Saltwater intrusion in coastal aquifers

Abstract:
The Coastal aquifers are the major sources of water throughout the world. About 50% of the world population lives along the coastal zones and the figure is likely to expand by 75% by the end of this century. India has a long coastline where 25% of the country’s population lives in the coastal zone. The coastal zone is the most urbanized and industrialized area in the country when compared with other parts. This in turn results in over-exploitation of groundwater resulting in decline of water levels, leading to saline water intrusion which is a natural phenomenon. Electrical resistivity imaging (ERI) is a well-known technique to monitor fresh-salt water transitions. ERI was conducted in Pondicherry coastal region using WDDS resistivity meter and interpretation was attempted using RES2DINV software. The profile lengths were 150 and 300m for 5m and 10m respectively with a maximum depth penetration of 55.3m. The apparent resistivity measured varied from 0.1Ωm- 1100Ωm and different litho units were demarcated by correlation with litho logs. From the profile, the northern parts of the study area were found to be less affected by saline water intrusion and maximum intrusion was recorded along the southern parts of the study area. In certain locations the coastal dunes act as a barrier to prevent saline water intrusion. The 2D ERI profiles gave a clear image of geoelectrical heterogeneities, associated with seawater intrusion in the coastal aquifers, contributing to future measures towards a rational management of ground water resources in the area. Groundwater samples were collected in site specific locations and analyzed for major and minor constituents and correlated with the ERI techniques. The regions recorded with higher EC and Cl/HCO3 ratios were found to be in good correlation with already demarcated saline intrusion zones. The extent of saline water intrusion aided with water quality was effectively demarcated.

Key Words:Coastal aquifer• Saline intrusion, Electrical Resistivity imaging • Puducherry.

Introduction
The Coastal aquifers are the major sources of groundwater throughout the world. A total of 50% of the world population thrives along the coastal zones and the figure is likely to get worse by the end of this century. Saline water influences in coastal aquifers is of major worry (Batayneh, 2006) because it constitutes the commonest of the pollutants in freshwater aquifers. India has a long coastline where 25% of the country’s population lives in the coastal zone. Ground water is the main source of water supply in the coastal region for multiple purposes like irrigation, domestic, drinking and industrial purposes. The over drafting of ground water triggers the sea water intrusion into the coastal aquifer.Semi Arid climate and uneven rainfall resulting in low replenishment of ground water is also one such concern for saline intrusion. Monitoring of saline intrusion is vital for the coastal water resources management (Ginzburg and Levanon, 1976). The Electrical Resistivity Imaging (ERI) methods are adequate for sea water intrusion mapping because of the very low resistivity value of sea water (0.2Ωm)(Griffiths and Turnbull, 1985; Griffiths et al., 1990; Griffiths and Barker, 1993; Chien and Shih, 2007, Satish et., al 2011).The 2D imaging gives the best result for sea water monitoring and the extent of the sea water intruded into the aquifer. ERI gives the best resolution and assist in sea water identification and to know the sub surface condition. Hence an attempt has been made in the coastal aquifers of Union Territory of Pondicherry, which is one of the major tourist places in India. Expanding in population, tourist activities, agriculture and industries have endangered the coastal groundwater. The monitoring of the sea water and fresh water interaction is the significant concern in the Pondicherry territory because they are mainly depends on the sub surface water for various purposes. The present study aim is to monitor the sea water intrusion in the coastal shallow aquifer along the Pondicherry coastal track.

Study area
The area demarcated for the ERI study falls along the coastal margin of Bay of Bengal with longitude 79o48’10” N to 79o52’9.5”N and latitudes 11o52’58”E to 12o2’18”E with a total area about 40 sq. km. The geology of the area comprises of sedimentary formation ranging in age from Tertiary to recent.The upper Tertiary in the area are repesented by Cuddalore formations of Mio- Pliocene age. The litho units of the cuddalore sandstone comprises of pebbles and gravels, coarse-grained sandstones with minor clays and seams of lignite.

The thickness of these formations varies from 30 to 130 m with a maximum thickness of 450 m observed in Manapattu.

Recent (Quaternary) Formations
The Recent (Quaternary) formations are represented by laterites and alluvium. Lateritic soil occurs as thin cap over the Cuddalore formations. Thick alluvial deposits are noted along river courses of Ponnaiyar and Gingee which confluences the sea in the Pondicherry region. The alluvium in the area is composed of sands, clays, silts, gravels and kankar.(CGWB).

Hydrogeology
Cuddalore sandstone and alluvial formations are the major aquifers of the study area. Water level in Cuddalore Sandstones (Upper Tertiary), composing of sandstones and gravels, occupy an extensive area in the region. The thickness of the aquifer ranges from 20 to245m bgl (Below Ground Level). Alluvial aquifers, comprises Sands and gravels, form the potential shallow aquifer with thickness ranging between 5 and 34 m bgl.Water table elevation ranges between 10 and 25 m bgl irrespective of seasons.

Methodology
The fig.2 illustrates the data acquiring process in wenner-α configuration.

The ERI was carried out in the present study by multi electrode resistivity imaging using WDDS-2, D.C Digital Resistivity Meter. In this study multielectrodes (32 Nos.) were connected with the resistivity meter. Wenner - α configuration has been attempted for this study. The ERI profiling was attempted at fivelocations, perpendicular to the coast with an electrode spacing of 5m and/or 10m with a maximum profile length of 300m and the depth of investigation was 55.7 m. The resistivity obtained wasinverted to create a pseudo-section using RES2DINV software.

Results and Discussion
A total of five ERI profiles were carried out in this study area along (Fig.3).

Profile A-A
The first profile (Fig.4) was carried out 300m away from the coast bearing longitude 79o52’2” E and latitude 12o1’50”N with 5m inter electrode separation of 155m length. The profile identified a sand dune (resistivity ranging from 705Ωm- 1100Ωm) along the eastern part of the profile spread direction. The influence of the sand dune was found to be extending up to a depth of 6.76 m BGL. The top layer with resistivity range of 138Ωm to 312Ωm indicates the lateritic soil depth varies from 1.25m to 6.76m BGL. The resistivity value 138Ωm - 256Ωm indicates the Sandstone deposit at a depth about 8m depth. Comparatively a very low resistivity zone with a resistivity range of 2.34Ωm-9Ωm is noted at a depth of 21.5m representing the influence of saline water intruded in Sandstone.

Profile B-B
This profile (Fig.5) was carried 50m away from the coast bearing latitude 79o50’53” E, longitude 11o59’2” N. The profile had 155 m length and 5m electrode separation. The higher resistivity (546Ωm to 612Ωm) measured near to the coastal line, indicates the beach sand extending up to a depth of 4m BGL. The low resistivity zone (<4Ωm) is noted along the profile line, identified as the saline water - fresh water interface extending from a depth of 6 to 27 m. The resistivity ranges (25Ωm to 100Ωm) indicate the presences of Sand Stone layer at depth varying from 5 to 15m. A thin layer of lateritic soil is observed along the western part of the profile with resistivity values ranging from 70Ωm to 125Ωm.

Profile C-C
This profile (Fig.6) has been conducted at a distance of 20mtowards inland from the coast, bearing longitude 79o50’24” E latitude 11o57’23” N. The profile length attempted was 155m with a 5m electrode separation. A very low resistivity from 6.76 m (0.5Ωm to 2.59Ωm), observed along the eastern side of the profile line confirms saline water intrusion up to a depth of 22 m. A decrease in resistivity value was noted towards the ocean confirming the interface with the aquifer. Higher resistivity range (69.2Ωm to 102Ωm) confirmed the presence of lateritic formations from depth of 1.5m to 4m. A patch of high resistivity is observed at 80m spread at a depth of 13.4 m might be interpreted as a perched water zone.

Profile D-D
This profile (Fig.7) was performed at 20m away from the coast bearing longitude 79o49’53” E, latitude 11o55’23” N. The top layer has been demarcated as a coarse sand layer with a high resistivity (400Ωm-1000Ωm). The second resistivity zone (0.152Ωm-2Ωm) was identified at a depth of 7m near to the coast might be the layer intruded with saline water. A thick low resistivity zone (0.152Ωm-2Ωm) noted at 15m depth, in the mid of the profile confirms saline water intrusion. The Western part of the profile demarcates another low resistivity zone (0.152Ωm-2Ωm) might be the sediments saturated with poor water quality.

Profile E-E
This profile (Fig.8) has taken with 10m electrode spacing and length of 300m, bearing longitude 79o49’24”E, latitude 11o53’9”N. This ERI demarcates sand formations with varying grain sizes with resistivity range from 19Ωm- 55Ωm at 2.5 to 13.5 m depth. A low resistivity zone (8Ωm -15Ωm) proved the presence of clay formation at a depth of 13.5m. The presence of sand layer is identified with resistivity value of 19.5Ωm - 42Ωm up to a depth of 35m.

Conclusion
The ERI has been attempted in the coastal region of Pondicherry demarcates the saline water intruded zones. The saline water intrusion was observed to occur in the sand stone aquifers (Cuddalore sandstone). The saline water intrusion was observed to be varying irrespective of depth of the bore holes identified in the study area. The Profile A-A’,B-B’ and D-D’ were showed a deeper level (27m) of saline water intrusion indicating the presence of deeper wells within the profile area extracting groundwater are responsible for the intrusion of saline water. In profile location C-C’ the shallow part of the aquifer (6m) shows the traces of saline water intrusion, might be due to the shallow well depth observed along the profile direction. In profile E-E’ no traces of saline water intrusion has been observed but the lower resistivity observed at shallow depth is due to the presence of clay beds and has been confirmed with the litho logs of wells drilled near by the profile direction. The best possible prevention from sea water intrusion is to reduce the groundwater withdrawal and to prevent further exploration activities which have been already practiced by the Pondicherry Government. This method gives the finest resolutions and good accuracy for monitoring and evaluation of saline water intrusion. By this method sea water influence zones have been evaluated in union territory of Pondicherry.