VLEACH approach best tool for environmental impact

By Imam Soeseno

JAKARTA (JP): Imagine a situation in which you, a hydrologist, are involved in conducting an environmental impact assessment (EIA) of a proposed golf course.

As a hydrologist, you would normally deal not only with the quantification of water dynamics in the proposed location, but also be responsible for the prediction of contaminant migration due to future activities related to the golf course. In the era of computers and system approach, it is quite logical for you to turn to mathematical models to help you in such a situation.

There are many mathematical models capable in simulating the dynamics of contaminants in soils and groundwater, but it is not that easy to choose one and incorporate it into your EIA project. In most cases you have to take a model which does not require extensive characterization of the system you are going to simulate. Then, forget about model calibration. There will be no time for this standard procedure.

With such limitations, VLEACH, a vadose zone leaching model developed by Varadhan Ravi and Jeffrey A. Johnson of Dynamac Corporation for the United States Environmental Protection Agency (USEPA), is one of the best choices. With limited data, but mostly available in any EIA study, the model can simulate the dynamics of contaminants in soils (the so-called vadose zone) resulting in quantifications of contaminant load in your soils and groundwater.

VLEACH was initially developed by the consulting group CH2M Hill for the USEPA. The model was designed specifically on the Phoenix-Goodyear Airport Superfund site in Arizona where in 1993 it was used successfully to evaluate groundwater impacts and volatilization of volatile organic contaminants by J. Rosenbloom and co-workers. Since that time, VLEACH has been used at numerous sites in the United States to assess the potential groundwater impacts from existing soil contaminants.

What to expect

VLEACH output provides information regarding the physical nature of the vapor, liquid, and solid contaminant mass balances in the soil, groundwater impacts from contaminants, and the concentration of contaminants within the soil at any time during the simulation time. Within the context of our golf course EIA, we could predict how long and how much a pesticide applied on the golf green would take to reach the groundwater. Since some part of the pesticide would evaporate in the soil, be absorbed by the soil minerals and dissolve in the soil water, the content of each phase (gas, solid and liquid) inside the soil at any depth can be estimated after a given time. Consequently, by adjusting the application rate of the pesticide and for a given soil, we could recommend a suitable application rate which would not pollute the groundwater. This is really an advantage of using a simulation model.

Limitations

VLEACH describes the movement of a contaminant within and between three different phases: as a solute in water, as a gas in the vapor phase, and as an absorbed compound in the solid phase. Equilibration between the phases occurs according to the user- given distribution coefficients. VLEACH, then, simulates vertical transport by advection in the liquid phase and by gaseous diffusion in the vapor phase.

As a dissolved pollutant migrates not only by advection (the movement at the same velocity as water flow), but also by dispersion (the mixing and spreading caused by diffusion and variations of flow velocity in the soil), the migration of the contaminant will be simulated as a plug. VLEACH simulations will result in higher dissolved concentrations and lower travel time predictions than would occur in reality. There are other assumptions made in the development of VLEACH, but still the model is useful in making preliminary assessments of the potential impacts of contaminants.

Requirements

VLEACH does not require extensive data collecting to be able to simulate the soil-groundwater system of any land use type, in our example, a golf course. Basically, you only need the information on the recharge rate of the groundwater, the soil bulk density, effective porosity, water content, organic carbon content, contaminant concentration in the recharge water, and other data normally available in conducting an EIA project. If any of the above data is not available, an estimation can be done and clearly described in the VLEACH manual.

Hardware and software requirements for running VLEACH can be fulfilled by any IBM-compatible currently available on the market. Here are the minimum requirements written in the manual: an IBM-compatible PC with INTEL 8086, 256 K RAM, CGA graphic board, a floppy drive, and DOS 2.0. Certainly you have a better PC and your system is completely qualified to run VLEACH!

Imam Soeseno is a hydrologist.