CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 GROUNDWATER Groundwater is a distinguished component of the hydrologic cycle. Groundwater is the water which occupies the voids in the saturated zone of earth s crust (rocks). It moves and stores in pore space (voids) of sedimentary rocks or in the fractures and joints of hard rocks. The uncertainty about the occurrence, distribution and quality aspect of groundwater and the energy requirement for its withdrawal impose restriction on exploitation of groundwater. In spite of its uncertainty, groundwater is much protected from pollution; it requires little treatment before it use; it is available almost everywhere; it can be developed with little gestation period and can be supplied at a fairly steady rate. 1.2 GROUNDWATER SITUATION IN INDIA India is a vast country having diversified geological setting. Variations exhibited by the rock formations, ranging in age from the Archaean crystalline to the recent alluvia, are as great as the hydrometeorological conditions. The land forms vary from sea level at the coasts to the lofty peaks of snow clad Himalayan Mountains attaining staggering altitude upto about 9000 metres. Variations in the nature and composition of rock types, the geological structures, geomorphological set up and hydrometeorological conditions have correspondingly given rise to widely varying groundwater situation in different parts of the country.

2 2 Groundwater plays an important role in augmenting water supply to meet the ever increasing demands for various domestic, agricultural and industrial uses. It is the only source of drinking water for about 90% of Indian population. The rapid growth rates of population, industry and agricultural practice have not only increased the exploitation of groundwater but have also contributed towards the deterioration of its quality. Therefore, the preservation and improvement of groundwater quality is of vital importance for human well being as well as for the sustainability of clean environment. 1.3 GROUNDWATER POLLUTION Sources of groundwater pollution can be classified mainly into two groups. These are anthropogenic (or man made) sources and natural sources. Anthropogenic sources include industrial wastes, municipal wastes, fertilizers, pesticides etc. Natural sources are mainly hazardous substances such as arsenic, fluoride, nitrates, carbonates, trace elements etc. present in the host geological formations. Some times natural sources are also activated by human interference with the groundwater system such as pumping of groundwater in excess to natural replenishment of aquifers which disturbs the regional water balance which is otherwise in the equilibrium state. It may lead to the pollution of fresh water due to encroachment of polluted water from the surrounding polluted groundwater resources. Groundwater moves very slowly on the order of 1 cm per day, so it takes a long time for contaminants to reach a drinking water aquifer. The residence time of the water in the surficial aquifer is likely to be on the order of decades, and deep aquifer water are thousands of years old. So the good news is that it takes a long time to pollute an aquifer, but the bad news is that once the aquifer is contaminated, it will probably take a very long time for natural restoration.

3 3 1.4 BACKGROUND OF THE RESEARCH PROBLEM Amaravathi River The Amaravathi river is a tributary of river Cauvery, South India. The 175 km long Amaravathi river begins at the Kerala / Tamil Nadu State border at the bottom of Manjampatti valley between the Annamalai Hills and the Palani Hills. It descends in a north eastern direction through Amaravathi reservoir and Amaravathi dam at Amaravathinagar. It is joined by Kallapuram river at the mouth of the Ajanda valley in Udumalaipettai of Coimbatore district and Nanganji river near Aravakurichi and Kodaganar river near Kodaiyur. Finally Amaravathi river joins with river Cauvery at Kattali near Karur. The river irrigates over 60,000 acres (240 km 2 ) of agricultural lands of in Coimbatore, Erode and Karur districts. The Amaravathi river and its basin, especially in the vicinity of Karur are heavily used for drawing water for industrial process and for the disposal of wastewater. As a result the river and the groundwater in the basin are severely polluted Amaravathi River Basin in Karur District Amaravathi river basin is located between north latitude 11 o 00' N and 10 o 00'N, east longitude 77 o 00'E and 78 o 15'E. The Amaravathi river enters into Karur district near Chinnadarapuram (30km upstream of Karur) and merges with river Cauvery near Kattali village (10 Km down stream of Karur). The flow in the river is seasonal and is contributed by the northeast and southwest monsoon seasons. Amaravathi river is the main source of water for domestic, irrigation and industrial uses in Karur district. Till the river reaches Karur, it is free from pollution. Whereas the river is severely polluted in Karur due to the discharge of partially treated effluent from the textile bleaching and dyeing units located in and around Karur Town. The TDS level in

4 4 groundwater quality is getting increased. Agricultural production is in the down trend. Farmers are seeking compensation. 1.5 NEED FOR THE STUDY Karur is the head quarter of Karur taluk and district. It is located on the bank of river Amaravathi (Figure 1.1). During the last three decades the town emerged as a major textile centres with more than 1000 power loom and handloom units producing bed spreads, towels and furnishings. The main raw material is cotton yarn. This cotton yarn and the finished product (i.e) gray cloth are bleached any dyed by the industries located in this town. At present 487 bleaching and dyeing units are in operation. These bleaching and dyeing units are located on either side of Amaravathi river within 2Km from the river bank and total of 10 km stretch on either side of Karur. By average one unit generates 30 kilo litres per day (KLD) of trade effluent. These units have provided either individual effluent treatment plant (IETP) or joined in common effluent treatment plant (CETP). After treatment the effluent is discharged into river Amaravathi. The total dissolve solids (TDS) in the river discharge is in the range of mg/l and the chloride is in the range of mg/l. Daily KLD of partially treated effluent is discharged into the river. On average daily tonnes of TDS with tonnes of chlorides are let into river. This salt ultimately accumulates in the soil and increase the TDS level in the groundwater. The effluent reaching the river is dark brown in colour. During summer period there is no water flow in river, only effluent flow can be noticed. In monsoon period the colour in river water can be noticed upto Kattali where the river get confluence with river Cauvery. Hence a detailed study on the impact of industrial effluent on the groundwater quality in this area is highly needed.

5 5 Figure 1.1 Location of Study Area Figure 1.2 Amaravathi River Basin Figure 1.3 Google Map of Karur & Amaravathi River 1.6 OBJECTIVES AND SCOPE OF THE RESEARCH The farmers of Amaravathi Ayacut had filed a writ petition in the High Court of Madras seeking compensation for damage caused to their crops,

6 6 lands and the well water due to pollution by the textile bleaching and dyeing units in Karur. The Hon ble Court directed the Loss of Ecology (Prevention & Payment of Compensation) Authority, which is a Central Government organization to assess the loss to ecology and environment in the Amaravathi river basin. The LoEA had carried out an extensive study on environmental and ecological impacts in the Amaravathi river basin by collecting groundwater samples in the affected areas and established its linkage to the damage. The survey covered a boundary of about 3 km width on both sides of the Amaravathi river. Based on the TDS values in the ground water, the areas were classified as Class I to V. In all the water samples, the dominant cationic species was sodium which confirms that chloride is in the form of NaCl which mainly comes from bleaching & dyeing units. The over all study concluded that in three villages the TDS was in the range of mg/l and majority of villages are in range of mg/l. This study was conducted in Even after seven years, there is no improvement in the quality of CETP effluent discharge into the river. The problem is aggravated and the farmers are severely making complaints and filed writ petitions. This is because of continuous increase of TDS level in the ground water. This situation motivated for a detailed study on the issue and find out a solution for the same Objectives of the Research The objective of this research is as follows: i. To carry out detailed assessment of existing groundwater quality on either side of the Amaravathi river basin from Aravakurichi to Kattalai (40 km stretch) with special focus on 10 km stretch (i.e) from Karur to Kattalai which is highly affected due industrial discharge. ii. To predict the groundwater quality in this area over a period of next 15 years by using a mathematical model under various scenarios.

7 7 iii. To suggest remedial measures so as to improve the groundwater quality in this area Scope of the Research In order to achieve the above objective, it is decided to use a computer based mathematical model Visual MODFLOW. Visual MODFLOW (version 2.8.1) is the most complete modeling environment for practical applications in three-dimensional groundwater flow and contaminant transport simulations. With the available primary and secondary data, the model is calibrated and validated. The validated model is used to predict the groundwater level contours and the contaminate concentration contours of the study area in the next 15 years for the following scenarios. i. If the present scenario continues (i.e) CETPs discharging the partially treated effluent with TDS in the range of ,000 mg/l., what will be the impact on the groundwater quality? ii. If the CETPs meet the TDS discharge standards of 2100 mg/l and discharge the effluent into river, what will be the impact in the ground water quality? iii. If the quantum of effluent discharge is doubled with TDS level of 2100 mg/l, what will be the impact on groundwater quality? iv. If the dyeing units go for reverse osmosis plant and recycle the entire effluent and achieve zero discharge, what will be improvement in the groundwater quality? v. If the groundwater recharge is increased by 1.5 times from year 2009 and the dyeing units adopt zero liquid discharge concept, what will be the improvement in the quality of groundwater? By using the predictions results, suggestive measures for aquifer restoration and remediation are made.

8 8 1.7 GOVERNING EQUATIONS Transport of contaminants in groundwater involves several mechanisms. These include advection, dispersion, adsorption and ion exchange, decay, chemical reaction and biological process. Advection represents the movement of a contaminant with the bulk fluid according to the seepage velocity in pore space. Dispersion is the combined result of two mass transport processes in porous media, namely mechanical dispersion and molecular diffusion. The mass transport phenomenon occurs mainly due to heterogeneities in the medium that cause variation in flow velocities and in flow path, which is referred as mechanical dispersion. Molecular diffusion is caused by the non-homogenous distribution of the pollutant concentration. The combined effects of mechanical dispersion and molecular diffusion make the solute spread to an even larger area than pure advection. Adsorption and ion exchange occur at the interface between the solid and liquid phases. The solute in the liquid may be adsorbed by the solid. The mass in the solid may also get into the liquid by dissolution or ion exchange. There may exist chemical reaction among fluids with different chemical composition and between fluids and solid particles. Biological processes such as the putridity of organisms and reproduction of bacteria will also change the concentration. The radioactive components within the fluid will decrease the concentration as a result of decay over time Darcy s Law A French Hydraulic Engineer, Henry Darcy, developed an empirical relationship for flow through porous media. He found that the specific discharge was directly proportional to the energy driving force (the hydraulic gradient) according to the following relationship:

9 9 h v x (1.1) x where = specific discharge in the x-direction, LT -1 x h = the change in head from point 1 to point 2, L x = the distance between point 1 and point 2, L h = the hydraulic gradient in the x-direction, dimensionless x The specific discharge v is defined as the flow volume per time per unit area. x dh vx K x (1.2) dx where K = saturated hydraulic conductivity in the x-direction, LT -1. The x negative sign indicates water flows from areas of high head to low head (a negative hydraulic gradient). K is a measure of both fluid properties and x aquifer media properties. In general it is greatest for coarse sands and gravel. Specific discharge v x is sometimes referred to as the superficial velocity or the Darcy velocity. It has units of velocity (length per time), but it is not the actual velocity that water move through the aquifer. The actual velocity is the specific discharge divided by the effective porosity under saturated conditions. It is greater than the specific discharge because water is throttled through the narrow pore spaces, creating faster movement. v x u (1.3) x n e where u = actual fluid velocity, LT -1 x n = effective porosity. e In consolidated aquifers, the effective porosity can be smaller than the total porosity (void volume/total volume). Effective porosity reflects the interconnected pore volume through which water actually moves.

10 Flow Equations The movement of groundwater must be known so as to determine contaminant transport in an aquifer. Groundwater must satisfy the equation of continuity; water is conserved in flow through porous media. The equation of continuity is given below for nonsteady-state conditions in a confined or an unconfined aquifer. ( v v x) ( y ) ( v ) z ( n ) (1.4) x y z t where = water density, ML -3 v, v, v = specific discharge in longitudinal, lateral, and vertical directions, x LT -1 y z n = porosity of the porous medium t = time, T The units of each term of equation (1.4) are ML -3 T -1 or mass change of water per unit volume (in an elementary control volume) due to any changes of flow in three dimensions. The term on the right-hand side of equation (1.4) is for the change in mass of water due to expansion or compression (a change in density) or due to compaction of the porous medium (a change in porosity). ( n) n n S t t t where S = specific storage = g( n ), L -1 s s h (1.5) t =aquifer compressibility, LT 2 M -1 = fluid compressibility, LT 2 M -1 Units on compressibility are generally m 2 N -1 or ms 2 kg -1. Substituting equation (1.5) into equation (1.4) results in ( v v x) ( y ) ( vx) S x y z s h (1.6) t

11 11 Expanding terms on left-hand side of equation (6) results in terms with z and y x,,, which can be neglected in comparison to those terms shown below. A general form of groundwater flow in three dimensions in a porous media with the assumption that the fluid properties t h S z v y v x v s z y x (1.7) Divide by the fluid density and substitute Darcy s law into the left-hand side of equation (7) t h S z h K z y h K y x h K x s z y x (1.8) Equation (1.8) is a second-order partial differential equation in three dimensions that requires a numerical solution. Boundary conditions and an initial condition are needed as well as aquifer parameters K and s S. It is the basic equation for unsteady flow in aquifers Contaminant Solute Transport Equation Solute transport and reactions can be modeled using the advectiondispersion equation coupled with the flow equation. In tensor notation, the three-dimensional transport of solute is written n m m ij ij i i i r x C D x C u x t C 1 (1.9) Where C = solute concentration, ML -3 t = time, T u i = velocity in three dimensions, LT -1 x i = longitudinal, lateral, and vertical distance, L D ij = dispersion coefficient tensor, L 2 T -1

12 12 r m = physical, chemical, and biological reaction rates, ML -3 T -1 If the velocity and dispersion coefficients are constant in space and time, and if the principal components of dispersion are D, D, D for longitudinal, lateral, and vertical flow, Equation (1.9) simplifies to a partial differential equation in three dimensions with constant coefficients. C u t x C u x y C u y z C D z x 2 C 2 x D y xx yy 2 C D 2 y zz z 2 C 2 z r m (1.10) Velocity values can be obtained from the solution of flow equation, so it is desirable to solve the flow balance before contaminant transport modeling begins. The range of dispersion coefficients D, D, D can be obtained from literature and fix by model calibration. The best method to obtain velocity and dispersion coefficients of water is from injection of a conservative tracer and monitoring its arrival at observation wells. Potassium bromide, sodium chloride, tritium, fluoroscein, and Rhodamine WT occasionally used. x y z dyes are the tracers 1.8 GROUNDWATER MODELS In general, models are conceptual descriptions or approximations that describe physical systems using mathematical equations-they are not exact descriptions of physical systems or processes. The applicability or usefulness of a model depends on how closely the mathematical equations approximate the physical system being modeled. In order to evaluate the applicability or usefulness of a model, it is necessary to have a thorough understanding of the physical system and of the assumptions embedded in the derivation of the mathematical equations. Groundwater models describe groundwater flow and fate and transport processes using mathematical equations that are based on certain simplifying assumptions. These assumptions typically involve the direction of flow, geometry of the aquifer, the heterogeneity or anisotropy of

13 13 sediments or bedrock within the aquifer, the contaminant transport mechanisms and chemical reactions. Because of the simplifying assumptions embedded in the mathematical equations and the many uncertainties in the values of data required by the model, a model must be viewed as an approximation and not an exact duplication of field conditions. Groundwater models, however, even as approximations are a useful investigation tool that groundwater hydrologists may use for a number of applications. Among these are: Prediction of the possible fate and migration of contaminants for risk evaluation. Tracking the possible migration pathway of groundwater contamination. Evaluation of design of hydraulic containment and pump-and-treat systems. Design of groundwater monitoring networks. Wellhead protection area delineation. Evaluation of regional groundwater resources. Prediction of the effect of future groundwater withdrawals on groundwater levels. It is important to understand general aspects of both groundwater flow and fate and transport models to ensure that application or evaluation of these models may be performed correctly. 1.9 GENERAL CONCEPTS Groundwater Flow Models Groundwater flow models are used to calculate the rate and direction of movement of groundwater through aquifers and confining units in the subsurface. These calculations are referred to as simulations. The

14 14 simulation of groundwater flow requires a thorough understanding of the hydrogeologic characteristics of the site. The hydrogeologic investigation should include a complete characterization of the following: Subsurface extent and thickness of aquifers and confining units (hydrogeologic framework). Hydrologic boundaries (also referred to as boundary conditions), which control the rate and direction of movement of groundwater. Hydraulic properties of the aquifers and confining units. A description of the horizontal and vertical distribution of hydraulic head throughout the modeled area for beginning (initial conditions), equilibrium (steady-state conditions) and transitional conditions when hydraulic head may vary with time (transient conditions). Distribution and magnitude of groundwater recharge, pumping or injection of groundwater, leakage to or from surface-water bodies, etc. (sources or sinks, also referred to as stresses). These stresses may be constant (unvarying with time) or may change with time (transient). The outputs from the model simulations are the hydraulic heads and groundwater flow rates which are in equilibrium with the hydrogeologic conditions (hydrogeologic framework, hydrologic boundaries, initial and transient conditions, hydraulic properties, and sources or sinks) defined for the modeled area. Figure 1.4 shows the simulated flow field for a hypothetical site at which pumping from a well creates changes in the groundwater flow field. Through the process of model calibration and verification, which is discussed in later sections of this report, the values of the different hydrogeologic conditions are varied to reduce any disparity between the model simulations and field data, and to improve the accuracy of the model. The model can also be used to simulate possible future changes to hydraulic head or groundwater

15 15 flow rates as a result of future changes in stresses on the aquifer system (Predictive simulations are discussed in later sections of this report). Figure 1.4 Simulated Groundwater Flow Vectors and Hydraulic Head Fate and Transport Models Fate and transport models simulate the movement and chemical alteration of contaminants as they move with groundwater through the subsurface. Fate and transport models require the development of a calibrated groundwater flow model or, at a minimum, an accurate determination of the velocity and direction of groundwater flow, which has been based on field data. The model simulates the following processes: Movement of contaminants by advection and diffusion, Spread and dilution of contaminants by dispersion, Removal or release of contaminants by sorption or desorption of contaminants onto or from subsurface sediment or rock, or Chemical alteration of the contaminant by chemical reactions which may be controlled by biological processes or physical chemical reactions. In addition to a thorough hydrogeological investigation, the simulation of fate and transport processes requires a complete characterization of the following: Horizontal and vertical distribution of average linear groundwater velocity (direction and magnitude) determined by a calibrated groundwater flow model

16 16 or through accurate determination of direction and rate of groundwater flow from field data. Boundary conditions for the solute. Initial distribution of solute (initial conditions). Location, history and mass loading rate of chemical sources or sinks. Effective porosity. Soil bulk density. Fraction of organic carbon in soils. Octanol-water partition coefficient for chemical of concern. Density of fluid. Viscosity of fluid. Longitudinal and transverse dispersivity. Diffusion coefficient. Chemical decay rate or degradation constant. Equations describing chemical transformation processes, if applicable. Initial distribution of electron acceptors, if applicable. The outputs from the model simulations are the contaminant concentrations, which are in equilibrium with the groundwater flow system, and the geochemical conditions (described above) defined for the modeled area. Figure 1.5 shows the simulated migration of a contaminant at a hypothetical site. Figure 1.5 Simulated Contaminant Plume Migration

17 17 As with groundwater flow models, fate and transport models should be calibrated and verified by adjusting values of the different hydrogeologic or geochemical conditions to reduce any disparity between the model simulations and field data. This process may result in a re-evaluation of the model used for simulating groundwater flow if the adjustment of values of geochemical data does not result in an acceptable model simulation. Predictive simulations may be made with a fate and transport model to predict the expected concentrations of contaminants in groundwater as a result of implementation of a remedial action. The simulated containment of a contaminant plume is shown in Figure 1.5. Monitoring of hydraulic heads and groundwater chemistry will be required to support predictive simulations TYPES OF MODELS The equations that describe the groundwater flow and fate and transport processes may be solved using different types of models. Some models may be exact solutions to equations that describe very simple flow or transport conditions (analytical model) and others may be approximations of equations that describe very complex conditions (numerical model). Each model may also simulate one or more of the processes that govern groundwater flow or contaminant migration rather than all of the flow and transport processes. As an example, particle-tracking models, such as MODPATH, simulate the advective transport of contaminants but do not account for other fate and transport processes. In selecting a model for use at a site, it is necessary to determine whether the model equations account for the key processes occurring at the site. Each model, whether it is a simple analytical model or a complex numerical model, may have applicability and usefulness in hydrogeological and remedial investigations.

18 Analytical Models Analytical models are an exact solution of a specific, often greatly simplified, groundwater flow or transport equation. The equation is a simplification of more complex three-dimensional groundwater flow or solute transport equations. Prior to the development and widespread use of computers, there was a need to simplify the three-dimensional equations because it was not possible to easily solve these equations. Specifically, these simplifications resulted in reducing the groundwater flow to one dimension and the solute transport equation to one or two dimensions. This resulted in changes to the model equations that include one-dimensional uniform groundwater flow, simple uniform aquifer geometry, homogeneous and isotropic aquifers, uniform hydraulic and chemical reaction properties, and simple flow or chemical reaction boundaries. Analytical models are typically steady-state and one-dimensional, although selected groundwater flow models are two dimensional (e.g. analytical element models), and some contaminant transport models assume one-dimensional groundwater flow conditions and one-, two- or three dimensional transport conditions. An example of output from a one-dimensional fate and transport analytical model (Domenico and Robbins, 1985) is shown in Figure 1.6 Because of the simplifications inherent with analytical models, it is not possible to account for field conditions that change with time or space. This includes variations in groundwater flow rate or direction, variations in hydraulic or chemical reaction properties, changing hydraulic stresses, or complex hydrogeologic or chemical boundary conditions. Analytical models are best used for: Initial site assessments where a high degree of accuracy is not needed, Designing data collection plans prior to beginning field activities, An independent check of numerical model simulation results, or

19 19 Sites where field conditions support the simplifying assumptions embedded in the analytical models. Figure 1.6 Results from One-Dimensional Fate and Transport Model Numerical Models Numerical models are capable of solving the more complex equations that describe groundwater flow and solute transport. These equations generally describe multi-dimensional groundwater flow, solute transport and chemical reactions, although there are one-dimensional numerical models. Numerical models use approximations (e.g. finite differences, or finite elements) to solve the differential equations describing groundwater flow or solute transport. The approximations require that the model domain and time be discretized. In this discretization process, the model domain is represented by a network of grid cells or elements, and the time of the simulation is represented by time steps. A simple example of discretization is presented in Figure 1.7. The curve represents the continuous variation of a parameter across the model space or time domain. The bars represent a discrete step-wise approximation of the curve. The accuracy of numerical models depends upon the accuracy of the model input data, the size of the space and time

20 20 discretization (the greater the size of the discretization steps, the greater the possible error), and the numerical method used to solve the model equations. Figure 1.7 Example of Discretization Process Unlike analytical models, numerical models have the capability of representing a complex multi-layered hydrogeologic framework. This is accomplished by dividing the framework into discrete cells or elements. An example of representing a multi-layered aquifer system in a numerical model is shown in Figure 1.8. In addition to complex three-dimensional groundwater flow and solute transport problems, numerical models may be used to simulate very simple flow and transport conditions, which may just as easily be simulated using an analytical model. However, numerical models are generally used to simulate problems which cannot be accurately described using analytical models.

21 21 Figure 1.8 Example of Discretization of Complex Hydrogeological Conditions by a Numerical Model 1.11 COMPUTER BASED MODELS There are several types of computer based models for modeling the groundwater flow and contaminant transport. Some of them are listed below. 3DFEMFAT: It is a 3-D finite-element model of flow and transport through saturated-unsaturated media. It is applied for infiltration, wellhead protection, agriculture pesticides, sanitary landfill, radionuclide disposal sites, hazardous waste disposal sites, density-induced flow and transport, saltwater intrusion. It can do simulations of flow only, transport only, combined

22 22 sequential flow and transport, or coupled density-dependent flow and transport. AQUA3D: It is a 3-D groundwater flow and contaminant transport model. It solves transient groundwater flow within homogeneous and anisotropic flow conditions. It also solves transient transport of contaminants and heat with convection, decay, adsorption and velocity-dependent dispersion. AT123D: It is a analytical groundwater transport model for longterm pollutant fate and migration. It computes the spatial-temporal concentration distribution of wastes in the aquifer system and predicts the transient spread of a contaminant plume through a groundwater aquifer on a monthly basis for up to 99 years of simulation time. BIOF&T 2-D/3-D: It is for biodegradation, flow and transport in the saturated / unsaturated zones. It allows real world modeling not adsorption. Desorption, and microbial process based on oxygen -limited, anaerobic, firstorder, or Monod-type biodegradation kinetics as well as anaerobic or firstorder sequential degradation involving multiple daughter species. Chemflo: It simulates movement of water and chemical in unsaturated soils. The equations are solved numerically for one dimensional flow and transport using finite differences. Chem Flux: It is a finite element mass transport model. It predicts the movement of contaminant plumes through the processes of advection, diffusion, adsorption and decay. It provides an elegant and simple user interface.

23 23 FEFLOW: Finite element subsurface flow system. It simulates 3D and 2D fluid density-coupled flow, contaminant mass (salinity) and heat transport in the subsurface. It is capable of computing: a). groundwater systems with and without free surfaces (phreatic aquifers, perched water tables, moving meshes, b). problems in saturated-unsaturated zones, c). both salinity-dependent and transport phenomena (thermohaline flows), d). complex geometric and parametric situations. FLONET/TRANS: It is a 2-D cross sectional groundwater flow and contaminant transport model. It uses the dual formation of hydraulic potentials and streamlines to solve the saturated groundwater flow equation and create accurate flownet diagrams for any two-dimensional, saturated groundwater flow system. It also simulates advective-dispersive contaminant transport problems with spatially-variable retardation and multiple source terms. FLOWPATH: 2-D groundwater flow, remediation, and wellhead protection model. It is for groundwater flow, remediation, and wellhead protection. It is a comprehensive modeling environment transport in unconfined, confined and leaky aquifers with heterogeneous properties, multiple pumping wells and complex boundary conditions. Typical application includes a). Determining remediation well capture zones, b). delineating wellhead protection areas, c). designing and optimizing pumping well locations for dewatering projects, d). determining contaminant fate and exposure pathways for risk assessment. GFLOW: It is analytic element model with conjunctive surface water and groundwater flow and a modflow model extract feature. It is an efficient stepwise groundwater flow modeling system. It models steady-state flow in a single heterogeneous aquifer using the Dupuit-Forchheimer assumption.

24 24 GMS: Groundwater Modeling Environment for MODFLOW, MODPATH, MT3D, RT3D, FEMWATER, SEAM3D, SEEP2D, PEST, UTCHEM, and UCODE. It is a sophisticated and comprehensive groundwater modeling software. It provides tools for every phase of a groundwater, simulation including site characterization, model development, calibration, post-processing, and visualization. Groundwater Vistas (GV): It is a sophisticated windows graphical user interface for 3-D groundwater flow and transport modeling. It couples a model design system with comprehensive graphical analysis tools. GV is a model-independent graphical design system for MODFLOW, MODPATH, MT3DMS, RT3D, PARH3D, PEST, UCODE, SEWWAT, MODFLOWT- SURFACT, MODFLOW2000. HST3D: It is a 3-D heat and solute transport model and a powerful user-friendly interface for HST3D integrated within the Argus Open Numerical Environments (Argus ONE) modeling environment. HST3D allows the user to enter all spatial data, graphically run HST3D, and visualize the results. MicroFEM: Finite-element program for multiple-aquifer steadystate and transient groundwater flow modeling. It is a new program, based on the DOS package Micro-Fem. It demonstrates the whole process of groundwater modeling, from the generation of a finite-element grid through the stages of preprocessing, calculation, post processing, graphical interpretation and plotting. Confined, semi-confined, phreatic, stratified and leaky multi-aquifer systems can be simulated with a maximum of 20 aquifers. MOC: This is a 2-D model for solute transport and dispersion in groundwater. It is applicable for one-or two-dimensional problems involving

25 25 steady-state or transient flow. MOC computes changes in concentration over time caused by the processes of convective transport, hydrodynamic dispersion, and mixing (or dilution) from fluid sources. MOC is based on a rectangular, block centered, finite-difference grid. It can also be used for heterogeneous and/or anisotropic aquifer. MOCDENSE: Two-constituent solute transport model for groundwater having variable density. It simulates the flow in a cross-sectional plane rather than in an aerial plane. Because the problem of interest involves variable density, the model solves for fluid pressure rather than hydraulic head in the flow equation. The solution to the flow equation is obtained using finitedifference method. Solute transport is simulated with the method of characteristics. Density is considered to be a function of the concentration of the one of the constituents. MODFLOW: This is a three-dimensional ground water flow model. MODFLOW has become the worldwide standard ground-water flow model. MODFLOW is used to simulate systems for water supply, containment remediation and mine dewatering. The modular structure of MODFLOW consists of a main Program and a series of highly-independent subroutines called modules. The modules are grouped in packages. Each package deals with a specific feature of the hydrologic system which is to be simulated such as flow from rivers or flow into drains or with a specific method of solving linear equations which describe the flow system. MODFLOW SURFACT: It is a new flow and transport model and it is based on the USGS MODFLOW code, the most widely-used groundwater flow code in the world. MODFLOW has certain limitations in simulating complex filed problems. Additional computational modules have

26 26 been incorporated to enhance the simulation capabilities and robustness. It is a seamless integration flow and transport modules. MODFLOWT: This is an enhanced version of MODFLOW for simulating 3-D contaminant transport This includes packages to simulate advective-dispersive contaminant transport. It performs groundwater simulations utilizing transient transport with steady-state flow, transient flow, or successive periods of steady-state flow. MODFLOWwin32: (MODFLOW for Windows) The model has all the features of other MODFLOW versions and the new packages which include stream routing, aquifer compaction, horizontal flow barrier, BCF2, and BCF3 packages and the new PCG2 solver. In addition, it will create files for use with MODPATH (particle-tracking model for MODFLOW) and MT3D (solute transport model). MODPATH: It is a 3-D particle tracking post processing package for MODFLOW, the USGS 3-D finite-difference ground-water flow model. It is a widely used particle tracking program. MOFAT: It is multiphase (water, oil, gas) flow and multi component transport model. Simulate multiphase flow and transport of up to five non-inert chemical species in MOFAT. Simulate dynamic or passive gas as a full three-phase flow problem. Model water flow only, oil-water flow, or water-oil-gas flow in variably-saturated porous media. MS-VMS: It is a comprehensive MODFLOW-based visual modeling system for ground-water flow and contaminate transport. It over comes the limitations of MODFLOW.

27 27 MT3D: It is a comprehensive three-dimensional numerical model for simulating solute transport in complex hydrogeologic settings. MT3D has a modular design that permits simulation of transport processes independently or jointly. MT3D is capable of modeling advection in complex steady-state and transient flow fields, anisotropic dispersion, first-order decay and production reactions, and linear and nonlinear sorption. MT3D is linked with groundwater flow simulator MODFLOW and it is designed specifically to handle advectively-dominated transport problems without the need to consider refined methods specifically for solute transport. PEST: It is a nonlinear parameter estimation and optimization package. It can be used to estimate parameters for just about any existing model whether or not you have the model s source code. PESTAN: It is a pesticide transport model. It is a USEPA program for evaluating the transport of organic solutes through the vadose zone to groundwater. Input data includes water solubility, infiltration rate, bulk density, sorption constant, degradation rates, saturated water content, saturated hydraulic conductivity and dispersion coefficient. POLLUTE: It is a finite layer contaminant migration model for landfill design. It can be used for fast, accurate and comprehensive contaminant migration analysis. Landfill designs that can be considered range from simple systems on a natural aquitard to composite liners with multiple barriers and multiple aquifers. PRINCE: It is a well known software package of ten analytical groundwater models originally developed as part of an EPA 208 study. There are seven one, two and three-dimensional mass transport models and three two-dimensional flow models in PRINCE. These groundwater models have

28 28 been rewritten from the original mainframe FORTRAN codes. Two popular analytical models have been added to the original collection, and the ability to import digitized or AutoCAD-produced DXF site map files has been added. PRZM3: It links two models, PRZM and VADOFT to predict pesticide transport and transformation down through the crop root and vadose (unsaturated) zone to the water table. PRZM3 incorporates soil temperature simulation, volatilization and vapour phase transport in soils, irrigation simulation, and microbial transformation. PRZM is a one-dimensional finitedifference model which uses a method of characteristics (MOC) algorithm to estimate numerical dispersion. VADOFT is a one-dimensional finite-element code that solves Richards equation for flow in the unsaturated zone. The user may make use of constituting relationships between pressure, water content, and hydraulic conductivity to solve the flow equation. PRZM3 is capable of simulating multiple pesticides or parent-daughter relationships. PRZM3 is also capable of estimating probabilities of concentrations or fluxes in or from various media for the purpose of performing exposure assessments. RBCA Tier 2 Analyzer: It is a two-dimensional groundwater model with a comprehensive selection of contaminant transport simulation capabilities including single or multiple species sequential decay reactions such as reductive dechlorination of PCE instantaneous or kinetic-limited BTEX biodegradation with single or multiple electron acceptors and equilibrium or non-equilibrium sorption. SEAWAT: It is a three-dimensional variable-density ground-water flow model for porous media. The source code was developed by combining MODFLOW and MT3DMS into a single program that solves the coupled flow and solute-transport equations. The SEAWAT code follows a modular structure, and thus, new capabilities can be added with only minor

29 29 modifications to the main program. SEAWAT reads and writes standard MODFLOW and MT3DMS data sets, although some extra input may be required for some SEAWAT simulations. This means that many of the existing pre and post processors can be used to create input data sets and analyze simulation results. SESOIL: It is a seasonal compartment model which simulates longterm pollutant fate and migration in the unsaturated soil zone. SESOIL describes the following components of a user-specified soil column which extends from the ground surface to the ground-water table. a). Hydologic cycle of the unsaturated soil zone, b). pollutant concentrations and masses in water, soil, and air phases, c). pollutant migration to groundwater, d). pollutant volatilization at the ground surface, e). pollutant transport in wash load due to surface runoff and erosion at the ground surface. SESOIL estimates all of the above components on a monthly basis for up to 999 years of simulation time. It can be used to estimate the average concentrations in groundwater. The soil column may be composed of up to four layers, each layer having different soil properties which affect the pollutant fate. In addition, each soil layer may be subdivided into a maximum of 10 sub layers in order to provide enhanced resolution of pollutant fate and migration in the soil column. The following pollutant fate processes are accounted for : volatilization, adsorption, cation exchange, biodegradation, hydrolysis and complexation. SLAEM / MLAEM: It is based on analytic element method. The computer program is intended for modeling regional groundwater flow in systems of confined, unconfined, and leaky aquifers. SLAEM is single layer analytic model and MLAEM is a multi layer analytic model. All programs run under MS Windows 95, 98 and NT. The programs are native windows applications and are accessed via a modern and flexible Graphical User Interface (GUI), as well as via a command-line interface. The latter capability

30 30 makes it easy to drive the program from other programs such as Arc-View, ARC/INFO and PEST. The programs read DXF-files and produce BNA files that may be read by other programs such as SURFER. SOLUTRANS: It is a 32-bit Windows program for modeling threedimensional solute transport based on the solutions presented by Leij et al. for both equilibrium and non-equilibrium transport. The interface and input requirements are so simple that it only takes a few minutes to develop models and build insight and complex solute transport problems. With SOLUTRANS we can, in a matter of minutes, model solute transport from a variety of source configurations and build important insights about key processes. It offers a quick and simple alternative to complex, time-consuming 3-D numerical flow and transport models. SUTRA: It is a 2D groundwater saturated-unsaturated transport model, a complete saltwater intrusion and energy transport model. SUTRA simulates fluid movement and transport of either energy or dissolved substances in a subsurface environment. SUTRA employs a two-dimensional hybrid finite-element and integrated finite-difference method to approximate the governing equations that describe the two interdependent processes that are simulated: (i) fluid density-dependent saturated or unsaturated groundwater flow and either (iia) transport of a solute in the groundwater, in which the solute may be subjected to equilibrium adsorption on the porous matrix and both first-order and zero-order production or decay, or (iib) transport of thermal energy in the groundwater and solid matrix of he aquifer. SVFlux 2D: It represents the next level in seepage analysis software. Designed to be simple and effective, the software offers features designed to allow the user to focus on seepage solutions, not convergence problems or different mesh creation.

31 31 SWIFT: It is fully-transient, three-dimensional model to simulate groundwater flow, heat (energy), brine and radionuclide transport in porous and fractured geologic media. In addition to transient analysis, SWIFT offers a steady-state option for coupled flow and brine. The equations are solved using central or backward spatial and time weighting approximations by the finitedifference methods. In addition to Cartesian, cylindrical grids may be used. Contaminant transport includes advection, dispersion, sorption and decay, including chains of constituents. SWIMv1: It is software package (version1) for simulating water infiltration and movement in soils. It consists of a menu-driven suite of three programs that allow the user to simulate soil water balances using numerical solutions of the basic soil water flow equations. As in real world, SWIMv1 allows addition of water to the system as precipitation and removal by runoff, drainage, evaporation from the soil surface and transpiration by vegetation. SWIMv1 helps to understand the soil water balance so they can assess possible effects of such practices as tree clearing, strip mining and irrigation manganese. TWODAN: It is a popular and versatile analytic groundwater flow model for Windows. TWODAN has suite of advanced analytic modeling features hat allow you to model everything from a single well in a uniform flow filed to complex remediation schemes with numerous wells, barriers, surface waters, and heterogeneities. TWODAN has many capabilities: heterogeneities, impermeable barriers, resistant barriers, and transient solutions. It can be used for modeling remedial design alternatives, wellhead capture zones, and regional aquifer flow. VAM2D: It is a two dimensional finite-element groundwater model that simulates transient or steady-state groundwater flow and contaminant

32 32 transport in porous media. VAM2D analyzes unconfined flow problems using a rigorous saturated-unsaturated modeling approach using efficient numerical techniques. Accurate mass balance is maintained in VAM2D even when simulating highly nonlinear soil moisture relations. Hysteresis effects in the water retention curve can also be simulated. A wide range of boundary conditions can be treated in VAM2D including seepage faces, water-table conditions, recharge, infiltration, evapotranspirtaion, and pumping and injection wells. The contaminant transport option can account for advection, hydrodynamic dispersion, equilibrium sorption, and first-order degradation. Transport of a single species or multiple parent-daughter components of a decay chain can be simulated. This model can perform simulations using an areal plane, cross section, or axisymmentric configuration. Visual Groundwater: It is a 3-D visualization software package which can be used to deliver high-quality, three-dimensional presentations of subsurface characterization data and groundwater modeling results. Visual Groundwater combines state-of-the-art graphical tools for 3-D visualization and animation with a data management system specifically designed for borehole investigation data. This model also comes with a data conversion utility to create 3-D data files using X,Y,Z data, and girded data sets. Threedimensional images of complex site characterization data and modeling results can be easily created. VISUAL HELP: It is an advanced hydrological modeling environment available for designing landfills, predicting leachate mounding and evaluating potential leachate contamination. Visual HELP combines the latest version of the HELP model with an easy-to user interface and powerful graphical features for designing the model and evaluating the modeling results.

33 33 Visual MODFLOW: It provides professional 3D groundwater flow and contaminant transport modeling using MODFLOW-2000, MODPATH, MT3DMS and RT3D. It allows to: a). graphically design the model grid, properties and boundary conditions, b). visualize the model input parameters in two or three dimensions, c). run the groundwater flow, pathline and contaminant transport simulations, d). automatically calibrate the model using Win PEST or manual methods, e). display and interpret the modeling results in three-dimensional space using the Visual MODFLOW 3D-Explorer. This model (version2.8.1) is used in this research and it is explained in detail in Chapter-3. VISUAL PEST: It combines the powerful parameter estimation capabilities of PEST2000 with the graphical processing and display features of WinPEST. PEST2000 is the latest version of PEST, the pioneer in modelindependent parameter estimation. It has undergone continued development with addition of many features that have improved its performance and utility to a level that makes it uniquely applicable in any modeling environment. VLEACH: It is a one-dimensional finite-difference vadose zone leaching, model developed by US EPA. It describes the movement of an organic contaminant within and between three phases: i) as a solute dissolved in water, (ii) as a gas in the vapour phase, and (iii) as an adsorbed compound in the solid phase. The leaching is simulated in a number of distinct, userdefined polygons vertically divided into a series of user-defined cells. At the end of the simulation, the results from each polygon are used to determine an area-weighted ground-water impact due to the mobilization and migration of a sorbed organic contaminant located in the vadose zone on the underlying groundwater resource.

34 34 VS2DT: It is a USGS program for flow and solute transport in variably-saturated, single-phase flow in porous media. A finite-difference approximation is used in VS2DT to solve the advection-dispersion equation. Simulated regions include one-dimensional columns, two-dimensional vertical cross sections, and axially-symmetric, three-dimensional cylinders. The VS2DT program options include backward or centered approximations for both space and time derivatives, first-order decay, equilibrium adsorption (Freundlich or Langmuir) isotherms, and ion exchange. Nonlinear storage terms are storage terms are linearized by an implicit Newton-Raphson method. Relative hydraulic conductivity in VS2DT is evaluated at cell boundaries using full upstream weighting, arithmetic mean or geometric mean. WHI UnSat Suite: It combines SESOIL, VLEACH, PESTAN, VS2DT and HELP in a revolutionary graphical environment specifically designed for simulating one-dimensional groundwater flow and contaminant transport through he unsaturated zone. All five of these models are compiled and optimized to run as native Windows applications and are seamlessly integrated within the WHI UnSat Suite modeling environment. It can be used for (i) simulate long-term pollutant fate and transport (VOCs, PAHs, pesticides and heavy metals) in the unsaturated zone under seasonally variable conditions using SESOIL, (ii) predict vertical migration of volatile hydrocarbons through the vadose zone using VLEACH, (iii) estimate agricultural pesticide migration through the unsaturated zone using PESTAN, (iv) simulate groundwater flow and transport processes through heterogeneous, unsaturated soil using VS2DT, (v) predict seasonal recharge rates through heterogeneous soil conditions under variable weather conditions using the HELP model, (vii) generate up to 100 years of statistically reliable climatologically data for virtually any location in the world using the WHI weather generator.

35 35 WinTran: It couples the steady-state groundwater flow model from WinFlow with a contaminant transport model. The transport model has the feel of an analytic model but is actually an embedded finite-element simulator. The finite-element transport model is constructed automatically by WinTran but displays numerical criteria (Peclet and Courant numbers) to allow the user to avoid numerical or mass balance problems. Contaminant mass may be injected or extracted using any of the analytic elements including wells, ponds, and linesinks. In addition, constant concentration elements have been added. WinTran displays both head concentration contours, and concentration may be plotted versus time at selected monitoring locations. The transport model includes the effects of dispersion, linear sorption (retardation), and first-order decay. WinTran aids in risk assessment calculations by displaying the concentration over time at receptor or observation locations. WinTran will display the breakthrough curves after the simulation is finished GROUNDWATER MODEL DEVELOPMENT PROCESS Hydrogeologic Characterization Proper characterization of the hydrogeological conditions at a site is necessary in order to understand the importance of relevant flow or solutetransport processes. With the increase in the attempted application of natural attenuation as a remedial action, it is imperative that a thorough site characterization be completed. This level of characterization requires more site-specific fieldwork than just an initial assessment, including more monitoring wells, groundwater samples, and an increase in the number of laboratory analytes and field parameters. Without proper site characterization, it is not possible to select an appropriate model or develop a reliably calibrated model. At a minimum, the following hydrogeological and geochemical information must be available for this characterization:

36 36 Regional geologic data depicting subsurface geology. Topographic data (including surface-water elevations) Presence of surface-water bodies and measured stream-discharge (base flow) data Geologic cross sections drawn from soil borings and well logs. Well construction diagrams and soil boring logs. Measured hydraulic-head data. Estimates of hydraulic conductivity derived from aquifer and/or slug test data. Location and estimated flow rate of groundwater sources and sinks. Identification of chemicals of concern in contaminant plume*. Vertical and horizontal extent of contaminant plume*. Location, history, and mass loading or removal rate for contaminant sources or sinks*. Direction and rate of contaminant migration*. Identification of downgradient receptors*. Organic carbon content of sediments*. Appropriate geochemical field parameters (e.g. dissolved oxygen, Eh, ph)* Appropriate geochemical indicator parameters (e.g. electron acceptors, and degradation byproducts)* * Required only by fate and transport models. These data should be presented in map, table, or graph format in a report documenting model development Model Conceptualization Model conceptualization is the process in which data describing field conditions are assembled in a systematic way to describe groundwater flow and contaminant transport processes at a site. The model

37 37 conceptualization aids in determining the modeling approach and which model software to use. Questions to ask in developing a conceptual model include, but are not limited to: Are there adequate data to describe the hydrogeological conditions at the site? In how many directions is groundwater moving? Can the groundwater flow or contaminant transport be characterized as one-, two- or three dimensional? Is the aquifer system composed of more than one aquifer, and is vertical flow between aquifers important? Is there recharge to the aquifer by precipitation or leakage from a river, drain, lake, or infiltration pond? Is groundwater leaving the aquifer by seepage to a river or lake, flow to a drain, or extraction by a well? Does it appear that the aquifer's hydrogeological characteristics remain relatively uniform, or do geologic data show considerable variation over the site? Have the boundary conditions been defined around the perimeter of the model domain, and do they have a hydrogeological or geochemical basis? Do groundwater-flow or contaminant source conditions remain constant, or do they change with time? Are there receptors located downgradient of the contaminant plume? Are geochemical reactions taking place in onsite groundwater, and are the processes understood? Other questions related to site-specific conditions may be asked. This conceptualization step must be completed and described in the model documentation report.

38 Model Software Selection After hydrogeological characterization of the site has been completed, and the conceptual model developed, computer model software is selected. The selected model should be capable of simulating conditions encountered at the site. The following general guidelines should be used in assessing the appropriateness of a model: Analytical models should be used where: (i) field data show that groundwater flow or transport processes are relatively simple, (ii) an initial assessment of hydrogeological conditions or screening of remedial alternatives is needed. Numerical models should be used where: (i) field data show that groundwater flow or transport processes are relatively complex, (ii) groundwater flow directions, hydrogeological or geochemical conditions, and hydraulic or chemical sources and sinks vary with space and time. A one-dimensional groundwater flow or transport model should be used primarily for initial assessments where the degree of aquifer heterogeneity or anisotropy is not known and for sites where a potential receptor is immediately downgradient of a contaminant source. Twodimensional models should be used for (i) problems which include one or more groundwater sources/sinks (e.g. pumping or injection wells, drains, rivers, etc.), (ii) sites where the direction of groundwater flow is obviously in two dimensions(e.g. radial flow to a well, or single aquifer with relatively small vertical hydraulic head or contaminant concentration gradients), (iii) sites at which the aquifer has distinct variations in hydraulic properties, (iv) contaminant migration problems where the impacts of transverse dispersion are important and the lateral, or vertical, spread of the contaminant plume must be approximated. Three-dimensional flow and transport models should generally be used where the hydrogeologic conditions are well known, multiple aquifers are present, the vertical movement of groundwater or contaminants is important. The rationale for selection of the appropriate model software should be discussed in the model documentation report.

39 Model Calibration Model calibration consists of changing values of model input parameters in an attempt to match field conditions within some acceptable criteria. This requires that field conditions at a site be properly characterized. Lack of proper site characterization may result in a model that is calibrated to a set of conditions which are not representative of actual field conditions. The calibration process typically involves calibrating to steady-state and transient conditions. With steady-state simulations, there are no observed changes in hydraulic head or contaminant concentration with time for the field conditions being modeled. Transient simulations involve the change in hydraulic head or contaminant concentration with time (e.g. aquifer test, an aquifer stressed by a well-field, or a migrating contaminant plume). These simulations are needed to narrow the range of variability in model input data since there are numerous choices of model input data values which may result in similar steady-state simulations. Models may be calibrated without simulating steady-state flow conditions. At a minimum, model calibration should include comparisons between model-simulated conditions and field conditions for the following data: hydraulic head data, groundwater-flow direction, hydraulic-head gradient, water mass balance, contaminant concentrations (if appropriate), contaminant migration rates (if appropriate), migration directions (if appropriate), and degradation rates (if appropriate). These comparisons should be presented in maps, tables, or graphs. A simple graphical comparison between measured and computed heads is shown in Figure 1.9. In this example, the closer the heads fall on the straight line, the better the goodness-of-fit. Each modeler and model reviewer will need to use their professional judgment in evaluating the calibration results. There are no universally accepted goodness-of-fit criteria that apply in all cases. However, it is important that the modeler make every attempt to

40 40 minimize the difference between model simulations and measured field conditions. Typically, the difference between simulated and actual field conditions (residual) should be less than 10 percent of the variability in the field data across the model domain. A plot showing residuals at monitoring wells (calibration targets) is shown in Figure A plot in this format is useful to show the goodness-of-fit at individual wells. The modeler should also avoid the temptation of adjusting model input data on a scale which is smaller than the distribution of field data. This process, referred to as "over calibration", results in a model that appears to be calibrated but has been based on a dataset that is not supported by field data. For initial assessments, it is possible to obtain useful results from models that are not calibrated. The application of uncalibrated models can be very useful in guiding data collection activities for hydrogeological investigations or as a screening tool in evaluating the relative effectiveness of remedial action alternatives. Figure 1.9 Comparison between Measured and Computed Hydraulic Heads

41 41 Figure 1.10 Residuals from Comparison of Measured and Computed Heads at Calibration Targets History Matching A calibrated model uses selected values of hydrogeologic parameters, sources and sinks and boundary conditions to match field conditions for selected calibration time periods (either steady-state or transient). However, the choice of the parameter values and boundary conditions used in the calibrated model is not unique, and other combinations of parameter values and boundary conditions may give very similar model results. History matching uses the calibrated model to reproduce a set of historic field conditions. This process has been referred to by others as model verification. The most common history matching scenario consists of reproducing an observed change in the hydraulic head or solute concentrations over a different time period, typically one that follows the calibration time period (Figure 1.11). The best scenarios for model verification are ones that use the calibrated model to simulate the aquifer under stressed conditions. The process of model verification may result in the need for further calibration

42 42 refinement of the model. After the model has successfully reproduced measured changes in field conditions for both the calibration and history matching time periods, it is ready for predictive simulations. Figure 1.11 Comparison between Computed and Observed Heads with Time Sensitivity Analysis A sensitivity analysis is the process of varying model input parameters over a reasonable range (range of uncertainty in values of model parameters) and observing the relative change in model response (Figure 1.12). Typically, the observed changes in hydraulic head, flow rate or contaminant transport are noted. The purpose of the sensitivity analysis is to demonstrate the sensitivity of the model simulations to uncertainty in values of model input data. The sensitivity of one model parameter relative to other parameters is also demonstrated. Sensitivity analyses are also beneficial in determining the direction of future data collection activities. Data for which the model is relatively sensitive would require future characterization, as opposed to data for which the model is relatively insensitive. Modelinsensitive data would not require further field characterization.

43 43 Figure 1.12 Simulated Change in Hydraulic Head Resulting from Change in Parameter Value Predictive Simulations or Evaluation of Remediation Alternatives A model may be used to predict some future groundwater flow or contaminant transport condition. The model may also be used to evaluate different remediation alternatives, such as hydraulic containment, pump-andtreat or natural attenuation, and to assist with risk evaluation. In order to perform these tasks, the model, whether it is a groundwater flow or solute transport model, must be reasonably accurate, as demonstrated during the model calibration process. However, because even a well-calibrated model is based on insufficient data or oversimplifications, there are errors and uncertainties in a groundwater-flow analysis or solute transport analysis that make any model prediction no better than an approximation. For this reason, all model predictions should be expressed as a range of possible outcomes which reflect the uncertainty in model parameter values. The range of uncertainty should be similar to that used for the sensitivity analysis. The following examples demonstrate, in a limited manner, how model predictions may be presented to illustrate the range of possible outcomes resulting in model input data uncertainty. Figure 1.13 shows the range in computed head at

44 44 calibration targets at a particular point in time resulting from varying a model parameter over its range of uncertainty. If the purpose of the predictive simulations is to determine the future hydraulic head distribution in an aquifer, the upper and lower estimate of head should be presented so that appropriate decisions may be made. Figure 1.13 Simulated Uncertainty in Hydraulic Heads at Calibration Targets In the following example (Figure 1.14), hydraulic heads are predicted for a future time interval in response to changing stresses on the aquifer system. The predicted results using the calibrated flow model are presented as the black line. The red and blue lines show the hydraulic heads predicted using slightly different model input parameters which result in values of hydraulic head which are five percent higher and lower than the heads predicted using the calibrated model. Again, the range in predicted heads should be presented so that appropriate, or conservative, decisions may be made regarding the groundwater resource.

45 45 Figure 1.14 Predicted Range in Hydraulic Heads Figure 1.15 shows an example of delineating a wellhead protection area (WHPA) for a public water-supply. These WHPAs were simulated using the range of hydraulic conductivity values reported from an aquifer test at a municipal well. The WHPA delineated using the low hydraulic conductivity value (shown in brown) is wider than and not as long as the WHPA delineated using the high value of hydraulic conductivity (shown in gold). In order to be protective of the public water supply, it's important that the delineated WHPA is conservatively estimated. In this example, the final recommended WHPA is a composite of the two previously simulated WHPAs. Figure 1.15 Simulated Wellhead Protection Areas Using Range of Hydraulic Conductivities