VLEACH approach best tool for environmental impact

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.