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Published on January 1, 2008

Author: smith

Source: authorstream.com

Modelling of Seawater Intrusion :  Modelling of Seawater Intrusion C. P. KUMAR National Institute of Hydrology Roorkee (India) 17 June, 2005 Slide2:  Seawater Intrusion A natural process that occurs in virtually all coastal aquifers. Defined as movement of seawater inland into fresh groundwater aquifers, as a result of higher seawater density than freshwater groundwater withdrawal in coastal areas Densities:  Densities Freshwater: 1000 kg m-3 Seawater: 1025 kg m-3 Freshwater: 0 mg L-1 Seawater: 35,000 mg L-1 Slide5:  Salt Water Intrusion Pumping causes a cone of depression and... …draws the salt water upwards into the well. Slide6:  PROPER MANAGEMENT WILL PREVENT SALINIZATION OF WELLS! Not PREVENTING sea water intrusion, but CONTROLING sea water intrusion. Slide7:  Presence of salinity in coastal aquifers can be detected by (a) Geophysical methods - Resistivity method (b) Geochemical investigations - Chemical composition of groundwater - Isotope studies (age of water to identify the source of salinity) Slide8:  Field surveys (geophysical and geochemical studies) can only reveal the present state of seawater intrusion but can not make impact assessment and prediction into the future. Mathematical models are needed for these purposes. Ghyben-Herzberg relation is a highly simplified model. Dynamic movement of groundwater flow and solute transport needs to be considered. A density-dependent solute transport model including advection and dispersion is needed for the modelling. Slide10:  Most popular models for seawater intrusion SUTRA SEAWAT HST3D FEFLOW Recently released Visual MODFLOW Pro 4.1 now integrates SEAWAT-2000 to solve variable density flow problems, such as seawater intrusion modeling projects. USGS:  USGS HST3D Three-dimensional flow, heat, and solute transport model MOCDENSE Fluid density and viscosity are assumed to be a linear function of the first specified solute. SEAWAT A computer program for simulation of three-dimensional variable-density ground water flow SHARP A quasi-three-dimensional, numerical finite-difference model to simulate freshwater and saltwater flow separated by a sharp interface in layered coastal aquifer systems SUTRA 2D, 3D, variable-density, variably-saturated flow, solute or energy transport Others:  Others 3DFEMFAT 3-D finite-element flow and transport through saturated-unsaturated media. Combined sequential flow and transport, or coupled density-dependent flow and transport. Completely eliminates numerical oscillation due to advection terms, can be applied to mesh Peclet numbers ranging from 0 to infinity, can use a very large time step size to greatly reduce numerical diffusion. FEFLOW FEFLOW (Finite Element subsurface FLOW system) saturated and unsaturated conditions.  FEFLOW is a finite element simulation system which includes interactive graphics, a GIS interface, data regionalization and visualization tools. FEFLOW provides tools for building the finite element mesh, assigning model properties and boundary conditions, running the simulation, and visualizing the results. FEMWATER - 3D finite element, saturated / unsaturated, density driven flow and transport model. Slide15:  Numerical Dispersion Numerical approximations of the derivatives of the non-linear solute transport equation may introduce truncation errors and oscillation errors. The truncation error has the appearance of an additional dispersion-like term, called numerical dispersion, which may dominate the numerical accuracy of the solution. Oscillations may occur in the solution of the solute transport equation as a result of over and undershooting of the solute concentration values. If the oscillation reaches unacceptable values, the solution may even become unstable. Slide17:  The complex density-dependent ground water flow and mass transport models provide stable and accurate results when employed with proper spatial and temporal discretization. The grid Peclet Number (ratio of the spatial discretization and the dispersion length) and the Courant Number (ratio of the advective distance during one time step to the spatial discretization) should match the following constraints: where Px, Py and Pz are the Peclet Numbers; Cx, Cy and Cz are the Courant Numbers; x, y and z are the grid spacings; L and T are the longitudinal and transverse dispersivity, respectively; and t is the time step. Slide18:  Expertise and Studies at NIH Modelling of Seawater Intrusion Dr. Anupma Sharma Dr. S. V. N. Rao Mr. C. P. Kumar Dr. Vijay Kumar Mr. P. K. Majumdar Dr. M. K. Jose (on deputation) Nuclear Hydrology Group Kakinada Regional Centre Slide19:  UNDP Training: Two scientists were trained under UNDP Project (Vijay Kumar, 1997 & C. P. Kumar, 1998) - Application of SUTRA model. Ph.D. Thesis: Numerical Modelling of Seawater Transport in Coastal Aquifers (Anupma Sharma, University of Roorkee, 1996) Planning Models for Water Resources Management in Coastal and Deltaic Systems (S. V. N. Rao, IIT Madras, 2003) Research Project: Freshwater-Salinewater Interrelationships in Multi-Aquifer System of Krishna Delta, Coastal Andhra Pradesh (Hydrology project in collaboration with Ground Water Department, Andhra Pradesh) Slide20:  Recent Publications (excluding national conferences) Simulation of Sea Water Intrusion and Tidal Influence C. P. Kumar ISH Journal of Hydraulic Engineering, March 2001. New MOC Model of Seawater Transport in Coastal Aquifers Anupma Sharma, Deepak Kashyap and G. L. Asawa Journal of Hydrologic Engineering, September/October 2001. Numerical Simulation Models for Seawater Intrusion C. P. Kumar Journal of Indian Water Resources Society, July 2002. Simulation of Seawater Intrusion in Ernakulam Coast Dipanjali D. Bhosale and C. P. Kumar International Conference on "Hydrology and Watershed Management", 18-20 December 2002, Hyderabad. Slide21:  Modelling Strategies to Simulate Miscible Transport of Seawater in Coastal Aquifers Anupma Sharma, Deepak Kashyap and G.L. Asawa Hydrology Journal of IAH, March-June 2003. SUTRA and HST3D Modeling and Management of Saltwater Intrusion from Brackish Canals in Southeast Florida Manfred Koch and Anupma Sharma The Second International Conference and Workshop on Saltwater Intrusion and Coastal Aquifers Monitoring, Modeling, and Management (SWICA-M3), March 31-April 2, 2003, Mexico. Effect of Various Parameters on the Size of Fresh Water Lens in Home Island Vijay Kumar and John L. Luick AHI Journal of Applied Hydrology, 2004. Slide22:  Constraints in the Numerical Modelling of Salt Water Intrusion C. P. Kumar Journal of Soil and Water Conservation, December 2004. Aquifer Restoration from Seawater Intrusion: A Field Scale Study of Minjur Aquifer System, North Chennai, Tamilnadu, India. S. V. N. Rao 18th Seawater Intrusion meeting in Cartagena, Spain Few other papers on groundwater development and management in coastal aquifers by Dr. S. V. N. Rao Slide23:  SIMULATION OF SEA WATER INTRUSION AND TIDAL INFLUENCE Objective: Simulation of sea water intrusion in Nauru Island and examine the effect of tidal forcing on the fresh water resources. Nauru Island is a coral island in the central Pacific Ocean, very near the equator and occupies a land area of 22 km2. The Nauru aquifer was simulated in two-dimensions using vertical section with SUTRA. Slide25:  The water table is at an average elevation of 0.3 m above sea level and ground water flows radially outward to the sea. The island is underlain by a lens of fresh water as much as 7 m thick with average thickness of 4.7 m. Fresh water overlies a thick mixing zone which in turn overlies sea water. The unusually thick mixing zone of brackish water is due to the high hydraulic conductivity of the limestone. Slide27:  Quantitative estimates of hydraulic conductivity have not been undertaken in Nauru Island, but by analogy with similar raised limestone islands elsewhere, hydraulic conductivity is estimated to be about 800 - 1,000 m/d. Tidal fluctuations may also have some effect on the distribution of salinity in the mixing zone, particularly in areas near the coastline. Oceanic tides have an amplitude of 0.8 m. Mean annual rainfall is 1,994 mm and annual rainfall has a high degree of variability. For this study, a uniform recharge rate of 540 mm/year was assumed. Slide28:  Discretization A vertical section of the aquifer along the line C-C’ - 6400 m long and 120 m deep, was discretized to 832 rectangular elements and 891 (27 x 33) nodes. The horizontal spacing was kept constant as 200 m. The vertical spacing was made variable, being 2, 3, 5 and 10 m from top of the aquifer to depths of 20, 35, 60 and 120 m, respectively, below mean sea level (MSL). Slide29:  Boundary Conditions A no-flow boundary condition is specified along the bottom of the mesh at a depth of 120 m where the limestone is considered to be impervious. A recharge boundary due to rainfall is specified at the top of the aquifer. Along the left and right vertical boundaries, a hydrostatic pressure defined by p = s g d was imposed. Here, p is the hydrostatic pressure, s is the density of sea water, g is the acceleration due to gravity, and d is the depth. Any inflow, occurring through the specified pressure boundaries, has a sea water concentration of 35,700 mg/L TDS (i.e. C* = 0.0357 kg TDS/kg fluid). Slide31:  Model Parameters The Nauru aquifer is reported to be not under any major stress such as pumping, it was therefore assumed to be in a steady state condition. Only one set of salinity data, measured during 1987, was available. No measurement of hydraulic parameters has been undertaken in the island and therefore estimated by trial and error using relevant information from similar cases. Slide32:  Values of Hydraulic Parameters for Nauru Island for Simulation with SUTRA Hydraulic Parameter Value Horizontal hydraulic conductivity, Kh 900 m/d Anisotropy, Kh/Kv 50 Recharge rate 540 mm/year Porosity 0.30 Longitudinal dispersivity, L 65 m Transverse dispersivity, T 0.15 m Molecular diffusivity 1.0x10-10 m2/s Slide33:  The following fixed values were used in the computations: Fresh water density  = 1,000 kg/m3 Sea water density s = 1,025 kg/m3 Fluid viscosity  = 10-3 kg/m/s Coefficient of fluid density change with concentration ¶/¶C = 700 kg/m3 Slide34:  Simulation of Ground Water Salinity The 1997 version of SUTRA (2D) was used for the simulation. To obtain a steady state solution, the simulation run was divided into 1,000 time steps of 15 days each, which corresponds to a total simulation period of about 41 years. Figure 4 presents the measured salinity concentrations along section C-C’ and figure 5 presents the ground water salinity obtained in the present study. The ground water salinity contours for the concentrations 0.005, 0.01, 0.02 and 0.03 in figure 5 are found to compare well with measured. Slide37:  The results indicate that the model represents the behaviour of the aquifer quite well under the existing conditions. The model is very sensitive with respect to changes in hydraulic conductivity and recharge. Higher values of hydraulic conductivity facilitate intrusion of sea water, whereas increased recharge has the opposite effect, diluting saline water within the aquifer. The model is also sensitive to changes in porosity, anisotropy and dispersivity but less sensitive to changes in molecular diffusivity. Slide38:  Tidal Influence The tidal signal is manifested as a pressure wave that propagates inside from the coastal boundaries towards the centre of the model area. Sinusoidally varying pressures were applied at the boundaries to simulate tidal forcing. The amplitude of sine wave function (assumed for sea water tides) was taken as 0.80 m with frequency of two cycles per day. The tidal influence on sea water intrusion has been shown in figure 6 which can be compared with figure 5 (without tidal forcing). Slide40:  A dramatic reduction of the fresh water lens was observed when tidal influence is also considered. The area of fresh water (concentration less than 0.0005 i.e. 500 mg/L TDS) was reduced by approximately one half in figure 6 (with tidal forcing). This result highlights the importance of including tidal forcing in numerical studies of coastal and island aquifers. Location of the earthquakes / tsunami:  Location of the earthquakes / tsunami Tsunami Animation:  Tsunami Animation Slide43:  The 26-12-04 tsunami has affected groundwater systems in the low-lying coastal zones of the stricken areas. Schematic representation of the possible effects of the 26-12-04 tsunami on coastal groundwater systems: * Upconing of brackish groundwater under abstraction wells, * Intrusion of brackish or saline water from ponds, * Fingering of brackish water from pools, * Reduction in freshwater volume due to shoreline retreat, etc. Slide44:  There are three primary modes through which the saltwater may enter the underlying aquifers. The first is direct contamination of wells, both large-diameter dug wells and small-diameter tubewells that were either open at the top or damaged during the flooding. The second contamination pathway is widespread infiltration of seawater into the aquifer from the land surface through the unsaturated zone, the quantity controlled by the permeability of the surface sediments and the depth to the water table. Following drainage to the sea, some seawater may remain inland as surface-water bodies in local low-lying areas. It acts as long-term point sources of saltwater to the groundwater system. Numerical models can be used to analyse the impact of tsunami on groundwater resources. Slide45:  Potential Remediation Approaches Widespread infiltration of a dense non-reactive contaminant is difficult to remediate. Removal of bodies of standing saline water and purging of wells. Allow natural recharge to flush salt from the aquifer. If the seawater is isolated in a particular aquifer horizon, it may be pumped out of the aquifer and discarded. However, application of this method near the coast may induce classical seawater intrusion. If saltwater contamination is contained in shallow aquifers which are isolated from deep aquifers by confining units, the deep confined aquifers may become an alternative source of fresh water through installation of deeper tubewells. Slide46:  Future Action Data collection and long-term monitoring is necessary to assess and manage the impact of the tsunami-induced saltwater contamination. Measurements of well salinity levels over time as well as salinity profiles with depth at selected locations should be obtained. Generic cross-sectional or three-dimensional numerical groundwater models of variable-density flow and solute transport can be constructed to better understand contamination mechanisms and the effectiveness of different remediation strategies.

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