon season.

Salinity
Salinity values of the well water samples are generally higher in the pre-monsoon. The salinity values of the water samples ranges from 0.03 ppt to 20.17 ppt in the pre-monsoon 2010 season. It varies from 0.02 ppt to 1.32 ppt in the monsoon 2010 season.

Dissolved Oxygen
During monsoon season, the DO content of the well water samples varied from 0 to 284 mg/L. In the premonsoon 2010 season, the DO content varied from 0.07 mg/L to 13.21 mg/L. The DO content is higher in the middle portions of the study area in the pre-monsoon season compared to monsoon season in which higher in the southern parts ( Figure 9). The total hardness values of the well water samples varied from 15 mg CaCO 3 /L to 475 mg CaCO 3 /L in the post-monsoon season. TH is higher towards the southern and western part of the study area compared to northern and eastern part ( Figure 10).

Calcium (Ca 2+ )
The calcium content of the well water samples in post-monsoon 2008 varied from 2 to 90 mg/L. Calcium content is higher shoreward and towards southern part of the study area ( Figure 11).

Magnesium (Mg 2+ )
The magnesium content of the well water samples varies from 1.1 mg/L to 92.7 mg/L in the postmonsoon 2008. Magnesium content is higher towards the southern and northern part of the study area ( Figure 12).

Sodium (Na + )
The sodium content of the well water samples varied from 3.1 mg/L to 710 mg/L in the post-monsoon 2008. Sodium content is found higher in the southern part of the study area ( Figure 12).

Potassium (K + )
The potassium content of the well water samples varied from 0.65 mg/L to 43 mg/L in the post-monsoon 2008 season. Potassium content is higher in the southern part of the study area compared to northern part ( Figure 13).

Carbonate (CO 3 2-)
The carbonate content of the well water samples varied from 0 to 24 mg/L in the post-monsoon 2008 season. Carbonate content is higher in the middle part of the study area compared to the northern and southern parts (Figure 14).

Bicarbonate (HCO 3 -)
The bicarbonate content of the well water samples varied from 7 mg/L to 734 mg/L in the post-monsoon 2008 season. Bicarbonate content is higher in the southern part of the study area compared to the northern part ( Figure 15).

Total Alkalinity (TA)
The total alkalinity in the well water samples varied from 6 to 602 mg CO 3 /L in the post-monsoon 2008 season. Total alkalinity is higher in the southern part of the study area compared to the northern part ( Figure 16).

Iron (Fe)
The Fe content in the well water samples varies from 0 to 9.22 mg/L in the post-monsoon 2008 season. Fe content is higher in the middle and southern part of the study area compared to the northern part ( Figure 17).

Chloride (Cl -)
The chloride content in the well water samples varied from 1.27 mg/L to 186 mg/L in the post-monsoon 2008 season. Chloride content is more or less uniform, but slight increase is noted in the northern and middle part of the study area ( Figure 18).

Sulfate (SO 4 2-)
The sulfate content in the well water samples varies from 0 to 161 mg/L in the post-monsoon 2008 season. Sulfate content is generally higher in the eastern part of the study area compared to the western part ( Figure 19).

Nitrate (NO 3 -N)
The nitrate content in the well water samples varied from 0.01 mg/L to 6.05 mg/L in the post-monsoon 2008 season. The nitrate content shows higher values in the northern and southern part of the study area as well as eastern part in the middle portion ( Figure 20).

Turbidity
The turbidity of the well water samples varied from 0 to 11 mg/L in the post-monsoon 2008 season ( Figure 21).

Hill-Piper Diagram
Hill-piper diagram of the post-monsoon 2008 water samples was plotted and 8 water types were found. Majority of the water samples are Ca-HCO 3 type (55 Nos) followed by Na-Cl type (19 Nos). It is followed with Na-HCO 3 type (13 Nos.). 6 samples show CaCl type and 5 samples show Mg-HCO 3 type. Two samples each are CaSO 4 type and MgCl water type. One sample belongs to MgSO 4 type.

Geophysical Investigation
Geophysical investigation (VES) was carried out in eight cross-shore traverses in the study area with schlumberger con guration. Eight cross-shore pro les were constructed. Maximum current electrode separation is 100 m.

Vertical Electrical Sounding
Vertical Electrical Soundings were carried out at 43 locations. It is found that majority of the curves are Qtype (16 nos) followed by H-type curves (13 nos). K-type curves are obtained from 3 locations. KA-type and QA-type curves are seen from one locations each.  Table 1.  The pro le B-B' shows a similar trend with high resistive zone in the shoreward direction and low resistive zone in the landward direction. The presence of freshwater lens in the shoreward region and saline wedge in the landward region is prominent.

Pseudo section
The pro le C-C' indicate the presence of high resistive freshwater zone in the western side in the shoreward zone and low resistive zone towards the landward side. Low resistive saline wedge is seen towards depth.
In the pro le D-D', the high resistive zone is seen in the shoreward side with a saline wedge in the bottom extending towards landward side towards east.
The pro le E-E' shows the high resistive freshwater lens in the middle of the land area and low resistive zones in the eastward and westward direction. The saline wedge is at depth in the shoreward direction.
In the pro le F-F', a clear high resistive freshwater zone is found in the landward side whereas the low resistive saline zones are seen towards the shoreward direction.
In the pro le G-G' high resistive zones are seen in the topmost portions and towards east. Saline zones are seen towards depth and in the shoreward direction.
In the pro le H-H'. high resistive zones are seen in the western side into the middle and is thinning towards east. Saline wedge is seen in the bottom and towards the eastern parts.

Conclusions
Spatially, it is observed that in majority of the areas, brackish water intrusion into the freshwater aquifers is the major reason for salinity. The seasonal variation in TDS, salinity and anion content is in uenced by the seasonal riverine and tidal ux. In some of the areas along the coast where sand ridges are predominant, freshwater aquifers are not affected by saline water intrusion. Hill-piper diagram of the well water samples indicates the presence of dominant bicarbonate type (Ca-HCO 3 ) water followed by NaCl type and Na-HCO 3 type water. The presence of Q-type curve in most of the locations of resistivity sounding indicate the presence of saline water upconing. Presence of H-type curve in rest of the locations indicate fresh water zones towards depth. Cross-sectional pro les indicates variations in apparent resistivity across the coast from beach to landward direction. The pro les indicate the presence of high resistive zone in the seaward direction at most of the places indicating the freshwater lens in the seaward zone. Low resistive saline zone is indicated towards east and also shows a thickening trend due to saline water upconing.