Identification of seawater intrusion by Cl/Br ratio and mitigation through Managed Aquifer Recharge in aquifers north of Chennai, India

JGWR (ISSN: 2321- 4783), Vol.2, Issue1, June 2013 Published by AGGS, India

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Identification of Seawater Intrusion by Cl/Br Ratio and Mitigation through Managed Aquifer Recharge in Aquifers North of Chennai, India

Indu S. Nair, S. Parimala Renganayaki and L. Elango

Department of Geology, Anna University, Chennai-600 025, India

Email: [email protected] Abstract

Groundwater chemistry is of great interest in coastal regions due to varying degree of mix-ing of groundwater with seawater. The objective of this study is to demarcate the seawater intruded areas of coastal aquifers located in Arani-Korattalai river basin just north of Chennai, India by geochemical signatures and identify the efficiency of the check dam as a method of managed aqui-fer recharge for mitigating seawater intrusion. Groundwater samples were collected and analysed for major and minor ions from over fifty wells located in this area during the month of July 2011. Ratio of Cl to Br in groundwater samples from the Arani-Koratallai river basin were compared with the ratio of Standard Mean Ocean Water (SMOW), which are about 288. Groundwater with Cl/Br ratio similar to that of SMOW is considered to be affected by seawater intrusion. Whereas, groundwater with Cl/Br ratio more than the value of SMOW is likely to be derived by mixing of domestic waste water and if this ratio is below SMOW is derived from the agricultural activities. The Cl/Br ratio was used to delineate seawater intruded region and up to 10 km distance from the coast is affected due to seawater intrusion. Further, a study carried out in the western part of this area to understand the effectiveness of managed aquifer recharge by check dam indicated that this structure is capable of increasing rainfall recharge by 37% and it is efficient in improving the groundwater quality. Hence, construction of check dams across the rivers at frequent intervals will assist in mitigating the problem of seawater intrusion. Keywords: alluvial aquifer, Cl/Br ratio, seawater intrusion, prevention, check dam, recharge 1. Introduction

Coastal aquifers are vulnerable to seawater intrusion especially in regions were groundwa-ter is over pumped. It is important to understand the extent of seawater intrusion in order to plan for remediation measures and to prevent deterioration in groundwater quality. Heavy demand of water for various domestic, irrigation, industrial purpose and rainfall recharge less than the groundwater abstraction are the main causes of the decrease in groundwater level in coastal aqui-fer. Decline in groundwater level below mean sea level causes the reversal of hydraulic gradient near the coast which results in seawater intrusion. Salinity in coastal aquifers is not only due to the seawater intrusion, but also may be due to other geogenic and anthropogenic sources such as eva-porate dissolution, leaching of fertilizer and pesticides from irrigated water, domestic waste water and salt pan activities. Seawater intrusion in coastal aquifer can be identified based on the hydro-geochemical signatures (electrical conductivity, ionic ratios), isotopic techniques and resistivity surveys (Gnanasundar and Elango, 1999; Bergelson et al., 1999; Kim et al., 2003; Marimuthu et al., 2005; Appelo and Postma, 1994; Richter and Kreitler 1993; Davis et al., 1998; Sathish et al., 2011). Among these methods, Cl/Br ratio are particularly good indicators of seawater intrusion and they both possess chemically similar characteristics. These ions are not affected by redox reac-tion, not adsorbed onto mineral or organic surface and they do not form insoluble precipitates, hence they are conservative in nature. The main purpose of this study is to identify the extent of


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Fig.1. Location of the study area with groundwater sampling wells

seawater intrusion and other causes of salinity in aquifers north of Chennai, India based on the Cl/Br ratio and to identify the efficiency of the check dam as a method of managed aquifer re-charge for mitigating seawater intrusion. This paper is based on the preliminary results obtained from two ongoing research projects.

2. Study Area

The study area forms coastal part of Arani- Koratalai river basin located north of Chennai, Tamil Nadu (Fig 1). The eastern side of the area is bounded by Bay of Bengal. The north and southern boundaries where fixed where the thickness of the alluvial formation is less than 5m. The western boundary was fixed arbitrarily. The annual average rainfall is about 1200mm, most of it falling during the north east monsoon (October-December). The Arani River is on the northern part of the area while the Koratallai river occupies the southern region (Fig 1). These two rivers are non perennial and normally flow only for a few days during north east monsoon period. Near the east-ern boundary of the area Buckingham Canal runs parallel to the coast and this carries saline water. After the monsoon period saline waters occupy the river up to a distance of about 6 km.

2.1Geology and Geomorphology

The maximum elevation of the study area is about 10 m in the west and sea level in the east. Thus the area gently slopes towards east. There is no remarkable elevation difference in the north south direction, except for the two river courses. Important geomorphic units include alluvial plains, shallow/deep pediments, pediplains, coastal plains/dunes and a number of paleo channels. Most of the area consists of alluvial plains, especially between the two rivers.

Geologically, the gneiss and charnockite rocks of Proterozic era occur as basement at ranging between 65 and 105 m (UNDP 1987). These rocks are overlain by the Gondwana forma-tion consisting of clay, shale, sandstone, conglomerate and boulders. Tertiary formations overlay these with clays, shale and sandstone. Alluvium of quaternary to recent period consisting of clay, slit, sand, gravel and pebbles occur at the top. The thickness of alluvium varies from 45m to 60m and the thickness is high between the rivers and increases towards the coast.

The alluvial deposits of about 60m thickness are water bearing and function as aquifers. Groundwater in the upper part of the alluvial formation with more amount of finer material with


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relatively low hydraulic conductivity occurs in unconfined condition. The groundwater table is gen-erally at a depth of about 5 to 7m. The interlaying clay and slit of varying thickness (2-5m) func-tion as aquitard, which is at depth of about 15m. The lower part of the alluvium with relatively high hydraulic conductivity functions as semi-confined aquifer. The piezometric surface is at a minimum elevation of about -20 m msl.

2.2 Land use

The entire area has been divided into consists of six classes of land use/land cover of which most area used for agricultural purpose is the maximum. Fig 3 shows that almost 75% of the area has been used for agriculture and the rice is the most important crop grown. Most of the farm sizes varied from 2 to 3 acres. The main cropping season is from September to January. The eastern part of the area (near the coast) is being used for salt pan activities which are classified as wetlands. The saline groundwater as well as seawater is pumped for these activities. Most of the settlements are located on the south-eastern region. 3. Methodology

Initially a well inventory survey was carried out to understand the types of wells and pumping pattern to choose sampling wells. Then, fifty representative wells (25 open and 25 bore wells) located were chosen for periodical monitoring of groundwater quality. Groundwater sam-ples were collected in clean polyethylene bottles of 500 ml capacity during June 2011. These sam-ples were collected from wells which are used for domestic purpose and for agricultural purposes. Samples were collected after pumping the wells for sufficient time so as to collect the formation water. Apart from this, a sample from the water stored by the check dam located 35km away from the coast and wells located surrounding the check dams were also collected during February 2012. The EC of the samples were measured in the field immediately after sampling using a portable

Fig.3. Land use/Land cover map of the study area

Fig.2. Geology of the study area


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multi parameter probe (YSI 556) which was calibrated before the analysis. Cl and Br concentra-tion in water samples were analysed using Metrohm 861 advanced compact ion chromatograph.

4. Results and Discussions

The two chemically conservative ions Cl and Br are good indicators of the seawater intru-sion and other salinity sources. These ions are considered to be conservative because they are not affected by redox reactions and they are not adsorbed into any mineral or organic surfaces (Fetter 1993). The plot on Cl/Br vs Cl of groundwater samples of bore well (> 10 m depth) and open well (< 10 m depth) are shown in Fig 4 and Fig 5 respectively. Cl concentration in this area varied from 22 mg/l to 17600 mg/l. Br concentration of this study area varied from 0.2 mg/l to 59 mg/l. Groundwater samples may have derived salinity from more than one source such as seawater from the Bay of Bengal, salt pan activities, agricultural return flow and domestic waste water. This plot of Cl/Br Vs Cl was used to demarcate the sources of groundwater salinity in the study area.

4.1 Seawater intrusion

Cl is the most abundant chemical constituent in seawater and the concentration of Br is very less. Ratio of Cl / Br in groundwater samples from the Arani - Korattalai basin were com-pared with the ratio of Standard Mean Ocean Water (SMOW). The Cl/Br weight ratio of SMOW

Fig.4. Cl versus Cl/Br ratio of groundwater from bore wells

Fig.5. Cl versus Cl/Br ratio of groundwater from open wells


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Fig.6. Demarcation of region affected by seawater intrusion based on Cl/Br ratio

is about 288 (Seawater concentration of Cl = 19,352 mg/l and Br = 67.3 mg/l). Groundwater with Cl/Br ratio similar to that of SMOW is affected by seawater intrusion (Richter and Kreitler 1993; Morris and Riley 1966; Manheim 1972; McCaffrey et al 1987). Whereas, groundwater with Cl/Br ratio more than the value of SMOW is likely to be derived by mixing of domestic waste water and if this ratio is below SMOW, it is derived from the agricultural activities.

This improper disposal of untreated domestic waste is the cause for high Cl / Br concen-tration in the study area. Figure 4 and 5 shows that, the points plotted above the SMOW line indi-cates the effect contamination by domestic waste water. Intensive agricultural activities of this study area may also contaminate the source groundwater due to the return flow from these activi-ties. The points plotting below the SMOW line in figures 4 and 5 with increased Br may be due to the enrichment of gypsum fertilizer and hydrolysis of methyl bromide in the soil, which releases in-organic Br to the shallow groundwater as reported by Vengosh et al. (2002). Two samples of this area are having high Cl and Br, which is due to salt pan activities (Fig 5).

4.2 Delineation of seawater intruded region

Based on the Cl / Br ratio the region affected by seawater intrusion and other anthropogen-ic activities are identified. Groundwater samples with Cl/Br ratio similar to that of SMOW are considered to be affected by seawater intrusion. The region comprising of wells with groundwater having Cl/Br ratio similar to the SMOW are indicated in Fig 6. This region is affected by seawater intrusion which is due to over pumping of production wells located just west of this intruded re-gion.


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5. Mitigation of Seawater Intrusion-Effect of a Check Dam across Arani River

Withdrawal of groundwater greater than the recharge from this coastal aquifer has resulted in seawater intrusion up to a distance of about 10 km as seen in Fig 6. There are many methods such as reduction in pumping, rearrangement of pumping wells, increase of groundwater recharge, row of injection wells, row of seawater pumping wells and subsurface barriers to prevent seawater intrusion. Out of these methods, increasing groundwater recharge is more suitable to control sea-water intrusion, as it can be implemented with reasonable cost and it do not require reduction or rearrangement of pumping wells. Increasing groundwater recharge in coastal aquifer system can be done by check dams, recharge ponds/tanks and spreading channels. It is proposed to construct a se-ries of check dams across Arani and Korattalai Rivers to mitigate the problem of seawater intru-sion. One such check dam in Arani river as a measure of managed aquifer recharge is taken up for the present study to identify its impact on groundwater system (Fig 7) (Parimala Renganayaki and Elango 2013a). This check dam is located at Paleshwaram village at a distance of about 35 km from the Bay of Bengal. This check dam is of 260 m length and 3.5m height. The storage capacity of this dam is 0.8 Mm3.

About 37% percentage of additional recharge is induced by this structure. Fig 8 shows the

plot between chloride and electrical conductivity values of water in the check dam and groundwater for the month of March 2012. The electrical conductivity and chloride value of water in the check dam is 596 µS/cm and 128 mg/l respectively. Well no 1, 5 and 4 are located closer to the check dam and electrical conductivity and chloride value of these wells varied from 700 µS/cm to 1100 µS/cm and 143 mg/l to 158 mg/l. Well no 3 and 29 are located away from the check dam and elec-trical conductivity and chloride values of these wells varied from 1555 µS/cm to 1770 µS/cm and 220 mg/l to 345 mg/l respectively.

The electrical conductivity and chloride values of the wells located far away from the check dam is much higher than the wells located closer to the check dam. The improvement in the

Fig.7. Location of Paleshwaram check dam across Arani River


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electrical conductivity and chloride concentration in well no 1, 4 and 5 is due to the recharge of wa-ter stored in the check dam (Parimala Renganayaki and Elango 2013b).

6. Conclusion This study was carried out to identify the seawater intrusion using the geochemical signa-tures like Cl/Br. The analysis using Cl/Br ratio indicated that the seawater is intruded up to 10 km from the coast. Study on managed aquifer recharge structure indicated that check dam is improving the groundwater recharge and groundwater quality. Hence, construction of check dams in this area across the rivers at frequent intervals will assist in mitigating the problem of seawater intrusion by increasing groundwater recharge. Acknowledgment

We wish to thank the Department of Science and Technology, New Delhi for funding this research under the Women Scientist Scheme (Grant SR/WOS-A/ET-49/2010(G)) and Water Technology Initiative (Grant AR DST/WAR-W/WSI/05/2010).

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Identification of seawater intrusion by Cl/Br ratio and mitigation through Managed Aquifer Recharge in aquifers north of Chennai, India

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