1.1 Objectives of the Study
1.2 Justification for and Significance of the Study





1. INTRODUCTION


Groundwater is defined as water in the vadose or unsaturated zone and saturated zone. Groundwater is an important source of water supply for municipalities, agriculture, and industry. Estimates of withdrawals by source indicate that during 1990, total groundwater withdrawals were 81 billion gallons per day. Based on these estimates, groundwater withdrawals increased by 9 percent from 1985 to 1990, while surface-water withdrawals increased by 1 percent over the same period (Solley, 1993). Although the rate of increase in fresh-water use in the United States has slackened over the past several decades, the total demand for groundwater has continued to increase.

Contamination of groundwater systems is a growing and pressing problem. A wide variety of chemicals (Appendix B) has been identified as contaminants found in underground water system. In 1988, the National Water Quality Inventory reported data on the relative importance of various sources of contamination and various types of contaminants. State inventories showed that more than half the states and territories listed underground storage tanks, septic tanks, agricultural activities, municipal landfills, and abandoned hazardous waste sites as major threats to underground water system. Other sources include industrial landfills, injection wells, road salt, saltwater intrusion, and brine pits from oil and gas wells (USEPA, 1990b).

Major factors contributing to underground contamination include; weather and rainfall condition (intensity and time), soil profile condition (soil structure, temperature, moisture content, pH values, organic matter content, conductivity), recharge or discharge (irrigation or well), chemical property (solubility, persistence), and so on (Fetter, 1993; Fetter, 1994; Freeze, 1979; Novotny and Chesters, 1981; Dominico and Schwartz, 1990).

1.1 Objectives of the Study

The purpose of this research is to develop a vadose zone leaching, saturated zone mixing, and groundwater flow model to analyze movement of contaminant in a heterogeneous aquifer. The concepts and models of the fate and transport of organic contaminants in the vadose zone, “that body of the soil system extending from the soil surface through the root zone to the saturated zone interface,” (Wagenet and Rao, 1985) were reviewed in recent literature. The developed model was evaluated through laboratory soil column tests. The soil property parameters necessary for the model calculations were estimated through conventional or modified methods. Also, the approximate time until the contaminant reaches surface water may be estimated from flow rates in the saturated zone beneath the vadose zone.




1.2 Justification for and Significance of the Study

Groundwater contamination is a very serious problem, and therefore its mechanism must be studied quantitatively in order to help in tracing the source of contamination or predicting the future effects. Some groundwater contaminants come from chemicals spilled onto the ground or contained in landfill materials through vadose zone. An existing vadose-zone leaching and saturated-zone mixing model program (VLEACHSM 1.0a) from EPA simulates pollutant migration through the homogeneous vadose zone and subsequent mixing within the saturated zone (Lee, 1995; 1996). Because most of the actual vadose zones consist of layers of different soil properties, a vertically-heterogeneous vadose-zone leaching model needs to be developed. Therefore, soil property parameters used for each layer in the model calculations need to be estimated through laboratory tests. Also, because there is no model which can deal with contaminant movement in both vadose zone and saturated zone at the same time, except for a very limited way as in VLEACHSM, a separate program which deals with a heterogeneous saturated zone needed to be developed to estimate the time from the contaminant release to the saturated zone until it discharges to a surface water body. In addition, because graphic user interface (GUI) and graphic representation of the simulation results were not commonly available except in expensive commercial softwares, such graphic capability is desired in the programs developed in this study.

Because the first and second part (VLEACHSM 2.0) of the vadose-groundwater (VG) program will be able to deal with vertically-heterogeneous vadose zones, it can be used to simulate a wider range of actual contamination sites, which will help better monitoring, managing, and predicting contamination of soil and groundwater systems in various geologic situations. Also, an attempt to estimate the contaminant movement within the underlying saturated zone will help in estimating the time it takes to reach surface water such as rivers, lakes, and ocean.

Most existing models, including VLEACHSM 1.0a, were based on ASCII text files, and each user was expected to create an input file using an editor, execute the program, and import the output files into a separate graphic software program to plot the simulation results. Because creating such an input file with an editor requires careful attention in positioning each parameter value within the specified columns, it is desirable that a user-friendly preprocessor with GUI be provided to create the appropriate input data file. Also, because importing the simulation output results into another graphic software requires laborious reformatting of the data structure, it is also desirable that a postprocessor be provided to plot the output results in x-y graphs using GUI programming by Visual Basic or C++.





Last modified: Oct 15, 1999
VG Model / Samuel Lee / VADOSE.NET