Simulation of Saltwater Intrusion in a Coastal Aquifer ... - Science Direct

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ScienceDirect Aquatic Procedia 4 (2015) 700 – 705

INTERNATIONAL CONFERENCE ON WATER RESOURCES, COASTAL AND OCEAN ENGINEERING (ICWRCOE 2015)

Simulation of Saltwater Intrusion in a Coastal Aquifer in Karnataka, India. Lathashri.U.Aa* and A.Maheshab a,b

Department of applied mechanics and hydraulics, National Institute of Technology Karnataka, Surathkal, Dakshina Kannada District, India,Pincode-575026.

Abstract A grid based variable density numerical model, SEAWAT-2000 is used to conceptually simulate groundwater flow and transport for a coastal stretch in Karnataka state, India. SEAWAT is a coupled version of MODFLOW and MT3DMS designed to simulate three-dimensional, variable density groundwater flow and multi-species transport. The variable density flow process uses the familiar and well established MODFLOW methodology to solve the variable density groundwater flow equation. The aquifer considered for the present study is bounded by Arabian sea on the west, the ridge line along the east andShambhaviand Pavanjerivers along the northern and southern sides respectively. The study has its focus on managing the available data in the most efficient manner to develop a reliable and sophisticated simulation model. The aquifer parameters are estimated by calibrating the model for two year period with daily time step. The aquifer can be categorized as unconfined having good groundwater potential with aquifer transmissivity and specific yield ranging from 10 to 810 m2/day and 0.0008 to 0.0122 respectively. The model evaluation in terms of the accuracy is carried out bycomparingwith the measured data on seasonal basis.From this, the model is found to be scientifically sound for further management applications. The model so developed can be applied in predicting the saltwater intrusion in coastal aquifers for various developmental and climate change scenarios like sea level rise. © byby Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license © 2015 2015The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015. Peer-review under responsibility of organizing committee of ICWRCOE 2015 Keywords:Coastal aquifer ;Flow and transport; MODFLOW; Saltwater intrusion; SEAWAT.

* Corresponding author. Tel.: +91 9448726769; fax: +91-0824-2474039. E-mail address:[email protected],

2214-241X © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015 doi:10.1016/j.aqpro.2015.02.090

U.A. Lathashri and A. Mahesha / Aquatic Procedia 4 (2015) 700 – 705

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1. Introduction Unplanned exploitation of freshwater aquifers has led to severe groundwater quality problems worldwide. This problem is predominant in the coastal aquifer system, as the coastal groundwater systems are sensitive to impacts such as decreased recharge, contamination from natural and manmade sources and over-exploitation (Essink. 2001) considering the threat of seawater intrusion.Under natural conditions, there exists equilibrium between saltwater and freshwater due to seaward freshwater gradient resulting in nominal saltwater intrusion into freshwater aquifer. Due to excessive pumping activities, seaward freshwater gradient gets reduced and even sometimes may be reversed to landward direction. This leads to aggressive saltwater intrusion from the sea contaminating inland freshwater aquifers to large extent which may take several years to get remediated. To address this issue effectively,density dependent groundwater model is required to track the movement of the solute in coastal aquifers (Lin et al. 2009).Three dimensional groundwater flow and solute transport models namely FEFLOW, 3DFEMFAT, HST3D, AQUA3D, FEMWATER, SEAWAT and MOCDENS3D are in useworldwide.SEAWAT is one of the widely used codes to simulate saltwater intrusion (Werner et al. 2013). The objective of the present work is to use the SEAWAT code to develop a solute transport model for a coastal aquifer in Karnataka, India which is yet to be investigated. The present investigation may form the preliminary data for exploring the possibilities of further groundwater development for the region. 1.1. Study area The basin considered for the study lies between 74°46’ E to 74°49’ E and 13°1’ N to13°5’ N covering a spatial extent of about 26km2 (Fig. 1).The basin has Arabian sea on the west and the rivers Shambhavi and Pavanje along the northern and southern sides respectively. The river Pavanje meanders along the coastline for about 6kmsat a distance of about 500m from the sea to form the river mouth along with the river Shambhavi. The study area has tropical, humid type of climate. Coconut and paddy arethe major crops of the basin. Since the majority of the population in this area depends on agriculture, surface water and groundwater in this regionareutilized to meet the irrigational water demand.Since, surface water supply partially fulfils the freshwater requirement of the area, the thrust on groundwater resources is more. According to the investigation carried out by Harshendra (1991), the present aquifer can be categorized as unconfined with rich lateritic formation, having a good groundwater potential. The aquifer transmissivity and specific yield ranges from 10 to 810 m2/day and 0.0008 to 0.0122 respectively. Adding to this, the vertical electrical sounding survey conducted in the area shows that the aquifer depth in the region varies between 20 to 30m. 2. Methodology 2.1. Conceptual model development To simulate the variable density effects on transient groundwater flow, a numerical model SEAWAT-2000 is used. SEAWAT is a coupled version of MODFLOW 2000 (Harbaugh et al., 2000) and MT3DMS 5.2 (Zheng and Wang, 1999 and Zheng, 2006) designed to simulate three-dimensional, variable density groundwater flow and multispecies transport. The variable density flow (VDF) process uses the familiar and well established MODFLOW methodology to solve the variable density ground water flow equation. The MT3DMS as part of SEAWAT, referred as the Integrated MT3DMS Transport (IMT) process solves the solute transport equation of the process of saltwater intrusion in coastal aquifers. The model grid has its origin at 4,75,646 in the x- direction and 14,39,602 in the y- direction (UTM WGS 1984, zone 43). There are 65 cells in the x direction and 95 cells in the y direction. The model grid consists of 2,587 numbers of active cells, with an approximate cell dimension of 100×100m in the horizontal plane. The vertical section is represented by a single grid of varying dimension. The top surface of the model grid is interpolated to the terrain elevation which ranges from 0 to 60 m above msl and the bottom elevation is set to 30m below mean sea level.

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Fig. 1. Study area

2.2. Input parameters and boundary conditions Due to the scarcity of data, the available data are compiled in the most appropriate way to build the model. The Arabian sea bordering the model area in the west is represented by a constant head boundary of 0m and constant TDS concentration of 35 kg/m3 .The ridge line located along theeasternboundary of the model area is fixed as a no flow boundary. The rivers are assigned with the river boundary condition available in the MODFLOW code.For this, the river stage data, river bed conductance and river bed thickness are assigned based on the available data and field visits. The river water is assigned a TDS concentration equal to that of the sea water during non-monsoon season (October to May) and zero concentration during the monsoon (June to September). This is because,the field studies conducted by Harshendra (1991) have shown that, the chloride concentration of Pavanje river water is enormously high starting from October as compared to that during the month of June to September. The annual average rainfall of the region is about 3900mm and rainfall recharge is the main source of replenishment for the aquifer.From earlier investigations, it was found that the recharge coefficient appropriate to this region varies from 10 to 20%. Hence, a groundwater recharge co-efficient of 20% is assumed for the present investigation. A total of 587 wells are considered in the model domain based on available data. The draft per well is calculated based on the irrigation requirement and domestic need. The aquifer domain is categorized into 5 zones of aquifer parameters, based on the transmissivity values obtained from pumping tests conducted by Harshendra (1991). Fig.2 shows the model grid arrangement, boundary conditions adopted for the model and the aquifer zonation.


Fig.2. Model domain

2.3. Model simulation The model is calibrated for hydraulic conductivity, specific yield, porosity, groundwater recharge co-efficient and dispersivity. The model is run under steady state condition for the month of October 2007. The aquifer parameters are randomly assigned initially and calibrated by trial and error method, until an acceptable match between the observed and calibrated heads are obtained. The water level data of two observation wells, maintained by the Central Ground Water Board and the Department of Mines and Geology, Govt. of Karnataka are used to calibrate the steady state model. The head obtained from this simulation is used as the initial head condition for the transient groundwater flow simulation. The transient simulation is carried out for two year period (September 2011 to August 2013) with daily time step. The transient calibration is performed until a good match between observed and simulated groundwater head and TDS are obtained. For this purpose,data fromfour wells are utilized, which were monitored by Honnanagoudar (2012) during the calibration period. The locations of all the observation wells are shown in fig.1. 3. Results The transient calibration is carried out on seasonal basis to get a better understanding of the model performance. The scatter plots of the observed versus simulated groundwater head and TDS showing the respective R2 values for the months of October (Post-monsoon), December (Winter), April (Summer or Pre-monsoon) and August (Monsoon) are presented in fig.3. The results indicate that the model is scientifically sound with R2 ranging from 0.60 to 0.91and can be used for further applications. The average error between the model simulated and observed data exitsto the extent of about 18%due to measurement errors and the model limitations. In the SEAWAT-2000 documentation (Langevin et al. 2003), the limitations of the applicability of the model is stated clearly.

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Fig .3 The scatter plots of the observed versus simulated groundwater head (A,B,C,D) and TDS concentration(E,F,G,H) for the months of October, December, April and August respectively.


The model is validated for the period September 2013 to August 2014 and the statistical evaluation results of simulated versus observed groundwater heads are obtained with satisfactory accuracy (R2 =0.627). The flow simulation results show that, the hydraulic head during the month of August ranges between 0and 30m and that during the month of May ranges between 0 and -2.5m. The contour shows a rising trend from the coastline (0m) which is low-lying area, towards the high elevated area towards the east. The transport simulation shows that the TDSisoline of 2kg/m3 encroaches the area by 200 to 400m more landwardin certain areas near the coastline during the dry season as compared to that during the monsoon period because of the fall in the hydraulic head. This is also indicated by the water budget outcome, which shows a total inflow and outflow across the constant head boundary (sea) of 664.40 m3/day and zero respectively during the month of May (dry period) and zero inflow and 6724.50 m3/day outflow during the month of August (monsoon). The parameter sensitivity analysis carried out in the present study indicate that, hydraulic conductivity of zones 1 and 3 and specific yield of zones 2 and 3 are highly sensitive and the hydraulic conductivity of zone 4 and specific yield of zones 4 and 5 are less sensitive tothe modelperformance. There is a negligibly small increase (0.002%) in the extent of saltwater intrusion due to the increase in longitudinal dispersivity, hence is less sensitive to spatial movement of saltwater intrusion. 4. Conclusion Groundwater is one of the major sources of freshwater for the population of the coastal, tropical basin considered for the study. Hence, in order to have the sustainable development of the available fresh groundwater resources, scientificassessment of the coastal hydrodynamics of the aquifer is necessary. The SEAWAT code is a sophisticated tool whichproves to be a good alternative in understanding the aquifer system and the spatial extent of saltwater intrusion for the study area considered for the study with the available data. The model applied can be considered reasonably accurate with R2 values ranging from 0.60to 0.91. The calibrated values of hydraulic conductivity and specific yield for the basin range between 4.5 to 14.5 m/day and 0.005 to 0.0122 respectively. In the present study, a single layered model is developed due to the lack of hydro-geological data of the deeper aquifer. However, according to the model documentation, without many layers and extensive database accurate simulation of flow and transport near the freshwater/saltwater interface is not possible. But, the transient simulation results indicate that the model replicates the field situations with reasonable accuracy and hence can be applied in predicting the lateral movement of saltwater intrusion for similar coastal aquifers subjected to various developmental activities and climate change effectslike sea level rise. References Essink, G.O., 2001. Improving fresh groundwater supply: problems and solutions. Ocean Coast Management 44, 429–449. Harbaugh, A.W., Banta,E.R .,Hill,M.C., McDonald,M.G., 2000. MODFLOW-2000, the U.S. Geological Survey Modular Ground-Water Model—User guide to modularization concepts and the ground-water flow process, U.S. Geological Survey Open-File Report 00-92, pp.121. Harshendra, K., 1991. Studies on water quality and soil fertility in relation to crop yield in selected river basins of D.K. District of Karnataka state. Ph.D. thesis, Mangalore University, Karnataka, India. Honnanagoudar, S.S., Reddy,D.V., Mahesha,A., 2012. Terrain analysis and hydrogeochemical environment of aquifers of the southern West Coast of Karnataka, India. International Journal of Earth Sciences and Engineering 05, 1619-1629. Langevin, C.D., Shoemaker, W.B., Guo,W., 2003. MODFLOW-2000, the U.S. Geological Survey modular groundwater model: Documentation of the SEAWAT-2000 version with the variable-density flow processes (VDF) and the integratedMT3DMS Transport Processes (IMT). U.S. Geological Survey Open-File Report 03–426. Lin, J., Snodsmith, J.B., Zheng,C., and Wu,J., 2009. A modeling study of seawater intrusion in Alabama Gulf Coast, USA. Environmental Geology 57, 119–130. Werner,A.D., Bakker ,M., Post ,V.E. A., Vandenbohede ., Lu,C., Ataie-Ashtiani,B., Simmons,C.T., Barry,D.A., 2013. Seawater intrusion processes, investigation and management: Recent advances and future challenges. Advances in water resources 51, 3-26. Zheng,C ., Wang,K., 1999. MT3DMS - A modular three dimensional multispecies transport model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater systems, Contract report SERD99-1, U.S. Army Corps of Engineers, United States. Zheng,C., 2006. MT3DMS v5.2 supplemental user’s guide, Technical report to the U.S. Army Engineer Research and Development Center, Department of Geological Sciences, University of Alabama, 24 p.

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