ABSTRACT
The phenomenal increase in population of Chennai city has resulted in water demand in excess of regular source of water supply. The water demand was 1026 million litres per day (MLD) in 2001 and would become 1980 MLD (2021), while supply is expected to be around 1586 MLD, if full capacity of the reservoirs and the groundwater supply are to be projected (Dayamalar, 2011). The deficit in water supply is being managed at many a places through groundwater extraction. The southern Chennai coastal aquifers have been a sourceof water supply for more than a decade and there is lot of apprehension that coastal aquifers would be affected by seawater ingress. An attempt has been made to study the hydrodynamics in coastal aquifers of southern part of Chennai Metropolitan Area using groundwater flow modelling. The model has been calibrated and validated with the observed data. For the period between 2000 & 2005 predictive simulation was carried out for another ten years up to 2015. The various scenarios, viz., continuation of present groundwater development, increased groundwater development, reduction in groundwater development,augmentation coupled with changes in groundwater development were studied in the developed model. The recommendations have been made taking into consideration of groundwater potential and acceptability of the strategy by the local population. It is seen that reduction of the groundwater extraction without the participation of all the stake holders including public & government would be difficult. It would be ideal to reduce the groundwater extraction but being on the pragmatic side, it is suggested that the strategy comprising continuance of existing groundwater draft with augmentation in the select cells by 10%during the monsoon period may be adopted for the management coastal aquifers in the southern part of Chennai Metropolitan area.
INTRODUCTION
The phenomenal increase in population of Chennai city has resulted in water demand in excess of regular source of water supply. The water demand was 1026 million litres per day(MLD) in 2001 and would become 1980 MLD (2021), while supply is expected to be around 1586 MLD, if full capacity of the reservoirs and the groundwater supply are to be projected(Dayamalar, 2011). In order to meet the deficit the private entrepreneurs have resorted to marketing of water by mining water. The phenomenon of seawater ingress in the coastalaquifers in Minjur in north has diverted the entrepreneurs to exploit the coastal aquifers in the southern part of Chennai Metropolitan Area (CMA) and there is lots of apprehensionabout the fate of the coastal aquifers in the southern part of CMA in the coming years. The mushrooming of dwellings on the southern part of Chennai has resulted in the more stress on the fragile coastal aquifer system. In this scenario, an attempt has been made to study the hydrodynamics in the coastal aquifer systems so as to formulate strategies for groundwater management.
BACKGROUND INFORMATION
The coastal area on the southern side of Chennai city in Tamil Nadu, India, has been considered for the study. It is bounded by Bay of Bengal in the east and Kovalam Creek on the south while in the north Tiruvanmiyur area was taken as boundary and about 1–3 km west of Buckingham canal was taken as western boundary (Fig 1). The area is characterized by sub tropical climate and has an average maximum daily temperature of 430C during May/June and an average minimum daily temperature of 250C during January/February. The area receives rain under the influence of Southwest and Northeast monsoons. The southwest monsoon stretches from June to September while northeast monsoon is active between October to December. The normal annual rainfall (1971-2005) is 1458 mm of which 33% is received from SW monsoon and 60% from NE monsoon. Rainfall received during winter (January – March) and summer (April – May) months account for 3% and 4% of annual rainfall respectively. The area is characterised by gently sloping terrain, with sand dunes in general formed parallel to the coast. The general elevation of the terrain is between 3 and 10 m above mean sea level. The area is underlain by sand-clay admixtures followed by crystalline basement of Charnockites.
In general, the aquifer system comprises fine to coarse-grained sand, sand – clay admixtures and weathered basement comprising Charnockites. The unconsolidated formation can be further segregated locally into layers comprising Grey sand- clay intercalation (Zone III), Grey sand with argillaceous intercalations and characterised byshells (Gastropods and Lamellibranches) (Zone II) and brown/red sand (Zone I). Charnockites are the rock types constituting the consolidated formations. Drilling of about 10 m in the basement has revealed that the weathered and fractured rocks are encountered down to the drilled depth.
OBJECTIVE
The objective was to study the groundwater flow pattern in the coastal aquifer system, using the techniques of groundwater flow modelling.
CONCEPTUALIZATION
In order to carry out the groundwater flow modeling, it is essential to delineate the aquifer extent, its interconnection, source of recharge etc., which can be termed as conceptualization of the aquifer system. The multi aquifers system can be visualized using the techniques of hydrogeology, hydrochemistry or isotope hydrology. Mazor (1973), Cotecchia et al., (1974), Mazor (1979), Hem (1985), Eriksson (1985), Mazor and George (1992) and Mazor (1997) have used hydrochemical techniques to study interaction between systems. Rajmohan & Elango (2004) have studied the chemical processes to bring out theprocess of mixing of water, water rock interaction, etc to bring out the causes for the chemical composition of water. The intermixing phenomenon has been clearly brought bythe works of Mazor & Verhagen (1983) and (Mazor, 1997). In India, isotope techniques have been used to study the aquifer-aquifer interaction, lake-aquifer interaction, seawater intrusion studies by Sukhija and Shah (1976), Rao et al (1987), Navada & Rao (1991), Navada et al (1993), Nachiappan (2000), Shivanna et al (2006), Vaithianathan (2003), Gouthaman (2003) and Suresh & Lawrence (2006). In the present study, the techniques of hydrogeology, hydrochemistry and isotope hydrology have been utilised in the conceptualization of the aquifer (Fig 2).
Number and Nature of Aquifers: The aquifer system in the study area can be considered as two-aquifer system, viz., top sandy aquifer and weathered and fractured aquifer (basement). The groundwater occurs under unconfined condition in top sandy aquifer and under unconfined to semi-confined condition in the underlying basement.
Aquifer Boundary (Lateral Extension): The top sandy aquifer extends through out the area underlain by weathered and fractured Charnockites.
Aquifer Boundary (Vertical Extension): The thickness of the aquifer encountered during drilling of piezometers has given the vertical extension of the aquifer and has been interpolated to get the spatial variation within the study area.
Aquifer-Aquifer Interconnection: The presence of clay-sand admixtures in varying proportions throughout the area between top sandy aquifer and weathered & fractured aquifer deems it possible to have interconnection between the two aquifers. The vertical hydraulic conductivity also is not uniform as the clay-sand admixture is not uniform. It is supported by the results of hydrochemical and isotope studies and it can be inferred that the interconnection between the aquifers varies from limited to a complete interconnection (Suresh, 2006).
Source of Recharge: The seasonal variation in the elevation of water table and piezometric surface indicate a large contribution from precipitation. This is also supported by the plot of chemical species, ionic ratios and isotopic composition. The overall uniformity of the quality (brackish to saline) of the groundwater in weathered and fractured aquifer indicates that in addition to the precipitation, the lateral flow across the boundary of the study area could also form a source of recharge (Suresh, 2006).
Boundary Conditions: The area bounded by Bay of Bengal will have a constant head boundary on the east, while on the south, west and north, as there are no conspicuous hydrogeological barrier, they will be a varying head boundary.
MODELING
The modelling techniques have been used extensively in formulating sustainable management strategies for conjunctive use of surface water and groundwater, solute transport model for regulation & control of contaminant migration and for regulated groundwater development in coastal aquifers (Anderson & Woessner (1992),Lawrence (1995), Sorek and Pinder (1999), CGWB (1999), William and Turner (2000), Jha et al (2003), Mohan (2003), Satheesh (2005).
Processing Modflow Version 5.3 was used for groundwater flow modelling in the present study. The top sandy layer was only considered for modeling purposes and initially, the model has been run in steady state condition. The parameters were adjusted and smoothened head was taken as initial piezometric head for transient flow Simulation.
The Model was started from the period July 2000 and initially the transient state condition was run for 11 months period ending May 2001. The length of each stress period was taken as 30 days period with one time step for each stress period and unit of time as day. The model was calibrated and validated with the observed water level data. The simulation was extended to 59 stress periods and model was run for the period July 2000 to May 2005.
The model was run and the results were studied. The water budget was taken up to 11th Stress Period, covering the period from July 2000 – May 2001. The cumulative water budget for the distributed model is given as Table 1.
A perusal of the table shows that there is an inflow of 13.700743 M.Cu.m while there is an outflow of 13.700729 M.Cu.m with a change in storage of 0.000014 M.Cu.m.
In order to validate and calibrate the model data, spatial variation of both observed and simulated water table elevation data of the study area at the end of stress period 11 (May 2001) has been compared and furnished in Fig.3. A perusal of the figure shows that there is a good match between the two values. Subsequently, the calibration and validation has been extended up to 59th stress period (May 2005) and plot of both observed and simulated water table elevation data is furnished as Figure 4. A comparison of the simulated and observed values showed there is a good match between the two.
Further, the temporal variation of observed and simulated water table elevations has beencompared using hydrograph analysis. The plot of observed and simulated for the select hydrographs have provided as Figure 5 along with the variance in the computed andobserved values. A perusal of the figure shows that the difference in the observed and simulated values are less than 0.5m and the variance of the computed and observed values also range between 0.16 to 0.6, thereby indicating a sufficiently good match. Inthe present study, the model has been validated for the period of Jun 2000 to May 2005 for a period of 5 years. It was decided to carry out predictive simulation for additional 10 years, i.e., up to 2015, as the coastal system is fragile and sensitive to changes. Hence the predictive simulation has been restricted only up to 2015.
TESTING OF MANAGEMENT OPTION
The predictive simulation was extended up to 179 stress periods (May 2015). The various option consisting of increasing draft, increasing recharge and its combination have been tested. The increase in draft has been uniformly spread all over the area in all the stress periods but the recharge has been restricted to the monsoon month (June-December) and only in the selected cells eastern side of Buckingham canal, which border the sanddune formations so as to have the maximum impact of recharge.
The various options tested are as follows.
1. Recharge using Normal rainfall & same draft – STRATEGY1
2. Recharge using normal rainfall using 10% addition in draft - STRATEGY2
3. Recharge using normal rainfall using 25% addition in draft – STRATEGY3
4. 10% augmentation with 25% additional draft - STRATEGY4
5. 25% augmentation with 25% additional draft - STRATEGY5
6. 10% augmentation with the same draft - STRATEGY6
In order to study the impact of groundwater management strategy, the inflow-outflow at end of each year (May 01, May 02, May 15) have been compared for each strategy and presented in Table 2 & as Fig 6.
A perusal of the table shows that in Strategy 1 & 6, where the groundwater draft has not been increased shows positive inflow-outflow, while Strategy 3, 4 & 5 show negative inflow-outflow, where groundwater draft has been increased. It is also to add that with increase in draft by 10% in Strategy 2, still the inflow-outflow has remained positive. In strategy 4 & 5, the recharge has also been augmented by 10% & 25% respectively and even then the inflow-outflow component is negative. The same results have been pictorially represented to visualize the changes in inflow-outflow component over the years.
It is very difficult to reduce the groundwater extraction with out the participation of all the stake holders including public & government. It would be ideal to reduce the groundwater extraction but being on the pragmatic side, it is suggested that the strategy 6 comprising continuance of existing groundwater draft with augmentation in the select cells by 10% during the monsoon period may be adopted for the management coastal aquifers in thesouthern part of Chennai Metropolitan area.
CONCLUSION
The aquifer system in the study area can be considered as two-aquifer system, viz., top sandy aquifer and weathered and fractured aquifer (basement). The thickness of the aquifer encountered during drilling of piezometers has given the vertical extension of the aquifer. It is found that the thickness of sandy aquifer varies and the same has been interpolated to get the spatial variation within the study area.
The modeling has been carried out using PM 5.3 version. The area has been modelled as single layered aquifer considering only the top sandy aquifer. The model has been run initially for 11 stress period for the period June 2000 to May 2001 and subsequently extended up to 59 stress periods (May 2005). The model has been calibratedand validated using the water table elevation data of observation wells by comparing both hydrographs (temporal variations) and water table elevation contours (spatial variation). The observed and simulated values are found to match reasonably well, there by indicating that regional model so developed is reasonably representing the field situations.
The simulation has subsequently extended as predictive simulation to year 2015 (15years as modeling period) to study the groundwater flow in and out of the system. Further, different management options of increasing the groundwater draft and effecting groundwater augmentation has been studied to work out a feasible management option considering the present socio economic conditions.
It is very difficult to reduce the groundwater extraction without the participation of all the stake holders including public & government. It would be ideal to reduce the groundwater extraction but being on the pragmatic side, it is suggested that the strategy 6 comprising continuance of existing groundwater draft with augmentation in the select cells by 10% during the monsoon period may be adopted for the management coastal aquifers in thesouthern part of Chennai Metropolitan area.
ACKNOWLEDGEMENT
I would like to express my gratitude to Chairman, CGWB for all his encouragement and guidance in preparation of the paper. I take this opportunity to thank Dr N.Varadaraj, who was Regional Director, SECR at the time of work and Shri D.S.C.Thambi, who was Regional Director, SECR at that time of publication of the report for their moral support and encouragement. The data of CGWB, SG&SWRDC & Metrowater have been used in the studies and all responsible for collection, storage and dissemination of data in these departments are gratefully acknowledged.
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S.Suresh, Central Ground Water Board, Faridabad
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