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Mahesha, A.

Vyshali

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Conference Paper, Published VersionVyshali; Mahesha, A.Tropical, Coastal Aquifer Management - A Case StudyZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:Kuratorium für Forschung im Küsteningenieurwesen (KFKI)Verfügbar unter/Available at: https://hdl.handle.net/20.500.11970/109957Vorgeschlagene Zitierweise/Suggested citation:Vyshali; Mahesha, A. (2010): Tropical, Coastal Aquifer Management - A Case Study. In:Sundar, V.; Srinivasan, K.; Murali, K.; Sudheer, K.P. (Hg.): ICHE 2010. Proceedings of the9th International Conference on Hydro-Science & Engineering, August 2-5, 2010, Chennai,India. Chennai: Indian Institute of Technology Madras.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Proceedings of ICHE2010, IIT Madras, Aug 2-5,2010 Tropical, Coastal Aquifer Management - A Case Study                         TROPICAL, COASTAL AQUIFER MANAGEMENT - A CASE STUDY  Vyshali1 and A.Mahesha2  Abstract: The coastal aquifers constitute important sources of fresh water worldwide. Management of tropical, coastal aquifer is a challenging task due to uneven distribution of rainfall, increased water consumption due to urbanization and proximity to the sea. The present work focuses on the issues related to saltwater intrusion in a regional aquifer system between the rivers Gurpur and Pavanje of southern Indian peninsula. The characterization of the aquifer was carried out through pumping tests, electrical resistivity and water quality analysis. The results indicate that the aquifer is a shallow, unconfined aquifer with good groundwater potential. Even though the water table gets recharged up to the saturation level every year, it gets drained quickly due to prolonged summer, increased demand and nearness to the sea. The extent of saltwater intrusion into the aquifer is simulated through Saturated-Unsaturated TRAnsport (SUTRA) model. The simulation results were verified through field observations and the model is used for long term prediction on saltwater intrusion due to probable groundwater withdrawals. The results also reveal that a few locations are being seriously affected by saltwater intrusion which requires special attention. The safe withdrawals are estimated for long term operations considering the probable groundwater development in the region.  Key Words: coastal aquifers, saltwater intrusion, saturated-unsaturated transport (SUTRA), pumping tests, specific yield, transmissivity.  INTRODUCTION  Groundwater extraction in the coastal region changes the dynamic balance between the flow of freshwater and saltwater so that the saltwater-freshwater interface will move and attain an equilibrium position governed by the quantity extracted and the balance outflow of freshwater to the sea. When the groundwater withdrawals are large from the locations close to the saltwater interface, the saltwater upconing may turn water saline. A case study is being presented here from tropical, coastal basin of Karnataka, India which spreads between the rivers Gurpur and Pavanje.  The chief occupation of the people here are agriculture and fishing. However, due to the developmental activities and urbanization in the recent years, mainly in the fields of industry, commerce and trade, the demand for fresh water has enormously increased.  At present, the domestic as well as industrial water requirements are mainly met by Nethravathi river water which is being supplied by the Mangalore city corporation and groundwater. However, in the near future, an alternative source for the water supply is essential out to meet the ever increasing   1 Senior Lecturer, Dept. of Civil Engineering, Sona College of Technology, Salem-636005;      Email:vyshalik@gmail.com. 2 Professor, Dept. of Applied Mechanics and Hydraulics, National Institute of Technology Karnataka, Surathkal, Mangalore-575 025; Email: maheshamai@yahoo.com.  Proceedings of ninth International Conference on Hydro-Science and Engineering (ICHE 2010), IIT Madras, Chennai, India. 2 - 5  August 2010 546Proceedings of ICHE2010, IIT Madras, Aug 2-5,2010 Tropical, Coastal Aquifer Management - A Case Study                      demand for fresh water. Under the above scenario, it is very much essential to consider the effective and optimal utilization of coastal aquifer for fresh water supply.  In the present study, an attempt has been made to quantify the extent of saltwater intrusion and safe freshwater withdrawals over a longer period for sustainability.  The present area of study is the coastal basin stretched between the Pavanje and Gurpur rivers of Dakshina Kannada district (Fig.1). The areal extent of the region is about 30 km2.  A total of 41 monitoring wells are considered in the area as shown in fig. 1 which include open wells and ponds. The south-west monsoon is the principal rainy season for the region during June-September bringing in about 80% of the annual rainfall of about 3500mm. The post-monsoon season usually experiences scanty rains with increasing water demand. This area’s freshwater requirement even though partially met by surface water supply, greater thrust at present and in future would be on groundwater resources.                       Fig.1. Study Area  METHODOLOGY        The groundwater investigations should necessarily involve in the assessment of the aquifer parameters. The aquifer parameters required for the simulation are evaluated from the pumping tests conducted at about 17 well locations during the period November, 2006 – May, 2007. The saturated thickness of the aquifer in the study area varies from 15m to 24m as per the investigations (Department of Applied Mechanics, 1996) and the electrical resistivity tests carried out using the Schlumberger technique (Nath et al, 2000) at 22 locations. In the present study, three dimensional numerical simulation of the basin was carried out using SUTRA (Voss and Provost, 2002). The governing equations are solved with the use of appropriate boundary 547Proceedings of ICHE2010, IIT Madras, Aug 2-5,2010 Tropical, Coastal Aquifer Management - A Case Study                      conditions and initial conditions for the distributions of pressure and concentrations throughout the aquifer. The numerical scheme is based on a hybridization of finite-element and integrated finite-difference methods used in the framework of a method of weighted residuals (Voss, 1984). The groundwater extraction and other relevant data used for simulation are collected from the field tests and from similar works reported elsewhere (Bhosale and Kumar, 2001). The probable freshwater draft rates anticipated due to the developmental works are predicted and are considered during the simulation to know the aquifer response. The simulation is transient in both pressure and concentration. In the present work, a time period of four months is selected to suit the pre-monsoon (February – May), monsoon (June – September) and the post-monsoon (October – January) seasons in a year.         For the simulation of saltwater intrusion, the study area is divided into three sub-basins i.e. coastline basin, Pavanje basin and Gurpur basin. For the Gurpur river sub-basin, the section of the aquifer is 4575m long and 25 m deep and was discretized into 63 hexahedral elements and 88 nodes. Another section of the aquifer is 4550m long and 25 m deep and was discretized into 75 elements and 104 nodes.  Therefore, the total number of hexahedral elements and nodes considered are 1575 and 2288 respectively. The vertical spacing was kept constant at 5m from top of the aquifer to 25 m depth below msl. The spatial and time discretizations are checked for convergence. The details of the discretization, number and type of elements, initial and boundary conditions for the other sub-basins are available elsewhere (Vyshali, 2009).                                                    RESULTS AND DISCUSSION     In the present study, an attempt was made to characterize the aquifer from the field tests i.e pumping tests, vertical electrical sounding and water quality analysis. In total, about 18 pumping tests are conducted in the area. The wells are generally of large diameter (>3m) and of shallow depth (<10m). The maximum pumping duration was about 2½ hours. The pumping test analysis was carried out using Aquifer Win32 software based on Neuman (1974) method. From the tests, the aquifer is found to be having moderate to good groundwater potential with the transmissivity ranging from 34.9m2/day to 461.4 m2/day and specific yield from 0.0685 to 0.2805. Transmissivity values were found to increase towards the northern boundary of the study area and it was low near the coast. Lowest specific yield values were found near the coast and highest values were found near the eastern boundary of the study area. These results are in conformity with the earlier studies conducted by Raghunatha et al., (2001).        From the vertical electrical sounding studies conducted, it is evident that the study area consists of shallow unconfined aquifer with the thickness ranging from 18m to 24m. In addition, the presence of saltwater was detected for regions having low resistivity (< 70 ohm-m). The results on the water table observations indicate that the water table variation over a year was quite significant and was found to be about 0.75 -11m. The water table reaches it’s saturation level by the end of the monsoon season every year and recedes then onwards till the onset of the monsoon (rainy) season. The extent of saltwater intrusion into the aquifer assessed based on the field tests are given in table 1. The water quality results indicate that the wells which are very close to the seacoast with a distance of less than about 350m, were found to be saline throughout the year. Also, the wells along the banks of the Pavanje river with a distance less than 500m show considerable salinity in the summer period (Feb to May). From the field studies, it was 548Proceedings of ICHE2010, IIT Madras, Aug 2-5,2010 Tropical, Coastal Aquifer Management - A Case Study                      evident that the locations at Chelaru Meginaguthu, Mukka, Padre, Idya, Chitrapur, Biakampady, Kenjar and Tokur are affected by saltwater intrusion during the summer period at a depth range of about 15m – 60m. Out of the above, locations at Mukka, Chitrapur, Baikampady and Tokur are affected almost throughout the year.   The saltwater intrusion was simulated in the study area under transient and steady state conditions through SUTRA model. The simulated values are compared with the observed values of the water quality parameters. The model results are found to over-estimate the observed results and approximate in terms of distribution of concentration. Further fine tuning or calibration of the model is required which requires additional data. The work in this direction is in progress. The model predicts that a few wells located in the Pavanje river basin were found to be having the salinity exceeded the standard limit during the post-monsoon and the summer periods. A couple of wells in the coastal zone were found to be saline throughout the year. In the Gurpur river basin also summer salinity exceeds in a few locations and these areas need to be properly developed to prevent further intrusion of saltwater. In the coastline study area, saltwater was found to intrude up to 350m from the seacoast. During the summer period, the NMPT region is the most vulnerable zone. In the Pavanje study area, saltwater intrusion was observed in the isolated region of 850 to 1000m from the river due to the region’s proximity to a tributary and the lower elevation.    Table 1. Status of saltwater intrusion based on field investigation Well No. VES Ω-m @ depth Salinity, mg/lt (=0) Cond. µsimens /cm (<750)TDS, mg/lit (<500)Cl/HCO3 (<1.5) RL of well bottom, m Distance to sea/river, m 07 70.5@ 18m 800* 1382* 967* -- -1.62 351-r 19 25.12@35m 400* 751 529 3.70 0.41 459-s 24 3.48@ 16m 400* <750 501 <1 -1.59 285-s 25         -- 1000* 1763* 1120* 4.07* -0.65 284-r 26 28.8@ 16m 300* 686 <500 <1 -1.87 313-s 31 18.66@24m 400* 966 607 3.86* -3.48 641-r 32 53.94@16m 500* 1222 784 4.58* -4.28 826-r 33 36.54@16m 500* <750 700 2.25 -3.52 662-r 37        -- 100 <750 <500 4.44* -2.11 1090-s  * - Saltwater affected throughout the year; -r – distance from river; -s – distance from the sea  Besides, the long terms effects of the operation of the wells in the region are also investigated in the present study to facilitate proper aquifer management. The future possible 549Proceedings of ICHE2010, IIT Madras, Aug 2-5,2010 Tropical, Coastal Aquifer Management - A Case Study                      groundwater withdrawal rates are incorporated in the simulation and the results are analyzed. In the National Institute of Technology Karnataka campus, where the freshwater withdrawal is significant, the pumping rate should be limited to 300m3/day in the summer and up to 500m3/day during the monsoon to prevent the saltwater intruding either from the sea or from the tributary of the Pavanje river.  The area around NMPT may have severe saltwater intrusion problem over a longer period of operation of the wells beyond 2000-2500m3/day. For Pavanje study area, the combined optimal pumping rate on either side of the banks should be preferably below 300m3/day during the summer to avoid the problem of saltwater intrusion. The Gurpur study area may have serious problems in the long run if the combined summer pumping from the wells exceeds about 300m3/day. The above restrictions should play a vital role in the management of coastal aquifers.    CONCLUSIONS  The investigations carried out on the tropical, costal basin of Karnataka have indicated that the aquifer can be considered to be having good groundwater potential. The field explorations have revealed that the coastal aquifer is shallow and unconfined in nature with depth up to 24m. The water table fluctuation in the basin is significant ie. up to about 11m over different seasons of the year. The saltwater affected areas are identified through field test and about 20% of the region is permanently affected due to low elevation and proximity to sea. About 25% of the region is affected by saltwater intrusion only during the summer months due to tidal water along the river courses. The optimal rates of withdrawal for the region are estimated over a long term basis considering the possible increase in freshwater demand. This outcome may be used as input to the detailed investigations while planning the coastal aquifer management over the coming years.  ACKNOWLEDGEMENTS The authors are grateful to the Indian National Committee on Hydrology (INCOH), Ministry of Water Resources, Govt. of India (Grant No. 23/24/2002-R&D/2056 dtd. 16th Sept., 2004) for the financial support to carry out the investigation.  REFERENCES  Bhosale,D. and Kumar,C.P.2001. Simulation of SeaWater Intrusion in Ernakulam Coast, http://www.angelfire.com/nh/cpkumar/publication/ernac.pd Department of Applied Mechanics and Hydraulics, 1996. Testing and Analysis of Soil Samples, a Report, NITK Surathkal, 17pp. Nath, S.K., Patra, H., and Shahid, S. 2000.  Geophysical Prospecting For Ground Water, Oxford and IBH Publishing, New Delhi. Neuman, S.P.1974. Analysis of pumping test data from anisotropic unconfined aquifers considering delayed gravity response, Water Resources Research, Vol.11, No.2, pp.329-342. Raghunath, R, Murthy, S. T.R. and Raghavan, B.R. 2001. Aquifer Parameter Evaluation of Nethravathi River Basin, Karnataka State, India. J. Applied Hydrology, 12(1), 46-51.  Voss C.I. 1984. SUTRA – A finite-element simulation model for saturated-unsaturated, fluid-density-dependent ground-water flow with energy transport or chemically-reactive single-species solute transport: U.S. Geological Survey Water-Resources Investigations Report 84-550Proceedings of ICHE2010, IIT Madras, Aug 2-5,2010 Tropical, Coastal Aquifer Management - A Case Study                      4369, pp. 409.    Voss, C.I. and Provost, A.M. 2002. SUTRA-A Model for Saturate-Unsaturated variable- Density ground water Flow with solute or Energy Transport: U.S. Geological Survey Water-Resources Investigations Report, 02-4231, 260pp.  Vyshali, 2009. Studies on Saltwater Intrusion in the Coastal Aquifer of D.K. District, Ph.D. Thesis, Dept. of Applied Mechanics & Hydraulics, NITK Surathkal, 195pp.   551

Tropical, Coastal Aquifer Management - A Case Study

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Tropical, Coastal Aquifer Management - A Case Study

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